US Army Course - Refrigeration and Air Conditioning ( Courses 1 - 4)

SUBCOURSE OD1747

EDITION A

REFRIGERATION AND AIR CONDITIONING I

REFRIGERATION AND AIR CONDITIONING I (Fundamentals) Subcourse OD1747 Edition A United States Army Combined Arms Support Command Fort Lee, VA 23801-1809 10 Credit Hours INTRODUCTION This subcourse is the first of four subcourses devoted to basic instruction in refrigeration and air conditioning. This subcourse explains the fundamentals of electricity and their application in the refrigeration process. It discusses circuits, motors, and troubleshooting. This is followed by a discussion of fundamentals and the maintenance of the gasoline engine. The theory of refrigeration is also explained based on the characteristics of refrigerants. Unless otherwise stated, whenever the masculine gender is used, both men and women are included.

INTRODUCTION WITHIN THE LAST 20 years refrigeration has become a vital part of American economy. Not only does nearly every household have its own private machine for the manufacture of ice and cold, but the vast industry of transporting, storing, and selling fresh foods would collapse overnight without the facilities to preserve fruits, meats, and vegetables. Furthermore, many amazing therapies of medical science depend upon refrigeration. All over the world the Army maintains bases equipped with the latest war materiel for keeping the peace or for defending our country. The men who man these bases must have suitable working conditions, proper food, and the best hospital treatment possible. In accomplishing these tasks, the Army makes use of every phase of refrigeration. Consequently, it must have men who will make a career of installing and maintaining the many refrigeration units it owns. This course is offered to personnel who wish to improve their knowledge of the science of refrigeration. This memorandum explains the fundamental reactions which make up the process of present-day refrigeration. It should help the man who is interested in increasing his knowledge of refrigeration. Review exercises are at the end of each chapter.

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ACKNOWLEDGMENT Grateful acknowledgement is made to Allied Chemical Corporation; E. I. du Pont Nemours and Company, Inc., and the American Society of Heating, Refrigeration, and Air Conditioning Engineers for permission to use illustrations from their publications.

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CONTENTS Page Introduction......................................................................................................................................................................i Acknowledgement...........................................................................................................................................................ii Chapter 1 2 3 4 Principles of Electricity....................................................................................................................................................1 Fundamentals of Gasoline Engines...............................................................................................................................41 Physics of Refrigeration.................................................................................................................................................48 Refrigerants....................................................................................................................................................................58 Glossary.........................................................................................................................................................................63 Appendix........................................................................................................................................................................66 Answer to Review Exercises.........................................................................................................................................77

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CHAPTER 1 Principles of Electricity We all use electrical equipment, such a lights, radio, television, electric stove and heaters, refrigerators, air conditioners and many more. We use these items many times a day and accept them as a matter of course. As long as the electrical equipment operates properly, we accept it with little concern about what actually takes place. Each of these devices operates because electric current flows through it. 2. To understand how electricity functions, you need to know the theory of electricity. The word "electric" is derived from the Greek word meaning "amber." The ancient Greeks used the word describe the strange force of attraction and repulsion that was exhibited by amber after it had been rubbed with a cloth. By knowing what electricity does, people have long ago developed theories which now are proving productive. 3. After centuries of experimentation by the world’s greatest scientists, laws by which electricity operates are becoming more widely known and better understood. Also, the world has arrived at a generally accepted theory of the composition of matter. Therefore you must learn about “matter” and certain magnetic effects exhibited by matter. 1. Electrical Fundamentals electrons, which are in constant motion about the nucleus. 1-3. Electrons move at a high rate of speed in orbit around the nucleus and carry a negative charge. The electrons apparently do not bunch up as the protons do in the nucleus. An atom may be compared to our planetary system, with the sun as the nucleus and the earth and other planets representing the electrons. This is illustrated in figure 1, which shows the similarity between a hydrogen atom and our earth-sun system. More complex atoms have a larger nucleus and additional electrons. The electrons are considered to be relatively loose and are usually considered to be that which make up an electric current or flow. 1-4. Electricity is often referred to as static electricity or dynamic electricity. A generator is said to produce dynamic electricity, and from this comes the word “dynamo” as another name for a generator. This is a machine which converts mechanical energy to electrical energy. Generally speaking, we are able to control dynamic electricity so that it is a useful force which we can put to work. A battery is also a source of dynamic electricity which we can control. The chemical action in a battery produces electrical energy which has three useful applications in an automobile. It drives the electric motor which starts the engine. It supplies energy to the spark plugs as heat for ignition, and the car lamps also use electrical energy for light. The car's generator recharges the battery and supplies the electric power when the engine is running. Generators and batteries are the most widely used sources of dynamic electricity. Now let's discuss static electricity and its effects. 1-5. The effects of static electricity can be observed in dry weather when you run a comb through your hair. The crackling you hear is the result of small discharges of electricity, and in a dark room you can see the tiny flashes of light a mirror. Lightning in a summer storm is the violent discharge of tremendous static charges.

1-1. Matter means all substance - solids, liquids, and gases. Today, the accepted theory is that matter is composed of three long-lived particles and many more short-lived particle. We are concerned only with one of the three long-lived particles - the electrons. 1-2. Electron Flow. Where there is a general movement of electrons in one direction, an electric current flows. The electrons together with protons (positively charged particles) and neutrons (neutral particles), make up atoms, of which all substances are composed. The protons and neutrons are in the nucleus (center of atom) and generally do not move about within a substance. The remainder of the atom is composed of

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Figure 1. Structure of atoms compared to earth and sun. A charge accumulates over a period of time, and when it becomes great enough to overcome the resistance of the air, a bolt of lightning occurs. Static electricity is the result of friction which dislodges enough electrons to form a charge. When the charge becomes very great, the accumulated energy is released in the form of electrical energy accomplished by lightning and thunder. 1-6. The next discussion will cover the three most common terms in electricity: “voltage,” “current,” and “resistance.” These three words are probably the most important in electrical fundamentals. If you understand the relationship between voltage, current, and resistance, you will have a good foundation on which to build your knowledge of electricity. Therefore, it is important that you learn the meaning of these terms. Since electricity cannot be seen, we will present visual comparisons to help you in understanding the relationships. 1-7. Voltage is one of the several terms which mean the same thing. These terms are: “voltage,” “potential,” “electromotive force (emf),” “potential difference,” and “electrical pressure.” The last term, “electrical pressure,” comes close to telling what voltage is. For example, the voltage of a battery is like water pressure in a hose when the nozzle is closed. This is called potential energy, not performing work. When the nozzle is opened, the water is forced out by the pressure, thus doing work. This may be related to closing an electrical switch, such as turning on your automobile lights. The potential energy of your 2 battery is then released, performing the work of lighting the lights. The voltage is expended in the lights in the form of heat and light. Remember that voltage is electrical pressure. 1-8. The current flow is made possible by closing the switch which lowers the resistance to the voltage. Since this circuit has a relatively high resistance, the lamps could be burned for several hours before the battery would be discharged. The starter for the engine has a very low resistance, so it will draw a large current from the battery. It uses so much energy that the battery may become completely discharged by operating the starter for just a few minutes. This is reasonable because the starter is doing more work (converting electrical energy into mechanical energy) than are the car lights. With the foregoing discussion in mind, let us now consider concise definitions of our electrical terms. Voltage is electrical pressure. Current is the movement of electrons. Resistance is the opposition to current flow. 1-9. Voltage is measured in volts. Current is measured in amperes. Resistance is measured in ohms. One volt is the electrical pressure required to cause 1 ampere of current to flow through a resistance of 1 ohm. Scientists have made experiments which show that 6280 trillion electrons pass a given point each second when there is 1 ampere of current in a circuit.

1-10. Resistance to electric current is present in all matter, but one material may have much more resistance than another. Air, rubber, glass, and porcelain have so much resistance that they are called insulators and are used to confine electricity to its proper circuit. The rubber covering on the wires to an electric lamp prevents the wires from touching each other and causing a short circuit. The rubber also protects a person who is using the lamp so that he does not receive an electric shock. Air acts as an insulator whenever a light switch is opened. Air fills the gap between the open contacts of the switch, and no current flows because of the high resistance. However, even air may at as a conductor if the voltage is

high enough; otherwise, there could not be the electrical discharge which appears in a lightning strobe. 1-11. Metals are good conductors of electricity but some are better than others. Copper and silver are both good conductors of electricity because of their relatively low resistance. Aluminum is not as good, but is used for long overhead spans because of its light weight. Iron is a poor conductor, although it is used in combination with aluminum for added strength. Alloys of nickel and chromium are used in heater element to provide a specific resistance which passes enough current to heat the wires to a red glow. The alloy makes it possible to operate at high temperatures without melting. Copper is

Figure 2. Copper wire size and resistance. 3

relatively cheap and a good conductor; it is the most widely used for wiring circuits. 1-12. The resistance of a copper wire is determined by three things: the cross-sectional area, the length, and the temperature. In normal temperature ranges the change in resistance is very small. The main factors of resistance are the area or cross section of a wire and its length. A wire with a larger diameter will have a greater cross-sectional area than will a smaller wire, and consequently less resistance. A long wire will have more resistance than a short one. Figure 2 shows the relationship between wire size and resistance. The first column gives the wire by number. A No. 40 wire is about the diameter of a hair. Sizes larger than No. 4/0 (spoken as four aught) are given in thousands of circular mil (350 MCM is 350,000 circular mils). The column at the right gives the resistance in ohms for 1000 feet of wire. One thousand feet of No. 10 copper wire has a resistance of about 1 ohm. The safe current carrying capacity is given in three columns which show the effects of insulation and conduit on the heat radiation ability of the conductor. 1-13. Magnet Characteristics. Magnetism related to electricity as heat is related to light. Whenever light is produced, we have heat; and wherever electricity is produced in the form of an electric current, we have magnetism. However, heat can be made without visible light and magnetism can be detected without an electric current. The effects of magnetism make a good starting place toward an understanding of electricity. Many of the fundamental laws can be demonstrated by simple experiments which you can perform for yourself. 1-14. A magnetic compass needle, a bar magnet, and some iron filings are the main things required. The compass needle will point toward the magnetic poles of the earth unless iron or steel objects are close enough to

Figure 4. Pattern of a magnetic field. affect it. When the north pole of a bar magnet is brought close to the north pole of the compass needle, they will repel each other, as shown in figure 3; but there is a strong attraction between a north pole and a south pole. This illustrates the fundamental law of magnetism which says that like poles repel while unlike poles attract. Between two magnets there is a magnetic field made up of lines of force. 1-15. This field around a magnet can be shown by placing a sheet of glass or paper over a bar magnet. As iron filings are sprinkled over the surface, they assume a definite pattern, as shown n figure 4. The magnetic field is strongest at the poles of the magnet, where the lines of force are bunched closely together. Lines of force follow a uniform distribution and never cross each other. A magnetic field may be distorted by iron or influenced by another magnetic field. A piece of soft iron will concentrate the lines of force in a field. In the same manner, two unlike poles brought near each other will have their fields linked up in common with each other. 1-16. Lodestone is a natural magnet which has been known for many centuries. From it the first compass needles were fashioned. Artificial magnets are made by exposing metal to a strong magnetic field. Hardened iron will retain magnetism over a long period of time. Alloys of aluminum and nickel make even stronger magnets. 1-17. The relationship between electricity and magnetism can be demonstrated by a strong electric current passing through a conductor. If iron filings are sprinkled over a piece of cardboard, as shown in figure 5, they will show a pattern of rings surrounding the conductor. A sensitive compass held near the wire will line up at right angles to the wire, showing that the lines of force have a definite direction. The compass needle

Figure 3. Attraction and repulsion between magnets.

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will swing around 180° if the current in the conductor is reversed. This requires direct current (dc) such as we get from a battery. The current from a battery is said to have only one direction, so it is called direct current. By reversing the connections of a circuit to a battery, the current in that circuit may be made to take the opposite direction. 1-18. The magnetic field produced by a single straight conductor is relatively weak. However, the field can be concentrated by forming the conductor into a coil. In this form a coil carrying an electric current shows a magnetic pattern similar to that of a bar magnet. The coil develops a north pole at one end and a south pole at the other end. The polarity may be determined by the left-hand rule which states, “If the coil is grasped with the left hand with the fingers pointing in the direction of electron flow (negative to positive), then the thumb will point toward the north pole of the coil.” Electrons have a negative charge so they are attracted by a positive charge. Consequently, the electron movement in a circuit is from negative to positive. The electron movement in the conductor is indicated by two arrows figure 6. 1-19. Most coils are formed around an iron core because the core intensifies the magnetic field. The coil with its iron core is called a solenoid. Air offers resistance to the lines of force, which is called reluctance. Iron has less reluctance than air so that the lines of force will choose a path through iron rather than air when there is a choice. Forming the iron into the shape of a horseshoe makes less distance between the poles of a magnet, and the field is more concentrated. Soft iron is used for the core in electromagnets, as it will lose its magnetism when the current in the coil stops. The core is

Figure 6. Magnetic field produced by a coil. built up with thin sheets of soft iron which serve to insure the loss of magnetism when the magnetizing force is removed. An example of this is an electromagnet used for picking up and moving scrap iron in a salvage yard. The magnet is hung from a crane and may pick up a ten or more of iron at one time. When the load is moved into position to be dropped, the current to the coils is shut off. The loss of magnetism in the core allows the load to fall. The strength of the field of an electromagnet is determined by the number of turns of wire in the coils and the magnitude of the current. 1-20. Electron Movement and Effects. Electrons flowing through conductors cause several effects. We shall discuss some of these briefly. 1-21. Heat. Heat is generated as the electrons flow through the conductor. The electric coffeemaker, electric stove or heater, and such items are examples of this effect that we see each day. Light is a side effect of the heat generated. 1-22. Light. An incandescent lamp is made up of a filament (conductor) inclosed in an evacuated envelope. As current passes through the filament, it is heated to the point of glowing. If no air is allowed into the envelope, the filament will last a long time. 1-23. When electrons flow through an ionized gas at the right pressure and value, the gas will glow. Also, if a stream of electrons strikes certain compounds, the compounds will glow. Your TV picture gives a picture because of this effect. 1-24. Chemical. The chemical effect of electron movement is important. If electrons are forced to move through a solution of certain chemicals, one of the elements in the solution will come out of the solution in its natural state.

Figure 5. Electric current produces magnetic field. 5

Figure 7. Basic electrical symbols. 6

Thus, if an electric current is sent through a solution of copper sulphate, pure copper is deposited on one of the contacts immersed in the solution. A stream of electrons reaching a contact immersed in a solution can change the chemical makeup of the contact. 1-25. Magnetism. Magnetic field, identical to those discussed previously are produced as a direct result of electron movements within a conductor. 1-26. Electromagnetic Fields. The magnetic fields produced by electric currents are called electromagnetic fields and are composed of lines of force like all other magnetic fields. For example, in the field around a straight wire (conductor) carrying current the lines of force are concentric circles. The force of the field is strongest close to the wire, and it weakens rapidly the greater the distance from the wire. 1-27. To determine the direction of the magnetic field about a current-carrying wire, use the left-hand thumb rule which states, “Hold your left hand as if grasping the wire in such a way that your thumb points in the direction of the current (electron) flow. The fingers of your left hand will then point in the direction of the magnetic field about the wire.” 1-28. The magnetic field associated with a loop of wire is much the same as the field of a bar magnet. The loop has poles similar to those of a bar magnet, with lines of force emerging from the north pole and entering the south pole. The left-hand rule applied to the loop of wire will show you which is the north and which is the south pole. 1-29. If equal currents pass through a coil of wire consisting of 8 closely wound turns and through a singleturn loop of the same diameter as the coil, the magnetic fields will be almost identical in direction at every point. However, the magnetic field strength of the 8-turn coil will be approximately 8 times that of the single loop. This is because the fields of the 8 turns are virtually parallel to each other at every point and their effects are cumulative at every point. 1-30. If you spread out the 8 turns into a helical coil the magnetic field between the turns will be very weak. This is because the fields of adjacent turns will be opposite in direction and will tend to cancel each other. Inside and outside the coil they will be strong, for they will be cumulative. The net result will be a strong field of fairly uniform intensity, represented by nearly straight lines of force both inside and outside the coil. 1-31. Both of the coils, the one closely wound and the other spread apart, will each have a north pole at one end and a south pole at the other. The direction of the field will depend upon the direction of the current flow.

1-32. Safety. Anyone working with electricity must always be on his guard because of the dangers involved with electricity. Follow all rules. The basic rule is to keep clear of lines or equipment when they are energized. Do not put yourself in such a position that your body may become part of the circuit. Rules cannot be written to cover every situation; your own good judgment must govern your actions. The man who always practices safety will establish good working habits so that he will naturally do his work in a safe manner. The man who neglects safety is a menace to himself and to those working around him. Carelessness or a devil-may-care attitude should not be tolerated; either will eventually lead to the destruction of life or property. 1-33. Study the information in figure 7 so that you can recognize and identify each item. These symbols will be used in this chapter to make schematic diagrams of circuits. The purpose and application of these device will be explained in the discussion of circuits. 1-34. An example of the use of symbols is shown in figure 8. The upper part shows a picture of a toaster, a percolator, and a hot plate. Each of these has a resistance element which converts electricity into heat when the appliance is plugged into an outlet. The lower part of the figure show how these items would be repre-

Figure 8. Comparison between picture and symbol representation.

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Figure 9. Inducing voltage by moving a conductor through a field. sented in a schematic diagram. Each item is shown by the same resistance symbol and must be identified with labels to distinguish which is which. Notice how much simpler the schematic diagram appears and yet it conveys the same information from an electrical standpoint as the more complex picture. You could easily draw the diagram in less than a minute and it would tell another technician the same story - that there were three appliances connected to a suitable source. 2. Production of Electromotive Force 2-1. A generator is a machine which converts mechanical energy into electrical energy. First, the generator must have some source of mechanical energy. The type of machinery used to supply this energy to the generator is usually called the prime mover. 2-2. There are a number of methods used as prime movers. Water power (hydroelectric) normally has low operating costs, but high installation costs. Steam power (steam turbine) has a low installation and operating cost when used for plants of 15,000-kw capacity or more. Diesel engines are used a great deal h plants where the capacity required is from 2,000 w to 15,000 kw. However, there are low-speed and high-speed diesel engines. The high-speed diesel engine has a lower installation cost than the low-speed type, but its life is not as long. Gasoline engines should not be chosen to drive generators in plants which require continuous power because their fuel and maintenance costs are too high. The gasoline engines are usually used for small portable units. 2-3. The electrical power output from a generator may be either direct current (dc) or alternating current (ac), depending upon the construction. However, in principle, the rotating coils and the magnetic field through which they turn are the same for both types of generators. The primary difference between ac and dc generators is the method by which the current is taken from the machine. 2-4. In a generator we have two set of coils and a field: one set of coils is in motion and the other set of coils acts as an electromagnet to set up a magnetic field. Figure 9 shows how a conductor moving across a magnetic field has a voltage induced in it. The galvanometer connected to the conductor has the zero position of the pointer in the center of the scale so that it can read current in either direction. As the conductor is moved upward through the field, the galvanometer needle is deflected to the left. When the conductor is moved downward, the galvanometer needle is deflected to the right, showing that the direction of current in the conductor is reversed. 2-5. Direct-Current Generator. A simplified diagram of a dc generator is illustrated in figure 10. A loop of wire represents the conductor that rotates in the magnetic field. The ends the loop terminate in two copper half rings which are insulated from each other. Fixed brushes make a contact with the copper to conduct electricity to the external circuit. The loop is rotated a clockwise direction. In position A, the lines of force are not being cut by the armature conductors but no voltage is produced. In position , with the black half of the armature conductor

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toward the north pole and the black half-ring against the negative brush, the armature conductor is cutting the maximum lines of force. At this position maximum voltage is induced into the armature conductors with the current flow through the galvanometer as indicated in figure 10. At position C, the armature conductor has rotated 180° from position A and again no voltage is produced. In position D, with the white half of the armature conductor toward the north pole and with the white half-ring against the negative brush, the armature conductor again is cutting the maximum lines of force, with maximum voltage being induced into the armature conductors and with the current flow through the galvanometer in the same direction as position B. Check the black brush in the figure at positions B and D and you will see that the sides of the armature conductor change but the brushes are stationary; they deliver direct current because either armature conductor in contact with the black brush will have the same direction of motion across the field. 2-6. A direct-current generator is quite different from the working model shown in figure 10. Instead of permanent magnet, strong electromagnets are used. The strength of the field can be controlled by changing the current in the field coil A variable resistance in the field circuit makes it possible to control the voltage output of the generator. Instead of a single loop, there are many coils of wire in the rotor. The ends of each coil terminate in opposite copper segment. These copper segments are formed in a ring called the commutator. The rotor assembly illustrated figure 11 is an armature for a dc generator. 2-7. The ends of the armature shaft ride in bearings. The three main parts of a generator are the stator, the rotor, and the end bells. The main frame of the generator holds the stator or field. This frame supports the end bells which carry the bearings. One end bell contain the brush rig which holds the brushes. The voltage generated is controlled by a rheostat in the field circuit that changes the strength of the electromagnets. A change in speed would also change the voltage, but it is much simpler to control by resistance. 2-8. Alternating-Current Generator. A simplified diagram of an ac generator is shown figure 12. The difference between the dc generator and the ac generator is in the method used to deliver the current to the brushes. In the ac generator, sliprings are used instead of a commutator. This means that the same side of the loop delivers current to the same brush reFigure 10. Simplified diagram of a direct-current generator.

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Figure 11. DC generator armature. gardless of rotation; otherwise the operation is the same. 2-9. The illustration shows the loop turning in a clockwise direction. At position A, the lines of force are not being cut by the armature conductor so no voltage is produced. At position 3, the armature conductor is cutting the maximum lines of force, and the galvanometer indicates the direction of current flow by the needle pointing to the right. At position C, the galvanometer again shows zero because the lines of force are not being cut by the armature conductor. At position D, the armature conductor are again cutting the maximum lines of force, and the galvanometer again shows a current flow but in the opposite direction. What happened? At position B, the black side of the loop is moving down through the field and the black slipring is negative, sending current toward the meter. At position D, the black side of the loop is moving up through the field. Now the black slipring is positive. Current is directed from the white slip ring to the meter and back. The direction of current in the loop reversed itself and the same is true in the external circuit to the meter. The loop in the dc generator operated the same way but the commutator acted as a mechanical device to direct the current in only one direction to the external circuit. 2-10. The output frequency, or cycles, of an ac generator is determined by it speed and the number of poles. A two-pole machine must be driven at 3600 rpm to produce 60 cycles per second. A four-pole machine requires a speed of 1800 rpm for a frequency of 60 cycles. The formula for frequency is 2-11. The simple ac generator discussed here would produce single-phase current, as there is only one loop or winding. A three-phase generator requires three sets of windings and each winding produces one phase. The windings are physically displaced from each other 120° apart so that maximum voltage in one winding is generated at a different time from that in the other windings. At least three wires are needed to deliver three-phase electrical power from the generator to equipment. A single-phase voltage and current is developed between any two of the wires. Phases may be designated by number or as A, B, C, for identification. Figure 13 shows the pattern of a three-phase current for one complete cycle. A peak occurs every 60°, or 6 times for each cycle. The same pattern of rise and fall should be used to illustrate the cycle of three-phase voltage. 3. Direct Current Fundamentals 3-1. In order for current to flow, two things are essential: there must be a source of electrical pressure (voltage) and there must be a complete circuit. The source of voltage may be a battery, a generator, or some other device. The complete circuit requirement means that there must be a complete path from the negative terminal through the load and back to the positive terminal of the source. The complete path should allow the electrons to flow freely to the load, do their work in the load, and then move freely back to the source. 3-2. However desirable this condition is, it cannot be completely achieved since no material used as a conductor (wire) allows the electrons to move with complete freedom. There is always some resistance to the electron flow. All conductors have some resistance; just how much they have depends on the size and length of the con-

where f is the frequency, P is the number of poles, and S is the speed rpm. The output voltage is controlled in the same manner as described for a dc generator. A rheostat in series with the field is used to change the strength of the field magnet; the stronger the field, the greater the voltage generated. 10

Figure 13. One cycle of a three-phase current. ductors as well as on the materials of which they are made. 3-3. The source of voltage is any device which has an excess of electrons in one place over the number of electrons in another place. Connecting the two places by means of an electrical circuit, including resistance, permits the two places to try to equalize the number of electrons. The movement of electrons that results from this attempt is what is known as current. 3-4. Ohm’s Law and DC Circuit. Since Ohm's law contains two separate thoughts, it may be expressed in the following two statements: (1) Current in any electrical circuit is directly proportional to the voltage, and (2) current in any electrical circuit i inversely proportional to the resistance. Ohm's law is more generally stated as follows: The current in a circuit is equal to the voltage divided by the resistance. Mathematically, it is expressed as: (1) In this equation, I stands for the current in amperes, E for the voltage in volts, and R for the resistance in ohms. Thus, if the source of potential is a 6-volt battery and the electrical device is a bulb having 3 ohms of resistance, the current will be:

3-5. The equation for Ohm's law can be converted mathematically to read as follows: E= IXR (2) By use of this equation, you can determine the voltage across a component of a circuit if you know the unit's resistance and the current flow through it. Thus, if you know that the current through a lamp is 2 amperes and the resistance of the amp is 3 ohm, you know that the voltage across it must be 3 X 2, or 6 volts. 11

Figure 12. Simplified diagram of an alternating-current generator.

3-6. The equation for Ohm's law can be converted mathematically in still another way to read: (3) Using equation 3, you can determine the resistance of any circuit component if you know the voltage across it and the current flowing through it. Suppose you know that the voltage across a lamp is 6 volts and the current through it is 2 amperes. You can find the lamp' resistance by substituting in equation 3: Figure 14. Series circuit. Solution: a. Using equation 5: E = 8 + 12 + 4 = 24 volts b. Using equation 4: It = 4 = 4 = 4 amperes c. Using equation 3, the resistance of each unit is computed as follows:

3-7. Using these three equations enables you to find any one of the three quantities - voltage, current, or resistance - if you know the other two. 3-8. Series Circuits. A series circuit is one in which there is only one path through which the current can flow. In figure 14 three resistances and a battery are connected to form a series circuit. Since there is but one path for the current all of the current passes through each resistance and the current is the same throughout the entire circuit, or It = I1 = I2 = I3, etc. (4) 3-9. The total voltage drop in the series circuit is equal to the sum of the voltages (voltage drops) across the individual resistors, or Et = E1 + E2 + E3, etc. (5) 3-10. The total resistance of the circuit is equal to the sum of the resistances of the individual units, or Rt = R1 + R2 + R3, etc. (6) 3-11. If one of the devices in a series circuit burns out, there is no longer a complete path for the current and, therefore, the other devices the circuit will not operate. 3-12. Problem: In figure 14, three resistances are connected in series across a 24-volt power source. The voltages and currents are measured and found to be as indicated in the illustration. Find: a. The total voltage drop. b. The total current. c. The resistance of each unit. d. The total resistance.

d. Using equation 6: Rt = 2 + 3 + 1 = 6 ohms Check: Using equation 3, the total resistance can also be computed as follows:

3-13. Parallel Circuits. In a parallel circuit, two or more electrical devices provide independent paths through which the current may flow. The voltage across each device so connected in parallel is the same, or Et = E1 = E2 = E3, etc. (7) 3-14. The total current in the circuit is equal to the sum of the individual currents flowing through the parallel-connected devices, or It = I1 + I2 + I3, etc. (8) 12

3-15. Thus, the total amount of current in a parallel circuit is greater than the current in any one individual branch or leg, and consequently the total resistance must be less than the value of the smallest resistance in the circuit. The greater the number of electrical devices or resistors connected in parallel in a given circuit, the greater will be the total current, and the smaller will be the total resistance of the circuit. 3-16. Electrical devices are connected in parallel in any installation in order to: (1) decrease the total resistance of the circuit and (2) allow the units to operate independently of each other. In a parallel circuit, if one unit burns out it does not affect the operation of the other units; one path is broken but the other circuits are still complete. 3-17. There are several ways to calculate the total resistance of a parallel circuit. We shall show the simpler way first, which is the product over the sum method, and then give you the more complex general rule. 3-18. To calculate the total resistance of the parallel circuit shown in figure 15, use the following equation and solve for the equivalent resistance of only two paths at a time. (9)

c. Then, combing R(1 equation 9 again, you have

and 2)

with R3 and using

3-21. The general equation for finding the total resistance in a parallel circuit is known as the reciprocal method. It involves determining the reciprocal of the sum of the reciprocals of the individual resistances. In other words, find a common denominator and divide the resistances

3-19. When the load units that are connected in parallel all have the same resistance value, the previous equation may be simplified to read: (9A) 3-20. Problem: In the illustration accompanying the previous discussion, three load units are connected in parallel. Using the resistance values indicated, find the total resistance. Solution: a. Using equation 9, for the first two paths

b. Since 3 ohms is the equivalent resistance of the first two paths you may substitute a 3-ohm resistor for them, and adding the 6-ohm resistor of the third path redraw the circuit as shown at the right in the illustration. 13

Figure 15. Parallel circuit.

into the common denominator, then add and invert, and divide this sum to find the total resistance. (10) d. Since the total current must flow through the series resistor, the current flow through it must be 4 amperes. e. Since the total current flowing through the four lamps is 4 amperes and since they all have the same value of resistance, the current must divide evenly among the lamps and is therefore found to be 1 ampere through each lamp circuit. 3-25. Power. Besides the current, voltage, and resistance of a circuit, the power must also be considered. Power is defined as the rate of doing work, and it is measured in a variety of units. An electric motor, for example, is rated in terms of horsepower. One horsepower is the rate of doing work when a 550-pound weight is raised a distance of 1 foot in 1 second. Some motors develop 5000 or more horsepower. Electrical

3-22. Using equation 10, the total resistance can be computed as follows:

3-23. Series-Parallel Circuit. As shown in figure 16, in a series-parallel circuit some of the units are connected in series with each other, while other units are connected in parallel. To solve a series-parallel problem, first convert it to a series circuit by substituting an equivalent resistance for the parallel resistances; then solve the series circuit problem as explained previously. 3-24. Problem: In the illustration of the series-parallel circuit a resistor is connected in series with four lamps which are connected in parallel with each other. The voltage and resistances were measured and found to be as indicated. Find the current through the various parts of the circuit. Solution: a. The resistance of each lamp is 4 ohms. Therefore, using equation 9A, the equivalent resistance of the four lamps in parallel is

b. Substituting a 1-ohm resistor for the four lamps and using equation 6, you find the total resistance of the circuit as follows: R4 = 5 + 1 = 6 ohm c. Using equation 1, you compute the total current in the circuit to be

Figure 16. Series-parallel circuit.

14

Figure 17. Sine wave of current and voltage. power is generally expressed in terms of watts. A watt is the power consumed in a circuit through which 1 ampere flows under a pressure of 1 volt. One horsepower equals 746 watts. 3-26. Most electrical devices are rated according to the voltage that should be applied to them and also according to the amount of power they require. For example one lamp might be rated as a 115-volt, 40-watt lamp, while another might be rated as a 115-volt, 20-watt lamp. This means that both lamps are to be operated on a 115-volt circuit, but that twice as much power is required to operate the first lamp as the second. 3-27. You can compute the wattage of an electrical unit - that is, the power it requires - by multiplying the value of the current flowing through it by the value of the voltage applied to it. P= IXE (11) 4-2. Alternating current has largely replaced direct current for a number of reasons, namely: (1) ac voltages can be increased or decreased very efficiently with transformers, (2) ac devices are much simpler and consequently are less prone to trouble than are dc devices, (3) ac units are much lighter, and (4) they operate more efficiently. 4-3. Most electrical appliances manufactured in the United States have a small “data plate” which gives the electrical information necessary for connecting the appliances to the proper electrical circuits. This data plate usually gives the voltage, frequency (cycles per second), horsepower (size of motor) or watts (for heating units), amperes, ac or dc, and the power factor. If you connect electrical appliances per the information on the data plate, they usually give a long life of uninterrupted service. 4-4. Phase of Current and Voltage. When current and voltage pass through their zero value and reach their maximum value at the same time, the current and voltage are said to be in phase. If the current and voltage pass through zero and reach their maximum values at different times, the current and voltage are said to be out of phase. In a purely inductive circuit the current reaches a maximum value later than the voltage, lagging the voltage by 90°, or one-fourth of a cycle. In a circuit containing only capacitance, the current reaches its maximum ahead of the voltage, and the current leads the voltage by 90°, or one-fourth of a cycle. 4-5. Figure 17 shows graphically the in-phase condition and the effect of inductance and capacitance on this phase relationship. The current will never lead or lag the voltage by exactly

Thus, a starter motor drawing 70 amperes at a potential of 24 volts is using 1680 watts of electrical power. To convert electrical power (wattage) to horsepower, divide the electrical power rating by 746. Thus, by dividing 1680 watts by 746-watts (the electrical equivalent of 1 horsepower) you will find that the starter motor will develop approximately 2.25 horsepower. 4. Alternating-Current Fundamentals 4-1. In a dc circuit, current moves in one direction, from the negative terminal of the source through the circuit to the positive terminal. In ac circuits, the current flows first in one direction and then in the opposite direction, thus the name “alternating current.” 15

90° because of the resistance of the conductor. The number of degrees by which the current leads or lags the voltage in a circuit depends on the relative amounts of resistance, capacitance, and inductance in the circuit. 4-6. Inductance. When an alternating current flows through a coil of wire, it sets up an expanding and collapsing magnetic field about the coil. The expanding and collapsing magnetic field induces a voltage within the conductor proper which is opposite in direction to the applied voltage. 4-7. This induced voltage opposes the applied voltage, thus serving to lessen the effect of the applied voltage. This results in the self-induced voltage tending to keep a current moving when the applied voltage is decreasing and to oppose a current when the applied voltage is increasing. This property of a coil which opposes any change in the value of the current flowing through it is called inductance. 4-8. The inductance of a coil is measured in henrys, and the symbol for inductance is L. In any coil the inductance depends on several factors, principal of which are the number of turns of wire in the coil, the crosssectional area of the coil, and the material in the center of the coil, or the core. A core of magnetic material greatly increases the inductance of the coil. 4-9. Remember, however, that even a straight wire has inductance, small though it may be when compared to that of a coil. All ac motors, relays, transformers, and the like contribute inductance to a circuit. 4-10. Capacitance. Another important property of ac circuits, besides resistance and inductance, is capacitance. While inductance is represented in a circuit by a coil and resistance by a resistor, capacitance is represented by a capacitor. Any two conductors separated by a nonconductor constitute a capacitor. The capacitor is used in an electrical circuit to momentarily store electricity, smooth out pulsating dc, give more torque to a motor by causing the current to lead the voltage (see fig. 17), reduce arcing of contact points, and hasten the collapse of the magnetic field of an ignition coil to produce a hotter spark. 4-11. Power in AC Circuits. In a dc circuit, we calculate power by using equation 11, where the volts times the amperes equal the watts (power). Thus, if 1 ampere flows in a circuit at a potential of 200 volts, the power is equal to 200 watts. The product of the voltage and the amperage is the true power of the circuit in this case. 4-12. In an ac circuit, however, the voltmeter indicates the effective voltage and an ammeter indicates the effective current. The product of these two indicate what is called apparent power. The relationship between true power, reactive power, and apparent power is shown graphically in figure l8. Only when the ac circuit is made 16

Figure 18. Power relations in an ac circuit. up of pure resistance is the apparent power equal to the true power. 4-13. When there is capacitance or inductance in the circuit, the current and voltage are not exactly in phase with each other, and the true power is less than the apparent power. The true power is obtained by a wattmeter indication. The ratio of the true power to the apparent power is called the power factor of the load and is usually expressed as a percentage. In equation form the relationship is:

(12) 4-14. Problem: A 220-volt motor draws 50 amperes from the supply lines, but the wattmeter indicates that only 9350 watts are taken by the motor. What is the apparent power and what is the power factor of the circuit? Solution: a. Apparent power = 220 X 50 = 11,000 voltamperes. b. Using equation 12,

5. Transformers 5-1. A transformer is an apparatus which transforms electrical energy at one voltage into electrical energy at another voltage. It consists of two coils which are not electrically connected (except auto transformers) but are arranged so that the magnetic flux surrounding one coil cuts through the other coil upon buildup or collapse of the magnetic field. When there is an alternating current in one coil, the varying magnetic flux

Figure 19. Voltage and current transformer. creates an alternating voltage in the other winding by mutual induction. A transformer will also operate on pulsating dc but not on pure dc. 5-2. A transformer consists of three primary parts: an iron core, which provides a circuit of low reluctance for the magnetic flux; a primary winding, which receives the electrical energy from the supply source; and a secondary winding, which receives electrical energy by induction from the primary and delivers it to the secondary circuit. 5-3. The primary and secondary coils are usually wound, one upon the other, on a closed core obtain maximum inductive effect between them. The turns of insulated wire and layers of the coil are well insulated from each other by layers of impregnated paper or mica. The iron core is laminated to minimize magnetic current losses (eddy losses) and is usually made of specially prepared silicon steels since these steels have a low hysteresis loss. (Hysteresis loss is the portion of the magnetic energy converted to heat and lost to the system so far as useful work is concerned. It occurs with changing magnetic polarity.) 17 5-4. There are two classes of transformer - voltage transformers for stepping up or stepping down voltages, and current transformers which are generally used in instrument circuits. In voltage transformers the primary coils are connected in parallel across the supply voltage, as seen in figure 19. In current transformers the primary windings are connected in series in the primary circuit. 5-5. Of the two types, the voltage transformer is the more common. There are also power-distributing transformers for use with high voltages and heavy loads. Transformers are usually rated in kilovolt-amperes. 5-6. Principles of Operation. When an alternating voltage is connected across the primary terminals of a transformer, an alternating current will flow and selfinduce in the primary coil a voltage which is opposite and nearly equal to the connected voltage. The difference between these two voltages will allow just enough current to flow in the primary coil to magnetize its iron core. This is called the exciting (magnetizing) current.

Figure 20. Single-phase generator and load. 5-7. The magnetic field caused by the exciting current cuts across the secondary coil and induces a secondary voltage by mutual induction. If a load is connected across the secondary coil of the transformer, the load current flowing through the secondary coil will produce a magnetic field which will tend to neutralize the magnetic field produced by the primary current. This, in turn, will reduce the self-induced (opposition) voltage in the primary coil and allow more primary current to flow. 5-8. The primary current increases as the secondary load current increases, and decreases as the secondary load current decreases. When the secondary load is removed, the primary current is again reduced to the small exciting current sufficient only to magnetize the iron core of the transformer. 5-9. Connecting Transformers in an AC Circuit. Before studying the various uses of transformers and the different ways of connecting them, you should understand the difference between a single-phase circuit and a three-phase circuit. 5-10. A single-phase circuit is a circuit in which the voltage is generated by an alternator, as shown in figure 20. This single-phase voltage may be taken from a single-phase alternator or from one phase of a threephase alternator, as explained later. 5-11. A three-phase circuit is a circuit in which three voltages are generated by an alternator with three coils so spaced within the alternator that the three voltages generated are equal but reach their maximum values at different times, as shown figure 21. In each phase of a 60-cycle, three-phase generator, a cycle is generated every 1/60 second. 5-12. In its rotation, the magnetic pole passes one coil and generates a maximum voltage; one-third of a cycle (1/180 second) later, this same pole passes another coil and generates a maximum voltage in it; and onethird of a cycle later, it passes still another coil and generates a

Figure 21. Sine wave of voltage outputs of single- and three-phase generators. maximum voltage in it. This causes the maximum voltages generated in the three coils always to be onethird of a cycle (1/180 second) apart. 5-13. Three-phase motors and other three-phase loads are connected with their coils or load elements arranged so that three transmission lines are required for delivery of power. (See fig. 22.) Transformers that are used for stepping the voltage up or down in a three-phase circuit are electrically connected so that power is delivered to the primary and taken from the secondary by the standard three-wire system.

Figure 22. Three-phase generator with three conductors. 18

Figure 23. Step-down transformer, two-wire system. 5-14. However, single-phase transformers may be connected across any two phases of a three-phase circuit, as shown figure 23. When single-phase loads are connected to three-phase circuits, the loads are distributed equally among the three phases in order to balance the loads on the three generator coils. 5-15. Another use of the transformer is the singlephase transformer with several taps in the secondary. With this type of transformer, we can lower the voltage and also have several working voltages, as shown in figure 24. A center-tapped transformer powering a motor requiring 220 volts, along with for lights requiring 110 volts is shown in figure 25. The motor is connected across the entire transformer output, and the lights are connected from the center tap to one end of the transformer. With this connection we are using only half of the secondary output. 5-16. This type of transformer connection is used quite extensively because of the combinations of voltages that may be taken from one transformer. Various voltages may be picked off the secondary winding of the transformer by inserting taps (during manufacture) at various points along the secondary winding. The various amounts of voltage are obtained by connecting to any two taps or to one tap and either end, as shown a previous illustration. 5-17. Transformers for three-phase circuit can be connected in any one of several combinations of the wye (y) and delta (∆) connections. The connection used depends on the requirements for the transformer. 5-18. Wye connection. When the wye connection is used in three-phase transformers, a fourth or neutral wire may be used, as show in figure 26. The neutral wire serves to connect single-phase equipment to the transformer. Voltages (120 v) between any one of the three-phase lines and the neutral wire can be used for power for devices such as lights or single-phase motors. Single- and three-phase equipment can be operated simultaneously, as show in figure 27. 5-19. In combination, all four wires can furnish power at 208 volts, single and three-phase, for operating single- and three-phase equipment such as motors or rectifiers with the center tap used as equipment ground. When only three-phase equipment is used, the ground wire may be omitted. This leaves a three-phase, threewire system. 5-20. Delta connection. Figure 28 shows the primary and secondary with a delta connection. Between any two phases the voltage is 240 volts. This type of connection using the three wires - A, B, and C - can furnish 240volt, three-phase power for the operation of three-phase equipment. 5-21. Wye and delta connections. The type of connection used for the primary coils may or may not be the same as the type of connection used

Figure 24. Multivoltage transformer secondary. 19

Figure 25. Step-down transformer, three-wire system. for the secondary coils. For example, the primary may be a delta connection and the secondary a wye connection. This is called a delta-wye (∆-y) connected transformer. Other combinations are delta-delta, wyedelta, and wye-wye. 5-22. Current Transformers. Current transformers are used in ac power supply systems. 5-23. The current transformer is a ring type transformer using a current-carrying power lead as a primary (either the power lead or the ground lead of the ac generator). The current in the primary induces a current in the secondary by magnetic induction. 5-24. The sides of all current transformer are marked “H1” and “H2” on the unit base. The transformers must be installed with the “H1” side toward the generator in the circuit in order to have proper polarity. The secondary of the transformer should never be left open while the system is being operated; to do so could cause dangerously high voltages and could overheat the transformer. Therefore the transformer output connections should always be connected with a jumper when the transformer is not being used but is left in the system. 6. Electrical Meters 6-1. In the installation, inspection, maintenance, and operation of electrical air-conditioning equipment, you will often have to measure voltage, current, and resistance. A number of instruments have been developed for this purpose.

Figure 28. Wye-to-wye connection. 20

Figure 2. Four-wire, three-phase wye system.

Figure 28. Delta-to-delta connection. We will discuss these meters and their uses. 6-2. Galvanometer. In electrical systems the moving-coil galvanometer (D'Arsonval type) is used quite extensively. This movement is used in such instruments as voltmeters, ammeters, thermocouple thermometers, and electrical tachometer. 6-3. Voltmeter. A voltmeter is an instrument used to measure the difference in electrical potential, or the voltage, between two point. (See fig. 29.) Notice in figure 29 the rotary switch which may be connected to various size resistors. These are in series with the movable coil to limit the amount of current flow through Figure 30. Ammeter with an external shunt. the meter circuit. If an unmarked voltage is to be measured, set the rotary switch to the highest resistance and work down until the meter reads in a somewhat midposition of full scale. 6-4. Ammeter. An ammeter is an instrument that measures the amount of current flowing in a circuit. You may have a need for an ammeter with a range from a milliampere to 500 amperes. These meters may have an external shunt, as shown in figure 30, or they may be internally shunted. Question: What is a shunt for? Answer: Very fine wire is used in the coil. This wire can carry very little current without overheating - only a small fraction of an ampere. A low-resistance shunt is connected in parallel with the meter so that most of the current bypasses the meter; only a very small portion of the total current flows through the coil. For example: When a 300-ampere ammeter and a 300-ampere shunt are connected into a circuit carrying 300 amperes, only 0.01 ampere flows through the meter to give full-scale deflection; the remaining 299.99 amperes flow through the shunt. 6-5. By applying the basic rule for parallel circuits, you can easily compute the value of a shunt resistor needed to extend the range of an ammeter. 6-6. Ohmmeter. An ohmmeter is an instrument used to measure resistance in ohms. Combination voltohmmeters and other multipurpose meters are used more than simple ohmmeters. The principle of operation of an ohmmeter is

Figure 29. Voltmeter. 21

Figure 31. Ohmmeter. basically the same, regardless of whether the meter is a separate instrument or is part of a multipurpose instrument. 6-7. An ohmmeter contains a very sensitive galvanometer. The scale on the dial is calibrated in ohms. Maximum current flows through the circuit when there is a minimum amount of resistance between the ohmmeter terminals. For this reason, zero is at the righthand end of the scale. The ohmmeter does not have an evenly, graduated scale; frequently the right half of the scale will read to about 5000 ohms, while the left half will read 100,000 ohms or more. The left-hand end of the scale is sometimes marked “INF,” which means there is infinite resistance between the terminals. 6-8. Some ohmmeters have three or even four posts to which the leads may be attached. (See fig. 31.) These posts may be marked in different ways on different meters, but for purposes of explanation let us consider a meter on which the posts are marked “C,” “RX1,” “RX10,” and “RX100.” If the leads are connected to C and RX1, the resistance being measured is indicated directly on the scale. If the terminals are connected to C and RX10, the reading on the scale must be multiplied by 10 to give the actual resistance. If the terminals are connected to C and RX100, the reading on the scale must be multiplied by 100. Short the two leads together 22

and zero the meter with the zero adjustment. This must be done any time the lead is moved from one jack to another. CAUTION: Make sure the circuit to be measured is dead before using the ohmmeter. 6-9. Rectifier Meter. Alternating-current voltages are often measured by rectifier type meters. A rectifier meter is actually a dc meter with a rectifier added to change the ac to dc. Without a rectifier, of course, a dc meter would give no indication when applied to an ac circuit. Generally, a copper-oxide rectifier connected as a bridge provides the rectification. This is shown in figure 32. Values of ac voltages indicated on the rectifier meter are effective values. 6-10. Wattmeters. Power in an ac circuit is not always found by multiplying voltage by amperage as in a dc circuit. Such a power computation can be made for ac circuit only when the voltage and current are in phase, that is, when there is a purely resistive load. In practice this condition seldom exists, since in almost all ac circuits the load is reactive because of the presence of inductance and capacitance. The wattmeter, however, measures the true power consumed in a circuit by all electrical devices regardless of the type of load. 6-11. Wattmeters may be used to measure power consumed in either single-phase or three-phase circuits in which the load is balanced. The single-phase wattmeter has a high-resistance moving voltage coil for many turns of fine wire and stationary coils, called current coils, of low resistance with a few turns of heavy wire. Connect the current coils in the line in series with the load, and the voltage coil across the line. 6-12. A single-phase wattmeter may be connected to measure the power by a three-phase circuit. To do this, connect the current coil in one load line and the voltage coil between the line and ground. This will give the power in one phase. Multiply this by 3 to get the total power. 6-13. Three-phase wattmeters consist of two or more single-phase movement with all the moving elements mounted on one shaft. Separate single-phase wattmeters can be used to measure power in three-phase circuits by connecting two wattmeter in any two of the three phases. In this case, add the two wattmeter readings if the power factor of the load (motor) is greater than 50 percent (the power actor can be found on the nameplate or in the technical order). If the power factor is below 50 percent, the power input to the load (motor) is the difference between the two readings. 6-14. You can determine whether to add or subtract the readings by the following: If both of the scale pointers deflect toward the top of the scale, add the readings; if one tends to indicate a negative value, reverse either the voltage or

Figure 32. Rectifier meter circuit. current connections and subtract the reading of one wattmeter from the reading of the other. 6-15. Using Electrical Meters. Only two of the meters discussed in this chapter, the voltmeter and the ohmmeter, are used to locate troubles in an electrical circuit. How the meters are used for this purpose will be explained in detail. However, before attempting to use any of the meters which have been discussed, you should fix firmly in your mind certain precautions concerning their use. 6-16. General precautions. (1) Never connect a voltmeter to a circuit having a voltage that exceeds the voltmeter scale. If the voltage is unknown, start with a high scale and work down until you get the correct one. (2) Never connect an ammeter into a circuit carrying more current than the maximum reading on the scale of the meter. (3) Always connect an ammeter in series with the units in the circuit. (4) Never connect an ammeter across the terminals 23 of a battery or generator, or any other place where you provide a path through the meter from a source of voltage direct to ground. To do so would cause the meter to burn out immediately. (5) Always check the rating of a meter before you use it. (6) Never use an ohmmeter to check an electrical circuit until the source of voltage has been disconnected from all parts of the circuit to be checked. Using the ohmmeter in a live circuit would damage the meter. (7) Always connect the voltage coil of a wattmeter to the supply side of the current coil. 6-17. Voltmeter. The most common trouble found in electrical circuits that are inoperative is an open circuit. This means simply that there is not a complete path for the current to flow through as it should. The “open,” or the place where the circuit is open, can be located with either a voltmeter or an ohmmeter. If electrical power is available, use the voltmeter. 6-18. An open in a circuit may be located anywhere in the circuit. It my be in the switch,

Figure 33. Continuity testing with a voltmeter. 24

fuse, wiring, or in the unit itself. If the fuse is burned out, or open, you should inspect all of the circuit to determine what caused the fuse to blow. 6-19. A trouble known as a short might have caused the fuse to blow. A short is direct contact between the hot and negative or ground portion of the circuit. Since there is practically no resistance in this new or short circuit, the current flow increases immediately until it exceeds the capacity of the fuse and blows the fuse. 6-20. A voltmeter is always connected in parallel with the unit being tested - that is, across the unit - or to the points between which the difference of potential is to be measured. 6-21. If you should accidentally connect the voltmeter in series with the circuit, it wouldn't hurt the mete because the high resistance in the meter would limit the current flow. However, the units in the circuit would not operate because of the low current 6-22. Locating an open with a voltmeter is simply a matter of checking to see how far voltage is present in the circuit. Voltage will be present the circuit right up to the point where the circuit is open. 6-23. When you have to check a circuit to find an open, you can start at any point in the circuit. It is logical, of course, to check the fuse first and the unit second. As explained earlier, this will enable you to tell whether the trouble is an open or a short. 6-24. If the fuse and the unit are both good, you may have to check each end of each length of wire in the circuit to find the open. Use the wiring diagram of the circuit as a guide. The important things are to know what voltage reading you should have at each point in the circuit and to recognize an abnormal reading when you get one. Figure 33 shows the voltage readings obtained at different points in a circuit with an open fuse, an open lamp filament, and one with an open ground wire. 6-25. Ohmmeter. Before you use an ohmmeter to check a circuit, be sure there is no electrical power in the circuit. It was explained earlier in this chapter that using an ohmmeter in a live circuit could damage the meter. 6-26. If you use a multirange ohmmeter to check resistance, choose a scale on the ohmmeter which you think will contain the resistance of the element you are going to measure. In general, select a scale in which the reading will fall in the mid-scale range. Short the leads together and set the meter, with the zero adjustment, to read zero ohms. If for any reason you change scales, readjust the meter to zero ohms. 6-27. Connect the leads across the circuit. Infinite resistance indicates an open circuit. A reading other than infinite resistance indicates continuity. 6-28. Let's simulate locating the troubles with the ohmmeter. First we must be sure we have disconnected

the power from the circuit to permit use of the ohmmeter. Now, with one lead connected to negative or ground, check at various points with the other lead. If you start at the point where the circuit is grounded, the meter will read zero ohms. 6-29. After you pass the first resistance the meter will read that resistance. When you get the first reading of infinite resistance, this will indicate that the open is between that point and the point where you got the last normal reading.

Figure 34. Single-phase motor with capacitor starting winding. 25

6-30. When you check continuity in a parallel circuit, isolate the unit you are checking so the ohmmeter will not show the resistance of parallel paths. 7. Motors 7-1. In this section we will discuss some of the electrical motors that you may encounter in your job. We will discuss ac single and polyphase induction motors, ac/dc universal motors, and synchronous motors. 7-2. Principles of Operation. The speed of rotation of an ac motor depends upon the number of poles and the frequency of the electrical source of power:

7-3. Since an electrical system operates at 60 cycles, an electric motor at this frequency operates about 2 1/2 times the speed of the old 25-cycle motor with the same number of poles. Because of this high speed of rotation, 60-cycle ac motors are suitable for operating larger refrigeration systems. 7-4. Alternating-current motors are rated in horsepower output, operating voltage, full-load current, speed, number of phases, frequency, and whether they operate continuously or intermittently. 7-5. Single-Phase Induction Motors. All singlephase induction motors have a starting winding (see fig. 34) since they cannot be started with only the singlephase winding on the stator. After the motor has started, this winding may be left in the circuit or be disconnected by a centrifugal switch. 7-6. Both single-phase and three-phase motors operate on the principle of a rotating magnetic field. As a simple example of the principle

Figure 35. Production of a rotating magnetic field. 26

Figure 36. Squirrel-cage induction-motor rotor. of the rotating field, imagine a horseshoe magnet held over a compass needle. The needle will take a position parallel to the magnetic flux passing between the two poles of the magnet. If the magnet is rotated, the compass needle will follow. 7-7. A rotating magnetic field can be produced by a two- or three-phase current flowing through two or more groups of coils wound on inwardly projecting poles of an iron yoke. The coils on each group of poles are wound alternately in opposite directions to produce opposite polarity, and each group is connected to a separate phase of voltage. 7-8. You can understand this action with the aid of figure 35, which shows a four-pole stator field energized by two windings connected to two separate phase voltage. Winding No. 1 of the motor is 90° out of phase with winding No. 2, which causes the current in winding No. 1 to lead the current in winding No. 2 by 90°, or by 1/240 second, assuming the frequency of the ac power supply is 60 cycles per second. Winding No. 1 can be referred to as phase 1, and winding No. 2 as phase 2. 7-9. The direction of the magnetic field is indicated by a magnetic needle (considered as a north pole for clarity). The needle will always move to a position where it will line up with the magnetic flux passing from pole to pole. Notice the phase relationship of the two voltages which are applied to the two phase windings of the field. Phase 1 supplies current to the coils on poles A and A', and phase 2 supplies current to the coils on poles B and B'. The two currents are 90° out of phase, with phase 1 leading. 7-10. At position B, the current in phase 1 at a maximum and the poles of A and A' are fully magnetized. The poles of coils B and B' are not magnetized, since the current in phase 2 is zero. Therefore the magnetic needle points in the direction shown. At position C, the current coils A and A', phase 1, has decreased to the same value to which the current in coils B and B', phase 2, has increased. Since the four poles are now equally magnetized, the strength of the field is concentrated midway between the poles, and the magnetic needle take the position shown. 7-11. At position D, the current of phase 1 is zero through coils A and A', and there is no magnetism in these coils. There is maximum current through coils B and B', the magnetic field strength of B and B' is maximum, and the magnetic needle takes the crosswise position. This action is repeated during successive cycles of the flow of the alternating currents, and the magnetic needle continues to revolve in the same direction within the field frame as long as the two phase currents are supplied to the two sets of coils. 7-12. In an induction motor with two poles for each phase winding, the north pole would glide from one pole to the other in 1/120 second and make a complete revolution in 1/60 second, which would be at the rate of 3600 rpm. If the compass needle is replaced by an iron rotor wound with copper bar conductors (usually

27

Figure 37. Shaded pole motor stator windings. called a squirrel-cage rotor because the conductors resemble a squirrel cage, as shown in figure 36, a secondary voltage is induced in the conductors by mutual induction much in the manner that the secondary voltage is developed in a transformer. 7-13. Current flowing in the conductors produces a magnetic field which reacts on the rotating magnetic field and causes a rotation of the iron core similar to the rotation of the magnetic needle. The direction of rotation may be reversed by reversing the connections of one phase. 7-14. Shaded-pole motor. The stator windings of a shaded-pole motor differ from other single-phase motors by definitely projecting field poles (fig. 37). A lowresistance, short-circuited winding or copper band is placed across one tip of each pole, from which the name “shaded-pole” is derived. As the current increases in the stator winding, the flux increases. A portion of this flux cuts and induces a current in the shaded winding. This current sets up a flux which opposes the flux inducing the current; therefore, most of the flux passes through the unshaded portion of the pole, as shown in figure 38.

When the current in the winding and the main field flux reaches a maximum, the rate of change is zero, so no electromotive force is induced in the shaded winding. A little later the shaded winding current, which lags the induced electromotive force, reaches zero, and there is no opposing flux. Therefore the main field flux passes through the shaded portion of the field pole. This results in a weak rotating magnetic field with sufficient torque to start small motors. Because of the low starting torque, shaded-pole motors are furnished in ratings up to approximately 1/25 horsepower and are used with small fans, timing relays, small motion picture projectors, and various control devices. Shaded-pole motors are designed for a specific direction of rotation that cannot be changed after the motor is assembled. 7-15. Split-phase motor. Split-phase motors contain two windings, the main winding and the starting winding. The main winding is wound on the stator and the starting winding is wound on top of the main winding in such a

Figure 39. Schematic of a single-phase, split-phase motor. manner that the centers of the poles of the two windings are displaced by 90°. The windings are connected in parallel (fig. 39) to the same supply voltage; therefore, the same voltage is applied to both winding. The starting winding is usually wound with fewer turns of small size wire and has iron on only two sides. It, therefore, has less inductance than the main winding, which has a low resistance and is surrounded by iron on all sides except one. When the same voltage is applied to both windings, the current in the main winding lags the voltage more than the current in the starting winding. This produces a rotating field which starts the motor. As the motor approaches full speed, a centrifugal mechanism mounted on the rotor opens a centrifugal switch (fig. 39) and disconnects the starting winding from the line. If the centrifugal mechanism should fail to open the switch, the motor will run hot because of the high resistance of the starting winding and will burn 28

Figure 38. Flux path in a shaded-pole motor.

Figure 40. Schematic of a single-phase permanent-split capacitor motor. out the starting winding if allowed to run any length of time. This is the most frequent cause for failure of splitphase motors. The split-phase motors are usually furnished in ratings from 1/60 to 1/3 horsepower and are desirable for use in machine tools, office equipment, pumps, fans, blowers, oil burners, kitchen appliances, and laundry equipment. Split-phase motors may or may not have a built-in thermal overload relay for the protection of the motor during an overload. The relay is usually of the automatic type, opening when the current in the windings is above normal and automatically resetting when the current is restored to normal. To reverse the split-phase motor, reverse the loads of either the starting winding or the running winding. 7-16. Capacitor-start motor. The capacitor-start

motor is so called because a capacitor instead of resistance is used to split the phase. The capacitor, usually mounted on top of the motor, is connected in series with the starting winding to provide the necessary shift in time phase of the current flowing through it. This capacitor is usually intermittently rated and must be disconnected for normal operation, which disconnection is usually done by a centrifugal mechanism mounted on the rotor. When the motor is stopped, the switch closes and is in the correct position when the motor is started again. The capacitor-type motor has a higher starting torque at less current than the split-phase motor and also provides a greater thermal capacity. Capacitor-start motors are usually furnished in ratings from 1/6 to 1 horsepower and are used on compressors, pumps, fans, and machine tools. 7-17. Permanent-split capacitor motor. The permanent-split capacitor motor is similar to the capacitor-start motor, except that the permanent capacitor (fig. 40) is connected in series with the starting winding permanently and is not removed from the circuit during operation by a centrifugal switch. This eliminates the need for a centrifugal switch and switch mechanism. The capacitor is continuously rated and is selected to give best operation at full speed while sacrificing starting torque. Permanent-split-capacitor motors develop 40 to 60 percent starting torque and are used on easily started loads such fans and blowers. 7-18. Capacitor-run motor. The capacitor-run motor has two capacitors connected in parallel (fig. 41). One, a running capacitor, is a continuously rated capacitor and remains in the

Figure 41. Schematic of a single-phase dual voltage capacitor run motor. 29

Figure 42. Three-phase induction motor. 30

circuit while the motor is running. The other, a starting capacitor, is intermittently rated and is used in the circuit during starting. The starting capacitor is removed by a centrifugal mechanism and switch as the motor approaches full speed. Therefore the capacitor-run motor is a combination of the capacitor-start and the permanent-split capacitor motors. This motor has a high starting torque as well as good running characteristics and is generally furnished in ratings of 1/2 horsepower and larger. Capacitor motors may be reversed by changing the leads to the starting winding at the motor terminals. 7-19. Three-Phase AC Induction Motors. The three-phase ac induction motor is also called a squirrelcage motor. The rotating magnetic field of the threephase motor operates the same as a two-phase motor. The difference between a two-phase and a three-phase motor in the windings. The two-phase windings are placed 90° apart where the three-phase windings are placed 120° apart. This means that the currents that produce the magnetic field reach a maximum 1/180 second apart in a 60-cycle circuit. 7-20. Notice figure 42, which shows the connection of a wye-connected stator in a three-phase induction motor. The rotor of the motor is represented by the compass needle, which points in the direction of the magnetic field and revolves as the magnetic field revolves. The individual current waves are shown along the phase wires as they would actually be during operation. Notice the current in phase A reaches a maximum at position 1 and at that instant the currents in phases B and C are both negative. 7-21. At position 2, 1/180 second later, the current is at a maximum in phase B and is negative in phases A and C. At position 3, which is 1/180 second later than

position 2, the current is at positive maximum in phase C and is negative in phases A and B. In the diagrams the magnetic field caused by the maximum positive current is shown in heavy dark lines. The other poles are indicated with dotted lines. The rotor, like the single-phase motor, follows the rotating magnetic field of the stator winding. 7-22. The speed of the induction motor is always less than the speed of the rotating field of the stator. If the rotor were to turn at the same speed as the rotating field, the rotor conductors would not be cut by any magnetic field and no voltage would be induced in them. No current would flow; thus there would be no magnetic field in the rotor and, hence, no torque. 7-23. A three-phase induction motor exerts a torque when at rest and therefore starts itself when the proper voltage is applied to the stator field coil. To reverse the direction of rotation of a three-phase motor, reverse the leads of any two phases. 7-24. The three-phase spring (wound rotor) induction motor is wound with a three-phase drum winding. The windings are connected wye (y) or delta (wye connection is shown in fig. 43), and the three leads are brought out and connected to three electrical contact rings (sliprings) which are secured to the shaft. Brushes riding on the rings are connected to an external resistance through which the rotor circuit is completed. Motors containing wound rotors have a high starting torque with low starting current us adjustable speed. 7-25. Synchronous Motors. Synchronous motors are divided into two classes according to their size and application. The larger horsepower motors use threephase power and have separately excited salient pole rotors. The smaller motors are usually furnished as fractional-horse-

Figure 43. Schematic of a three-phase slipring induction motor. 31

Figure 44. Schematic wiring diagrams of universal motors. power motors and obtain their rotor-excitation current through induction. Although an induction motor is considered as a constant-speed motor, it is subject to approximately 10 percent variation in speed under various load conditions, since the operating torque depends upon the percentage of slip between the rotating magnetic poles and the magnetic flux of the rotor. The speed of a synchronous motor is controlled by the frequency of the alternating-current power source and is, therefore, maintained with a high degree of accuracy. The smaller

size synchronous motors are constructed as reluctance motors or hysteresis motors, which are described in following paragraphs. 7.26. Reluctance motor. The stator of a reluctance motor is similar in construction to that of the singlephase induction motor and may be of the shaded-pole, split-phase, or capacitor type. The squirrel-cage rotors have grooves cut to allow the addition of salient poles. The number of salient poles mounted on the rotor corresponds to the number of rotating stator poles. The motor starts as an induction motor, but, upon reaching a speed near synchronism, it pulls into step because of the salient poles and operates at exactly synchronous speed. The reluctance motor, unlike the larger size synchronous motor (which has on the rotor a field winding supplied with direct-current excitation and which operates at unity or at a leading power factor with high efficiency), operates at a lagging power factor and has a rather low efficiency. Therefore, the reluctance motor is used only where exact synchronous speed is required, such as in electric clocks, time switches, relays, and meters. 7-27. Hysteresis motor. The construction of the hysteresis motor is similar to that of the reluctance motor except for the rotor. The rotor does not have a squirrelcage winding. Instead the rotor core is usually made of a ring of metal having permeability, such as chrome or cobalt steel. The highly magnetic core material retains its magnetism over a period of time and this enables the rotor to reach its synchronous speed. Hysteresis motors develop a constant torque from zero synchronous speed and are used in a clock’s timing devices; they will operate unattended for long periods of time. 7-28 Universal Motors. Universal motors are designed for operation from either direct current or single-phase alternating current and are all of the serieswound type; that is, the field windings are connected in series with the armature windings. Universal motors are divided into two types: the straight series-wound universal motor and the compensated series-wound universal motor. 7-29. Straight series-wound universal motor. The straight series-wound universal motor has the field windings connected in series for opposite polarity, the same as the field winding of any direct-current motor, and then in series with the armature (fig. 44A). This type motor uses salient-type pole pieces (fig. 45) for mounting the field windings and is usually furnished in sizes up to 1/3 horsepower but can be furnished in larger sizes for special applications. The motor full speed is rated from 1800 rpm on the larger sizes to 5000 rpm on the smaller sizes and no-load speeds ranging from 12,000 to 18,000 rpm. Since these motors run at dangerously high speeds at no-load, they are usually built into the

32

Figure 45. Salient pole laminated steel core of a universal motor. equipment being driven. This type motor is used in portable machines and portable equipment in general. 7-30. Compensated series-wound universal motor. The compensated series-wound, distributed-field, universal motor contains a main winding and a compensating winding connected in series with the armature (fig. 44B). The core of this type motor is similar to the construction of the core of a split-phase alternating-current motor (fig. 46). The main winding is usually placed in the slots first and the compensating winding is placed over it, 90 electrical degrees away. The compensating winding reduces the reactance voltage present in the armature when alternating current is used. It has a better commutation and power factor than does the straight series-wound universal motor, and usually comes in higher horsepower ratings. Compensated series-wound universal motors are used with portable tools, office machines, vacuum-cleaning equipment, and portable equipment in general. 8. Motor Maintenance 8-1. Cleanliness is essential if we are to have trouble-free motor operation. Dirt, moisture, and excessive oil tend to restrict air circulation, deteriorate the insulation, and accelerate wear and friction. To increase the life of the motor, you should wipe all excessive dirt, oil, and grease from the surface of the motor. Use a cloth moistened with a recommended cleaning solvent.

CAUTION: Do not use flammable or toxic solvents for cleaning, as they may cause injury to personnel or damage to property. 8-2. The inside of the motor can be cleaned with a blower or with compressed air. Care should be exercised when using compressed air so the insulation is not damaged by the blast of air. 8-3. Motor Lubrication. You must be sure the motor has been properly lubricated. Lubrication should be done according to the applicable publication for the motor. 8-4. You should also make periodic checks for grease or oil leakage and for overlubrication. After lubricating a motor, be sure to wipe away any excess oil or grease. 8-5. Wiring. The wiring leads to the motor must be kept clean and secure and checked for wear. If the wiring becomes frayed, it must be replaced. 8-6. Mounting. Motors must be kept secure to perform efficiently. A loose mounting can cause a belt to slip and wear or can cause vibrations which tend to harden any copper component (wiring and tubing). 9. Circuit Protective and Control Devices 9-1. Electricity, when properly controlled, is of vital importance to the operation of refrigeration equipment. When it is not properly controlled, however, it can become dangerous and destructive. It can destroy components or the complete unit; it can injure personnel and even cause their death. 9-2. It is of the greatest importance, then, that we take all precautions necessary to protect

Figure 46. Compensated series-wound universal motor. 33

the electrical circuits and units and that we keep this force under proper control at all times. In this section we shall discuss some of the devices that have been developed to protect and control electrical circuits. 9-3. Protective Devices. When a piece of equipment is built, the greatest care is taken to insure that each separate electrical circuit is fully insulated from all others so the current in a circuit will follow its intended individual path. Once the equipment is put into service, however, there are many things that can happen to alter the original circuitry. Some of these changes can cause serious troubles if they are not detected and corrected in time. 9-4. Perhaps the most serious trouble we can find in a circuit is a direct short. You have learned that the term is used to describe a situation in which some point in the circuit, where full system voltage is present, comes in direct contact with the ground or negative side of the circuit. This establishes for current flow a path that contains no resistance other than that in the wire carrying the current, and these wires have very little resistance. 9-5. You will recall that, according to Ohm's law, if the resistance in a circuit is extremely small, the current will be extremely great. When a direct short occurs, then there will be an extremely heavy current flowing through the wires. 9-6. To protect electrical systems from damage and failure caused by excessive current, several kinds of protective devices are installed in the systems. Fuses, circuit breakers, and thermal protectors are used for this purpose. 9-7. Circuit protective devices, as the name implies, all have a common purpose: to protect the unit and the wires in the circuit. Some are designed primarily to protect the wiring. These open the circuit in such a way as to stop the current flow when the current become greater than the wires can safely carry. Other devices protect a unit in the circuit by stopping current flow to it when the unit becomes excessively warm. 9-8. Control Devices. The components in an electrical circuit are not all intended to operate continuously or automatically. Most of them are meant to operate at certain times, under certain conditions, to perform very definite functions. There must be some means of controlling their operation. Either a switch or a relay, or both, may be included in the circuit for this purpose. 9-9. Switches. Switches are used to control the current flow in most electrical circuits. A switch is used to start, to stop, or to change the direction of the current flow in the circuit. The switch in each circuit must be able to carry the normal current of the circuit and must be insulated heavily enough for the voltage of the circuit.

9-10. The toggle switch (as shown in fig. 47 along with the knife switch which is used to simplify the operation of a toggle switch) is used more than any other kind of switch, but there are others, such as pushbutton, microswitch, rotary selector, and even relays and magnetic motor starts, which can be classified as switches since they operate, start, and stop current flow in a circuit. 9-11. Magnetic motor starters. A magnetic motor starter is wired to satisfy a particular application; and there are numerous applications, so we will not attempt to cover all of them. Figure 48 shows a pump, air conditioner, and fan operating through motor starters. Look at figure 48 and notice the two single-pole single throw (SPST) switches, thermostat, holding coils, motor protectors, and step-down transformer. Also notice that three-phase equipment must have protective devices in at least two wires, but single-phase equipment may be protected by one protective device.

Figure 47. Various knife and toggle switches.

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Figure 48. Use of magnetic motor starters. 9-12. In figure 48, the air conditioner will not operate unless the fan and pump holding coils are energized, and the thermostat switch is closed. Notice that the control circuit for the air conditioner is wired in series through the auxiliary contacts of the fan and pump motor starters. Also, notice that a low voltage may be used to control a higher voltage with the use of a stepdown transformer. 9-13. If switches Nos. 1 and 2 are closed, the pump and fan will operate but the air conditioner will not until the thermostat completes the circuit for its holding coil. If an overload develops in the pump or fan, the heaters open the respective control circuit, which in turn breaks the control circuit for the air conditioner. 9-14. Maintenance and Troubleshooting. Most of the troubles in motor starters will be in the load contacts, holding coil, or heaters. A voltmeter can be used to check the load contacts, if the voltmeter leads are connected in parallel to each set of contacts and the holding coil is energized. The voltmeter should read zero. If it does not, then the contacts need to be cleaned or replaced. With power off, the heaters and the holding coil may be checked with the ohmmeter. Heaters should be sized correctly to give protection to the motor; if undersized they would cause nuisance tripping in normal current flow. 9-15. In this chapter you have studied the fundamentals of electricity, circuits, Ohm's law, transformers, magnetism, electrical meters, circuit protective and control devices, and motors that are used to drive refrigeration equipment. Let's continue with another type of drive for refrigeration equipment, the gasoline engine.

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REVIEW EXERCISES These review exercises are intended to assist you in studying the material in this memorandum. The figures following each question correspond to the paragraph numbers that contain information pertaining to the exercise. In order to obtain the most benefit from the review exercises you should try to work them before you look at the answers in the back of the memorandum. Do not send in your solutions to the review exercises.

CHAPTER 1 Objective: To show knowledge of the fundamentals of electricity, circuits, motors, circuit protectors, troubleshooting, and safety. 1. What type of electricity does a generator produce? (1-4)

2.

Define voltage, current, and resistance. (1-6, 10)

3.

Why are alloys of nickel and chromium used in heater elements? (1-11)

4.

The resistance of copper wire is determined by three things. What are they? (1-12)

36

5.

What type metal is used to make a permanent magnet? (1-16)

6.

What determines the output frequency of an ac generator? (2-10)

7.

Given an electrical potential of 110 volts and a resistance of 55 ohms, find the amperage draw. (3-4)

8.

Given a resistance of 12 ohms and a 20-amp current draw, find the electrical potential. (3-5)

9.

Given a 5-amp current draw and an electrical potential of 110 volts, find the resistance in ohms. (3-6)

10. If a dc motor draws 1 ampere of current when connected to 746 volts, what is the horsepower of the motor? (3-25, 27)

11. What is the electrical symbol for inductance? (4-8)

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12. What effect does a capacitor have on an ac motor circuit? (4-10; Fig. 17)

13. When will the apparent power be equal to the true power in an ac circuit? (4-12)

14. Will a transformer operate on any dc circuit? (5-1)

15. Name the three primary parts of a transformer. (5-2)

16. List the four types of transformer connections. (5-21)

17. What is the purpose of the various size resistors connected in series with the voltmeter movable coil? (6-3)

18. Why is a shunt connected in parallel with the ammeter meter circuit? (6-4)

19. When will maximum current flow through the ohmmeter circuit? (6-7)

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20. Before using a dc meter on an ac circuit, what must be added to the circuit? (6-9)

21. What the purpose of the wattmeter? (6-10)

22. How would a voltmeter be connected to check or a blown fuse? (6-20)

23. What must be done to the circuit before making a continuity check in a parallel circuit? (6-30)

24. What determines the speed of rotation of an ac motor? (7-2)

25. How many windings must a single-phase induction motor have? (7-5)

26. What would happen to the split-phase motor if the start winding failed to disengage? (7-15)

27. Which of the single-phase motors has the best running characteristics and highest starting torque? (7-18)

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28. How is a three-phase motor started? (7-23)

29. Why does a reluctance motor operate at exactly synchronous speed? (7-26)

30. What type motor may be used on either ac or dc? (7-28)

31. How often should a motor be lubricated? (8-3)

32. What are circuit protective devices used for? (9-7)

33. On three-phase equipment how many protective devices must be in the circuit? (9-11)

34. In figure 66 the air conditioner will not operate if the fan is not on. Why? (9-12)

35. Where will most of the troubles be located in a magnetic motor starter? (9-14)

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CHAPTER 2 Fundamentals of Gasoline Engines Occasionally you may be called upon to service engine-powered refrigeration units. You will find these engine-powered units on refrigerated vans, mobile field units, and some trailers used for electronic system maintenance. 2. Since you will be operating and servicing these units, you must possess a working knowledge of gasoline engine. Let's begin with a discussion of the four stroke cycle engine. 10. Principles of Operation 10-1. For a four stroke cycle internal-combustion engine to operate and deliver power, the following series of events must occur in the order illustrated in figure 49. A mixture of fuel and air must enter the cylinder and be compressed. The mixture must be ignited by some means, causing it to burn and expand. The expanding gases then force the piston down. The piston then must move upward, expelling the burned gases from the cylinder. This series of five events must take place time and time again in exactly the same sequence if the engine is to deliver power. To improve the efficiency of the engine, various valves are timed to open or close at a piston position slightly before or slightly after a deadcenter position. 10-2. The two stroke cycle engine is a one that completes its cycle of operation in only two strokes, instead of four as in the four stroke cycle. Mechanically the two stroke cycle engine is slightly different. Some have the intake and exhaust ports placed in the cylinder wall, while others may use a combination of intake ports and mechanically operated exhaust valves in the combustion chamber. When ports are used and the piston moves down on its power stroke, it first uncovers the exhaust port to allow burned gases to escape and then uncovers the intake port to allow a new air-fuel mixture to enter the combustion chamber. On the upward stroke, the piston covers both ports and at the same time compresses the new mixture in preparation for ignition and another per stroke. 10-3. Theoretically the two stroke cycle engine should produce twice as much power as a four stroke cycle engine of the same size. This is not true, because fuel is wasted and power is lost when some of the incoming fuel mixture mixes with the exhaust gases and is exhausted out of the engine. In this manner the volumetric efficiency of the engine is reduced considerably. Volumetric efficiency is the ability of an engine to take in enough air to insure complete combustion. However, a two stroke cycle produces more power output per unit weight than a four stroke cycle engine. 10-4. So far all we've discussed is the operation of gasoline engines. Now we will over the servicing of gasoline engines. We will start with the lubrication system. 11. Maintenance of Lubrication System 11-1. The lubricating system of an engine includes a number of different units. In this system the oil is picked up from the oil pan reservoir by the pump. The pump is usually driven by the camshaft. An oil strainer is placed in series with the pump to remove foreign substances, such as metal particles, dirt, etc, from the oil. The oil is forced through metal tube and galleries in the engine block to various parts of the engine. It is then either splashed or forced on the moving parts of the engine after which the oil returns to the oil pan reservoir, thus completing the cycle. 11-2. In order for the lubrication system to function properly, the operator must observe and record the oil pressure at predetermined intervals, maintain the proper oil level in the crankcase of the engine, and change the oil and oil filter element, as specified by the technical manual applicable to the specific engine. 11-3. Oil Pressure Gage. The oil pressure gage indicates the resistance of the oil being circulated through the engine. The resistance is generally

41

Figure 49. Four stroke five event cycle principle. measured in pounds per square inch. A pressure gage does not show how much oil there is in the crankcase. It merely shows that oil is being pumped sufficiently to create an indicated pressure. If the oil pressure gage does not show any oil pressure, the engine must be stopped, since it is an indication that the oil is not circulating and lubricating the moving parts. The engine will be severely damaged if it is allowed to operate without oil pressure for a short length of time. 11-4. Oil Level Gage. The oil level gage rod is usually of the bayonet type, similar to that used on automobiles, and is used to check the oil level in the crankcase. The gage rod is usually stamped at “add oil” and “full” levels. Oil level on the bayonet gage rod should be taken only when the engine is not operating and the engine oil is at normal operating temperature. Always keep the oil above the “add” mark. 11-5. Oil Filter. The primary function of the oil filter is to filter out contaminating substances as the oil passes through the filtering element. Two types of oil filters are used: one is the sealed element type and the other is the replaceable element type. 11-6. Most oil filters are designed with a bypass valve which permits free circulation of the lubricating oil f the filter element becomes clogged. Normally a filter element should be changed when the lubricating oil in the engine is changed. This change should be performed at such intervals as recommended by the applicable publications. 42 11-7. Use care when replacing the filter - to avoid damaging the oil lines or the oil line fittings. Our next discussion will be the maintenance of the fuel system. 12. Maintenance of Fuel System 12-1. A gasoline engine fuel system consist essentially of a storage tank for the fuel, a fuel filter to clean the fuel, a fuel pump to transfer the fuel from the tank to the carburetor, and a carburetor to mix the fuel with the air. 12-2. Fuel Filter. Fuel filters may be of various designs and located at any point between the fuel tank and the carburetor. In figure 50 the fuel enters the bowl and pass up through the filter screen before it flows out through the outlet. Water, or any solid caught by the screen, settles to the bottom of the bowl. The bowl can be removed and cleaned. 12-3. Fuel Pump. The fuel pump pumps gasoline from the fuel tank, through the fuel filter, to the carburetor float chamber, at approximately 3 psi. 12-4. Carburetor. The basic function of the carburetor is to meter the air and fuel in varying percentages according to the engine requirements. The most desired mixture has an air-fuel ratio of 15 to 1-15 parts of air to 1 part of fuel by weight. A 15 to 1 ratio is referred to as a normal or medium mixture. A mixture con-

carburetor shown in figure 51 is one used with a small air-cooled engine which operates a 25-cubic foot refrigerator. This carburetor has three adjustments: the main needle valve, the idle adjusting screw, and the throttle adjusting screw. The main needle valve meters gasoline to the engine at operating speeds, the idle adjusting screw meters gasoline to the engine at idle speed, while the throttle adjusting screw adjusts the idling speed of the engine. Most carburetors have only the latter two adjustments. In these carburetors the gasoline is metered to the engine automatically. 12-6. Air Cleaner. The carburetor air cleaner must be kept clean to prolong engine life. Two types of air cleaners are used. They are the wet and dry types. At certain intervals, as recommended by the applicable publication, the wet filter is disassembled, washed in nonflammable cleaning solvent, reassembled, and refilled or sprayed with oil. Dry type cleaners are replaced at

Figure 50. Fuel filter. taining less air is known as a rich mixture. Maximum horsepower is obtained at a ratio of 12 or 13 to 1; maximum economy, however, is obtained with a 15 to 1 ratio. The carburetor must automatically vary the proportion of air and fuel to meet the changing conditions under which the engine operates. 12-5. To procure maximum horsepower and maximum economy from an engine, it is sometimes necessary to make certain carburetor adjustment. The

Figure 52. Ignition system. prescribed interval. They must never be oiled; however, in emergencies they may be cleaned with compressed air. 12-7. We've mentioned previously that the fuel-air mixture must be ignited by an electric spark from a spark plug; let's discuss the system that causes this function. 13. Maintenance of Ignition System 13-1. The complete function of the ignition system is shown in figure 52, but let's discuss each one separately. 13-2. Spark Plugs. Spark plugs should be removed, cleaned, and inspected at intervals prescribed by the manual for the particular engine. This operation is important because dirty spark 43

Figure 51. Carburetor.

Figure 53. Adjusting spark plug gap. plugs and plugs that have insufficient or too large a gap between the electrodes will cause hard starting and irregular firing of the engine. 13-3. Using a thickness gage, as shown in figure 53, adjust the gap between the electrodes to the specified amount recommended by the manual for the particular engine. The electrodes are spaced properly when the correct thickness gage can be lightly drawn between them. 13.4. The coil (transformer) is used to step up the voltage to approximately 18,000 volts dc. To do this the primary of the coil is connected to a set of points. These points open and close to create pulsating dc that can be stepped up. The secondary side of the coil which also produces pulsating dc is connected to the rotor in the distributor and from there to each spark in turn. 13-5. The condenser (capacitor) is in the circuit to help collapse the magnetic field and reduce arcing at the points. 13-6. Distributor. The distributor with its components is shown in figure 54. The distributor points should be inspected periodically. To inspect the condition of the point, stop the engine and remove the distributor cap and rotor. Then examine the distributor breaker points for pits or evidence of overheating. If the points are badly pitted or burned, they should be replaced. 13-7. After the points are replaced, adjust the clearance when fully open as prescribed by the publication for the specific engine. Also, replace the rotor and cap. 13-8. Storage Battery. The storage battery is a very vital part of the electrical and ignition system and must be properly maintained for dependable automatic operation. 13-9. The most common type of storage battery is the lead and acid type. It is so called because the plates 44

are composed of lead and the electrolyte is a solution of acid. 13-10. A battery must be tested periodically to determine its state of charge. To test the specific gravity of the electrolyte of each cell, remove the filler caps, being careful to prevent dirt or foreign matter from falling into the cells. 13-11. Use a hydrometer to test the specific gravity of each cell. If the specific gravity is less than 1.175, increase the generator charging rate, or recharge the battery. 13-12. If the unit is being operated in a tropical or hot climate and the specific gravity is over 1.225, the charging rate should be reduced. If the unit is operating in a temperate climate, the charging rate should not be reduced unless the specific gravity is over 1.290. When operating the unit in a frigid or cold climate, always keep the battery fully charged. 13-13. The battery electrolyte level should be inspected daily. When available, distilled water should be used to refill a storage battery. If the water level is low, refill each cell so that the level is about one-half of an inch above the top of the plates. It is important that the electrolyte level be properly maintained at all times. It is also very important that the specific gravity be

Figure 54. Distributor.

maintained at sufficient strength to prevent freezing in extremely cold locations. 13-14. If the electrolyte level is too low to obtain a reading with the hydrometer, refill the battery with distilled water and allow the unit to operate for an hour or more before taking a hydrometer reading; otherwise an accurate, specific gravity test cannot be obtained. 13-15. Wash the battery terminals, cable clamps, and cables with a solution of water and soda. See that the vents in the filler caps are open. To keep the terminals and battery cable clamps from corroding, coat them with grease. Do not drop a battery, and don't pound on the terminal. At intervals, remove the battery from its cradle, clean the cradle, and coat it with rust preventive compound. 13-16. Now that we have oil in the engine, fuel in the tank, and voltage to the spark plug, we can start the engine. Wait, we've forgotten another important system - the cooling system. We must have a system that will keep the engine at a normal temperature. We had better discuss this topic a little further. 14. Maintenance of Cooling System 14-1. All internal combustion engines are equipped with some type of cooling system to dissipate the great amount of heat they generate during operation. About one-third of the heat generated by combustion must be dissipated by the cooling system. Cooling systems are classified into two categories - liquid cooling and air cooling. 14-2. Liquid Cooling. A simple liquid-cooling system consists of a radiator, a circulating pump, a fan, a thermostat, and a system of water jackets and water passages within the engine. 14-3. If the engine temperature runs abnormally high, clean the exterior of the radiator by blowing compressed air through the fins to dislodge any foreign material and dead insects. If the temperature still runs high, heating may be due to an accumulation of sludge in the radiator. It is then best to drain and flush the radiator and engine block with dear water. Refill the radiator with soft water if it is available. If treated water is not available, then use clear tap water, but drain and flush the system more often. 14-4. For operation below freezing or if the engine should be standing idle at temperatures below freezing without being drained, ethylene glycol or a similar antifreeze should be added in sufficient quantity to prevent freezing at the lowest anticipated temperature. 14-5. Air Cooling. Air-cooled engines are designed in such a manner that the engine cylinder and head are

cooled by forced circulation of air provided by vanes on the flywheel. The blower case inclosing the flywheel and the baffles around the cylinder control the flow of air. Keep the system clean to prevent overheating of the engine and to assure uniform air velocity for proper cooling. When the flywheel vanes, cylinder, and cylinder fins become coated with dust and dirt, the engine blower case must be removed to clean the units. Using a stiff bristle brush or a scraper, remove all traces of dirt from the flywheel vanes and the cylinder and cylinder head fin. When maintaining cylinder fins it is important that fins not become bent or otherwise damaged, as this will result in hot spots within the cylinder. 14-6. Well, we've got the engine running normally; now we'll connect it to the compressor and get some work done. 15. Maintenance of Drive Mechanism 15-1. All drive belts should be examined regularly for wear, breaks, and adjustments. A worn belt becomes bright and smooth and tends to ride the bottom of the pulley or to slip when under a load. Continuous rubbing of the side of a belt wears down the edges and decreases the efficiency of its drive. Excessive friction from the contact with abrasive dust causes internal breakdown of a rubber belt. The presence of stray lubrication near a rubber belt should be checked. Oil and grease soften and deteriorate rubber. However, some flexible V-belts are made of a special composition which is not affected by grease or oil. 15-2. A belt which runs loose may snap in two. Low belt tension causes reduced and unsteady output. Unusual tautness brings on rapid wear of the belt, motor bearing, and compressor bearings. 15-3. If a belt shows indications of wear and cracks, it should be replaced. Always replace belts in matched sets if at al possible. To check the tension of the drive belt, which operates small compressor units, deflect the belt at a point halfway between the engine pulley and the compressor pulley. The deflection at this point, with a 10-pound pressure, should be between 1/2 inch and 3/4 inch. Adjust the belt as required or replace with a new one of the correct size. During the inspection and maintenance of V-type belts it must be remembered that the driving force is on the sides of the pulley and not on the bottom of the pulley groove. 15-4. The -four stroke cycle engine is most common to the career field. The operation of the engine depends upon proper maintenance. Each subsystem - fuel, electrical, and cooling - must work in harmony for peak performance. We've

45

discussed the maintenance to be performed on each system. The most important service that can be given an engine is proper lubrication. A large percentage of powerplant breakdowns are a direct cause of insufficient lubrication. Lubricate each powerplant according to the recommendations prescribed by the applicable

publications. The frequency of maintenance is outlined in publications furnished by the manufacturers of the engines or by the TO when available. 15-5. Since we have covered the prime movers for refrigeration equipment, let’s study the physics of refrigeration so the prime movers can be put to use.

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CHAPTER 2 Practice Exercises Objective: To show knowledge of the fundamentals and maintenance of the gasoline engine. 1. List the series of five events a four stroke cycle engine must go through to delivery power. (10-1)

2.

When should the engine oil be checked? (11-4)

3.

To obtain maximum economy what should the air-fuel ratio be? (12-4)

4.

What type of electrical power is delivered to the ignition coil? (13-4)

5.

What is the purpose of the condenser in the engine ignition circuit? (13-5)

6.

What is the most common storage battery composed of? (13-9)

7.

When should ethylene glycol be used? (14-4)

8.

What is the best way to check a drive belt for correct tension? (15-3)

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CHAPTER 3 Physics of Refrigeration When venturing into the field of refrigeration, the first thing to learn is what goes on within the unit to produce the “cold.” When we talk about something being “cold” we simply mean that it has less heat in relation to something else. Every substance will have some heat until the substance reaches absolute zero. 2. Heat is not destroyed in producing the cold but is simply removed from the place where it is unwanted. Heat is also in a mechanical refrigeration system to help remove the unwanted heat. 3. The particular phase of natural science with which we are concerned involve the study of conditions under which certain changes take place; for example, when a solid melts or when a liquid boils. 16. Thermodynamics 16-1. Before we go into the study of thermodynamics let's see what it means. “Thermodynamics is the physics that deals with the mechanical action or relations of heat processes and phenomena.” One of the laws of thermodynamics is a formula which states that 778 foot pounds of work is equivalent to the heat energy of one Btu. Anther law is a statement that heat will only transfer from a higher temperature to a lower temperature. 16-2. Heat. All substances have heat; however, some will have more heat than others. Heat is the movement of the molecules within the substances. The more they move the hotter the substance becomes. To completely stop this movement the substance must be reduced in temperature to absolute zero. 16-3. Cold. We use this term to show that an object has less heat than something else. Cold is not produced but is merely a result of removing heat, which removal slows down the molecular movement. Many substances change their state from a solid to a liquid, a gas or vice versa, with the addition or subtraction of heat. Other substances change their state by sublimation; in other words, they change from a solid directly into a gas. There are different types of heat and different methods of transferring this heat; but first let's look at some types of heat. 16-4. Sensible Heat. Sensible heat is the amount of heat that can be added to or subtracted from a substance without changing its state. Sensible heat can be measured by a thermometer and detected by the body senses when present in appreciable amounts. 16-5. Latent Heat. Latent heat is hidden heat present in a substance. When ice at 32° F. melts into water at 32° F., a change of state takes place. During this change, a certain amount of heat is required to melt the ice to water at 32° F. This heat which causes the change of state is known as the latent heat of fusion. Now if the water at normal atmospheric pressure is heated until it reaches 212°, it will not rise above the temperature until it is all changed into steam (vapor). The heat that changes a substance from a liquid to a vapor is known as the latent heat of vaporization. 16-6. The graph shown in figure 55 indicates that the amount of heat required to change 1 pound of water from a solid to a liquid is 144 Btus. To change 1 pound of water from a liquid to a gaseous state requires a total of 970 Btus. 16-7. Specific Heat. The fact that it takes 1 Btu to raise the temperature of 1 pound of water 1° does not mean that this is true for all substances. Some substances require more heat while others call for less heat to raise their temperature equal amounts. Water is used for comparison, and the amount of heat required as compared to water is the specific heat of a substance. A few specific heat values are given for different substances in figure 56. 16-8. Heat Transfer. Heat can be transferred from a hot object to a cooler object until both are equal in temperature. Heat can be transferred by any one of three different methods - conduction, convection, and radiation - or by a combination of these same methods.

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Figure 57. Heat transfer by conduction. contracts, becomes more dense (heavier), and falls back to its source, where it is heated again. Thus, a circulation of air is set up which continues as long as heat is provided. Figure 58 shows how heat is transferred by connection. 16-11. Radiation. Heat may be transmitted from one place to another without the use of any material carrier. The best example of this method of transfer of heat is found in the radiation of energy from the sun to the earth. We know that the atmosphere of the earth is negligible at a comparatively short height above the earth and that the rest of the more than 90 million miles up to the place where the sun’s atmosphere begins is filled with little or nothing. Therefore we know that both light and heat energy from the sun must come through space. Such a method of transfer is called radiation. 16-12. Radiation is the process of emitting radiant energy in the form of rays or particles, as shown in figure 59. In this case, a person's hand feels warm, even though it is a considerable distance from the source of heat. The rays or particles pass through the air and heat the hand more than the air between. 16-13. The transmission of heat by these three mediums can be controlled according to the required needs. Conduction is aided-by providing

Figure 55. The three states of water and the heat required to make change state atmospheric pressure. 16-9. Conduction. When heat is transmitted from one part of a substance to another part of the same substance or from one substance to another in direct contact the process is termed “conduction.” To verify these two statements by experiment, use a metal rod, as illustrated in figure 57, placing one end over a flame. As the heat is absorbed, the molecules become active, and in a short time the cooler portion of the metal rod becomes warm. Metals are good conductors of heat; but other materials, such as glass or cork, aren't. Materials which offer resistance to the flow of heat are known as insulators or poor conducts. 16-10. Convection. Convection will be clear to you if you will follow the flow of air as it is transmitted through a heating system. When air is heated, it expands and becomes lighter because of the change in density. Cooler heavier air flows in under the warm air and forces it upward. Then, as the warm air becomes cooler it

Figure 56. Specific heat values. Figure 58. Heat transfer by convection. 49

Figure 59. Heat transfer by radiation. large conducting surfaces and good heat-conducting materials, such as iron, silver, or copper. Convection my be assisted by speeding up the flow of air, as in a forcedair circulation system. The flow of heat can also be controlled by dampers and thermostats according to one's desire. Dark colors usually absorb heat while light colors reflect heat. For this reason, a certain surface finish may radiate heat more efficiently than another. This is an aid to heating by radiation. 16-14. Temperature. The relative hotness of a body is termed “temperature.” This is not the quantity of heat in the body substance, but merely its degree of warmth. An ordinary thermometer is used for the measurement of temperature. 16-15. Two types of scales that are in general use for temperature measurement are the centigrade and the Fahrenheit. Figure 60 compare the two sales. Looking at the centigrade scale, you can see that 0° is the freezing point and 100° the boiling point of water. There are 100 divisions on the centigrade scale compared to 180 division on the Fahrenheit scale. Water freezes at the 32° point and boils at the 212° point on the Fahrenheit scale.

Figure 61. Density, volume, and weight. 16-16. It becomes necessary at times to convert from Fahrenheit to centigrade or from centigrade to Fahrenheit temperatures. A simple formula for converting these temperatures has been used by all members of the refrigeration trade. When converting Fahrenheit temperatures to centigrade, subtract 32° from the Fahrenheit temperature and multiply the remainder by .556 (5/9). To change centigrade temperatures to Fahrenheit, multiply the centigrade temperature by 1.8 (9/5) and add 32°. This formula should be memorized for use not only in the study of refrigeration but also in the study of air conditioning. 16-17. Density. The density of a substance is the ratio of its mass or weight to its volume. The upper portion of figure 61 shows that volume may be given either in liquid measure as gallons or in cubic measure as cubic inches. One gallon

Figure 60. Comparison of Fahrenheit and Centigrade scales. 50

Figure 62. Determining specific gravity.

of water has a weight of 8.337 pounds at a temperature of 62° F. 16-18. The relative weight of liquids and solids is determined by specific gravity. Pure water is used as a standard reference with a value of 1. The specific gravity of cast iron may be figured by the method illustrated in figure 62. The weight of water which is displaced by a 15-pound bar of cast iron is 2.1 pounds. Divide 2.1 into 15 to get the specific gravity, which is about 7.1 for cast iron. The specific gravity of a liquid may be measured with a hydrometer such as is used with a storage battery. The float in a hydrometer is calibrated so that the scale gives a direct reading of the specific gravity of the liquid being tested. 16-19. The density of a gas is expressed by specific volume. The specific volume is the volume of 1 pound of the given gas under standard conditions (temperature of 68° F. and pressure of 29.92 inches of mercury). Next we shall consider what is meant by pressure and some of the effects of it. 16-20. Pressure. Before a refrigeration can operate normally, a pressure difference must exist between different units of the system. Consequently, pressure and its laws are important. Pressure is the force per unit of area expressed in pounds per square inch or pounds per square foot. The pressure of air on one's body at sea level is approximately 14.7 pounds per square inch, or 2117 pounds per square foot. Since there are 144 square inches in 1 square foot, 14.7 is multiplied by 144 to find the pressure per square foot:

Figure 64. Illustration of horsepower. pressure on the sides and bottom of its container. A good illustration of gas pressure is the pressure exerted by the substance used to inflate an ordinary balloon. The gas pressure inflates the balloon, supporting all points of its surface. Figure 63 illustrates the different types of pressures explained in this paragraph. When you learn about the refrigeration cycle, you will find that mechanical power is used to increase pressure. 16-22. Work and Power. An understanding of energy relations is essential to a complete knowledge of refrigeration. Energy is “the capacity to do work,” and whenever energy is spent there will be some work done. Work is “the force in pounds multiplied by the distance through which it acts.” The unit of work is called the foot-pound. One foot-pound is the amount of work done in raising 1 pound vertically a distance of 1 foot. 16-23. Example. What amount of work is done in lifting 2000 pounds a distance of 10 feet? Force X distance = work 2000 X 10 = 20,000 foot-pounds Power is the time rate of doing work. Mechanical power is termed “horsepower.” One horsepower

16-21. A material exerts pressure on its supporting surface. For example, a desk (solid) exerts pressure on the floor through its legs. If the legs were removed, the desk would fall. A liquid, such as water in a pail, exerts

Figure 63. Types of pressure.

Figure 65. Unit of heat content. 51

does work at the rate of 33,000 foot-pounds per minute. 16-24. Referring to figure 64, you will see that if the 2000 pounds were lifted 10 feet in 2 minutes, the power required would be:

16-25. Energy. In addition to mechanical power, we are concerned with electrical and heat energy. You will find in refrigeration that changes in heat energy are the basis of cooling. Electrical and mechanical energy are combined in most systems to produce changes in heat energy. The relationship between these three types of energy is expressed in terms of the following equivalents: 778 foot-pounds = 1 horsepower = 1 horsepower = 1 Btu 2,545.6 Btus/hr 746 watts

Figure 66. High and low side of system. 52

16-26. Scientists have a theory that heat comes from the vibration of the molecules in a substance. The rate of vibration determines the temperature, while the total energy involved in the movement of all the molecules of a substance determines the heat. Heat is measured in thermal units. The British thermal unit (Btu) is defined as the amount of heat required to raise the temperature of 1 pound of water 1° F. 16-27. Looking at the example (see fig. 65), you can see by the rise in temperature how 1 Btu was added to the water, causing a change in its heat content. Note that the state of the water does not change even though it has a higher temperature. Heat may be added without a change of state until the boiling point of the water is reached. 16-28. Critical Temperature. We can liquefy any gas by lowering its temperature or by increasing the pressure. However, there are temperatures at which gases cannot be liquefied regardless of the applied pressure. These are called the critical temperatures. 16-29. Critical Pressure. The critical pressure of a liquid is the pressure at or above which the liquid will remain a liquid regardless of the applied heat. 16-30. Enthalpy. Enthalpy is the total heat (energy) in 1 pound of a substance. The enthalpy for water is accepted at 32° F., where the accepted enthalpy

for refrigerants is at -40° F. Example: To find the enthalpy of 1 pound of 70° F. water, subtract 32° F. from 70° F. Total heat at 70° F. = 38 Btu. 16-31. Entropy. Entropy is a mathematical constant that is used by engineers for calculations of the energy in a system. Again, 32° F. and -40° F. are the accepted bases used in these calculations. Most of the refrigerant performance charts will show the constant entropy lines. 17. The Mechanical Refrigeration Cycle 17-1. Before studying the various changes which take place in the refrigeration cycle, it is necessary to see just how latent heat and pressure changes have become the foundation of modern refrigeration. 17-2. Uses of Latent Heat. When ice melts, its degree of temperature remains constant; however, it absorbs a large amount of heat in the process of changing from ice to water. 17-3. In the evaporator of the modern refrigerator, the refrigerant changes from a liquid to a gas. To make this change of state, heat must be absorbed by the refrigerant. This heat can only come from the space to be cooled. We can say that the cooling action within the cabinet takes place in the evaporator.

Figure 67. Compression system.

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Figure 68. Absorption system. 54

17-4. The condenser is another area within the refrigeration cycle where latent heat of vaporization is used. The heat absorbed in the evaporator must be given up in the condenser. The condenser is surrounded by either air or water; and as the hot gas comes into contact with either of these mediums, it gives up its heat and condenses into a liquid. 17-5. You can see now that latent heat of vaporization plays an important role within the cycle, but let's not forget another important ingredient-pressure differences. 17-6. Utilization of Pressure Difference. In refrigeration, it is necessary to produce cold. This is made possible when differences of pressure are present. The high and low sides of a system and places where pressure varies during a cycle can be seen in figures 66 and 67. The reduction of pressure within the cycle takes place at the expansion valve. The refrigerant (of the R12 type) boils at -21.7° F. under atmospheric pressure. If the pressure is reduced to 11.999 psia, the boiling point is lowered to -30° F. The cabinet temperature is maintained above this temperature; therefore, the heat of the cabinet will be readily absorbed by the refrigerant. Now you should have an understanding of how pressure differences are used in obtaining the refrigeration effect. 17-7. Now that we've covered latent heat and pressure differences, we are ready to apply these to the refrigeration cycle. 17-8. The refrigeration cycle is common to all machines made for towering of temperature in our everyday living. The type of system used, however, depends upon the locality where the refrigeration is needed. 17-9. Compression System. Figure 67 illustrates the simplified refrigeration system. By applying the theory of latent heat and pressure differences, you can see what takes place in producing low temperatures. This illustration may be applied to any refrigerator regardless of size or shape. 17-10. Every system involves a cycle of one kind or another. We will trace through the entire cycle step by step. 17-11. As the piston moves down, low-pressure gas is emitted through the valve to fill up the cylinder. As the piston starts up, compression takes place because the gas is forced into a smaller space. As the gas is compressed, heat of compression is added. At the topmost position of the piston, the gas is forced through the exhaust valve into the condenser The gas is at its highest pressure. The condenser is a series of tubes surrounded by a cooling

medium (air or a water). As the gas is forced through the tubes, the heat of compression plus the latent heat of vaporization from the evaporator is dissipated into the surrounding cooling medium. 17-12. The removal of heat causes the gas to condense to a high-pressure liquid. This liquid flows into a receiver, which is merely a storage space for the refrigerant. The liquid leaves the receiver and moves up the liquid line to the expansion valve, where the pressure of the liquid is reduced. As a result, it absorbs heat through the walls of the evaporator, lowering the temperature of the compartment to be cooled. As the liquid boils, which is caused by the heat picked up from the cooling compartment, it changes into a low-pressure gas. This low-pressure gas now enters the suction line leading to the compressor. The cycle is now complete. 17-13. Absorption System. The absorption system differs from a compression system in that heat energy is used instead of mechanical energy to make a change in the conditions necessary to complete a cycle of refrigeration. Gas, kerosene, or an electrical heating element is used as the source of heat supply. 17-14. To better explain the operation of the absorption system, we have put figure 68 in block form. Also we have added a float in the condenser. Let's start the cycle by creating a vacuum in the absorber and evaporator, and starting these pumps. Water will boil at 40° F.-45° F. with a vacuum of 29.53 inches of mercury (Hg). As the refrigerant (water) is sprayed on the 55° F. chilled water coil, the refrigerant boils and absorbs the heat from the chilled water. The refrigerant vapor is then absorbed by the lithium bromide, and becomes weaker. To have continuous operations, the lithium bromide must be made stronger and the refrigerant must return to the evaporator. To do this the generator pump is started and a steam valve is opened. The generator pump forces the weak solution through the heat exchanger (where the weak solution is preheated and the strong solution from the generator is cooled), then into the generator. Steam is used to make the refrigerant (water) go into a vapor again where it condenses into pure water in the condenser. As the refrigerant level rises in the condenser the float opens to return the refrigerant into the evaporator for continuous operation. 17-15. We have discussed the physics of refrigeration and the cycle of the mechanical and absorption refrigeration systems. Now let's discuss the medium used in these systems to transfer the heat from where it is unwanted to a place where it is unobjectionable.

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CHAPTER 3 Practice Exercises Objective: To show knowledge of the physics of refrigeration and to apply the theory to the subtraction of heat. 1. At what temperature will all molecular movement stop? (16-2)

2.

When a solid changes directly from a solid to a gas, what is it called? (16-3)

3.

How is cold produced? (16-3)

4.

Describe the term “sensible heat.” (16-4)

5.

Describe the term “latent heat.” (16-5)

6.

What is the specific heat of water? (16-7)

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7.

Convert -40° centigrade to Fahrenheit. (16-16)

8.

How is the relative weight of liquids and solids determined? (16-18)

9.

What is the pressure per square foot at sea level? (16-20)

10. What amount of work is done in lifting 33,000 pounds a distance of 2 feet in 1 minute? horsepower? (16-23, 24)

What the required

11. What is 1 Btu equal to in foot-pounds? (16-25)

12. How many Btus are required to raise the temperature of 50 pounds of water 2°? (16-26)

13. What is the temperature called at which a liquid cannot be liquefied regardless of the applied pressure? (16-28)

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CHAPTER 4 Refrigerants Heat cannot be transferred from the inside of the refrigerator to the outside without some sort of medium or heat-carrying device. This medium is called refrigerant. 2. Just what is refrigerant? Well, the dictionary defines it as follows, “A substance, such as ice, liquid air, ammonia, or carbon dioxide, used in refrigeration.” We could define refrigerant as the medium (fluid or gas) used to transfer heat from the evaporator to the condenser. 3. The requirements for a refrigerant are almost self-explanatory. It is obvious that an automatic mechanism should be safe; that is, free from the danger of poisonous, flammable, or explosive gases. Refrigerants must be noncorrosive in order that the more common metal can be used in the construction of the machine part. It must also be such that its presence can be easily detected and traced to its source in the event of leaks. It is also desirable to keep pressures within the refrigeration cycle as close to atmospheric pressure as possible, for any great differences in pressures tend to cause leaks, overwork the compressor, and lower the overall efficiency of the system. Another desirable characteristic of a refrigerant is stability. If a refrigerant is to have this, then it must remain chemically unchanged while constantly going from a low temperature to a high temperature and back to a low temperature. It must not set up a chemical reaction with the lubricants used in the system. It must not chemically deteriorate if it comes in contact with air or moisture within the system. 4. There are various types of refrigerants used today. The choice depends upon the application. Each manufacturer attaches to his unit a nameplate which gives the type and amount of charge in the system. Changing to a different refrigerant should not even be considered, since most units are deigned for use with one specific refrigerant. Each refrigerant has a different pressure-temperature relationship. This relationship will be the topic of our next discussion. 18. Effect of Temperature and Pressure 18-1. As you learned earlier, we can liquefy any gas by lowering its temperature. At some temperatures the gas can be liquefied by increasing the pressure. However, there are temperatures at which gases cannot be liquefied regardless of the applied pressure. These are called critical temperatures. 18-2. For example, we can change steam to water by lowering its temperature below 212° F. or raising the pressure; but at 689° F. no amount of pressure will effect the change. Anyone living at a high altitude has noticed that boiled food must be cooked for a longer period of time or under pressure. Boiling temperatures of points are lower at lower atmospheric pressures and higher at higher atmospheric pressures. The critical pressure of a gas (water vapor) is the minimum pressure required to liquefy (condense) it at its critical temperature. 18-3. The critical pressure of a refrigerant must be above any condensing pressure that might be encountered during a cycle of operation; otherwise the high-pressure gas would not condense and the refrigeration machine would cease functioning. If the ordinary condensing pressures are up near the critical pressure, the amount of power required to compress the refrigerant is excessive; therefore the critical pressure of a refrigerant must be well above the normal condensing pressure. 18-4. If the critical temperature of a refrigerant is not higher than the condensing temperature, the hot gas coming from the compressor will not condense regardless of pressure. If the temperature differential is small, power consumption is excessive. 18-5. If the hot gas coming from the compressor doesn't cool, the refrigeration cycle is not complete. The heat transferred to the refrigerant in the evaporator cannot be dissipated at the condenser. What heat was transferred in the evaporator? The heat from the food and inclosed area.

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This caused the refrigerant to evaporate. Let's explain this heat and vaporization process thoroughly. 18-6. Latent Heat of Vaporization. With the exception of the comparatively small amount of heat absorbed by vapor superheated in the evaporator and in that part of the suction line within the refrigerator space, all of the heat-absorbing or refrigerating capacity that a refrigerant has comes from its latent heat of vaporization. In other words it depends on how much heat the refrigerant requires per pound to be changed from a liquid to a gas. Everything else being equal, the refrigerant having the highest latent heat of vaporization is the most desirable. 18-7. Boiling Point and Condensing Temperature. Each refrigerant is made up of a combination of chemical elements. The various components of each differ in reaching their boiling point or the temperature at which they condense. The boiling point of a refrigerant is that temperature and pressure at which it is changed from a low-pressure liquid to a low-pressure gas. The heat required comes from the area to be lowered in temperature. The evaporator is the heat-absorbing section of a system. As stated before, the refrigerant R12 has a boiling point of -21.7° F. at atmospheric pressure. This boiling point is well below the lowest evaporating temperature at which the system operates. 18-8. The critical temperature of a refrigerant is usually considerably higher than the condensing temperature and pressure required in an operational system. The critical temperature of R-12 is 233° F., and the critical pressure s 582 pounds pr square inch. The pressure temperature table for R-12, found in the appendix of this volume, will show the normal operating pressure corresponding to a given temperature. (Table 2) 18-9. The cooling medium, such as air or water, is cooler than the refrigerant as it enters the condenser. Heat is absorbed by the cooling medium and dissipated into the atmosphere which changes the state of the refrigerant from a gas to a liquid. 18-10. Classification of Refrigerants. Today there are a number of different refrigerants used by manufacturers of refrigeration machines. The following paragraphs are devoted to a discussion of a few different refrigerants, their characteristics, and the methods used in testing for leaks. 18-11. Ammonia (NH3). This refrigerant is used most in certain applications in industry and also in the absorption type refrigerator. Ammonia is colorless and has a pungent odor. It boils at -28°F. atmospheric pressure. When one volume of ammonia and two volumes of air are mixed, there is danger of explosion. Ammonia very toxic and requires heavy fittings. Units using ammonia must be water cooled. To detect ammonia leaks, the repairman uses a sulphur candle, the flame of which gives off a white smoke when it comes in contact 59

with an ammonia vapor. Still another means of detecting an ammonia leak is the phenolphtalein paper method. A mild concentration of ammonia causes the paper to turn pink; heavier concentrations turn the paper scarlet. (Table 1) 8-12. Refrigerant (R-12). Refrigerant-12 is colorless and odorless both as a liquid and as a gas. If a heavy concentration of this gas is present, a very slight odor is evident, but the vapor will not irritate the skin, eyes, nose, or throat. R-12 boils at -21.7° under atmospheric pressure. The presence of moisture in R-12 does not cause corrosion; only a mild discoloration of brass, copper and steel results. It is noncombustible and also mixes readily with oil. To detect R-12 and other halogen refrigerant leaks the halide detector (as shown in fig. 69) may be used. Other methods may also be used.

Figure 69. Halide leak detector.

18-13. Carbon dioxide. Carbon dioxide gas is harmless to breathe except, of course, in heavy concentrations when all the oxygen is excluded. In such cases, suffocation results. It has a slightly pungent odor and an acid taste. 18-14. Because of its low efficiency as compared to others, this refrigerant is seldom used in household refrigerators. It is used principally in industrial systems and on ships. 18-15. Other refrigerants. Other refrigerants used to a great extent in the refrigeration industry try are Refrigerant-11, Refrigerant-22, and Refrigerant-11. Less commonly used refrigerants are Refrigerant-21, Refrigerant-113, butane, ethane, propane, and methyl formate. (Tables 3-5) 18-16. You must become familiar with the safety precautions related to refrigerants, for as we've mentioned previously, working safely benefits both the equipment and you. 18-17. Transfer of Refrigerants. Refrigerants are obtainable in amounts from railroad carload to a 1-pound can. However, most of the refrigerant is in 145-pound cylinders. These cylinders are too heavy for the serviceman to move from place to place so the refrigerant must be transferred into smaller containers. This is done by obtaining a small cylinder designed for the particular gas which is to be transferred. Connect a charging line, weigh the empty cylinder and cool it if possible (set in ice or other methods), invert the full cylinder, and open both cylinder valves. Stop the transfer when the small cylinder becomes 80-85 percent liquid full. CAUTION: Never fill a cylinder over 85 percent liquid full and always wear protective equipment when transferring refrigerant. 18-18. Let’s look at some of the “do's” and “don'ts” while handling refrigerant cylinders. (1) Never drop cylinders or permit them to strike each other violently. (2) Never use a lifting magnet or a sling when handling cylinders. A crane may be used when a safe cradle or platform provided to hold the cylinders. (3) Cylinder valve caps should be kept on at all times except when the cylinders are in use. (4) Never fill a refrigerant cylinder completely full of refrigerant. The safe limit is 85 percent full. Overfilled cylinders are apt to burst from hydrostatic pressure. (5) Never mix gases in a cylinder. (6) Cylinders are made to hold gas - don't use them for a support or roller. (7) Never tamper with the safety device on a cylinder. (8) Open cylinder valves slowly and use a

cylinder valve wrench. Never use a monkey or Stillson wrench for this purpose. (9) Never force misfitting connections; make sure that the threads of regulators and unions are the same as those on the cylinder outlet. (10) Never attempt to repair or alter a cylinder or valves. (11) Never store cylinders near flammables. (12) Always keep cylinders in a cool place away from direct sun rays if possible and fully secured in place. (13) Do not store full and empty refrigerant cylinders together. They should be stored in different sections of the shop to avoid confusion. (14) Always insure that gas cylinders are secured in place both when empty and filled. 18-19. As we stated before, you should always wear protective equipment while charging or transferring refrigerant. However, if something happens when you do not have the protective equipment on and the refrigerant comes in contact with your eyes or skin, you should know the first aid that will help you. If the refrigerant comes in contact with the eyes they can be bathed in a 2percent boric acid solution. For frostbite on the skin the area can be bathed with cold water and massaged around the area until circulation is restored. Do not disturb the frost blisters. 18-20. A refrigerant is the carrier of heat in a system; consequently, it is found in different parts of the system in different states. How do we know which state the refrigerant i in within the system? Very easy; we use the refrigerant table. Using the table, we can check the pressures within the system and convert the pressures to temperatures. This can also tell us if the system is safe to open. Remember, even though you know a little first aid, it's better to be safe than sorry. 18-21. Tables have been compiled through experiment and research for each of the most commonly used refrigerants. These tables show the pressure, density, volume, heat content, and latent heat corresponding to certain temperatures. The charts are so designed that when you have one condition given you can determine the other relative factors. (Tables 1-6) 18-22. We have had a discussion on a few of the most important refrigerants and their purpose as heat carriers in a refrigeration system. A refrigerant is the bloodstream of any refrigerator; it removes heat at a low pressure as it evaporates, and gives up heat at a high pressure as it condenses. The properties of a few of the most common refrigerant gases are discussed and the characteristics noted, as well as the safety precautions which are essential and must be observed. You are the one who will be handling refrigerant, so don't be careless, for they can

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cause personal injury. The sections covering safety precautions, safe handling of gases, and first aid treatment list the “dos” and “don'ts” to be followed when dealing with refrigerants. Read and heed; these are for your own

benefit. Tables which will be used in every step of this course are contained in the appendix to this memorandum.

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CHAPTER 4 Practice Exercise Objective: To show knowledge of the characteristic of refrigerants and of safety practices in handling these refrigerants. 1. What is the critical temperature of water? (18-2)

2.

Why must the critical pressure be above the condensing pressures? (18-3)

3.

Which refrigerant would be the most desirable - one with the lowest or highest latent heat of vaporization? (18-6)

4.

What kind of a refrigerant gives off a white smoke when a leak is detected while using a sulphur candle? (18-11)

5.

What is the safe limit for filling a refrigerant cylinder? (18-17, 18)

6.

If refrigerant comes in contact with the eyes, they may be bathed in what? (18-19)

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Glossary ABSOLUTE HUMIDITY - The amount of moisture that is in the air; it is measured in grains per cubic foot. ABSOLUTE PRESSURE - Gage pressure plus atmospheric pressure (see pressure conversion table). ABSOLUTE TEMPERATURE - The temperature that is measured from absolute zero (-460° F., zero° R., and -273° C., zero° K.) ACCUMULATOR - A tank that is used to keep liquid refrigerant from flowing to the compressor. ACTIVATED ALUMINA - A chemical desiccant. ACTIVATED CARBON - Processed carbon that is used for a filter. ADIABATIC COOLING - Process of changing sensible heat for latent heat without removing heat (evaporative cooling). ANEMOMETER - An instrument used to measure the rate of airflow. ATMOSPHERIC PRESSURE - Pressure that is exerted upon the earth by the atmospheric gases. AUTOTRANSFER - Common turns serve both the primary and secondary coils. Different taps are used to step up or step down the voltage. AZEOTROPIC REFRIGERANTS - These are mixtures of refrigerants that do not combine chemically but provide good refrigerant characteristics. BACK PRESSURE - Low side pressure or suction pressure. BOYLE’S LAW - The volume of a given mass of gas varies as the pressure varies if the temperature remains the same. BRITISH THERMAL UNIT - The amount of heat required to raise the temperature of 1 pound of water 1° F. CALORIE - The quantity of heat required to raise the temperature of 1 gram of water 1° C. CASCADE SYSTEM - Refrigeration system where two or more systems are connected in series to produce ultra-low temperatures. CHARLES’ LAW - The volume of a gas varies directly with the temperature provided that the pressure remains constant. COEFFICIENT OF PERFORMANCE (COP) - The ratio of energy applied as compared to the energy used. COMPOUND REFRIGERATION SYSTEM - A system with two or more compressors or cylinders in series. CRITICAL PRESSURE - The pressure of the saturated vapor at the critical temperature. CRITICAL TEMPERATURE - The temperature at which the liquid and vapor densities of a substance become equal. CROSS CHARGED - Two different fluids used to create the desired pressure-temperature relationship. CRYOGENIC FLUID - An ultra-low temperature gas or liquid. CRYOGENICS - Refrigeration producing temperatures at or below -250° F. CURRENT RELAY - A relay which makes or breaks a circuit depending on a change in current flow. DALTON’S LAW - The total pressure of a mixture of gases is the sum of the partial pressures of each of the gases in the mixture.

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DENSITY - The mass of a substance per unit volume (consistency). DEWPOINT - The temperature at which a saturated vapor will begin to condense. DRY ICE - Solid carbon dioxide at approximately -109° F.; it is used in the shipment of produce. EBULATOR - A sharp-edged material inserted in a flooded evaporator for better efficiency. FLASH GAS - When changing from a high-pressure liquid to a low-pressure liquid some of the liquid flashes (evaporates) off and cools the remaining liquid to the desired evaporation temperature. FOOT-POUND - The amount of work done in lifting 1 pound 1 foot. GRAIN - A unit of weight; 7000 grains equals 1 pound. HEAD, STATIC - Pressure of a fluid measured in terms of height of the column of the fluid. HEAT LOAD - The Btus that are removed in 24 hours. HEAT OF COMPRESSION - The transformation of mechanical energy of pressure into energy of heat. HYDROMETER - An instrument used to measure the specific gravity of a liquid. HYGROMETER - An instrument used to measure the ratio of moisture in the air. INDUCTION MOTOR - An ac motor that operates on the principles of a rotating magnetic field. KATATHERMOMETER - An alcohol thermometer used to measure air velocities by means of cooling effect. KELVIN SCALE (K) - A thermometer scale that is equal to centigrade but using zero as absolute zero instead of -273° C. (absolute centigrade). LATENT HEAT - Hidden heat; heat energy that a substance absorbs while changing state. MANOMETER - A U-shaped tube filled with a liquid that is used to measure the pressure of gases and vapors. MEGOHM - One million ohms. MULLION HEATER - An electrical heating element used to keep the stationary part (mullion) of the structure between the doors from sweating or frosting. MULTIPLE EVAPORATION SYSTEM - A system with two or more evaporators connected in parallel. MULTIPLE SYSTEM - A system with two or more evaporators connected to one condensing unit. OIL SEPARATOR - A device used to remove oil from a gaseous refrigerant. OZONE - A gaseous form of oxygen, usually generated by a silent electrical discharge in ordinary air. PITOT TUBE - Part of an instrument used to measure air velocities. POTENTIAL ELECTRICAL - The electrical force which tries to move or moves the electrons in a circuit. POTENTIAL RELAY - A relay which is operated by voltage changes in an electromagnet. POWER FACTOR - Correction coefficient for ac power. PYROMETER - A device used to measure high temperatures. RANKIN SCALE (R) - A thermometer scale that is equal to Fahrenheit but using zero as absolute zero instead of -460° F. (absolute Fahrenheit). RELATIVE HUMIDITY - The percent of moisture in the air as to what it can hold at that temperature and pressure. SATURATION - When air is saturated it is holding the maximum amount of water vapor at that temperature and pressure. (It may also be applied to other substances.) SENSIBLE HEAT - Heat that can be measured and causes a change in temperature. SOLAR HEAT - Heat energy waves of the sun.

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SPECIFIC GRAVITY - Weight of a liquid compared to water. SPECIFIC HEAT - The ratio of the quantity of heat required to raise the temperature of a body or mass 1° to that required to raise the temperature of an equal mass of water 1°. SPECIFIC VOLUME - Volume per unit (one) mass of a substance. STANDARD ATMOSPHERE - When air is at a condition of 14.7 psia and 68° F. STANDARD CONDITIONS - 68° F., 29.92 inches Hg., and R. H. of 30 percent used in air-conditioning calculations. STRATIFICATION OF AIR - When air lies in different temperature layers because of little or no air movement. SUBLIMATION - When a substance changes from a solid directly into a gas without becoming a liquid. SUBCOOLING - Cooling of a liquid below its condensing temperature. SUPERHEAT - Adding heat to a vapor above its boiling temperature and at the same pressure. THERM - 100,000 British thermal units. THERMISTORS - An electrical resistor made of a material whose resistance varies with the temperature. TRANSISTOR - An electrical device used to transfer an electrical signal across a resistor. TRIPLE POINT - A condition of pressure and temperature where the liquid, vapor, and solid states can coexist. VAPOR PRESSURE - The pressure exerted by a vapor upon its liquid or solid form. VELOCIMETER - A direct reading air velocity meter, reading in feet per minute. WEB BULB - A dry bulb thermometer with a wick attached to the bulb that is used in the measurement of relative humidity.

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APPENDIX REFRIGERANTS Properties of Liquid and Saturated Vapor Tables 1 - 6

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67

68

69

70

72

73

74

75

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Answers To Practice Exercises

CHAPTER 1. 2. 3. 4. 5. 6. 7. 8. 9. A generator produces dynamic electricity. (1-4)

1

Voltage is electrical pressure; current is the movement of electrons and resistance is the opposition to current flow. (16, 10) These alloys make it possible to operate at high temperatures without melting. (1-11) The cross-sectional area, the length, and the temperature. (1-12) Hardened iron. (1-16) Number of poles and speed of rotation. (2-10) 2 amperes. (3-4) 240 volt. (3-5) 22 ohms. (3-6)

10. One horsepower. (3-25, 27) 11. The symbol for inductance is L. (4-8) 12. The capacitor gives the motor more torque by causing the current to lead the voltage. (4-10; Fig. 17) 13. Only when the circuit is made up of pure resistance. (4-12) 14. No, only on pulsating dc. (5-61) 15. Iron core, primary winding, and secondary winding. (5-2) 16. Wye-wye, delta-delta, and wye-delta. (5-21) 17. To limit the amount of current flow through the meter circuit. (6-3) 18. The shunt is connected in parallel with the ohmmeter circuit to bypass most of the current around the meter coil circuit. (6-4) 19. Maximum current will flow through the ohmmeter circuit when there is minimum resistance to flow. (6-7) 20. A rectifier must be added to change ac to dc. (6-9) 21. To measure the true power in an ac circuit regardless of the type load. (6-10) 22. To check for a blown fuse the voltmeter is connected in parallel with the fuse. (6-20) 23. To check for continuity in a parallel circuit the unit being tested must be isolated from the rest of the circuit. (6-30)

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24. The speed of an ac motor depends on the number of poles and the frequency of the applied electrical source. (7-2) 25. A single-phase induction motor must have two windings, a start and a run winding. (7-5) 26. The motor would run hot and burn out the start winding if allowed to run any length of time. (7-15) 27. A capacitor start, capacitor run. (7-18) 28. A three-phase motor exerts a torque when at rest, and therefore it starts itself when the correct voltage is applied to the stator field coils. (7-23) 29. The reluctance motor operates at exactly synchronous speed because of the salient poles. (7-26) 30. Universal type motor may be used on ac or dc. (7-28) 31. A motor should be lubricated according to applicable publications. (8-3) 32. Circuit protective devices are used to protect the unit and wires in the circuit. (9-7) 33. Two. (9-11) 34. If the fan circuit is not closed, the air conditioner holding oil circuit will be opened at the auxiliary contacts in the fan motor starter. (9-12) 35. Most troubles will be found in the load contacts, holding coil, or heaters. (9-14) CHAPTER 2 1. 2. 3. 4. 5. 6. 7. 8. Intake, compression, ignition, power, and exhaust. (10-1) The engine oil should be checked when he engine is stopped and the oil is at normal operating temperature. (11-4). An air-fuel ratio of 15 to 1 gives maximum economy. (12-4) Pulsating dc. (13-4) The purpose of the capacitor (condenser) in the engine ignition circuit is to help collapse the magnetic field and to reduce arcing at the points. (13-5) Lead and acid. (13-9) Ethylene glycol should be used when the water-cooled engine will be exposed to freezing temperatures. (14-4) With a 10-pound pressure at a point halfway between the compressor pulley and the drive pulley the belt should deflect 1/2 to 3/4 inch if the belt is correctly adjusted. (15-3) CHAPTER 3 1. All molecular movement will cease at absolute zero. (16-2)

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2. 3.

Sublimation. (16-3) Cold is not produced but is merely a result of removing heat. (16-3)

4. Sensible heat is the amount of heat that can be added to or subtracted from a substance without changing its state. (16-4) 5. Latent heat is hidden heat and is the heat that is added to or subtracted from a substance when it changes its state. (16-5) 6. 7. 8. 9. The specific heat of water is 1. (16-7) -40° centigrade is equal to -40° Fahrenheit. (16-16) The relative weight of liquids and solids is determined by specific gravity. (16-18) 2117 pounds per square foot. (16-20)

10. 66,000 foot-pounds; 2 horsepower. (16-23, 24) 11. 778 foot-pounds = 1 Btu. (16-25) 12. 100 Btus. (16-26) 13. The critical temperature. (16-28) CHAPTER 4 1. 2. 3. 4. 5. 6. The critical temperature of water is 689° F. (18-2) If the critical pressure is not above the condensing temperature the gas will not condense. (18-3) The most desirable refrigerant would have a high latent heat of vaporization. (18-6) Ammonia gives off a white smoke in the presence of a flaming sulphur candle. (18-11) A refrigerant cylinder must never be filled more than 85 percent. (18-17, 18) Boric acid solution may be used if liquid refrigerant comes in contact with the eyes. (18-19)

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SUBCOURSE EDITION OD 1748 A

REFRIGERATION AND AIR CONDITIONING II (COMMERCIAL REFRIGERATION)

REFRIGERATION AND AIR CONDITIONING II (Commercial Refrigeration) Subcourse OD 1748 Edition A United States Army Combined Arms Support Command Fort Lee, VA 23801-1809 14 Credit Hours INTRODUCTION This subcourse is the second of four subcourses devoted to basic instruction in refrigeration and air conditioning. This subcourse explains the components, operation, and repair of commercial refrigeration systems and provides a detailed explanation of the various uses of compressors. In addition, there is detailed instruction on the use and defrosting of storage cabinets, plant design, special systems, and vehicular refrigeration units. There are three lessons. 1. 2. 3. Commercial Refrigeration Systems. Commercial Refrigeration Systems (continued). Cold Storage, Ice Plants, Special Applications, and Vehicle Units.

Unless otherwise stated, whenever the masculine gender is used, both men and women are included.

PREFACE THE REFRIGERATION field includes a wide variety of refrigerators, and you must be able to implement the maintenance program that keeps these refrigerators operational. The first chapter is devoted to small commercial refrigeration units, mainly portable types, such as are used in homes, messhalls, and commissaries. We will discuss the absorption type refrigerator as well as the more common compressor type used in most domestic refrigerators and freezers. The components, their operation, and the troubleshooting procedures for both types are discussed. Braxing, welding, and cutting methods are explained, and the last section gives repairs and services. The subject is expanded in the second chapter to other commercial units, such as water coolers, ice cube machines, and larger equipment like walk-in cabinets and display cases. Large cold storage plants and ice plants merit treatment in a separate chapter. In Chapter 4 we will discuss systems designed for special application, such as those using multiple evaporators and multiple compressors. This chapter also includes a section on ultralow-temperature systems. As you will learn in the last chapter, there have been few changes in automotive air conditioning, but there are some brand new methods for refrigerating food transport trucks. By becoming familiar with the symptoms that lead to refrigerator breakdowns, you will be able, in many instances, to prevent such breakdowns. Regardless of the type and size of any refrigerator, a specific cycle is followed before the refrigerating effect takes place. Therefore, a thorough knowledge of this cycle and a clear understanding of applicable troubleshooting procedures should make your job less difficult.

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ACKNOWLEDGEMENT Acknowledgement is made to the following companies for the use of copyright material in this memorandum: Alco Valve Company, St. Louis, MO; Controls Company of America, Milwaukee, Wisc.; McGraw-Hill Book Company, New York, N.Y.; John E. Mitchell Company, Dallas, Texas; Mueller Brass Company, Port Huron, Mich.; Nickerson and Collins Company, Chicago, Ill.; The Trane Company, LaCrosse, Wisc.

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CONTENTS Page Preface.......................................................................................................................................................................... Chapter 1 2 3 4 5 Commercial Refrigeration Systems..................................................................................................................... Commercial Refrigeration Systems (Continued)................................................................................................ Cold Storage and Ice Plants................................................................................................................................ Special Application Systems ............................................................................................................................... Vehicle Refrigeration Units................................................................................................................................ Answers to Review Exercises.................................................................................................................................. 1 30 56 71 85 95 i

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CHAPTER 1

Commercial Refrigeration Systems
THERE WAS A TIME when Grandma use a block of ice to keep her food from spoiling. Later, a mechanical unit replaced the block of ice In those days a service call often meant airing out the kitchen before work could be started because the place was full of ammonia or sulfur dioxide fumes. You do not have that problem today, for the modern domestic refrigerator use a refrigerant which is practically odorless and harmless. 2. The domestic units explained in this chapter use one of two basic systems: the absorption refrigeration system and the compression refrigeration system. We will take up the construction of domestic refrigerator boxes first, since much of this information is common to the absorption refrigerator and the freezer boxes which follow. The discussion of components such as latches. ice cube makers, and other features will also include specific information relating to their operation, service, or maintenance. 3. A section on compression system components starts with a brief review of operating principles followed by the components which put these principles into action. The section concludes with refrigerator performance which is based on normal operation of the system. 4. Freezers are dealt with by supplying only that information which is necessary. For example, the construction and insulation of a freezer box is essentially like a domestic refrigerator box, so there would be no profit in repeating it. 5. Troubleshooting hermetic systems is divided into electrical troubles and mechanical troubles. The discussion is centered around those components which are essential to the main system. Other components which have been discussed previously are accessories or special features and, as such, they are treated separately because they will vary from one unit to another. 6. A comprehensive discussion of safety introduces the section on brazing, welding, and cutting. Emphasis is placed on those fluxes and alloys commonly used by a refrigeration repairman. This is followed by the repairs which you are normally expected to make on domestic refrigerators and freezers. The necessary services for charging a small hermetic system make the final discussion in this chapter. Since most of us are familiar with the domestic refrigerator, it is the logical starting point for your study. 1. Domestic Refrigerators 1-1. The recent trend has been to make larger domestic refrigerators. Consequently, many homes now use units as large as those found in small cafes or restaurants. Such larger units are quite expensive, but they have been accepted by the buying public because of their proven reliability and greater efficiency; 10 years of continuous service is not uncommon. 1-2. Construction and Components. A refrigerator is made of two steel shells. The outer shell consists of steel plates welded to a steel frame, which gives strength and rigidity. The inner shell is formed from a single sheet of steel, which must provide the mounting arrangement for the shelves and support the evaporator. The space between the two shells is filled with insulation, and the gap at the edge is closed by the breaker strips. The door is formed from a single sheet of steel and is given rigidity by the liner. The door gasket is installed so that it fits into the gap between the liner and the shell. The body of the door is filled with insulation. 1-3. Insulation. When temperature differences exist close to each other, they always try to equalize each other. Insulating materials can retard the transfer so that a cooled area will stay cold longer. From your studies, you may remember that heat is transferred by convection, conduction, and radiation. Convection is the transfer of heat by air currents. Cells of dead air space reduce convection by restricting the movement of air. Conduction is the transfer of heat by a medium acting as a bridge from one temperature zone to another. Material such as paper is a poor conductor. Radiation is the transfer of heat in the same way that light is propagated. Radiant heat can pass through a block of clear ice so that the heat can be sensed on the other side. Radiation

1

from a surface is determined by the color and texture of the material. 1-4. The greatest heat load in a refrigerator is the heat transferred through the door and walls of the box. Better insulation means less heat gain, greater efficiency, lower operating costs, and an extended life for the compressor. Among the basic insulating materials used, you will find spun glass, rock wool, cork, plastics, and metals. These materials are produced as sheets, fibers, cells, or a combination of these. For example, if you will look at the edge of a corrugated cardboard box you will see a combination of sheet and cell constructions. Cells are tubular in cardboard construction, but a honeycomb type of cell will insulate better. 1-5. The manufacture of insulation has been so improved in recent years that present-day boxes are built with a relatively thinner wall. Some new types of insulation will sufficiently reduce heat transfer with only one-fourth to one-half inch of insulation. Among the newer insulation materials are steel and aluminum. Thin sheets of metal are made to form a multilayer sandwich with dead air space between the sheets. However, with such metal insulation, in order to prevent the accumulation of moisture, as with other types of material, it is important that adequate sealing be provided. Among other new insulating materials are synthetic fibers or plastics which can be molded into a form that will fit between the outer and inner shells of a box. Such molded insulation has the advantage of eliminating corner and edge joints. 1-6. Any insulation must have an effective moisture barrier provided to insure its long life. Obviously, because some materials will lose their insulating value if they get wet, they must be waterproofed. To these materials, odorless tars are often applied to seal the surface and keep the moisture out. Why are such tars used? Because the taste of food would be ruined if aromatic tars were used for sealing. Among the methods of sealing are painting or dipping the insulation in a waterproof compound to close the pores in the surface against moisture. As a further protection against moisture, special rubber gaskets are used to close all spaces where wire or tubing passes through the insulation. In fact, every precaution is taken at the time of manufacture to keep moisture from getting into insulation. Also when foam insulations are used, they are nonburning if they are made to Federal Specification HH1-1-00530 (ASTM 1692). Note, too, that some synthetics are not only resistant to rot but also have no nutritional value which might support rodents, insects, or fungi. Finally, while all synthetics are not equally elective, a few have such a low K-factor that when used

for the same purpose, they are equivalent to twice the thickness of many natural materials. 1-7. By now, as a refrigeration specialist, it should be obvious that moisture can cause many troubles. In fact, in an area where air circulation is restricted and temperature is near 70°, conditions are right to both promote the growth of bacteria and result in corrosion of the metal. The effectiveness of modern sealing methods is shown by the rare occurrence of this trouble with insulation. Still, the complete replacement of insulation is necessary if flooding has resulted in the seal breaking down. Such seal breakdown occurs if a box is submerged under water too long. You can ordinarily make minor repairs to torn insulation with tape. When you do this, however, paint the patch with a waterproof sealer such as hydrolene, an odorless tar. 1-8. Breaker strips. The gap between the inner and outer shells is closed by means of the breaker strips. These are not required to make an airtight seal, because they are found inside the area of the door gasket. Yet, because the temperature in the box is colder than the insulation space, any moisture will tend to be drawn from the insulation. If you have ever removed or replaced a breaker strip, you will know from experience that those made from plastic are easily broken. Still, whether they are made of metal or plastic, these strips require careful handling, as kinking will permanently deform the strip and result in a gap. 1-9. Stile or mullion heaters. These heaters are found in back of the breaker strip on some boxes. They are low-watt linear elements which operate continuously to prevent frost creepage around the door. This is another reason for using care when removing a breaker strip, since it is possible to damage the wiring or the heater strip. 1-10. Door gaskets. These gaskets are made of rubber or plastic and follow the general shape shown in figure 1. Note the air pocket, which acts as an air cushion and helps to insure an airtight seal between the door and the cabinet. Most manufacturers today place the door-closing magnets in the air pocket of the gasket. In this position, the magnets not only hold the door closed but also insure the gasket making a seal throughout its life without ever needing adjustment. The strip magnet is installed in the top, bottom, and .latch side of the door but not at the hinge side, because if placed here, it would tend to close the door. Older models had one or two large magnets with steel plates which took the place of a door latch. 1-11. Door latches. In spite of widespread publicity, 44 children were reported as having lost their lives in unused refrigerators between 1 January and 17 November 1964. Therefore, whenever a refrigerator of the old style is to be stored, the

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Figure 1. Door gasket. latch should be removed and taped to the inside of the box. In shipment, the door can be secured by tape or a length of rope. If you remember these precautions, you will never have to worry about having contributed to the death of a child. In fact, in many states it is against the law to abandon a refrigerator without removing the door or the latch. 1-12. Adjustable door latches were used on refrigerators for many years before the introduction of permanent magnets as door closers. A door latch can be adjusted to compensate for a worn or compressed door gasket. To do so, release the locking screw and move the lip closer to the cabinet. Use a thin piece of paper between the gasket and the cabinet to check the seal. With the door closed on the paper, the amount of drag on the paper as you pull it indicates the amount of seal that the gasket furnishes. 1-13. Location and Power Supply. A domestic type refrigerator is often located without consideration of the operation of the equipment. As you know, these units are self-contained, with the condenser mounted under or in back of the box. Thus, the unit cannot operate efficiently if it is placed too close to an oven or to a space heater. Note, too, that although the original location of a refrigerator may have been quite satisfactory, subsequent installation of heating unit may inadvertently find the refrigerator in the hottest part of a room. Furthermore, consideration as to the suitability of a location should also take into account the distance from the power source. A domestic box is provided with a cord and plug which can be used in a convenience outlet. As there are usually several outlets on one branch circuit, it is possible that a branch circuit is being operated at close to capacity. A refrigerator will be operating on marginal current under these conditions. If it is connected to an outlet which is last on the circuit, the voltage drop may be so great that the unit will not give satisfactory performance.

1-14. At most overseas bases, where there are government furnished quarters, refrigeration equipment is designed for the voltage and frequency of the electricity in the local area. However, you may find that some refrigerators made for 60-cycle operation have been transported over seas and are being used on 50-cycle current. If the voltage is correct, the unit may give satisfactory operation on the lower frequency. Voltage and frequency are just two of the many new things you should be aware of when you are on an overseas assignment. Refrigerator made for overseas use with unusual electric requirements have a notice posted inside the box stating the specifications. 1-15. Combination Refrigerators. Since the freezer section has increased in size, practically all boxes made in the larger sizes today are combination refrigerators. Continued improvement has led to such things as automatic ice cube makers, forced-air circulation, frostfree operation automatic defrosting, and even ultraviolet lamps, which retard bacteria growth and reduce odor. New developments have resulted in more usable cubic feet in the box by reducing the size of the compressor and the insulation space. Most new units use Freon 22, because it is more efficient, requiring less horsepower for equivalent output. As the demand for refrigerators has increased, it has brought about the development of special-purpose storage compartments for different foods. 1-16. Special compartments. Meat storage compartments are kept at slightly above freezing. with high humidity-as high as 90 percent. These conditions are favorable for extended storage of unwrapped foods. High humidity prevents desiccation, and the near freezing temperature retards bacterial action. 1-17. High-humidity storage for leaf vegetables, such as lettuce and celery, is provided by drawers located in the bottom of the refrigerator section. Another specialpurpose compartment is the butter conditioner, which is located in the door. Butter is maintained at a warmer temperature here than elsewhere in the box so that it will not be too hard for serving. Two methods are used to get the proper temperature for this compartment. For one, when an electrical heater is used, a rheostat is varied to get the desired heat. The heater element is connected to the electric supply whenever the compressor is running. A flexible cord is used at the door hinge to bring the circuit into the door. For the other, when a heater element is not used. the butter conditioner depends on the heat passing through the door at that place to keep the butter from getting too hard. The insulation thickness at that place in the door determines the rate of transfer of heat. 1-18. Automatic ice cube maker. An automatic ice cube maker is a specialized item found

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in the freezer compartment of some boxes. One popular make uses a solenoid valve, an electric motor, an electric heater, a feeler bulb, and two thermostats. A mechanical valve (globe valve) in the water line is adjusted to reduce the line pressure so that the correct amount of water will be metered into the ice cube tray. This valve will need adjustment if there is a change in supply water pressure. One thermostat senses when the ice is made, and the other thermostat has a feeler bulb in the storage tray which senses when the tray is full. Let us consider the automatic operation, starting with the completion of a batch of ice cubes. Sensing that the water is frozen and that the cubes are ready to be used, the "cold" thermostat energizes the electric heater in the tray. The "hot" thermostat senses when the tray is warm enough to release the cubes, at which time this thermostat starts the electric motor. The motor accomplishes the following operations as it goes through one cycle: It resets the "cold" thermostat and turns off the heater. Mechanical fingers sweep the cubes out and into the storage tray, and the solenoid valve opens the water line to refill the freezer tray. The solenoid valve remains open for an interval determined by the motor operation which closes the solenoid valve at the end of the interval. The motor establishes its own holding circuit when it starts, and it opens this circuit when the cycle is completed. (NOTE: We are dealing here with a time sequence in which some things happen together, while in others there is an overlap of action involved.) Ice cubes will not be ejected if the storage tray is full, because the feeler bulb in the tray will interrupt the circuit of the "cold" thermostat. As soon as enough cubes have been removed, the heater will be turned on to release the cubes in the tray and start another cycle. 1-19. Automatic defrosting. One of the earliest schemes to defrost an evaporator was the manual pushbutton which started the cycle. When defrosting was completed, the compressor was automatically restored to normal operation. By contrast, today there are defrost systems which are completely automatic. We will discuss the more common such systems. One such, a mechanical system, uses a counter which registers each time the door is opened. After a certain number of openings, the device starts the defrost cycle. This system follows the idea that each time the door is opened the coils will accumulate some moisture, and after approximately 60 openings, defrosting will be necessary. 1-20. An electric clock is used in three basic systems. In one system, the clock may be wired in parallel with the compressor so that it measures the total running time of the compressor. The theory here is that after 6 hours of compressor operation, the coils should need defrosting.

In another such system, a clock is wired in parallel with the cabinet light so that the clock measures the length of time that the door remains open. The advantage of these two systems is that they indirectly reflect the heat load on the unit. In still another system, one utilizing hot wire defrosting, a 24-hour clock, which is set to defrost at 0300 hours, simultaneously opens the circuit to the compressor and turns on two heaters. Instant heat is supplied by one of the heaters, which is similar to that used on an electric range. The insulated heater wire is installed either alongside the evaporator coil or in the center of the tubing which makes the coil. The other heater is a low-wattage heater used to keep the drain free of ice accumulation. A 70° thermostat, when satisfied, resets the clock mechanism. The heaters are turned off, and the compressor circuit is made ready for operation. 1-21. Defrosting must be scheduled whenever the frost accumulation has reached 1/8 inch thick. Otherwise, the layer of frost will act like an insulating blanket which slows down the transfer of heat. 1-22. Hot gas defrosting, still another method, utilizes an electrically operated solenoid valve controlled by a clock or some other timing device. Figure 2 shows the solenoid valve open. This position allows the hot gas from the compressor to pass through the evaporator. When the defrosting period ends, the valve closes and the compressor will return to normal operation. Accumulated melt

Figure 2. Hot gas defrost system.

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water is caught in a drain trough and piped to a tray in the compressor compartment for evaporation. You should remember from your study of fundamental refrigeration principles, that the capillary tube, shown in figure 2, is universally used as the expansion device in a small system. It has the advantage of no moving parts, and the pressures are equalized after the compressor stops so that there is less starting torque on the motor. 1-23. Special Features. Two special features which are found in some boxes are ultraviolet lamps and circulation fans. Ultraviolet lamps are wired into the circuit for continuous operation. Their radiation kills bacteria and counteracts some disagreeable odors. Circulating fans are used to give positive ventilation and to insure frost-free operation with proper control of humidity. The ventilation channels, between the freezer and the refrigerator, must not be blocked by storage containers. The motor-driven fan is wired in parallel with the compressor so that the fan circulates air during the cooling period. A switch in the line to the fan opens the fan circuit when the refrigerator door is open. The fan switch may be incorporated with the door switch that controls the cabinet light. Do not confuse this fan with the one used for cooling a compressor and a condenser. Food stored in a box with forced circulation must be covered to prevent desiccation. Also, if much uncovered, moist food is stored in such a box, it will cause excessive moisture to accumulate in the freezer. This excessive moisture will, in turn, cause frequent defrost cycling and compressor operation, which may lead to complaints about "defective equipment." 2. Absorption System Refrigerators 2-1. For many years, the absorption system has proved satisfactory for domestic refrigerators. Large capacity absorption systems are covered in Volume 4. Boxes made for use in the United States are designed to use either natural, liquid petroleum, or artificial gas heat. (In Europe, boxes have been made which use electricity for heating.) Absorption system refrigerators are made with automatic defrosting and ice cube makers. The defrost system is the electric hot wire type, with a timer such as is found in conventional boxes. The automatic ice cube maker is of the type which we have explained in Section 1 under the same title. Identification plates are located in the frozen food or the control compartment. 2-2. Each burner must be provided with the correct size orifice for the gas supply to which the refrigerator is connected. Gases are rated in B.t.u. per cu. ft., with LP gas the highest at 1600 B.t.u. per cu. ft. If you will compare two nozzles for size. You will see that the one for use with LP will have the smaller orifice. The nozzle

with the larger orifice is used with natural gas. which has less heat value. 2-3. The most popular domestic refrigerator of the absorption type uses an ammonia-water cycle in an atmosphere of hydrogen. The system is pressure tested at 800 p.s.i.g., but normal pressure in the system is 200 p.s.i.g. A fuse plug will release the pressure in the system if temperature rises to above 175° F. This release of pressure prevents any accidental explosion of the system. 2-4. Construction and Components. The absorption refrigerator box is constructed and insulated in much the same way as we have explained previously in this chapter. The essential parts of an absorption system are a generator, a vapor separator, a condenser, an evaporator, and an absorber. Among the components are some items which may sound unfamiliar. The generator is that art of the system where a water-ammonia solution is heated. The vapor separator is a special chamber where the water is separated from the ammonia. The absorber is so named because water at this place absorbs ammonia vapors. A brief review of the principles involved is given next. 2-5. Operation. The principle of operation of an absorption system depends upon the strong affinity which water has for ammonia. When the water-ammonia solution is heated in the generator, a mixture of water and ammonia vapors is given off. This vapor mixture rises to the vapor separator, where much of the moisture is extracted and returned to the generator by way of the absorber. Ammonia vapors rise in an atmosphere of hydrogen to the condenser, which is cooled by air temperature, and the ammonia liquefies. Liquid ammonia falls into the evaporator. The area in the evaporator has a concentration of hydrogen, which encourages the ammonia to vaporize and absorb heat. The evaporator is located in the freezer compartment of the box. From the evaporator, the ammonia vapor falls to the absorber, where it joins the water (from the vapor separator) returning to the generator, thereby completing the cycle. The system is completely closed, and there are no adjustments possible, except to the flame which supplies heat to the generator. The flame operates continuously, and any changes in heat load are met by changing the size of the flame. This adjustment is made automatically by a thermostatic control which regulates the gas supply to the flame. The sensing element is located in the freezer compartment. You will find several items between the unit and the gas line in an installation of this type. Starting from the gas line, these are a shutoff valve, a filter, a pressure regulator, and a gas burner with an automatic control. The gas burner valve is provided with a safety feature which will

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shut the gas off if the flame should be extinguished. In the order in which the device is implemented, the safety consists of a heater, a snap button, and a poppet valve. The snap button is linked to the poppet valve. The snap button is a curved piece of metal which reverses its curve when it is heated. The heater is located in the flame and conveys heat to the snap button. As long as the snap button is hot enough, it will hold the poppet valve open. If the flame fails, the snap button will reverse itself and close the valve, shutting off the gas. To light the flame again, you will have to heat the heater with a match and press a pushbutton to rest the poppet. If the heater should be accidentally dislodged from the flame during cleaning, the flame would be extinguished. The simple remedy for this condition is to move the heater back into the flame path and relight the unit. 2-6. Maintenance. To maintain an absorption refrigerator, you need only to keep it clean. Especially, remove all soot from the flue and burner chamber, since it acts as an insulator when it accumulates and thus prevents the generator from getting enough heat to do its job efficiently. Also, keep the flue clear of obstructions, since the flue draft is designed to work with the burner for best operation. In addition, keep dust accumulations off the condenser so that it will be able to transfer heat. This is important, since the condenser in an absorption system is more sensitive than that in a conventional type refrigerator. The only adjustment that you can make is to the flame to insure that it is clean and that it gives a minimum of carbon. A flame which is too yellow gives low heat and will not give satisfactory service because the flue will gather excessive soot and need cleaning too often. 2-7. An important aspect of correct installation of an absorption refrigerator is to be sure that it is placed level. When checking with a level, be sure that you check the unit rather than the box. Why? Because unless the unit is level, the system cannot operate properly. On the other hand when a unit is level, it will work correctly even if the box is slightly out of plumb. 2-8. When a box is placed in service after having been left idle for an extended period of time, it may not function at a cool enough temperature to make ice cubes. One suggested remedy for this condition is to remove the unit from the box and turn it upside down for an hour. Then install the unit and reconnect it to the gas line. Thereafter, it should function properly when fired up. Of course, you might find it simpler to disconnect the gas line and turn the whole box upside down for an hour. Otherwise, the main reasons for poor unit operation are an incorrect flame and/or too much dirt clogging the heat exchanger surfaces.

3.

Compression System Refrigerator Components 3-1. The main parts of a domestic type refrigerator are discussed in this section. These are compressors, condensers, evaporators, and refrigerant controls. We will cover the common types of each and their various applications. The hermetic system is used with all domestic boxes. We will also discuss refrigerator performance as it relates to the components of compression system refrigeration. 3-2. Reciprocating Compressors. The operating principle of a compressor is closely related to the refrigeration cycle. A brief review of the principles of refrigeration is appropriate at this time. When a gas is compressed, it gets hotter. When pressure on a gas is lowered, it gets cooler. When a liquid becomes a gas, it picks up heat. The gas passing through a compressor gets hotter. It gives up this heat in the condenser, where it becomes a liquid. It changes from liquid to gas in the evaporator, where it picks up heat (cools the evaporator) and carries this heat through the compressor back to the condenser, where it is again cooled. The function of the compressor is to make the required pressure changes on the refrigerant so that it can do its work. High pressure is on the condenser side (high side) and low pressure on the suction side (low side). 3-3. The reciprocating compressor consists of a cylinder and head, a piston and connecting rod, intake and exhaust valves, servicing valves, fly-wheel, crankshaft and crankshaft seal, and suction strainer. Clearances as small as 0.0001 inch are possible between the moving pans, because the compressor is operating in a closed environment where the temperature range is relatively narrow. 3-4. The piston may be driven in a number of ways. In one, the crankshaft may be like the kind used in automotive engines. Another type uses an eccentric crankshaft which operates like a cam. Still another is the scotch yoke mechanism which uses a pin mounted off center to the crankshaft. A sliding member inside the piston permits the rotation of the pin to be translated into up-and-down motion. Variations such as these are possible because gases are being pumped which do not produce heavy bearing loads. The piston is made to come as close as possible to the head without touching. Clearance may be as little as 0.01 inch at top dead center. 3-5. Exhaust and intake valves are usually made of thin disks of steel which seat against shoulders in the valve plate. These valves are sometimes called flutter or reed valves. Pressure in the cylinder closes the intake valve and raises the exhaust valve on compression. On the intake stroke, pressure in the suction line opens the intake

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valve, while back pressure from the high side closes the exhaust valve. Valves are designed to operate at a maximum lift of 0.10 inch. Beyond this point, the valve gets noisy. 3-6. Rotary Compressors. Fewer moving parts and less vibration are advantages of the rotary, which is made in two styles. One style uses an eccentric shaft with blade which is forced against the shaft by a spring. The blade slides back and forth in a slot in the case between the intake and exhaust. As the shaft turns, it traps a gas charge at the intake and sweeps it around to the exhaust. Oil makes the seal for the blade so that the gas will be compressed. 3-7. In another style of rotary, vanes are mounted in slots on the shaft. The shape of the case around the vanes is eccentric. Centrifugal force holds the vanes in continuous contact with the eccentric wall. The inlet port is located in the wall furthest from the shaft, at the spot where a gas charge is picked up between two vanes. As the shaft turns, the space between the shaft and the wall becomes smaller, compressing the charge of gas. The exhaust port is set in the case where the shaft almost rubs against the case. The compressed charge of gas is forced out the exhaust at this point. 3-8. The exhaust valve is a flapper type made of spring steel. A muffler is placed in the high-pressure line to suppress the popping noise which accompanies the release of a gas charge. The suction line is provided with a check valve to prevent gas from leaking back when the compressor is stopped. The suction strainer prevents dirt particles from entering the compressor. 3-9. Condensers. Among the kinds of condensers which you should know are (1) the finned coil with forced convection, (2) the finned coil with natural convection, and (3) the plate condenser with natural convection. Coils may be mounted in an upright, an inclined, or a horizontal position. The first few turns of coil may be placed under the pan which collects water from defrosting. Evaporation of this water aids the condenser in dissipation of heat. Between the condenser and the evaporator is a capillary tube firmly soldered to the suction line in order to operate as a heat exchanger. 3-10. Evaporator Arrangements. While one condenser may serve a combination refrigerator, there are three general arrangements for the evaporator to cool the two areas. In one arrangement, the evaporator coil in the freezer is extended into the refrigerator compartment. A restrictor separates the two sections of the evaporator coil, as shown in figure 3. The capillary tube is omitted for simplicity. The refrigerant goes first to the cooling coil, which is forced by the restrictor to operate at a higher pressure and temperature. After passing the restrictor, the refrigerant is at a lower pressure and absorbs sufficient heat to drop the temperature in the

freezer to the desired low range. The restrictor used in some refrigerators is a weighted valve, and the method employed is called a weighted valve system. 3-11. A box which uses a weighted valve must be installed level to insure proper operation. If the position of the valve is disturbed from the correct position, the box will not perform properly. For example, certain boxes use a weighted valve which is designed to operate properly at an angle of 60° from the horizontal. If the valve is tilted too much toward the vertical, the food compartment will get warmer. On the other hand, too much tilt toward the horizontal will make the fresh food compartment colder. The indication of this trouble is continuous operation of the compressor and partial frosting of the food compartment evaporator coil. Either or both symptoms may be present. 3-12. The second arrangement uses an evaporator coil in the freezer compartment in combination with a secondary closed loop coil. This secondary coil acts as both a condenser and an evaporator. You can see in figure 4 that part of the closed loop is in the freezer. This part of the coil acts as a condenser, while the rest of the loop in the refrigerator acts as an evaporator. One major

Figure 3. Dual temperature system. 7

Figure 4. Closed secondary system. advantage of this is that the closed loop can be designed to operate frost free. 3-13. A third arrangement uses one of several combinations of the first two methods. These various combinations are designed with the object of either automatic defrosting or frost-free operation. Some of these combinations rely on air circulation to get better results. Slots between the two areas will allow convection currents to circulate, or a fan may be used for forced air to insure a more uniform temperature. 3-14. When only one coil forms the evaporator in the freezer compartment, it is called "air spill over." The method is called "refrigerant spill over" when two coils are used in series. When a separate evaporator is used to form a closed loop the method is known as a secondary refrigerant system. 3-15. Regardless of which arrangement is used for the evaporator, the system depends on a capillary tube with its heat exchanger function for proper operation. 3-16. Capillary Tubes. A capillary tube is used to control the refrigerant by placing a restriction in the liquid line. It is sometimes called a choke tube, a more descriptive name. The inside diameter and the length of the tube are critical factors. The diameter of the tube 8

determines the restriction which it will have to control the flow of refrigerant. The length of the capillary must be long enough so that the liquid will have started to change to a gas as it nears the end of the tube. A selected portion of the tube is soldered to the suction line forming a heat exchanger. The length of the heat exchanger is calculated in the design so that the capillary will deliver liquid at the proper temperature. If the solder connection is broken, the heat exchanger will be lost, and the unit will not perform properly. 3-17. Starting Relays. At one time, the thermostat was used directly to control starting and stopping the compressor motor. The feeler bulb in older units was located in the freezer compartment. In some refrigerators, the feeler bulb is above a small access panel in the top of the freezer. Now, you will find that most refrigerators are provided with a starting relay, which provides two advantages: First, the relay can be located close to the motor. Second, the relay can handle the motor current more easily. 3-18. Changes in temperature in a refrigerator cause operation of the thermostat, which controls operation of the relay. The starting and stopping of the compressor motor is under the control of the relay. The characteristics of the motor (current and internal resistance) will determine the size of the relay used with it. Since these relays are sensitive to temperature changes, they are located where they will be least subject to changes. The three most common types are (1) the current relay; (2) the voltage, or potential, relay; and (3) the hot wire relay. We will discuss the operation of each so that you will understand the troubles you might find. 3-19. Hot wire relays. One type of hot wire relay uses two bimetal strips and two heater resistors. In figure 5, there is a schematic diagram which shows the connections for the relay and motor. The motor terminals are identified by C for common, R for run, and S for the start winding. When the motor control closes, electricity is applied to both the running and the starting wind-

Figure 5. Hot wire relay.

Figure 6. Current relay. ings. The capacitor in the circuit of the starting winding gives the motor more starting torque. The resistor, in series with the bottom bimetal strip is designed to heat enough of the starting current so that it will cause the bimetal strip to bend up. This opens the circuit to the capacitor and the starting winding. The bleeder resistor across the capacitor keeps the relay contacts from being burned. The upper bimetal strip is heated by the upper resistor by the current going to the running winding. As long as the current to the running winding is normal, there will not be sufficient heat to make the upper bimetal strip open. Thus, you can see that the strip in the "run" circuit acts as an overload device. The current in the "run" resistor also serves to keep the bimetal strip in the start circuit from cooling so that its contacts remain open until the motor stops. If, for any reason, the motor current becomes excessive, the overload contacts will open and stop the motor. However, a disadvantage of this overload protection is that as soon as the device cools, the contacts will close and the motor will start again. It will then short-cycle until the circuit is opened. Failures such as this are rare, however. A variation of the hot wire relay uses a wire under tension to operate the contacts. In any case, the hot wire relay has been proved reliable in thousands of commercially made refrigerators. 3-20. Current relays. You will find that figure 6 is a schematic diagram of a current relay. Many refrigerators are equipped with this type of relay. For the sake of variety, this schematic shows a diagram which is typical 9 of the control circuit for a water cooler. The only essential difference between this and the circuit for a refrigerator is the freezestat. Its purpose is to prevent the chilled drinking water from being frozen. The freezestat will stop the compressor motor (if the thermostat does not) to prevent the formation of ice which could damage the tank. The thermal overload protects the motor from burning out by opening the circuit if the motor draws too much current. Excessive heat from the resistors makes the bimetal strip bend, which breaks the circuit. 3-21. Operation of the current relay occurs as soon as the thermostat closes the circuit to the motor. The inrush of current to the running winding is strong enough so that the coil pulls the armature up and completes the starting winding circuit. This happens in a fraction of a second, putting the starting winding and its capacitor in the circuit. As the motor approaches its operating speed, the motor current drops because of counter electromotive force (cemf). The coil in the relay is designed so that it will release when the motor r.p.m. passes three-fourths of its normal speed. The current in the relay coil is not sufficient to hold the weight of the armature when the motor is operating at its normal speed; therefore, the relay contacts open. A bleeder resistor is connected in parallel with the capacitor. Its purpose is to prevent a high-voltage discharge from the capacitor, which would burn the contacts in the relay as they open. An advantage of using a current relay is that the contacts in the thermostat are

only required to close on the inrush current to the running winding. Furthermore. the contacts in the relay are only required to carry the inrush current to the starting winding. 3-22. Servicing (i.e. troubleshooting) a current relay consists of checking the circuits for proper operation. As there are no adjustments, a relay which does not operate properly must be replaced. If the relay contacts are badly burned, the bleeder resistor should be checked for correct resistance. Some capacitors have the bleeder resistor made as an integral part of the capacitor. If the resistor is found defective, a new resistor of the correct wattage and resistance can be connected across the capacitor to make a repair. 3-23. Testing a current relay can he confusing. At least one maker uses a metal washer as the armature. When the coil is not energized, the washer lies (on the bottom of the case. You can hear it rattle when the case is tapped. If you did not know better, you might assume that something had broken loose and that therefore it was defective. Also, a relay of this make might be defective even though you could hear no rattle as the armature could be welded to the contacts. In addition, current relays are sensitive to position; consequently, they

must be mounted horizontal or level to insure proper operation and normal life. 3-24. Potential relays. A voltage sensitive, or potential, relay will have its coil connected in parallel with the starting winding of the compressor motor. The schematic diagram in figure 7 is typical of the wiring of a motor provided with this type of relay. The defrost switch and circuits associated with it have been discussed already in the first section of this chapter. The contacts of the thermostat must be heavy enough to carry the inrush starting current to the compressor motor. An advantage of the potential relay is that the relay contacts are normally closed. Even so, the capacitor discharge across the relay contacts when they open may cause them to burn and be welded together. Thus, a bleeder resistor is required across the capacitor to prevent any welding of the contacts. The value of resistance for a bleeder is usually 15,000 ohms, and it is rated at 2 watts. 3-25. Operation of the potential relay does not occur until the motor passes three-fourths of its speed. The relay coil is voltage sensitive and requires considerably more than the line voltage to make it operate. When the thermostat contacts close, the motor starts, because both motor windings are energized. The contacts in the relay are

Figure 7. Potential relay. 10

normally closed. As the motor approaches its operating speed, the current decreases. Induced volt age in the starting winding now becomes greater than the applied voltage. The reason for this lies in a transformer action between the two windings in the motor. The voltage sensitive coil in the relay is sufficiently energized to open the contacts in the starting winding. The relay remains energized by the voltage induced in the starting winding. When the thermostat opens, the motor stops, and the relay contacts return to the closed position. 3-26. Servicing a potential relay consists of checking the coil and the contacts for continuity if troubles arc suspected. Like other relays, the potential relay is sensitive to position and must be mounted level to insure proper operation and normal life. Since there are no adjustments, a defective relay must be replaced. A badly burned set of relay contacts would indicate that the bleeder resistor should be tested and replaced if it is open. 3-27. Resistors. A bleeder resistor should have a resistance between 15,000 and 30,000 ohms, depending on the size of the motor and the capacitor. A 2-watt rating is satisfactory for a refrigerator. The wattage rating is stamped on the body as "2 W." A resistor which is color coded will have bands of color painted on the body. Here are four examples: Ohms 15,000 20,000 25,000 29,000 First band Brown (l) Red (2) Red (2) Red (2) Second band Green (5) Black (0) Green (5) White (9) Decimal multiplier Third band Orange (000) Orange (000) Orange (000) Orange (000)

3-28. Refrigerator Performance. So that you can evaluate the operation of a refrigerator, analyze troubles, and take the most economical means of repair, you must know its performance characteristics. Some of the factors which you should consider are these: electrical consumption, percentage of compressor running time, relative temperatures and types of use, vibration or noise, and continuous operation. Relative temperatures, of course, include room temperature and room humidity (mentioned below) and the temperature, respectively, of the freezer section and the refrigerator section (not discussed below). Continuous operation, although discussed below, is caused by troubles not normally considered in refrigerator performance. 3-29. Electrical consumption is figured in kilowatthours per 24-hour period. You may obtain a recording type meter from the electric shop on the base or from the local utility company when an actual test is necessary. You may compare the results with standard test curves

furnished by the maker of the box. A modern refrigerator may use from 2 kilowatt-hours to 8 kilowatthours in a 24-hour period, depending on the size of the cabinet and the difference between cabinet and room temperature. 3-30. The percentage of compressor running time is affected by various variable conditions, but normal use in a 75° F. room will require the compressor to operate one-third of the time. Any large increase in running time indicates there are abnormal conditions. For example, setting the thermostat for a refrigerator temperature of 20° F, when a temperature close to 38° is considered normal will increase the compressor running time. 3-31. Relative temperatures and type of use are just two of the variables which affect performance. Increased room temperature and room humidity put a heavier load on the compressor. A difference of 20° room temperature may double the running time and electric power used. Higher humidity will produce frost at a greater rate causing a reduction in evaporator efficiency. Type of use includes frequency and duration of door openings as well as the foods stored in the refrigerator. Uncovered liquids will cause an automatic defrost to cycle more frequently. Unusual load would also result if the freezer is required to make a great many ice cubes. 3-32. Vibration or noise is rare in the modern refrigerator as the capillary tube system used by most manufacturers requires no moving parts except for the compressor and the motor. Vibration may result from a defective mounting. For instance, one of the more common causes is failure to release the holddown bolts for the compressor, which were installed before shipment of the cabinet. On the other hand, a unit will be noisy if it is low on oil. Thus, if a leak has developed, requiring that refrigerant must be replaced, then it is likely that oil has also been lost and must be replaced. One clue to watch for is this: A noisy capillary tube will usually indicate low refrigerant, and this results in gas noises such as bubbling or hissing. Of course, low refrigerant is also indicated by a partially frosted cooling coil and continuous compressor operation. In the latter event, low- and high-side pressures will be lower than normal. You should also not that a capillary tube which is clogged or pinched will show a high vacuum on the low side and an abnormally high head pressure. 3-33. Continuous operation can be caused by a defective thermostat, one which does not cut the compressor off. Any abnormal condition which causes continuous operation may be obscured at the time you are called because overheating may cause the motor overload protection to operate. Thus, when you get to look at the unit, you may find it short-cycling; that is, the overload device will be shutting the unit off. When it cools enough, the unit will start again. A fast check of

11

this can be made by feeling the temperature of the motor and compressor with your hand. Many compressors now have the overload protector inside the shell, where the heat from the motor will prevent the unit from shortcycling. One last item which is often overlooked is low line voltage, which will make a motor run slow and overheat. Where a television set is used in the same building as the refrigerator, low line voltage is indicated if the picture fails to fill the tube. This is one way of verifying low voltage when a voltmeter is not available. You must remember to consider all of the factors when you evaluate the performance of a unit. 4. Freezers 4-1. Many homes today have domestic freezers for the storage of large amounts of frozen foods. These freezer cabinets are either of the upright style or of the chest style and range in size from 15 to over 25 cubic feet. In such freezers the capillary type system with a hermetic unit is widely used. Also, in many cases, the compressor employed is similar to those used for refrigerators. Upright freezers are supplied with forcedair circulation when they are made for frost-free operation. Automatic defrosting is done by the hot gas or the hot wire method used in combination refrigerators. Because of the similarity of the two systems, troubleshooting a freezer can be done by the same rules as for a refrigerator. Troubleshooting a hermetic system follows this section. The construction features of a freezer are often the same as a refrigerator. Therefore, the information here is concerned with those features which apply to freezers. 4-2. Operation and Care. The operating temperature for domestic freezers is in the range of 0° F. to 10° F. Each such freezer is equipped with a thermostat so that it can be adjusted to satisfy the user's desires. Most foods are already frozen when placed in storage, but when nonfrozen foods are placed in the freezer, it would be desirable for the user to move the control to a colder position in order both (1) to insure faster freezing and (2) to prevent the thawing of other foods. Food must be stacked so as not to interfere with air circulation, as this would cause warm spots to develop in the cabinet. Food must also be wrapped in moisture proof and vaporproof material to prevent desiccation. 4-3. A freezer cabinet in normal usage will require defrosting about twice a year. Frost may be removed by means of a scraper, such as a wooden paddle, or with a stiff fiber brush. Use care not to damage the finish. When you desire a complete defrosting, however, remove all frozen foods and store them in dry ice to prevent

thawing. Then shut off the power and use warm water to hasten melting of the ice. Of course, you should never try to chip ice from coils, as such action might damage them beyond repair. Also, wash the box with a solution of baking soda or ammonia, and always dry the inside of the box before putting it back in service. 4-4. Construction Features. To prevent frosting around the door, a mullion heater of low wattage is installed in back of the breaker strip. By this means, too, sweating around the door is virtually eliminated, because the heater is operating continuously. Fiberglas, rock wool, and certain synthetics are used for insulation of present day freezers. Consequently, where formerly a 3to 4-inch thickness of insulation was used in cabinets, you may now find them built with walls less than 2 inches thick. Yet, because of the colder chest temperature, freezer insulation must be better protected against moisture than that in a refrigerator. The most effective method for accomplishing this has been found to be a combination of venting and sealing. Venting is provided to prevent a partial vacuum which would occur in a sealed box as the temperature of the air is lowered. If the insulation scaling is to be effective, however, it must have greater resistance than the venting. Thus, the vent will allow box pressure to equalize while the seal remains intact. 4-5. Freezer Failure and Alarms. Within 24 hours after a freezer fails, the foods it contains will start to thaw. If this is discovered in time, dry ice can be used to prevent the thawing while repairs are made to the unit. Otherwise, meats and fresh frozen fruits must be used quickly, as it is new satisfactory-or safe-to freeze them a second time. To avoid this situation, alarm devices are made which will signal a rise in temperature to 15° F. in the cabinet. The alarm is given by both visual and audible means. Two kinds of alarms are available. One is made for operation on a 6-volt battery. The other is made for operation on a 110-volt circuit. The latter should be plugged into a branch circuit different from that used for the freezer. The thermal element should be located in the upper part of the cabinet, where it will reflect a rise in temperature quickly. While such alarm devices are not generally supplied for domestic units, they can be installed in any freezer cabinet. 4-6. In the next section, we will deal with troubleshooting hermetic systems. You will not find any distinction between refrigerators and freezers, because they both use the same kind of hermetic systems. The main difference between refrigerators in general and freezers is that only part of a refrigerator is held at near zero temperature.

12

5.

Troubleshooting Hermetic Systems 5-1. The sealed system preferred for freezers and refrigerators is called a closed or hermetic system. The shell which contains the motor and compressors is welded shut, thus the name “sealed unit?” The motor leads pass through a glass insulator, which is bonded to the metal to insure a joint that will never leak. The one big advantage of a hermetic compressor is that there are no seals where leaks can develop. This eliminates at least one trouble spot from the system used in domestic refrigerators and freezers. But, as you know, there are still enough other trouble spots to keep a serviceman busy. 5-2. The best troubleshooter puts his brain to work before reaching for the toolbox. The first action on the job should be to question the user. Ask him, for example, these things: • When did you first notice this trouble?

• •
• •

How often does it happen? Does it happen at night as well as during the day? Has the unit been making a strange noise? Is this condition intermittent or is it continuous?

Does this happen on just certain days of the week? The answers to such leading questions should enable you to determine whether the trouble is being caused by misuse. By eliminating outside factors at the beginning, you will know that you are dealing with a fault in the equipment itself. After this, consider the possible electrical troubles first, as they can usually be checked easily and quickly. 5-3. Electrical Troubles. There is a logical sequence which should be followed in making tests on the electrical system. The first check seems so simple that it is often overlooked. Remember, the unit cannot operate without electrical power. A quick reference for common faults is given in table 1, together with the possible causes and their remedies. Such a trouble chart is most useful, since it presents a great many facts in a small space. In addition, you will sometimes find the solution to a problem while studying a troubleshooting table, even though the specific fault does not appear in the table. Often, in fact, a common fault is passed by because it seems too obvious. The serviceman may think that a common fault is too easy and could not possibly be the trouble he was hunting. Do not prejudge; instead, make reasonable "guesses" from what you see and the trouble chart, then test to find out the practical results. In the following paragraphs, we have given you a detailed explanation of the most likely troubles to be found in the electrical system.

•

5-4. Power supply. Check the source of power for voltage to the unit. How? With a voltmeter or a multimeter! In the case of a refrigerator, open the door. If the light does not come on, there are several possibilities: (1) the power circuit is incomplete to the unit; (2) the lamp is burned out; (3) the door switch is defective; (4) the circuit to the unit may be good, but the wires to the lamp and the door switch are broken somewhere in the box. If the lamp lights, you will know that there is power to the unit. However, check the voltage with an accurate voltmeter when you suspect low voltage. Remember: The line voltage may vary 10 to 15 volts with changes in the load during the day. Most units will not show difficulty unless the voltage drops below 105 volts. 5-5. Overload protection and controls. Make sure that the extension cord is disconnected be, fore making a continuity test on the protector. With an ohmmeter or a test lamp, check for a continuous circuit through the overload protector. If it tests open, you have found at least one trouble which will prevent the compressor motor from operating. Replace a defective overload protector and check the unit for normal operation. Many compressors have the overload protectors located inside the shell. A distinctive label on the compressor is used to indicate an internal mounting. Placing a protector inside the shell has the effect of extending the cooling period after an overload trips. Remember, when checking an overload protector mounted inside, allow the compressor sufficient time to cool so that the protector has a chance to automatically reset itself. How long is "sufficient time"? When you can rest your hand comfortably on the shell, the compressor should have cooled enough for you to make a valid test of the overload contacts. 5-6. Check, too, all control switches for proper operation, since one open switch will prevent the unit from operating. Such items as thermostats, defrost controls, and freezestats are all designed to open and close the primary circuit. Remember the function of the item which you are checking, because an open circuit may not mean that the device is defective. A thermostat should show an open circuit if the feeler bulb is colder than its operating point. A defrost control will be open if the timer is in the defrost cycle. Some defrost systems have a reset which is actuated by an increase in temperature above a set point. A freezestat will show an open circuit when it senses a temperature lower than its operating point. You can check the operation of a device by raising or lowering its temperature. A thermostat is checked by placing the feeler bulb in a glass of ice and water. An ohmmeter or test light is connected across the contacts so that the time of opening and closing can be observed. A thermometer is placed in a glass of water and its temperature is

13

Table 1

read at the time the contacts open. Remove the ice and add warm water slowly till the contacts close. Again read the temperature of the water. Replace the thermostat if it does not conform with manufacturer’s specifications. 5-7. Motor circuit. A hermetic system can be checked quickly with a motor-start analyzer. It will check for continuity in motor windings, for shorted windings, and for grounded windings. It can also be used to start a motor and can reverse the direction of rotation. The analyzer contains capacitors which can be used in the 14

motor circuit to increase its starting torque. Higher starting torque or momentary reversing are two ways of unlocking a compressor which for some internal reason cannot be started normally. When an analyzer is not available, plug the refrigerator cord into an outlet and test for voltage at the terminal block where the cord terminates. There should be voltage at the terminals if the cord is good. If the motor runs, you should use a clamp type ammeter to check for correct motor current. Next, you must unplug the cord and make some continuity checks with a test light or an ohmmeter.

Unless you are familiar with the electrical system, you will need a wiring diagram for the unit which you are testing. 5-8. Some compressors have an electric heater in the crankcase to prevent condensing of the refrigerant during off time. Liquid refrigerant can cause slugging and damage the compressor. One manufacturer used a capacitor with one of the motor windings to act as a heater during compressor off time. Such a motor would always be "live" even when not running. Be sure of the type of motor used before you attempt to trace the motor circuit. Determine the type of relay (hot wire, current, or potential relay) which is installed in the unit After you have checked the diagram and understand the circuit, you will be ready to check out that specific motor. 5-9. For purposes of our explanation, refer to figure 7, which illustrates the circuit for a potential relay. We will use the compressor motor circuit shown in figure 7 to identify the motor's terminals in the following discussion. Make a continuity check from C to S and between C and R. A test lamp should light almost normal in each case if the windings are good. An open circuit is indicated when the lamp fails to light. Note that this test is valid only if direct current is used to energize the circuit. If alternating current is the only power available for the test lamp, the common connection at C must be opened. Otherwise, the closed contacts of the relay and the capacitor will make a complete circuit. Opening C is not necessary when checking with an ohmmeter, because it uses direct current from self-contained batteries. The reason is that a capacitor blocks direct current while it allows alternating current to flow. See the paragraph for testing capacitors, where the capacitor is explained more fully. 5-10. To test for a grounded motor winding, check from terminal C to an unpainted part of the compressor-motor shell. An ohmmeter must be used to measure the resistance of the motor windings to test for a shorted coil. Readings should compare closely to the specifications of the manufacturer. A severely shorted coil would be indicated by tripping of the branch circuit breaker or by blowing of the fuse when the unit is plugged into the voltage outlet. If tests indicate that the motor windings are at fault, the hermetic unit must be replaced. 5-11. If the motor runs but overheats during operation, a current draw test with a clamp type ammeter will give an indication of conditions. Motor current should be within 10 percent of nameplate rating on the unit. The nameplate may give two amperage figures, such as FLA 3.5 and LRA 18.0 The FLA stands for "full load amperage," while LRA stands for "locked rotor amperage.” If the current exceeds the nameplate rating

by more than 10 percent, it is considered unsatisfactory, and the hermetic unit must be replaced. A motor drawing its LRA rating indicates that the rotor is not turning. Conditions inside the sealed unit will also be indicated by unusual vibration and noises. For tests of the refrigeration system, see Mechanical Troubles, paragraph 5-17. 5-12. Testing capacitors. We will discuss two methods for testing a capacitor. When the capacitor can be disconnected from the circuit and the bleeder resistor, a reasonable test is to charge and then discharge it with its normal voltage (of over 120 v). Charge it by momentarily applying voltage to its terminals. Then use a piece of insulated wire to short circuit the terminals. A hot spark indicates that the capacitor is able to hold a charge. Some capacitors have a bleeder resistor of between 15,000 and 30,000 ohms which is in the form of an integral part that cannot be disconnected. This type of capacitor may be checked by connecting an ammeter and a 10-, 15-, or 20-amp fuse in series with the capacitor. Apply 120 volts to the capacitor just long enough to read the ammeter. If the fuse blows, the capacitor is shorted and must be replaced. Use a fuse large enough to carry the current and make sure that the current will not be so great as to drive the ammeter needle off the scale. For example, a 20-mfd capacitor at 120 volts should draw less than 1 ampere, while a 400mfd capacitor at 120 volts will draw 18 amperes. When making a test, apply voltage to a capacitor just long enough to read the ammeter. The current measured should be within 20 percent of that determined by the formula given where mfd is the rating in microfarads and v is the normal applied voltage. The number 2650 is a constant for 60-cycle current, while 3180 is the constant used when calculating a circuit using 50-cycle current.

A defective capacitor must be replaced by one of the voltage and mfd rating or the equivalent as specified by the manufacturer. 5-13. Testing relays. Before testing a relay, you must know the type. "You may have a schematic diagram which shows the hookup of the relay but does not identify it by name. You should be so familiar with the common types that you know their characteristics well enough to identify them. A fan motor is used in some units for forced-air circulation. The diagram in figure 6 shows an example of a relay and a fan motor in

15

the same circuit. The fan motor must be disconnected before testing the relay. 5-14. Referring to figure 5, you will see a type of hot wire relay that has two bimetal strips, two resistors or heaters, and two set of contacts. Both sets of contacts should be dosed when the relay is not energized. The start contact should open soon as the motor reaches operating speed. You can verify opening of the start contacts with a voltmeter which should read line voltage across the start contacts. A zero reading will indicate that the contacts are not opening. 5-15. The current relay shown in figure 6 can be checked for continuity through the coil and for an open circuit across the contacts when it is not energized. The contacts close on starting but should remain open while the motor is running. Use direct current, such as with an ohmmeter, to test across the relay contacts, as a.c. can feed around through the motor winding and the capacitor, giving a false reading of continuity. 5-16. The potential relay, which is shown in figure 7, must be isolated from the compressor motor before testing. Open the R and S leads at the terminals on the relay. Check the relay connects between R ad S for continuity. The contacts are normally closed; thus, the test should show a complete circuit. A test between S and terminal L should also show a complete circuit through the coil of the relay. If either test shows an open circuit, the relay is defective and must be replaced. 5-17. Mechanical Troubles. The mechanical troubles found in a refrigerator can be divided into two categories: (1) those which are caused by defect in manufacture, and (2) those which are the result of mishandling or accident. A quick review of mechanical troubles is furnished in table 2, where possible causes are listed with their related faults. As the repair or replacement of components involves the use of soldering and welding equipment, a review of this subject is presented before we discus the procedures to follow with specific items. 6. Braxing, Cutting, and Welding 6-1. Before you can join metals or make repairs you must know how to use welding equipment properly. Safety for yourself is stressed so that you can do this work without danger to your self. First, the safety rules which you should know will be presented. Then we will discuss the equipment and explain procedures for brazing with alloys, silver brazing, welding copper, and cutting metal. 6-2. Safety Rules. Study the following rules so that you will understand them. Apply the rules in your work so that you will set a good example for others to follow. You will avoid accidents this way: • Never drop a cylinder or allow it to fall.

•

•

• • • • • • •

• • •
•

infusorial earth, which will get into the regulator and valves if the tank is placed on its side. Also, safety plugs in the bottom of the tank will pass harmlessly into the floor if the cylinder is standing up when they blow out. Never allow oil or grease to come into contact with oxygen; specifically, never direct a jet of oxygen at an oil-soaked surface. Spontaneous combustion may result. Never lay an oxygen cylinder on its side. The top of the cylinder carries the safety plug. If it blows while the cylinder is on its side, the exhaust pressure released will propel the cylinder like a rocket. Never use oil, grease, or any lubricant on a torch. Never hang a torch or hoses on regulators or cylinder valves. Never use matches for lighting a torch, as your hand may be seriously burned as a result. Use a friction igniter or a suitable pilot light. Never light the torch from hot metal when working in a confined space. Accumulated fumes can flare or explode. Never weld where hot sparks can set fire to material or where sparks can fall on your legs or on the hoses. Always wear goggles designed for the welding work or brazing which you are doing. Never block yourself from the cylinders when you are working; make sure that you can get to them easily and quickly from your working position. Never store cylinders in direct sunlight or near heaters. A valve clogged with ice may be thawed with warm water; however, never use a flame or boiling water for this purpose. Never test for acetylene leaks with a flame because of the danger of a flareback and a cylinder fire. Use soapy water, instead. Always open valves slowly.

• •

Never bump a cylinder or otherwise handle it roughly. Never lay an acetylene cylinder on its side. In addition to acetylene, the tank also contains

Always keep the special wrench used to turn acetylene on and off near the valve so that it can be turned off quickly in an emergency. • Never hammer or beat on a valve; furthermore, do not attempt to adjust a valve or a gauge which does not work. • Replace protective caps on the cylinders whenever gauges have been removed. 6-3. Oxygen and Acetylene Apparatus. In figure 8 you will find an illustration of oxygen and acetylene cylinders and the accessories used with

•

16

TABLE 2

a soldering and welding torch. The rules for properly setting up this apparatus are as follows: • Place the oxygen and acetylene cylinders on a level floor and secure them so that they cannot be • accidentally knocked over. Then remove the protecting caps. • Crack each cylinder valve just enough to blow out dirt or foreign matter. Close the valve as soon as the throat is clear, then wipe off the seats. (NOTE: Do not stand in front of a valve when cracking it.) • First, connect the acetylene regulator to the acetylene cylinder; then, second, connect the 17

•

•

oxygen regulator to the oxygen cylinder. Use a close-fitting wrench to tighten the connections sufficiently to prevent leakage. Connect the red hose to the acetylene regulator. As you do this, note the left-hand threads on the acetylene hose connections. Next, connect the green hose to the oxygen regulator. Screw the connections tight enough to prevent leaking. Release the regulator screws to avoid damage to the regulators and gauges and open the cylinder valves slowly. Read the high-pressure gauge to check the pressure of the contents in each cylinder.

Figure 8. Oxygen and acetylene apparatus.

•

•

Blow out the oxygen hose. By turning in the regulator screw; open each regulator (two gauges) so as to blow out the hose; then release the regulator screw. If it is necessary to blow out the acetylene hose, you must do the work in a place which is both well ventilated and free from sparks or flame. Connect the red acetylene hose to the torch needle valve stamped "AC" and the green oxygen hose to the torch needle valve stamped "OX." Test all hose connections for leaks at the

•

•

torch and at the regulator by turning in both regulator screws with the torch needle valves closed. Release the regulator screws after testing and drain both lines by opening the torch needle valves. Slip the tip nut over the mixing head, screw the tip into the mixing head, and assemble it in the torch body. Then tighten the assembly by hand and adjust the tip to the proper angle. Secure the adjustment by tightening it with the tip-nut wrench. Adjust acetylene working pressure by open-

18

TABLE 3

ing the acetylene torch needle valve and turning the regulator screw to the right for the required pressure according to the size of the tip. Adjust oxygen working pressure in the same manner, according to tables 3 and 4. For tip sizes in the low-pressure or injector type of torch use table 3. For tip sizes in the medium-pressure or balanced-pressure type of torch, use table 4. (NOTE: In table 4, each of the first three sizes requires 1 pound of pressure, while the others take the same number of pounds of pressure as the tip size. The size of the tip that you choose is determined by both the thickness of the metal or tubing and the area which must be heated.) 6-4. Shutting down the torch safely involves the following 6-step procedure: • First close the acetylene valve on the torch. • Second, close the oxygen valve on the torch.

•

Third, close the acetylene and oxygen cylinder valves.

Fourth, drain both the regulators and hoses. Open the torch acetylene valve until gas flow stops; then close the valve. Drain the oxygen regulator and hose in the same manner. Both the high- and low-pressure gauges on the oxygen and acetylene regulators should now read "zero." • Fifth, release the tension on both regulator screws by turning them to the left until they rotate freely. • Sixth, coil the hose and suspend it in a suitable holder, being careful to avoid kinking the hose. 6-5. Use of Alloys for Braxing. The alloys used for silver brazing all have a melting point above 1000° F. This is, however, still below the melting point of the base metals to be joined. When properly made, the joint will be at least strong as the metals joined. You must swage one end of tubing which is to be joined. The swaging tools must be clean and free of oil. This will produce a swaged end which does not require added cleaning. The most important factor in

•

TABLE 4

19

joining tubing is to have proper clearance between the parts. An easy slip fit with tubing should approximate the range .0015 to .005 inch, which is recommended. To insure proper centering of the male pat, insert it so as to evenly contact the shoulder of the swaged member. This procedure will insure a uniform distribution of the alloy with no voids and prevent the alloy from dripping into the inside of the tubing, where it would cause an obstruction. Position the angle of the joint in tubing so that solder or flux will not drop inside. Rely on capillary action to pull the solder throughout the joint. 6-6. The ends of tubing to be joined must be square and uniformly round. The surfaces must be free of oil, scale, grease, and dirt. If you find any oil or grease, you can remove it with hot caustic soda. You can remove scale with an acid pickle bath; however, you must then remove all traces of acid after such treatment, since any trace of acid left in the tubing will cause trouble in the future. Avoid handling surfaces after they have been cleaned. 6-7. The alloys listed in table 5 are all suitable for working with copper, but Easy Flo3, Sil-Fos, and Phos-Copper are considered best. Many manufacturers make alloys for soldering. Sil-Fos is for use with nonferrous metals only. ProsCopper will make good joints in copper without flux. Easy-Fo3 is 50 percent silver alloy, with the addition of 3 percent nickel. The other Easy-Flo numbers give the percentage of silver in the alloy. These alloys are available from many manufacturers. In using silver solder on fittings, you should be careful to observe installation instructions to insure good joints. One manufacturer specifies that alloys containing phosphorous, such as Sil-Foe or Silvaloy 15, not be used on fittings which are copper-plated steel. They recommend instead, Silvaloy 45 or Easy-Flo. 6-8. It is important that you heat the work to the flow point of the alloy before applying the alloy. Alloys with a large spread between melting and low point are

easier to work with, since the alloy with a large spread has a better chance of making a joint before it sets. 6-9. Use asbestos paper or wet cloth to keep heat from pans which might be affected by it. Some valves are provided with neoprene seats, and these must be removed if they are too close to where heat will be applied. Otherwise, the heat will destroy the value of the seat, and the valve will leak. 6-10. When you use flux in making a joint, you should observe the following: Apply the flux evenly to the metal surfaces which are to be protected from oxidation. If the flux wets the surface easily, this indicates that it is clean. If it balls up and spreads unevenly, the surface is oily and requires cleaning. In addition, the behavior of the flux can be used as a temperature gauge. One popular brand used with brazing becomes white and puffy about 600° F. A 800° F. it smoothes out with a milky color, while at 1100° F. it turns clear, and the bright metal surface should show through the flux. 6-11. Silver Brazing. This method of joining metals is properly called low-temperature brazing, but it is often incorrectly referred to as silver soldering. It is necessary to heat the metals only to the melting point of the silver solder, 1175° F. A low-melting-point alloy, such as Easy-Flo, is used with a suitable flux, such as that made by Handy and Harmon. The melting point of the flux, 1125° F., gives a good indication of when the metals to be joined are near the correct heat. A carburizing or reducing flame should be used to insure a good point where brazing copper with silver solder. (See fig. 9.) Phos-Copper may be used to join copper to copper without using a flux. No flux is an advantage when tubing or parts are joined in a system which should be kept clean. Remember that a clean, dry refrigeration system is one that keeps working year after year without giving trouble. A slightly carburizing or reducing flame is required when working with Phos-Copper. 6-12. Welding Copper. A welding rod should have approximately the same composition as the

TABLE 5

20

Figure 9. Torch flame. base metal being handled. Employing such a rod, you must use a slightly oxidizing flame when you are welding copper. Remember, too, that copper will absorb carbon monoxide gases from a carburizing flame and in a porous weld (since silver solder is worked at a lower temperature, a carburizing flame is used). 6-13. Welding rods, such as Airco 23 Deoxidized Copper or Oxweld 19 Cupro-Rods, should be suitable for

welding copper. The choice of tip for the torch should be about two sizes larger than that which you would choose for steel of the same gauge. A large flame is required because copper conducts heat away much faster than steel. No flux is required to make the weld; therefore the weld must be made fast before oxidation occurs. 6-14. Metal is heated for about 3 inches along the seam to a full red heat. The weld should be started inside and worked to the nearest edge. The torch should be held at about a 60 angle to the base. Speed should be uniform and the end of the filler rod should be kept in the molten puddle. During the welding operation, the molten metal is protected by the outer flame envelope. If the metal ceases to flow freely, the filler rod must be raised and the work must again be heated to a full red. 6-15. Cutting Torch. Metal is properly cut with a cutting torch which is quite different from the welding torch just discussed. However, it is joined to the hoses in the same manner as the latter, and the same safety rules apply. The main differences are found in the body of the torch and in the tips. Thus, the cutting torch has a compound head which directs a hollow flame of oxyacetylene. Also, a trigger valve in the body controls an added jet of oxygen. When the valve is pressed, this jet of oxygen in the center of the flame makes the cut by superheating the metal at the point of contact. A part of the cut metal is burned in this operation. Stainless steel is difficult to cut with a torch because it is resistant to oxidation. However stainless steel may be cut by laying a steel welding rod on the cut to be made. The heat developed by oxidation of the rod is sufficient to melt a slot in stainless steel plate. Table 6 gives the pressures and tip size to use with steel plates of various thickness. The oxygen pressure is set higher to supply the added oxygen necessary for cutting. Steel is heated to incandescence before the cut is started. As soon as the oxygen jet is applied, the cut will appear at the edge of the plate. The cut should be made at a uniform pace as fast as the metal is removed. If the torch is moved too slow, there will be a waste

TABLE 6

21

of both fuel and metal. If the torch is moved too fast, the cut will fail, because the metal is not hot enough. 6-16. Hydrocarbon Torch. Smaller torches which use LP gas can be used for silver solder work, but they are limited to smaller work. They do not produce as large a flame, and the temperature of the flame is not as hot as that of the oxyacetylene torch. Otherwise, the same rules and techniques apply to both kinds of torches. 7. Repairs and Service 7-1. Very few refrigerators have service valves or fittings. You can remedy this by installing an accessory service valve or line tap, clamping it to a line where it becomes a permanent part. A gasket makes a seal between the valve and the line, while the valve stem is provided with a piercing tool which breaks into the line. However because these gaskets leak in time and the valves are expensive, most shops will maintain a valve kit and a set of adapters. Having such a kit, you can use the adapters to install valves and gauges in a system when it becomes necessary for you to make pressure tests. These adapters, valves, and gauges do not become a permanent par of the system, and you can remove them after completing the tests. You may also install a gauge manifold in the system to make tests. The use of a gauge manifold is illustrated in Chapter 2, figure 23, where we have described cleaning of the system with a circulator. 7-2. The repair of refrigerators and freezer cabinets is generally limited to service and minor repair work. However, because you may be required to make more extensive repairs, the explanations are made as complete as time and space allows. The material covers leak detecting and repairing, pressure testing, replacing of a capillary tube, a condenser, a compressor and an evaporator, the cleaning of a system, the removal of moisture from a system and the charging of a small hermetic system. 7-3. Detecting Leaks. When a hermetic system has lost some of its charge, you can be sure that there is a leak which must be found and repaired. We will discuss here only the two detectors which are most commonly employed: the halide and the electronic leak detectors. But first, several fundamentals should be clarified. Some detectors are so sensitive that they can prove unreliable in air contaminated with a low concentration of halogens. In such a situation, the system should be charged with nitrogen or carbon dioxide at 60 to 80 p.s.i.g. and checked with a soap solution. If a pressure test of the system is also required, follow the recommendations of the American Society of Refrigeration Engineers as given in the American Code and explained in paragraph 7-12.

7-4. Halide leak detector. The halide leak detector (see volume 1) uses bottled gas to heat the reactor plate. A sampling tube is used to pull air and the refrigerant vapor (in case of a leak) into the flame. To test for leaks, light the torch and let the reactor plate turn cherry red, then hold the open end of the rubber tube near the joint to be tested. A leak is indicated when the flame color changes from blue to blue-green or bright green.

7-5. Electronic leak detector. Several companies produce electronic detectors which will detect a Freon leak as small as 1/2-ounce per year. The general rules of operation for this equipment are as follows: • Be sure to use the detector with the correct voltage or the correct batteries. Follow instructions for the detector you are using. • Allow the unit sufficient time to warm up before testing. • Do not exceed the normal duty cycle if the tester is limited to intermittent duty. • Keep a record of the hours of operation. The element has a limited number of hours of life in some instruments. • Make correct adjustments for background contamination. Even after proper adjustment, contaminated air may cause erratic operation of the instrument. • Do not place the detector probe in heavy concentrations of refrigerant, as it overloads the instrument. WARNING: Be sure to observe the warning that some instruments carry stating that their use is prohibited in a combustible or explosive atmosphere. • Always turn the detector off as soon as you have finished testing. 7-6. Repairing Leaks. Unless someone has punched an ice pick through a line, the only place a leak should occur is at a joint. Before a joint can be soldered, the pressure must be released from the system. Prepare the system as for charging by connecting a "T" and stub into the suction line. Purge the system to atmospheric pressure. Sweat the joint just as if you were making a new joint, but do not overheat it. Use asbestos paper to protect adjacent surfaces from the flame. Wrap the tubing with wet cloth where you wish to keep the heat from spreading. After you sweat the joint, partially charge the system and again test for a leak. If the joint holds, charge the system

22

to its normal capacity and triple pinch the stub to seal it. 7-7. Small pinhole leaks in tubing on the low side of a refrigerator may be repaired in several ways. For example, either cold solder plastics or special resin glues made for refrigeration work may be used to make a patch. At least one resin glue on the market has the same thermal characteristics as aluminum. A glue like this will expand and contract with the metal and will not crack. However, when making a patch with glue, be careful not to force the material through the hole in such a way as to cause an obstruction in the tubing. Also, the surface must be kept free of any traces of oil so that the glue will bond properly and completely seal the hole. 7-8. Larger holes in aluminum tubing can be repaired by soldering with 95-5 solder, which can be worked at a lower temperature than that which is required for brazing. Here, the right flux to use is just as important as the right solder, because flux also has a critical temperature. Some men may be able to use higher temperature alloys to solder aluminum tubing; but as the work gets hotter, the danger of melting out a chunk of material becomes greater. 7-9. Repairs to tubing on the high-pressure side are made with appropriate solder and flux. The higher pressure in the high side-30 pounds or more-makes it advisable to repair with hot solder rather than attempt a cold glue patch. Remember that the methods we have discussed here apply to systems using R-12, which include domestic refrigerators and freezers. If, for example, you should try to cold patch a leak in a system using R-22, your results would probably be unsatisfactory. Furthermore, the higher operating pressure of R-22-about 250 p.s.i.g. requires more strength than can be obtained by cold patch methods available at the present time. 7-10. Flare connections and fittings. Flare connections may be provided with seal. These cannot be removed without tearing them. When opening such a flare connection, cut away the seal with a knife, being careful not to nick or scratch the flare. A new seal is installed when the flare is reconnected. Always slip the flare nut over the tubing before making a flare. If the nut is too loose, look closely at it size; you may have picked a nut which is a size too large. 7-11. Flaring. In making a flare, by placing a drop of refrigerant oil on the flaring cone, you can produce a smoother flare. Also, apply refrigerant oil to the nut and the flare surfaces before assembling them. The oil will allow the flare connection to be drawn up tight without overstraining the nut. It will also lubricate the threads so that there will be no doubt in your mind as to when the nut is snug. TABLE 7

7-12. Pressure Testing and Leak Testing with Nitrogen or Carbon Dioxide. There may be times when refrigerants are not readily available or in short supply. To save refrigerant, you can make pressure checks of a system using dry nitrogen or CO2. If the pressure test is satisfactory, you can conduct a leak test in two ways. One method calls for charging the system with a small amount of refrigerant and raising the pressure in the system by means of nitrogen. A halide leak detector is then used to check for leaks. The other method calls for charging the system to the desired pressure, then testing it for leaks, using a solution of soap and water. With both methods the system must be evacuated to bleed off all nitrogen or CO2 after tests are satisfactory and before charging with refrigerant. 7-13. Replacing Capillary Tubes. A broken or plugged capillary tube requires a replacement which is exactly the same as the original in its length and inside diameter. Approximately the same length of the replacement should be soldered to the suction line to make a heat exchanger. The variations in diameters of some capillary tubes are shown in table 7. 7-14. Many refrigerators have a capillary size of . 114-inch OD and .049-inch ID when used with R-12. Of course, the outside diameter of each can readily be checked with a micrometer, while the inside diameter can be checked with a wire. Notice that both the gauge and diameter of wire is compared with capillary tube diameters in table 7. Do not try to force a wire into a capillary to check the inside diameter. Also, make certain that the wire has not been burred on the end as a result of being crushed by cutters. You can check the diameter of a wire with a wire gauge or a micrometer. In any case, the correct size wire should slip easily into the capillary. 7-15. When exact replacements are not available, you may install an adjustable capillary tube in the system. In such an event, the capillary tube should be cut to equal the length of the one which it replaces. A heat exchanger of the same length is made by soldering the capillary to the suction line. Note that the ends of the capillary should be cut with a tube cutter to get a uniform end. Also,

23

swage appropriate ends of the tubing so that the capillary can be soldered into the system. (NOTE: Keep ends taped or plugged with rubber caps to keep moisture out while the system is open.) If fittings are available, the tubing may be quickly joined. However, because such fittings are expensive, most shops will use a torch and solder the connections. 7-16. After installation, test the unit for leaks, evacuate it, dry it, charge it with refrigerant, and test it. Set the capillary adjustment so that the evaporator frosts evenly. Then make a final check for proper adjustment by seeing that the lines to and from the evaporator are not frosted. 7-17. Replacing the Condenser, Compressor, and Evaporator. Normal service for the condenser, compressor, and evaporator is to clean them with a stiff bristle fiber brush or with compressed air. The replacement of these would involve the detailed procedures just described. Open the system and cap the ends with tape or rubber plugs to keep air and moisture out. Then prepare the ends to be soldered and assemble the system. After that, charge the system and test it for leaks. Next, dry and evacuate the system. Then charge the system with refrigerant and make an operational check. 7-18. System Cleaning. After a hermetic motor has burned out, the system will be contaminated with burned pieces of metal and insulation. The dark color and pungent odor of an oil sample will give mute evidence of such a motor burnout. When it happens, remove the compressor and thoroughly clean the system before putting it back into service. As most smaller units in this condition should have the entire system replaced, the discussion of cleaning is related in Chapter 2, where it is most appropriately related to larger systems. 7-19. Removing Moisture. Since a comprehensive explanation of procedures for removing moisture from both large and small systems is more appropriate to the equipment discussed in Chapter 2 of this volume, it is given there instead of here. The procedures described there for systems under 5-ton capacity apply equally well to domestic refrigerators and freezer cabinets. 7-20. Charging a Small Hermetic. Charging a small hermetic system with refrigerant is a simple procedure which generally requires three steps: (1) dry the system, (2) install a suction line stub and a high-pressure line stub, and (3) add the refrigerant. As you know, a few other jobs related to charging must also be done at the same time. Thus, if the system has a leak, the system must be repaired and then tested. The amount of oil lost can be estimated by the size of the oil spot, which will not evaporate. Since most of the oil will remain in

the compressor, the presence of a large amount of oil will indicate a leak in that area. Similarly, if the system has been opened and you suspect that an appreciable amount of moisture has entered it, evacuate the system. First charge the system with a small amount of refrigerant and then evacuate it to 50 microns (about 29 inches of vacuum) for from 5 to 30 minutes. Remember, too, that when a system has lost a part of its charge, you should assume that some moisture has been drawn into the system. To counteract this problem, your first step is to install a new drier-strainer. 7-21. Replacing the drier-strainer. The drierstrainer is located between the condenser and the capillary tube. To replace it, cut the old drier-strainer out of the system and install a new drier-strainer in its place. The new drier will be able to hold the small amount of moisture which might have entered the system. 7-22. Installing stubs or process tubes. A system which has no provision for charging and purging must have stubs installed. To do this, prepare a “T” with about a 1-foot stub connected to the foot of the "T." Provide the stub with an appropriate fitting and cap the fitting until you are ready to use it. The stub and "T" can be heated to insure that they are dry. Cut the suction line at a convenient location and install the “T” and stub permanently in the line. Also, cut the high-pressure line at a convenient place and install a "T" and stub there too. Note that some servicemen claim that this is not necessary if the system is still under pressure. In any event, if the stub is connected at the highest part of the condenser, it will work best for purging air from the system. 7-23. Adding refrigerant. When adding refrigerant, first install a valve and a gauge in the high-side stub. Then connect a charging line to the low-side stub. Do not tighten the stub connection until you have purged the charging line by cracking the refrigerant cylinder valve long enough to blow out trapped air. Do this by cracking the valve in the high-side stub and slowly opening the valve in the charging line. Next, star the compressor and shut the purging valve when refrigerant appears there. Be sure to observe any frosting of the evaporator and shut the charging valve when the coil has become frosted completely. 7-24. Continue to observe operation of the unit. High head pressure in excess of 160 p.s.i.g. indicates that there is still air in the system which requires purging. Remember that the pressure will vary with the ambient temperature. Air in the system will also be indicated by a lack of uniform warmth of the condenser coil. A check with your hand will reveal spots which are cooler near the top of the coil, where air pockets are displacing warm liquid. From 15 minutes to half an hour may be required to properly charge the system. At

24

the end of this period of observation, check the low side of the evaporator coil. If the frost line extends too far beyond the evaporator coil on the suction side, the system has been overcharged and some refrigerant should be bled from the system by cracking the valve on the high side. If frost shows on the tubing at the inlet (high side), too far from the evaporator coil, increase the size of the heat exchanger. Solder another 2 inches of the capillary to the suction line. This correction would apply when a new capillary tube has been installed. 7-25. When the performance is satisfactory, close all of the valves and triple pinch the stubs to seal them. Gauge lines and fittings can then be removed. Remember the necessary steps to prepare a system for service when moisture is present in it. First, partially charge and purge the system of air. Second, pump down to 29 inches of mercury to remove the moisture. Third, install a new drier-strainer. Fourth and fifth, charge the system with refrigerant and start the compressor. In the charging step, no purging of air will be necessary if the system has been evacuated. Obviously, pulling a vacuum on the system removes air as well as moisture. Review Exercises The following exercises are study aids. Write your answer in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the test. Do not submit your answers for grading. 1. What are the main construction features of a modern domestic box? (1-2)

5. What must be used with insulation make it effective in a refrigerator? (1-6)

6. What are some of the big advantages of new synthetic insulation? (1-6)

7. Why do breaker strips require careful handling? (1-8,9)

8. A stored or abandoned refrigerator should be treated in what way?(1-11)

9. How is the door gasket checked for a seal? (112)

10. What factors should you consider in the location of a refrigerator (1-13)

11. How can you distinguish a refrigerator made for use overseas? (1-14)

2. Insulation must be able to reduce what three forms of heat transfer? (1-3)

12. An automatic ice maker in a refrigerator may be provided with two thermostats. What is the function of each? (1-18)

3. What puts the refrigerator? (1-4)

greatest

heat

load in

a 13. Why is a second electric heater used in the drain with an automatic defrost system? (1-20)

4. What has been the result of using improved insulating materials in a refrigerator? (1-5)

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14. Explain a simple automatic defrost system which uses only one valve for defrosting with hot gas. (1-22)

23. Why can a compressor operate so well with such fine clearances? (3-3)

15. What are the methods of heating used in an absorption system refrigerator? (2-1)

24. How close may a compressor's piston approach the head? (3-4)

16. With regard to exercise 15, what is the main distinction with on fuel? (2-2)

25. At what point may compressor valves get noisy? (3-5)

17. Give the principle of operation of the absorption system refrigerator. (2-5)

26. What advantages do rotary compressors have over the piston type? (3-6)

18. Since the flame burns continuously, how does the absorption refrigerator meet changes in heat load? (2-5)

27. What is one way of helping to cool the condenser without having to use a fan? (3-9)

28. A restrictor placed between two sections of an evaporator serves what purpose? (3-10) 19. After cleaning, what might prevent the burner from operating, and how can this problem be solved successfully? (2-5) 29. A weighted valve consideration? (3-11) 20. Describe the maintenance for a refrigerator with an absorption system. (2-6) 30. What are the critical factors in the makeup of a capillary tube? (3-16) 21. How can installation result in poor or faulty operation of an absorption refrigerator? (2-7) 31. Where is a bleeder resistor used and how does it protect a relay? (3-19) 22. When a refrigerator with an absorption system is placed in service after an idle period of 6 months, it may refuse to cool. How might this be corrected? (2-8) requires what special

32. What functions are performed by a hot wire relay? (3-19)

26

33. How does a current relay operate? (3-21)

43. What is indicated when a test shows a motor drawing its LRA rating? (5-11)

34. Where would you first check for the cause of badly burned relay contacts? (3-24)

44. Give two methods for checking a capacitor. (512)

35. What does a noisy capillary tube indicate? (3-32) 45. Describe the main characteristics of a current relay. (5-15, also 3-21) 36. How could low voltage cause excessive electrical consumption? (3-33) 46. Describe the main characteristics of a potential relay. (5-16, also 3-24) 37. What are the proper methods of manually defrosting a freezer? (4-3) 47. What are some of the causes of vibration in a refrigerator? (5-17, Table 2) 38. What would you suspect if you found heavy frost had frozen a freezer door shut? (4-4) 48. Why must an acetylene cylinder be secured in an upright position? (6-2) 39. When troubleshooting, what common fault is often made by a serviceman? (5-3) 49. Why must an oxygen cylinder be used in an upright position? (6-2) 40. State the advantage of locating an overload protector inside the compressor's shell. (5-5) 50. Which is the correct test for an acetylene leak? (6-2) 41. What is the difference in operation between a thermostat and a freezestat? (5-6) 51. Why must oil and grease be kept away from oxygen? (6-2) 42. Why is it better to use direct current for checking a motor circuit? (5-9) 52. How can you always identify the acetylene valve in a torch? (6-3)

27

53. Why must regulator screws be released before the cylinder valves are opened? (6-3)

62. What are the disadvantages of using a line tap? (7-1)

54. In soldering tubing, what is considered the most important factor in making a leakproof connection? (6-5)

63. How could a leak detector be too sensitive? (7-3, 5)

55. How hot should the work be heated before you apply the alloy when you are brazing cooper tubing? (6-8)

64. Why should a heavy concentration of halogen be avoided when you are using an electronic leak detector? (7-5)

56. What precautions must be observed when you are brazing certain valves to tubing? (6-9)

65. Where may cold solder or glues be used successfully for repairs? (7-7)

57. When you are brazing using a flux, what clue tells you how hot the joint is? (6-10)

66. What precautions must be observed when you are patching a hole in tubing? (7-7)

58. In view of the last question, how is it that when you are brazing copper with silver solder, you should use a carburizing flame? (6-11)

67. Why is it important to use the right flux with the right solder? (7-8)

68. Cold solder or special glues are limited to which systems? (7-9) 59. Why must you use a slightly oxidizing flame when you are welding copper? (6-12) 69. In what two ways can a system be leak tested with dry nitrogen? (7-12) 60. Why does the welding of copper require a larger flame than that required for welding steel? (613) 70. What are the most important factors in a replacement capillary tube? (7-13) 61. How is a cutting torch used to cut stainless steel? (6-15) 71. How can you measure the inside diameter of a capillary tube? (7-14)

28

72. What is the purpose of using tape or caps, and when are they needed? (7-15, 17)

76. When frost extends out on the suction line beyond the evaporator coil, what condition is indicated? (7-24)

73. After installation of a major part, what are the proper steps toward placing a system back in service? (7-16)

77. After replacing a capillary tube, you find that the frost line extends too far on the inlet line or high side of the evaporator. What action will correct this condition? (7-24)

74. If you find a very large oil spot, where would you expect to find a leak? (7-20) 78. Why must a refrigerator serviceman be able to make joints like an expert-quickly and correctly? (7-1-24) 75. What would happen if you forgot to purge the charging line? (7-23)

29

CHAPTER 2

Commercial Refrigeration Systems (Continued)
GRANDPA MAY HAVE had problems with his old ice box, but the problems of his neighborhood grocer trying to keep meats and vegetables fresh were much greater. His corner druggist also had trouble keeping the ice cream hard and had to keep a cold drink chilled with ice. By way of contrast, today's grocer has large storage cabinets which are automatically cooled, and the presentday druggist has refrigerated cases which keep ice cream hard and cold drinks really cold. In addition, many stores now have water coolers for the comfort of their customers. 2. Continuing the discussion which we began in Chapter 1, we will now explain different applications of compressors for water and beverage coolers, for ice making machines (such as ice cube makers and flake ice machines), and for soda fountains. Other applications which we will discuss are those involving storage cabinets (such as reach-in, walk-in, and display cabinets) and the defrosting for such cabinets. (Of course, these larger systems may use a hermetic unit, such as we have discussed in Chapter 1 in reference to refrigerators and freezers. ) You will find the open type compressor discussed in Section 13, under the heading "System Components." Similarly, you will find system cleaning explained (under the same name) in Section 17. The troubleshooting and repair sections are a continuation from the preceding chapter. 8. Water Coolers 8-1. Package units for cool drinking water are used in offices, shops, and messhalls. They are rated in gallons of water cooled per hour, with capacities ranging from 3 to 20 gallons per hour. The control of each is adjusted to supply water at a temperature of 50° F. These range from the bottle type through the bubbler type to the remote unit multiple type. All, of course, develop troubles which require servicing or repairing. 8-2. Bottle Type. The bottle type water cooler is installed where drinking water connections are not readily available. A freezestat is employed in the control circuit with the thermostat to insure against ice damage to the tank. The thermostat is set so that a thin coat of ice will form on the coils before the compressor is stopped. The freezestat opens the control circuit before ice formation damages the tank if the thermostat should fail to stop the compressor. The schematic diagram shown in figure 6, Chapter 1, is typical of the control circuit using a current relay for a unit type water cooler. The major components in such a water cooler are the same as in a simple refrigerator. 8-3. Bubbler Type. The bubbler type water cooler has a tap water inlet connecting to the water service line and a drain outlet connecting to the waste system. The unit may be designed to take advantage of cold waste water in a bubbler fountain by using a precooler. Typically, the warm water coming into the unit first passes through a coil which wraps around the drain sump. Then the water passes into an accumulator tank, where the evaporator coil is located. The design is calculated to cool the unit enough to produce a small amount of ice in the tank. This procedure thereby assures a reserve carryover during compressor off time. 8-4. Remote Unit Multiple Type. You will find the remote unit multiple type water cooler in a large modem hospital or office building. A compressor unit of the required size will be located at some remote place in the building. It may be a hermetic unit, but if the heat load is great enough, an open type compressor may be installed. From the heat exchanger at the remote location, insulated pipes carry the cold water to bubbler fountains and other outlets throughout the building A large installation may require a 10-ton unit to insure an adequate supply of cold water in hot weather. 8-5. Troubleshooting. Troubleshooting procedures are the same as those described for the hermetic unit in a refrigerator, with these additions. You may be called to service a unit which has a faulty valve or a plugged drain. One of the troubles with a valve is a slip in the linkage of the foot pedal which requires readjustment and lock-

80

ing. Another trouble is salt formation in the valve. Disassembly and cleaning of the seat and washer will solve the latter problem in most cases. However, you must replace the washer if it is grooved or warped. Also, a plugged drain requires disassembly and cleaning, whereas a leaking water tank or line may be repaired with a plastic or synthetic glue, provided it is not toxic and not poisonous. 9. Beverage Coolers 9-1. There are some persons who would have you believe that beverage coolers are a "big deal." However, there is nothing mysterious about them. Their purpose is to cool bottled beverages in the range of 27° to 40° F., depending on the freezing point of the liquid. The cooling system is designed to meet the following factors: • Anticipated heat load. • Freeing point of beverage. • Desired temperature of beverage. 9-2. If a box is loaded with a different beverage from that for which it was designed, it may be necessary to change the "cold setting" to prevent freezing the product. The position of the feeler bulb and the thermostat setting determine what range of temperature a box will hold. 9-3. A selfcontained unit of the horizontal type will use a hermetic system layout similar to that of a freezer chest, whereas a vertical unit will have a layout like that of a refrigerator. A heavy duty hermetic unit will be provided with an oil pump and an oil cooler. The condenser shown in figure 10 illustrates a heavy duty type which includes a coil for the oil cooler. The oil cooler coil will normally be hotter than the condenser. The larger display type beverage cooler has a remote compressor and condenser located in a compressor room or in an outside shed. The one major difference between a horizontal and an upright case lies in the style and layout of the evaporator. The horizontal case will have a wall type evaporator, while the upright case will use a plate type with forced air. Repairs and service for a hermetic system are the same as those which you studied for a refrigerator. Remote compressors of the open type are explained in Section 13. 10. Ice Making Machines 10-1. Several types of machines are manufactured for making ice cubes or flakes. For example, one type of automatic ice cube maker has already been explained in our discussion of domestic refrigerators. Ice cube makers are classed according to the evaporator as of the tray type, the tube type, the cell type, or the plate type. In comparison, machines for making ice flakes are classed as of the plate type, the rotating cylinder type, and the flexible membrane type. These latter machines

Figure 10. Condenser coil with oil cooler all have the same purpose, but they employ different types of evaporators. 10-2. Components. Whatever the type, the parts ordinarily found in most icemaking machines are a hermetic compressor; a condenser cooled by air, water, or a combination of both; and a receiver-drier-strainer. The refrigerant control can be a capillary tube, a constantpressure expansion valve, or a thermostatic expansion valve. Of these, a system using a thermostatic expansion valve will require a receiver. Where evaporator heat is used to loosen the ice, you will find a hot gas solenoid valve. The water-handling system will be continuous flow or intermittent. Also, the control of the ice forming and harvest cycle will be on a continuous basis in the rotating cylinder type, with the starting and stopping of the unit being the main control function. An automatic tray type will follow a cycle which is timed in the manner which we have already described for an automatic ice cube maker in a refrigerator. 10-3. Ice Cube Evaporators. The biggest difference found among ice making machines lies in the evaporator used. Because the tray type evaporator has already been described, it should give you no difficulty. Since the main difference among 31

such machines lies in the method of ejecting the cubes from the tray, we will not discuss it here; tube type evaporators will be taken up first instead. 10-4. Tube types evaporators. This arrangement has the evaporator form a bank of tubes. Water flows down the inside of the tube and is frozen. As more ice is formed, the hole in the center becomes smaller and restricts the flow of water until finally the excess water triggers the harvest cycle. A hot gas solenoid valve operates to allow the evaporator to release the ice. In one machine the long rods are cut into suitable lengths. In another machine, the evaporator tubes are chilled in sections so that rods of the desired length are formed between the warm spots. Still another method uses accumulated water pressure to eject the ice rod with enough force to break it. 10-5. Cell type evaporator. There are two major variations of the cell type evaporator. In one the cell operates under water, and when the ice is released it floats to the surface where it is forced from the tank by a current of water. In the other type, inverted cells are used which have water sprayed against them. The ice forming period is set by a timer, which then frees the ice by hot gas. 10-6. Plate type evaporators. Among the variations of the flat plate type, there is one main distinction: the plate may be either horizontal or vertical. For instance, in one type of machine, the horizontal plate produces a slab of ice, which is then moved on to a hot wire grid which is heated electrically. Here the slab melts into individual cubes, which then fall through into a storage bin. 10-7. Another type of such a machine uses a grid which is moved into position against a vertical plate. After the cube is formed, the grid is moved against a knockout plate which ejects the cubes. In one design, two vertical plates have matching cold spots which face each other. A unique feature of this model is a variable control over the length of the period for forming ice. Within a short period, the ice produced will be like a lens. If the period is long enough, the two opposite lenses will build a bridge to each other and produce a piece of ice which looks like a Yo-Yo. 10-8. Blowdown. Units are provided with a siphon the water pan to blow down the water system each time the unit stops. A complete flush removes the accumulated salts. The water left behind from each freezing cycle concentrates the salts in the water. Where the water is very hard, a manual blowdown may be necessary to move the salt and insure proper formation of ice. 10-9. Flake Ice Machines. These units use some evaporators which are similar to those of the cube makers, but the harvesting method employed is different.

10-10. Plate type evaporator. In this type of evaporator a thin sheet of ice is formed on the plate. When the desired thickness is reached, hot gas is directed to the plate to loosen the ice, which then passes through a crusher or grinder. Another arrangement freezes the slab in a spring-metal grid. After it is free of the plate, the flexible grid is drawn over a sharp bend, causing the ice to fracture into small pieces. A variation of this last method uses a flexible belt or membrane which passes over a plate or a refrigerated roller. The belt breaks up its cargo by passing around a sharp bend. 10-11. Cylinder type evaporator. Again, in this type, there are many variations, but the essential items are the refrigerated cylinder and a cutter-scraper for harvesting. Water may be flowed or sprayed on the cylinder continuously. Harvesting occurs when the ice becomes thick enough to contact the cutters. The machine will continue to make ice until a level is reached in the storage bin, where the ice contacts a feeler which, in turn, will stop the machine. The operation of the feeler is the same as that in the storage bin of an automatic cube maker. The position of the feeler determines the amount of ice which will be stored in the bin before the machine is stopped. 10-12. Troubles in Ice Makers. With so many different types of icemaking machines being used, you will find that it is necessary to have the right service manual for the equipment on hand when you are dealing with mechanical trouble or needed adjustments. You will find, too, that after mechanical problems, the water supply is probably the next greatest source of trouble. Sediment, scale, and salt formation are problems which vary widely from one locality to another. In fact, under severe conditions, water treatment may be the only means of keeping an automatic ice maker in satisfactory operation. On the other hand, in some localities, the domestic water supply contains so much salt that crystals lodge in the seat of a faucet, causing it to drip. Thus, such faucets in everyday use require that incrustation be removed from the stem and gasket every 2 or 3 months. 11. Soda Fountains 11-1. A recent addition to your responsibilities is the maintenance of soda fountains. A complete fountain has an ice cream compartment, cold bottle storage, syrup cooler, and a beverage cooler. Among other things, we will discuss a dry type eat exchanger coil aid a carbonator system. A typical soda fountain is shown in figure 11, with one compressor attached to a multiple evaporator. The syrups and the drinking water are kept at 45° to 50° F, while the ice cream compartment is held between 0° and 10° F. The heat exchanger, at the left in figure 11, serves to cool the liquid

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Figure 11. A typical soda fountain. refrigerant so it can absorb more latent heat as it changes to a gas. As you know, a lower refrigerant temperature also reduces the tendency of the liquid to ash to a gas as it passes the control valve. Flashing at the valve reduces the valve's capacity and also reduces the efficiency of the system. 11-2. Dry Type Coil. The beverage cooler, shown at the right in figure 11, is a dry type heat exchanger. It consists of an aluminum casting which contains at least two sets of coils. Additional coils are provided when more than one beverage is to be cooled. One coil is an evaporator, and the other coil (or coils) carries the liquid to be cooled. A heat exchanger of this type must be large enough to meet the cooling demands of the system. At the same time, it must provide sufficient transfer of heat so that it does not increase the pressure drop in the low side too much. To prevent this condition, a suction pressure regulating valve may be placed in the suction line from the heat exchanger. A component which is often part of a soda fountain is the carbonator which makes soda water. 11-3. Carbonator System. The carbonator which is included in some soda fountains is not as complicated as some people suppose. 33 With the information which follows, you should be able to maintain a carbonator in satisfactory operation. It has four essential parts: a CO2 tank, a mixing tank, a water pump, and an electrical control to operate the water pump. The CO2 tank is used to charge the mixing tank with gas at 80 p.s.i.g. The correct pressure is adjusted by means of a pressure regulating valve. The water pump is used to deliver a high-velocity jet into the mixing tank. The turbulence is so great that the water readily absorbs several times its own volume of CO2. The water from the pump should be chilled before it enters the mixing tank, since cold water absorbs CO2 much more readily than warm tap water. The mixing tank is also refrigerated to ensure delivery of cold soda at the valve, because warm soda water loses its charge very quickly. Carbonated water is drawn from the bottom of the tank below a baffle, which keeps turbulence from this area. 11-4. The pump motor is started and stopped by a magnetic contactor which is controlled by a float or by an electrode circuit in the tank. Operation of an electrode circuit is illustrated in figure 12. However, the CO2 tank and charging connections are not shown. The transformer serves to isolate the control circuit from the house electrical service. Of course, the tank ground and the trans-

Figure 12. Carbonator pump motor control. former secondary ground need not be connected to each other if both connections are made to a cold water pipe. The figure shows the position of the switch contacts with the pump running and the tank being filled with water. When the water level reaches the upper electrode, a circuit is completed for the holding coil by way of the water and the ground path. This energizes the holding coil, which pulls the armature down and opens the circuit to the pump motor. At the same time, another pair of contacts close a circuit to keep the holding coil energized by way of the bottom electrode. The pump motor will start again when water drops below the lower electrode, because then the holding coil will no longer be energized and the spring will pull the armature up, closing the circuit to the pump motor. 12. Storage Cabinets 12-1 The types of cabinets which you will find used at military installations are reach-in, walkTABLE 8 in, and display cabinets. The temperature range for storage of different foods is shown in table 8. From the information in this table, you can see that a display cabinet designed for fresh meats would not have the cooling capacity for frozen foods. Thus, while two cabinets may appear to be similar, they may be quite different in design and performance. Defrosting problems related to these cabinets are also discussed later in this section. 12-2. Reach-In Cabinets. This type of cabinet is familiar to many of us as the self-service refrigerator used for dairy products at the neighborhood grocery store. In appearance it looks like an oversize refrigerator with glass doors. If it uses wooden shelves, they must be made of spruce or maple, as these woods have no appreciable odor. The larger sizes of such cabinets may have as much as 100 cubic feet capacity. Either self-contained or remote condensing units are available. Evaporators are forced air or natural convection, depending on the purpose of the cabinet A modern reach-in cabinet for a messhall has forced-air circulation and automatic defrosting. The defrost cycle is designed so that the unit will give frost-free operation. However, manual defrosting is necessary when the equipment is subjected to such adverse conditions as operation in a highly humid atmosphere. Remember, too, that reach-in cabinets used for low-temperature service will accumulate frost at a much higher rate than those operated at temperatures above freeing. 12-3. Walk-In Cabinets. These are used to provide temporary cold storage of food in messhalls and commissaries. A large consolidated

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messhall has three walk-ins operated at temperatures appropriate to the food held in them. Older models use hot water defrost in which the unit is turned off and water is flushed over the evaporator coils until the frost is washed off. Since evaporators are of the forced-air type, the unit must be shut off before defrosting to prevent water being blown all over the cabinet. Late model cabinets have automatic defrosting controlled by an electric clock; they use the hot gas method. The compressors for these cabinets are mounted in a shed outside in mild climates. However, a compressor room is preferred in cold climates to insure operation of all compressors. 12-4. For many years wooden cabinets with cork-fill insulation were standard for walk-ins. In contrast, new construction methods now use metal panels with porcelain or enamel finish for the cabinet. This change has led to more sanitary conditions and easier maintenance. This is especially true since some synthetic insulating materials are as good as cork. In fact, if production costs drop low enough, these synthetics may replace cork. Unlike organic materials, synthetics are vermin-proof and are not readily susceptible to fungus when moisture gets past the seal. The application of modern insulation is explained in detail in Chapter 3, Cold Storage and Ice Plants. 12-5. Display Cabinets. These are known as open or closed and singleor double-duty cabinets. In the double-duty case, both the display section and the base section are refrigerated. The type of food to be stored will determine operating temperature of the case and the design of the evaporator. The evaporator may be of either the plate type or a finned tube with forced-air circulation. Sections may be joined together to make a cabinet of any desired length. A cabinet made of several sections will usually have a multiple evaporator system, such as we will discuss in Chapter 4. 12-6. Display cabinets are constructed of steel panels with baked enamel and porcelain finishes. Corkboard, glass wool, or synthetics may be used for insulation. In some type of construction, certain parts may rely on formed insulation for some of the strength and rigidity of the cabinet. The insulation may be blanket type, batts, panel, or formed member. Electric heater strips are provided around doors or access openings to prevent frost which could freeze a door shut. 12-7. Open type display cabinets are successful because of the development of the air curtain which keeps heat gain at a minimum. Careful design of the forced-air system has led to an ideal combination of fans and ducts to produce a curtain of cold air. Anything which interferes or disrupts the flow of air would result in excessive operation of the compressor. Thus, the open type display 35

case must be in an air-conditioned space for satisfactory operation. Otherwise, the unit will accumulate frost at an abnormal rate so that manual defrosting is required. This latter is necessary because the frost layer disrupts the air current when the frost gets too thick and must be removed by manual means, such as flooding with hot water. However, the defrosting methods which we discuss next will function properly if the cabinet is used in an area which is air conditioned. 12-8. Defrosting Methods. The methods for defrosting storage cabinets are (1) compressor off time, (2) hot gas, (3) hot wire, (4) hot water, and (5) secondary solution. If you are stationed at an older base where equipment has been purchased over a long period of years, you may find all of these defrost methods being used. 12-9. Compressor off time. The compressor off-time method is limited to cabinets operating at temperatures above 28°. Ambient temperature is relied on to bring the evaporator coil temperature up to where the frost will melt. 12-10. Off-time defrost may be controlled (1) by suction pressure, (2) by time clock, or (3) by a combination, with a time clock used to start the cycle. The first, suction pressure control, has two disadvantages which affect operation of the unit. For one thing, under an increased heat load, ice forming on the evaporator will cause the unit to stop for a defrost period. Another drawback is found in cold weather, when low outdoor temperature can make the compressor cooler than the evaporator. Under this condition the suction pressure can remain below the cutin point and the unit will remain idle. This last condition would occur in a normally mild climate when a cold wave has sent temperatures to below freezing level. The second control method, time clock control defrost, is independent of temperature variations when it has both the start and terminate function. However, when timer start is combined with suction pressure termination., you can expect to find the difficulty we have just described. 12-11. Hot gas defrosting. When this method is used for large display cabinets, it requires some modification from the simple system that we have explained for a domestic refrigerator. One disadvantage (among many) of this system is that in cold weather the compressor may not deliver enough heat for the rapid defrosting which is expected from a modem unit. Consequently, a number of variations of the simple system are used to overcome the disadvantages as follows: (1) Meter the hot gas to the evaporator so as to prevent formation of liquid which could get back to the compressor. (Just a small amount of liquid entering the compressor will cause pistons to hammer.) (2) Use a liquid receiver and meter the liquid into the suction line. (3) Add sufficient heat to insure that the refrigerant will be a gas

when it returns to the compressor. (4) Use a four-way valve in the system to completely reverse it so that the evaporator functions as a condenser and the condenser serves as an evaporator during the defrost cycle. The obvious disadvantage to avoid here is that hot gas can defrost the coils so rapidly that the drain lines may require heating to prevent melted water from freezing in the drains and plugging them. 12-12. Hot wire defrosting. This method has the big advantage of being unaffected by changes in ambient temperature. The heater wire may be laid in contact with the evaporator, or it may be hung in the form of a grid between the evaporator and the fan when forced-air circulation is used. A fan switch is a necessary part of the automatic defrost system where the forced-air may drive melted water out of the drains. The hot wire defrost cycle is so short that the drains require heating to prevent freezeup. Improved electrical heating elements account for the speed, because heating is almost instantaneous through the whole evaporator. 12-13. Hot water spray. Defrosting with hot water uses a water bath or spray aimed directly on to the evaporator. This system requires that the compressor and air circulation fan be shut off before the water is turned on. The cycle must be long enough to insure drainage of water from the evaporator before the unit is restarted. 12-14. Secondary solution. This method of defrosting uses a refrigerant which is heated and passed through a secondary coil in the evaporator. You should recognize that this system is similar to hot wire defrosting in that it will not be affected by changes in ambient temperature. It appears that at the present time the secondary solution method has generally been replaced by the hot wire system. 12-15. Hightemperature control. Safety controls are an important part of automatic defrosting systems applied to large commercial cabinets. You will find that a hightemperature control is used to terminate the defrost cycle, thereby preventing the cabinet temperature from going too high and thus endangering the food in storage. This is an added safety feature which will take over if the defrost cycle should be interrupted and fail to complete itself. 12-16. High-pressure control. A system which uses hot gas defrosting may have a pressure cutout switch to keep the unit from operating at too high a pressure. The defrost valve will have an auxiliary outlet connected by capillary tubing to the pressure control. When the defrost valve is open, it supplies pressure to a bellows in the pressure control.. The pressure control is set to open at 180 pounds and close at 155 when it is used on a system charged with R-

12. The contacts in the pressure control will open the compressor motor circuit if pressure exceeds its setting. 13. System Components 13-1. In the last section we discussed cabinets which are often made in large sizes. A walk-in cabinet for milk products handled at a big commissary store may require enough capacity to cool a room 20 by 40 feet. The refrigerant flow in such a system is shown in figure 13. The valves and accessories of the system are discussed in this section. 13-2. Open Type Compressors. So far, we have discussed the hermetic compressor, which, normally, you will not be able to repair. The welded case of a hermetic unit is beyond the capability of the repair shop. However, a semi-hermetic unit has a bolted case which can be disassembled to make repairs to the compressor. The one big difference is that a semi-hermetic does not require the shaft seal which an open type compressor must have. If you have not had the opportunity of working with a larger system, you will probably benefit greatly from a review of the major components. The following discussion is related to the items illustrated in figure 13, which shows a low diagram of a refrigeration system. 13-3. Separator. The oil separator is a simple trap designed to remove the oil from the hot refrigerant gas and return the oil to the compressor. A float is used to open a valve which allows the accumulated oil to return to the sump. 13-4. Service Valves. The suction service and the discharge service valves are provided with fittings so that they may be connected to gauges and to charging lines. The valves also serve to isolate the compressor from the system if it is necessary to replace the compressor unit. 13-5. Condensers. Several types of condensers may be found with large installations. The choice is dictated by the cooling load of the unit and the weather factors of the locality. 13-6. Air-cooled condensers. This condenser is the most simple type and gives the least amount of trouble. For heavy duty, the condenser is enclosed in a shroud, and a fan forces air across the coils to cool them. 13-7. Water-cooled condenser. Water-cooled condensers are of the shell-andtube type or the tube-within-a-tube type. In the first type, water circulates through the tubing while the shell serves as both condenser and receiver. In contrast, the doubletube type circulates the refrigerant through the outer tube to take advantage of the air cooling the refrigerant. When a compressor is also water cooled, the exhaust water from the condenser is circulated on through the cylinder heads. Where water is at a premium, a spray pond or cooling

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Figure 13. Flow schematic of refrigeration system. tower is used to cool the water so that it can be used over again. 13-8. Receivers. The receiver in a system must be large enough so that it can hold all of the refrigerant in the system. The receiver is the tank where the refrigerant is stored after a system is pumped down. The receiver outlet valve is a quill type, with its inlet tube (quill) reaching to the bottom of the receiver. It is referred to as a king valve, because this is the valve which is closed while a system is being pumped down. The receiver inlet valve is closed after pumping down is completed. 13-9. DrierStrainer. The drier-strainer is a cartridge type with direction flow indication on the case. Direction flow must be observed as it is arranged so that the strainer will hold particles of drier which might be dislodged. Also, the unit is properly baffled for liquid flow in the direction indicated. 37 13-10. Sight Glass. The sight glass enables you to see the flow of refrigerant in the system. Bubbles will appear when the charge gets low; they indicate that the system is losing refrigerant. 13-11. Refrigerant Controls. The principles of refrigerant control are the same for valves as for capillary tubes. However, valves provide a variable control over a wider range of load. Modern valves are designed so as to modulate refrigerant flow to meet variations in load. The valve must not starve the evaporator; this is not good economy. Likewise, the valve must not cause flooding, since this can damage the compressor. As you study these valves, see if you can find examples of the types mentioned here which are present in the equipment used at your installation. NOTE: Although figure 13 illustrates an automatic expansion

valve, such as is covered first below, other types of valves are also discussed including thermostatic expansion valves, high-side float valves, and low-side float valves. 13-12. Automatic expansion valves. The first valves to be developed as automatic were known as the constant-pressure type. The automatic expansion valve has a spring on each side of a diaphragm. Evaporator pressure under the diaphragm acts with the closing spring to close the valve when the pressure rises. This kind of valve has been used in systems with ammonia. It does not modulate, so it is not used as a refrigerant control where the load change is great. One quite common application of the automatic expansion valve is in drinking water coolers, because their heat load is fairly constant in a narrow temperature range. The automatic constant-pressure expansion valve has also been used successfully as a pilot valve for larger valves. One such application is for the control of a suction pressure regulator. As a pilot, the automatic expansion valve may even be used to operate a suction service stop valve to prevent freezing. An equalizer line is used to compensate for the pressure drop across the valve. 13-13. Thermostatic expansion valves. Valves of this type use a bulb and capillary tube to transmit pressure to a spring-loaded bellows or diaphragm. Such valves are identified by (1) the size of connections, (2) the length of the capillary, (3) the internal or external equalizer connection, (4) the capacity, and (5) the type of refrigerant charge. The type of charge is indicated by the color used on a valve according to the following list: Refrigerant Color 12 yellow 22 green 500 orange 502 orchid 40 red 717 white The capacity is the nominal capacity of the valve in tons of refrigeration. There are three kinds of refrigerant charge used in thermostatic expansion valves: liquid charge, gas charge, and cross charge. a. Liquid charge. This valve has the remote bulb and capillary charged with the same refrigerant (R-12 for example) as that which is used in the system with which the valve is to be used. The liquid charge is sufficient so that some liquid will be left in the bulb under all conditions. The advantage of such a charge is that it will control the refrigerant even when the valve or diaphragm is colder than the bulb. Among the disadvantages of such a charge are possible flooding and hunting. Its main application is found in low-temperature systems of large capacity.

b. Gas charge. This valve has the bulb and capillary charged with the same refrigerant as that present in the system with which it is to be used However, the amount of the charge employed is smaller than that found in the liquid charge; thus at a predetermined point, all of the liquid will become vapor. This point is the maximum operating pressure of the valve. The disadvantage of such a charge is that the control will be lost if the diaphragm and case are colder than the bulb, since refrigerant will then condense in the valve. For this reason the application is only suitable to a system which operates at temperatures above freezing and where the evaporator pressure drop insures that bulb temperatures will be colder. c. Cross charge. The cross charge expansion valve uses a liquid charge in the bulb and capillary which is different from the refrigerant found in the system with which it is used. The pressure-temperature curve of the charge is such that it will cross the pressure-temperature curve of the refrigerant used in the system. By careful selection of the refrigerant used for the cross charge, the manufacturer can make a valve which will perform best in any desired range or for any set of conditions. Some of the advantages of such a charge are these: (1) the valve closes quickly when the compressor stops; (2) the valve exercises control at high suction temperature, preventing floodback; and (3) the valve is more sensitive to pressure changes rather than bulb temperature changes, which reduce hunting. 13-14. High-side float valves. This control is normally used with a single evaporator, but it can control several evaporators if they are connected in series and if each is provided with an individual bypass. The high-pressure float valve, used with a flooded evaporator, has two advantages: First, all of the refrigerant is liquid when it enters the evaporator, so there is no cooling lost from expansion taking place in the delivery line. Second, all of the refrigerant passing through the valve is liquid, so the capacity of the valve is not subjected to changes from flashing. The evaporator may be a bunker type with forced-air circulation or a shell-and-tube type for brine water chilling. A surge drum is installed at the evaporator to prevent flooding of the compressor during changes in load. The amount of refrigerant charge is critical, for if the system is charged beyond its capacity, flooding will damage the compressor. The evaporator is provided with an oil drain and return line to the compressor. 13-15. Low-side float valves. Such valves are each connected into the low-pressure side of the system, but the function of this valve is almost the same as that of the high-side float valve. The difference is that a part of the evaporator space is taken up by the tank and float control. Adjust-

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ment of the low-side float is critical. If the refrigerant level is too low, oil may accumulate in the float chamber, leaving the compressor with insufficient oil to lubricate it. Another effect of the layer of oil on top of the refrigerant is that it may cause the refrigerant to refuse to boil until a much lower temperature-pressure point is reached. Ebullators are used as catalysts to insure boiling of the refrigerant at its normal point. 13-16. Compressor Pressure Switch. In figure 13 you will find that item 12 is a switch with capillary lines to the high side and low side of the compressor. In such a situation, a combination switch can be used in many ways. One application would be as a high-pressure and low-pressure safety switch. Another application would be as a high-pressure safety switch and low-pressure motor control. Specific functions are, of course, determined by the system involved and the purpose intended. 14. Troubleshooting and Repairs 14-1. When a service call is received, it usually means trouble. If you were the boss and had a choice, who would you send? Would you choose the most experienced man, the one best able to do the job quickly? But what about the man with little experience? He needs the opportunity to learn. The solution is to, perhaps, send the inexperienced man along as a helper. As you do this, you can make sure that the challenging jobs will go to the better qualified, while the simpler jobs will go to the less qualified. After all, whether you are in military or civilian life, it is usually the best qualified man who gets the most pay and the most interesting assignments, isn't it? In this section we will discuss many problems you would encounter in troubleshooting the larger systems which use an open type compressor. Then, in Sections 15, 16, 17, and 18, we will take up several aspects of servicing, each of which is important enough in itself to be studied separately rather than as subparts of servicing. But first, let us consider the basic rules of electrical safety which you must know in order to avoid getting into trouble. 14-2. Electrical Safety. In spite of repeated warnings many servicemen forget the safety rules and become involved with a live circuit. Then they learn the hard way-perhaps even fatally-that memorizing the safety rules is not enough; these rules must be practiced-consistently, automatically! Briefly they are: • Do not wear rings or metal watchbands at work.

• •

Do not wear shoes with metal clips or hobnails. No horseplay. Distractions cause accidents.

When testing or working on a live circuit, use the buddy system. A loner may lose his life. Remember, the man who practices safety will develop habit patterns which will protect him. Then -having such habits-such a man can devote more of his attention to the particular problem he is trying to solve. 14-3. Electrical Troubles. Let us discuss normal operation first. For a brief review of electricity, consider what a circuit does: Something happens when a circuit is completed. Something happens when a circuit is opened. Keep these two things in mind when you are looking for a trouble, and the solution will be easier to find. Actually, you are looking for the answers to a series of unspoken questions. Yet, their answers will become more obvious if you will state the questions to yourself. For example, "Why doesn't the compressor motor start?" Answer! "An open circuit!" "Where?" This is what you are really seeking. "Where is the open circuit?" Here are your six possible answers: • At the circuit breaker or fuse box. • At the motor starter. • At the control switch. • At one of the safety or lockout switches. At an open connection or a loose terminal (which may be one and the same thing). • At a broken wire. 14-4. Electrical Repairs. Several specific remedies are available, depending on where a fault is located. To name a few: (1) At a circuit breaker, pressing the "Reset" button will restore the circuit if it has opened because of overload. (2) A blown fuse calls for a replacement of the same size. (3) A loose terminal can be tightened. (4) A broken wire can be spliced, soldered, and then insulated with electricians' tape. 14-5. When several safety controls are used in one circuit, they are connected in series with each other. The operation of any one of the safety control devices opens the circuit. When more than one control switch is used to complete the circuit to a motor from different locations the control switches must be connected in parallel. Thus, you must know the purpose and function of a control before you start to troubleshoot it. 14-6. Three-phase motors are preferred in units larger than 5 horsepower. Tests on a three-phase motor are quite different from a single-phase. When a threephase motor will not start, one or more phases are open. (See mechanical troubles for a locked rotor.) To test, you must first check all three phases for voltage on the source

•

•

• •

Treat all circuits as live circuits unless you know they are dead. Be sure switches are of and tagged before working on a circuit.

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side (top) of the switch. Using a voltage tester, check with the test prods from A to B, B to C and C to A. To check a three-phase switch or starter, you must have it closed to a live circuit. Then cross-check by going from A phase (input or top connection) to B phase (output or bottom connection). If this test shows no voltage, B phase is open at the switch. Cross-check the other two phases in the same manner. CAUTION: Do not attempt the above test if the motor hums but does not start. It is possible to damage the motor while making the test. The trouble is not in the motor if it starts when belt tension is released. 14-7. If the switch and the control circuit tests show that they are operating correctly, the fault may be either in the terminal block of the motor or in one of the motor windings. Serious trouble in the motor will be indicated by evidence of overheating, such as charred insulation or the smell of burned insulation. Such damage would call for the services of the electric shop, which may be able to supply you with a replacement motor of the right horsepower and direction of rotation. 14-8. Mechanical Troubles. Mechanical troubles also concern you. For example, when there is evidence that a trouble is in the compressor, here are two mechanical causes which are most common: (1) A locked rotor may be caused by a frozen bearing or it can be the result of high head pressure. (2) A frozen bearing occurs in the compressor from lack of oil more often than from a faulty oil pump. Of course, a mechanical failure in the compressor is possible, but this will seldom cause a locked rotor. The usual symptoms are an inability to cool sufficiently and noisy operation of the compressor. 14-9. Abnormal pressures. When the cause of a trouble is not obvious and the compressor will operate, gauge readings are necessary to help spot the cause. Abnormally high head pressure indicates a restriction in the high side. The cause may be (1) air in the system, (2) moisture in the system, (3) dirt or sludge. (4) a kink or a pinched line, and-last but often unsuspected-(5) a partly closed valve. 14-10. An abnormally low head pressure on the high side would not be a positive indication of compressor failure because it would also depend partly on the state of charge in the system and what the suction pressure gauge measures. Low charge is checked by looking for bubbles in the sight glass, which should show a solid flow of liquid under normal conditions. 14-11. Refrigerant controls. The refrigerant control is the source of many troubles. Indications of a restriction at the control are low suction pressure and an inability of the evaporator to pull the temperature down. The compressor may run continuously or it may cycle on

safety controls. Whenever the refrigerated area is too warm, the thermostat will be calling for compressor operation all the time. The thermostat may be checked by turning its setting toward warmer to see that it will operate properly. The cause of trouble at a thermostat can be loss of charge, a pinched capillary tube, or improper adjustment. The first two troubles would require a replacement. An ice bath and thermometer are necessary to make the correct adjustment of a thermostat. 14-12. Troubles at a refrigerant control valve are restrictions or improper adjustment. Ice at the valve seat or needle reduces the capacity of the valve and causes abnormal readings of pressure gauges. Dirt or metal particles in the strainer can clog it to produce the same effect. A flare fitting which is not frostproof or one in which the seal has failed can cause hidden trouble. Ice will accumulate under the nut slowly, crushing the tubing. Thus, a careful inspection is necessary to reveal the defective fitting. It is for this reason that solder joints are preferred in below-freezing areas. Check for ice by warming the suspected trouble spot. 14-13. Adjustment of controls. Before attempting the readjustment of an expansion valve, you should make sure that one of the following is not a cause of your trouble: • A worn needle and seat. • A leaking bellows. • Ice forming on the bellows. When a valve will not close completely, the condition indicated might be a worn needle and seat, which can be replaced with new parts. A leaking bellows generally requires that the valve be replaced. Ice forming on the bellows may be prevented by coating them with vaseline, but this will not work in zero cold areas. A better solution is to keep moisture out of the housing by sealing it. After a valve has been repaired, it can be tested with an expansion valve test assembly. This consists of a refrigerant tank and service valve, two gauges for highand low-pressure readings, and a cooling chamber with crushed ice. A source of dry air at 100-pound pressure can be used in place of refrigerant. For low-temperature work, dry ice may be used for the cooling chamber and a low-reading thermometer to check the temperature. A test assembly setup is shown in figure 14. The connection for the gauge on the outlet side of the valve is left loose enough for escape of pressure to simulate refrigerant low. As the valve adjustment is changed, the closing and opening pressures are noted on the gauges. A valve that will not adjust to its required specifications must be replaced.

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Figure 14. Expansion valve test assembly. 41

14-14. If replacements are made to a float valve, there is one consideration which must be observed. Operating levels of the float must be the same as they were originally to insure proper operation. Any change in the operating levels will change the operation of the system. 14-15. Condenser and Evaporator Service and Repair. Dry type condensers and evaporators are cleaned with a stiff fiber brush or compressed air. The direction of air should be opposite to the normal flow. The frequency of cleaning recommended is based on a time interval for average conditions. The interval will be shorter when conditions require more frequent cleaning. Bent fins must be straightened to maintain adequate airflow. An outdoor mounted condenser can have a large part of its air passages closed by hail driven against the fins by the wind. Pinhole leaks in tubing can be repaired by brazing. Use flux sparingly as excess flux may pass through the hole into the tubing. 14-16. Pressure Testing. After extensive repairs or replacement of major components in a system, you may be called on to check the system with high pressure. Pressure testing of a system may be done with nitrogen or carbon dioxide to determine the strength of tubing and joints. Specified test pressures as recommended by ASRE in the American Code should be followed. Be careful to apply the test pressure by building up pressure slowly. Avoid sudden shock loads to the system. Take reasonable precautions to protect yourself and other personnel with suitable barricades so that no one will be injured if a line should rupture. 15. Preparing To Open the System 15-1. Before you can open a system, you must take the necessary steps to save the refrigerant in the system and prevent air from entering. The usual procedure is to pump down. First, close the receiver outlet valve and operate the compressor until the suction pressure gauge reads 3-5 PSIG and levels off at this pressure. This indicates that pumping down is complete. At this point, the receiver inlet valve is closed and the compressor is stopped. Of course, it may be necessary to allow some refrigerant back into the system, because the system should not be opened without a positive pressure. Such a procedure will keep air out and leave you less work to do when the time comes for you to put the system back into operation. 15-2. When equipment, such as a strainer, is provided with valves and a bypass line, it is not necessary for you to pump down the system. One precaution is necessary as it may be possible to trap liquid refrigerant in the equipment when it is isolated from the system. To avoid this, you must be careful that you do not unload a dangerously high pressure accidentally, when you open the

Figure 15. Spring caliper. system in these circumstances. The moral? Always purge equipment of refrigerant in the manner specified and you will be on the safe side. 16. Open Type Compressor Overhaul 16-1. Among the causes of compressor failures, valve trouble and a lack of oil are probably the most common. We will not discuss valve trouble at this time. As for a lack of oil, it can result in trouble with all of the moving parts in a compressor which require lubrication. This section deals first with the use of a micrometer to check dimensions and then with the special knowledge you should have to enable you to overhaul a compressor. Of course, the manufacturer's service book is indispensable, since it contains the overhaul instructions for the compressor. In it you will find those rules with which you should approach every job. Any exceptions to the rules will be found in the specific instructions for each piece of equipment. Now, let us begin by explaining special measuring tools. 16-2. Measuring Tools. Among the measuring tools which you must use are several types of micrometers and calipers. Let us consider first those tools which are least accurate, then move on to a discussion of the more accurate measuring tools. 16-3. Spring type caliper. Look at figure 15, which is an illustration of spring type calipers. The accuracy of this type is limited to how close you can read the measurements on a scale; it is also 42

Figure 17. Outside micrometer. shaft is parallel with the anvil face. Use the rachet stop to drive the spindle in against the shaft. But note that the rachet stop is a drive which will slip. However, it always applies the same driving force to the spindle so that results will be uniform. Figure 18 shows three examples of making a correct reading on a 1-inch micrometer. Each number on the barrel marks one-tenth of an inch (0.1 inch). Between adjacent lines on the barrel, there are twenty-five hundredths of an inch, 0.25, marked off by 25 divisions around the thimble. Each complete revolution of the thimble moves the spindle twenty-five thousandths of and inch, 0.025. This is the distance marked off between two adjacent lines on the barrel. Four of these divisions, from the zero mark on the barrel will bring up the number “1” on the barrel. This indicates 0.1 inch, or one-tenth of an inch. The bottom example in figure 18 shows a micrometer set to measure 0.224 inch. The 0.2 is read from the barrel, and the 0.024 is read from the thimble. In the middle example, the correct reading is 0.226 inch, even though the next line on the barrel does not show under the thimble. The position of the zero on the thimble indicates that it has more than completed one additional revolution. The top example shows the barrel exposed beyond the 0.30

Figure 16. Caliper rule. dependent on the amount of feel (or pressure) which you place on the ends of the caliper when you are checking a piece of work. 16-4. Caliper rule. As shown in figure 16, the caliper rule has one fixed jaw and one movable jaw which slides on the rule. The sliding arm is marked with two lines labeled “OUT” and “IN”. For measuring an outside diameter, the scale is read where the OUT line matches it at 51/64 inch, as shown in the bottom half of the illustration. To measure an inside diameter, the width of the jaw must be taken into account. To measure the ID of a cylinder as shown in figure 16, read the scale where the IN line matches it at 42/64 inch. The smallest ID that a caliper rule can measure is the width of the jaws. 16-5. Micrometers. Four types of micrometers are used in making precision measurements of machine parts. These are the outside micrometer, the inside micrometer, the depth micrometer, and the dial micrometer. The accuracy of these tools is no better than the skill of the user. They must be used correctly to give precise measurements. a. Outside micrometer. An outside micrometer is used to measure the diameter of a shaft or the thickness of a sheet. The outside micrometer, shown in figure 17, is used to measure diameters of less than 1 inch, which is the limit of movement of the barrel. Common sizes of the micrometer are 1 inch, 2 inches, and 3 inches. The rachet stop in the base of the handle is used to drive the spindle when you are taking a measurement. The first step is to have the micrometer set wider than the shaft to be measured. Next, the shaft should be held firmly against the anvil (fixed face) so that the 43

Figure 18. Reading a micrometer.

Figure 19. Reading a vernier scale. line, indicated by the position of the thimble. The zero has already passed the revolution line on the barrel. A micrometer with a vernier scale can measure to ten thousandths of an inch. Figure 19 gives an enlarged picture of the scale of a micrometer set to measure 0.2862 inch. The vernier scale is etched in the barrel, the lines being parallel with its length. The vernier reading is made by locating the line on the thimble which matches the vernier line on the barrel. In this case it is the 16th thimble line which matches the vernier line 2, which gives the reading of 0.0002, or two ten-thousandths. b. Inside micrometer. To make accurate meas-

Figure 21. Depth micrometer with extensions. urements with an inside micrometer, you must give careful attention to several details. Figure 20 shows the extension rods and their use with an inside micrometer. An extension rod must be absolutely clean before it is mated to the micrometer. Any particles of dirt which prevent the extension from bottoming would cause inaccurate readings. Its length can be checked with an outside micrometer of sufficient size. The micrometer must be held parallel with the diameter line of the cylinder being gauged. The "feel" or drag of the tool should be only slight and is checked by holding one end firm against the cylinder wall while the other end is moved straight up and down. Out-of-round and runout is checked by taking sample readings at several points of a cylinder for comparison. The barrel and the thimble are marked and read in the same manner as an outside micrometer. c. Depth micrometer. The use of a depth micrometer and its extension rods are shown in figure 21. The same rules apply to the assembly of this tool as those given for the inside micrometer. The extension rod must be clean, and it 44

Figure 20. Inside micrometer with extensions.

Figure 22. Dial micrometer. must mate exactly to give accurate measurements. You must hold both shoulders of the gauge flush against the edge of the opening while making a measurement. The spindle is driven by the rachet stop so that its travel will be arrested as soon as the rod touches bottom. d. Dial micrometer. A dial micrometer is a precision tool in which measurements are read directly on a dial. The dial micrometer, shown in figure 22, is provided with a handle so that it can be easily handled to check a cylinder wall. The steel spring at one end of the micrometer provides a two-point contact with the wall to insure better accuracy. Runout is easily checked, as the tool gives a continuous reading while it travels down the cylinder. The dial micrometer is mounted in a fixed holder when it is used to check a shaft to see whether or not it is true. 16-6. Compressor Disassembly. After you pump down a system you must frontseat the compressor suction and discharge valves. You can then remove the compressor from the system. However, there is one note of caution which you should observe. Before a compressor is removed from the system, the crankcase should be vented and the oil drained. The drain plug should be loosened slowly, and pressure should be bled from the crankcase before the plug is taken completely out. The oil should be inspected and a note made as to its color and condition. You will find signs of wear indicated by the presence of fine particles of metal or a substance like grit in the oil or in the bottom of the crankcase. If the oil appears good and if there are no

signs of grit in the crankcase, you could safely assume that the bearings were in good condition. A compressor teardown is not necessary if trouble is caused by a defective oil pump. With the oil pump system, however, there are two items which might also cause a trouble. These are the oil pressure regulating valve and the oil pressure safety switch. Failure of the regulator or switch can cause much needless work if tests are not made properly. Parts which cannot be adjusted to factory specifications should be replaced. 16-7. What is it that we are trying to emphasize in our last paragraph? Just this! The condition of the oil is a reliable and accurate gage of internal conditions in the compressor. Even if a mistake were made in the interpretation of conditions, that is no reason to make a major overhaul out of a simple replacement. An exception to this would be where a compressor was approaching the end of the time interval when a major inspection would be required. As a supervisor, you will be expected to make wise decisions in matters of this kind. One important item which may be overlooked is the cause of a failure. Repairing or replacing a part is not enough unless you know the cause of a failure and take measures to correct it. 16-8. Oil Seals. A seal is used where the crankshaft passes through the crankcase. This seal must be able to hold the pressure in the system whether the shaft is moving or at rest. One type of seal uses a bellows made from a thin metal tube. One end of the tube has a flange which is secured to the crankcase, while the other end has a metal and graphite ring which is forced against a shoulder on the crankshaft by a spring. In this mounting, the tube is stationary. A variation on this type has the tube and bellows mounted on the shaft, and the seal is pressed against a shoulder on the crankcase. The main disadvantages of this type of seal are that it requires that the shoulder which the seal presses against must be hardened and that a scratch across the shoulder can cause the seal to leak. New developments in synthetics have led to the use of materials which provide a seal without needing a specially hardened surface. 16-9. One new type of seal is the rotating seal head, which is precision built and assembled at the factory. It has a neoprene bellows, a lapped carbon seal washer, a retainer shell, a driving band, and a flange retainer. The seal washer mates with a stationary seat on the cover plate. The seat is also precision lapped. The driving band presses the bellows against the shaft to insure its turning with the shaft. The bellows is made so that it will ride along the shaft, permitting end play of the shaft while maintaining contact of the seal with the seat.

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16-10. On some compressors, a seal nut must be removed from the shaft first before the cover plate or seal guard is removed. Check the threads carefully before attempting to remove the nut. A nut with a lefthand thread must be turned clockwise to loosen it. When the seal has been removed, check all of the seal faces for scratches and signs of wear which could prevent a new seal from doing its job. Remove all packing compound and rust preventive from the new seal with approved cleaning solvent. Failure to clean the seal thoroughly will introduce foreign grease into the system, which can cause trouble. After cleaning and drying the new parts, mating surfaces must be coated with refrigerant oil before assembly. 16-11. After installing a new seal, proper operation may be checked as follows: Frontseat the suction service valve and run the compressor until the vacuum levels off. Then frontseat the compressor discharge valve and watch for an increase in discharge pressure. A rise in discharge pressure indicates that air is being drawn into the system. 16-12. Valves and Plates. You will find that suction and discharge valves in a large compressor are mounted in a valve plate. You can replace these as a complete assembly for each cylinder when necessary. Replacement is necessary whenever the limits specified by the manufacturer are exceeded. Check the depth of the seat for both suction and discharge valves to see that the amount of wear is within limits. Check the valve disks to see that they are not worn too thin. A depth micrometer will serve to check the depth of a seat from the face. An outside micrometer is used to measure the thickness of disks for evidence of wear beyond recommended limits. 16-13. If a spring steel valve is replaced, the new valve must seat properly. Some replacement valves will be found with a slight burr on one side. This burr side must be placed up or away from the seat. Otherwise, the valve will not seal properly, and the seat will be scored. The burr should be removed if it is heavy enough to break up and cause metal chips in the system. A slight feather edge should not produce chips but does indicate the side which should be up when it is installed. Valve seats that are of the raised type may be lapped with fine compound if the seat is worn. Care must be used that the lapping tool is flat. The tool must not be allowed to rock during the lapping as the seal surface would be lost. All compound must be removed after the lapping operation is completed, as a small amount of the compound will quickly ruin every bearing in a compressor. Unless well-qualified personnel are available to perform precision lapping, it would be advisable for you to make valve and valve plate replacements and save work, such as lapping, for experts. Always use new gaskets with valve plates and cylinder heads. Remember

that new cylinder head gaskets must have the same thickness when installed as the old gaskets, or the compression ratio will be changed. A gasket that is too thin will cause the compressor to be noisy, while one that is too thick will reduce compressor efficiency. 16-14. Connecting Rods, Pistons, and Cylinders. These parts should be checked for wear against the table of specifications for the compressor. Wear limits vary from as little as 0.001 inch for wrist pins and bushings to as much as 0.003 inch for cylinder sleeves. Label the caps and rods so that they can be reinstalled in the same positions from which they were removed. Some compressors are provided with removable cylinder sleeves. If the connecting rod will not pass through the sleeve, then the rod, piston, and sleeve must be removed together. You must be careful that the piston does not come through the top of the sleeve during removal, as the rings will give you trouble. Check bearing surfaces for correct measurements and for scratches or other signs of damage. Lubricate pans with refrigerator oil before reassembling them. Bearing caps should not be filed unless this action is specifically directed by the manufacturer. 16-15. Ring gap is checked by inserting the ring about 3/8 inch from the top of the cylinder. Compression rings have a taper toward the top of the ring. If installed upside down, the compressor will be noisy in operation, indicating that oil is being pumped. The top of the ring (marked "TOP") must face up so that it will be toward the cylinder head when it is installed. Oil rings which have no taper may be installed with either side up. Check the ring gap with a feeler gauge after the ring is inserted into the cylinder about 3/8 inch below the top. Ring gaps must be staggered around the piston. Side clearance between the piston and ring should be about 0.001 inch, and the ring must be free to move. When the new rings are installed, be sure to break the glaze on the old cylinder wall so that the rings will wear in properly. 16-16. Crankshaft. A crankshaft whose bearing surfaces are worn but otherwise are in good condition may be used if undersize bearing inserts are available. Remember that worn bearings for the connecting rods may mean worn parts at other places also. So be sure that you take all factors into account before attempting a repair that may not prove satisfactory all around. 16-17. Oil Pump and Accessories. Oil pumps are gear type positive displacement to insure delivery of oil at the pressures required for refrigeration compressors. For example, a system designed for oil pressure of 45 to 55 p.s.i. above suction pressure should have an oil pressure gauge reading between 85 and 95 if the suction pressure is

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40 p.s.i. An oil regulating valve of the spring-loaded type insures correct oil pressure automatically. Some of these valves are adjustable, while others are factory set. When pressure cannot be adjusted high enough, it may be an indication of badly worn bearings. 16-18. An oil safety switch operates on differential pressure, which is the difference between pump discharge and crankcase pressure. If oil pressure drops too low, the compressor motor is shut off. Low oil pressure can result from low oil, pump failure, worn bearings, and crankcase dilution by refrigerant. Diluted oil may be caused by worn rings. 16-19. After repairs are made to a compressor, the system may require cleaning before it can be put back in operation. While the following section is written for a hermetic motor burnout, the cleaning procedures can be applied to any system, large or small, which requires cleaning. The process also applies to a new system which has just been assembled or installed and which requires cleaning before it can be considered ready for operation. 17. System Cleaning 17-1. This section is in part reprinted from April 1961, and June 1962, Refrigeration Service and Contracting. A new system requires cleaning before operation to insure removal of any foreign material left in the system. Also a system requires cleaning after burnout of a hermetic motor. In either case the method you use for cleaning a system will be determined by prevailing conditions and by the equipment you have available. With a new system the first step is to install a filter-drier in the suction line to prevent damage to the compressor. The filter-drier is changed as often as necessary. Such a change is called for whenever suction pressure drops, as the drop indicates a clogged filter. The following methods are generally considered to do a satisfactory job of cleaning. 17-2. Cleanup Procedure for Small Capacity. After you have established that a burnout has occurred, follow the cleanup methods recommended by the equipment manufacturer. If the manufacturer's service manuals are not available, you may follow the procedures outlined in this section. The procedures that we will discuss in the following paragraphs apply to most hermetic compressors. On larger systems a single filterdrier, connected in the suction line, may cause pressure drops. If this occurs, you must use parallel driers. CAUTION: Acid burns can result from touching the sludge in a burned out compressor. You must wear rubber gloves when handling any contaminated pan. 17-3. To clean a system with a liquid line filterdrier and less than 10 pounds of refrigerant charge, you must follow these procedures: • Evacuate the system from the high side (discharge shutoff valve). • Flush the system completely with new refrigerant. • Install the new hermetic compressor-motor.

Install a filter-drier in the liquid line, using a size larger than specified. • Charge the system with new refrigerant. • Start the system according to the manufacturer's instructions or local SOP's. • Check the oil and filter-drier on followup calls to establish the need for replacement. • On the first followup call, install a moisture and liquid indicator (liquid eye) in the liquid line, after the filter-drier and before the expansion valve. This will indicate when moisture content is within acceptable limits. 17-4. Cleanup Procedure for Large Capacity. The procedure to follow if the system contains more than 10 pounds of refrigerant is: • Bleed some refrigerant from the high side. If the refrigerant has a burned or acid odor, it must be evacuated to the atmosphere. If there is not a strong odor, you can evacuate the refrigerant into a clean, dry drum. • If the system did not have a drier in the liquid line, you must clean the expansion valve strainer and the internal expansion valve parts. • If the system has a drier in the liquid line, remove and discard it. • Install the new compressor.

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Install a filter-drier in the suction line. Install an oversized filter-drier in the liquid line. Evacuate the entire system with a vacuum pump capable of pulling. 0.2 inches of H g. Break the vacuum with refrigerant.

Re-evacuate and charge the system with the original type of refrigerant. Charging should be accomplished through the low side of the system. • Change the liquid line filter-drier and remove the filter-drier from the suction line after 48 hours. • The filter-drier in the suction line can be changed sooner if the pressure drop affects system capacity, but it is considered good practice to leave it in the system for at least 6 hours. • Install a moisture and liquid indicator in the liquid line and check the oil color and odor. It must be changed if it appears dirty or smells burned. • In 2 weeks, recheck the oil and change it if necessary. The moisture indicator will show whether or not the drier needs replacement. 17-5. Flushing the System. One of the accepted methods of cleaning the system after a hermetic motor burnout is flushing. Dry air, nitrogen, or

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Make sure that the compressor motor is burned out; then remove the electrical leads from the motor terminals. Remove the refrigerant and oil as a liquid, but do not purge off in the gaseous state. Since, at this time, the acid content may be high, be careful to avoid contact of it with your skin or eyes. Remove the filter-drier and expansion device. Install bypass where components were removed, except at the suction and discharge lines to the compressor. Attach the circulator to the system. Circulate the solvent for at least 4 hours. Change filter-drier in the circulator as frequently as it is necessary to insure removal of moisture. Shut off the circulator and blow the system out with R-12 or R-22. Get the system as dry as possible. Install the new compressor, filter-drier, and expansion device. Partially charge the system and make a leak test with a halide torch. Evacuate the system three times. Break vacuum with refrigerant each time. Charge the system with new oil and refrigerant after the third evacuation is complete. Check the system after 48 hours of operation. Change the oil if dirty, and replace the filterdrier. Check the system again in 40 to 60 days. Again, change the oil if necessary and replace the filterdrier.

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Figure 23. Circulator setup. carbon dioxide gases are often used for this purpose if them is no sludge in the system. However, most manufacturers recommend the use of refrigerant (the same as that used for system charge) the flushing agent. Purging with gases (air, nitrogen, carbon dioxide, etc.) does little good, because the sludge adheres tightly to the internal surfaces of the system. The solvent action of the refrigerant is essential for adequate system cleanup. Refrigerant 11 has been found to be the best solvent. It remains in liquid form at normal room temperature, because its boiling point is 74° F. at atmospheric pressure. Its cost is low, and it is readily available from local wholesalers. 17-6. Circulation of the solvent (R-11) can be accomplished with the setup shown in figure 23. To flush the system, you would open valves B and C and close valve A and D. Backwashing is accomplished by opening valves A and D and closing B and C. The flow of solvent through the strainer and filter-drier is always in the same direction, as shown in figure 23. The pump is of the diaphragm type. All of these components are usually available so that you could build your own circulator and mount it on a dolly or two-wheeled cart for mobility. 17-7. Procedure for Cleaning a System with a Circulator. We have discussed procedures for system cleanup. The procedure that we will discuss now is considered much more efficient, but to use it, a circulator is needed. The setup for such a circulator has been illustrated in figure 23. The procedure for cleaning a system with a circulator follows:

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18. Removing Moisture from the System 18-1. The amount of moisture in a refrigeration system must be kept at a minimum to provide satisfactory operation. The main sources of moisture are low-side leaks, contaminated oil or refrigerant, and leakage in a water-cooled condensing unit. Moisture may enter the system whenever it is open, such as during installation or when you are making repairs. 18-2. Moisture Troubles. You will find that moisture in the system will cause one or more of the following undesirable effects: • Freezing at the expansion device. • Corrosion of metals (this forms sludge). • Copper plating. Chemical damage to the motor insulation or to other system components. • A restricted or plugged filter. We will discuss two methods of dehydration, one

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Figure 24. Cutaway of a filter-drier. using driers with desiccants and the other using vacuum pumps. 18-3. Driers. The drier unit contains desiccant, screens, and filters. Figure 24 shows the internal construction of a nonrefillable drier. The distribution baffle prevents a solid stream of refrigerant from passing through the desiccant block. It also eliminates turbulence and insures a smooth flow of refrigerant through the drier. The spun glass filter is held in place by the antibypass ring. The ring forces a of the refrigerant to flow through the desiccant block and then through the porous bronze filter. The filter casing carries model identification and positive directional flow arrows or inlet and outlet markings. The model number indicates the size of the filter. The total charge in a system is the basis for the size drier selected. If the total charge is unknown, you can assume that it is 8 pounds per ton for R-12 and 6 pounds per ton for R-22 and R-502. NOTE: The following general rule may be used to estimate the system charge in relation to the horse-power rating of the condensing unit: R-12 = 8 lbs/hp R-502 = 6 lbs/hp R-22 = 6 lbs/hp The directional indication must be observed during installation. If it has been connected in backwards, it will cause a restriction. Why? Because particles of desiccant will get into the system, since the screen will be on the wrong side. 49 18-4. Desiccants. A desiccant is a compound capable of absorbing the moisture in the refrigerant-oil mixture. We will discuss three commonly used desiccants: activated alumina, silica gel, and calcium sulfate. You must be familiar with the correct use of these desiccants. All three are rated as highly acceptable, but the use of the wrong desiccant in a system can cause trouble and breakdowns. 18-5. Activated alumina. Alumina removes moisture by absorption. It is used on systems containing sulfur dioxide (SO2), methylene chloride, methyl chloride, R-11, and R-12. It can be used in the suction or liquid line for all these refrigerants except sulfur dioxide. However, on an SO2 system, you can install it in the suction line only. Activated alumina is available in granular, ball, tablet, and solid core form. 18-6. Silica gel. Silica gel is a glasslike silicon dioxide gel which removes moisture by absorption. It also removes acids and does not dust. It is available in granular, bead, and solid core form and is used on most refrigerating systems, since it is compatible with most refrigerants. 18-7. Calcium sulfate. The anhydrous form of calcium sulfate is also used to remove moisture from a system. Although it forms dust somewhat more than activated alumina, it can still be left in the system permanently. It is available in granular,

Figure 25. Temperature pressure relationship. (reprinted from April 1964 Refrigeration Service and Contracting). stick, and special block form. Calcium sulfate cannot be used with SO2 refrigerant. 18-8. The reactivation temperature for these desiccants are: • Activated alumina 350°-600° F. Calcium sulfate 450°-480° F. 18-9. Installation of Driers. Before installation, a drier should have both ends open and be baked in an oven at 300° F. for 24 hours. The drier should be capped after baking to prevent accumulation of moisture. Caps are removed just before the drier is placed in the system. Even a drier which has been sealed by the manufacturer should be dried before installation if there is any reason to suspect that moisture may have passed the seals. 18-10. Dehydrating with a Vacuum Pump. This material is in part reprinted from April 1964 Refrigeration Service and Contracting. To start our discussion, let us 50

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Silica gel 350°-600° F.

think of pressure as the column of mercury it will support. Think of atmospheric pressure as equal to 29.92 inches of mercury instead of 14.7 p.s.i.g. at sea level. This will permit us to use the pressure-temperature relationship shown in figure 25 when we determine the vacuum which must be attained to boil water at various ambient temperatures. 18-11. Referring to figure 25, a vacuum pump capable of eliminating all but 1 inch of Hg is able to remove moisture at an ambient temperature of 80° F. or more. While a pump pulling within 1 inch of H g, can eliminate moisture, it must also be capable of holding this vacuum throughout the dehydration process. Before we consider the variables that affect a vacuum pump's performance, we should first review some general classifications of pumps relative to their ability to remove moisture by the boding process. 18-12. The piston type compressor might pull a vacuum of 28.2 inches of Hg, which is actually 1.7 inches of Hg on a manometer. Quite a number of these pumps have been used for vacuum work, but they are impractical in removing water by the boiling method. Under normal ambient temperature, moisture contaminates the crankcase oil. 18-13. A rotary type compressor can pull a vacuum of 29.63 inches of Hg,. This pressure will cause the water to boil at an ambient temperature of approximately 45° F. For these reasons, the rotary compressor is practical for use as a vacuum pump. 18-14. The compound two-stage high vacuum pump is capable of pulling down to about 50 microns for prolonged periods of time. Because such pumps are twostage pumps, they can be equipped with a gas ballast or vented exhaust. The gas ballast or vented exhaust feature is a valving arrangement which permits relatively dry air from the atmosphere to enter the second stage of the pump. This air mixes with the air being passed from the first to second stage and helps to prevent the moisture from condensing into a liquid and mixing with the vacuum-pump oil. 18-15. Frequent vacuum-pump oil changes should be anticipated and recognized as the single most important factor in preventive maintenance. Even a pump equipped with a vented exhaust cannot handle large amounts of moisture without having some of it condense into the oil. If the water is allowed to remain in the pump, the moisture will attack the metal pans and result in a loss of efficiency or capacity. The oil should be changed after each major evacuation or dehydration process. 18-16. When, you are determining the size of the pump (c.f.m. capacity) to meet your needs, you must remember that the length and diameter of the line being dehydrated dictates the size pump to be used. The 75c.f.m. capacity pump will handle most applications, since the system is normally dehydrated through a 1/4-inch line.

Review Exercises The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the test. Do not submit your answers for grading. 1. Why would a bottle type water cooler have a freezestat in the control circuit when the thermostat is t so that ice will form? (8-2)

8. Why is frequent blowdown necessary in ice making machine? (10-8)

9. In addition to mechanical troubles, what is the biggest source of trouble with ice making machines? (10-12)

2. What is the purpose of a precooler in a bubbler fountain? (8-3)

10. Why is a heat exchanger necessary in the liquid line of the soda fountain shown figure 11? (111)

3. Of several methods you may use when patching a leak in a water tank or line, in which method must precautions be observed for good health? (8-5)

11. A dry type coil like the beverage cooler in figure 11 must be correctly sized for what reason? (112)

12. What is the function of the water pump in a soda fountain carbonator? (11-3) 4. Describe the adjustment you would make to a beverage cabinet used to cool four different kinds of bottled beverages. (9-2) 13. If you find the water pump for a carbonator is running continuously, where would you check first to try to locate the trouble? (11-4) 5. When checking a condenser for warm spots, how might a warm coil be misleading to you? (9-3) 14. What simple test can be made to prove that a break in the ground circuit has caused the carbonator pump motor to run continuously? (11-4)

6. What is the big difference that distinguishes types of ice making machines? Give some examples. (10-1)

15. What might cause constant complaints of having bad taste or flavor from a storage cabinet which has been properly refrigerated? (12-2) 7. What are three types of evaporators you may find in an ice cube machine? (10-4-6)

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16. On older models not provided with automatic defrosting. what must be done before water is used for evaporator defrosting? (12-3)

25. The service valves in a system serve what purpose? (13-4)

17. What is meant by a “double duty” display cabinet (12-5)

26. Give the advantage or characteristic of two types of water-cooled condensers. (13-7)

18. In addition to special illumination, what other feature may be found around the door of a display cabinet? (12-6)

27. Why must a receiver be sized to the system? (13-8)

28. What is peculiar about a receiver outlet valve? (13-8) 19. Why must there be a continuous flow of cold air in the display section of a refrigerated open display case? (12-7) 29. How might reversed flow affect a drier-strainer? (13-9) 20. Give the five methods you have studied for storage cabinet defrosting. (12-8) 30. What are the three features considered essential in an expansion valve? (13-11) 21. What is the limitation on compressor off-time defrosting? (12-9) 31. Give the reasons why a water cooler can use an automatic constant pressure expansion valve. (13-12) 22. Describe the reverse cycle used for cabinet defrosting. (12-11) 32. What is the purpose of an equalizer line with an expansion valve? (13-12) 23. What is one purpose of using a high-temperature control in a low-temperature cabinet? (12-15) 33. Give the descriptive items you would use to identify a thermostatic expansion valve. (13-13) 24. What is the purpose of the capillary tube connected to a defrost valve? (12-16) 34. What are the three kinds of charge used in the bulb and capillary of a thermostatic expansion valve? (13-13)

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35. Explain the meaning of a cross charge. (13-13)

45. You find a compressor unit continuously running yet unable to cool sufficiently. What checks would you make to locate the cause of the trouble? (14-10, 11)

36. Give the arguments for and against the liquid charged bulb. (13-13) 46. How would a compressor act if the thermostat controlling it had lost the charge from its capillary? (14-11)

37. Give the pros and cons for a gas-charged bulb. (13-13)

38. What are the advantages of a high-side float valve? (13-14)

47. How would you make a quick check for ice blocking a refrigerant control? (14-12)

39. What effect is produced by a layer of oil on top of the refrigerant? (13-15)

48. What is the indication of a worn needle and seat in an expansion valve? (14-13)

40. Before making tests on a "live" circuit, why should you remove rings ad not wear a metal watchband? (14-2)

49. When adjustments or repairs are being made to a float valve, what consideration must you observe? (14-14)

41. When you find a circuit breaker tripped, what else should you do besides resetting the breaker? (14-4)

50. Where a drier-strainer is provided with valves and a refrigerant bypass, what precautions must you observe before you remove the item from the system? (15-2)

42. A compressor motor will not start and an ammeter test reads full LRA. For what purpose would you release belt tension? (14-5, 6; also 511)

51. What is the purpose of a rachet stop in an outside micrometer? (16-5)

43. Why is it poor practice to try repeatedly to start a motor which does not turn over? (14-6, 8)

52. In figure 18, the top illustration shows an arrow at the right on the thimble. If the thimble is turned 12 divisions in the direction of the arrow, what will be the reading of the micrometer? (1615)

44. List the causes of abnormally high head pressure. (14-9)

53. How can you check to see that an extension rod with an inside micrometer is properly seated to give correct readings? (16-5)

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54. What is the best tool to use to check a crankshaft to see whether or not it is true? (165)

63. Unless cylinder sleeves are replaced, what must also be done when a new set of rings is installed? (16-15) 64. What other factors must be taken into account when bearing inserts are replaced? (16-16)

55. Give five causes for loss of oil pressure in a compressor. (16-6, 18)

65. When is system cleaning required? (16-19) 56. When installing a new oil seal, what precautions should you observe? (16-8-10) 66. After a hermetic motor burnout, why must you use precautions when cleaning the system? (17-2, 7)

57. How is a compressor operated to check a new seal for a leak? (16-11)

58. What are two important checks which you should make when you are inspecting compressor valves? (16-12)

67. In system cleaning, where is evacuation equipment connected and why? (17-3)

68. Why is a new filter-drier installed after a system is cleaned? (17-3) 59. Replacement valves made of spring steel should be inspected before installation. How should you perform this inspection? (16-13) 69. What is the purpose of using refrigerant to break the vacuum when cleaning a system? (17-4, 7) 60. What is the difference between most oil rings and compression rings used in a refrigeration compressor? (16-15)

70. What is the limitation on the use of activated alumina drier? (18-5)

61. How would you obtain indications that a set of compression rings has been installed upside down? (16-15)

71. Which system is not compatible with anhydrous calcium sulfate as a drier? (18-7)

62. How would you check the ring gap of the rings used in a refrigeration compressor? (16-15)

72. What are the general procedures recommended before a drier is installed? (18-9)

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73. What vacuum is necessary to boil water at a room temperature of 80° F.? (18-11)

74. Why are frequent oil changes recommended for the oil in a vacuum pump? (18-15)

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CHAPTER 3

Cold Storage and Ice Plants
DO YOU SUPPOSE that this chapter should be introduced with the importance of preserving food? Perhaps. But for a change, let us consider your job if you are assigned to work at a cold storage warehouse or an ice plant. You can make the job dull and boring or very interesting. In the operation of a plant there will usually be some time each day when you will have nothing to do but observe the equipment. The wise man will use this time to study the equipment. He will learn how it is laid out and where the various parts are located. While you are taking readings, you should make a mental note of changes which you observe and try to determine the causes of the changes. In this way you will both pass the time quickly and become more familiar with your equipment's proper operation. Remember, too, the man who is alert to operating conditions will recognize the symptoms of impending trouble and can thus often prevent a major breakdown. The larger plants are attended by operators on shift duty 24 hours a day. 2. This chapter is divided into two major headings. The first covers the design, insulation, layout, and operation and maintenance of large cold storage plants. The second covers the layout, operating principles, components, and the ammonia system operation and maintenance. We will undertake the discussion of a large cold storage plant first. 19. Large Cold Storage Plants 19-1. Have you ever observed the different types of cold storage in a commissary store? You will find similar areas in a large cold storage plant. Let us consider the effect of these areas on the design of a plant. 19-2. Plant Design. In the layout of a refrigerated warehouse, consideration is given to the operating temperatures by using the center of the building for the coldest operations. Warmer rooms are located around the center and act as a buffer for the colder areas. The temperatures of the rooms which follow are presented as examples of one warehouse. 19-3. Rail rooms. A refrigerated warehouse is usually laid out in the shape of a rectangle. The two longest sides of the building are occupied by rail rooms. Meat hooks suspended from an over head rail provide the means for moving heavy loads. The rail room is normally maintained at about 35° F. Inside doors provide access to the processing room and meat cooler room, which are also part of the overhead rail system. 19-4. Processing room. This room is held at 45° F., and because of its warmer temperature, it is located at one end of the building. This is where the butcher cuts a carcass into smaller parts before storage or distribution. Such processing rooms are dangerous areas. During normal operations the floors become extremely slippery; therefore, you must be careful in order not to fall whenever your work requires that you pass through these rooms. 19-5. Meat cooler. A temperature of 30° F. should be maintained in the meat cooler for holding fresh meat prior to distribution. A meat holding and issue room at 30° F. provides additional storage area for the same purpose. 19-6. Milk, butter, and egg room. This area is normally held at 35° F. and is one of the colder areas. 19-7. Fruit and vegetable room. An average temperature of 40° F. is recommended for general storage, so this room is also eligible for location by an outside wall. 19-8. Potato room. You may already know that potatoes in storage give off heat and carbon dioxide. Consequently, the potato room should have a high ceiling and must have positive ventilation to insure the safety of personnel. Otherwise, the carbon dioxide will form a dense layer at the floor if the fans are turned off. An unsuspecting person would not realize his danger in an atmosphere so heavy with carbon dioxide until he felt himself fainting. Then it would be too late for him, as asphyxiation follows quickly. Furthermore, potatoes should not be piled more than 6 feet high to insure removal of heat. If the pile is too high, the heat will not be removed fast enough, and they

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will spoil rapidly. Outside air ducts are used to bring fresh air into the room by means of fans. The fresh air is directed across the evaporator coils for cooling. To insure the safety of personnel, operating instructions for this room must be strictly observed. Operating temperatures from 35° to 45° F. may be used, depending on the type of potatoes in storage. Early crop potatoes keep best 50° F. but must be considered for short-term storage of less than 3 months. Late-crop potatoes will age best and keep from 5 to 8 months if the storage room is kept between 35° and 40°. In very cold weather, heaters are turned on to keep the products from being frozen. 19-9. Freezer room. Occupying the center of the warehouse are the freezer rooms. A below-zero room is used both for freeing products and for long-term storage. Products already frozen may be kept in a 10° F. room, where storage is for a short period, such as a week. If the insulation is installed properly in the cold rooms, your job will be easier, because the refrigeration equipment will not be overloaded. The total heat load of the warehouse will be lighter than if the rooms were poorly insulated. 19-10. Vapor Barriers and Insulation. Information on insulation is in part reprinted from January 1964 Refrigeration Service and Contracting. There was a time when a cold storage warehouse was insulated with sawdust. But now, new materials and methods of installation produce improved construction. You must be familiar with these improvements so that you can properly maintain equipment in rooms with modern insulation. There are four basic fundamentals to be considered in construction of a modern cold storage room. These are: • Design the structure so the room and the building can move independently of each other. • Apply a continuous vapor barrier on the warm side of the insulation, using a plastic film or laminate with the lowest permeability. • Select insulation which has a permeability rating considerably higher than the vapor barrier but has a low permeability to air. • Select a finish with a permeability rating considerably highest than the insulation. 19-11. The vapor barrier should consist of a plastic film or laminate with the lowest "permeability value" available. The value in permeability equals the number of grains of moisture that will pass through 1 square foot of the material in 1 hour under a vapor pressure differential of 1 inch of mercury. It has been found that a building using plastic film as a vapor barrier is more efficient thermally and less costly as compared with a building using the

common adhesive or hot asphalt treated insulation. A structure constructed according to the four fundamentals will allow the insulated room to move independently of the building structure. This means that when proper techniques are followed for the installation of the vapor barrier, the vapor seal at the wall-ceiling juncture remains intact, and no leakage occurs at the joint. Modem construction allows the outer shell of the building to breathe with changes in ambient temperature while the cold storage room is held stationary in a narrow temperature range. If the original vapor barrier adhesive was inadequate or ruptured, a moisture vapor and air leak would occur and cause deterioration of the insulation. 19-12. With a new construction which follows the four basic fundamentals, the vapor barrier is installed prior to the insulation. This vapor barrier is not cemented to the walls of the building structure or the insulation but is supported independently of them. This is very important, since it allows building and cold room movement to have no effect upon the integrity of the vapor barrier. In effect, the insulation is enveloped in a vapor barrier on the warm side. This vapor barrier can be inspected after installation prior to the application of the insulation. Do not overlook the importance of being able to inspect the vapor barrier to insure adequate protection from vapor and airflow. Any damage can be repaired a this time, and overlaps can be carefully inspected to se whether specifications have been followed. 19-13. Insulating materials. Materials which have been used successfully include 6 to 8 mil polyethylene film in wide sheets, laminates of aluminum foil with polyester film, and creped paper. The width is sufficient to greatly reduce the number of laps and joints. Laps and joints require sealing with vapor barrier pressure sensitive tape. Some contractors use rolls of foil laminates because of its case of application. The insulation is installed dry without adhesives. A vapor permeable finish is applied to hold it in place. The finish protects it from damage and provides a sanitary washable surface. In some cases the finish is fire resistant. With proper materials, any small amount of vapor which does pass through the vapor barrier can flow through the insulation without changing its state. It remains as a vapor to be condensed on the cooling coil. The moisture does no condense within the insulation to impair its thermal efficiency, and the cooling load due to heat gain trough the insulation remains relatively constant at or near its original U-factor. 19-14. Sealing fasteners. The fastener locations on the framing should be premarked and a strip of caulking ribbon placed behind each location. This will seal the hole in the vapor barrier caused by the fastener. After the framing for the ceiling is installed, the vapor barrier sheet is spot

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stapled to the inside of the framing. Seal the ceiling vapor barrier to the wall barriers with vapor barrier pressure-sensitive tape. The ceiling framing is securely fastened to the top of all wall framing members so that movement of the building will not affect the wall-ceiling juncture. The framing for the self-supporting walls should also rest on a plate securely fastened through the floor vapor barrier into the subslab. 19-15. After the necessary bracing is in place, the vapor barrier is installed on the self-supporting walls. It is installed on the outer side of the framing and sealed to the overlap of the ceiling vapor barrier. The first layer of insulation is force-fitted between the wall studs and nailed to the underside of the ceiling. The second layer is installed horizontally on the walls and perpendicular to the ceiling framing under the suspended ceiling. This layer is held in place with plastic skewers or nails. 19-16. Lay polyethylene vapor barrier over the subslab and tape all joints and seal with tape to the ends of the wall vapor barrier. You will then be ready to lay in the insulation. The joints must be tight. Then lay a vapor permeable sheet, such as 15-pound roofing felt, over the floor insulation and up over the wall insulation, above the height of the concrete curb that will be poured later. Also pour the concrete wearing slab and install a curb of the proper height. After this, cut off the 15-pound felt even with the top of the curb. The felts acts as a slip sheet. It allows the floor and curb to move independently of the walls. It also prevents excess water in the concrete from wetting the floor insulation. 19-17. At this point, the job is ready for installation of the permanent fasteners and the stabilizers. Fasten 4inch-wide galvanized metal lath over each framing member. Apply a portland cement finish in two coats over the walls and ceilings. The finish should be cured with water spraying in accordance with recommendations for wiring. This treatment reduces shrinkage and gives maximum strength to the finish. 19-18. Plant Layout. In order to understand the layout of a cold storage plant, you should be able to read blueprints. Standard drawings are often used with additions and note added so that the diagram may be applied to a specific installation. You should recognize that this is the case with the illustrations used in this section. At the time when a facility is built, an accurate drawing is made of the equipment. These drawings are referred to as "as-built drawings." Changes or modification should be entered in the drawings; thus the drawings should be kept up to date by appropriate entries of modifications or additions to the plant. 19-19. Machine room. If you have ever seen the machine room in a cold storage plant, you were probably

impressed by the array of tubing. Some of it you recognized without a doubt, while other parts of the tubing seemed strange to you. Look at figure 26 and you will see a diagram of the layout of tubing and equipment in a machine room. Appropriate labels are provided so that you can identify each item. Notice how a pitch is specified for horizontal lines to prevent pockets of liquid from being trapped in the lines. A few words of explanation may help you to read such a diagram. 19-20. Look at the upper right part of figure 26 and find a line which is labeled "LL to HT System coolers." The abbreviation means liquid line to high-temperaturesystem coolers. At the upper left you will find a line marked "S.L. 4," which means "4-inch suction line." Beside the legend "Sight glasses" you will see a number "1" in a small triangle. This 1 refers to an item which was added after construction has been completed. When items are added, such as symbols with special meaning., a notation should be made under "added notes" or "revisions" on the blueprint. Appropriate forms, such as status and operational records, component replacement, and historical records, must be maintained for plant equipment as directed by inspection TO's. 19-21. Under the label "Liquid Receiver" is a note telling you to "See detail." Several details may be shown on the same blueprint. A detail of a typical receiver is shown in figure 27. Note the two driers, which makes it possible to replace one without shutting down the system. You can see many things in the detail which could not be included in figure 26 because of lack of space. 19-22. Cold rooms and evaporators. The various cold rooms are provided with evaporators of appropriate size. In figure 28 you will find a detail of typical connections to an evaporator. Each large cold room may have one or more evaporators, and valves must be located so that an evaporator can be isolated while the rest of system remains operating. Let us now consider the operation and maintenance of a cold storage plant. 19-23. Operation and Maintenance. For purposes of this discussion, we will start with the motor controls and compressors and follow the flow of refrigerant. Thus, we can explain the operation of the plant in a logical sequence. 19-24. Compressors, controls, and accessories. From our previous discussion of a machine room, you will remember the four compressors shown in figure 26. Two 15-hp compressors supplied the high-temperature evaporators, while two 20-hp units furnished the lowtemperature evaporators. In either side, the loss of one compressor will not halt operations completely. When possible, compressors of the same size are installed so that the

58

Figure 26. Machine room diagram. 59

Figure 27. Detail of liquid receiver. system can be standardized and spare parts will be interchangeable. The inventory of spare parts will thus be smaller because there will not be a need for duplication. a. Pressure control. Generally, motors larger than 5 hp are three phase and require a starter. The motor starter is controlled with a pressure motor control tapped into the low-side suction line. The cutout pressure is set for 10° F. below the coil temperature, while the cut-in pressure should be set for the desired coil temperature. The spread between the two settings determines the frequency of compressor operation. Operation in very cold weather may produce conditions which will prevent some systems from operating automatically because of abnormal pressure. The receiver outlet valve (king valve) may be throttled so as to force the pressure to increase. As pressure builds up to a near normal level, the valve should be opened more. Be sure, however, to restore valves to normal operating positions after the condition is corrected. Remember, a pressure control's function is to close and open a set of contacts in the circuit to the operating coil of the starter for the motor. b. Magnetic starter. The motor control is essentially a three-phase magnetic switch which is closed when its coil is energized. The coil serves to close the switch and to hold it closed. The switch is closed against spring pressure, which throws the contacts open as soon as the circuit to the coil is interrupted. Two overload protection provide protection against shorts in all three phases. Protectors are located in A and C phases, which should be the two outside wires. Remember, when testing a switch for supply voltage, the upper terminals are connected to the feeders. Check the upper terminals from A to B, B to C, and A to C for voltage on all three phases. If one phase is dead, the trouble is in the feeder or supply. The lower three terminals are connected to the motor. When a small fourth contact is part of a switch,

Figure 28. Detail of evaporator connections. 60

it provides a holding circuit to the closing coil. The wiring diagram inside the cover of the switch box will give the wiring diagram for that particular switch. Maintenance consists in replacement of overload protectors which have failed or a magnetic coil which has an open circuit. In both cases an exact replacement or a substitute as specified by the manufacturer must be used. If the switch contacts are severely burned, the entire switch may require replacement. Excessive heat may damage other parts, such as the springs, which would cause more trouble in the future if the switch was partly rebuilt. c. Time-delay relay. You will remember that in the illustration of a machine room, each system was supplied by two compressors. If both these motors were started at the same time, there would be twice the load on the electrical system. For example, a three-phase, 220-volt, 15-hp motor will draw about 100 amperes starting current. If both motors started together, the combined load of 200 amperes would cause a voltage drop, resulting in slower starting. This would extend the starting period, and the motor windings would be subjected to unnecessary overheating. Such a situation is avoided by using a time-delay relay with one motor so that it is not started until the other motor reaches operating speed. A time interval of from 3 to 8 seconds is sufficient. The time-delay relay consist of a solenoid coil and plunger with a set of contacts which close the circuit to one of the motor starters. The plunger operates in a dashpot filled with oil. Maintenance requires that the relay be kept clean. If oil needs replacing, be sure to use oil that is approved for use in a dashpot. Dash-pot oil keeps the same viscosity over a wide range of temperature. The time interval can be changed by turning the adjustment screw. An ordinary light switch maybe wired in parallel with the time-delay relay if the relay fails. The second compressor can then be started manually after the first compressor starts. Operation can thus be continued until the relay can be replaced. d. Motors and drives. Three-phase motors have the high starting toque required by large compressors. Once a motor is installed properly, all it needs is cleaning and oiling. Motor bearings should be checked daily for normal temperatures. They should not be so hot that you cannot hold your hand comfortably on the bearing shell. Always open the motor switch before checking the belt driver end. Lubrication of motor bearings it done on a regular schedule as directed. Proper installation means that the motor must be lined up so that its pulley drum is parallel with, and in the same plane as, the pulley on the compressor shaft. An initial test of the direction of rotation of the motor should be made before the terminal’s connections are made permanent and insulated

with tape. If at the test, the motor rotates incorrectly, its direction is reversed by interchanging two of the motor leads with the supply leads. By transposing two of the phases, a three-phase motor will have its rotation reversed. You may have to remove a motor at some time for repairs. Before disconnecting the wires, be sure that you attach labels to all of the supply leads and the motor leads. Then, when you are ready to reconnect the motor, wire it the same as before. A large compressor is driven by multiple V-belts, which should be provided with a safety enclosure. Belt guards are for your protection and must always be in place during normal operation. Loose or damaged guards must be repaired or replaced. Always open the compressor switch so that the motor cannot start while you are working around or in the immediate area of the belts. If the oil is spilled on V-belts, they must be cleaned immediately. Again, open the compressor switch so that the unit cannot start while you are working on it. If the oil soaks into the belts before they can be cleaned, plan on replacing the set soon, because oil causes the material to deteriorate rapidly. Multiple drives of four or five belts are common. When one belt becomes too loose or worn, it may be removed and operation continued temporarily. You should expect some drop in output from the compressor because of belt slippage. It is impractical to replace one belt at a time, because V-belts are supplied in matched set. However, you will always find some variation in individual length. This variation becomes apparent when you try to adjust the tension on a newly installed set. When you need to replace a set of belts, be sure to follow all relevant safety rules. First, open the electrical supply switch to the motor. Then hang a caution or danger tag on the switch handle if there is any possibility that someone might close the switch while you were working on the motor. Next, release the holding bolts and the locknut on the jackscrew. Also, back off the jackscrew to release tension on the belts, remove the old set and install a matched set of belts on the pulleys. After this, take up the jackscrew until belt deflection indicates correct tension; then, set the locknut on the jackscrew and tighten the holding bolts. You will find some variation in belt tension recommendations, but these are determined by operating conditions. For instance one beltmaker recommends a belt deflection of from 1/2 to ¾ inch for each foot between the pulley shafts. In contrast, another recommendation calls for a deflection of 1/64 inch per inch of span between pulley shafts. Experience is probably your best gauge as to the correct tension. In operation, too little

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tension is indicated when most of the belts show a pronounced flutter. As an example, the right tension on a matched set of belts may be most nearly correct when one belt shows a small flutter while the rest are steady during operation. The object of adjustment is to get a minimum amount of vibration without getting the belts too tight. Belts which are too tight will wear out quickly. On the other hand, belts which are too loose will slip and result in lost efficiency and reduced output. In any case, most V-belts are made of material which will not glaze. Belt dressing should never be used on V-belts unless it is recommended by the manufacturer. When adjustment will not correct belt slippage, it would at best be a temporary measure to apply a dressing, and you should plan to replace the set. e. Gauges and records. The gauge for high pressure and low pressure are located on a single panel so that the operating conditions can be observed quickly. Alarms and indicator lights are also mounted on the same panel. Oil pressure gauges may be on the panel but are generally mounted on the compressor. Readings are taken at regular intervals to check for normal operation. You, as a supervisor, may require readings made continuously to monitor the system during unusual conditions, such as when the system is first placed in operation after major repairs. The period of close observation may be long or short, depending on what changes have been made ad how long it will take the system to settle down into a normal pattern of operation. Abnormal pressure changes are indications which must be interpreted correctly. A gradual change in pressure may be from a change in ambient temperature, from loading or unloading work in the cold rooms, or from a trouble developing. A rapid change in pressure is usually an indication of trouble. The possibility of this latter, rapid change points up the advantage of making frequent readings and recording them. If the gauges have not been checked in several hours and a big change has been noted in pressure, the operator may have no way of knowing whether the change is sudden or gradual. For example, when the ambient temperature drops low during a cold winter's night, you an expect below-normal gauge readings. If you are fortunate enough to pull an assignment at a plant equipped with automatic temperature and pressure recorders, you will be able to read a continuous record of plant operation. An automatic recorder has one or more pens which trace a line in ink on appropriately marked graph paper. The pens are delicately balanced and driven electrically by a bridge circuit. Be careful, however, when changing charts or putting ink in the pen reservoir, since rough treatment will damage the pen’s mounting and

cause an error in the recording. You may still be required to make one or more readings of the gauges each shift. Your records serve as a double check on system operation. The best part about a graph chart is that the frequency and duration of each cycle of operation can be seen clearly at a glance. The chart is a useful tool for the supervisor because, by comparing charts, he can detect day-to-day changes. f. Oil receivers. The oil receivers provide a means of temporary storage of the oil returned from the oil separators. Sight glasses provide a means of checking the oil level in the receiver. Oscillation of the oil level indicates that the oil separators are performing their function of returning oil to the receiver. The oil level will drop each time that the float valve opens to return oil to the compressors. In case of low oil level, do not add oil to the system until you have determined that oil has been lost. Why? Because the low level can be an indication that oil is accumulating in one of evaporators. 19-25. Condensers and water towers. Two types of condensers are favored for large systems. These are the evaporative and the tube-and-shell types of condensers. a. Evaporative condenser. You will find the first type illustrated in figure 29. The upper section of the unit contains the blowers, which pull air through the coils. The center section contains an array of spray headers, which spray water over the condenser coils. The combination of air and water acts to produce a water temperature considerably lower than the air because of evaporation. Efficiency of the condenser falls off as humidity goes up. The water pump circulates water from the sump up to the spray headers. Makeup water is added to the sump by means of a valve and float control located in the sump near the pump intake. Bleed-off water is tapped from the far side of the sump ad discharged to the sewer. Bleed-off and makeup connections are not shown in figure 29. Bleed-off water must drain at a rate sufficient to insure that salts do not accumulate in the unit. The condenser operation would be seriously lowered by even a small salt deposit. If salts start to appear, it is advisable to increase the amount of makeup water by opening the bleed-off valve wider. When scale appears on the condenser coils, they must be cleaned with a stiff bristle brush. Bristles must be hard enough to remove the scale but not so hard as to damage the tubing. Water which has an acid pH will not cause scale, but too much acid will cause corrosion. Operation in cold weather may require the pump to be turned off. In extreme cold the fans may have to be stopped and the water drained to prevent freezing. Capacity control of a condenser is necessary to

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Figure 29. Layout of an evaporative condenser. insure efficiency of the system. There are four ways to control the capacity of the condenser. One method controls the water pump by a pressure-operated switch. When head pressure drops below a predetermined point, the pump is stopped and the unit acts as an air-cooled condenser. Another method is to use a pressure-operated switch to turn off the blowers when pressure drops to a predetermined point. Still another method is to use a two-coil condenser, with a solenoid valve to cut one coil off when a pressure switch operates at its low-pressure setting. A fourth method uses modulating dampers on the air inlet. The dampers are designed to close when the compressor is idle. This last method has advantages for cold weather operation as the dampers may be operated so that freezing of the water is not a problem. b. Tube-and-shell condenser. Another type of condenser is the tube-and-shell, which circulates water through the tube. The condenser shell may also serve as a receiver for the system. The condenser water is circulated through a cooling tower, where evaporation drops the temperature of the water. The spray headers in a cooling tower are similar to those shown in figure 29. A water bypass line around the tower is the usual method of capacity control. This also serves for cold weather operation, but in extreme cold the system may have to be drained to prevent freezing and breaking of pipes. Makeup water is added to the system automatically by means of a float control in the sump. Bleed-off water may be used as a method of scale control. Algae control and water treatment are discussed in Volume 4. Capacity control may also be attained by means of a water63 regulating valve located n the condenser inlet water pipe. The valve is controlled by compressor discharge pressure so that head pressure remains relatively constant within a predetermined range. The water regulating valve is a modulating type which controls the volume of water through the condenser. Where water is cheap and plentiful, it may be discharged overboard after passing through the condenser instead of recirculating it through a cooling tower. Both evaporative condensers and cooling towers accumulate sludge which must be flushed. The unit must be taken out of service, the sump drained, and the mud removed. Twice a year cleaning should be sufficient, except where dust conditions are very severe. 19-26. Liquid refrigerant receivers. The liquid receiver must be large enough to hold all of the refrigerant in the system. Any time the system is to be opened, it must first be pumped down. This is done by closing the receiver discharge (king) valve and operating the compressor until the suction pressure levels off at 3-5 PSIG indicating that the system has been pumped down. The receiver inlet valve is then closed, and all the refrigerant is stored in the receiver. Repairing of leaks, testing and major repairs has been explained in the preceding chapter. 19-27. Expansion valves. Expansion valves should be checked during each walk-through inspection. Look for unusual signs of frosting or extension of the frost line on the tubing. Frost on the high side of the valve indicates a restriction in the liquid line. Check fans for operation. Note also any unusual noises, such as hissing, which would indicate a low refrigerant change. When a

expansion valve begins to get plugged, a from ice which is formed by moisture in the system, one of the observable signs will be an unaccountable rise in the temperature of the room. When this happens, you must do more than just clean out the valve. In addition, the system drier should be changed and the system carefully checked to find where the moisture is getting in. The leak must be on the suction side at a joint, unless the tubing has been punctured. 19-28. Evaporators. The evaporator used in a cold room is a coil and fin type with a fan or blower for cooling. Hot gas defrosting is the most practical method for zero operation, but some older units may still require manual defrosting with water. These methods are the same as those you studied in the last chapter. 19-29. Evaporators and blowers should be inspected for security of their mountings. Any unusual noises should be investigated and traced to their source. Unusual odors in a cold room may be caused by a hot motor or a bearing which needs lubrication. Conditions which you cannot correct should be reported so that proper action can be taken. 19-30. A large evaporator in a zero cold room may become sluggish in operation over a period of time. These conditions are caused by a gradual accumulation of oil in the coils. A hot gas defrost system will usually prevent the oil from condensing in the evaporator. However, this discussion of oil condensing in the evaporator brings up one of the big advantages of defrosting with hot gas. Should oil accumulate in one of the evaporators, the first indication would be a drop in oil level at the oil receiver with no external signs of the loss. By operating the hot gas system for an additional period of time, the oil can be picked up by the refrigerant gas and moved out of the evaporator. The success of this operation will be reflected by a rise in the level at the oil receiver. 20. Ice Plants 20-1. Both portable and permanent type of ice plants are used by the military. You may find the portable plant in sizes ranging from 1-ton to 15-ton units using standard halocarbon refrigerants. The operation of a system using such a refrigerant has already been explained. While this section will deal with a permanent type plant using an ammonia system, ice making is done by the same procedure regardless of which refrigerant is used for freezing the water. Building design will generally call for insulation similar to that used for cold storage. The discussion which follows is centered around an ice plant using an ammonia system. 20-2. Plant Design and Layout. A permanent ice plant requires special construction of a building, as

illustrated in figure 30. You will see that part of the floor has been omitted from the drawing. Below the floor is a large tank which is filled with a brine (salt) solution. The evaporator is a flooded type with the coil weaving back and forth between rows of ice can set in the brine tank. The floor above the tank has removable slabs. Look at the left in figure 30; you can see where one of the slabs has been removed so that the ice can filler can be used to fill a fresh can with water. Just to the right, you can see where the hoist has been used to lift a can with a frozen block of ice out of the brine tank. In the foreground, a can dump is illustrated. Warm water may be sprayed over the can to loosen the ice. 20-3. In figure 30 you can see an illustration of a compressor with a shell and tube condensing unit. A cooling tower would be necessary unless fresh water is very plentiful. At the opposite corner of the room, you will see the agitator motor. This motor circulates the brine to increase the rate of transfer of heat between the evaporator coil and the ice cans. To the right of the agitator is the accumulator. This is a large vertical tank which extends down into the brine tank. It serves to provide liquid ammonia to the bottom header of the evaporator. The ice storage room is not illustrated here; but in some plants, the overflow from the main evaporator is used to cool the ice storage room. 20-4. Operating Principles and Application. Let us start at the compressor for a brief review of the operating cycle. Coming from the compressor is a hot gas under high pressure. The gas is cooled and becomes a liquid in the condenser. At the float valve, the liquid passes into a reduced pressure area-the evaporator-where it boils and absorbs heat as it changes to a gas. From the accumulator, the suction line returns low-pressure vapor to the compressor. The refrigeration cycle is illustrated in figure 31, where we have also shown the cooling water path through the tubing of the condenser and the compressor heads. The temperature of the brine is maintained at 15° F. This will free a 300-pound block of ice in 45 to 48 hours. At warmer brine temperature, the freezing period becomes too long to be economical. At temperatures colder than 15° F., the ice is too brittle and fractures easily. 20-5. There are three factors to make a good block of ice: (1) A brine at 15° F. has the lowest practical temperature which will make good ice. (2) The ice water should be agitated continuously while the main part of the block is being frozen. This is done by means of a small rubber hose which puts a jet of low-pressure air into the water. The pressure is adjusted so that the hose pulses back and forth. This insures a good grade of clear ice. (3) The core water should be sucked out and re-

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Figure 30. Layout of an ice plant. placed by fresh water when the block is about two-thirds complete. The core water will have salts and other impurities concentrated in it, and the ice will have a disagreeable taste unless the core water is discarded. Also, the fresh water in the core will freeze faster than the water it replaces, so it is also a measure of economy. Let us consider next certain new components which are peculiar to an ice plant. 20-6. Components. The usual accessories, such as strainers, are just as vital in an ammonia system as in any other similar system. Of more specific concern to you here are such new components as ice cans, can fillers, agitators, core suckers, ice can hoists, dip tanks, and can dumps. These are discussed, in the order named, below. 20-7. Ice cans. The ice cans used at an ice plant are all of the same size in order to standardize operation and handling. Cans are made in different sizes, with the smallest one capable of making a 50-pound block of ice. The largest cans are made to produce a block of ice weighing 300 pounds. The lifting arrangement at the top of the can must be standardized so that one hoist attachment will fit all of the cans. 20-8. Can fillers. A permanent type can filler uses a tank of sufficient capacity and a float valve to shut off the water automatically at the right level. The tank is 65 mounted on a raised platform higher than the top of the ice can. When a can is placed under the tank, a dump valve is tipped and the ice can is filled with the right amount of water. The water supply to the tank is shut off while the dump valve is tripped. When the dump valve is shut off, the tank is automatically refilled. A portable type of can filler is shown in figure 32. You must be sure that the float ball and trip level operate properly to shut off the water when the ice can is full. 20-9. Agitators. The brine agitator consists of an electric motor, a shaft, and an impeller. This motor runs continuously to insure transfer of heat from the cans to the evaporator. Another agitator consists of a small air pump and the necessary hose to reach the ice cans. Air is used to agitate the water during the early stages of ice formation to insure a good grade of ice. 20-10. Core suckers. A core sucker is a pipe long enough to reach the bottom of the ice can. A hose connects it with an injector or suction pump. The sucker is used to remove the core water, which contains a concentration of salt and other impurities, after most of the ice block has formed. The core water is discharged to the sewer system, and fresh water is added to fill the core so that freezing of the block can be completed. This

Figure 31. Refrigerant cycle and cooling water path. operation insures purity of the ice and freedom from objectionable taste and smell. 20-11. Ice can hoists. The can hoist is used to load and unload ice cans from the brine tank. A small plant may use a portable band winch of the type you saw in figure 30. A large volume plant usually has a rail-mounted overhead hoist which can be moved to any position over the brine tank. 20-12. Dip tanks. A large tank of water may be used to free the finished block from the can. The can is lowered into the tank, where it remains long enough to melt the surface of the block. Tap water temperature may be warm enough so that it is not necessary to add any more heat to the dip tank. 20-13. Can dumps. After the block is free, the can is removed from the dip tank and placed on a dump rack. The rack tips the can at an angle to allow the ice to slide out. When a dip tank is not available, the ice can may be positioned on the dump and warm water sprayed over the can to loosen the ice. 20-14. Ammonia System Operation and Maintenance. The operation of an ammonia system requires temperatures and pressures different from those for halogens refrigerants. Also new to you is the makeup of the brine solution used in an ice plant. 66 20-15. Brine solution. If you have operated a car in a cold climate, you know that an antifreeze solution insures that a liquid will remain liquid at below freezing temperatures. Although glycol or alcohol can be used as an antifreeze additive, the most common additive is salt, which gives the name "brine" to the solution. A salt solution may be checked for its freezing point with a hydrometer if the temperature of the solution is taken into account. Four brine solutions and their specific gravity are given in table 9. The specific gravity is given for a solution temperature of 60° F. A sodium chloride solution reaches its eutectic point at -6° F. Beyond this point, the addition of more salt will cause the solution to thicken. 20-16. A brine solution made with 2 1/2 pounds of calcium chloride to 1 gallon of water will not freeze at 0° F. Two pounds of ordinary table salt (sodium chloride) per gallon of water will freeze at slightly below 0° E A 40-percent solution of alcohol is good at 0° F., while an ethylene glycol solution requires about 45 percent glycol to inure that it will remain a liquid at 0° F. You will note that any of the above solutions would be adequate, as a brine temperature of 15° F., can usually be maintained with an evaporator temperature of 5° F

TABLE 9

20-17. Ammonia temperatures and pressures. Let us first deal with normal operating conditions which you would expect in warm weather. With a suction gauge pressure of 20 p.s.i.g., you should have an evaporator coil temperature of about 5° F. A normal head pressure of 185 p.s.i.g. should carry a vapor temperature of 238° F. Since temperatures approach 250° F., water jackets are used to cool the compressor head. At this head pressure, ammonia will condense at 96° F. In colder weather, head pressure may drop to 155 p.s.i.g., with the temperature of the vapor dropping to 212° F. At a gauge pressure of 155 p.s.i.g., ammonia condenses at 86° F. It is necessary to raise the operating pressure on the system. You may use a manual valve in the liquid line to throttle the system. As pressure builds up, be sure to open the valve and restore it to its normal operating position. 20-18. Condenser cleaning. Even with proper treatment, the tubes in a shell and tube condenser may accumulate scale. A temperature rise of 5° above normal in condenser output is an indication of scale formation. We will discuss next the most effective method for removing scale. The necessary equipment for cleaning is shown in figure 33. The drum or barrel should have a capacity of 50 gallons. It may be wood, stone, porcelain, or metal. Galvanized metal must not be used, as it will react too fast with the acid used for cleaning. The fine mesh screen (bronze or copper) in the barrel serves to prevent scale from entering the pump. Look at the position of the pump in the suction line and note how scale from the condenser will be held in the drum. The circulator pump must be made of acid-resistant parts. The vent pipe provides a way of voiding hydrogen gas from the system. This gas is evolved as part of the chemical reaction between the acid and the scale. Galvanized pipe or fittings must not be used in the setup.

20-19. Goggles, rubber gloves, and aprons must be worn while you are mixing acid or handling the solution. Furthermore, bicarbonate of soda (baking soda) should be immediately available to counteract any spills or neutralize accidental skin burns. (NOTE. Always add acid to water when mixing, particularly sulfuric acid. It gives off a large amount of heat, which the water can absorb and dissipate. Avoid inhaling acid fumes as they can easily damage mucous tissues.) An inhibited acid solution is prepared by using commercial grade hydrochloric acid (muriatic acid) with a specific gravity of 1.190. The solution may be prepared in the barrel used with the cleaning setup. Each 10 gallons of water requires that 3 2/5 ounces of inhibitor powder be dissolved in it. Add acid to this solution at the rate of 11 quarts of acid for each 10 gallons of water. Commercial grade sulfuric acid may be used if muriatic is not available, but the solution will not do as satisfactory a job of cleaning the tubing. 20-20. The inhibited acid should be circulated through the tubes for about 12 hours to remove scale deposits of average thickness. Cleaning time will be slower if the solution is cold, whereas cleaning will be faster with a hot solution. The strength of the solution may be checked by applying a few drops to some baking soda. The rate of bubbling or gassing indicates how much strength remains in the solution. When the solution becomes very weak, there is little point in continuing to circulate it through the condenser. Be sure to flush out the condenser tubing immediately after cleaning. Flush the condenser tubes with fresh water until the discharge water becomes clear. The effectiveness of the cleaning may be indicated by the amount of scale deposited in the barrel. The final measure, of course, is the return of the condenser to normal temperature range during operation.

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Figure 33. Equipment setup for scale removal. Review Exercises The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the test. Do not submit your answers for grading. 1. Where are the coldest rooms located in a refrigerated warehouse? (19-2) 2. Outside of the machine room you will find certain areas which may be dangerous. Explain the dangers in these areas. (19-4, 5, 8)

3. What will probably happen if potatoes are piled too high in storage? (19-8)

4. Modern methods of construction have introduced what two new basic fundamentals to a cold storage building? (19-10)

Figure 32. Portable can filter.

5. With modem construction of a refrigerated warehouse, to what should the vapor barrier be attached? (19-14, 15)

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6. How should you use the blueprint of a cold storage plant? Give several uses and applications. (19-18-21)

15. Your personal safety is at stake when you work on V-belt. What precautions must you observe to keep from injury? (19-24)

7. What are details on a blueprint? (19-21, 22)

16. How should you treat V-belts which get splashed with oil? (19-24)

8. A cold storage plant has what operating advantages when four or more compressors are installed? (19-24)

17. How would you adjust the tension when one Vbelt of a set shows a more pronounced flutter than that of the others? (19-24) 18. How would you interpret a gradual drop in head pressure as compared with a sudden drop? (1924) 19. What is the purpose of a recording chart? (1924)

9. What factor is used to determine the setting of a suction side pressure control which starts and stops the compressor motors? (19-24)

10. (True)(False) Evaporator coil temperature is a more reliable indicator of system operation than suction pressure. Why? (19-24)

20. In an evaporative condenser what is the purpose of bleed-off water? (19-25) 21. How can bleed-off water affect the volume of makeup water? (19-25)

11. What are two ways of building up head pressure during cold weather operation? (19-24, 25)

12. Describe the proper way to check a three-phase magnetic switch for voltage. (19-24)

22. What is probably the best method of capacity control of an evaporative condenser during cold weather operation? (19-25)

13. Where is dashput oil used? (19-24)

23. What are the main disadvantages of a cooling tower? (19-25)

14. What test should you make of a motor when you install it? (19-24)

24. What are the checks which should be made on a walk-through inspection of the freezer rooms in a cold storage plant? (19-27-29)

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25. What is a big advantage of a hot gas defrost system for a large evaporator in a zero cold room? (19-30)

29. Why is it necessary to use a core sucker? (20-5, 10)

26. Why is an agitator necessary in the brine tank of an ice plant? (20-3)

30. How is it that the brine solution is operated at 15° F. but it is made up so as not to freeze at 0° F.? (20-15, 16)

27. Why is the head of an ammonia compressor cooled with a water jacket? (20-3, 17)

31. How is inhibited acid prepared for cleaning scale from the tubes of a condenser? (20-19)

28. What are three important factors in making a good grade of ice? (20-5)

32. Why is baking soda necessary when you are cleaning scale with inhibited acid solution? (2019, 20)

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CHAPTER 4

Special Application Systems

SOMETIMES we try to make things more complicated than they are. You can avoid confining yourself if you keep in mind that a basic system is modified to make it work for a special application. Yet, not one of the system’s basic principles is changed. This chapter deals with multiple evaporators and multiple compressors and concludes with systems for producing ultralow temperatures. Technical information on these systems is reproduced from Commercial and Industrial Refrigeration, by C. Wesley Nelson, copyright 1952, McGraw-Hill Book Company; used by permission. You are already familiar with the name of items which will be discussed, but differences result from the conditions under which this equipment operates. Let us consider first the problems of a system using several evaporators. 21. Multiple Evaporator Systems 21-1. A multiple system is one in which several evaporators are operated from one compressor. A variation is the operation of two or more compressors with one evaporator. Multiple units are installed in restaurants, soda fountains, bars, and in other places where more than one refrigeration fixture is used. Capacity control is obtained by using two or more compressors for one evaporator. An example is an ice plant when the compressors are started or stopped according to the load demand. 21-2. Classification of Multiple Evaporator Systems. Fundamentally there are only two classifications of multiple-unit systems. The first is the one in which all the evaporators operate at the same temperature. This is the simplest, although not the most common. The second is the one in which the temperatures in the different refrigerators are not the same. In order to control the temperature in a multitemperature installation, various combinations of valves and controls must be used. The correct selection and installation of these valves and controls have a decided bearing on the success of the installation.

21-3. Cord Valves for Multiple Units. When two or more evaporators are operated from the same compressor and the temperature of the warmer refrigerator is not more than 5° F. higher than the colder, no special valves are necessary. When the temperature difference is greater than 5° F., some sort of valve or control for the warmer refrigerator is essential. A thorough knowledge of the operation and application of the types of available control valves is necessary before a satisfactory multiple system can be laid out. 21-4. Suction-pressure regulating valve. This valve, also called an evaporator-regulating valve, is placed in the suction line of the warmer evaporator, and controls its pressure (and consequently its saturation temperature) so that it will remain substantially constant and not go below the predetermined setting of the valve. Thus, when two or more evaporators are operated with one compressor, the desired temperature in the warmer evaporators can be maintained by the proper setting of the valve. The locations of the valves in a system are shown in figure 39. (See paragraph 21-19.) 21-5. Bellows type. A bellows type evaporatorregulating valve has the inlet connection from the evaporator and the outlet connection to the compressor. The evaporator pressure acting under the bellows and the force of a small spring under the valve are both opposed by a spring. When the forces are equal, the valve is in equilibrium and maintains a definite opening. A reduction in evaporator pressure will cause an unbalancing of forces, and the valve will throttle. The resulting decrease in the flow of vapor will prevent the pressure from going too low. The condensing unit continues to operate on the other evaporators at reduced suction pressure. This valve has a fitting where the evaporator pressure may be taken. A gauge adapter valve must be used. The cap is removed and the adapter is screwed onto the fitting with the key engaging the needle valve. The gauge is connected to the

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Figure 34. Method of bypassing evaporator-regulating valve. adapter and, when the valve is opened, the pressure may be read. This fitting is also used in pumping down the evaporator. The regulating valve must be bypassed when pumping down, because it would close on reduction of suction pressure. To bypass the valve, a line is run from the adapter to the compressor’s suction-valve gauge port. If the distance is too great, a connection should be provided on the compressor side of the valve, as shown in figure 34. 21-6. Diaphragm type. Another type of evaporatorregulating valve is illustrated in figure 35. This valve has a diaphragm instead of a bellows. One of the features of this valve is the collar with pressure graduations under the adjusting knob. By turning the knob until the bottom lines up with the graduations, you can make the correct setting without reading the pressure or without waiting for the refrigerator to arrive at the desired temperature. The operation of this valve is similar to the one previously described in that the valve remains open when the warmer evaporator pressure is high and then throttles as the compressor lowers the pressure. This valve has a gauge port for checking pressure and for bypassing. To get a reading, you attach the gauge to the port, remove the cap, and open the gauge shutoff valve. The valve range is from 40 p.s.i. to 0 in. Hg vacuum, and it can be used on Freon units up to 1/2 ton and up to 3/4 ton on methyl chloride and SO2 units. 21-7. Two-temperature type with manual closing. Still another type of two-temperature valve is the one shown in figure 36. You adjust this valve by the adjusting nut; the handwheel is used only for closing the valve without affecting the setting. You use the auxiliary valve when attaching a gauge. You may also use it to bypass the main valve when you want to pump out the coil. During normal operation, the auxiliary valve is closed, and when the coils being evacuated, the valve is turned into midposition. The valves men-

(Courtesy Controls Company of America) Figure 35. Diaphragm type evaporator-regulating valve.

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21-9. Thermostatic type. A thermostatically controlled suction-pressure valve is shown in figure 37. This valve is used where close, nonelectrical control of single evaporators is desired. Such applications are sweetwater baths, water coolers, beverage coolers, and soda fountains. In multiple installations, you place the valves in the suction line from the warmer evaporator, with the thermal bulb in the refrigerated space or liquid. This valve gives closer temperature control than does the pressure type. The valve illustrated in the figure is of the snap-action type, but the thermostatic type is also available in a valve which has throttling action. Before the refrigerator has reached the desired temperature, the valve is wide open and the coil is subject to refrigeration from the condensing unit. When the desired temperature is reached, the valve snaps shut and the coil is isolated from the rest of the system. If the refrigerator temperature is above freezing, the coil will defrost while the valve is closed. You must install this valve in a horizontal part of the suction line, and place a strainer ahead of it. The bulb should be located where it will reflect the average conditions, and in water baths it should be placed in the liquid but not too close to the coils. 21-10. Check valves. When evaporators are connected in multiple and the temperatures are different, the pressure in the warmer evaporator is higher than that in the colder. When the control valve opens, the highpressure vapor in the

Figure 36. Two-temperature valve with manual closing arrangement. tioned above should be installed in a location where frosting will not occur and should be reasonably close to the refrigerator which is to be controlled. These valves may be used on flooded or direct-expansion evaporators, providing defrosting is not required. 21-8. Snap-action type. A suction-pressure regulating valve of the snap-action type is not a throttling type and can be set to cut in and out at definite predetermined pressures. This valve is either wide open or closed tightly. You use it when you want to operate an evaporator on a defrosting cycle, and when a shorter operating time than that provided by the condensing unit is required. The effect of using a snap-action valve on an evaporator in a multiple system is the same as if it were connected to a separate compressor. This valve should be used only with a low-side float or with a thermostatic expansion valve. Most valves of this type have a gauge port to which a gauge may be attached to aid the proper setting. 73

Figure 37. Thermostatically controlled suction-pressure valve.

warmer evaporator would back up into the colder evaporator if no means were provided to prevent it. This vapor would cause a warming up of the colder evaporator and would impair its efficiency. To prevent this, you install a check valve which will permit the vapor to flow in only one direction. See figure 39 for the proper location of check valves in a multiple system. Next, let us review some important points before discussing the use of solenoid valves in a multiple system. 21-11. Important Points for Multiple Installations. Because of the large number of possible multiple combinations, it is impossible to give a set of rules and expect them to apply to all cases. Although there are exceptions to the rules, the eight rules explained next should be adhered to whenever applicable. (1) The coldest evaporator or evaporators must comprise more than one-half of the total load on the condensing unit. If the warmer evaporator were the major part of the load, the condensing unit would be operating at the higher suction pressure a greater portion of the running time and would not be able to bring the colder refrigerator down to the desired temperature. (2) The capacity of the condensing unit is selected at the suction temperature or pressure of the colder evaporator. Since the colder evaporator constitutes the major part of the load, the compressor will be operating at its pressure most of the time, although the pressure in the warmer refrigerator will be higher. This is another case where we must remember that the capacity of a compressor is less at lower suction pressures. (3) The evaporator for each refrigerator is selected at the suction pressure which will give the correct temperature and humidity for the particular application. The selection is made just as if each evaporator were to be connected to its own compressor. (4) When the temperature difference between the colder and the warmer fixture is greater than 5° F., a control for the warmer evaporator or evaporators is necessary. This control may be either a suction valve of the pressure or temperature type or a solenoid valve. In some cases, although the temperatures are the same, one refrigerator will be used much more than the other. In instances like this, a control valve should be used and placed in the suction line of the refrigerator with the least usage. (5) A snap-action type of suction-pressure control should be used if defrosting on the off cycle is required. This cannot be done, even though a snap-action valve is used, unless the refrigerator temperature is above 35° F. (6) The coldest evaporator should always be directly connected to the compressor, and a check valve should

be located in the suction line between the outlet and the first takeoff. In following the rule that half of the total load should be the cold evaporator, there is sufficient load on the compressor to eliminate low back pressure even though the control devices have isolated all the warmer evaporators. (7) In general, thermostatic expansion valves should be used as the liquid control when direct expansion evaporators are installed in multiple. The control of temperature in any refrigerator should not be by the adjustment of any expansion valve. This should be done by the adjustment of the suction-pressure valve in order to obtain the best operating conditions. (8) The liquid and suction lines should be sized according to the amount of refrigerant flowing and according to the load on each branch or main. These important points place a limitation on the applications in which a multiple installation should be made. When the temperature difference between the warmer and the colder refrigerators is greater than 25° F., multiplexing should be considered the exception rather than the rule unless humidity is not a factor. When high humidities are to be maintained, such as in florist's boxes, it is better to use a compressor for each evaporator. 21-12. Evaporators with forced convection. Although multiple installations can be made with all expansion coils, all low-side float coils, or with a combination of both, you must give extra thought when you want to use forced-convection coolers. A forced-convection unit must be caused to defrost during the OFF cycle unless some means for artificial defrosting is provided. In some multiple installations where the forced-convection unit is the colder evaporator, a sufficiently low suction pressure may occur so that the unit cannot defrost, and it thus become inoperative in a short time. If the forcedconvection unit is the colder evaporator, successful multiplexing may be done if suction-control valves are used. 21-13. Low-pressure cutout. Multiple installations, where suction-pressure or temperature-control valves are used, have the colder refrigerator controlled by the low-pressure cutout at the condensing unit. The warmer refrigerator, in this type of arrangement, does not require any electrical control. The low-pressure electrical control is set at the proper cut-in and cut-out points just as if it were operating on a single evaporator. Some of the modern suction-control valves that are used on the warmer evaporators are calibrated so that the temperature setting can be made in advance. If this is not so, you must operate the system until the refrigerators come

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down to the desired temperature; you then adjust the control to shut off the evaporator from the compressor at this point. In any event, the final adjustment of the controls is governed by the thermometer readings in the various refrigerators. 21-14. Solenoid Valves in Multiple Systems. Solenoid valves are used extensively in a multiple system. They may be placed in either the liquid line or the suction line. Figure 38 shows a multiple hookup using solenoid valves in the liquid line. As may be seen from the diagram, there is a thermostat in each refrigerator, and each thermostat is connected to the solenoid valve that is in the liquid line leading to its refrigerator. The operation of the system is as follows: Assume that all the thermostats call for cooling and that the compressor is operating on all of the evaporators. When one thermostat is satisfied, it opens the circuit and the solenoid valve closes the liquid line to the evaporator. The compressor pumps out the refrigerant from that evaporator and continues t operate on the others. As each thermostat is satisfied, its solenoid valve closes; and finally, when all valves are closed, the compressor is stopped by a low-pressure cut-out. 21-15. There are several important factors that must be considered in connection with multiple systems using solenoid valves. As each solenoid valve closes, its

evaporator is pumped out and the refrigerant is returned to the receiver. The receiver, therefore, must be large enough to hold the entire charge in the system. When the compressor is operating on all the evaporators, it is at full load and the suction pressure is high. As one evaporator after the other stops refrigerating, the suction pressure drops progressively lower. This drop in suction pressure has the disadvantage of continually upsetting the balance between the refrigerator temperature and the coil temperature. The result affects the humidity in the box and, in some cases, prevents the defrosting of the coil. A suction-pressure regulating valve placed in the suction line near the compressor will hold the pressure at the desired point in the evaporation although the crankcase pressure will be low when only one evaporator is operating from the compressor. The low-pressure control, which shuts off the compressor after the last solenoid valve closes, must be set lower than the suction pressure when the compressor is operating on only one evaporator. The setting of the cut-in point should be made so that the opening of one solenoid valve will start the compressor. 21-16. Solenoid valves that are used in the suction line from the evaporator should be selected so that the pressure drop through the valve will not be over 2 p.s.i. When a solenoid valve is so used, the refrigerant remains in the

Figure 38. Multiple system using solenoid valves. 75

coil and does not return to the receiver. If the valve is placed in the suction line, there is danger of accumulated liquid flooding over into the compressor in the event of a leaking float or expansion valve. 21-17. Installation. A multiple system is installed like a simple one insofar as the coils and condensing units are concerned. In order to simplify service operations, a manifold is usually placed on the wall near the condensing unit. Three-way valves are sometimes used to allow the refrigerant to pass through the valve unobstructed on the run of the valve, while the branch line from the side outlet may be shut off. Prefabricated manifolds may also be obtained. 21-18. Evaporators at same temperature. When the temperatures are the same in all of the refrigerators, there is an expansion valve on each coil. When two coils are located in the same case, it is not good policy to connect coils in series, because the second coil will not maintain its rating. Two coils should not be connected to one expansion valve, regardless of whether they are in series or parallel. There are no pressure-control valves needed when all evaporators are operated at the same temperature. The entire low side of the system is controlled by a low-pressure control located on the base of the compressor unit.

21-19. Evaporators at different temperatures. The line diagram in figure 39 shows the connections for three coils at different temperatures. Note the presence of a suction-pressure control valve in each of the warmer coils. The suction line from the coldest evaporator is directly connected to the compressor with a check valve in the line. In this diagram, the -10° evaporator should constitute the major portion of the load. A check valve is also found in the suction line from the 35° evaporator, since this evaporator is colder than 45° and there would be the possibility of the warm vapor backing up and condensing, 22. Multiple Compressors 22-1. Compressor units are sometimes connected in parallel to obtain greater flexibility or to use small units on one evaporator where a larger one that will balance properly is not obtainable. The practice of operating compressors in multiple is not new; in fact, it is quite common in ammonia plants and cold storage warehouses. The principle difficulty in interconnecting condensing units is in the oil return to the crankcase. Ammonia does not present any problem in this regard, since oil and ammonia are not miscible. Freon, methyl chloride, and other oil-miscible refrigerants, on the other hand, present a real difficulty when condensing units that use them are

Figure 39. Multiple systems using evaporative-regulating valves. 76

Figure 40. Balanced lines between compressors. interconnected. Multiple installations of condensing units should be avoided and made only when it is not possible to split up the units that each evaporator has its own condensing unit. 22-2. Connections Between Compressors. Compressors that are to be interconnected should be of the same manufacture and preferably of the same size. A good installation is one in which each condensing unit will carry its share of the load and the oil return will be such that the proper level will be maintained. To get this balance of the load, the liquid, suction, and discharge lines are interconnected and it is necessary that oil- and gas-equalizer lines are installed between the crankcases. The compressors should be placed close to each other so that the connecting lines will be as short as possible. 22-3. Balanced refrigerant lines. Lines should be connected as shown in figure 40 so that the elbows and length of pipe from the tee to one compressor are the

same as to the other compressor. Thus the frictional resistance between each receiver and the manifold should also be approximately the same. Both suction and discharge lines should be balanced in the same manner as we have just described. Suction lines should be connected so that the oil which is returning through the line will divide between the compressors as evenly as possibly. Suction lines from a manifold to the compressor, if used, should be taken off at the side to facilitate oil return. 22-4. Oil-equalizer line. The crankcase oil-equalizer line may be connected in one of two ways, A or B, a shown in figure 41. The A method is preferable although the B method is more often used because of the location of plugs in the crankcases of some compressors where the equalizer lines may be connected. In the method shown in A, the oil-equalizer connection should be made at the lowest safe oil level. Condensing units should be placed on their foundations so that the oil level in each is in the same horizontal plane. The gasequalizer connection is made above the maximum oil level. All equalizer lines should be level. The size of the manifold to which the various branches are connected should be at least equal in area to the sum of the branches. Crankcase oil- and gas-equalizer lines should be not less than 3/4-inch ID up to 10 tons capacity and linch ID over 10 tons. 22-5. Control of Multiple Compressors. When condensing units are interconnected for purposes of capacity control, usually you have to provide a means for starting and stopping the compressor according to load demands. The compressors are manually operated only when an operator is in attendance and when load changes can be anticipated beforehand. For liquid or air cooling, if close temperature control is not required,

Figure 41. Oil equalizer lines. 77

two temperature controls may be used, one being set a degree or two higher than the other. A more common method is to use pressure controls on the common suction line, set in sequence so that the compressors will start and stop according to changes in suction pressure. When this method is used, thermostats and solenoid valves are almost always installed. 22-6. When a number of compressors are connected together or when an installation consists of a number of single evaporator and condensing-unit installations in one refrigerated space, situations occur in which all compressors will start at the same time. This puts a very heavy load on the electric lines and power system. A timer, which will delay the starting of compressors until after the first one, should be installed. The action of the timer is such that when the contactor for the first compressor has closed, there is a delay of 10 or 15 seconds before the timer closes the control circuit of the second compressor and allows it to start. If there are more than two compressors, a timer may be used on each except the first one. 23. Ultralow Temperature Systems 23-1. The use of ultralow-temperature refrigeration in industrial work has increased tremendously in the past few years. Commercial units are now manufactured to produce temperatures below -100° F. for various applications. An example is the production testing of various instruments and appliances, such as radios, cameras, clocks, and meters which may be subject to low temperatures in arctic climates or in outer space. Ultralow temperatures find application in various kinds of metal treatment. The hardness of certain kinds of steels has been materially increased after a conventional hardening process by lowering the temperature to about -110° F., allowing them to warm to room temperature, and then tempering. Another development is the shrinkfitting of parts by using cold instead of heat. The male part is reduced in temperature to -100° F., after which it is fitted to the female part and the unit allowed to warm to room temperature. The extensive use of aluminum riveting in aircraft and other metal work has led to the use of special alloy rivets which may be prevented from age hardening and kept soft by holding them at -40° to -45° F. until ready for use. 23-2. Test chambers which are designed for simulating conditions encountered by military and other aircraft are being used increasingly. They are used for testing instruments, clothing, military weapons, and equipment that is normally carried in an airplane. Weather bureau information shows a temperature of -50° to -60° F. at

elevations around 50,000 feet, and cabinets for testing are usually kept between -60° and -70° F. Some cabinets are equipped so that any temperature from +100° to -100° F. may be obtained. 23-3. Insulation Requirements. Ultralow-temperature cabinets require more consideration in regard to insulation and construction than do zero cabinets. Insulation thicknesses of 10 and 12 inches are needed, and extra care must be taken with vaporproofing to prevent the entrance of moisture. When a refrigerator is intended to maintain a low temperature at all times, it is usually desirable to use an insulating material which has a high thermal capacity such as cork. Any interruption in refrigeration will not be so serious because of the slow warming up of the box. On the other hand, when rapid fluctuations in temperatures are desired, such as in simulated flight, an insulation of low heat capacity should be used. Ferrotherm (a number of thin steel sheets with air spaces) and Santocel (silica aero-gel) are two such insulating materials. Tests at Wentworth Institute using dry ice in a box insulated with nine sheets of Ferrotherm with 36-inch air space between the sheets gave a temperature reduction from +70° to -70° F. in 45 minutes. 23-4. Refrigerant and Compressor Problems. A simple refrigeration cycle is neither suitable nor economical for ultralow-temperature application. In order to obtain heat extraction from a box at a temperature of, say, -70° F., it is quite evident that a coil temperature less than -70° F. is necessary. If a 10° F. temperature difference were assumed, a design temperature of -80° F. for the coil would be used. If Freon 12 were the refrigerant selected, the corresponding absolute pressure and volume would be 2.885 p.s.i.a. and 11.57 cubic feet respectively. This would give a compression ratio of over 30 to 1 with normal condensing temperatures. This is much higher than is possible with a conventional compressor. As a matter of fact, a compressor operating with any such ratio would not discharge vapor but would simply compress and expand the vapor in the cylinder without doing any useful work. The compression ratio for units working under average commercial conditions is approximately 4 to 1. Ratios up to 8 to 1 are considered as being satisfactory for a single compressor. When ratios are above these values, because of extremely low temperatures, you must employ staging in order that there will be less work required per ton of refrigeration. 23-5. Staging. In a simple compression system, the heat that is absorbed at the low level of temperature is rejected at a higher level in one

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step. In a low-temperature application where this is either impossible or impractical, the heat may be "pumped" in two or more steps. This is called staging. Staging may be done by two methods in refrigeration work: (1) by using compound compression in which the vapor is removed from the evaporator by the low-pressure compressor and discharged by it to the high-pressure compressor, which discharges to the condenser in the usual manner; (2) by using an arrangement called cascading which is, in effect, two cycles operating at different heat levels. The low-pressure compressor discharges into a condenser which is the evaporator for the high-pressure cycle. The final heat is rejected to the cooling water as in the simple system. In direct staging the same refrigerant is used throughout, while in the cascade system different refrigerants may be used in the high- and low-pressure stages. 23-6. Compound Systems. Although a compound compression system consists essentially of two or more compressors in series, the addition of intercoolers and subcoolers will increase the efficiency and reduce the cost of operation. Of the several arrangements, two of the most common, the direct compound and the cascade systems, are included here. 23-7. Direct compounding. Direct compounding with an intercooler is illustrated in figure 42. This is a twostage compression in which the refrigerant vapor is drawn

from the evaporator through the heat exchanger by the first-stage compressor. The discharge from this compressor passes through a water-cooled intercooler, which is located between stages, and from there to the suction of the high-pressure compressor (second stage). The vapor is then liquefied in the condenser and flows through the other side of the heat exchanger to the expansion valve. The use of an intercooler reduces the superheat and the work of compression. The piston displacement of the low-pressure compressor is greater than that of the high-pressure compressor because of the greater volume of the vapor at low pressure. The proper sizing of the high- and low-pressure cylinders is such that the desired capacity will be obtained, and the compression ratios for each compressor will be approximately the same and within reasonable limits. When two individual compressors are used, they should be at the same level, and oil-equalizer lines between them must be provided as shown in the illustration. The intermediate pressure which exists between stages is not controlled and will fluctuate within small limits as the load varies. 23-8. A compound compression system with a liquid subcooler is shown in figure 43. The colder the refrigerant when it enters the expansion valve, the less flash gas will be formed when the refrigerant cools down to the evaporator

Figure 42. Direct compounding with an intercooler. 79

Figure 43. Direct compounding with a subcooler. temperature. The purpose of the subcooler is to subcool the refrigerant and thus increase the refrigeration effect. Referring to the diagram, you see that the liquid leaves the condenser, and a potion of it expands in the subcooler coil at the suction pressure of the high-pressure compressor. The remainder, at a much lower temperature, leaves the subcooler and goes to the expansion valve. What actually happens here is that the flash gas generated in the coil of the subcooler need only be compressed in the high-pressure compressor. The vapor has a smaller volume at this intermediate pressure, and for these reasons, a considerable amount of work may be saved. Without the subcooler, the flash vapor would have to be compressed from the evaporator pressure, where the volume is high, through both the first- and second-stage compressors. This arrangement, or one which will attain similar results, is a necessity in ultralow-temperature refrigeration. In place of single compressors, a V-design of a compound compressor is used in smaller size units. This type of compressor requires only one motor and eliminates the problem of oil level equalizing. 23-9. Cascade systems. A cascade system consists of two or three separate simple cycles operating in conjunction with each other at different temperature levels. The connecting point is a heat exchanger between the stages. This interstage heat exchanger is the 80 condenser for the first stage and the evaporator for the second stage. An elementary diagram of a cascade system is shown in figure 44. Beginning with the lowpressure cycle, the vapor from the evaporator is compressed in the first-stage compressor and goes to the interstage heat exchanger, where it gives up its heat to the second-stage evaporator coil. The condensed liquid then flows to the first-stage expansion valve and the evaporator, completing the low-pressure cycle. The vapor which is generated in the coil in the heat exchanger because of the heat it has absorbed is compressed in the second-stage compressor, and the high-pressure vapor is condensed, its heat going to the cooling water. Each stage is an independent simple cycle, and for this reason has some advantages over the compound compressors. There is no problem of oil equalizing, and a different refrigerant may be used in each stage. There is some loss in the cascade system because a temperature difference must exist in the heat exchanger in order that the heat from the first stage will flow into the second stage. At the present time, the use of Freon 22 in the low stage and Freon 12 in the high stage will produce temperatures down to -90° F. 23-10. A two-stage cascade system with a second heat exchanger for subcooling the liquid and with an oil separator in the discharge of the first

stage compressor is show in figure 45. There are a number of variations of this arrangement as far as the location of the second heat exchanger is concerned. The interstage heat exchanger, however, is always located in the same place- between the two stages. 23-11. Refrigerants for Compound Systems. At the present time, the number of refrigerants that are suitable for ultralow-temperature applications are few. Of the common refrigerants, ammonia SO2, methyl chloride, and CO2 are not used. Ammonia and methyl chloride have higher specific volumes than either Freon 12 or Freon 22, and SO2 has a freezing point of -98° F., in addition to a high boiling point. Carbon dioxide becomes a solid when it expands to a temperature below -70° F. Freon 12 and especially Freon 22 possess the best characteristics for low-temperature applications. The condensing pressure of Freon 12 at 80° F. is 98.76 p.s.i.a., whereas for Freon 22 it is 159.7 p.s.i.a. Compressors designed for Freon 12 may ordinarily be used with Freon 22, but in the large sizes particularly, compressors designed for Freon 22 should be used. The displacement of the compressor is les for Freon 22 than it is for Freon 12. Therefore a Freon 12 compressor would have a greater capacity when using Freon 22 than when using Freon 12. Under some conditions, then, a larger motor would be required when using Freon 22. In systems under 10 h.p., other parts such as liquid and suction lines would be the same as for

Freon 12. Ethane, ethylene, and methane are hydrocarbon refrigerants which are occasionally used in applications where the temperatures are below -100° F. These refrigerants are explosive and are, therefore, unacceptable where there would be a hazard. Freon 13 is a refrigerant which is replacing the hydrocarbons for ultralow temperatures. All of the low-boiling-point refrigerant have high head pressures, and pressure-relief valves must be provide wherever they are used. 23-12. Controls. In an ultralow-temperature system, the flow of refrigerant to the evaporator may be controlled either by an expansion valve or by a float control. Ordinary expansion valves are not suitable because excessive superheating at the bulb location is necessary to operate the valve. A differential-temperature expansion valve designed for ultralow application has two power element operated by thermal bulbs. When an expansion valve is used, a solenoid valve is always placed ahead of it, and the coil is pumped down at the end of the running cycle. 23-13. The high-side float is used quite extensively and is simple and inexpensive. When used with an accumulator, the charge is not too critical, and since the float valve is in the high side of the system, any moisture is less subject to freezing.

Figure 44. Elementary cascade system. 81

(Copyright 1952. McGraw-Hill Book Company. used by permission.) Figure 45. Two-stage cascade system. 23-14. There are various ways to wire the controls in a compound compression system, depending on the particular application. In a system which consists of one condensing unit and one evaporator, the simplest arrangement is to use a solenoid valve and a low-pressure cutout with a temperature control in the refrigerated space. The temperature control actuates the solenoid valve, and the pressure control operates the compressor. A special cutout must be used since the conventional tools are not satisfactory for vacuums over 20 inches Hg. Design of these control circuits should consider overload protection because of heavy loads and excessive pressures which occur during pulldown. The electrical controls for cascade systems consist fundamentally of a set of controls for each cycle. 23-15. Lubricating Oil for Low Temperature. It is very important that the lubricating oil which is to be used for low-temperature applications be an oil from which no wax will separate at or below the lowest expected operating temperature. The Freons as well as other chlorinated refrigerants possess solvent-extraction properties which remove wax from oil. With a refrigerant-oil mixture, there are two conditions which bring about wax separation. These are low temperatures and a high percentage of oil in the mixture. The use of a high-grade oil which has been processed especially for low-temperature refrigeration and the use of oil separators will minimize if not eliminate wax formation. 23-16. An expansion valve or other refrigerant control in which there has been wax formation acts somewhat like one in which moisture has frozen. With wax formation, the valve will "let go" at a temperature lower than 32° F. if heated. Also, if the valve is taken apart, the wax may be seen in the orifice or at the outlet. Such conditions would indicate that a poor grade of oil had been used in the system. REVIEW EXERCISES The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the test. Do not submit your answers for grading. 1. How is capacity control obtained in a large system with a variable heat load? (21-1)

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2. Give two applications of multiple evaporator systems. (21-2)

11. What two equalizer lines are necessary for proper lubrication of multiple compressors? (224)

3. Name five type of evaporator-regulating valves. (21-4-9)

12. In the case of multiple compressors, what is done to reduce heavy electrical loads on starting? (22-6)

4. Where are check valves installed in multiple evaporators at different temperatures? (21-10) 13. List two applications of an ultralow-temperature system to Air Force problems or testing. (23-2) 5. Why must the coldest evaporator make up more than half of the heat load? (21-11) 14. What is the main requirement of an insulation for an ultralow-temperature chamber which must respond to rapid changes in temperature. (23-3)

6. With multiple evaporators at different temperatures, which evaporator will be controlled by a low-pressure cutout at the compressor? (21-13)

7. How is the size of the receiver affected in a multiple evaporator system with solenoid valves in the liquid line? (21-15)

15. What are two methods of “staging” to produce ultralow temperatures. (23-5)

8. What is the possible danger to the compressor when solenoid valves are used in the suction line? (21-16)

16. Describe a direct compound system which has good efficiency. (23-6, 7)

9. What is the main difficulty with compressors connected in parallel? (22-1)

17. Why may two different refrigerants be used in a cascade system? (23-9)

10. What are the installation requirements for satisfactory operation of compressors in parallel? (22-2)

18. Name three refrigerants used for ultralow temperatures. (23-11)

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19. Why is an ordinary expansion valve unsatisfactory at ultralow temperatures? (23-12)

21. How is wax prevented from causing trouble in an ultralow-temperature system? (23-15)

20. Why is a special cutout necessary for ultra-low temperatures? (23-14)

22. How can you tell the difference between wax and moisture as the cause for a frozen expansion valve? (23-16)

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CHAPTER 5

Vehicle Refrigeration Units

FROM MARKET TO warehouse and from warehouse to dining hall, refrigerated Air Force trucks transport many tons of perishable foods safely without risk of spoiling. The safe delivery of these foods is dependent on the operation of refrigeration units. Part of your job is proper maintenance of truck-mounted units to insure the delivery of good food to all the troops. This chapter discusses the units for truck refrigeration and car cooling. 24. Refrigeration Unit for Trucks 24-1. Since trucks and semitrailers transport perishable or frozen foods long distance, they require some type of refrigeration. Refrigeration units for trucks and semitrailers are of the same type, design, and size as units used in reach-in and walk-in refrigerators. Specifications will depend on the demand. 24-2. Refrigerator trucks and semitrailers have specially designed bodies adapted for the transportation of material under refrigeration. These bodies are designed with a double wall or shell, with Fiberglas insulation between them. The refrigerator unit is installed where it will give the best circulation of refrigerated air for the demand. The source of power and control operation of the condensing unit and engine will be explained in the following section. 24-3. Types of Units. The refrigeration units used today are called package units. In such a package unit, the refrigeration unit, gasoline engine, starter-generator, and battery are all mounted on one frame as a single selfcontained unit. Older type units may, however, have the gasoline engine and the starter-generator mounted on a separate frame with a belt to the compressor. 24-4. There are two types of compressor drives which get their power from the truck's engine: (1) the enginedriven electric generator and motor and (2) the transmission shaft-driven compressor. If either of these types is used, the refrigeration unit stops when the truck engine stops, thus requiring an outside source of refrigeration during layovers.

24-5. On the other hand, the package type gasolineengine-driven units are automatically controlled to start and stop as the system may require. The space, or opening for the refrigeration unit, is designed for the demand, as are the special bodies. Both the size of the bodies and the type of material to be under refrigeration determine the type, size, and number of refrigeration units to be installed. 24-6. Installation. Refrigeration units for trucks and semitrailers are usually mounted on the front of the bodies, either at the top or bottom. When the unit is mounted other than on the front, the trailer may have a rack or platform designed for the purpose, as well as mounting bolts to secure the unit to the trailer body. 24-7. Power Connections. Power is furnished by a gasoline engine, which is usually of the 2-cylinder, 4cycle, L-head, air-cooled type. This engine is equipped with a 12-volt, d.c., combination starter-generator, with the armature mounted on the engine crankshaft. The engine is started electrically by the power from two 6-volt storage batteries, connected in series, that are mounted on the front of the truck or trailer body near the refrigeration unit. Most of these engines are governor controlled so that the engine runs at a speed of 2400 r.p.m. This speed will give a maximum horsepower of 9.4. The power is transmitted from the engine to the evaporator blower, the condenser fan, and the compressor by the use of V type belts and pulleys. The compressor is set to run at 1800 r.p.m. when engine speed is 2400 r.p.m. 24-8. Starting Procedures. The starting procedures are the same for most truck refrigeration units. After checking for leaks, valve settings, compressor oil level, engine oil, and fuel level, etc., turn the refrigeration unit thermostat until the pointer indicates the temperature to be maintained within the truck or semitrailer. Then set the heat-cool switch to the cool position. (NOTE: Some units have an electric heating coil mounted

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in the evaporator blower which is used in the refrigeration system to cool the trailer. The blower forces air across the heating coil surfaces to raise the temperature in subzero weather. On other units the cooling element of the thermostat is bypassed when the heat-cool switch is on HEAT. The heating element of the thermostat completes the circuit to start the engine and to energize the defrost solenoid valve. When the defrost valve opens, hot refrigerant gas is permitted to flow to the evaporator to provide heat, which will raise the temperature a few degrees above freezing inside the trailer.) 24-9. Electrical Circuit Operation. The starter switch is turned to the ON position in order to close the circuit and energize the thermostat, which operates the starter-relay coil. This action set up a magnetic field that attracts the relay contractor bar to complete the circuit to the starter-generator. When the relay circuit is opened or broken, the relay coil is deenergized, and the contact bar springs away from the coil and opens the circuit to the starter-generator circuit. This stops the engine. The thermostat bellows expands or contracts with the temperature change at the feeler bulb to automatically open or close the contacts of the switch within the thermostat. In turn, the opening or closing of this switch opens or closes the circuit to the starter-relay coil. 24-10. When the starter-relay closes, it also completes a circuit to the choke-solenoid relay. This relay armature closes the contacts that operate the automatic choke and the defrost solenoid valve. When the engine starts, the current flowing through the choke-solenoid relay reverses direction and decreases in magnitude so that the relay is deenergized. This releases the relay armature and opens the contacts to the automatic choke and the defrost solenoid valve. 24-11. When the automatic choke coil is energized, it attracts an armature lever attached to the carburetor choke valve by a linking rod. When the lever is drawn to the magnetized coil, the carburetor choke valve closes. As the engine starts, a reverse flow of current is set up in the choke-solenoid relay circuit, and the relay contact point break the circuit to the automatic choke. 24-12. The starter-generator has a charging rate resistor placed in the generator field circuit to regulate the generator charging rate, which is controlled by the voltage regulator. As the batteries near a fully charged state, voltage in the field circuit of the voltage regulator rises. At 17 volts, magnetism created by the regulator winding insufficient to pull the armature down. This opens the contacts of the normal control circuit, causing the current

to flow through the resistor in the field circuit and forcing the generator to operate at a minimum output. 24-13. The defrost thermostat is energized when the defrost switch is pressed and completes the circuit to the defrost holding relay. This permits an increase in the evaporator coil temperature to 50° F. This temperature increase causes a bimetallic disk in the thermostat to snap to a reverse position. The thermostat switch contacts open and deenergize the defrost holding relay. This last action returns the refrigeration unit to its normal cooling cycle. 24-14. Operational Check. All of the controls which we have been discussing operate automatically, except the starter switch, the heat-cool switch, and the defrost switch. Also, the gasoline gauge registers continually when the starter switch is on. 24-15. Blast Chilling. The technique known as blast chilling saves time, investment, and fuel. In blast chilling, liquid carbon dioxide or nitrogen is injected into mechanically refrigerated trucks, resulting in quick cooling. Paragraphs in this section identified by an asterisk (*) are in part reprinted from January 1964 Refrigeration Service and Contracting by permission of Nickerson and Collins Company. 24-16. Do not confuse this type of cooling with total truck refrigeration by liquid CO2 throughout the run. We will discuss total liquid CO2 cooling later in this chapter. Blast chilling is used at maximum cooling demands. Such a demand occurs, for example, after loading has been completed. 24-17. Blast chilling is also an excellent means of auxiliary refrigeration in transit, especially after partial unloading or extended periods of parking (meal times, vehicle servicing, etc.). In addition, it can supplement mechanical cooling if the mechanical refrigerating system should fail. *24-18. Let us compare blast chilling with mechanical cooling in regard to initial temperature pulldown. Thus, while some tests made by some users and reported in trade publications have shown that blast chilling with liquid CO2, started at +40° F., drops trailer temperature to -40° F. in 3 minutes, mechanical cooling required 12 hours to cool the same trailer from +40° F. to -10° F. *24-19. There are two supplemental benefits when blast chilling is used. First, the refrigerants (carbon dioxide and nitrogen) are inert gases, which will not harm most cargoes but will in many cases benefit the cargo by blanketing it against contact with the oxygen and moisture of the air. Secondly, the quick temperature reduction minimizes initial thawing and cuts down or eliminates refreezing. Blast chilling cools the entire

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trailer quickly, including the "hot spots" near the doors. *24-20. The equipment for blast chilling is very simple. It consists of CO2 tanks, a liquid control valve, flexible hose, and a nozzle arrangement. A manifold is necessary if a bank of tanks is used. *24-21. Safety measures. Adequate vents or openings must be provided to relieve any buildup in pressure and facilitate the complete displacement of warm air with cold CO2 vapor. During the first half minute or so of blast chilling, we recommend that you leave a door partially open. Special clothing should be worn during blast chilling. This may include gloves, coveralls, face shield, etc. You must be conscious at all times that you are working with liquids at very low temperatures and substantially high pressures. Blast chilling fills the entire cargo space with refrigerant gas, CO2 or gaseous nitrogen, and, at the same time, decreases the oxygen concentration to a point where the atmosphere is no longer safe for breathing. During this operation, no one may be present in the space. Furthermore, the compartment must be vented before anyone enters it after the operation is completed. *24-22. Dry ice versus liquid. The use of liquid CO2 (dry liquid nitrogen) for blast chilling must be compared to the more conventional use of solid CO2 (dry ice). One pound of liquid CO2 does produce less refrigerating effect (B.t.u./lb.) than a pound of dry ice, but several factors make the liquid a more suitable refrigerant for blast chilling. These are: (1) the immediate vaporization of liquid CO2 can pull the temperature down much faster than dry ice can; (2) the injection of liquid CO2 can be controlled automatically and at much higher rates than are feasible with dry ice; and (3) liquid CO2 is easily stored and is ready for use at any time. Also, (4) dry ice requires delivery shortly before it is to be used and cannot be stored (for practical purposes) for long periods of time. Finally, (5) and (6) liquid CO2 can be handled more easily and at a lower cost than dry ice can. 24-23. Complete Liquid CO2 Truck Refrigeration. The use of liquid CO2 for truck refrigeration has been developed recently. Carriers have long desired a refrigeration system that would require less maintenance and fewer breakdowns than is usual with mechanical systems. Tests have shown that a liquid CO2 system is practical. It has only one moving part, which is the control valve used to turn the system on or off. A pressure reduction valve is also necessary for safe operation of the system. Besides the ducts and nozzles, there are the storage tanks, which present the main drawback of the system. These tanks are very heavy,

have a charge which is limited by the size of the tank, and need special equipment for recharging. 24-24. For continuous cooling, the control valve would be operated by a thermostat. For blast chilling, the same system would serve by operating the control valve manually to attain the desired cooling. The same precautions and safety rules for blast chilling must also be observed with continuous cooling. Remember, when either liquid carbon dioxide or dry ice is used, the refrigerated area becomes dangerous to life because of the displacement of oxygen. 25. Automotive Air Conditioning 25-1. There are various types of automotive airconditioning installations. Among these are the dash, trunk, dash-and-roof, and dash- and trunk-mounted units. The basic components in each of these installations remains the same as those for the reach-in and walk-in refrigerator. Paragraphs in this section which are marked by an asterisk (*) are reprinted in whole or in part from the Mark IV Service Manual by courtesy of John E. Mitchell Company, Dallas, Texas. This source is used so that specific information can be given on the latest components and procedures. Let us first consider the refrigerant and oils recommended for Mark IV units. *25-2. Refrigerant. R-12--clean, dry and free from contamination-is the only fluid to be used in Mark IV units. It is nontoxic, noncorrosive, nonflammable, nonexplosive and odorless under ordinary usage. CAUTION: There is one exception to the above: R-12 released in the presence of an open flame will form phosgene gas, a lung irritant. Although it is a safe refrigerant, certain precautions must be observed when handling it or when servicing any unit in which it is used. At normal atmospheric pressure it will, in a liquid state, evaporate so quickly that anything it contacts will freeze. *25-3. Refrigerant Oil. The Yolk or Tecumseh compressor may be used in a Mark IV unit. Either of two types and grades of compressor oil may be used with York compressors: Suniso No. 5 or Texaco "Capella E." The Tecumseh model HA compressor uses a special dual inhibited oil: Suniso 3 G Dual inhibited or Texaco Capella B inhibited. These oils have been highly refined and sealed against moisture contamination. • DO NOT transfer to any other container for use or storage. • DO NOT at any time, other than when pouring, allow the oil container to remain uncapped or loosely capped because moisture from the surrounding atmosphere will be absorbed.

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It is especially important to use only recommended oils in a compressor during the warranty period. Use of other oils will void the warranty in the event of failure. *25-4. Safety precautions. The following list of safety precautions is intended for you, the automobile airconditioner serviceman. Observance of these points may avoid personal injury to you, damage to your equipment and to your customer's car-as well as possible lost manhours. a. Never remove the automobile radiator pressure cap when the engine is hot. b. Never close the compressor discharge valve with tie unit in operation. c. Keep your hands clear of the automobile engine fan and belts when the engine is running. This should also be considered when opening and closing the compressor service valves. d. Be sure gauge manifold hoses are in good condition. Never let them come in contact with the engine fan or exhaust manifold. e. Make sure refrigerant hoses are clamped so that they cannot come in contact with any sharp metal or with the exhaust pipe or manifold. f. Always wear goggles when opening the refrigeration system. Refrigerant liquid or gas can permanently damage the eyes. (See paragraph 25-5 for first aid treatment.) g. Never apply heat from a torch to a sealed refrigeration system. Refrigerant will expand rapidly with heat and could cause an explosion. h. Refrigerant 12 in the presence of an open flame produces phosgene gas. This is toxic. Never breathe it. i. Do not use refrigerants other than R-12. j. Extreme care should be taken never to use methyl chloride refrigerants, because a chemical reaction between methyl chloride and the aluminum pans of the system will result in the formation of product which burn spontaneously on exposure to air or decompose with violence in the presence of moisture. k. Be sure all engine capscrews are tight and are of the correct length for their particular application. Pulleys coming off at high speed can cause costly damage to the automobile and possible injury to the occupants. l. Wear goggles when using a hole saw or portable jig saw. This is cheap insurance for eye protection. m. Use extreme caution when drilling holes in the automobile. Holes drilled into the electrical wiring or into the gasoline tank can cause fire or explosion. n. Do not run the automobile engine in an area not well ventilated. Carbon monoxide displaces oxygen.

o. Keep hands away from moving evaporator fans and blower wheels. High-speed motors have enough power to cause painful injury. p. Use caution when working around exposed evaporator coil fins. Painful lacerations can be inflicted by the fins. q. Do not run the automobile engine with automatic transmission fluid lines disconnected or costly damage to the transmission may result. 25-5. First Aid. The skin and eyes should also be protected from contact with R-12 liquid or vapor. Since R-12 is readily absorbed by most oils, a small bottle of sterile mineral oil and a small quantity of boric acid should be located near the service stall. Should R-12 contact the eyes, wash them immediately with a few drops of mineral oil, followed by a thorough cleansing with a weak solution of boric acid. See a physician if irritation continues. FOR YOUR OWN PROTECTION, WEAR GOGGLES when opening the refrigeration system. 25-6. Components. As you know, the parts of an auto air conditioner do the same things as the parts in a refrigerator. However, because of location and environment, the installation and operation of some parts are quite different in an auto. 25-7. Condenser. The condenser is mounted ahead of the radiator for the car's engine. For this reason the engine tends to run hot, particularly at low speeds. This may be compensated for by using a larger fan or one with more blades. When air conditioning is added to a car, it may also be necessary to change the standard radiator and install a larger size to keep the engine from overheating. The addition of an air conditioner imposes these three new factors on the car's operation: (1) the condenser reduces the volume of air to the radiator and the engine compartment, (2) the compressor requires the engine to produce more horsepower while it is engaged, and (3) the heat output from the engine is more for any given speed. The use of ethylene glycol in the car's radiator will help to get rid of heat from the engine faster. Also, the engine cooling system is pressurized to increase cooling. The radiator cap is spring-loaded to retain a certain operating pressure, usually from 4 to 16 pounds, in the system. Failure of the cap to maintain cooling system pressure will result in the engine's overheating. 25-8. Evaporator. Most units are made for mounting under the dash or on the firewall. However, some units have been mounted in the trunk or in back of the rear seat The disadvantage of the rear mounting is the long runs of tubing required to connect the unit. On the other hand, a

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mounting under the dash makes the lines much shorter but fills the center space between the dash and the floor. In any case, copper lines are not satisfactory for automotive work, because vibration over a long period of time will cause copper to harden and become brittle. Consequently, refrigerant lines for an automobile airconditioning installation are made from high-pressure neoprene hose. 25-9. Receiver, drier, and strainer. On most automobile air-conditioning systems, the receiver, drier, and strainer a- combined into one compact unit which is installed in the liquid line. The function of these components is similar to those previously mentioned. A sight glass is the means of observing refrigerant leaving the condenser. Under normal operating conditions, a fully charged system will deliver a solid stream of liquid refrigerant to the liquid line leading to the expansion device. A clear sight glass usually indicates a fully charged system, unless the system is completely discharged. Bubbles in the sight glass are an indication of a partially charged system. Some units also have a moisture indicator in back of the sight glass. The moisture indicator should be blue if moisture is not present in the system. Moisture in the system would cause the blue indicator to turn pink. Do not confuse this type of indicator with those which you may find in large refrigerator systems. (For example, one indicator is green but turns bright yellow with moisture, while others show different color combinations.) *25-10. Expansion valve. Control of the liquid refrigerant entering the evaporator coil is done by a thermostatic expansion valve. The power clement or thermobulb and connecting tube will be charged with either a liquid or gaseous refrigerant, usually of the type used in the air-conditioning system. The power element is connected to the open area above the diaphragm by means of a small capillary tube. The lower side of the diaphragm actuates a ball check valve by means of push rods. Thus movement of the diaphragm provide control of the valve inlet opening. The power element clamped to the evaporator coil outlet responds to the suction as temperature. An increase of this temperature increases the pressure and temperature of the refrigerant in the power element. Conversely, a decrease of suction gas temperature decreases the pressure and temperature of the power element refrigerant. It can be seen from this that increased suction gas temperature causes the expansion valve to open, admitting more refrigerant, while a decrease in suction gas temperature causes the expansion valve to move toward the closed position. *25-11. RoboTrol valve. The RoboTrol valve replaces the SelecTrol valve which was used in older models of Mark IV units. The RoboTrol valve is, like the SelecTrol, a

suction line flow control valve; but, unlike the SelecTrol, its operation is entirely automatic with no provision for manual temperature control. The function of the RoboTrol is to control evaporator pressure and volume flow to the compressor to provide the coldest possible consistent air temperature without allowing condensate to freeze on the evaporator coil fins. *25-12. Refer to figure 46, which is a cross-sectional view of the RoboTrol revealing its internal construction. Actually the valve is quite simple, operating through. a balance between a coil spring (6) and a sealed metal bellows (7) which is subjected to suction line pressure and flow pressure drop at the valve. Increased pressure collapses the bellows against the spring, thus moving the conical valve head away from its seat. Reduced suction line pressure allows the spring to overcome the bellows action, thus pushing the valve head back toward its seat.

Figure 46. RoboTrol valve.

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*25-13. Spring tension is regulated at the factory to close the valve when suction line pressure indicates a temperature in the evaporator coil that would cause freezing of condensate on the fins. An adjusting screw (1) is provided for changing spring tension in the field if specific humidity conditions require a different setting from that made at the factory. Turning the adjusting screw clockwise increases spring tension to provide a higher pressure setting. This may be necessary in areas of extreme humidity where ice can form on the coil rapidly. *25-14. It should be understood that adjustment of the RoboTrol must not be made unnecessarily. Many miles of highway driving may be required to prove an inadequate valve setting. A low setting will be satisfactory at city driving speeds where ice has little chance of forming but will be very unsatisfactory at road speeds when refrigeration capacity is in surplus. A high setting may rob the unit of efficiency at all speeds. *25-15. Correct setting of the RoboTrol will maintain about 26 p.s.i.g. in the evaporator coil. Because the valve is connected directly to the compressor and suctionpressure readings are normally taken at the compressor, pressure drop in the suction line must be considered. For normal operation in all areas the valve should be set to close at 17 p.s.i.g. and to open at 19 p.s.i.g. pressure with readings taken upstream from the valve. *25-16. Other than adjustments described above, no service to the valve can be carried out. A leaking valve must be replaced. When installing a RoboTrol, use the backup flats provided on the inlet side fitting to avoid strain and possible distortion of the outlet connection at the compressor. This distortion will cause icing of the evaporator regardless of the adjusting spring setting. *25-17. Thermostatic controls. Three types of thermostats have been used in Mark IV units, beginning with the Commuter and Sportsman Evaporators in 1961. These units used a bimetal type thermostat responsive to discharge air temperature. The thermostat as applied in the 1962-63 Monitor has a control lever and pin. When replacing this lever-actuated thermostat, move the control lever to the extreme right end of the slot. Position the thermostat so that the lever and pin lightly touch one another. Tighten the mounting screws in this position. Low temperature cut off is 28° F. *25-18. Electrical system. All wiring is stranded copper with plastic covering. Most 12-volt units will have No. 16 wire, while No. 12 wire is furnished for 6-volt units. The Sportsman unit uses No. 14 wire for either voltage. *25-19. Fuses. Prior to the 1962 Monitor, all units provided for a fuse or fuses between the automobile

ignition switch accessory terminal and the evaporator switch. Fuse requirements are a 20-amp fuse for 14-volt circuits and a 30-amp fuse for 6-volt circuits. Instead of a conventional fuse, the 1962-65 Monitor has a 15-amp circuit breaker, mounted at the rear of the evaporator. The circuit breaker has a current rating of 15 amps at 20 percent overload for 30 minutes. *25-20. Motors. Three motor types have been used since 1958 with Mark IV units. While differing in size and number as well as length of shaft extension, all are series wound D.C. Both 6- and 12-volt applications are used. No service to the motors should be required. Bearings are porous bronze with oil saturated wicks and are designed to last the lifetime of the automobile without additional lubrication. Occasionally a motor may chatter audibly, especially on rough roads or when the automobile is driven rapidly around a corner. This can usually be attributed to excessive armature end play. Correct end play, when the motor shaft is moved by hand, should be about 1/64 inch. Additional spacing washers placed on the armature shaft inside the motor will reduce end play. The motor housings must be separated for installation of these washes. 25-21. Magnetic clutch. A magnetic clutch has a field coil which is stationary in one type. Another type has a field coil which rotates, and this type requires brushes and two collector rings to supply electricity to the coil. The rotating field has the possibility of trouble from poor contact between the brushes and rings. The stationary coil is not subject to such trouble and is therefore considered to be more reliable. *25-22. Operation of the magnetic clutch is very simple. When the current to the clutch is off, the rotor pulley idles free on the clutch bearing. The compressor shaft does not rotate. When current flows to the field of the clutch, the rotor-pulley and armature (attached to the compressor shaft) are "locked" together magnetically. The compressor shaft rotates and refrigeration is provided. Note the following instructions for refrigerant lines which will conclude our discussion of components. *25-23. Refrigerant lines. Always use grommets where rubber refrigerant lines pass through the radiator yoke, firewall, or trunk compartment floor. All holes for refrigerant lines should be cut with a hole saw of the proper size, as indicated by the instruction sheet. Be sure that rubber lines are not against the exhaust manifold or any sharp metal edges and that they are clamped properly in enough places to keep them from sagging under the car. Clamps should be

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attached to the car with No. 10 sheet metal screws. Use a 1/8-inch drill for these screws. 25-24. Control of Refrigeration. There are three methods to control the cooling. Some units may be more sophisticated than others, combining two of these methods. Here is a brief discussion of three types of control. 25-25. Thermostat. A thermostat which operates a switch is used on some units. The feeler bulb is located in the airstream next to the evaporator. The switch closes the circuit to the magnetic clutch when cooling is called for. With this type of control the compressor will only operate when cooling is demanded. 25-26. Pressure-operated bypass. This method of control uses a bypass valve which is operated by pressure on the low side of the system. A diaphragm in the valve controls the bypass by responding to changes in pressure on the low side. As the temperature in the car become lower, the pressure in the low side reduces, and this reduced pressure on the diaphragm causes it to open the bypass so that refrigerant no longer flow to the evaporator. Operation of the valve is adjusted by a linkage which change spring pressure on the diaphragm. 25-27. Solenoid-operated bypass. The solenoid valve is controlled by a thermostat set in the air-stream from the evaporator. The valve opens the bypass line when the thermostat senses that the temperature in the car is cold enough. When the air becomes warm enough, the thermostat will cause the solenoid valve to close the bypass line, and the unit will again operate to cool the car. 25-28. Servicing and Adjusting. In this area we will present service information which can be applied to most installations. The owner's service manual is required when it is necessary to make exact adjustments. It is not practical to adjust valves without the specification, because you can do more harm than good. Let us begin with the drive pulley. *25-29. Crankshaft drive pulley. The seating and centering surfaces, both on the air-conditioner crankshaft pulley and the original pulley hub, or balancer to which t is to be attached, must be wiped free of all dirt and grit before installation. Any foreign material on these surfaces an prevent the pulley from seating properly, resulting in a wobble which may permanently damage the pulley and balancer or cause the V-belt to fail prematurely. This is especially true with respect to pulleys of the type secured to the crankshaft with only one retaining bolt. Where a key is employed with this type of pulley, make sure the key length is correct to just fill the key-way without causing any pulley wobble. File

or mind the key to length if necessary. To check pulleys which appear to be out of line, hold a straightedge against the faces. *25-30. Magnetic clutch removal. In most units you can remove the clutch as follows: Remove the center bolt and washer from the crankshaft. Screw a 5/8-inch NC (National Coarse) capscrew into the threads in the end of the clutch hub and tighten it against the crankshaft until the clutch comes free. Use a centering disc when you reinstall a clutch. If no centering disc is available, check the edge of the clutch against a fixed point on the compressor while the clutch is slowly turned. No runout will be observed when the clutch is properly centered. 25-31. Belts. While exact specifications are given for some belts, you will seldom have the equipment to make exact adjustments. The general rule calls for 1/2-inch belt deflection between pulleys. If a belt shows rapid wear, check the pulleys for dents or scratches in the grooves. A damaged groove will tear up a new belt. If the belt shows signs of the cord separating from the rubber, it indicates the belt has been stretched in attempting to force it over the pulleys. This is the main cause of ruined belts and pulleys. The fan pulley is designed to have at least 3/8-inch clearance between the fan and the radiator. Make a check for the cause if there is much departure from the correct clearance. *25-32. Oil. Occasionally, after having been in service some length of time, some units may show a grayish discoloration of the refrigerant and oil. This can be observed through the sight glass which may become coated on the inside until it is opaque. This condition is caused by moisture contamination and should be rectified immediately. Usually, replacement of the drier is sufficient, but in cases of extreme coating the expansion valve should be removed and cleaned out manually. Then replace the valve along with a new drier. Check the compressor oil for severe discoloration. Drain and refill with clean, dry refrigeration grade of oil if required. *25-33. Expansion valve. Perhaps the most important thing to remember here is to make sure the thermobulb is good tight metal-to-metal contact with the copper suction line. When replacing the expansion valve, sand the bulb and suction tube mating surfaces. Then tighten the bulb clamp securely. To avoid twisting the copper tubing, always use a backup wrench on the valve when loosening or tightening connections. *25-34. Inadequate compressor oil, aside from causing possible damage to the compressor, will result in improper lubrication of the valve needle. Lack of oil at the needle and seat materially affects the liquid seal, resulting in excessive wear. *25-35. Refrigerant lines. All hose assemblies

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are manufactured to rigid specifications. Each length of hose is thoroughly cleaned and dried before being cut to length for installation of couplings. When servicing, use clean refrigeration oil on all fittings-nothing else. The use of refrigeration oil on all fittings will aid in making leakproof connections. DO NOT USE SEALANT COMPOUNDS. If these compounds (blue or red in color) are introduced into the system they will clog strainers. The result will be complete failure or lowered efficiency and a voided warranty. *25-36. Lines must be clamped to prevent their contact with exhaust manifolds, carburetors, linkage, etc. Be sure grommets are installed to protect hoses where they pass through metal partitions. *25-37. Switches and rheostats. Defective switches should be replaced. Attempted repairs are seldom satisfactory. Intermittent unit operation may be caused by a defective rheostat. Rheostat switches with built-in clutch circuits have, on rare occasions, been known to cause intermittent cooling. This could result from an uneven bedding of the resistor coil in the ceramic switch base. The sliding contact shoe, being moved along the resistor coil as the switch knob is turned, may ride up on a section of the coil that is sufficiently high to lift the shoe almost clear of the clutch circuit ring. The resulting poor contact may cause the clutch to slip or cut out. If a condition of this sort is suspected, check the clutch circuit with a voltmeter while turning the rheostat knob slowly back and forth. If the voltage varies sharply, replace the rheostat switch. *25-38. Evacuation with a vacuum pump. The following procedure is given for Mark IV units. However, they may be generally applied to most auto air conditioners when it is necessary to remove air or moisture from a system. a. Remove protective caps from gauge ports of compressor service valves. Connect gauge manifold hoses to appropriate compressor service valves. b. Schrader gauge line adapters are required for all 1964 compressors. c. Connect gauge manifold center hose to refrigerant container. OPEN refrigerant container valve. (Use only refrigerant grade R-12.) d. Crack open high-pressure gauge manifold valve and allow refrigerant vapor to enter system until a pressure of 50 p.s.i.g. is observed on low-pressure gauge. CLOSE high-pressure gauge manifold valve. CLOSE refrigerant container valve and disconnect hose from container. e. Using a leak detector, thoroughly check all connections, the compressor, evaporator, condenser, and service valve operating stems or Schrader fittings with protective caps in place. Repair any leaks at this time.

f. Connect gauge manifold center hose to vacuum pump. OPEN both gauge manifold valves and start vacuum pump. g. After vacuum pump has run at least 15 minutes, CLOSE both gauge manifold valves and stop vacuum pump. Low-pressure gauge should indicate at least 28inch vacuum. High-pressure gauge should read zero (0) p.s.i.g. or below. h. Disconnect gauge manifold center hose at vacuum pump and connect to refrigerant container. OPEN refrigerant container valve. Loosen gauge manifold center hose at gauge manifold. Refrigerant released will purge air from hose. Tighten center hose connection at gauge manifold. i. Crack open high-pressure gauge manifold valve and allow refrigerant vapor to enter system until a pressure of 0 to 5 p.s.i.g. is observed on low-pressure gauge. CLOSE high-pressure gauge manifold valve. CLOSE refrigerant container valve and disconnect hose from container. j. Repeat steps f. and g. This will complete double evacuation procedure necessary for thorough moisture and air removal. k. Disconnect gauge manifold center hose at vacuum pump. l. Connect portable charging cylinder filled with R12 to the center gauge manifold hose. Open charging cylinder valve and purge center hose. Open both gauge manifold hoses and admit 34 ounces of R-12 for the monitor. OR An alternate method of charging the system involves the use of cans or factory filled drums of refrigerant. Connect the container(s) to the center gauge manifold hose. Purge hose and admit refrigerant until the system is at container pressure. Do not invert the container. m. CLOSE charging container valve and both manifold valves. n. Start engine and set idle to approximately 1500 rpm. If shop temperature is 90° or above, place a fan in front of radiator to simulate ram airflow. o. With temperature control knob or lever turned to maximum cold position, allow unit to operate for 2 minutes with blower(s) on. (Monitor evaporator only must have a jumper wire connector from a 12-volt source to the clutch.) Observe the sight glass located in top of receiver-drier or in expansion valve body. If bubbles appear, OPEN low-pressure gauge manifold valve and container valve. Add charge until bubbles disappear. p. CLOSE low-pressure gauge manifold valve

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and turn blower(s) off. Bubbles should not a pear in sight glass until low-pressure gauge reading reaches 12 p.s.i.g. At low-pressure gauge reading of 8 p.s.i.g., it is normal for bubbles to appear in sight glass. If bubbles do not appear between 12 and 8 p.s.i.g., disconnect gauge manifold center hose from container and purge a small amount of refrigerant from system. After purging refrigerant from system repeat this step until bubbles appear within a low-pressure range of 12 to 8 p.s.i.g. q. Turn blower(s) on. When low-pressure gauge reading indicates 25 to 30 p.s.i.g., sight glass should not show bubbles. Turn blower(s) off. Observe sight glass; bubbles should not appear at low-pressure gauge reading of 12 p.s.i.g. or above. Make at least two complete cycles of this step. If bubbles appear above 12 p.s.i.g., add refrigerant as described in steps o. and p. r. Close container valve and disconnect gauge manifold hose from contains. Remove clutch jumper wire. s. Place a thermometer inside of discharge or cold air outlet. Turn blowers on full. Run unit 10 to 15 minutes. Thermometer should read 50° dry bulb or below, with a return air temperature of 80° dry bulb or below. t. If equipped with conventional service valves, BACKSEAT both valve operating systems and open gauge manifold valve to purge charging hoses. If equipped with Schrader type service valve, slowly loosen charging hoses at valve to purge. Replace all protective caps on compressor service valves. *25-39. If a unit has lost its charge, follow the above procedure to recharge and BE SURE TO LEAK TEST THOROUGHLY. REVIEW EXERCISES The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the test. Do not submit your answers for grading. 1. Why do some trucks and semitrailers require refrigeration? (24-1)

3. Which source of power (see question 2) has a disadvantage and why? (24-3, 4)

4. What is the relationship between the governed speed of the engine and the speed at which it drives the compressor? (24-7)

5. Explain the two methods of supplying heat to the storage area in very cold weather. (24-8)

6. Is the gasoline-engine-operated refrigeration unit on trucks automatic? Explain. (24-9-13)

7. Explain the use of liquid for blast chilling a trailer. (24-15-20)

8. Are there dangers as well as advantages to blast chilling? Explain. (24-15-21)

9. How does the use of dry ice compare to liquid CO2 for cooling a trailer? (24-22)

10. What would be the advantages of liquid CO2 refrigeration of a trailer as compared with those of mechanical refrigeration? (24-23)

2. What are the two prime sources of power which may be used to drive a compressor for a refrigerated truck unit? (24-3, 4) 93

11. Why should you use only the manufacturer’s specified grade of refrigerant oil in a compressor for an automotive air conditioner? (25-13)

12. Give two conditions which require you to wear goggles when working on an auto air conditioner. (25-4)

19. What should always be installed where refrigerant lines pass through a metal wall? (2523)

13. How can a pressure radiator cap cause an engine to overheat? (25-7)

20. How could failure to clean the crankshaft or pulley hub result in wobble of the pulley? (2529)

14. Why are copper lines unsatisfactory in an automobile? (25-8)

21. Describe how a magnetic clutch with a threaded hub can be removed without resorting to a puller. (25-30)

15. Where would you look for a sight glass in an auto air conditioner? (25-9) 22. What are two causes of belt failure? (25-31) 16. Compare the operation of an expansion valve with the RoboTrol valve. (25-10-12) 23. The thermobulb should be checked for what condition if you suspect improper operation of an expansion valve? (25-32) 17. What are the two means of protecting the electrical system of an automobile air conditioner? (25-19) 24. Why must sealant compounds never be used on fittings? (25-35) 18. What two types of fields may be used in a magnetic clutch? (25-21) 25. To insure a clean system, what is the most important operation to perform when you connect lines for charging? (25-38)

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Answers to Review Exercises
CHAPTER 1 18. 1. 2. 3. 4. A modern refrigerator is constructed of two metal shells separated by a layer of insulation. (1-2) The insulation in a refrigerator must reduce heat transfer by convention, conduction, and radiation. (1-3) The greatest heat load is usually from the heat outside the box. (1-4) 20. Improved insulating material has resulted in molded insulation which is very effective and yet takes much less space than older types. (1-5) A moisture or vapor barrier must be used to seal the insulation. (1-6) (1) New synthetic materials can be molded to fit. (2) They have such a low K-factor that only half the space is needed as for some natural products. (3) The synthetics are more resistant to rot. (4) They have no food value to attract rodents. (1-6) Breaker strips are often brittle and may be broken or kinked. Consequently, you should know the proper way to remove each type to prevent damage to it from forcing. Also, carelessness may break the wiring or heaters in the stile or mullion. (1-8,9) The latch of such a refrigerator should be removed so that the door cannot be locked. (1-11) The seal of a door gasket is checked with a sheet of thin paper for uniform drag. (1-12) The refrigerator should not be placed near an oven or heater and should have its own branch circuit, where possible. (1-13) Refrigerators for use overseas will have a special notice (usually posted in a conspicuous place inside the box) stating the voltage and frequency of the current for which each is designed. (1-14) One thermostat senses when ice is made and starts the harvest cycle. The other thermostat senses when the storage tray is full and holds off the harvest cycle. (1-18) Automatic defrosting with an electric heater can be completed so quickly that the melted water would freeze in the drain pipe if a second heater were not used to warm the drain. (1-20) Automatic defrosting with hot gas can be done with a solenoid valve, which allows hot gas from the compressor to pass directly through the evaporator. (1-22) Electricity supplies the heat for some units made in Europe but absorption systems in America are made for LP or natural gas. (2-1) The main distinction regarding fuels is that the burner orifice used with LP gas is smaller, because LP gas has much more heat value. (2-2) The absorption cycle is based on the principle that water has a strong affinity for ammonia. (2-5) 21. 22. 23. 19. Changes in heat load are reflected by a thermostat in the freezer compartment which regulates a valve to vary the size of the flame. (2-5) If the heater should be dislodged from the flame during cleaning, the pushbutton would not reset the poppet. To correct this, move the heater back into the flame, and the reset will hold. (2-5) An absorption system must be kept clean. Dust and soot must be removed periodically from all heat exchangers, and the flame must be properly adjusted for maximum heat and minimum carbon. (2-6) If installation is made so that the unit is not level, the system will not perform properly. (2-7) If the fault is in the ammonia-water cycle, it may be corrected by turning the unit upside down for about an hour. (2-8) Clearances in a compressor may be as little as 0.0001 inch, because it runs in a closed environment (no moisture, no acids) with a relatively narrow temperature variation. (3-3) A piston may approach the head as close as possible without touching. Clearance may be only 0.01 inch at top dead center. (3-4) Compressor valves may get noisy when their maximum lift is greater than 0.10 inch. (3-5) Rotary compressors have fewer moving parts and produce less vibration than piston types. (3-6) To do this, part of a condenser coil may be placed so as to evaporate the water collected from defrosting. (3-9) A restrictor placed between two evaporator sections forces the first coil to operate at a higher pressure and temperature than the second. (3-10) Because a weighted valve is sensitive to its position, any departure from the correct mounting angle will cause improper operation. (3-11) The critical factors in the makeup of a capillary tube are its internal diameter, its length, and the length of the heat exchanger portion. (3-16) A bleeder resistor is connected in parallel with a capacitor to help absorb the discharge of the capacitor when the relay contact open. This arrangement prevents burning of the relay contact. (3-19) A hot wire relay opens the circuit to the starting winding after the motor is started and provides overload protection if the motor draws too much current. (3-19) A current relay is sensitive to current and is designed to release when the current in the relay falls below a certain point. (3-21) You would first check for an open circuit at the bleeder resistor when you have found relay contacts badly burned.

5. 6.

7.

24.

8. 9. 10. 11.

25. 26. 27. 28.

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33. 34.

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35. 36.

Bubbling noise or hissing from a capillary tube is usually an indication of low refrigerant. (3-32) Low voltage will cause a motor to run slow so that a compressor might have to run continuously to cool a refrigerator, resulting in high electric bills. (3-33) A freezer may have frost accumulations scraped off with a wooden paddle or with a stiff fiver brush. However, ice should never be chipped off; it should be melted with warm water. (43) For a freezer door to become frosted shut, the electric heater strip would have to be out of operation. (4-4) A mistake made by many servicemen when troubleshooting is to pass over one of the more common faults because it seems too obvious or too easy. (5-3) The advantage of placing the overload protector inside the shell is to extend the off time in case of an overload operation which keeps the unit from short-cycling. (5-5)

53. 54. 55.

Regulator screws must be released before valves are opened, to avoid damaging the regulators and the gauges. (6-3) The most important factor in making a leakproof solder joint in tubing is to have correct clearance between the parts. (6-5) You should heat the work to flow point of the alloy before applying it to the joint. (6-8) Valves with neoprene seats must have them removed before you begin any soldering; otherwise, the heat will destroy the valve seat, and it will leak. (6-9) Flux changes its appearance with temperature. At 600° F. it may appear puffy, and it will smooth out with a milky color at 800° F., while at 1100° F. it will turn clear. (6-10) In silver brazing, the copper is not heated to as high a temperature as it is in copper welding; thus the copper would not tend to absorb carbon monoxide from a carburizing flame. (6-11) The reason is that the oxidizing flame is used to prevent formation of carbon monoxide which copper would absorb, forming a porous weld. (6-12) Copper conducts heat away faster than steel; thus the welding of copper requires a larger tip for the torch, which will give a larger flame. (6-13) In stainless steel, the cut is covered with a length of welding rod. When heated the welding rod will burn, supplying the added heat necessary to melt out the cut. (6-15) A line tap is expensive and can only be used once. Also, as the gasket hardens, it will start to leak; then the leak will have to be repaired. (7-1) In a contaminated atmosphere a leak detector may be so sensitive that results continue to be erratic even after adjustments have been made for the background. (7-3, 5) If the probe is exposed to a concentration of halogens, the electronic leak detector will be overloaded and may be damaged. (7-5) Small holes in the low side of a refrigerator or freezer can be patched with a cold solder or glue made for refrigeration work. (7-7) Before cold patching a hole, the surface must be absolutely free of oil so that the patch will bond to the metal and make a complete seal. Do not pack the material or force it into the tubing, where it would form an obstruction. (7-7) Flux has a critical temperature. If a high-temperature flux is used with a lower temperature solder, the solder will flow easily long before the flux. In contrast, the right flux will flow at about the same temperature as the solder (7-8) Cold solders or special glues are limited to systems charged with R-12 and should be used for patching only in the low side. (7-9) A system can be pressurized with dry nitrogen and leak tested with soapy water. If the system is partially charged and then pressurized with nitrogen, a halide leak detector can be used. (712) These are that a capillary tube should have the same length and diameter as the one which it re-

37.

56.

38. 39.

57.

58.

40.

59. 41. A thermostat closes its contacts when temperature rises, while a freezestat opens its contacts when temperature drops below its operating point. (5-6) 60. 42. Checking a motor circuit with direct current is better where there is a capacitor, because alternating current passes through a capacitor easily and may lead to a false conclusion. (5-9) 61. 43. 44. 45. 46. When a test shows that a motor is drawing current equal to its LRA rating, it indicates that the rotor is locked. (5-11) A capacitor may be checked either (1) by charging and discharging it or (2) by measuring the current through it. (5-12) The current relay is current sensitive, and its contact first close and then open in normal operation. (5-15, also 3-21) The potential relay is voltage sensitive, and its contacts are normally closed. The contacts should be open when the motor is running normal. (5-16, also 3-24) Some of the causes of vibration in a refrigerator are loose motor or tubing mounts, failure to remove shipping bolts, and uneven floor or refrigerator feet. (5-17, Table 2) An acetylene cylinder must be secured upright because: (1) It must no be allowed to fall. (2) If the safety plugs blow, they will pass harmlessly into the floor. (3) In any but an upright position, the material in the cylinder may become dislodged and foul the gauges and valves. (6-2) For two reasons: (1) The safety plug is at the top of the cylinder, and if it blows in an upright position the plug will be blown through the roof. (2) The tank will vent itself harmlessly if upright, but if lying flat, it will be jet propelled. (6-2) Soapy water is the correct test for an acetylene leak, since a flame could cause a flareback resulting in a cylinder fire. (6-2) The reason is that even a small amount of oil or grease in contact with pure oxygen can result in spontaneous combustion or an explosion. (6-2) The red hose identifies it. The acetylene valve can only be attached to the red hose because of the left-hand threads of the connection. (6-3) 70. 62.

63.

64.

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48.

66.

49.

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50. 51.

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69.

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places. Also, the length soldered to the suction line should be the same as that of the original heat exchanger. (7-13) 71. Gauge a wire to be sure that it is slightly smaller than the ID of the capillary tube. If it is the correct size, it should slip inside the capillary easily without forcing. (7-14)

9.

Aside from mechanical troubles, the water supply is the biggest source of trouble, because it produces sediments, scale, and salt crystals, which affect both metals and nonmetals adversely. (10-12) Where multiple evaporators are used, a heat exchanger is necessary to insure that the line will deliver liquid refrigerant to all of the expansion valves. (11-1) A dry type heat exchanger must be correctly sized so that it can cool the liquids sufficiently without producing too much pressure drop in the low side. (11-2) The water pump supplies a high-velocity jet of cold water so that it will absorb CO2. Water pressure must exceed the CO2 pressure which charges the tank to 80 pounds. (11-3) If the ground connection is broken at the isolating transformer, the water pump motor would run continuously until stopped by operation of an overload device. (11-4) A simple test is to connect a jumper wire from the tank to the ground side of the isolating transformer. When the ground circuit is completed, the motor should stop if the tank is full. (11-4) The use of aromatic woods, which will spoil the flavor of foods, could do this. This spruce and maple are used inside a cabinet, since they are hard and do not have an obnoxious odor. (12-2) On such models, not only must the system be shut down but also, where forced air is used, the fan must be turned off to prevent the blowing of water all over the cabinet. (12-3) A "double duty" display cabinet is one in which the lower compartments under the display section are also refrigerated. (12-5) The area around the door of a display case may be warmed with a heater strip to prevent the formation of frost. (12-6) The flow of cold air must be continuous across the display section of an open case because it must have a curtain of cold air in order to operate properly. (12-7) Storage cabinet defrosting methods are (1) compressor off-time, (2) hot gas, (3) hot wire, (4) hot water, and (5) secondary solution. (12-8) Compressor off-time defrosting is limited to cabinets operating above 28° F. (12-9) Reverse cycle defrosting is a hot gas method, using a four-way valve so that the evaporator becomes the condenser and the condenser becomes the evaporator. (12-11) A high-temperature control is a safety device to terminate the defrost cycle before the cabinet temperature rises too high. (1215) The capillary tube from a defrost valve supplies pressure to operate a high-pressure safety control to limit pressure in the system. (12-16) The service valves provide connection points for gauges and charging and permit isolating the compressor from the system. (13-4) The shell-and-tube condenser uses the shell to serve

10. 72. 73. At any time that a system is opened, the ends should be taped or capped to keep moisture and air out. (7-15, 17) After major replacement, the system should be leak tested, evacuated, dried, charged, and checked for proper operation. (716) The leak must be in the low part of the system at or near the compressor, because most of the oil is stored in the compressor. (7-20) If the charging line is not purged, air will be forced into the system when the charging valve is opened. (7-23) When the suction line shows frost extending too far from the evaporator, the system is overcharged, and some refrigerant should be bled from it. (7-24) When frost extends too far on the inlet line to the evaporator (after the capillary has been replace), increase the size of the heat exchanger by soldering more capillary tube to the suction line. (7-24) The refrigerator serviceman must be able to make a leakproof joint quickly and correctly so that moisture and air may be kept at a minimum. The shorter the time that a system is open, the better are your changes of purging air quickly. (7-1–24) CHAPTER 2 1. The thermostat is set so that a thin coat of ice will form before the compressor is stopped. This ice provides a cushion so that the unit will not operate for each drink which is drawn. The freezestat insures that the unit will stop before ice gets thick enough to damage the tank if the thermostat fails. (8-2) The waste water from a bubbler fountain is already cold, so it is made to pre-cool the warm water before it enters the cold tank. (8-3) 20. 3. Patching a water tank or line used for drinking purposes requires approved materials only. Certain plastics or synthetics are very poisonous. (8-5) 21. 4. When different bottled beverages are cooled in the same cabinet, the thermostat would have to be set high enough for the one kind most liable to freeze. (9-2) A warm coil could lead you to a wrong conclusion if you mistake an oil cooler for a condenser coil. (9-3) 23. 6. The big difference between ice making machines lies in the evaporator. Examples are the tray, plate, tube, channel, and cell types. (10-1) 24. 7. 8. In an ice cube machine, you may find a tube type evaporator, a cell type evaporator, or a plate type evaporator. (10-4–6) Dissolved salts concentrate in the water that is left from ice making. These salts would make unpalatable ice or could lower the freezing temperature so that the machine would not make ice properly. (10-8) 25. 22. 11.

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75. 76.

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as both condenser and receiver. The tube-within-a-tube type circulates refrigerant through the outer tube to take advantage of air cooling also. (13-7) 27. 28. This is done because the receiver must be able to hold all of the refrigerant charge when the system is pumped down. (13-8) The receiver outlet valve, sometimes called a king valve, is provided with a quill, or inlet tube, which reaches to the bottom of the receiver to insure the picking up of liquid rather than gas. (13-8) Reversing the flow of a drier-strainer might allow particles of the drier to get into the system. (13-9) A good expansion valve should have modulating action, it should not starve the evaporator, and it should not cause flooding. (13-11)

43.

Trying repeatedly to start a motor with a locked rotor can cause more damage because of excessive current in the circuit. (14-6, 8) Causes of abnormally high head pressures are restrictions caused by pinch, air, a clogged screen, a frozen expansion valve, a partly closed valve, or a think head gasket. (14-9) When a compressor continuously runs but does not cool, check for high suction pressure, which would indicate a low-side restriction; or check for bubbles in the sightglass, which would indicate a low charge. (14-10, 11) If the capillary in a thermostat had lost its charge, the bellows could not expand. Since warming of the charge expands the bellows to make the compressor run, the unit would remain idle. (14-11) A quick check for ice blocking a refrigerant control is to warm the control and watch to see whether or not the pressure gauges return to normal readings. (14-12) When a valve is supposed to be shut and it continues to leak refrigerant, it indicates that the needle and seat are worn. (1413) When you make adjustments or repairs on a float valve, be sure to restore its operating point to the original level of the fluid. (14-14) Equipment which can be isolated from the system by valves must have been purged by bleeding off pressure before it is removed from the system. (15-2) The ratchet stop in a micrometer makes it possible to exert the correct driving force on the spindle when a measurement is made. (15-5) In figure 18, the top illustration will read 0.012 inch less if the thimble is moved 12 divisions in the direction of the arrow. Thus, the reading would be 0.292 inch. (16-5) You can check the proper mating of an extension rod with an inside micrometer by measuring it with an outside micrometer. (16-5) The best tool for checking a crankshaft to see whether or not it is true is a dial micrometer. (16-5) Loss of oil pressure may be from (1) a low oil supply, (2) worn bearings, (3) a defective oil pump, (4) a defective oil pressure regulator, or (5) oil diluted with refrigerant. (16-6, 18) When installing a new oil seal, be sure to clean all grease and preservative from the seal, apply refrigerant oil to the seal, and carefully inspect all seal surfaces for scratches which would cause a leak. (16-8–10) To check a new seal for a leak, operate the compressor with the suction line closed till the vacuum gauge levels off. Then close the compressor discharge line and watch the high-pressure gauge for a rapid rise, which would indicated that air is being drawn into the compressor. (16-11) Check the depth of the valve seat for too much wear, and check the valve to see that it is not worn too thin. (16-12) Check for a slight burr or feather edge on one side of a spring steel valve. The burr could damage the sea; therefore the valve is installed with the

44.

45.

29. 30.

46.

47. 31. The automatic expansion valve works well with a water cooler because the load is uniform in a narrow temperature range and because the valve is not required to modulate. (13-12) 48. 32. 33. The equalizer line is used to compensate for pressure drop across the valve. (13-12) You can identify a thermostatic expansion valve by: (1) the size of the connections, (2) the length of the capillary, (3) the type of charge, (4) the internal or external equalizer, and (5) the capacity in tons. (13-13) Three types of charge used in a thermostatic expansion valve are the liquid, the gas, and the cross charge. (13-13) The cross charge is a refrigerant different from that used in the system so that the cross charge temperature-pressure curve will cross the curve of the refrigerant used in the system. (13-13) The liquid-charged bulb will always have some liquid left in the bulb; thus it will continue to hold control even when the valve is colder than the bulb. Its drawbacks are the possibility of flooding and/or of hunting. (13-13) The gas charge is smaller than the liquid charge; therefore the maximum operating pressure of the valve can be determined by fixing the amount of the charge. Its disadvantage is that control is lost if the diaphragm is colder than the bulb, since gas will condense away from the bulb. (13-13) The advantages of a high-side float valve are that the capacity of the valve is not subject to change from flashing and all of the refrigerant enters the evaporator as a liquid, so there is no lost cooling. (13-14) A layer of oil on top of the refrigerant may cause the refrigerant to refuse to boil unless it is agitated or unless an ebullient is used. (13-15) Before making tests on a "line" circuit, you should remove rings and metal watchbands, because safety records show many sever burns from metal jewelry which has caused a short circuit. (142) When a circuit breaker is reset, you should determine what caused the trip to operate, as you can often find and correct a minor defect before it becomes a major problem. (14-4) A compressor motor which draws full LRA may be good if it starts normally with belt tension released. The indication is that there is a locked compressor. (14-5, 6 also 5-11) 57. 51. 49.

50. 34. 35.

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burr side up. A heavy burr should be removed, as it could produce metal particles in the system. (16-13) 3. 60. In a refrigeration compressor, the compressor ring usually has a taper which slopes to the top of the ring (marked “TOP”). The top side must be installed facing the head. Oil rings which do not have a taper may be installed with either side up. (16-15) Two indications of upside-down compression rings would be low oil in the sight glass and noisy compressor operation – knocking- because of pumping oil. (16-15) Ring gap can be checked with a feeler gauge after the ring has been inserted into the cylinder about 3/8 inch below the top. (16-15) When a new set of rings is installed in an old cylinder, the glaze must be broken from the cylinder wall so that the rings will wear in quickly. (16-15) When a compressor has worn to the point of requiring new bearing inserts, other moving parts must also be inspected for signs of wear beyond specified limits. (16-16) 7. 65. System cleaning is required on a new installation before it is placed in service as well as on a system which has suffered a hermetic motor burnout. (16-19) 8. 66. While cleaning a system after a hermetic motor burnout, avoid contact with the sludge, as it may contain acid. Also avoid bleeding contaminated refrigerant into the air, as the acid may be strong enough to burn your eyes. (17-2,7) system cleaning is done by evacuating from the high side. The reason for doing this is to reverse-flush the system. (17-3) After a system is cleaned, the drier will have absorbed considerable moisture, and the installation of a new drier will insure a dry system. (17-3) When cleaning a system use refrigerant to break the vacuum in order to keep air and moisture from enter the system. (17-4, 7) Activated alumina may be used as a drier only on the suction side of a system charged with SO2 (18-5) Anhydrous calcium sulfate must not be used as a drier in a system charged with SO2. (18-7) Before installing a drier, it should be opened and baked at 300º F. for 24 hours to insure dryness. (18-9) 12. 73. 74. A vacuum of about 1-inch mercury is required to boil water at 80º F. (18-11) 13. A vacuum pump requires that the oil be changed to get rid of the moisture which accumulates in the oil during service. (1815) 14. Chapter 3 1. The coldest rooms are located in the center of a refrigerated warehouse surrounded by warmer areas which act as a buffer and make it easier to maintain zero temperatures in freezer rooms. (19-2) During normal work, the floors in meat processing an storage rooms become very slipper. Also, where large quantities of potatoes are stored, you must have positive ventilation, as 15. 11. 10. 9.

accumulations of CO2 can cause asphyxiation. (19-4, 5, 8) If potatoes in storage are piled too high – more than 6 feet – heat will accumulate in the center of the pile, and they will spoil rapidly. (19-8) Using modern methods of construction provides a cold storage room with a continuous vapor barrier to keep out moisture; also, a room which can move independent of the building. (1910) In the construction of a modern refrigerated warehouse, the vapor barrier must be attached only to the cold room walls, because they can move. (19-14, 15) A blueprint can be studied to learn the meaning of symbols and give one a mental picture of the layout of equipment. For example, a blueprint will show changes and additions to the plant. Also, a blueprint will often help you find the location of equipment. Finally , you should use a blueprint to record modifications to the plant as they are made. (19-21, 22) Blueprint details are enlargements of small parts of a drawing to show fine points which would be lost in a small-scale diagram. (19-21, 22) When four or more compressors are installed, the plant can be split into two systems, with two compressors for high temperature and two for low temperature. Either system can continue operation at reduced capacity, even if one compressor fails. (19-24). The determining factor for setting the operating points of a pressure control in the suction side is temperature. Adjustment is made so that the control cuts in when the evaporator coil is at its desired operating temperature and cuts out when coil temperature has dropped 10º F. While it is essentially a pressure control, its adjustment is most satisfactory when made according to temperature. (19-24) True. Evaporator coil temperature is more reliable because our concern is to keep a room within temperature limits. Suction pressure is more susceptible to variations which occur during the operation of the system. (19-24) In cold weather when compressor discharge pressure drops, you can build up pressure in the system by (1) throttling the king valve and (2) reducing the capacity of the condenser. (19-24, 25) In checking for voltage at a three-phase magnetic switch, check the upper terminals from A to B, B to C, and A to C. (19-24) Dashpot oil is used in time-delay relays. Use of other oils in a dashpot would result in erratic operation with changes in temperature. (19-24) On the installation or replacement of a motor, you should test it for correct rotation and line up the pulleys. (19-24) When working on or around V-belts, you should plan on replacing them soon, as oil causes the belt material to rot. (1924) If oil cannot be cleaned off a set of V-belts, you should plan on replacing them soon, as oil causes the belt material to rot. (1924)

4.

61.

62.

5.

63.

6.

64.

67. 68.

69. 70. 71. 72.

16. 2.

99

17.

When one belt shows a flutter more pronounced than that of the other belts in a set, the condition indicates that the one belt has stretched in service, and the belt tension should be readjusted before the vibration gets too severe. (19-24) A gradual drop in head pressure over a few hours would accompany a drop in ambient temperature. Over a few days, a continual drop would indicate trouble. To contrast, a sudden drop in pressure usually indicates trouble. (19-24) The purpose of a recording chart is to provide a continuous record of plant operating conditions. (19-24) The purpose of bleed-of water is to get rid of some of the water in which salts are concentrating. (19-25) When makeup water is controlled automatically by a float valve, the rate of bleed-off water will determine the amount of makeup water added. (19-25) In cold weather operation, the best method of capacity control of an evaporative condenser is by means of modulating dampers in the air inlet. (19-25) The disadvantages in operating a cooling tower are (1) scale formation, (2) algae growth, and (3) that it must be protected from freezing in cold weather. (19-25) When making a walk-through inspection of a freezer room, make these checks: (I) Look for unusual sins of frost on expansion valves and extension of frost on the lines. (2) Check to see that fans are operating. (3) Note any unusual noise, vibration, or odor. (19-27-29) A hot gas defrost system may be operated to drive out oil which has accumulated in an evaporator by operating the defrost for a longer period. (19-30) In an ice plant, an agitator is used to keep the brine moving to insure transfer of heat from the ice cans to the evaporator coil. (20-3) Water jackets are used to cool the head of an ammonia compressor because of the high operating temperature of around 250° F. (20-3, 17) Three factors in making a good grade of ice are (1) a brine at 15° F., (2) agitation of the ice water during freezing, and (3) removal of the core water and replacing it with fresh water at the proper time. (20-5) A core sucker is necessary to remove the core water that contains a large concentration of salts which, if left, would take much longer to freeze and would make ice with a disagreeable taste and odor. (20-5, 10) While the brine temperature is 15° F., it must circulate around the evaporator coils which are at 5° F.; consequently, the solution is adjusted low enough to keep it from forming ice around the evaporator. (20-15, 16) Inhibited acid should be prepared in a crock or wooden barrel. Goggles, rubber gloves, and apron must be worn. Inhibitor powder is first dissolved in water at the rate of 3 2/3 ounces of powder for each 10 gallons of water. Muriatic acid is added slowly at the rate of 11 quarts of acid to each 10 gallons of water. Use commercial grade acid of 1.190 specific gravity. (20-19)

32.

Baking soda should be at hand for instant use to neutralize an acid burn of the skin. It can also be used to check the strength of the acid solution during the cleaning process. (20-19, 20) CHAPTER 4

18.

1. 2.

Capacity control for a variable heat load is obtained by using multiple compressors. (21-1) Two applications are multiple evaporators operated at the same temperature and multiple evaporators at different temperatures. (21-2) Types of evaporator-regulating valves are: (1) bellows, (2) diaphragm, (3) two-temperature, (4) snap-action, and (5) thermostatic. (21-4-9) With multiple evaporators at different temperatures, check valves are installed in the suction line of each of the colder evaporators. (21-10) The coldest evaporator must make up more than half of the heat load or it will not pull down to the desired temperature. (21-11) Since the coldest evaporator has the lowest suction pressure, it will be controlled by the low-pressure cutout. (21-13) When the solenoid valves are closed the evaporators are pumped down, so the receiver must be large enough to hold the total system charge. (21-15) A solenoid valve in the suction line may allow liquid refrigerant to accumulate and flood into the compressor, causing damage when the valve opens. (21-16) The main difficulty with compressors in parallel is insuring equal division of the oil. (22-1) For good operation in parallel, compressors should be the same make and size and all interconnecting lines should be balanced. (22-2) Oil equalizer and gas-equalizer lines must be connected between crankcases of multiple compressors to insure lubrication. (22-4) With multiple compressors, a time delay allows a 10- or 15second interval between the starting of the first compressor and the second. (22-6) An ultralow-temperature chamber is used to test aircraft and weapons. (23-2) A test chamber which makes rapid changes to ultralow temperatures must have an insulation such as Ferrotherm, which has a low heat capacity. (23-3) Two methods of staging are compound compression and compressors in cascade. (23-5) A direct compound system has two compressors in series with an intercooler between, for better efficiency. (23-6, 7) Since there are two separate systems in a cascade system, two refrigerant having different temperature ranges may be used. (23-9) Three refrigerants for ultralow temperatures are R-12, R-13, and R-22. (23-11)

19. 20. 21.

3.

4.

22.

5.

23.

6. 7.

24.

8.

25.

9. 10.

26.

27.

11. 12.

28.

13. 29. 14.

30.

15. 16.

31.

17.

18.

100

19.

At ultralow temperatures, an ordinary expansion valve requires excessive superheating at the bulb location to operate the valve. (23-12) Ultralow temperature requires a special cutout because a conventional model is not satisfactory for a vacuum over 20 inches Hg. (23-14) To avoid problems from wax in an ultralow-temperature system, a high grade oil and oil separator are used. (23-15)

10.

The liquid CO2 refrigeration of a trailer is more reliable because it has a minimum of moving parts and reduces temperature faster. (24-23) The manufacturer's warranty may be void if a compressor fails while operating with an unrecommendable oil. (25-3) You should wear goggles when opening the refrigeration system and when using a hole saw or portable jig saw. (25-4) Failure of the radiator cap to maintain pressure can cause an engine to overheat. (25-7) Vibration causes copper to harden and become brittle. (25-8) You would look for a sight glass in the receiver-drier-strainer, located in the liquid line. (25-9) An expansion valve operates by means of a thermo-bulb to control liquid refrigerant to the evaporator. A RoboTrol valve operates by means of suction line pressure to control evaporator pressure and volume flow to the compressor. (25-l0-12) Either fuses or a circuit breaker may protect the electrical system. (25-19) A magnetic clutch may use a stationary field coil or a rotating field coil with brushes and collector ring. (25-21) Always install grommets on a line where it passes through a metal wall. (25-23) Failure to clean dirt from the shaft or hub may prevent proper mating causing a pulley to wobble. (25-29) A magnetic clutch can be removed without a puller if its hub is threaded. Screw a 3/8-inch NC capscrew into the bulb and tighten it against the crankshaft until the clutch comes free. (2530) Two causes of belt failure are damaged pulley grooves and separation of belt material because of being stretched too tight. (25-31) When you suspect improper operation of an expansion valve, check the thermobulb to see that it is securely clamped and makes metal-to-metal contact with the suction line. (25-32) Never use sealant compounds on fittings as the sealant may clog the strainer and void the warranty on the unit. (25-35) When connecting lines to charge a system, always purge the lines with refrigerant before you tighten the fittings. (25-38)

20.

11. 12.

21. 22.

13. Warm the frozen valve carefully and if it is released at a temperature colder than 32° F., the difficulty is the result of wax formation. (23-16) CHAPTER 5 1. 2. 3. Trucks which must transport perishable foods re- quire refrigeration to prevent spoilage. (24-1) The primary source of power may be either a small gasoline engine or the main engine which powers the truck. (24-3, 4) Using the main engine as the primary power source is less advantageous, since the compressor stops when the truck engine is stopped. (24-3, 4) When engine speed is governed for 2400 r.p.m., the compressor is driven at 1800 r.p.m. (24-7) 19. 5. Heat may be furnished by a blower and an electric heating coil, or it may be supplied from operating the unit, with the defrost valve open so that hot gas flows through the evaporator. (24-8) Yes. All of the controls operate automatically, except the starter switch, the heat-cool switch, and the defrost switch. (249-13) Liquid CO2 is released into a trailer as a fog, which removes cargo heat very quickly. (24-15-20) 22. 8. Yes. While blast chilling can cool a trailer in a few minutes, the atmosphere will have too little oxygen for breathing, and protective clothing should be worn to protect against frost burns. (24-15-21) The liquid CO2 vaporizes faster than dry ice, so it will cool a given area in less time than the water. Also, the liquid can be controlled easily by a valve. Finally, the liquid is stored in tanks for use at any time, while it is impractical to store dry ice for an extended period of time. (24-22) 20. 21. 17. 18. 4. 14. 15. 16.

6.

7.

23.

9.

24. 25.

101

SUBCOURSE OD 1749

EDITION A

REFRIGERATION AND AIR CONDITIONING III (AIR CONDITIONING)

REFRIGERATION AND AIR CONDITIONING III (Air Conditioning) Subcourse OD 1749 Edition A United States Army Combined Arms Support Command Fort Lee, VA 23801-1809 18 Credit Hours INTRODUCTION This subcourse is the third of four subcourses devoted to basic instruction in refrigeration and air conditioning. This subcourse discusses airflow properties and temperature response including an explanation of air and temperature measuring devices. Instruction is also provided on the installation, operation, and maintenance of self-contained airconditioning units. In addition, the subcourse covers the electric motor, pneumatic controls of refrigeration, the installation of ventilation systems, and the types and components of heat pumps. There are four lessons. 1. 2. 3. 4. Temperature, Airflows, and Measuring Devices. Self-Contained Units and Duct Systems. Controls. Evaporation, Ventilation Systems, and Operation of Heat Pumps.

Unless otherwise stated, whenever the masculine gender is used, both men and women are included.

PREFACE THIS subcourse deals with another phase of your specialty description-air conditioning. Since the principles of refrigeration and air conditioning are similar, your mastery of the subject will come easy. You will find that we discuss several components peculiar to air-conditioning systems. To qualify you in the area of air conditioning we discuss the following systems in this subcourse: 1. 2. 3. 4. 5. 6. Self-contained package air conditioners Mechanical ventilating systems Fresh sir and air duct systems Control systems Evaporative cooling systems Heat pump systems

We're also going to refresh your knowledge of the following: 1. 2. 3. Components of sir Temperatures, airflows, and their measuring devices Design and installation factors

*** IMPORTANT NOTICE ***

THE PASSING SCORE FOR ALL ACCP MATERIAL IS NOW 70%. PLEASE DISREGARD ALL REFERENCES TO THE 75% REQUIREMENT.

i

ACKNOWLEDGMENT Grateful acknowledgment is made to the American Society of Heating, Refrigeration ad Air-Conditioning Engineers; Johnson Service Company; Honeywell, Inc.; and Taylor Instrument Companies for permission to use illustrations and text material from their publications.

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CONTENTS Page Preface----------------------------------------------------------------------------------------------------------------------------Acknowledgment-----------------------------------------------------------------------------------------------------------------Chapter 1 2 3 4 5 6 7 8 9 Undesirable Properties of Air ------------------------------------------------------------------------------------------------Temperatures, Airflows, and Their Measuring Devices------------------------------------------------------------------Design and Installation Factors----------------------------------------------------------------------------------------------Self-Contained Package Air-Conditioning Units---------------------------------------------------------------------------Fresh Air and Air Duct Systems---------------------------------------------------------------------------------------------Controls--------------------------------------------------------------------------------------------------------------------------Evaporative Cooling------------------------------------------------------------------------------------------------------------Mechanical Ventilation--------------------------------------------------------------------------------------------------------Beat Pumps----------------------------------------------------------------------------------------------------------------------Answers to Review Exercises----------------------------------------------------------------------------------------------------1 11 18 25 43 53 91 104 118 133 i ii

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CHAPTER 1

Undesirable Properties of Air
HOW DOES THE atmosphere of a hospital operating room differ from the atmosphere of your work area? Well, the atmosphere of the operating room must be free of foreign matter, humidity controlled, and airconditioned. But most work areas are not air-conditioned at all. Some may have fans to ventilate the area. Since your duties will bring you to areas in which the atmosphere is conditioned, you must know how to control the various undesirables you might find in the air. 2. The elements you will study in this chapter are foreign material, odors, and moisture. 1. Foreign Material 1. Normal air contains varying amounts of foreign materials commonly referred to as permanent atmospheric impurities. These materials can arise from such natural processes as erosion, wind, and sea water evaporation. Such contaminants will vary considerably in concentration but will range far below those caused by manmade activities. 2. Some manmade contaminants are: smoke caused by transportation and industry, chemical sanitizers, and various dusts and sprays used in agriculture. 3. Dusts. Dusts are solid particles projected into the air by wind, grinding, drilling, shoveling, screening, and sweeping. Generally, particles are not called dust unless they are smaller than approximately 100 microns. Dust may be of mineral type. such as rock, metal, or sand; vegetable, such as grain, flour, wood, cotton, or pollen; or animal, including wool, hair, silk, feathers, and leather. 4. Fumes. Fumes are solid particles commonly formed by the condensation of vapors from normally solid materials. Fumes may be formed by sublimation, distillation, galvanization, or by chemical reaction whenever such processes create airborne particles predominantly smaller than 1 micron. Fumes which are permitted to age tend to flocculate into clusters of larger size. This characteristic is often made use of when we want to remove fumes from the air. 5. Smokes. Smokes are extremely small solid particles produced in incomplete combustion of organic substances such as tobacco, wood, coal, oil, and other carbonaceous materials. Smoke particles vary considerably in size, the smallest being much less than 1 micron and often in the size range of .l to .3 micron. 6. Air Filters. Air filters are ordinarily used to remove particles such as those found in outdoor air. Filters are employed in ventilating, air-conditioning, and heating systems where the dust content seldom exceeds 4 grains per 1000 cubic feet of air. Since the purpose of a filter is to remove as much of the contamination as practical, it is obvious that the degree of cleanliness required is a major factor in determining the type of filter design to be used. The removal of these particles becomes progressively difficult as the particle size decreases. The installation of air filters will justify their cost through a reduction of equipment failure and housekeeping, and by providing dust-free air for critical manufacturing processes. 7. Air filters can generally be classified into three groups, depending upon their principle of operations: (1) viscous impingement, (2) dry, and (3) electronic. 8. Viscous impingement type filter. This filter consists of relatively coarse media constructed of fiber, wire screen, woven mesh, metal stampings or plates, or sometimes a combination of these. The filter may be of (1) the unit or pane) type, which is manually cleaned; (2) the disposable type, which is replaced after it has accumulated its dirt load; or (3) the automatic moving curtain type, which changes its media in the airflow when the pressure across the filter reaches a predetermined pressure. 9. The viscous impingement filter derives its name from the fact that the medium is treated with a viscous substance, frequently referred to as an oil or adhesive. In the operation of the filter, the airstream is broken down into small columns which are made to change directions, depending upon filter construction. At each change of direction, the larger dust articles continue in a straight line

1

because of their momentum, and when they impinge against the medium they are held by the adhesive surface. The viscous material used on this type filter requires careful selection. In general, it is considered good practice to follow the manufacturer's recommendations. However, desirable characteristics of a suitable adhesive are: (1) a low percentage volatility so as to have negligible evaporation, (2) a viscosity that varies only slightly with normal temperature change, (3) the ability to inhibit the growth of bacteria and mold spores, (4) high capillarity, or the ability to wet and retain the particles, (5) high flash and fire point, and (6) freedom from odor. 10. The arrangement of the filter medium is one of two types. The high velocity type has the filtering medium placed on edge, perpendicular to the base of the duct, so as to offer low resistance to airflow. Filters in this category carry a face velocity rating of 480 to 520 f.p.m. This filter does not have any recommended direction of airflow. 11. The progressive pack or progressive density design, in which the medium is packed more densely on the leaving air side, permits the accumulation of dirt throughout the depth of the media. Filters of this design are rated at a face velocity of 300 to 350 f.p.m. 12. Unit filters generally have metal frames which are riveted or bolted together to form a filter bank or section. The rate at which they need cleaning depends upon the type and concentration of the dirt in the air being handled. Various cleaning methods can be used, but the most widely used procedure is to wash the filter with steam or water (frequently using a detergent) and then dip or spray the filter with its recommended adhesive. Excessive adhesive should be allowed to drain off before you reinstall the filter in the air-stream. 13. Manometers or draft gauges are often used to measure the pressure drop across the filter and thereby indicate when the filter requires servicing. Unit filters are serviced when the pressure drop reaches 0.5 inches water gauge pressure. A visual inspection should be made periodically if a manometer or draft gauge is not installed in the system. 14. The disposable filter is constructed of inexpensive materials and is to be discarded after one period of use. The cell side of this design is a combination of cardboard and metal stiffeners. 15. The moving-curtain viscous filters are available in two main types. In one design the filter medium is installed on a traveling curtain which intermittently passes through an adhesive reservoir, where the medium gives up its dirt load and takes on a coating of new adhesive. The medium used in the design consists of metal panels or sections made of screen wire, stamped plates or baffles, or reinforced mesh, which is attached to a pair of

chains. The chains are mounted on sprockets located in the top and bottom of the filter housing. The medium thus forms a continuous curtain which moves up one face and down the other. 16. Automatic filters of this design may often utilize a timer for periodical filter movement. The timer is so set that it allows the curtain to make one revolution every 24-48 hours. 17. The precipitated dirt must be removed from the adhesive reservoir. This is done by scraping the dirt into a tray which can be conveniently suspended from the reservoir lip. The frequency of dirt removal is variable, but in normal operation, this type of filter will require attention approximately once every 3 months. Where it is desirable to eliminate this maintenance, the adhesive may be pumped through oil clarifiers or can be allowed to circulate through large settling tanks. 18. The moving-curtain filter is also available in roll form, which is fed automatically across the filter face. The dirty medium is rewound on a spool at the bottom of the filter housing. Movement of this type filter is controlled by a pressure switch control. 19. Filters of this type are considered to be fail safe, as they have a trip switch that indicates that the filter medium is exhausted. This switch also opens the circuit to the filter drive motor. 20. At this time you must remove the old filter and spool and insert a new one. The old filter is not reusable. 21. Most automatic types of viscous filters are equipped with a fractional horsepower motor operating the drive mechanism through a gear reducer. The operating period is adjustable so that the media travel can be adjusted for changes in dust concentration. In operation the resistance of an automatic filter will remain constant as long as proper operation is obtained. A resistance of 0.4 to 0.5 inch water gauge pressure at a face velocity of 500 f.p.m. is typical of this class filter. 22. Dry type air cleaners. The media used in dry type air filters are usually fabriclike or blanketlike materials of varying thicknesses. Media of cellulose fiber, bonded glass, wool felt, asbestos, and other materials are used. The medium is frequently supported by a metal frame in the form of pockets or V-type pleats. In other designs the media can be constructed to be self-supporting. The pockets and pleats provide a high ratio of filter area to face area. 23. The efficiency of the dry filter is higher than that of the viscous impingement type. The wide choice of filter media makes it possible to supply a filter for any cleaning efficiency desired. The life or dust holding capacity is lower than the viscous impingement filter, because the dust tends to clog

2

the fine pore or openings. Dry filters have a large lintholding capacity because of the large surface area exposed by the pleated arrangement of the media. 24. Types of media which provide extremely high cleaning efficiency consist of pleated cellulose-asbestos paper, sand beds, compressed glass fibers in the form of paper, or glass fiber blanket material. The use of these filters is limited to concentrations in the range of outdoor air and where efficiencies to 99.95 percent on submicron particles are required. 25. In some designs of dry type air filters, the filter medium is replaceable and is held in position in permanent metal cell sides. Other dry air filters are discarded after one period of use. 26. The initial resistance of a dry type filter will vary with the medium being used. A number of commercial designs have an initial resistance of 0.1 inch water gauge pressure and are replaced when a final resistance of 0.5 inch water gauge pressure is reached. The more cleaning efficiency the filter offers, the more resistance there will be to airflow. In any event, the filter should be compatible with the resistance against which the fan will be called upon to operate. 27. Automatic dry filters are similar to the roll type viscous impingement filter. These filters are not recommended for handling of atmospheric dust, but are used in such applications as textile mills, drycleaning establishments, and printing press room operation. 28. Electronic air filters. The two types of electronic air filters are the ionizing type collectors and charged media type collectors. 29. The ionizing type electronic air filter uses the electrostatic precipitation principle to collect particulate matter. 30. In a typical case, a potential of 12,000 volts may be used to create the ionizing zone, and some 6000 volts between the plates upon which the precipitation of dust occurs. Safety devices are used to protect personnel from shock. The door to the filter section is outfitted with a switch that will open the circuit to the filter plates. 31. The voltage necessary for operation of the equipment is obtained from high-voltage, direct-current power packs which operate from a 120-volt, 60-cycle, single-phase power supply. Power consumption is approximately 12 to 15 watts per 1000 c.f.m. plus about 40 watts required to energize the rectifier tube heaters. 32. Filters of this type have very little resistance to airflow. Therefore, care must be exercised in arranging the duct approaches on the entering and leaving sides of the filter in order to evenly distribute the air across the entire area of the filter. The efficiency of the filter is sensitive to air velocity. In most systems, resistance is deliberately added in the form of a perforated plate,

prefilters, or afterfilters for the purpose of obtaining a uniform distribution of air. The resistance generally ranges from 0.15 to 0.25-inch water gauge pressure at velocities of 300 to 400 f.p.m. Screens of 16 mesh should be installed across outdoor air inlets to prevent larger foreign objects from entering the system. Special devices must be installed in front of the ionizing filter to remove excessive lint. 33. The ionizing type electronic filter is very efficient. It is available in either fixed or moving collector types. The fixed collector plates are often coated with a special oil which acts as an adhesive. Cleaning is accomplished by washing the cells in place with hot water from a hose or by means of a fixed or moving nozzle system. The bottom of the filter chamber is made watertight and is provided with a drain. 34. In one moving-plate type the grounded elements on which the dirt collects are mounted so as to form a traveling curtain. The traveling curtain intermittently passes through a reservoir containing a fireproof chemical adhesive. This unit is equipped with wipers which remove the collected dirt from the plates. The dirt then settles as a sludge in the bottom of the reservoir from which it must be removed periodically. 35. The charged media type electronic filter consists of a dielectric filtering medium, usually arranged in pleats, as in the typical dry type filter. The dielectric material may consist of glass fiber, cellulose, or other similar materials. The medium is supported on or is in contact with a gridwork consisting of alternately grounded and charged members, the latter being held at a potential of 12,000 volts d.c. so that an intense and nonuniform electrostatic field is created through the dielectric medium. Airborne particles approaching this field are polarized and drawn toward filaments or fibers of the media. 36. The precipitator of this type offers resistance to airflow. The resistance, when clean, is approximately 0.10-inch water gauge pressure at 250 f.p.m. velocities. The resistance of this type filter increases as dust accumulates on the media. Like the typical replaceable media dry filter, the charged media precipitator is serviced by replacing the medium. The dielectric properties of the media become impaired when the relative humidity exceeds 70 percent. 2. Odors 1. Odor is defined as that property of a substance which excites the sense of smell. To be odorous, a substance is usually in a gaseous or vapor state, or possesses a vapor pressure. Some odors are pleasant, others unpleasant, depending

3

upon their psychological and sociological association. 2. The sources of odor that cause discomfort to individuals are many. They may be introduced from the outdoor atmosphere and contain a high percentage of hydrogen sulfide, industrial effluents or smog. In enclosed areas, odors may be caused by the human body, tobacco, etc. Odors may also be caused by wet, dirty airconditioning coils. The metals and coatings used on coils materially affect the possibility of producing objectionable odors. 3. Odor removal may be done by physical or chemical means. Ventilation with clean air, air washing or scrubbing, charcoal adsorption, and masking are physical methods; while chemical adsorption, destruction of odor sources, vapor neutralization, and catalytic combustion are chemical methods. 4. Washing and scrubbing, like filtering, are applicable to the removal of particulates and, in some cases, are means of recovery of a valuable product. Odors associated with the particulates are removed indirectly by this process. Combustion is employed to alleviate the effects of harmful exhaust gases and particulates on people, vegetation, and property. 5. Ventilation, charcoal adsorption, and masking are effective in air-conditioning for odor control. We will limit our discussion to these three. 6. Mechanical Ventilation. Ventilation systems supply fresh air where natural ventilation is insufficient; remove heat, vapor, or fumes from a building; and discharge these undesirables to the atmosphere. It has been found that 30 c.f.m. per person is necessary for effective ventilation in sports arenas to avoid eye Irritation, odors, and impaired visibility. 7. The actual oxygen requirement per person varies with the activity. It is normally about 0.89 cubic feet per man-hour when the activity is walking at the rate of 1 mile per hour. 8. Smoke and other solid or liquid particulates can be effectively removed by electronic precipitators or absolute liters. Odors, gases, and vapors can be removed effectively by charcoal adsorbers. Considerable fuel and power savings can result from the use of charcoal adsorbers as compared to ventilation. 9. Charcoal Adsorption. Charcoal adsorption is the physical condensation of a gas or vapor on the charcoal sorbent. The charcoal or carbon is especially prepared from coconut shells, peach kernels, or other materials. To increase the surface area and thereby increase the adsorption capacity, the charcoal or carbon is activated. 10. The preparation of activated charcoal is usually done in two steps: first, the carbonization of the raw material; second, the high temperature oxidizing process. The purpose of the oxidizing process is to remove from 4

the capillaries of the raw material those substances which cannot be carbonized. This is done to create extensive surfaces on which adsorption can take place. 11. Coconut charcoal, properly prepared, is considered the standard high quality material for air or gas purification in air-conditioning systems. The quality of charcoal as an adsorber of gases is rated on the breakthrough time when subjected to the standard Accelerated Chloropicrin Test. The capacity of charcoal to adsorb gases or vapors depends primarily on the types of gases and vapors being adsorbed. Some are readily adsorbed, while others are not. Improved adsorption of various gases and vapors can be obtained by impregnation of the charcoal with certain mineral salts. 12. Masking. Odor masking is the process of hiding one odor by superimposing another odor to create a more overpowering sensation, preferably pleasant. The masking agent, which can be in spray cans, wick bottles, etc., does not alter the composition of the pre-existing odors. It simply covers such odors during the period of its addition to, or presence in, the air. 13. The application of a pleasant masking agent to an offensive atmosphere may result in a final combination that is still objectionable to the sense of smell. Therefore the objectionable odor concentration must not be so intense that the masking agent is itself required in objectionable quantities. 3. Air and Water Vapor 1. As we have stated previously, air is made up of various mixtures, including gases and moisture. We will discuss moisture in the air and its relation in psychrometry, and the means used to add or remove it from the air. 2. Psychrometry. Psychrometry literally means the measurement of cold. It is the name that has been given to the science that deals with air and water vapor mixtures. The amount of water vapor in the air has a great influence on equipment cooling and human comfort. Such atmospheric moisture is called humidity, and the common expression "It isn't the heat, it's the humidity" is an indication of the discomfort-producing effects of moisture laden air in hot weather. 3. The water vapor in the air is not absorbed or dissolved by the air. The mixture is a simple physical one, just as sand and water are when mixed. The temperature of the water vapor is always the same as the air. 4. When the air contains all the water it can hold, it is called saturated air. The amount of moisture present at the saturation point varies with the temperature of the air. The higher the temperature, the more moisture the air can hold.

5. Moisture Removal. The moisture in the air may be removed by various methods. We will discuss the mechanical and chemical methods. 6. Dehumidifying coils. Air can be cooled and dehumidified by passing it over the cold surfaces of cooling coils. The efficiency of this process may have to be checked if the desired values are changed or the system becomes unbalanced. In a later chapter you will become acquainted with methods of checking the efficiency of this type of system. 7. When dehumidification is accomplished with cooling coils, the coil temperature must be below the dewpoint temperature of the humid air. This low coil temperature causes the moisture in the air to condense out. The air is then reheated to lower the relative humidity. For example, an entering air condition of 100° F. dry bulb and 67 percent relative humidity could be conditioned to 70°-72° F. dry bulb and 40-50 percent relative humidity by the following procedure: a. The air being drawn into the system is preheated if it is below 70° F. b. The air then passes over the cooling coil. (Coil size is calculated by c.f.m., approximately 400 c.f.m. per ton of refrigeration.) The air is now cooled to approximately 33° F. and has a relative humidity of 100 percent. c. It now passes to the reheat coil, where its temperature is increased to 65°-70° F. The relative humidity is now 20-30 percent. d. We can now add humidity to the air if desired. Adding humidity to the air will be discussed later in this chapter. 8. Chemical dehumidification. Sorbents are solid or liquid materials which have the property of extracting and holding other substances (usually gases or vapors) brought in contact with them. The sorption process always generates heat, which is the major factor in dehumidification. All materials are sorbents to a greater or lesser degree. However, the term "sorbent" refers to those materials which have a large capacity for moisture as compared to their volume and weight. We will discuss the liquid absorbents and the solid adsorbent. 9. The liquid adsorbent (sulfuric acid, lithium chloride, lithium bromide, etc.) can adsorb moisture from or add moisture to the air, depending upon the vapor pressure difference between the air and the solution. 10. For dehumidification, the strong adsorbent solution is pumped from the sump of the dehumidifier to the sprayers. The sprayers distribute the solution over the contactor coils.

11. The solution, at the required temperature and concentration, comes in contact with the humid air which is flowing over the coil surface in the same direction as the liquid absorbent. Equipment is also available for counterflow operation. 12. Moisture is absorbed from the air by the solution and is maintained at a constant condition by automatic regulation of the flow of water through the cooling coils by means of a water regulating valve. 13. The heat generated in absorbing moisture from the air consists of the latent heat of condensation from the water vapor, the heat of the solution, or the heat of mixing of the water vapor and absorbent. The heat of mixing varies with the liquid absorbent used and the concentration and temperature of the absorbent. The solution is maintained at the required temperature by cooling with refrigerated or cooling tower water, or refrigerant flowing inside the tubes of the contractor coils. The quantity of coolant required is a function of the temperature of the coolant and the total heat removed from the air by the absorbent solution. 14. The dry-bulb temperature of the air leaving the liquid absorbent contactor at a constant flow rate is a function of the temperature of the liquid absorbent and the amount of contact surface between the air and the solution. In most commercial equipment the dry-bulb temperature of the air leaving the dehumidifier will be within 1° to 5° F. of the absorbent solution temperature. 15. The liquid absorbent is maintained at the proper concentration by automatically removing the water vapors condensed from the air. Approximately 10 to 20 percent of the solution supplied by the pump passes over the regenerator coil. The coil heats the solution with steam or other heating mediums. The liquid absorbents commonly used can be regenerated with 2 to 25 p.s.i.g. steam. The vapor pressure of the solution at temperatures corresponding to 2 p.s.i.g. steam is considerably higher than that of the outdoor air. The hot solution at the relatively high vapor pressure is in contact with outdoor air in the regenerator, where water is absorbed from the solution by the scavenger air. The hot moist air is discharged to the outdoors and the concentrated solution falls to the sump. The solution is then ready for another cycle. 16. The steamflow to the regenerator coil is regulated by a control responsive to the concentration of the solution circulating over the contactor coils. 17. Dehumidification by solid adsorption systems may be performed under static or dynamic operation. These desiccants can be silica gel, activated alumina, etc. 18. In the static method there is no forced circulation of air into or through the desiccant.

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Instead, the air surrounding the adsorbent is initially dried. Subsequently, through convection and diffusion, water vapor (humidity) passes into the air surrounding the desiccant and then to the desiccant, where it is stored. Since considerable time is required for dehumidification, this method is used quite often in shipping and storing delicate instruments that are sensitive to moisture. Various foods, such as potato chips, also use the static method of solid adsorption dehumidification. 19. On the other hand, dynamic dehumidification is operated with forced passage of air through a desiccant bed. The only prerequisites for a dynamic dehumidifier are a desiccant bed, a fan to force the humid air through the bed, and a heater to reactivate the adsorbent. 20. As the air passes through the desiccant bed it gives up a certain amount of its moisture. The rate of moisture pickup and the humidity condition of the leaving air are functions of a great many variables. Some of these variables will be discussed later. 21. The ratio of adsorbed moisture to entering air moisture content is known as adsorption efficiency. The adsorption efficiency in dynamic uses remains constant and at a relatively high level until some point within the cycle, at which time the efficiency begins to drop. This point is known as the breakpoint, and the amount of moisture adsorbed until this point is called breakpoint capacity. It is considered ideal to have the breakpoint capacity coincide with the equilibrium capacity. In actual operation, breakpoint capacity can be a small portion of the equilibrium capacity, depending on operating conditions. High inlet temperature and humidity, small bed depths, and high airflow rates will all tend to decrease the breakpoint capacity. Regeneration of the desiccant bed should be accomplished at breakpoint capacity, but adsorption can still be carried on. The adsorption is now done at a slower rate until the desiccant is completely saturated. This saturation point is called completion. 22. To regenerate the desiccant bed, the heater is energized and the airflow through the bed is usually reversed. The temperature of the effluent air rises rapidly at first, and then virtually levels off for a period of time. This period of time represents the period during which the major portion of the heat input is being used to boil off the adsorbed water. When the latent heat (evaporization) requirements begin to diminish, the heat input goes into sensible heat gain to the passing airstream. This period, measured from the start of desorption, is called temperature rise time. Although additional regeneration can be accomplished beyond this point, it is considered uneconomical because of the slower rate of desiccant activation. Regeneration past temperature rise time, until the adsorbent is in moisture

equilibrium with the airstream, is known as complete desorption or desorption to completion. The energy used in the heater per unit weight of water desorbed for any given time is called economy of desorption and is usually expressed in kilowatt hours per pound of water desorbed. 23. Some of the many variables that influence the results of a dynamic dehumidification operation are as follows: a. Variables concerning the desiccant bed: (1) Type of desiccant. (2) Dry weight of desiccant. (3) Particle size. (4) Bulk density. (5) Shape of bed. (6) Area of bed normal to airflow. (7) Depth of bed. (8) Packing of desiccant in the bed. (9) Pressure drop through the bed. b. Variables concerning the air to be dried: (1) Flow rate. (2) Temperature. (3) Moisture content. (4) Pressure. (5) Contact time between air and desiccant. c. Variables concerning reactivation: (1) Reactivation temperature. (2) Rate and magnitude of heat supply. (3) Heat storage capacity of the bed. (4) Temperature gradient of the bed. (5) Amount of insulation. (6) Amount of sweep gas. 24. Solid adsorption dehumidifiers are usually of the stationary dual-bed type. One bed absorbs while the other is being reactivated. The cycle time for the dualbed operation is normally specified by the manufacturer and is controlled by a timer. Larger units may have adjustable time cycles that can be changed for various operating conditions. Still others are operated from either manual or automatic reading of the effluent moisture content. 25. Humidifiers. There are many different types of humidifiers available for adding moisture to a conditioned area. The types that you will study in this section are the steam, atomizer, impact, forced evaporation, and air washer. Of these humidifiers, the steam type is the only one which puts vapor into the air. All the others consist of arrangements for exposing large surfaces of water, in the form of small droplets or wet surfaces, to the air. The water will evaporate and humidify the air. 26. Before we continue our discussion, let us review the principles of humidification. Humidity refers to the amount of moisture (water vapor) in the air Absolute humidity is the actual weight

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of water vapor per unit volume of air. Do not confuse the term "absolute humidity" with specific humidity. It is a common error. Specific humidity is the actual weight of water vapor per unit weight of dry air in the mixture. Specific humidity is dependent upon dewpoint temperature only and is expressed in grains of moisture per pound of dry air. The continual changes in volume which takes place with changes in temperature and in water vapor content make it very difficult to base any calculations upon the volume of the mixture. Through all of these changes, the pound of dry air remains a known factor and a suitable basis for our measurements. As you work with air and water vapor mixture calculations, you will realize why this basis was chosen. 27. Relative humidity refers to the amount of moisture actually in the air as compared to saturated air. Relative humidity depends only upon the vapor pressure of the water vapor present in the air and the dry-bulb temperature. The presence of air or any other gas has nothing to do with the relative humidity of a given space. 28. Now let us get back to humidification. In order to change water to vapor we must add 1050 B.t.u.'s to each pound of water evaporated. The heat may come from the air being humidified. This procedure will cause the air to cool at the same time that its being humidified. In any humidifying process in which no external heat source is used, the wet-bulb temperature will remain constant throughout the process. 29. Let us consider a sample of air at 80° F. drybulb temperature, 17 percent relative humidity, which is to be humidified to 100 percent. Using a psychromatic chart, you will find that the wet-bulb temperature is 55 F. The air will become cooler until it has reached 55° F. dry-bulb temperature. At this point the air is completely saturated and the relative humidity is 100 percent. You will also notice that the dewpoint temperature has risen from 31.3° to 55° F. and that the moisture content has increased from 25.5 to 64.7 grains of vapor per pound of dry air. You have added 39.2 grains at 0.0056 pounds of vapor to the air. Let us now discuss the various types of humidifiers that use this principle. Remember, the steam humidifier is the only type that uses an external heat source. 30. Steam humidifier. The simplest type of steam humidifier contains a nozzle or a set of nozzles through which live steam is allowed to escape into the air. This means of humidification is seldom used because steam carries odors that are objectionable in an air-conditioning installation. It is also difficult to eliminate the hissing sound produced by the escaping steam. Another fault is that the stream humidifier often provides more heat than is desired in the conditioned area.

31. Atomizer humidifier. The atomizer humidifier is very effective, because water is taken from a supply tank and blown into the air in the form of a fine mist. The atomized water vapor may be sprayed into the conditioned area or into a duct leading to the area. It functions much like a can of spray deodorant or a perfume atomizer. 32. Instead of the plunger arrangement found in a perfume atomizer, compressed air passes through a narrow section of pipe at a high velocity. This movement of air causes the water to be lifted out of the tank and be blown into the room or area. The tank is usually connected to a water supply line and is kept full by a float valve. 33. This humidifier adds no heat to the conditioned area. The atomized water vapor readily evaporates by the addition of heat taken from the air within the space. The evaporation will cause the dry-bulb temperature to decrease while the relative humidity increases. The wetbulb temperature (total heat) will remain constant. 34. While the atomizer humidifier is efficient because it uses all the water supplied to it, it is objectionable in areas where noise cannot be tolerated. The noise is caused by the high velocity air passing through the pipe. A drainpipe is not needed because the atomizer uses all its supplied water. 35. Impact humidifier. This type of humidifier uses an arrangement similar to an air washer. Fine jets of water are directed against a hard surface. The impact of the spray upon the surface causes the water to break up into a finer spray. The conditioned air is brought past the surface to pick up by evaporation as much of the spray as possible. 36. Eliminator plates are placed downstream from the spray to restrict large water droplets collected in the air. The water is thus prevented from entering the conditioned area and damaging the contents. 37. Twenty to fifty percent of the water supplied to an impact humidifier is actually evaporated and carried off in the conditioned air. This percentage varies because of the speed of the water leaving the jet, the entering air temperature and humidity, and the mixing of air and water vapor ahead of the eliminator plates. Greater jet velocity, higher air temperature, lower relative humidity, and better mixing of air and water will increase the percentage of evaporation. 38. Forced-evaporation humidifier. You know that evaporation takes place continuously from any water surface. But, do you know which factors determine the rate of evaporation? First, the rate of airflow plays an important role. If

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more air is brought into contact with the water in a given length of time, more evaporation will occur. When you heat the water you are actually increasing its vapor pressure. This heating effect will allow the water to evaporate more readily. 39. Those are the two factors-airflow and heat. Now let us apply these factors to the forced-evaporation humidifier. The forced-evaporation humidifier is so named because it provides a means by which water may be evaporated into the air more than would be normal. Most of these humidifiers consist of a large shallow pan in which a steam coil is immersed. A fan blows air across the pan at a high velocity. The water level is maintained by a float valve. 40. This humidifier does not waste any water. It is simple, in that there are no moving parts. If no heat is applied to the water, the water will evaporate by the heat in the air, or adiabatically. When the Water is heated, both the wet-bulb temperature and the total heat will increase. For example, if you added 40 B.t.u.'s of heat per hour to the water, and if the air is passed over the surface of the water at the rate of 20 pounds of dry air per hour, the total heat of the air will be increased by 40/20, or 2 B.t.u.'s per pound of dry air. Thus, if the air enters the humidifier with a total heat of 250 B.t.u.'s per pound of dry air and a wet-bulb temperature of 57.8° F., it will leave with a total heat of 27.0° B.t.u.'s per pound of dry air and a wet-bulb temperature of 60.8° F. 41. Air washer humidifier. You have studied air washers earlier in this chapter as a method of odor removal. Now you will learn how they are used to humidify air. 42. The spray type air washer is a very effective humidifier. Two banks of sprays are directed against the airflow and one is directed with the airflow. This arrangement of spray banks is 100 percent efficient, because all the air passing through the air washer will leave saturated. 43. If fewer spray banks are used, the efficiency will decrease. A general comparison of the saturation efficiency is: Number of banks Direction Efficiency 1 ...........................downstream ......... 50-70% 1 ...........................upstream .............. 65-75% 2 ...........................downstream ......... 85-90% 2............................opposing ............... 90-95% 2............................upstream .............. 92-97% 44. The efficiency of a washer is usually measured by the drop in dry-bulb temperature relative to the entering wet-bulb depression. For example, if the entering air conditions are 95° F. dry-bulb and 75° F. wet-bulb temperature with a leaving air temperature of

76° F., the efficiency is 95 percent. The principal factors affecting the efficiency are air velocity, the quantity of water sprayed per unit volume of air, the length of the chamber, and the fineness of the spray. 45. Most standard rating tables are based on a velocity of 500 f.p.m. through the air washer. Velocities above 750 or below 350 f.p.m. often result in faulty elimination of the entrained moisture. The quantity of water sprayed per 1000 c.f.m. of air varies between 1.5 and 5 g.p.m. per bank. The fineness of the spray depends upon nozzle design and the water pressure supplied to the nozzle. The pressure will vary between 20-40 p.s.i.g. You will find that the air resistance of an air washer is usually 0.2 to 0.5 inches of water. 46. The material most commonly used in the construction of air washers and the other types of humidifiers is galvanized sheet steel. The maintenance that you will be required to accomplish on humidifiers consists mostly of cleaning and painting. 47. The nozzles used on impact and air washer humidifiers are designed to produce a dense spray. To do this with a reasonably low water pressure, the body of the nozzle may be designed to give the water a swirling motion as it enters the nozzle cap. The cap is cupped to give an accelerated action to the water before it emerges into a spray at the orifice. The capacities of several standard size nozzles at different pressures are:
Shank diameter 1/4 3/8 3/8 3/8 3/32 1/8 3/16 1/4 Orifice diameter 10 0.33 0.59 1.27 2.01 Capacity of nozzle at indicated pressure (g.p.m.) 20 25 30 40 0.47 0.52 0.57 0.66 0.83 0.93 1.02 1.18 1.79 2.01 2.20 2.54 2.84 3.18 3.48 4.02

48. Flooding nozzles are often used to provide continuous flushing of the eliminator plates. These nozzles may also be used to flush the inlet baffles when lint-ladden air is being handled. Under this condition, they operate at 3 to 10 p.s.i.g. and are spaced to handle 3 to 6 g.p.m. per foot of humidifier width. 49. Corrosion is often encountered in humidifiers. When corrosion exists you must clean the humidifier and treat the water to prevent or retard further deterioration of the equipment. Chemical treatment should be a means of maintaining a pH of 7.5 to 8.5. A corrosion inhibitor may also be used, if allowable. 50. Humidifier operation. A humidistat is normally used to control the operation of a humidifier. A low humidity condition is sensed by the humidistat, which in turn will start the humidifier pump, position dampers, open valves, or start fans. The maintenance, adjustment, and calibration of humidistats will be discussed later.

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Review Exercises NOTE: The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the text. Do not submit your answers for grading. 1. What factor determines the type of filter design you would use on a particular installation? (Sec. 1, Par. 6)

8. How many watts will an ionizing filter consume when 3800 c.f.m. of air is being handled? (Sec. 1. Par. 31)

9. How much would it cost to operate the filter in question 8 for 1 hour at 3 cents a kilowatt? (Sec. 1, Par. 31 and Question 8)

2. Which filter arrangement would you use in a duct system having a velocity of 500 f.p.m.? (Sec. 1, Par. 10)

10. The conditioned air passing through a charged media filter is not being cleaned. The dry-bulb temperature is 50° F. and the dewpoint temperature of the air is 50° F. What has caused the air to remain dirty? (Sec. 1, Par. 36)

3. The pressure drop through a duct system is 2 p.s.i.g. What has occurred? (Sec. 1, Par. 13)

11. A complaint is submitted to your shop about an air conditioner giving off a peculiar odor. What condition most likely caused the air to become odorous as it passed through the duct? (Sec. 2, Par. 2)

4. Which type of filter requires the least amount of attention? (Sec. 1, Par. 17)

12. Air at 70' F. and 100 percent relative humidity is ____________. (Sec. 3, Par. 4)

5. Which type of filter would you install in a critical area such as a missile complex? Why? (Sec. 1, Par. 19)

13. How many c.f.m. can be handled effectively by a 5-ton cooling coil? (Sec. 3, Par. 7)

14. How is the quality of a liquid absorbent controlled? (Sec. 3, Par. 12) 6. How can you increase the surface area of a dry filter? (Sec. 1, Par. 22) 15. The temperature of the air leaving the dehumidifier is 10° below the absorbent temperature. How can you correct this condition? (Sec. 3, Par. 14)

7. The fan motor on an air-conditioning system overheats. The filter is clean and the fan is not malfunctioning. What has caused the motor to overheat? (Sec. 1, Par. 26)

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16. What is the adsorption efficiency of a dynamic dehumidifier when the adsorbed moisture is 20 grains and the entering air moisture content is 25 grains? (Sec. 3, Par. 21 )

21. What is maximum efficiency of the impact humidifier as compared to the atomizer type? (Sec. 3, Par. 37)

17. To regenerate a filter bed, 400 watts per pound of water is used. The amount of water desorbed is 3 pounds and the cost of electricity per kilowatt is 2.5¢. What is the economy of desorption and the cost of desorption? (Sec. 3, Par. 22)

22. Why does the rate of airflow play an important role in evaporation? (Sec. 3, Par. 38)

23. If you added 100 B.t.u.'s of heat per hour to a forced-evaporation humidifier and air is passing through it at the rate of 20 pounds of dry air per hour, how much heat will be added to each pound of dry air? (Sec. 3, Par. 40)

18. How many B.t.u.'s are required to evaporate 9 pounds of water? (Sec. 3, Par. 28) 24. The air leaving an air washer, used for humidification, is carrying water droplets out with it. The air velocity through the washer is 800 f.p.m. How can you correct this condition without altering the washer or changing the velocity? (Sec. 3, Par. 44)

19. Adding moisture to the air with an atomizer humidifier will ____________ the wet-bulb temperature. (Sec. 3, Par. 28)

20. How can you control the amount of humidity added to the air with an atomizer humidifier? (Sec. 3, Par. 32)

25. The resistance of the air passing through an air washer is 2 p.s.i.g. What has caused the pressure to rise and how can it be prevented in the future? (Sec. 3, Pan. 45 and 48)

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CHAPTER 2

Temperatures, Airflows, and Their Measuring Devices
MOST MANUFACTURERS of automobiles in the past few years have installed warning lights to indicate heating of the engine, low amperage output, and low oil pressure. Many times you' have probably wished to know the rate the battery was charging or the temperature of the engine. The trend in some automobiles is back to the gauges which will tell the owner more precisely how his automobile is performing. 2. Lets fit this to our situation. Can you tell how hot or cold a surface is or how much air is flowing out a ceiling outlet by placing your hand on it? No, you must u some type of instrument that will indicate the true condition of the component being checked. In airconditioning troubleshooting, you will find that the thermometer, psychrometer, and airflow measuring devices are valuable tools. 4. Temperature 1. Temperature is defined as the heat intensity or heat level of a substance. Temperature alone does not give you the amount of heat in a substance. It is an indication of the degree of warmth, or how hot the substance is. 2. The methods and scales used to measure temperatures have been arbitrarily chosen by scientists. The most common scale that you will use is the Fahrenheit scale, but we will also discuss the centigrade scale. as you may come in contact with it during an overseas tour. The Fahrenheit scale is so fixed that it divides the temperature difference from the melting temperature of ice to the boiling temperature of water into I80 equal divisions. It sets the melting point of ice at 32 divisions above the zero indication on the scale. Therefore, ice melts at 32° F., and water boils at 212° F. (32° F. + 180° F. = 212° F.) under an atmospheric pressure of 14.7 p.s.i.a. 3. The centigrade scale has coarser divisions than the Fahrenheit scale, and the melting point of ice is set at 0°. The boiling point is 100 divisions above this point, or 100° C. 4. It may be necessary to convert a Fahrenheit reading to a centigrade reading or vice versa. For this purpose, formulas have been developed. The formula to convert Fahrenheit to centigrade is: C. = 5/9 (F. -32) Centigrade may be converted to Fahrenheit by using this formula: F. = 9/5C. + 32 5. Sensible Heat. Sensible heat is the heat added to a substance that causes a temperature change. Likewise, heat may be removed from a substance; and if the temperature falls, the heat removed is sensible heat. 6. Specific Heat. The sensible heat required to cause a temperature change in substances varies with the kind and amount of the substance. This property is called the specific heat of a substance and is the amount of heat required to raise 1 pound of the substance 1° F. This value is good for computations, provided no change of state is involved. If a change of state should occur, the specific heat of the substance changes. To determine the amount of heat necessary to cause a temperature change in a substance, multiply the weight of the substance by its specific heat. Then multiply that answer by the temperature change (B.t.u. = specific heat X weight X temperature change). 7. Latent Heat. Latent heat is the heat that is added or taken from a substance, causing a change of state. These changes of state occur without any changes in temperature or pressure. Latent heat is commonly referred to as hidden heat. latent heat of fusion, latent heat of vaporization, and latent heat of condensation. 8. Total Heat. Any mixture of dry air and water vapor (atmospheric air) does contain both sensible and latent heat. The sum of these two heats is called total heat and is usually measured from 0° F. 9. Now that we've covered the various types of heat, we're ready to discuss temperature, or the intensity of heat.

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Figure 1. Thermometer. 10. Dry-Bulb Temperature. In air conditioning, the air temperature is listed more accurately as the drybulb temperature. This temperature is taken with the sensitive element of the thermometer in a' dry condition. Figure 1 shows a thermometer common to the airconditioning trade. Unless otherwise specified, all air temperatures are dry-bulb temperatures. 11. Wet-Bulb Temperature. A wet-bulb thermometer is an ordinary thermometer with a cloth sleeve of wool or flannel placed around its bulb and then wet with water. The cloth sleeve should be clean and free from oil and thoroughly wet with clean, fresh water. The water in the cloth sleeve is evaporated by a current of air at high velocity. The evaporation withdraws heat from the thermometer bulb, thus lowering the temperature. This temperature is now measured in degrees Fahrenheit. The difference between the dry-bulb and wet-bulb temperatures is called the wet-bulb depression. If the air is saturated, evaporation cannot take place, and the wet-bulb temperature is the same as the dry bulb. Complete saturation, however, is not usual and a wet-bulb depression is normally to be expected. 12. The wet-bulb thermometer indicates the total heat of the air being measured. If air at several different times or different places is measured and the wet-bulb temperatures found to be the same for all. the total heat would be the same in all, though their sensible heats and respective latent heats might vary considerably. In any given sample of air, if the wet-bulb temperature does not change, the total heat present is the same even though some of the sensible heat might be converted to latent heat or vice versa. 13. Dewpoint Temperature. The dewpoint depends upon the amount of water vapor in the air. If air at a certain temperature is not saturated that is, if it does not contain the full quantity of water vapor it can hold at that temperature-and the temperature of that air then falls, a point is finally reached at which the air is saturated for the new lower temperature, and condensation of the moisture then begins. This point is 12 the dewpoint temperature of the air for the quantity of water vapor present. 14. Relation of Dry-Bulb, Wet-Bulb, and Dewpoint Temperatures. The definite relationships between the three temperatures should be clearly understood. These relationships are: a. When the air contains some moisture but is not saturated, the dewpoint temperature is lower than the dry-bulb temperature and the wet-bulb temperature lies between them. b. As the amount of moisture in the air increases, the difference between the temperatures grows less. c. When the air is saturated, all three temperatures are the same. 5. Relative Humidity 1. The water vapor mixed with dry air in the atmosphere is known as humidity. The weight of water vapor, expressed in pounds or grains. occurring in each pound of dry air is called specific humidity. The amount of moisture that 1 cubic foot of air does hold at any given time is its absolute humidity. 2. When a gallon bucket contains 1/2 gallon of liquid, it is 50 percent full. If a cubic foot of air that could hold 4 grains of moisture holds only 2 grains, it is 50 percent full, or 1/2 saturated. The ratio of the amount of moisture which the air does contain to what it could contains is called its relative humidity. Expressed in general terms, relative humidity is defined as the actual absolute humidity divided by the absolute humidity of saturated air at the temperature being considered. The simple equation would be: % R. H. = actual gains per pound X (100) max. grains per pound that could be held at the given temperature. 3. Psychrometers. Instruments for measuring wetand dry-bulb temperatures are known as psychrometers. A sling psychrometer, shown in

Figure 2. Sling psychrometer. figure 2, consists of two thermometers mounted side by side on a holder with provisions so that the device can be whirled in the air. The dry-bulb thermometer is bare, and the wet-bulb is covered with a wick which should be kept wet with clean water. After whirling for a minute or two, the wet-bulb thermometer reaches its equilibrium point, and both the wet- and dry-bulb thermometers should then be quickly read. The difference between the two thermometer readings will depend on the relative humidity of the air. By a series of experiments, the effect of different relative humidities has been found through a wide range of temperatures. From these values, tables and charts have been constructed from which, when the wet-bulb and dry-bulb temperatures are known, both the relative humidity and the dewpoint temperature can be found. 4. In the aspiration psychrometer, a small fan is used to blow the air past the mounted wet- and dry-bulb thermometers to bring about the wet-bulb equilibrium. 5. There are several types of direct reading hygrometers. A hygrometer is a device that measures the relative humidity by use of a wet- and dry-bulb thermometer. In the hygrometer a pointer is actuated by some material sensitive to changes in the moisture content of the air. The pointer moves across a dial graduated in relative humidities. While these hygrometers have the advantage of reading relative humidity directly, they are not sufficiently accurate for most industrial purposes. 6. At some points in an air-conditioning system, recording psychrometers are used to provide a continuous record of both the temperature and relative humidity. Thus they eliminate the necessity for frequent sling psychrometer readings. Distilled water should be used for wetting the wet-bulb wick. If ordinary tap water is used, the dissolved solids could clog the capillaries in the cloth and the wick could become dry, resulting in an incorrect record. The air which is circulated over the bulb should be as free as possible from dust, dirt, and lint for the wick to retain its capillary action and give an accurate reading. At some locations where a low relative humidity is combined with dust-laden air and water, the wet-bulb wick may have to be changed twice daily to obtain proper accuracy. Wicks may be used over again after they are washed. 7. It is necessary to locate the sensitive wet and dry bulbs in ducts and chambers remote from the recording instrument. Remote panels are installed in ducts where natural circulation is adequate. 8. Psychrometric Charts. The psychrometric chart may be used in conjunction with psychrometers to determine the relative humidity of any particular space. These charts consist of straight lines and curves showing relationships between the relative humidity, dry-bulb temperature, wet-bulb temperature, dewpoint temperature, specific humidity, effective temperature, and air velocity (generally a fixed factor for each chart). 9. While relative humidity must be given consideration by all persons concerned with the conditioning of air, all major determinations for its specific control will be up to your supervisor. The amount of moisture in a space may have to be reduced, the dry-bulb temperature may have to be changed, or the source of moisture may have to be controlled. Wherever it is necessary, mechanical machinery and controls are installed to effect a continuous automatic control of this relative humidity. 10. The use of the psychrometric chart involves no more than knowing the wet-bulb thermometer reading and the dry-bulb thermometer reading. Various charts are constructed for different altitudes and situations where abnormally low surface pressures are encountered. Figure 3 illustrates the use of a psychrometric chart. 11. The procedures listed in the following paragraphs may be used to find the properties of air if two of the properties are known: a. Dry-Bulb Temperature. The dry-bulb temperature is found by following the vertical lines down to the bottom scale of figure 3. detail A. b. Wet-Bulb Temperature. The wet-bulb temperature is read directly at the intersection of the wetbulb line with the 100 percent relative humidity line (saturation curve). as shown in figure 3. detail A. The scale is marked along the 100 percent line. c. Relative Humidity. The relative humidity is read directly from the curved lines marked "relative humidity," as shown in figure 3, detail B. For points between the lines, estimate by distance. d. Moisture Content. The moisture content or absolute humidity is read directly from the horizontal lines, as shown in figure 3, detail C. It is the weight of water vapor contained in a quantity

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e. Dewpoint Temperature. The dewpoint temperature is read at the intersection of a horizontal line of a given moisture content with the 100 percent relative humidity line, as shown in figure 3, detail C. f. Total Heat. The total heat is read directly by following the wet-bulb line to the scale marked "Total Heat," as shown in figure 3, detail A. Total heat refers to a quantity of air and water vapor mixture which would weigh 1 pound if all water vapor were extracted, including the heat of the water vapor. g. Specific Volume. The specific volume is read directly from the lines marked "cubic foot per pound of dry air," as shown in figure 3, detail D. For points between the lines, estimate by distance. Specific volume is the volume occupied by a quantity of air and water vapor mixture which would weigh 1 pound if all water vapor were extracted. h. Vapor Pressure. The vapor pressure corresponding to a given moisture content is read directly from the left-hand scale marked "pressure of water vapor," as shown in figure 3, detail C. 12. Assume that readings in an area taken with a sling psychrometer were 85° F. dry bulb and 70° F. wet bulb. Use foldout at end of this memorandum and follow along with this example. The dry-bulb temperature (85) is located at the bottom of the chart. Following the 85 line upward until it intersects the 70.5° F. wet-bulb line (slanting downward from left to right), the user marks this point. In this example, there is no wet-bulb line for 70.5° F., so it is necessary to mark the point of intersection. It will be found that under these conditions the relative humidity is 50 percent as shown by the curved line also running through this point Projecting the point horizontally to the left of the wetbulb scale will give a dewpoint of 64.4° F. The steep diagonal line running through the point of intersection indicates that a pound of air under these condition will occupy 14 cubic feet. Projecting the point horizontally to the right shows that there are 90 grains of water per pound of dry air. The total heat, found by following the wet-bulb line upward to the left, is 34 B.t.u.'s per pound of air, which is the heat represented by the dry air plus the latent heat present at this degree of partial saturation (50 percent relative humidity). If the sensible temperature were 85° F. and the relative humidity were 70 percent the total heat would be 39.5 B.t.u.'s per pound of air. 6. Figure 3. Segment o! psychrometric chart. of air and water vapor mixture which will weigh 1 pound if all water vapor were extracted. Airflow 1. Most air-conditioning systems are designed to specifications, but you will find changes made and accepted that will be entered on as-built blue prints. The specific volumes of air in each individual section should be checked with the specifications or as-built drawings. 2. Air passages provide the means for air to 14

Figure 5. Slant type manometer. throughout the duct system is called the dynamic or total pressure. The pressure required to overcome losses due to friction of duct systems is called static pressure. Figure 6 shows a slant gauge used to measure pressure differences. 9. The various pressures of air applied to an airconditioning system are relatively small and cannot be measured in pounds for an accurate reading. Total, static, and velocity pressures for airflow in airconditioning systems are usually measured in inches of water. The pressure of air is measured by the application of a gauge calibrated in inches of water on the scale for accurate reading, as shown in figure 4. 10. The gauge in figure 4 contains a free-flowing liquid. The connections are extended by flexible tubing to the air passages and the atmosphere. The difference in pressure of the air will cause the liquid in the tube to rise or fall, indicating the pressure on the scale in inches of water according to the connections of the gauge. 11. Pitot Tube. Total and static pressure for airflow through a duct system can be measured by a pitot tube. Use of a pitot tube is illustrated in figure 7. The static pressure can be found by subtracting the velocity pressure from the total pressure. The static pressure subtracted from total pressure will equal velocity pressure for air in a duct system. The total (dynamic) pressure is equal to the static and velocity pressures. 12. The use of the pitot tube for pressure indication should be done with care. The tube should be located so that an average condition of airflow

Figure 4. U-type manometer. flow. With constantly changing requirements, a system designed for a specific need may have an entire new heat load or distribution requirement. Many of us have faced this situation, so you must understand how to make adaptations to correct the balance. 3. Air passages for air-conditioning systems in an installation contain the following: air intake, return, mixing, recirculating, exterior and exit division ducts, heating, cooling, humidification, dehumidification, air washing, filtering equipment, and inlet and discharge of the fan. The air passages from air-conditioning equipment to the spaces served are called duct systems. The distribution duct system includes the chamber, branches, risers, inlets, dampers, registers, returns, recirculating, mixing, baffling, and exit systems. 4. The factors that can affect air volume are the number of occupants, various heat transfers in building equipment. and temperature differences of interior and exterior spaces. All these factors are considered by engineers in their design of an air-conditioning system. 5. If the air weight or volume is determined and the load requirements are known, then duct and distribution systems are calculated according to velocities, pressures, and pressure drop in the duct system. 6. We will now discuss the devices used in measuring airflow and static and total pressures. 7. Manometers. Two types of gauges that can be used to measure air pressure are shown in figures 4 and 5. 8. The pressure required for the velocity of airflow necessary to overcome all the losses

Figure 6. Measuring resistance of air filters.

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will be measured in the interior of the duct. The tube should not be placed in a sharp turn in the duct, in a restricted area, in an offset, or in a section having varying airflow. Also avoid contact with material or substances that would obstruct air pressure to the pitot tube opening. 13. The pitot tube consists of two tubes, one sealed within the other. The opening of the pitot tube which faces toward the airflow measures the total pressure. The other opening of the pitot tube has airflow sweep across the opening so as to measure the static pressure. The velocity is determined by the differential reading on the gauge which is connected to the double pitot tube. 14. A certain pressure for airflow in ducts is required to cause motion and to overcome resistance and friction in the ducts. 15. Static pressure will vary according to the surface area with which the air is in contact. Some factors are: condition of air passages, construction and installation, dampers, interior design, length of duct system, leakage, eddy currents, and pulsation of airflow throughout the installation. 16. Anemometer. The anemometer, shown in figure 8, is used to measure air velocities at the opening of the air duct. The anemometer is moved across the entire area of the duct opening for a period of 1, 2, or 3 minutes and the average velocity in feet of air is calculated or measured. 17. The anemometer should be used with precaution and care. It requires frequent calibration or adjustment to maintain it for accurate measurement of air velocities at duct openings.

Figure 8. Use of the anemometer. 18. Kata Thermometer. A kata thermometer is an alcohol thermometer developed for determining very low air velocities. The bulb of the thermometer is heated in water until the alcohol rises to a reservoir above the graduated tube. The time for the liquid to cool 5° F. is observed by the use of a stopwatch, and this time is a measure of the air movement. 19. Velometer. The velometer is an instrument that is calibrated to read directly in feet per minute. The velocity pressure readings are converted by use of a formula without the necessity for timing. The velometer may be placed directly in the airstream or may be connected through a flexible tube to special jets which permit taking velocity readings in locations where it would be very difficult to use an anemometer or pitot tube. The velometer accuracy is within 3 percent and is much quicker to use than other instruments designed for this purpose.

Figure 7. Measuring airflow with a pilot tube.

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Review Exercises NOTE: The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the text. Do not submit your answers for grading. 1. What degree on a centigrade thermometer is equivalent to 60° on a Fahrenheit thermometer? (Sec. 4, Par. 4)

the dry-bulb thermometer if the wet-bulb wick was dry while whirling? (Sec. 4, Par. 11; Sec. 5, Par. 3) 7. Will the difference in the dry-bulb thermometer reading and the wet-bulb thermometer reading become greater or less as the relative humidity decreases? (Sec. 4, Par. 11; Sec. 5, Par. 3)

8. What two factors must be known in order to determine the relative humidity? (Sec. 5, Pa. 3) 2. What degree on a Fahrenheit thermometer is equivalent to 40° on a centigrade thermometer? (Sec. 4, Par. 4) 9. What type of water should be used to wet the wick of a wet-bulb thermometer? (Sec. 5, Par. 6) 3. How many B.t.u.'s would be required to raise the temperature of 8 pounds of cast iron 4°? (Specific heat of cast iron is 0.119)(Sec. 4, Par. 6) 10. If the total pressure of an air-conditioning system remains constant and the air ducts become partially clogged, will the static pressure increase or decrease? (Sec. 6, Pars. 8-11)

4. What term is applied to the sum of sensible heat and latent heat? (Sec. 4, Par. 8)

5. If the air is dry, which thermometer will indicate the highest temperature, a dry-bulb or a wet-bulb thermometer? (Sec. 4, Pars. 10 and 11)

11. If the total airflow pressure is equal to 20 inches of water and the static pressure is equal to 4 inches of water, what is the velocity pressure? (Sec. 6, Par. 11)

6. After whirling a sling psychrometer, how will the wet-bulb thermometer reading compare to

12. Is it possible to determine static pressure with a velometer? (Sec. 6, Pars. 11 and 19)

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CHAPTER 3

Design and Installation Factors
COST REDUCTION has become an important part of military policy. How can you contribute to this program ? You may be called upon to install an air conditioner or to calculate the heat load of a building. In either case, your skill can determine the savings. If your unit is undersized, it will have to be supplemented with another unit. If it is oversized, the running cost will be high. You may install the correct size unit but use poor duct insulation. Once again you are defeating the purpose of cost reduction. 2. In this chapter you will study heat loads, the selection of a good location for the condensing unit, insulation, and calculating a heat load. 7. Heat Sources 1. Heat that must be removed from a building arises from various sources. Some of this heat is gained through walls, doors, partitions, windows, ceilings, and roofs, and is caused by the difference in the temperature between the conditioned and unconditioned areas. Remember, heat always travels from the warmer to the cooler mass. Engineering formulas are used to calculate heat transmission. Glass and door areas are not used in calculation of wall area but are considered in heat transmission calculations in a separate formula. 2. In calculating wall area, use the length (ft.) of exposed wall measured on the inside and the height of ceiling. 3. Consideration must be given to construction materials of the walls and ceilings. For example, brick construction has a different heat transfer characteristic than wood. 4. Special tables are used in determining heat transmission through various building materials. Listed below are some heat sources that must be considered in cooling load computations. 5. Solar Effects. Heat that is transmitted by radiation through glass is absorbed by inside furnishings and surfaces. When the sun's rays strike glass, the glass will absorb a small percentage of the sun's energy, but most of the energy passes through the glass and causes an increase in heat. 6. The solar heat which passes through glass is absorbed by interior furnishings, walls, and floors. This heat is quickly given up to the air. Some of the solar heat absorbed by thick walls and doors will not dissipate its heat readily, but will continue radiating heat even after the sun has set. Because of this, the heat load is more continuous. 7. The intensity of the sun radiation on walls and glass varies with the time of day, the season, and the direction the walls and windows face. All the above factors must be considered when you are computing solar heat transmission. 8. Heat delivery by sun radiation through glass may be reduced by use of awnings, Venetian blinds, or shades. Any type of shade will help reduce the load on airconditioning equipment. 9. Special formulas and tables have been devised to determine solar heat transmission through glass and walls. 10. Infiltration and Ventilation. Heat and moisture are transmitted into a building by infiltration and ventilation. Air enters by leakage through window and door cracks, through doors or windows opened, and through porous walls. Special tables have been devised to determine the infiltration of air through window openings and for finding the volume of air entering from door traffic. All the above factors must be considered in cooling load computations. 11. Occupants. Heat load from occupants will cause an increase in sensible and latent heat. The amount of heat transmitted will vary with the degree of activity of the occupants. 12. Equipment. Heat load transmitted by equipment used in a building will vary with the type and operation. Electrical and mechanical equipment will have a major effect on total heat load. Tables have been developed that give values of common heat sources. All of the tables referenced in the preceding paragraphs may be found in the American Society of Heating, Refrigerating, and Air Conditioning Engineers' Guide. 13. Listed above are a few of the major heat transmission sources that must be considered when installing an air-conditioning unit. The total heat

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load must be established before an air-conditioning unit can be installed in a building. All heat transmission sources must be used in figuring out the total heat load. When the heat load has been determined, an airconditioning unit of correct tonnage can be installed and the proper cooling can be maintained in the building. 8. Selecting Location 1. The following are major considerations in selecting the location for air-conditioning equipment. 2. Availability of Space. Equipment should be located in the place most suitable for it. It may be necessary to compromise the ideal location with the actual one and to locate equipment in the space which is available. 3. Ambient Temperatures. Ambient temperature refers to the air temperatures surrounding the refrigeration equipment, such as the condensing unit and other parts of the system. Avoid extreme ambient temperatures, either too warm or too cold. 4. Excessively warm locations result in high heat leakage and service loads. High ambient temperature can give high condensing pressures with consequent loss of capacity. An ambient temperature of 10° F. above normal may increase the heat load and decrease equipment capacity to the extent where the operating time increases 25 percent above normal. 5. Consideration must be also be given to low temperature to which the equipment may be exposed. Do not install water-cooled equipment in locations colder than 40° F. to avoid frozen water-lines in condensers. 6. Ventilation. Proper ventilation is very important for carrying heat away from the condensing equipment. It is most important to air-cooled equipment which uses air to carry heat away from the condenser. It is also important to water-cooled equipment, even though water removes heat from the condenser. The heat from the compressor and motor must be carried away by the surrounding air, and this cannot be accomplished without adequate ventilation. Keep all ventilation facilities such as doors and windows free of barriers and other obstacles so that the air can be properly circulated. 7. Radiant Heat. Part of the heat given off by a hot object such as a hot stove, boiler, furnace, or even a hot brick is radiant heat. Avoid installing refrigeration equipment near such objects. It is not Sways possible, but the extra load of radiant heat should be avoided whenever possible. 8. Electric Supply. Be sure that the proper power is furnished before selecting a location. Check the

electric supply to be sure there is correct voltage, frequency (cycles per second), phase, and capacity of the wiring. If the equipment has a motor requiring 220-volt, 60-cycle, 3-phase alternating current, it will not run on 110-volt, 60-cycle, single-phase alternating current. Consult a qualified electrician on suitability of electric supply for motors installed on equipment. Motors should never be connected to a source of electric current until you are sure that available current is the same as that specified on the nameplate. 9. Water Supply. Before selecting a location for water-cooled refrigeration equipment, check the water supply for available capacity and maximum temperature. The capacity of a water-cooled condensing unit depends upon whether or not it is supplied with enough cool condensing water. Rated capacities of condensing units are usually based on 75° F. condensing water being available. Higher temperature water requires more water supplied. If enough water is not available, the capacity of the condensing unit may be reduced 5 percent for each 5° F. higher than the correct temperature of condensing water. 10. Drain. Check location of suitable drain and its capacity before installing equipment. Drain lines are connected to sewers through an open sight connection. If possible, trap and vent the sewer branch to guard against entry of sewer gas into the rooms. 11. Accessibility. When selecting the location of equipment, consideration must be given to its accessibility for cleaning and servicing. It is not always possible to find all the room you actually need. Whenever possible, leave enough room for a workman to get at all sides of the unit, and enough room to permit removal or replacement of any major assemblies, such as motor, compressor, and condenser. 12. Accessibility to those parts of equipment subject to preventive maintenance and inspections, or requiring readjustment, repair, or replacement must be given special preference. See that oil wells of motors, belts, air-cooled condensers, service valves, and especially suction service valves on compressor, gauge, and gauge ports, controls, and nameplate data are readily accessible. 9. Insulation 1. Insulation represents the composite covering which consists of the insulating material, lagging, and fastening. The insulating material offers resistance to the flow of heat; the lagging, usually of painted canvas, is the protective and confining covering placed over the insulating materials; the fastening attaches the insulating material to the piping and to the lagging.

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2. Insulation Temperatures. Insulation covers a wide range of temperatures, from the extremely low temperatures of the refrigerating plants to the very high temperatures of boilers. No one material could possibly be used to meet all the conditions with the same efficiency. Cork, rock wool, or hair felt is used for low temperatures. Such basic minerals as asbestos, carbonate of magnesia, diatomaceous earth, aluminum foil, argillaceous (clay-like) limestone, mica, fibrous glass, and diatomaceous silica are employed for high temperatures. Because of its high degree of refractoriness, diatomaceous silica forms the base of practically every high temperature insulating material. 3. Insulating Material Requirements. The following quality requirements for the various insulating materials are taken into consideration in the standardization of these materials: a. Ability to withstand highest or lowest temperature to which it may be subjected without its insulating value being impaired. b. Sufficient structural strength to withstand handling during its application, and mechanical shocks and vibrations during service without disintegration, settling, or deformation. c. Stability in chemical and insulation characteristics. d. Ease of application and repair. e. No hazard in case of fire. f. Low heat capacity, when used for boiler wall insulation, so that starting-up time may be minimized. 4. Insulating Materials. Listed below are a few of the more popular insulations that you may encounter in air-conditioning work. 5. Cork: Cork in block sections or compressed board form, coated with a special retardant cover, is used (where authorized) for temperatures below 50° F. It use is generally limited to refrigeration spaces where it will not be a serious fire hazard. Molded cork pipe covering, treated with a fire-retardant compound, is used on refrigerant piping. 6. Mineral or rock wool. Mineral or rock wool is a fiber made by sending a blast of steam through molten slag or rock. The rock fibers are usually from dolomite rock, composed of calcium and magnesium oxides and silicates. The fibers are brittle, of low tensile strength, light in weight, and resistant to moisture. The fibers are used in wire-reinforced pads for insulating large areas. 7. Hair felt. Hair felt, 1 inch or more in thickness, may be used in any service where the temperatures do not rise above 119° F. When combined with heavy asphalt-impregnated paper, this material is used on coldwater lines where the temperature range is from 50° to

90° F. When suitably waterproofed, it may be used for refrigerator piping. 8. Asbestos. Molded sheets, pads, blankets, or tapes of long asbestos fibers are suitable for insulating temperatures up to 850° F. This insulation material is cheaper and lighter than the diatomaceous earth type and is durable and rugged. The pads or blankets are used for insulating flanges or valves which must be taken down fairly often, as well as for turbine casings. The pads are molded to fit any shape, and the outer surface is fitted with metal hooks to facilitate their installation and removal. The blankets are generally made 1 inch thick, 40 inches wide, and fitted with hooks. The tapes are used for covering ½a- inch and smaller piping with curves and bends. They can be used for temperatures up to 750° F.; they tend to reduce fire hazards, but they have poor insulating quality. 9. Magnesia and asbestos. The magnesia and asbestos mixture, of which about 85 percent is magnesia, is the most common material used for hot piping. It is obtained commercially in pulverized form, in sheets, in shaped blocks, and in cylindrical sections for standard pipe sizes. Its principal features are low heat conductivity, ease of application, light weight, low cost, and chemical inactivity. The chief disadvantage is the limited temperature range, as the mixture calcines and decomposes at about 500° F. 10. Diatomaceous earth. The diatomaceous earth (sand formed from skeletons of certain microscopic plants) materials are combinations of the earth and magnesium or calcium carbonates, bonded together with small amounts of asbestos fibers. These materials are heavier, more expensive, and less insulating than others, but their high heat resistance allows their use for temperatures up to 1500° F. When practical, pipe coverings are made up with this material as an inner layer and with an outer layer of the magnesia-asbestos material. This lightens the overall weight. 11. Aluminum foil. Aluminum foil is the most effective insulating material for high temperatures. The foils are produced commercially from pure aluminum and are supplied in long, thin sheets, some 12 inches to 16 inches in width. The covering is light, particularly for large piping for which it is best suited. There is very little uncleanliness connected with the installing or removing of aluminum foil; it is easy and economical to manufacture and, because of its light weight and low inertia, it stands up well under vibration or shock. 12. There is more than one method of applying aluminum foil. The following is the most common of these methods. One or more layers of foil are wrapped about the material to be insulated, leaving a 3/8-inch airspace between each layer, and with a sheet metal cover to protect the foil. The airspace between the layers of foil is kept by 20

first hand-crinkling the foil so that its surface becomes uneven. This type of insulation serves to reduce to a minimum any convection current present in the air pockets. 13. The chief objections to the use of aluminum foil for insulation are the weight of the sheet metal cover necessary to cover the assembly and the high skill necessary for its application or repair. 14. Fibrous glass slabs. Fibrous glass slabs are used widely for insulating living quarters. The glass fibers in the pressed slabs are 4 inches or more in length and 0.0005 to 0.0008 inch in thickness. The slabs have a low moisture-absorbing quality and offer no attraction to insects, vermin, fungus growth, or fire. The slabs are first cut to shape, then secured in place by mechanical fasteners (as quilting pins), and finally covered with glass cloth facing and stripping tape (held in place by fireresistant adhesive cement). 15. The insulating cements. Insulating cements are composed of many varied materials. These materials differ among themselves as to heat conductivity, weight, and physical characteristics. Typical of these variations are the asbestos cements, diatomaceous cements, and mineral and slag wool cement. These cements are less efficient than other high-temperature insulating materials. They are valuable for patchwork emergency repairs and for covering small irregular surfaces (valves, flanges, joints, etc.). The cements are also used for a surface finish over block or sheet forms of insulation, to seal joints between the blocks, and to provide a smooth finish over which asbestos or glass cloth lagging may be applied. 16. Insulation Application. In applying insulating material, care should be taken that air does not circulate through the insulation, that moisture is kept from reaching the insulation, and that the insulation will not move or slip. 17. All sections or segments of the pipe coverings should be tightly butted at joints and secured with wire loops, metal banks, or lacing. Block insulation should be secured with 1/8-inch steel wire and galvanized mesh wire or expanded metal lattice. Insulating cement is used to fill all crevices, to smooth all surfaces, and to coat wire netting before final lagging is applied. 18. Moistureproofing is important for insulation over heated surfaces. Even though the temperature of the insulation dries off moisture, the heat loss is increased due to the evaporation. Moisture also impairs many insulating materials. This moistureproofing is also very important for low temperatures. At very low temperatures, the insulation should be air-sealed. Moisture drawn into low-temperature insulation condenses and freezes, thus lessening the efficiency and eventually causing disintegration. 21

19. The same insulating material employed on the piping may be used on pipe fittings, flanges, and valves. These components require additional consideration during installation. 20. When a permanent type of insulation is applied to a piping 4 inches and larger in size, a block insulation 1 inch thinner than that on the adjacent piping may be used for the bodies of flanged fittings and valves, for the entire surface of a threaded fitting, for the entire surface up to the bonnet of screwed valves, and for the flanges. The total thickness of insulation on the valve or fitting is made equal to that on the adjacent piping by applying insulating cement. The pipe insulation should be stopped short of the flanges and leveled off to enable the flange bolts to be removed. On piping under 4 inches in size, the insulation of the fittings may consist entirely of insulating cement, the same thickness as that of the adjacent piping. 21. When a removable type of insulation is applied, the flanges should be insulated with asbestos felt pads, sectional pipe insulation of the same thickness as that on adjacent piping, or block insulation 1/2 inch thinner than that on the adjacent piping and covered with 1/2 inch of insulating cement. 22. Installation Precautions. The following general precautions should be observed with regard to the application and maintenance of insulation: a. Fill and seal all air pockets and cracks. Failure to do this will cause large losses by conduction and convection currents. b. Seal the ends of the insulation and taper off to a smooth, airtight joint. Sheet metal lagging should be used at joint ends and at other points where insulation is liable to damage. Flanges and joints should be cuffed with 6-inch lagging. c. Cotton duck covering, fitted over insulation, should be smooth and well sewn (not less than three stitches per inch). It should be covered with two coats of lead and oil paint. Too much paint will cause the cotton duck to crack and split. d. Keep moisture out of all insulation work. Moisture is an enemy of heat insulation as much as it is of electrical insulation. Any dampness increases the conductivity of all heat-insulating materials. e. Insulate all hangers and other supports at their point of contact with the pipe or other units they are supporting. Failure to insulate these supports will cause a considerable quantity of heat to be lost by conduction through the support. f. Sheet metal covering should be kept bright and not painted unless the protecting surface has been damaged or worn off. The radiation from bright-bodied and light-colored objects is con-

siderably less than from rough and dark-colored objects. g. Once installed, heat insulation requires careful inspection, upkeep, and repair. Any lagging and insulation that is removed to make repairs should be replaced just as carefully as when originally installed. Old magnesia blocks and sections broken in removal can be mixed with water and reused in the plastic form. Save all old magnesia for this use. h. Insulate all flanges with removable forms. The forms can be made up as pads of insulating material wired or bound in place, and the flange can be covered with sheet metal casings which are in halves and easily removable. 10. Making Survey for Air-Conditioning Installation 1. We will now-relate the facts we have discussed to a specific air-conditioning installation. This installation is at Denver, Colorado. You may be called upon to determine the size of a unit needed at your installation. The primary difference between this example and your base would be the mean wet- and dry-bulb temperatures. 2. Cooling Load Requirement. In this chapter we have discussed heat sources in an area that is to be conditioned. These heat sources are as follows: a. Solar heat load on walls, roofs, and glass. b. Human heat load. c. Infiltration and ventilation. d. Machinery, 3. Upon adding the values of the heat sources listed, you would have a load that is called total internal cooling load. 4. Special consideration must be given to location of a room or building when cooling load calculations are being made. The interior load may change from one portion of the buildings to another. Varied heat loads from equipment, solar radiation, and occupants will alter the calculations. When calculating heat load, always consider the peak load that could be reached in the building. 5. With a building that may have a changeable heat load. an experienced air-conditioning man cannot state definitely the time of day that the building cooling load would be at a maximum. Therefore, it is necessary to calculate cooling load for this establishment at several different periods during the day. These times should be chosen when the values of the various heat sources are at their maximum. 6. Calculation of Cooling Load. Problems in calculating cooling load can be done with mathematical

exactness or by rough approximate estimates. The method that is used to calculate a cooling load depends upon the purpose for which the results will be used. Rough estimates are very inaccurate, and your plant may not meet cooling load requirements. 7. The mathematical exactness method is quite complicated and requires very accurate information regarding construction materials. It may require the drilling of holes through walls, roof, and floors to determine construction materials. Some principles used in determining the size of refrigeration plant required for a typical installation are mentioned below. 8. Rate of Heat Flow Through Walls. The rate at which heat is transmitted through walls, floors, and roofs is dependent on the following factors: a. Unit heat transfer coefficient (U-factor). The U-factor depends on wall material and thickness. b. Area of the heat-transmitting surface. a. The temperature difference between the sides of the wall. 9. The heat transfer coefficient is the combined rate of transmission of any substance expressed in B.t.u. per hour per square foot of area per degree Fahrenheit mean temperature difference. It combines the amount of heat transmitted by radiation, conduction, and convection into a single quantity referred to as the U-factor. 10. The basic heat transfer formula is expressed as Q = UA (T1 – T0). This formula is used in most calculations. Q = solar radiation in B.t.u. per hour. U = coefficient of heat transfer in B.t.u. per square foot taken from tables. A = area of transmitting surface in square feet. T1 = inside building temperature. T0 = outside building temperature. 11. Another formula that is used in calculations for radiation through glass is expressed as Qg = Ag Ig Fg Ag = area of glass in square feet. Ig = coefficient of heat transfer for glass taken from table. Fg = glass radiation factor. The following is a list of heat loads: a. Load from solar radiation. b. Sky radiation. c. Outdoor-indoor temperature differential for glass areas, exterior walls, partitions, ceilings, and floors. d. Load due to ventilation. e. Load due to heat sources within the conditioned spaces such as occupants, lights, fans, power, and other heat-generating equipment

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12. This data is available in ASHRAE Guides and manufacturers' manuals. 13. Specifications of Building To Be Air Conditioned.
Denver, Colo. Southern exposure. South wail: Front 20 ft. inside, 12 ft. high; plate glass 10 ft. x 6 ft. high; door 3 ft. x 7 ft. (glass). North wall: 20 ft. inside, 12 ft. high; plate glass, 2 windows 5 ft. x 3 ft. high; wooden door 3 ft. x 7 ft. East wall: 20 ft. inside, 12 ft. high; plate glass, 2 windows 5 ft. x 3 ft. high. West wall: 20 ft. inside, 12 ft. high; plate glass. 2 windows 5 ft. x 3 ft. high. Floor: Wooden lath and covered with 1/4-inch linoleum laid on wooden floor. Ceiling: 4-inch wooden rafters, metal lath plaster below with 1-inch wooden roof deck, covered with roofing paper. Occupancy: 5 employees (manual labor). Equipment: Electric lights 2000 watts, two 1/2-hp. motors, stove burner heating water, coffee urn (12 inch). Outside design condition: Denver, Colo., 95° F. dry bulb and 78° F. wet bulb. Inside design condition selected as 81° F. dry bulb and 68° F. wet bulb. All walls are constructed of 12-inch brick plastered inside. Heat gain calculation at peak load approximately 1:00 p.m. South wall 20 ft. X 12 ft. = 240 sq. ft. gross. Area glass l0 ft. X 6 ft. = 60 sq. ft. Area door 3 ft. X 7 ft. = 21 sq. ft. Transmission coefficient for glass 1.13. Transmission coefficient for brick .34. Heat transmission through wall = 757 B.t.u./hr. Heat transmission through glass = 1280 B.t.u./hr. Heat transmission by solar radiation = 3300 B.t.u./hr. South wall heat gained = 5337 B.t.u./hr. North wall 20 ft. X 12 ft. = 240 sq. ft. gross. Area glass 5 ft. X 3 ft. (2) 30 sq. ft. Area door 3 ft. X 7 ft. = 21 sq. ft. Heat transmission through wall = 900 B.t.u./hr. Heat transmission through glass = 475 B.t.u./hr. Heat transmission through door = 332 B.t.u./hr. North wall heat gained = 1707 B.t.u./hr. Fast wall 20 ft. X 12 ft. = 240 sq. ft. gross. Area glass 5 ft. X 3 ft. (2) 30 sq. ft. Heat transmission through wall = 1000 B.t.u./hr. Heat transmission through glass = 475 B.t.u./hr. Fast wall heat gained = 1475 B.t.u./hr. West wall 20 ft. X 12 ft. = 240 sq. ft. gross. Area glass 5 ft. X 3 ft. (2) 30 sq. ft. Heat transmission through wall = 1000 B.t.u./hr. Heat transmission through glass = 475 B.t.u./hr. West wall heat gained = 1475 B.t.u./hr. Floor 20 ft. x 20 ft. = 400 sq. ft. Transmission coefficient is .24. Heat transmission through floor = 1344 B.t.u./hr. Ceiling 20 ft. x 20 ft = 400 sq. ft. Location:

and roof Transmission coefficient is .32 Heat transmission through roof = 1790 B.t.u./hr. Solar radiation through roof = 3460 B.t.u./hr. Total heat gain through roof = 5250 B.t.u./hr. Total heat gain through walls and roof is equal to 5337 +F 1707 + 1475 4- 1475 + 1344 + 5250 = 16,588 B.t.u./hr. Heat gain from occupants. Sensible heat loss = 200 B.t.u./hr. Latent heat loss = 460 B.t.u./hr. Total heat gain sensible (5 men) = 1000 B.t.u./hr. Total heat gain latent (5 men) = 2300 B.t.u./hr. Total heat gain from equipment. Heat gained from coffee urn. Latent heat = 1200 B.t.u./hr. Sensible heat = 1200 B.t.u./hr. Electric motor = 640 B.t.u./hr. (sensible) Electric light = 7816 B.t.u./hr. (sensible) Stove burner = 3150 B.t.u./hr. (sensible) = 3850 B.t.u./hr. (latent) Total sensible heat gain from equipment = 18.566 B.t.u./hr. Total latent heat gain from equipment = 5.050 B.t.u./hr. Approximate total cooling load in B.t.u./hr. Sensible Latent 1. Through walls and solar radiation 16.588 2. Human load 1.000 2,300 3. Equipment 18,566 5.050 36,154 7,350

14. An additional factor of 10 percent is often added as a safety factor to the sensible load to take care of additional energy that may be added to the internal system. Therefore, total cooling load requirements become:
Sensible load = 36.154 X 1.10 Latent load Total cooling load = 39,769 B.t.u./hr. = 7,350 B.t.u./hr. = 47.119 B.t.u./hr.

47,119 Refrigeration = 12,000 = 3.9 tons of refrigeration equivalent to cooling load. This calculation is approximate because there are other factors that must be considered in cooling load calculations. Ventilation, infiltration, and duct losses are part of your cooling load requirements and will change in value with each installation. 15. Cooling load formulas and tables can be found in American Society of Heating, Refrigerating and AirConditioning Engineers' Guide, Fundamentals and Equipment, 1963. Review Exercise NOTE: The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the text. Do not submit your answers for grading. 23

1. Calculate the following wall areas. The outer dimensions are 10' x 14' and the walls are 8" thick. (Sec. 7, Par. 2)

8. A new barracks is being built and you are called upon to insulate it. What type of insulation would you use and why would you select that particular type? (Sec. 9, Par. 14)

2. A complaint is received from one of the base housing units. The user tells you that the air conditioner will not cool down the house sufficiently.. After questioning her, you find that she has all the drapes on the windows open and also that she didn't start the unit until noon. What directions should you give the user? (Sec. 7, Pars. 5-8)

9. The temperature of the hot water at the heating coil is 130° F. The design coil temperature is 180° F. and the supply temperature from the boiler is 185° F. Why is the temperature dropping from 185° to 130° F.? (Sec. 9, Par. 18)

3. Which type of heat load will affect humidity the most? (Sec. 7, Par. 11)

10. You are insulating a 2-inch pipe with 3/4-inch insulation. How much insulation and what type of insulation should you put on a globe valve? (Sec. 9, Par. 20)

4. The normal ambient temperature for a condensing unit was 80° F. when it was installed. Additional units have been installed in the area and the ambient temperature is now 105°. This increase in temperature is affecting the operating time of the units. How could you correct this situation? (Sec. 8, Pars. 4 and 6)

11. Find the solar radiation through a brick wall 20' x 40' which has a 30° F. temperature differential. (Sec. 10, Pars. 10 and 13)

12. Find the heat gain of a brick wall 10' x 12' which has two 2' x 4' glass windows. The outside temperature is 94° F. and the inside design temperature is 72° F. (Sec. 10, Par. 13)

5. How much efficiency would a condenser lose if the water supplied to it was 85° F. instead of 75° F.? (Sec. 8, Par. 9)

13. Which type of heat load will give off the most latent heat gain? (Sec. 10, Par. 13)

6. Which type of insulation should you use on a 40° F. cold storage room? (Sec. 9, Par. 5)

14. Find the total cooling load when the sensible load is 42,156 B.t.u.'s and the latent heat load is 8,750 B.t.u.'s. (Sec. 10, Par. 14)

7. The strainer on a low-pressure steam coil installation must be removed periodically for cleaning. How would you insulate the strainer? (Sec. 9, Par. 8)

15. What size unit would you install if the sensible load is 57,150 B.t.u.'s and the latent load is 9,170 B.t.u.’s? (Sec. 10, Par. 14)

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CHAPTER 4

Self-Contained Package Air-Conditioning Units

MOST SERVICEMEN call these air conditioners window- and floor-mounted units. You will find that they are identified in this manner throughout the chapter. The self-contained units differ from the remote units discussed in another volume (Equipment Cooling) of this course in that one housing contains all the components. 2. These units are usually found in offices or in a portion of a building that is separate. One example is a panel room in a heat and power building. It would be impractical to air-condition the entire building because of the heat load from the diesel engines, furnaces, refrigeration equipment, etc. The panel room houses gauges, recorders, and various instruments that the duty engineer observes to oversee plant operation. 3. You will study the window- and floor-mounted units. Included under these topics are installation, operation, maintenance, and various components peculiar to these systems. 11. Window-Mounted Units 1. The window-mounted air conditioner is a factory-made incased assembly, designed as a unit for mounting in a window or through a wall. It is designed fir free delivery of conditioned air to an enclosed space without ducts. 2. This air conditioner has a prime source of refrigeration and dehumidification, and a means of circulating and cleaning the air. It may also include means for ventilating and heating. The basic function is to provide comfort by filtering, cooling, dehumidifying, and circulating the room air; and to provide ventilation by introducing filtered outdoor air into the room or exhausting room air to the outside. If heating is provided, steam coils, hot water coils, or electric resistance heaters may he used, or the conditioner may be designed as a heat pump unit. We will discuss the heat pump later in this volume. 3. Sizes and Classifications. The cooling capacities of window-mounted air conditioners range

approximately from 4000 to 36,000 B.t.u./hr. or 1/3 to 3 tons. Remember, whenever you want to convert B.t.u.'s to tons, 12,000 B.t.u.'s equals 1 ton. The sizes had commonly been designated in terms of horsepower, but this proved to be inaccurate because various refrigerants differ in cooling efficiency. Capacities (sizes) are now measured in B.t.u./hr. 4. Most of these air conditioners are designed as household appliances and are equipped with line cords that may be plugged into a standard I15-230 plug receptacle with a ground. Conditioners requiring 115 volts are usually limited to a current load of 12 amperes, which is the maximum allowable load of a single -outlet 15-ampere circuit. This is in compliance with the National Electric Code (N.E.C.). 1959. A very popular I 115-volt model is one which is rated at 7.5 amperes. This rating allows the unit to he plugged into any standard 115-volt 15-ampere circuit. large units, generally over 10.000 B.t.u./hr. are designed as 230-volt units, which can be plugged into a 230-volt circuit within the limitations set forth by the National Electric Code. 5. There are also units which are designed for ; application to the particular power supplies you may encounter in countries outside the United States. Remember, always read the nameplate before plugging in a unit. 6. Many mounting designs are available for particular applications of window-mounted air conditioners. A few of the various mountings are: a. Inside flush mounting. The interior face of the conditioner is approximately flush with the inside edge of the window sill. b. Balance mounting. The unit is installed approximately half inside, and ha1f outside the window. c. Outside flush mounting. The outer face of the unit is flush or slightly beyond the outside wall. d. All-in-mounting. The unit is completely inside the room so that tie window can be closed. e. Upper sash mounting. The unit is mounted in the top of the window.

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f. Built-in mounts. The mounts are used for installing units in the walls of hotels, motels, residences, etc. 7. There are many special mounts that can be used. We will not discuss each one. A special mounting may be used for casement windows, swinging windows, and office windows with swinging units, to permit window washing. Special mounts are also used for transom windows over doorways. 8. Installation and Operation. Installation procedures vary because units can be mounted in several ways. It is important to consider the most suitable mounting for the installation, the user's desires, and existing building codes. 9. Electrical system. We've already discussed the electric power source needed for a 115- or 230-volt unit, but we didn't cover proper grounding of the unit. All window type air conditioners, regardless of voltage or amperage rating, must be grounded. Most units are equipped with grounding type male plugs. These plugs are used with a grounded (three-prong) 115-volt receptacle. 10. The National Electric Code states that noncurrent-carrying metal parts which are liable to become energized shall be grounded under one or more of the following conditions: a. Where permanently connected to metal-clad wiring. b. When in a wet location and not isolated. c. When within reach of a person standing on the ground outside the building. d. When in a hazardous location. e. When in electrical contact with metal or metal lathe. f. Where the voltage is more than 150 volts to ground. 11. Can you think of any installation that wouldn't require grounding? It's very doubtful that you can, so remember, whenever you install a unit, make sure it's grounded. 12. Can you plug the unit into any receptacle? Yes, if the total load of the air-conditioning equipment does not exceed 80 percent of the current rating of the branch circuit, provided the voltage rating is satisfied. If the branch circuit also feeds lighting units or other appliances, the total load of the air conditioner shall not exceed 50 percent of the current rating of the circuit. 13. If a question about the power source or grounding arises, contact an electrician. He is a specialist in electricity, as you are in refrigeration. The manufacturer includes instruction sheets with his unit. You will find these helpful in mounting and installing the unit.

14. Through-the-wall units with steam or hot wire coils must be wired in or connected with armored cable or conduit. The electrician should complete this task. When the cooling unit can be removed without disturbing the heating system, it is customary to provide enough wire to facilitate installation and servicing without disconnecting the entire air conditioner. 15. The electrical system of an air conditioner consists of an appliance cord, plug, thermostat, fan motor (s), starting relay, starting capacitor, running capacitor, compressor motor, overload protector, and switches that control the flow of current to the various electrical portions of the system. Now we will discuss each electrical component. 16. The appliance cord and plug are manufactured as one unit. The cord is usually a three-wire cord with two current-carrying conductors and a ground wire. The conductor should be the correct size to carry the current that is necessary to operate the unit. The round third terminal of the service plug (1 15-volt) is the grounding terminal and should never be removed. 17. The control switch (es) mounted on the control panel of the unit directs electric current to various portions of the system to satisfy the desires of the user. All functions of the switch (es) are clearly marked. 18. The thermostat automatically controls the operation of the compressor motor, fan motor, and accessories to provide the comfort conditions required by the user. This control is accomplished by a feeler bulb located in the return airstream. The ambient temperature of the feeler bulb causes a bellows in the thermostat to expand or contract. This in turn causes the thermostat switch to open or close electrical contacts to the compressor and fan motors. Some thermostats have positions which afford the user constant operation. 19. The fan motor(s) and fans provide the forced air through the evaporator and condenser coils. The fan motor always operates when the compressor is running. 20. The starting relay, used frequently on 115-volt units, may be of the voltage operated type with normally closed contacts. The relay magnetic coil is wired in parallel with the starting winding of the compressor motor. Voltage developed by motor operation at 80 to 90 percent of full speed is impressed on the relay coil, which opens its contacts. With the relay contacts closed, the starting and running capacitors are wired in parallel with each other and in series with the compressor motor starting winding. When the relay contacts open, the starting capacitor is disconnected, but the running capacitor remains in series with the starting winding. 21. The starting capacitor stores electricity and provides power for extra compressor motor starting torque at the starting instant. This capacitor remains in the circuit for only a brief 26

Figure 9. Room air-conditioner wiring diagram. interval at startup. If the starting relay fails to quickly take the starting capacitor out of the circuit, it is possible that the starting capacitor will fail. 22. The running capacitor, which can be a heavyduty, oil-filled capacitor, is used in the circuit to reduce the current requirements of the compressor motor power factor. 23. The compressor motor may he of various types. Two common types arc the capacitor start-capacitor run and the permanent-split capacitor motor. The permanent-split capacitor motor doesn't use a starting relay. 24. The motor overload protector is used on all compressor motor:; to protect them against excessive current draw and abnormal heat. The overload protector is usually mounted either under the terminal block cover or on top of the compressor against the shell. The overload protector consists of a bimetal strip with contacts and, in some cases, a heater clement. Excessive current draw will cause the heater element or the bimetal strip itself to heat up, thereby causing the contacts to open. Excessive compressor shell temperature can also cause the bimetal strip to open the contacts. 25. When the bimetal contacts open, they remain open until the temperature of the heater and/or compressor shall have cooled enough to cause a reset action. When making replacements, never use an unknown type of overload protector. Replace the overload protector with a like unit, as shown in an illustrated parts breakdown for that specific air conditioner. Be certain, also, that the overload protector has good metal-to-metal contact with the compressor shell to protect the compressor motor. 26. The operation of the electrical system for all window-mounted air conditioners are similar. We will use an air conditioner with a two-speed fan motor as an illustration. Figure 9 shows the wiring diagram for this air conditioner. With the pushbutton switch in the cooling position, current is applied to the fan motor Remember, the fan motor always operates when the compressor motor is running. Current also passes the thermostat. If the thermostat is calling for cooling, the compressor motor is energized. The compressor starts to rotate, helped by the starting capacitor. The start capacitor is in the circuit when the compressor motor starts, because the starting relay contacts on the voltage type relay are always closed when the relay coil is not energized. When the compressor reaches 80 to 90 percent of full speed, the voltage developed in the start winding is impressed upon the relay coil. The voltage developed at this speed is enough to pull the relay contacts open and take the starting capacitor out

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of the circuit. The compressor continues to run on the run winding and the running capacitor. Now we'll turn our thoughts to the refrigeration cycle. 27. Refrigeration cycle. The refrigeration cycle of an air conditioner consists of a compressor motor, condenser coil, evaporator coil, capillary tube, strainer assembly, and its interconnecting tubing. 28. The fan motor(s) circulates air to remove the heat picked up by the refrigerant system. The condenser fan brings in outside air and forces it through the condenser coil, where it picks up heat and carries it to the outside air. The evaporator blower wheel recirculates room air, passing it through the cold evaporator, where moisture in the air condenses and its heat is absorbed by the refrigerant in the cooling system. The refrigerant system is hermetically sealed. 29. The compressor pumps the low-pressure gas from the interior of the compressor shell into the discharge line. The high-pressure gas, with its heat concentrated by compression, is forced into the condenser. The high-pressure gas is raised in temperature, at the compressor, above the outside air temperature which is being used to cool the condenser. The hot gas gives up its heat as it passes through the condenser coils. The hot refrigerant gas gives up enough heat to condense to a liquid. High-pressure liquid refrigerant leaves the condenser. It now passes through a strainer assembly. The strainer is an enlarged tube with a very fine mesh screen to remove any foreign particles. The high-pressure liquid now enters the capillary tube. The capillary tube acts as a restrictor (metering device) and separates the high side of the system from the low side. 30. The high-pressure liquid refrigerant is reduced in pressure by the restrictive action of the capillary tube. The liquid enters the evaporator low-pressure area, which was created by the suction stroke of the compressor. The liquid refrigerant exposed to this reduced pressure begins to boil and absorb more heat from the recirculated warm air. The boiling action of the liquid refrigerant progresses throughout the evaporator tubes, picking up heat as it travels. This low-pressure liquid now changes to lowpressure gas which is drawn out of the evaporator and back to the compressor, where the cycle is repeated. 31. Airflow system. The airflow system consists of a fan motor(s), evaporator blower wheel, condenser fan(s), and their housings. Two-speed or variable-speed fans and controls may be used. Many air conditioners have two separate airflow systems. These systems are the room air cooling, ventilating circuit and the condensing or outside air circuit. They are separated by a bulkhead and gasket 32. To maintain peak performance, it is important that the filter, evaporator coils, and condenser coils be

kept clean. Any restriction of airflow to these components will result in reduced unit capacity. 33. The evaporator blower wheel draws the room air through the louvered grille of the cabinet, through the filter, then through the evaporator coils. It is then discharged back into the room. 34. The condenser fan draws its air from the outside. It then passes this air over the compressor and electrical controls and out the condenser, where it carries off heat collected from the room air and the various components of the air conditioners. The condenser fan is often equipped with a slinger ring that removes condensate water collected from the evaporator. This is done by slinging the condensate on the condenser where it evaporates. 35. Air seals are provided around the outer edge of the evaporator housing and in the front grille to restrict airflow to its proper path. Now that we've discussed the various systems, we can relate the troubleshooting techniques you might use on them. 36. Trouble Diagnosis and Testing Procedures. There are various pieces of test equipment you could use to diagnose trouble. These are the ohmmeter, voltwattmeter, load checker, test starting set, and psychrometer. 37. Electrical system. If you find the air conditioner inoperative with the service cord plugged into a power supply, check the electrical outlet with a test lamp or voltmeter. If power is not available, you must check the possible faults which we will discuss in the next paragraphs. 38. First, examine the fuse box for blown fuses (circuit breaker for tripped breakers) and be certain fuses are of the time-delay type and of the correct size as indicated on the front of the air conditioner. If no power is available at the line side of the fuse box, tell the user and advise the electric shop. 39. To determine if the power supply is adequate, check the voltage at the power source with a voltmeter. The voltage must be within ±10 percent of the voltage required with the air conditioner on maximum cooling. A load checker may be used to simulate the wattage that the air conditioner will draw. 40. If the correct voltage is available at the receptacle, examine the service plug to be sure it is making good contact with the receptacle. Remove the service plug from the receptacle and check the service cord with an ohmmeter. Full continuity should exist the length of the service cord. You must check all the connections within the air conditioner to insure that they are securely fastened and making good contact. 41. Another possible fault could be grounding. The third wire of the service cord (green) is grounded to the chassis and will eliminate the 28

Figure 10. Test starting set. shock hazard. Internal grounds will cause fuses to blow or, if of a minor extent, cause excessive power consumption and breakdown of components. Grounded current-carrying paths in the electrical system should not be allowed to exist. 42. Grounds nay he eliminated by testing each wire or component with an ohmmeter. With the ohmmeter on a high scale, test between the wire and any bright metal of the chassis, such as the copper tubing. No continuity should exist between the wire or electrical components and the chassis. Continuity should exist between the round prong of the service plug and the chassis. This is the grounding line. 43. To check the starting relay, disconnect the service cord from tie power source and expose the relay. With the ohmmeter, test the relay switch contacts for continuity. If there is no continuity, replace the relay. Another malfunction that may exist within the relay is a grounded relay coil. If an ohmmeter check indicates no continuity across the coil, the relay must he replaced. 44. The starting and running capacitors may be checked with an ohmmeter. The capacitors must be removed from the circuit and fully discharged before making a test. You can discharge the capacitors by shorting the capacitor leads. The first indication you should observe with the ohmmeter is a short circuit (needle will swing toward 0) and then the reading should slowly change to indicate a resistance reading of approximately 100,000 ohms. 45. To test the compressor motor, you must first disconnect the service plug from the power source. Now attach the test starting set, shown in figure 10, to the compressor motor. You may use the starting capacitor on the air conditioner if you've already tested it and found it not malfunctioning. The test set plug should be connected to the same or equivalent power supply used for the air conditioner. Push down the push button switch, then release it. The compressor motor should start. If it didn't start or if it blew fuses repeatedly, replace the compressor. If the compressor starts, the

trouble is in one or more of the other electrical components. 46. One of the components that may be faulty is the overload protector. The contacts in the overload protector are normally closed. An ohmmeter check between any two of the terminals should indicate continuity. If no continuity exists, the overload protector must be replaced. If the overload protector opens repeatedly and the voltage, wattage, and temperature of the compressor are normal, substitute a known good protector. If the opening continues, the trouble lies elsewhere. 47. Another component that may be malfunctioning is the thermostat. The thermostat test is very easily made. Set the thermostat to its coldest position and test for continuity between the two terminals. If there is no continuity, the thermostat must be replaced. Make sure that the thermostat feeler bulb is above 70° F. To make it that warm, hold the bulb in your hand. 48. We have discussed the troubleshooting techniques that you may use on the electrical system of an air conditioner. Remember that all conditioners are not alike. Therefore you should always refer to the manufacturer’s manual and wiring diagrams. 49. Refrigeration system. In the event a user complains of insufficient cooling or no cooling, there are some logical checks you should make before troubleshooting the refrigeration system. One would be the electrical system and the other the airflow. We’ve already discussed the electrical system, so we’ll discuss what to check in the airflow. 50. Check the airflow system for cleanliness of the filter, evaporator coils, and condenser coils. If these are clean, check the fan speed. The fan speed may be checked with a portable tachometer. These malfunctions are the primary causes of low or no cooling. If no electrical or airflow fault is found, you must then troubleshoot the refrigeration system. 51. The correct refrigerant charge will be indicated by normal amperage draw. The amperage may be found on the data plate. Low amperage draw is an indication of low refrigerant charge, while a high draw may be an overcharge or dirty condenser.

52. Another test you may accomplish to check refrigerant charge is the "frost back test." With the unit running, block the evaporator air inlet with a piece of cardboard. After a period of time, the suction line should frost back to the compressor. A partial frost back indicates a low refrigerant charge. If the suction line does frost back to

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Figure 11. Thermometer placement for performance tests. the compressor, the air conditioner should be given a performance test to determine that it is operating to its fullest efficiency. 53. Two dry-bulb thermometers, one psychrometer, and a wattmeter are needed for this test. Before you make the test, allow the unit to operate at full capacity for one-half hour. Be sure that the damper doors in the unit and any doors or openings to the room are closed so that no outside air is allowed to enter the room and no cooled air is allowed to leave the room. 54. Next, you must position the louvers on the front of the air conditioner so that the conditioned air flows upward. Now you place the various instruments that you will use to obtain values for comparison with performance data and tables in their pertinent places. One of the dry-bulb thermometers is suspended in the condenser inlet airstream. You must be careful not to allow it to make contact with any metal parts and must keep it out of the direct rays of the sun. The remaining dry-bulb thermometer is supported in the approximate center of the evaporator air outlet stream. The psychrometer is placed in the center of the evaporator inlet airstream. Be certain that you wet the wet-bulb wick. The wattmeter is connected in series with the power supply to the air conditioner. Figure 11 shows the various locations of these instruments during the performance test of a window air conditioner. Read the 30 temperatures and wattage draw when the lowest wet-bulb temperature is obtained. All readings should be taken as nearly simultaneously as possible. 55. At this point you need a helper stationed outside to read the inlet condensing air temperature. These readings are now compared to the performance table values. Each manufacturer has these tables available for each model he produces. You cannot perform the test accurately without them. These tables contain information such as condenser inlet air temperature, evaporator inlet air temperature (wet bulb), evaporator inlet-outlet air temperature differential, total wattage, low side pressure, and high side pressure. 56. Let's assume the following readings were taken from an Anthony make air conditioner, model 21-958806. This is a 1-H.P. 115-volt unit. Figure 12 is the performance table for this air conditioner. The condenser air inlet temperature is 95° F. and the evaporator air inlet temperature is 67° F. You must now refer to figure 12. Under column A we find the 95° F. condenser air inlet temperature, and in column B we find the 67° F. evaporator air inlet temperature. When we follow to the right from the 67° F. reading, we find the following values: Evaporator Inlet-Outlet Air Temperature Differential 17°-24° F.

Figure 12. Performance chart. Watts Total to Unit 1290-1470 Watts. Low Side Pressure 71-75 p.s.i.g. High Side Pressure 300-330 p.s.i.g. 57. From these performance values, we find that the normal air temperature drop across the evaporator is 17°24° F. If the temperature drop exceeded 24° F., you could Suspect a dirty filter, incorrect fan speed, or a restriction in the evaporator airflow. A temperature less than the minimum (17° F.) could indicate low line voltage, air leakage from normal paths, or a dirty condenser 58. Well, let's say that we've found the air conditioner operating at its fullest capacity and the user still complains of insufficient cooling. One possible remedy would be to replace the unit with a unit of larger capacity or install an additional unit. 59. If you have determined, by use of the performance test, that the air conditioner is not operating at its fullest capacity, you must troubleshoot the unit. The last possible fault you should troubleshoot is a low refrigerant charge. If leak testing is necessary, you could use the following test procedures. a. Expose the unit and the refrigeration component. b. Examine all components and tubing for breaks, cracks, and traces of oil. (Since a small amount of oil travels through the system with the refrigerant, a trace of oil would be a good indication of a leak.) c. With a halide leak detector, probe every joint for a leak source. 60. A soap-water solution may also be used for finding leaks. Finding a leak is one of three conditions that may make entry into the sealed unit necessary. The other conditions are restrictions and compressor and/or compressor motor failure. 61. Service Procedures. The service literature published by the manufacturer contains replacement diagrams for each model he produces. The knowledge you will gain from a close examination of these diagrams will help make the replacement of any part of an air conditioner readily possible. A step-by-step procedure is usually given on more difficult part removal items. This procedure is usually brief and keyed to a picture by numbers. To replace parts that you've removed, reverse the sequence you used to remove them. 62. Filters. Dirty filters, along with low voltage, are the major causes of poor performance of an air conditioner. You should familiarize the user with the location of the filter and with the fact that it should be inspected frequently for cleaning or replacement. Aluminum mesh type filters may be cleaned as often as necessary without damage. The filter should be cleaned with hot soapy water, then flushed thoroughly. After the filter is dry, it should be recoated with a domestic mineral oil or commercial type metal filter coating. You should explain to the user that the entire surface of both faces of the filter requires recoating. This procedure increases the dirt enhancing quality of the filter. 63. Condenser and evaporator. The coils of the condenser and evaporator should be cleaned periodically. You may accomplish this task with a soft brush or a vacuum cleaner. 64. Condensate disposal system. In order to reduce the humidity in the conditioned area, an appreciable amount of air-conditioner capacity is required. This is referred to as latent load,

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Figure 13. Sectional view of an air-cooled floor-mounted air conditioner. while the actual reduction of air temperature is called the sensible load. 65. As the room air passes through the evaporator coils, the moisture in the air condenses. This condensate then drains back to the condenser housing sump, where it is drained or picked up by the slinger ring of the condenser fan. The slinger ring blows the water into the hot surfaces of the condenser coil. This moisture helps to cool the condenser coil as it is vaporized and blown into the outside air. It is important to keep the condensate drain clear to allow free drainage of this water. Proper slope of the air conditioner to the rear provides for this drainage. 32 12. Floor-Mounted Units 1. The floor-mounted or console air conditioner may have either water-cooed or air-cooled condensing units. The air-cooled model is gaining in popularity due to water restrictions. Figure 13 shows an air-cooled console unit. Note how the condensate from the evaporator coil is entrained in the condenser air. We also find that two separate fans and motors are used to move the air, while the window-mounted air conditioner normally uses one Fresh air or ventilating air is bypassed from the condenser air just as it leaves the condenser fan. 2. This air conditioner is applied to either resi-

dential or commercial use. The latter is used for comfort cooling and control of temperature and humidity for manufacturing purposes. In residences it is used for comfort only. 3. We won't discuss each component of the various systems which make up the air-conditioner, as they are directly related to components that we've already discussed. Instead, we will discuss each system briefly with more thought concentrated on components peculiar to this unit. 4. Refrigeration System. The refrigeration system consists of a compressor, cooling coil, condenser, expansion device, and the necessary interconnecting tubing. The components peculiar to this system are the different types of condensers, expansion devices, and compressor capacity controls. 5. Condensers. We will discuss the various types of condensers that you may find on this air conditioner and how they are cleaned. The most common is the aircooled condenser. The air-cooled condenser consists of coils over which air is blown. Refrigerant cooling is obtained by adequate condenser surface and maximum sir circulation over the outside coil and fin surface. 6. Ordinary brushes and mild soap cleaning solutions will remove the usual dirt and dust deposited on air-cooled condensers. However, in some applications, the materials that may be deposited on the condenser cannot be removed by these means. If this situation arises, you can make a good cleaning agent by mixing 1/2 pound of trisodium phosphate with 1 gallon of water. After you use an acid or alkaline solution, you should rinse or flush the condenser with large quantities of clear water. 7. Another problem that you may have to cope with is carbon deposits. There is more danger to restricted airflow in using a solution which would loosen, or partially loosen, the carbon deposit than there would be in using a solution which would not remove all of it. The loosened particles could plug the condenser. The most satisfactory method of cleaning the inner portion of the condenser is to use superheated steam. You will find that this method will do a thorough job of removing all the loosened material from the inside of the condenser, and will prevent formation of any oxide or other material on the coils and fins. One precaution you should apply when using superheated steam is to be sure that the temperature of the steam is not above the melting point of any of the materials from which the condenser is constructed. 8. One water-cooled condenser (shell-and-tube) consists of a gas type sealed shell containing a copper coil or tubes. The hot refrigerant gas is admitted into the

condenser shell and flows down over the condenser tubes in which cooling water is circulated. The gas condenses on the surface of the tubes and runs to the bottom of the condenser shell. These condensers are used frequently where the cooling load is heavy and the ambient temperature may rise over 90° F. 9. The tubes in a shell-and-tube condenser have a tendency to become coated, and sometimes even filled, with deposits (magnesium, calcium, etc.) from the water that passes through them. The safest method that you may employ for cleaning a water tube is soft metal brushes. You should start with a small diameter brush and increase the diameter of the brushes until one that is just the diameter of the inside of the tube is used. Do not attempt to apply force to the brush rod with a hammer. A large piece of scale or deposit could cause the rod to deflect and rupture the tubing wall opposite the deposit. Most tubes are not galvanized or coated with a surface-protecting material, so you should oil each tube after it has been cleaned. This may be done by drawing an oil-soaked cloth through the tube. The film of oil will prevent oxidation and will be washed off in a short time after water has run through the tube. 10. Another water-cooled condenser is the doublepipe condenser. This type of water-cooled condenser has become very popular because of its performance and its convenience of manufacture. The double-pipe condenser consists of one pipe inside a large pipe. The ends of the larger pipe are sealed against the inside pipe so that liquids or gases may be directed through its entire length. 11. The water usually passes through the inner pipe, and the refrigerant through the outer. The counterflow principle is usually employed so that the lowest temperature water comes in contact with the lowest temperature refrigerant This type of operation could lower the leaving refrigerant temperature to the same temperature of the incoming. water. However, a 10° differential is considered satisfactory. 12. Liquid cleaning with solutions of strong caustic soda or mild muriatic acid is practically the only cleaning method you can use on this type of water-cooled condenser. You should test the strength of the solution before you use it, as it may weaken the tube. The manufacturer usually recommends the strength of solution you should use on his condenser and how to test it. When you mix the solution, always add the acid to the water and wear the appropriate safety equipment. 13. On large equipment, double-pipe condensers are made of iron pipe. These are so constructed that the return ells can be removed. This exposes the inner tube so that it may be cleaned with a brush, as were the tubes in the shell-and-tube condenser.

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Figure 14. Evaporative condenser. 14. The repair of water-cooled condensers is usually performed by the manufacturer, unless it is otherwise specified in the manufacturer's service publication. In this case a detailed breakdown and procedure are given. 15. The evaporative condenser, shown in figure 14, consists of a fan and motor, eliminators, condensing coil, waterpump, spray headers and nozzles, a water sump, and a makeup water valve. 16. The operation of the evaporative condenser is similar to that of the cooling tower except that condensing coils are installed in the airflow. The two types of evaporative condensers are the draw-through and the blow-through. We will limit our discussion to the draw-through type, as it is the most popular of the two. 17. First, we'll discuss the operation of the evaporative condenser. In the draw-through type, we find that the air enters at the sump plenum, then flows up through the condenser coils, spray nozzles, eliminators, and out to the atmosphere. The entering air causes the water on the condensing cods to evaporate. The evaporation process removes heat from the refrigerant within the coils, causing the high-pressure gas

to condense. The moisture-laden air then passes on to the eliminator plates, which are closely spaced surfaces that provide abrupt changes in airflow direction. The moisture particles are deposited on these surfaces and drained back to the sump. Effective elimination of moisture from the leaving air is essential to prevent projection of mist which can deposit moisture on surrounding surfaces. The carryover of water particles will also tend to form scale on the fan blades, thereby causing operational difficulties. 18. Scale is formed by low soluble salts. Polyphosphate chemicals may be added to the water, thereby enabling the water to become supersaturated without precipitation of scale-forming solids. Let's look at figure 14 again. We haven't mentioned the bleed tube which allows some of the pump discharge water to -drain off. What does that have to do with the formation of scale? Before we answer that question, let's state a few facts. a. The water contains suspended solids (salts, iron, etc.). b. The cycles of concentration increase each time the water circulates through the system (evaporization of water). c. The dissolved solids are less soluble because of changes in the water temperature (water heated by condenser coils). 19. With these facts in mind, we find that bleeding off some of the recirculated water and replenishing it with makeup water will decrease the amount of solids suspended in the cooling water. Remember, the cycle of concentration is the ratio of bleedoff water hardness to makeup water hardness. The bleedoff water rate is directly proportional to the amount of water being evaporated. Continuous bleedoff, with rates based on condenser evaporation rate and makeup water hardness, decreases the precipitation of scale on the condenser coil. Scale on the condenser coil surface decreases the heat transmission through the surface and may reduce airflow. 20. Normally the next component in the refrigeration system is the receiver. Since you are already familiar with the receiver, we will bypass our discussion of it and proceed to the expansion device. 21. Expansion device. The most common expansion device used on this air conditioner is the thermostatic expansion valve with an external equalizer and distributor. The external equalizer is used to compensate for the pressure drop across the evaporator. A valve with an internal equalizer would cause the evaporator to starve because the pressure sensed under the valve diaphragm would be the inlet evaporator pressure. Let's set up an example to clarify our discussion. 22. Figure 15 shows a thermostatic expansion valve (for refrigerant -12) with an internal 34

Figure 15. Thermostatic expansion valve with an internal equalizer. equalizer. We are going to give the evaporator a 10p.s.i.g. pressure drop across it. We find that the pressure acting on the lower side of the diaphragm is 37 p.s.i.g., which is causing the valve to close. With the valve superheat spring set at a compression equivalent to 10° F. superheat or a pressure of 9.7 p.s.i.g., the required pressure above the diaphragm to equalize the forces is 46.7 p.s.i.g. (37 + 9.7). Using your temperature-pressure chart, you will find that this pressure corresponds to a saturation temperature of 50° F. Therefore, the refrigerant temperature at point C must be 50° F. if the valve is to be in equilibrium. Since the pressure at this point is only 27 p.s.i.g. and the corresponding saturation temperature is 28° F., a superheat of 22° F. (50 - 28) is required to open the valve. This increase in superheat makes it necessary to use more of the evaporator surface to produce this higher superheated refrigerant gas. Therefore the amount of evaporator surface available for absorption of latent heat of vaporization of the refrigerant is reduced. The evaporator would be starved before the required superheat is reached. This starving effect increases as the load increases. 23. To compensate for an excessive pressure drop through an evaporator, you should install a valve of the external equalizer type, with the equalizer line connected into the evaporator (at a point beyond the greatest pressure drop) or into the suction line (on the compressor side of the remote bulb installation). The most common installation is in the suction line. When this valve is used, the true evaporator outlet pressure is exerted under the diaphragm. The operating pressures on the valve diaphragm are now free from any effect of the pressure drop, and the valve will respond to the superheat of the refrigerant gas leaving the evaporator. 24. The same pressure drop still exists through the evaporator; however, the pressure under the diaphragm is now the same as the pressure at point C, or 27 p.s.i.g.

The required pressure above the diaphragm for equilibrium is 27 + 9.7, or 36.7 p.s.i.g. This pressure corresponds to a saturation temperature of 40° F., and the superheat required is now 40 - 28, or 12° F. The use of an external equalizer has reduced the superheat from 22° F. to 12° F. Thus the capacity of a system will be increased. 25. The expansion device shown in figure 16 is the thermostatic expansion valve and pressure drop distributor. This arrangement is used on the multicircuit evaporator to assure that an equal mixture of gas and liquid refrigerant reaches each evaporator circuit An external equalizer is used to compensate for the pressure drop caused by the distributor. An internally equalized valve would limit the action of the valve and cause the evaporator to starve. 26. The maintenance and servicing of this valve is similar to that of the common thermostatic expansion valve. 27. Our next discussion will cover the various capacity controls that may be used on the system. These are the hydraulic cylinder unloader and the compressor bypass valve. 28. Capacity controls. The hydraulic cylinder unloader is used to improve compressor capacity control during light load conditions. The unloader accomplishes this by holding open the suction valve on some cylinders and allowing the piston to draw gas on the downstroke, but on the upstroke it returns the gas to the suction line without compressing it. 29. On single-step unloader systems, one-half of the cylinders are unloaded, while on multistep unloaders the cylinders are unloaded in increments. These increments depend on the number of cylinder in the compressor. Figure 17 and 18 illustrate a typical unloader mechanism, loaded and unloaded. The bottom portion of each figure shows the capacity control actuator. 30. To understand the complete operational cycle, you should think of the unloader mechanism

Figure 16. Single outlet expansion valve with a pressure drop distributor.

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against a combination of atmospheric pressure and force from a spring (5). The amount of spring tension is adjustable by a set screw (6). When the system requires less than full-refrigeration load, the suction pressure will fall below the predetermined point, causing an unbalance within the device, and the unloading cycle will commence. The drop in suction pressure permits the bellows (3) to expand, forcing the plunger (7) against the lever (8), and moving it downward. The downward movement of this lever opens the regulated orifice (9). The opening and closing of this orifice controls the action of the valving mechanism. 33. The function of the valving mechanism is to supply each of the cylinder unloaders with oil under pump pressure when full compressor capacity is required and to relieve this pressure when the cylinders are to operate unloaded. This valving mechanism consists of a hydraulic cylinder, containing an annularly grooved, floating piston

Figure 17. Hydraulic cylinder unloader (loaded). as two distinct components, the capacity control actuator and the cylinder unloader mechanism. 31. The capacity control actuator reacts to variations in refrigeration load requirements and transmits them to the cylinder unloader mechanisms which load or unload the cylinders. To perform this dual function, the capacity control actuator consists of a pressure-sensing device, which is sensitive to variation in suction pressure; and a valving mechanism, which regulates the oil pressure to the various cylinder unloader mechanisms. 32. The pressure-sensing devices (fig. 17) consist of a chamber (1) connected to the suction line (2) and a bellows (3), which is vented to atmosphere (4). The function of the pressure-sensing device is to maintain, as nearly as possible, a predetermined suction pressure. This pressure is the maximum pressure required to satisfy the refrigeration system. The specific set point is maintained by a balance of forces. Suction pressure is balanced 36

Figure 18. Hydraulic cylinder unloader (unloaded).

(11). The annular grooves are constantly fed with oil through a line (23). 34. Above the piston is a chamber (12) vented to the crankcase through an orifice (10). Below the piston is another chamber (13) connected to the annular grooves in the piston by an orifice (14). It is also connected to crankcase pressure through a regulated orifice (9). Located within the hydraulic cylinder is a spring (15), which tends to move the floating piston toward the lower chamber. 35. Under full capacity operation, as shown in figure 17, the regulated orifice (9) is shut off and the oil pressure in the lower chamber (13) increases because oil under pump pressure is being supplied through orifice (23). This pressure overcomes the force of the spring (15) and the floating piston (11), which rises in the cylinder. As it rises, the annular grooves in the floating piston coincide in sequence with lines 16-1, 16-2, and 163 to the cylinder unloaders, providing them with full oil pressure and permitting them to operate at full capacity. To make figures 17 and 18 as simple as possible, only line 16-1 is connected to a cylinder unloader mechanism. Lines 16-2 and 16-3 are, in reality, connected to identical mechanisms; and while this discussion is concerned with only one unloader mechanism, we could extend it to cover them all. 36. When full compressor capacity is not required, the regulated orifice (9) is opened through the movement of the lever (8); oil bleeds through it, and pressure within the lower chamber approaches crankcase pressure, as shown in figure 18. Under these circumstances, the force of the spring (15) overcomes the pressure in the lower chamber, and the floating piston (11) is moved downward so that lines 16-1, 16-2, and 16-3 become connected in sequence to crankcase pressure through the orifice (10). The spring-loaded ball (24) permits the piston to move only in distinct increments, one groove at a time. 37. In this manner the valving mechanism supplies or withdraws from each cylinder unloader the oil pressure that operates the unloader mechanism. 38. When oil from the forced feed lubricating system flows through line 16-1 from the valving mechanism to the cylinder unloader, it enters the annular chamber (17). The inner wall or unloader cylinder is firmly anchored to the cylinder liner; the unloader piston (18), however, is free to move. The up and down movement of this unloader piston raises and lowers the takeup ring (19), which, in turn, raises and lowers the suction valve lift pins (20). 39. Under full capacity operation (fig. 17), oil flows into the annular chamber (17) under pressure sufficient to contract the unloader piston springs (21 ). When oil

pressure forces the springs to contract, the unloader piston (18) moves down, and takeup ring (19) and the suction valve lift pins (20) move with it. This permits the suction valve (22) to function normally and the cylinder operates at full capacity. When the compressor is to operate at less than full capacity (fig. 18), crankcase pressure flows through the orifice (10), which allows the pressure in the annular chamber (17) to dissipate; the cylinder unloader springs (21) expand, lifting the unloader piston (18). This raises the takeup ring (19) and the valve lift pins (20), and holds the suction valve (22) open so that the controlled cylinder is operating in an unloaded condition. 40. You will find that the compressor unloader is sensitive to variations in suction pressure. It may be desirable to unload the compressor in response to variations in air temperatures. This can be accomplished also through the use of pneumatic or electric controls. By introducing controlled air pressure from a pneumatic thermostat to the inside of bellows (3), in place of normal atmospheric pressure, the suction pressure at which unloading begins can be varied, thus making compressor operation responsive to variations in air temperature as sensed by the pneumatic thermostat. When electric control of unloading is desired, the screw (6) is replaced by a mechanical device. It resets the suction pressure which causes unloading to begin. This device is driven by an electric motor that is positioned by an electric thermostat. 41. The hydraulic cylinder unloader may be adjusted to maintain a balance between the load and compressor capacity. This adjustment is usually made after an installation. We will discuss this adjustment, but you should follow the manufacturers recommendations while performing this task on your specific piece of equipment. Before you adjust the capacity control, you must load the system either naturally or artificially until design suction pressure is reached with the adjusting screw (6) turned all the way out. Now, slowly open the suction shutoff valve until the suction pressure is 2 p.s.i.g. below the design pressure. 42. The next step is to turn the adjusting screw clockwise until the first cylinder unloads. Just before it unloads, the control oil pressure will drop to approximately 26 p.s.i.g. below oil pump pressure. The control oil pressure is present in line 16-1. When the cylinder unloads, there will be a distinct change in the sound of the compressor and in the amperage being drawn. The remaining cylinder are automatically unloaded as suction pressure drops. 43. Maintenance of the cylinder unloader can be found in the manufacturer's publications. We

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Figure 19. Direct-acting solenoid valve. will not discuss it as it may not be applicable to your equipment and would only tend to confuse you. 44. The next capacity control device you will encounter is the compressor bypass valve. A special solenoid hot gas valve, installed in a bypass line around one or more cylinders, will provide compressor capacity control. The valve may be operated either by a thermostat or a switch. A check valve is required in the discharge line beyond the bypass line to prevent a reverse flow of discharge gas from other cylinders. 45. There are two types of solenoid valves you may find on this installation: the direct-acting, shown in figure 19, and the pilot-operated, illustrated in figure 20. In the direct-acting type, the pull of the solenoid coil opens the valve port directly by lifting the pin out of the valve seat. Since this valve depends solely on the power of the solenoid coil for operation, its port size for a given pressure differential is limited by the solenoid coil size. 46. Therefore large solenoid valves are usually of the pilot-operated design. In this type the solenoid plunger does not open the main port directly, but merely opens the pilot port (A). Pressure trapped on top of the piston (B) is released through the pilot port, thus creating a pressure unbalance across the piston (B). The pressure underneath is now greater than that above and the piston moves upward. This opens the main port (C). To close port C, the coil is deenergized, causing the plunger to drop and close the pilot port (A). Now the pressures above and below piston (B) equalize. The piston (B) will now close the main port (C). The pressure difference across the valve, acting upon the area of the valve seat, holds the piston in a tightly closed position. 47. You may have to select a solenoid valve while performing your routine duty. You'll find that there are many applications for this device. When you select a

valve, you should know the fluid to be controlled, cap city (in tons of refrigeration), maximum operating pressure differential, maximum working pressure, and electrical characteristics. The capacities of solenoid valves are given in tons of refrigeration at standard conditions for the various refrigerants, with a pressure drop across the valve of 2 or 4 p.s.i.g. for liquids and 1 p.s.i.g. for gas. Most manufacturers publish tables extending these capacities for higher pressure drops. 48. You'll find that all solenoid valves are rated in terms of the maximum operating pressure differential (m.o.p.d.) against which the valve will open. Let's use an example here to clarify our discussion. With the valve closed and an upstream pressure of 150 p.s.i.g. against a downstream pressure of 50 p.s.i.g., the pressure differential across the valve would be 150 - 50, of 100 p.s.i.g. The m.o.p.d. of this valve must be equal to, or in excess of, the valve (100 p.s.i.g.). 49. Now that you've selected the valve, the next step is to install it. Remember, most solenoid valves are designed to operate in a vertical position and, therefore, must be installed in a horizontal line. Special valves are available to be installed in any position. When installing the valve, be sure the arrow on the valve body points in the direction of refrigerant flow. The final step in the installation of the valve is the electrical wiring. You must be sure that the voltage, type of current, and frequency marked on the valve nameplate are compatible with the system voltage, current, and frequency.

Figure 20. Large pilot-operated solenoid valve.

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50. Should the valve fail to function after installation, the following are some of the probable causes of failure and suggestions for correcting them: a. Solenoid valve fails to open. (1) System operating pressure too high. The m.o.p.d. rating of the valve may be lower than the actual differential. A valve with a higher m.o.p.d. must be used. (2) Valve body or internal parts are warped. These faults are caused by excessive wrench torque or high brazing temperatures.. You must replace the damaged part or the entire valve as required. (3) Dirt or sludge causing valve to stick. You must dismantle the valve and completely clean the interior and component parts. Use an approved cleaning agent. (4) Low voltage. Check the power supply with a voltmeter. The applied voltage must be at least 85.percent of the rated voltage given on the nameplate. For example, a 115-volt solenoid requires at least 97.75 or 98 volts. If the voltage Is lower than 98 volts, the cause of the voltage drop must be determined and corrected. Common causes of voltage drops are undersized supply lines, other loads connected in series with the coil, loose or faulty connections, and faulty control switches or devices. (5) Coil burnout. Excessive voltage is the primary cause of coil burnout. Coils should not be subjected to voltages higher than 10 percent above the rated nameplate voltage-126.5 volts for a 115-volt rated coil. High ambient temperatures can also cause coil burnouts. Use a special high-temperature coil if your evaluation established overheating as the fault. b. Solenoid valve fails to close. (1) Valve body or internal parts are warped. Cause and correction same as (2) under subparagraph , Solenoid valve fails to open. (2) Dirt or sludge causing valve to stick. Cause and correction same as (3) under subparagraph a. (3) Electrical circuit closed. Troubleshoot electric circuit and repair or replace the faulty component (switch, relay, thermostat, etc.). (4) Congealed oil causing valve to stick. Refrigerant oil should be of the proper type for temperature range of the system. Corrections is the same as (3) under subparagraph . 51. These service hints are general in nature and can be applied to most solenoid valves. If problems arise beyond this scope, you should consult the manufacturer's service manual. 52. There are many other components that we might discuss but they are peculiar to one manufacturer only.

53. We will now enter into discussion of the airhandling system. The components that will be discussed are fans, motors, and drives. 54. Air-Handling System. The air-handling system on the floor-mounted air conditioner in similar to that on the window-mounted type except that the components are larger. 55. Fans. The two types of fans common to this air conditioner are the propeller and centrifugal fans. The propeller fan consists of a propeller, or disk wheel, within a mounting ring or plate; the centrifugal fan consists of a fan rotor, or wheel, within a scroll type of housing. 56. In some air-conditioning systems, it is desirable to vary the volume of air handled by the fan. This may be done by a number of methods. Where the change is made infrequently, the pulley, or sheave, on the drive moor or fan may be changed to vary the speed of the fan and alter the air volume. Dampers may be placed in the duct system to vary the volume. Variable-speed pulleys or transmissions, such as fan belt change boxes, or electric or hydraulic couplings, may be used to vary the fan speed. Fan volume can also be varied with the use of variable-speed motors and variable-inlet vanes. 57. From the standpoint of power consumption, the reduction of fan speed is most efficient when a direct mechanical method is employed. Inlet vanes save some power, while dampers save the least 58. When considering first, or initial cost, you'll find that damper are usually the lowest. Air supply demands and noise will dictate which type of control to install 59. Fan selection as to size and type depends on capacity, static pressure or system resistance, air density, type of application, arrangement of system, prevailing sound level, nature of load, and type of motive power available. To help you make your choice, a fan manufacturer will furnish you information on different sized fans working against different static pressures. Some of the important information is as follows: (1) volume of air (c.f.m.), (2) outlet velocity, (3) r.p.m., (4) brake horsepower, (5) tip or peripheral speed, and (6) static pressure. The most efficient operating point is usually shown by either boldface or italicized figures in the capacity tables. 60. Many fan applications and the corresponding types of fans commonly used are listed in the following paragraphs. 61. Unitary systems are equipped with centrifugal or propeller fans,, the later usually being limited to the relatively small suspended type (window-mounted) where no duct work is involved. Fans for units having considerable internal, or possible external resistance, amostly of the forward-curved blade, or so-called mixedflow centrifugal type. The latter is really a centrif-

39

ugal type with axial inlets, having a pressure curve resembling a backward-curved-blade centrifugal fan. Both types have the high capacities requisite for a compact unit. 62. Cooling tower fans are usually of the propeller type, but axial types are used for packed towers, and occasionally a centrifugal fan is used on the forced-draft tower. 63. Circulating fans are invariably of propeller, or disk type, and are made in a vast variety of blade shapes and arrangements. They are designed for a pleasing appearance as well as for efficiency. 64. We will discuss the maintenance of fans in Chapter 5. Now let's move on to the motors used on the floor-mounted unit. 65. Motors. Room air conditioner use motors ranging from 1/6 horsepower to 6 horsepower. Motors in most cases are controlled directly by the system controls, such as thermostats, timers, pressure switches, or other automatic devices. The most common voltage application is 115 or 230 volts. 66. The necessity for low current draw on a 115-volt circuit also makes it necessary to use permanent-splitcapacitor motors for the fans on room air conditioners. Shaded pole motors are used when current draw isn't a problem. 67. The hermetic compressor design makes it impossible and unnecessary to service compressor motors. Fan motors are accessible and you should service them as recommended by the manufacturer's instructions. 68. Permanent-split-capacitor motors do not require a starting switch, but the capacitor-start induction-run and the capacitor-start capacitor-run motors use a starting switch. 69. Motor protection is similar to that described in the previous section. Most compressor motors and airmoving motors are equipped with thermal protectors. These may be hermetically sealed for installation within the compressor shell or open for mounting on the outside compressor shell. Hermetically sealed protectors provide better protection where conditions such as loss of charge, obstructed suction line, or low ambient temperatures on stalled rotors can be troublesome. 70. When applying an electric motor, the following characteristic are important: a. Mechanical arrangement including the position of motor and shaft. b. Speed range. c. Horsepower. d. Torque. e. Inertia.

f. Frequency of operation. 71. The torque required to operate the driven machine (compressor) at every moment between initial startup and eventual shutdown is an important factor in determining the type of motor to be used. 72. The torque available at standstill, the starting torque, is usually welt above the torque at rated full load. The starting torque may be less than I00 percent, or as high as 300 percent of full load torque. 73. The starting current is usually 400 to 600 percent of the current at full load. 74. Full-load speed also depends upon the design of the motor. For induction motors, a speed of 1725 r.p.m. is typical for 4-pole motors, and a speed of 3450 r.p.m. for 2-pole motors (60-cycle). Full-load torque is the torque developed to produce the rated horsepower at the rated speed. Motors have a maximum or breakdown torque which cannot be exceeded without causing an abrupt change in speed. The relation between breakdown torque and full-load torque varies with motor design. 75. The required horsepower also determines the motor rating. The horsepower delivered by a motor is a product of its torque and speed. Since a given motor will deliver increasing horsepower up to a maximum torque, a basis for horsepower rating is needed. The National Electrical Manufacturers Association bases horsepower rating upon breakdown torque limits. Full-load rating of general purpose open type motors is related to the winding temperature rise, with a temperature rise limit of 40° C. by thermometer and of 50° C. by resistance for Class A insulation. Higher temperature rises may be allowed for enclosed and special-purpose motors. 76. The service factor of a general purpose motor is defined as "a multiplier which, applied to the normal horsepower rating, indicates a permissible loading which may be carried under the conditions specified for the service factor." Motors, other than general purpose motors, have a service factor of 1.0. 77. In this chapter the last subject we shall discuss is that of drives. 78. Drives. The fans and compressor motors are usually direct drive. Fans over 30 inches in diameter are belt driven to reduce the speed below that of the driving motor and to meet the sound level requirements. Housed fans in the smaller sizes (up to l0 inches) are either direct driven or belt driven, in accordance with sound level and other application requirements. Largersize housed fans are exclusively belt driven. 79. Adjustable pitch pulleys are usually provided to permit you to balance air delivery against system resistance.

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Review Exercises NOTE: The following exercises are study aids. Write your answer in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the text. Do not submit your answers for grading. 1. Before you plug in an air-conditioning unit you should read the _____________. (Sec. 11, Par. 5)

7. What three components of an air-conditioning unit will collect dir and thus restrict airflow through the unit? (Sec. 11, Par. 32)

8. Before you check a capacitor with an ohmmeter you should __________________ the capacitor. (Sec. 11, Par. 44)

2. When proceeding to plug in an 155-volt airconditioning unit you find that the receptacle is for a two-prong plug and the A/C unit hat a three-prong plug. You cut off the round prong and make your connection. What condition exists with the round prong removed? (Sec. 11, Pars. 9, 10, and 16)

9. During the process of troubleshooting an inoperative. compressor motor, you check the overload protector with an ohmmeter. What is the condition of the protector if. the meter indicates zero? (Sec. 11, Par. 46)

10. Does a low- or high-wattage draw indicate a low refrigerant charge? (Sec. II, Par. 51)

3. Is it permissible to connect a 9.5-ampere rated air conditioner to a 15-ampere circuit if other equipment connected to the same circuit uses 4 amperes? (Sec. 11, Par. 12)

11. What are the two major causes of poor performance of an air-conditioner? (Sec. 11, Par. 62)

4. You receive a work order to replace an airconditioner compressor motor that has burned out due to an overload. What other unit should you replace? (Sec. 11, Pars. 24 and 25)

12. What precaution should be observed when cleaning an air-conditioner condenser with superheated steam? (Sec. 12, Par. 7)

5. As the room air passes through the its heat is absorbed by the refrigerant. (Sec. 11, Par. 28) 13. When mixing a liquid acid cleaning solution should you add the water to the acid or the acid to the water? (Sec. 12, Par. 12) 6. At what component of the air-conditioning unit is the temperature of the refrigerant gas raised above the outside air temperature? (Sec. 11, Par. 29)

14. What will .result if the water bleed tube of the evaporative condenser should become clogged? (Sec. 12, Pars. 18 and 19)

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An air-conditioning compressor was written up because it would not unload during light load conditions. What could prevent the compressor from unloading? (Sec. 12, Pars. 31-39)

18. Should the air-conditioning compressor be loaded or unloaded when adjusting the cylinder unloader system? (Sec. 12, Par. 41)

What part of the capacity control actuator regulates the oil pressure to the compressor cylinder unloader mechanisms? (Sec. 12, Par. 31)

19. When you are preparing to install a solenoid valve, what two things on the valve should you check? (Sec. 12, Par. 49)

What pressure in the cylinder unloader mechanism will hold the compressor suction valves open? (Sec. 12, Par. 39)

20. You have been assigned the task of replacing a solenoid valve that has a burned out coil. What should you check for before installing the new valve? (Sec. 12, Par. 50)

21. What are the two methods of varying the volume of air handled by an air-conditioning system? (Sec. 12, Par. 56)

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CHAPTER 5

Fresh Air and Air Duct System
YOU HAVE probably heard this statement many times: "This room is smoky." As an air-conditioning mechanic you should be particularly interested. Why is the room smoky? Why can't the air-conditioning system handle the smoke? What can you do to correct this situation? 2. These questions and many others you may come upon will be answered in this chapter. Such subjects as dampers, fans, coils, and air duct systems are discussed. You will also learn how to determine duct sizes and how to balance a system. 13. Dampers 1. Dampers of the following types are usually installed in an air-conditioning system: a. Bypass damper (A of figure 21). b. Mixing damper (B of figure 21). c. Air intake damper (C of figure 21). d. Volume damper, as shown in figure 22. e. Recirculating damper (D of figure 21). 2. Bypass dampers control and regulate the airflow from return ducts and the intake openings. The air is diverted in specific directions to avoid airflow through a certain duct area. Mixing dampers are usually installed at duct and intake openings to provide a mixture of air from the exterior and interior spaces to the air-conditioning equipment. Mixing dampers control and regulate the airflow from room areas and the fresh air intake. The purpose of a mixing damper is to mix these volumes of air in the proper proportion. The volume damper is installed in the interior of the duct, as shown in figure 22. It provides a division for air volume to the openings by changing the area of the passageway. The volume damper is generally constructed of one blade and is fastened to the side of the duct with indicating sections on the exterior side of the duct surface for adjustments. 3. Intake dampers are installed at the opening connections to the air-conditioned equipment. These dampers usually control and regulate the airflow from the duct system connecting the spaces served to the equipment. The recirculation damper and exhaust damper are usually connected to a common motor by linkage. The linkage is arranged so that when one damper is open, the other damper is closed. Usually they can be at any position, from fully open to fully closed. 4. Dampers are installed in numerous ways to regulate and control air movement. They may have either one or several blades and may have automatic or manual controls. 5. Operation and Controls. All dampen operate either manually or through the use of automatic controls. Automatically controlled dampers are generally used in large air-conditioning systems and are operated by a motor. The motor operates pneumatically or electrically, and moves the dampers to various positions. The size, material, methods of operation, leverage, location, and signs of the motor vary with the manufacturer's specifications and installation requirements. Motor control and operation was covered in the preceding chapter. 6. Manual damper control is done through the use of rope or cable extensions. They are generally used as exit or intake dampers located in remote locations or inaccessible places in the system. 7. Maintenance. A few defects that might occur in the operation of a damper or louver follow: a. Bent shafts. b. Binding of blades or operating mechanism. c. Bent rods or levers. d. Air leakage. The most common defect that causes erratic damper operation is binding blades. 8. Wherever possible, you must inspect damper operation; and if any of the above defects are found, immediate action must be taken to repair the unit. 9. Most large dampers are built as a single unit and constricted with a frame to fit the duct and chamber opening. If the unit needs to be replaced, the complete damper unit can be taken out and a new damper installed. Whenever the services of a sheet metal worker are needed, the civil engineering section should be notified according to local procedures.

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Figure 21. Typical air-conditioning system.

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7. Supply and Booster Fans. Depending on their use in a duct system, a fan may be referred to as a supply fan or a booster fan. If a fan is furnishing or supplying a large volume of air, it is often referred to as a supply fan, its name being derived from the fact that it supplies the air. A booster fan is used in distributing air to a certain portion of the duct system. This fan helps in

Figure 22. Volume damper. 10. You should refer to construction drawings and specifications for location of dampers and installation details. 14. Fans 1. Fans are used for circulation of air in duct systems and are usually of one of the following types: a. Multiblade fans with blades curved backwards. b. Multiblade fans with blades curved forward. c. Multiblade fans with blades curved backward at the tip and forward at the heel. d. Multiblade fans with radial blades. e. Propeller or disk type. 2. The forward blade fan is a commonly used fan. It operates at a relatively low tip speed for a given pressure. It is compact in size and quiet in operation. The motor used with this type of fan should have a greater capacity than is actually needed by the fan. Forward blade fans do not operate well in parallel. The backward blade fan requires higher speeds for equivalent efficiency. 3. The propeller, or disk type fans are seldom used in duct systems. They develop relatively low pressures. The propeller fan is used for moving large quantities of air against low pressure with free exhaust. 4. The following information is required in selecting a fan for a given installation: a. The number of c.f.m. of air to be moved. b. The static pressure required to move air through the system. c. The motive power available. d. The operation of fans in parallel or singularly e. The degree of noise permissible. f. The nature of the load. 5. Knowing the above information, you can refer to the manufacturer's manuals to determine the specifications of each size fan. 6. Figure 23 illustrates two different type fans that may be used in an air-conditioning system.

Figure 23. Types of fans.

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be inspected frequently for lubricant. During periodic inspections, you may find the blower surfaces rusted. You must clean it and apply a coat of rust-resistive paint. 10. Fan noise may be caused by improper fan selection. The tip speed required for a certain capacity and pressure varies with the type of blade. You must remember that a fan has a rated capacity and if it operates beyond this, it will become noisy. Other factors that could cause noisy operation are loose or worn belts; improper construction of ducts and airways; and loose fan mountings. 15. Evaporator or Chilled Liquid Coil 1. Chilled water coils are designed to conform to all the specifications and to handle the necessary chilled water that may be required by changing load conditions in a building. The fin metal is generally made of copper or aluminum, as these metals readily conduct heat. Fin type construction gives more surface area. The plate-fin coil used for the refrigerant circulating system and the chilled water circulating system are similar, differing only in minor construction aspects; one system is filled with refrigerant and the other with chilled water. 2. Operation and Controls. The duct type, platefin cooling coil is supplied with a coolant control assembly, which is fitted with a drip pan and drain. The related duct thermostat bulb is fitted within the air inlet duct. The coil tubing passes through plates or fins of thin metal stacked six to the inch, the entire length of the coils. The airflow through the coil is parallel to the fins which may be curved slightly to create a turbulent flow of the air. Thus, all the air is caused to come in contact with the cooling surfaces. The coils are preferably installed on the intake side of the recirculating fan in the system so that the coil inlet face is always open and free for cleaning. Slight deviations in air quantity from the cooling ducts will materially alter the performance of the system. The designated air quantities should be limited to plus or minus 5 percent so that the proper operating speed for the circulating fan in the air ducts is maintained. 3. Maintenance. In most installations, cooling units are provided with filters in front of the coils to prevent the coil from becoming coated with foreign materials. These filters must be inspected and cleaned at frequent intervals. Where filters are not provided, a linty, matlike accumulation will form at the intake side of coils. Too much of this accumulation would result in a marked decrease of airflow over the coils. A film of dust, organic material, and grease will also form on the entire cooling surface, reducing the rate of heat transfer and creating a source of objectionable

Figure 24. Blower bearing. moving a specific amount of air to a portion of the building. 8. Maintenance. Once a year you should completely disassemble and inspect fans for defects. Bearings on fan shaft or motor are cleaned, checked for wear, and replaced if necessary. Ball bearings are repacked with grease and sleeve bearings are lubricated with oil or grease as prescribed by the manufacturer. When handling fan wheels, you must be careful not to bend them. This will cause them to wobble. Rusted surfaces should be cleaned and painted to reduce corrosion. 9. The blower wheel is inspected for proper alignment and freedom of rotation. Bent vanes must be repaired. Axial clearance is checked to insure that the wheel is not binding on the scroll. The adjustment is made by relocating the position of the shaft thrust collar, as shown in figure 24. Total axial movement of the shaft after final adjustment should be approximately 1/32 inch. The thrust collar is locked in place with a thrust collar set screw; worn thrust washers must be replaced. Periodic soakings of the washer in oil prolongs its life. The blower shaft sleeve bearings are normally lubricated with oil, while the ball bearings are packed with grease. Grease cups are generally refilled once a year, but should

46

odors. Therefore, the coils should be inspected frequently and cleaned as often as necessary. 4. Cleaning. In cleaning coils, a regular check-off list is generally followed for the entire air duct system. First, you must stop the fans, then open the coil access plates. After this is done, brush the intake side of the coil to loosen the lint and dirt. If this material is caked on the fins, use the special combs with teeth spaced to fit between the fins. Wire brushes may be used if care is taken to avoid damage to the fins. 5. Leaks. Coils that develop leaks must be repaired immediately. Repairing a leak can develop into a major job. It may require the coil to be drained so that proper maintenance can be performed. Manufacturer' maintenance manuals generally give coil structural material and recommended procedures for repairing coils. 6. Replacement. Every air-conditioning Installation has its own peculiarities as to design and installation; therefore it is impossible to give specific procedures on coil replacement information relative to your particular installation can probably be located in the constructed prints and manufacturers specifications and handbooks. 16. Brine and Heating Coils 1. Brine coils are used in air-conditioning systems that require low temperatures for dehumidification purposes. The brine (usually ethylene glycol) flowing through coils is designed to withstand low temperatures. Location of the brine coils for your installation can be found in construction drawings. 2. Operation and Controls. Brine coils operate on the same basic principle as the chilled water coils except for minor engineering differences. Refer to construction drawings and manufacturer's manuals and handbooks for detailed information concerning operation and controls. 3. Maintenance. Maintenance for the brine coil is accomplished in the same manner as maintenance on chilled water coils, as explained previously. One maintenance precaution that must be taken in repair or replacement of the brine coil is the preservation of the brine. 4. Heat Coils. Heating coils are used to heat the air pressure through coils for humidification purposes. These coils are supplied with hot water or steam. Hot water coils should not be operated with final air temperature below 50° F. Construction characteristic and location of the heating coils can be found in installation drawings, manufacturer's manuals, and handbooks. 5. The operation, control, and maintenance of heating coils are very similar in principle to that of the chilled water coils explained previously. Many heating

coils are controlled by an automatic temperature control; this control throttles the steam or water to the coils to help protect against subfreezing air coming into direct contact with the hot coil. This can become very dangerous and would result in damage to the equipment. A bypass damper control is used on hot water coils under the above circumstances, which allows the maximum amount of water to flow into the coil. 6. Heating coils are constructed to withstand high pressures and may or may not be of the self-draining type, therefore provisions are made for draining in case of repair or replacement. 17. Air Duct Systems 1. The duct system is used to distribute conditioned air from one location to another. This system may cost 25 percent of the initial investment. The resistance of a duct system is a substantial portion of the static pressure against which the fan operates-an important item in annual power cost. For this reason, in larger installations economies can be realized by designing the ducts to balance first cost against operating cost rather than by using the rule-of-thumb methods sometimes permissible on smaller installations. 2. Pressure Losses. Along an ideal frictionless duct system, total pressure-the sum of static and velocity pressures-remains constant in an actual system, losses occur due to two effects: friction losses and dynamic losses. Friction losses are primarily from surface friction, while dynamic losses result from sudden changes in velocity or direction, or from other eddy sources. Most of the pressure drop in a straight duct is caused by surface friction. You will find that various equations are used to calculate losses. These formulas are slanted toward design engineering. You will not be required to study them. 3. Duct Sizing. Ducts are sometimes sized by selecting a velocity at the fan discharge and by making arbitrary reductions in velocity down the run, usually at each branch or takeoff. 4. This method, called the velocity reduction method, has simplicity to recommend it, but it takes no account of the relative pressure losses in various branches. It use is acceptable only for estimating simple layouts. The method is not recommended for actual design. 5. The following table presents maximum design velocities considered good practice for conventional systems in various applications. Quality and size of installation, power costs, space limitations, and noise are the factors which should be considered in the selection of the proper velocity.

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RECOMMENDED MAXIMUM DUCT VELOCITIES, IN F.P.M.

APPLICATION Trunk and Large Rises Residences Apartments and hotel bedrooms Theaters Private offices-deluxe Private offices-average General offices Restaurants Shops-small Department storeslower floors Department storesupper floors 800 1,500 1,600

SUPPLY DUCT Small Rises Return and Mains Branches 600 600 1,100 1,200 1,100 1,300 1,400 1,400 1,500 1,600 1,400 1,000 1,200 800 1,000 1,200 1,200 1,200 1,200 1,200

2,200 1,800

2.100 1,800

6. High-Velocity and High-Pressure Air Distribution. In recent years there has been a trend toward higher duct velocities to reduce duct size at the expense of increased friction. Any system with velocities greater than 2,000 f.p.m. is usually considered to be a high-velocity system. Because of the higher average static pressure (5 to 10 in. w.g. at the fan), these systems are frequently called high-pressure systems. 7. In these systems, relatively high duct pressures are necessary to obtain stable control of variable volume outlets, or to obtain the required velocity for high induction terminal units. Increased stability is inherent in outlets with high design pressure drops (1/2 in w.g. or greater), since a given change in duct static pressure, due to throttling of a portion of the outlets, has a decreasing effect on the airflow through the remaining outlets as the design pressure drop increases. For example, a duct static pressure increase of 0.20 in. w.g. will increase the airflow through an outlet designed for a 0.20-in. drop by 41 percent, but by only 10 percent through one designed for a 1.0-in. drop. 8. When high outlet discharge velocities are used, high-temperature differentials between room and supply air may be employed, since induction within the room will afford adequate mixing of the supply stream before it enters the occupied zone. For example, a 30° F. difference may be used instead of the 20° F. differential common to conventional systems, with a one-third reduction in the supply-air volume. Some systems employ high velocity in the main ducts, with soundattenuation boxes where the velocity is reduced. 9. The space saved as a result of using high velocities should be balanced against increased first and operating costs. It is necessary to use fans of heavier construction for the higher static pressures. Great care is needed in the construction of duct work to prevent

leakage, and it is common practice to seal all joint and seams with sealing compound, tape, or by welding or soldering. Round duct is preferred to rectangular because of its greater rigidity, which allows the use of lighter gauges and avoids the need for reinforcing members. Spiral conduit, made from 2 1/2- to 6-in. zinc-coated steel or aluminum strip spirally wound with a doublelocked seam, is light, tight, and strong. Particular care should be given to the selection of fittings to avoid excessive pressure drops and noise generation. Avoid using 90° fittings, or fittings that are sleeved into the inside diameter of the main duct. The problem of maintaining satisfactory sound levels is magnified, and outlets with a low level of noise generation and a high degree of sound attenuation are required. The higher sound level of a high-pressure fan ordinarily requires the use of a sound absorber immediately downstream. Lined duct may be used where space permits; otherwise a baffle, cell, or plenum type absorber is required. Special attention should be given to the design of fan isolation and the use of flexible connections. 10. It is good practice to examine the critical (maximum pressure drop) run of conduit after preliminary sizing is completed and to reduce velocity at selected points if a significant reduction in fan horsepower can be effected. Sometimes the use of more costly special fittings of low dynamic loss can be justified for such runs. Conversely, where excess static pressure is to be dissipated in shorter runs, it may be desirable to size certain portion for higher velocities. 11. Duct Materials. The composition of ordinary galvanized-steel sheets includes approximately 0.10 percent carbon, 0.40 percent manganese, and minute quantities of phosphorus, sulphur, and silicon, with a heavy zinc coating. Of somewhat superior resistance to atmospheric corrosion (where high moisture conditions are encountered) is galvanized copper-bearing steel with a copper content of about 0.20 to 0.30 percent. Aluminum duct should be fabricated from 2S or 3S 1/2 or 3/4 hard stock; 3S is preferred for larger ducts. 12. Exhaust ducts for chemical laboratories and other applications involving corrosive fumes use copper, stainless steel, monel metal, lead-coated, or lead itself when necessary. Intake and exhaust hoods are frequently made of copper although this refinement is necessary when galvanized-steel construction is accessible for inspection and painting. Materials other than metal may be used in ducts for reasons of appearance or cost. 13. Board material. Such materials are cut to desired size and fastened by various means, with corner trim or edges and band trim along the seams. Most materials in use include in-

48

sulation value as a property. Besides filling the requirements for any ducts, the material should be fireproof, verminproof, moldproof, free from odor, and not subject to deterioration from water or vapor penetration. 14. Prefabricated materials. These ducts and fittings are available in standard even-inch dimensions in the smaller sizes and are designed primarily for the residential and small commercial market. They must meet standards similar to those specified for the board materials. 15. Sheet-Metal Standards. Ducts and sheet-metal connections may be fabricated according to several methods of construction. It is not too important which method we use, but the construction must meet the following standards: a. Materials of suitable quality for the purpose. b. Proper gauge for strength. c. Cross breaking and reinforcement, where needed, for rigidity and freedom from mechanical noise induced by vibration. d. Tightness of seams and corners to minimize leakage. e. Freedom from sharp internal edges to avoid noise regeneration. f. Conformance with design standards to permit desired airflow. 16. To insure desired airflow without excessive frictional and dynamic losses design standards are essential to govern the fabrication of shapes, fittings, vanes, and connections to equipment. Nearly all of these standards are based on two fundamentals of airflow: a. Air flowing from the chamber or conveyor of smaller section area into one of larger area tends to continue in a straight line. Air will not diverge, unless changed by vanes, at an included angle greater than about 20°. b. Air flowing from a chamber or conveyor of larger section area into one of smaller area tends to converge uniformly and follows the laws of entrance to orifices in fluid flow. 17. Duct Heat Gains and Losses. Whenever the air inside the duct is at a temperature different from the ambient temperature, heat will be transmitted outwardly or inwardly. The gain or loss, if of appreciable magnitude, may be important because: a. Transmission to or from a space not being treated, but through which the duct passes, is a total loss of heating or cooling effect. b. Transmission to or from the same space being conditioned may put too large a part of the heating or cooling effect where it is not wanted. The correction in the first case is either insulation (or dead air space) or a greater investment in heating or cooling capacity, or both. 49

The correction in the second case is a redistribution of the air to the various supply grilles to compensate for the cooling or heating effect of the duct surface. 18. Heat Insulation. Insulation is employed for two reasons: (a) to reduce loss of heating or cooling effect or (b) to prevent sweating of the duct. Determination of the first effect is computed by use of the following equations: Q = UA (t1 – t0.) where Q = heat loss (B.t.u./hr) U = B.t.u./(hr)(sq ft.)(°F. diff.) average values values A = duct surface (sq. ft.) t1 = air temperature inside duct (°F.) t0 = air temperature outside duct (°F.) 19. The economic value of insulation depends upon the total annual cost of the heating or cooling effect saved. A precise answer can be obtained only by a study of the particular application. 20. Sweating occurs when the temperature of the duct surface is below the dewpoint of the air touching it. For bare-sheet-metal duct: f0 (t0 – t3) = U (t0 – t1) t3 = t0 - U (t0 – t1) 1.6 where t3 = duct surface temperature (°F.) U = overall transmission coefficient f0 = surface conductance of outside duct surface 21. Air Leakage and Duct Maintenance. Air leakage varies over wide limits, depending on air pressure, type of construction, and workmanship, principally the last. Actual tests on typical supply systems have shown leakages from 5 to 30 percent. Corner holes normally account for only a small portion. 22. The largest source is at transverse seams located against the wall or ceiling in such manner that tight joints are almost impossible. Allowance should be made for leakage, depending on job conditions. For supply systems with static pressure in excess of 1 inch w.g., calking, felting, or soldering is recommended. 23. Ventilating and air-conditioning ducts normally require little maintenance. When they are dry the deterioration by corrosion is usually negligible. Periodic cleaning is important because even with comparatively efficient air-cleaning devices, dirt accumulates over a period of time. A shift in dampers will frequently blow a cloud of dirt into the room. More important is the real fire hazard of such an accumulation, which has been recognized by the Fire Underwriters. Ducts should be provided with access doors to allow for cleaning. 24. System Balancing. With the fan in operation, adjust the damper op the air-intake trunk until the velometer shows an air intake equal to

one-half the dwelling's cubic volume per hour. After you have done this, you must lock the damper in place. 25. A formula for calculating air quantities required for sensible cooling load is: Quantity of air (c. f. m.) = sensible heat load (B.t.u./hr.) 1.08 X temperature change 26. Temperature change is the difference (°F.) between the room temperature and the temperature of the entering air. Another rule of thumb that you may use to estimate the quantity of cooling air required is to figure on eight air changes per hour in the area to be conditioned. Quantity of air (c.f.m.) = 8 X room volume (cu. ft.) 60 min. /hr. 27. You should not use this rule if the risers are smaller than standard (3 1/4" x 14") or if the branch ducts are less than the equivalent of an 8-inch round duct. Another exception is when unusually large amounts of glass or exposed wall are present. 28. At all T-type duct transitions, check the first grille downstream from each branch with all of the grille frets or louvers in straight-flow position. Adjust the branch dampers until the velometer registers equal velocity through the grilles on both trunks. After you have equalized the velocities, you must lock the dampers in position. Continue this procedure along the duct system to any addition T-type transitions until all the Ttype transition dampers are adjusted. 29. If splitter dampers are installed, follow the same procedure until approximately the same air velocity over the entire duct and grille system has been reached. 30. Proper quantity of return air. After you know the total quantity of air required for cooling the conditioned area, you must adjust the air-conditioner blower drive for delivery of the design airflow. Check the delivery as close to the fan outlet as possible. The air velocity times the total outlet area will determine the total air volume being circulated. For example: 80 f.p.m. X 8 sq. ft. = 640 c.f.m. being circulated 31. Once the airflow is adjusted to the desired cubic feet of air per minute, you will have to tighten the nuts on the fan sheave for permanent adjustment of airflow. At this time you should check the current the blower motor is drawing to b certain that it is capable of driving the fan without overloading and burning out. 32. You must be sure that the return-air grille is large enough to handle all the air supplied to the space. Any lack of return air can seriously affect system operation.

33. Balancing air discharge grilles. High-side-wall, double-deflection supply-air grilles predominantly used in average sized systems will be discussed first. You must use the design data to find the amount of air required for each room or area. The grilles are usually dampered. Equalize and properly deflect the air delivery to the various areas by adjusting the louvers or grille frets. 34. Starting with the grilles closest to the blower, adjust the front horizontal grille frets so that you achieve the proper blow and drop. Blow and drop will be discussed later in this volume. A lighted match, warm thermometer, or rubbing alcohol on the exposed surface of your arm will help you to determine just how accurately the desired vertical flow of air is being achieved. Constant association with airflow will enable you to tell just what the air deflection is doing by the sensation of the airflow over your body. By adjusting the rear frets of the grille, the horizontal width of the air pattern is established. The ultimate goal is to achieve an even air pattern about 5 ½ to 6 feet above the floor level over the greatest amount of room area while attaining as close as possible the cubic feet of air required for the particular room. Then continue to the next closest grille to the supply blower, and so on to the last grille. It is possible that proper airflow from distant grilles cannot be attained. It is then necessary to return to the grilles closest to the blower outlet and partially close some of their rear frets, thereby forcing more air to the distant parts of the system. Continue the adjustment of rear grille frets in the same sequence until there is ample air supply to all rooms. 35. In checking the air volume from the grille, play the recording air-measuring instrument over several locations on the grilles and average the readings for final tabulation of total airflow. In setting the supply-air delivery patterns, refrain from projecting supply air directly toward a return grille. 36. The ceiling diffuser must be adjusted to pattern the airflow over most of the ceiling area. The diffuser usually has adjustable rings, dampers, or diffusing grids which will do the same thing as a high-side-wall grille when it is adjusted for patterning air delivery to the conditioned area. 37. The flush-floor diffuser with bars or frets is adjusted to pattern air sweep in an arc toward the ceiling. This will blanket the conditioned area just as high-sidewall grille does. 38. The baseboard type of diffuser has a balancing damper for controlling airflow. This diffuser does a good job, even though it has a minimum of adjustments that you can use for balancing. 39. The low-side-wall diffuser has grids, vanes, or frets for producing air patterns. It can produce an excellent cooling or heating condition when the

50

air is deflected properly. You will encounter few problems with high-side-wall or ceiling diffusers. Care should be taken with the other types of diffusers, especially in relation to obstruction which interfere with the required air pattern. Such devices must be balanced differently for summer and winter conditions. Review Exercises NOTE: The following exercises are study aids. Write your answer in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the test. Do not submit your answers for grading. 1. What type of dampers regulate airflow from return ducts? (Sec. 13, Par. 2)

7. How many fins would a 2-foot coil contain? (Sec. 15, Par. 2)

8. How would you straighten the fins on a coil? (Sec. 15, Par. 4)

9. Why is a brine solution used as a coolant in an air-conditioning system? (Sec. 16, Par. 1)

10. Which type of pressure loss is caused by an elbow in the duct? (Sec. 17, Par. 2)

2. The damper in a duct is operating erratically. What is the most probable cause? (Sec. 13, Par. 7)

11. Why isn't the velocity reduction method used for sizing duct for complex systems? (Sec. 17, Par. 4)

3. Which type of fan is most commonly used in a duct system? (Sec. 14, Par. 2)

12. A system with a velocity rating of 2400 f.p.m. is considered a ______________ system. (Sec. 17, Par. 6)

13. How are duct joints sealed? (Sec. 17, Par. 9) 4. What type of fan would you install in an area where large amounts of air are to be exhausted? (Sec. 14, Par. 3)

14. What type of material would you construct a duct with if corrosive fumes are to be handled? (Sec; 17, Par. 12)

5. How is the axial clearance adjusted on a blower wheel? (Sec. 14, Par. 9)

15. What occurs when air flows from a small chamber to a large area? (Sec. 17, Par. 16)

6. Why are the fins a cooling coil made of aluminum-or copper? (Sec. 15, Par. 1)

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16. What is the loss of cooling effect of a 12-square foot duct with a temperature differential of 10° and a U-factor of 1.14? (Sec. 17, Par. 18)

19. How can you determine the vertical flow of air from a grille? (Sec. 17, Pa. 34)

17. Where does most duct air leakage occur? (Sec. 17, Par. 22)

20. The horizontal airflow pattern is controlled by the ______________ ______________ of the grille. (Sec. 17, Par. 34)

18. How much air is required when the sensible heat load is 49,000 B.t.u./hr. and a temperature change is 15°? (Sec. 17, Par. 25)

21. Which type of diffuser is the hardest to use for balancing? (Sec. 17, Par. 38)

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CHAPTER 6

Controls
YOUR BRAIN is a control system. It controls your movements and it responds to various situations. Have you ever touched something hot? You really let go of it fast, didn't you? The control system of an air conditioner acts like a brain. It senses a change and responds with a corrective action. Three types of control systems will be discussed. These are motor, electric, and pneumatic controls. Responsive devices sensitive to temperature, pressure, and humidity will also be studied. 18. Responsive Devices 1. Most automatic controls function because they are responsive to changes in temperature, pressure, and humidity. We will discuss the various responsive devices that you will encounter in your control system. 2. Temperature-Responsive Devices. Many of the automatic control units such as the thermostat, fan switch, etc., must be responsive to temperature changes. The temperature change actually makes and breaks electrical contact within each unit. This action is an indicating signal transmitted to the primary control for specific action such as starting or stopping the operation of a piece of refrigeration equipment. 3. Bimetal strip. To accomplish the above specific action, the automatic control unit may be equipped with a bimetallic strip. This strip is made by welding together two pieces of dissimilar metals such as brass and "Invar," as illustrated in figure 25. At a certain predetermined temperature, this strip does not deflect or bend. However, when the strip is heated, it will tend to bend in the direction of the metal which has the least amount of expansion, as shown in figure 26. 4. By welding two electrical connections and contacts to the arrangement shown in figure 27, an electrical switch is constructed. This switch can be used to control an electrical circuit responding to temperature changes. 5. The bimetal strip is the basic principle of operation of many of the temperature responsive automatic units. However, some units may be operated by a bimetallic strip in the form of a

Figure 25. Bimetal strip. Figure 26. Bimetal strip being heated.

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Figure 27. A bimetallic switch. spiral, a U-shape, a Q-shape, or even a helix, as shown in the illustrations in figure 28. 6. Vapor-tension device. Another very common type of temperature-responsive device is one in which the effects of the temperature changes are transmitted into motion by a highly volatile liquid. The most commonly used vapor-tension device of this type is a simple

compressed bellows, shown in figure 29. It is made of brass and partially filled with alcohol, ether, or some other highly volatile liquid not corrosive to brass. When the temperature around the bellows increases, the heat gasifies the liquid, causing the bellows to extend and close a set of electrical contacts, as shown in figure 30. When the bellows cool, they contract and open the electrical contacts. This vapor-tension principle also is used to operate some of the automatic control units. 7. Remote-bulb device. Not all the liquid-filled devices are limited to just a simple bellows as described above. There are remote-bulb type devices that not only have bellows but also have a capillary tube and a liquid or gas-filled bulb, as shown in figure 31. When the liquid or gas in the bulb is heated, part of the liquid gasifies or the gas expands and forces its way through the capillary tube into the bellows. The increase of pressure inside the bellows causes the bellows to extend and close a set of electrical contacts. When the bulb cools, the gas liquefies, causing a decrease of pressure in the bellows. This causes the bellows to contract and open the set of electrical contacts. 8. Pressure-Responsive Devices. Pressureresponsive devices are incorporated in refrigeration and air-conditioning systems to operate and regulate valves, controllers, operators, etc.

Figure 28. Various shapes of bimetal strips. 54

Figure 31. Remote bulb device. 12. Humidity-responsive devices are designed with sensing elements which am very sensitive to humidity changes. Usually these sensing devices activate the action of a switch. A typical humidity-responsive device is shown in figure 33. The sensing element in this device is a number of human hairs which lengthen when the humidity is high and shorten when the humidity is low. The lengthening and shortening action of the hairs moves the lever, which. in turn opens and closes the contact points to a humidifying unit. 19. Motor Controls 1. A motor control is similar to a switch installed in a motor circuit that opens and closes the power lead to the motor. The major difference is that the motor control acts automatically in response to temperature or pressure changes. 2. Function of Motor Controls. The function of any motor control is to maintain a relatively constant temperature within the refrigerated space.

Figure 29. Bellows contracted when cooled. 9. Bellows. One type of pressure-responsive device uses the action of a bellows in a similar way to the remote-bulb device mentioned previously. In this case the bellows extends and contracts in response to the changes in pressure. The action caused by the movement of the bellows opens and closes a set of electrical contacts. 10. Bourdon tube. Another pressure-responsive device used in a pressure gauge is illustrated in figure 32. In this unit the pressure acts inside a hollow, flattened, bent tube called a Bourdon spring tube. The pressure inside the tube tends to straighten it, moving the mechanism which turns the pointer. The pressure gauge measures pressures in pounds per square inch. 11. Humidity Responsive Devices. Humidityresponsive devices e regularly used to cause the opening or closing of solenoid or motorized valves which, in turn, control the flow of water or steam to the humidifying equipment.

Figure 30. Bellows extended when heated. 55

Figure 32. A pressure gauge showing the Bourdon tube.

Figure 33. A humidity responsive device. This may be done by starting the unit when the temperature rises and stopping it when the temperature reaches its set point (control is satisfied). The temperature is constantly rising and failing between predetermined set points (cut-in and cut-out). 3. Low-Pressure Motor Control. We can control the temperature of a refrigerated space through low side pressure. If we control low side pressure, we ultimately control the temperature of the refrigerant in the evaporator. A pressure-temperature relationship chart is needed for the following example. Our system contains R-12, and the desired space temperature is 40° F. Now, using the chart, we find that we must control the low side pressure at 37 p.s.i.g. Therefore, we have controlled temperature by pressure. 4. A bellows, diaphragm, or Bourdon tube is used to motivate the points in the low pressure control (LPC). 5. The differential, cut-out minus cut-in, is set by regulating the amount of force exerted upon the bar by the adjusting spring. There are many variations in the characteristics of individual types of motor controls. Each control has an adjustment of one kind or another. This allows the control a wide range of applications. 6. One of the more useful tools in control adjustment is the pressure control setting chart. The settings for most applications can be found by referring to this chart. 7. If the particular application desired is not found on this chart, the next approach to use is the pressuretemperature relationship chart. 8. Assume that the desired cut-in pressure is 25 p.s.i.g. and that the cut-out pressure is 10 p.s.i.g. The differential would be 15 p.s.i.g. Since most controls have only two setting. to make, cut-in and differential, we find that knowing the cut-out pressure is an important factor. 9. The two scales (one for cut-in and the other for differential) are located on the front of the control. The adjusting screws are located on top of the control. The adjusting screws are turned until the pointers indicate the pressures desired. Make sure that you read the scales very carefully. The unit should be allowed to cycle once. The cycling ON should take pace when the low side pressure is 25 p.s.i.g., and the OFF cycle should occur

when the pressure drops to 10 p.s.i.g. These pressures may be observed on a gauge installed in the suction line. 10. Thermostatic Motor Control. The thermostatic motor control (TMC) operates on temperature rather than on pressure. These motor controls can be used on most types of refrigeration and air-conditioning units. They must be used on all units that utilize high or low side floats and capillary tube refrigerant control devices. 11. Principles of operation. The operating principle of the thermostatic motor control is based on a physics law which states that matter will expand when heated and contract when cooled. The power element uses this law in its function. 12. The element is a sealed container filled with a liquid, gas, or combination of the two. Any change in temperature surrounding the element will cause a pressure change within the element. The power element consists of three parts-a feeler bulb, capillary tube, and bellows. Any leak in the power element will render it useless. 13. The feeler bulb of the power element is located in suck a position as to be sensitive to any change in the temperature of the controlled space. For domestic units this location is right on the evaporator so as to control the evaporator temperature. It might also be fastened inside the refrigerated space. 14. Any rise in temperature will heat the bulb, causing the charge to expand. This expansion will be transmitted through the capillary tube to the bellows, causing the bellows to expand. Attached to the bellows and inserted into the housing to rest against one end of the lever system is a short push rod. The pressure of the power element will expand the bellows, pushing the rod against the lever. This lever will cause other levers to move, and the net result will be a set of electrical contact points closing. Closing of the points will cause the motor to start, and the unit will be in operation. 15. As the temperature at the feeler bulb drops, so will the temperature of the bulb. This causes a drop in power element pressure and will reduce the "push" on the lever system. Part of the lever system consists of a spring which counteracts the power element pressure. As the element pressure drops, the spring will pull the points apart and stop the unit. 16. When you turn the adjusting knob clockwise, the spring will become compressed, causing the cut-in temperature to rise. Compressing the spring puts more pressure in opposition to the power element and demands that the element heat up even more to overcome this increased pressure, closing the points. The converse (turning the knob counterclockwise) will decrease spring tension and lower the cut-in point 17. The TMC has a second spring that works in conjunction with the power element instead of

56

against it. This spring is used to set the "cut-out" temperature (on some controls) or the differential (on other type controls). 18. Location of feeler bulb. Normally, the feeler bulb will be tightly clamped to the evaporator. The TMC will operate on the evaporator temperature. The bulb can be located elsewhere if the situation demands it; some bulbs will be located in the cold box rather than on the evaporator. In this case the TMC will operate on the cold box temperature. On ice-making machines the feeler bulb should be located in the ice bin. In this application the TMC will shut the unit off when the ice level reaches the feeler bulb and cools it. When the ice level falls below the bulb, the bulb will warm up and turn the unit on. If the refrigerated space has forced air circulation, the TMC bulb should be mounted in the return airstream. 19. Replacement. The control mechanism is so delicately built that it is impractical to repair in the field. If operation is erratic because of power element failure, mechanical (lever) action failure, or contact point failure, you should replace the entire switch. 20. Checking thermostatic motor controls. Thermostatic motor controls are very delicate instruments. However, if they are not misused, they will give years of trouble free service. Thermostatic motor controls are subject to several troubles, each of which usually requires replacement of the complete control. Some of the more common troubles are treated in the following paragraphs. a. Loss of Charge. Occasionally the power element will lose part or all of its charge. This charge is very small and any loss at all will cause the unit to fail. A kinked or clogged capillary tube will give the same indication as a loss of charge. Usually a power element failure requires replacement of the complete control. However, it is possible to get replacement power elements for some controls, although care must be taken to insure that you have the correct replacement item. b. Burned Contacts. Even though there is snap action when the points open and close, they will burn. In such cases the points will either stick closed or become pitted and never close. In some cases the point can be filed and the control will operate satisfactorily for a period of time. Filing the points should be considered a temporary repair and the control should be replaced as soon as possible. c. Wear. The parts of a TMC are light and do not move very far, but they do move many, many times each day. One should not become too concerned about wear unless the unit has been in use a long time. However,

the TMC will wear out; when it does, remove and replace it. d. Electrical. Low and high voltages, high current flow, frayed insulation, bad electrical contacts, and various other electrical malfunctions will cause the TMC to fail. Electrical troubles can often be determined and repaired without having to replace the control. 20. Electric Controls 1. There are many devices that may be used in single- or three-phase circuits. We will cover each of these devices and how they operate. 2. Switches. The two types of switches used in electric controls are the snap action and mercury. These switches help. to reduce the problem of arcing when a circuit is open or closed. 3. Relays. There are many types of relays used in electrical application. The type that we are interested in is the control relay. These are relays that open or close an electric circuit of higher voltage by the use of lower voltage. These relays might be defined as an electrically operated switch. The main use of this relay is to remotely control an electrical device such as a fan motor or pump. 4. Control Transformers. A control transformer steps voltage down to operate different kinds of electrical controls. Line voltage applies to wiring or devices using 110 or 220 volts. In control terminology, low-voltage control takes in all controls and controlled devices that use 25 volts or less. 5. Magnetic Starter. Three-phase motors must have at least two of the leads open to stop their operation. The device that opens these wires (circuit) is a line starter or magnetic starter. A magnetic starter is nothing more than a larger control relay which electrically operates two or more switch contacts. 6. Trouble Analysis. Before you can effectively troubleshoot a control circuit or system, you should know the circuit and how it operates. You can study the circuit in a wiring diagram of that particular circuit. Studying the diagram will give you a knowledge of the circuit as it should normally operate. If the system does not function properly, the circuit is defective and an analysis of the trouble and its location must be made. 7. Types of trouble. In practically all defective circuits, one .of the following types of trouble will exist: a. Open-A circuit that has a break in any part of the circuit between the load and source. b. Short-A circuit in which a conductor comes in contact with a point or object that it is not supposed to touch.

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(1) Direct short-A circuit in which one of the hot conductors comes in contact with a neutral conductor. This type of short circuit will "blow" a fuse or trip a breaker because there isn't any resistance in the circuit. (2) Cross short-A circuit in which two of the hot conductors make contact. This short will cause an electrical feedback even though one of the conductors is open. c. Low power-This trouble causes units to operate improperly. Two effects are sluggish motors and dim lights. Low voltage may be caused by loose, dirty, or corroded connections as well as a low power source. 8. Location of trouble. As soon as you have studied the wiring diagram, the next step is to check out the circuits with the appropriate test equipment. 9. Opens may be checked with a voltmeter or by a continuity Lest. The continuity of a circuit may be checked by a continuity meter or light, an ohmmeter, or a bell. The power must be off when making continuity tests. An ohmmeter will indicate infinity across an open circuit, while the light or bell would not function. 10. A short can be located with an ohmmeter or by a continuity test. When checking a circuit with an ohmmeter, a zero-resistance reading indicates a short, and an infinity reading indicates an open. Remember, when using an ohmmeter to test a circuit, the circuit power source must be off. 11. Major Advantages of Electrical Controls. Electrical energy is commonly used to transmit the change in space condition sensed by the controller to other components of the system. This signal from the controller will translate into work at the final control element. For this purpose, electricity has the following major advantages: a. Electric controls are available wherever there is a source of power. b. Electric wiring is usually easy to install. c. Electric power readily amplifies the relatively feeble impulse received from the sensing element. d. The impulse received from the sensing element can be applied directly to produce one or several combinations or sequences in electrical output. This allows one actuator to perform several desired functions. e. It readily permits remote control. The controller can be a considerable distance from the controlled space or element. 12. Modes of Electric Controls. All control systems do not use the same types of action to accomplish their purposes. The method by which a control system acts is called the control mode. We will

discuss the two-position, proportional position, and floating controls. 13. Two-position control. In two-position controls the final control element occupies one of two possible positions (open or closed). The following is a list of systems that can use two-position control operation. a. Domestic heating systems. (You may be called upon to calibrate and troubleshoot controls used on heating systems.) b. Electric motors on unit heaters and refrigeration machines. c. Water sprays for humidification. d. Electric strip heaters. 14. There are two values of the controlled variable which determine the position of the final control element. Between these values there is a zone in which the controller cannot initiate an action of the final control element. This zone is called the differential gap. As the controlled variable reaches the higher of the two values, the final control element assumes one of its two positions, which corresponds to the demands of the controller, and remains there until the controlled variable drops back to the lower value. The final control element then travels to the other position as rapidly as possible and remains there until the controlled variable again reaches the upper limit. 15. There are two varieties of two-position control which have been developed. The first, and oldest, may be called simple two-position control. This has been more or less standard in the past and, as its name implies, it is fairly elementary. The second, which may be called timed two-position control, is a later development which is rapidly replacing simple two-position control. 16. In simple two-position control, the controller and the final control element interact in the manner previously described without modification from any source, either mechanical or thermal. The result is cyclical operation of the equipment under control. The controlled variable fluctuates back and forth between two values determined by the magnitude of the differential and the lag in the system. Since the action of the controller is such that it cannot change the position of the final control element until the controlled variable reaches one or the other of the two limits of the differential, these limits become the minimum possible swing of the controlled variable. 17. In simple two-position control, the controller never catches up with the controlled condition. Thus it corrects a condition that has already passed, rather than one which is taking place or is about to take place. Consequently, simple two-position control is applicable only to systems in which total system lag (including not only transfer lags but also measuring and final control

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element lags) is small. For this reason, simple twoposition control rarely finds application in comfort heating control, but lends itself to the control of certain industrial processes and auxiliary processes in air conditioning. 18. There is no single control point in simple twoposition control. Rather, the controlled variable cycles back and forth between two extremes. It is convenient to think of the control point as being midway between the two extremes and offset as being a sustained deviation of this control point. Thus, offset is a shifting of the whole curve either up or down, and the mean value is either raised or lowered so that it no longer corresponds to a point midway between the upper and lower limits of the controller differential. 19. Offset (on a temperature control system, for example) is caused by the fact that the average value of the controlled variable must be lower under heavy load conditions and higher under fight load conditions in order that heat can be supplied at the lower or higher rate needed. At peak load the burner must remain on 100 percent of the time. Therefore the controlled variable cannot rise to the upper limit of the thermostat differential; otherwise the burner would be shut off. Likewise, under minimum load the controlled variable cannot fall to the lower limit of the differential or the burner would be turned on. 20. Since the amount of offset is limited in this way by the differential, it is usually a serious problem in simple two-position control unless it happens that a wide differential must be used. 21. The ideal method of heating any space is to replace lost heat in exactly the amount needed. With two-position control, this is obviously impossible since the burner is either "full-ON" and the heat delivered at any specific instant is either too much or too little. However, a close approximation of the ideal method .of heat delivery can be had by using timed two-position control. In this method of control, heat is delivered in measured quantities on a "percentage ON-time basis" so that fluctuations of the control point are, for all practical purposes, eliminated. 22. For example, suppose we have a domestic heating system with a two-position control which is required to make up a heat loss of 20,000 B.t.u. in 1 hour at a certain load. The total capacity of the burner is 40,000 B.t.u. per hour. This means that the burner will have to operate 30 minutes out of the hour, whether it is on for 30 minutes and off for 30 minutes, on for two 15minute periods and off for two 15-minute periods, on for six 5-minute periods and off for six 5-minute periods, or any other combination in the same ratio. 23. In many cases the longer cycles would be unsatisfactory because the variations in temperature 59

would be too great. Dividing the heat into the correctly sized packages, so to speak, and delivering them at the right time gives a closer approximation of the desired result. 24. In timed two-position control, the basic interaction between the controller and the final control element is the same as for simple two-position control. However, the controller responds to gradual changes in the average value of the controlled variable rather than cyclical fluctuations. The gradual changes modify the timing action to meet the changes in load. 25. Timing action may be provided mechanically, for example, by a cam mechanism. The chief disadvantage of this method is that only the relative duration of the ON and OFF periods may be varied with changes in load. The frequency remains fixed. 26. Thermal timing devices are more convenient and flexible. Placing side-by-side a heating element and a temperature-sensitive element controlling the power supply to the heating element creates a thermal timer. As long as the ambient temperature is within certain limits, the thermal timer will cool on its ON point, energize the heater, heat to its OFF point and deenergize the heater, again cool to its ON point, and repeat the cycle. As the ambient temperatures decline, the time required for the timer to heat to the OFF point increases, and the cooling time decreases. Thus the timer automatically changes the ratio of ON to OFF time. Moreover, the nonlinear shape of the heating and cooling curves may be utilized to vary the total cycle time also, and therefore the frequency of the cycles. 27. In the latest models of domestic heating thermostats, this principle is utilized by taking full advantage of the effect produced by the artificial heater long included as standard in similar thermostats. The ON and OFF points of the thermostat are fixed by adjustment of the setting dial, and a small offset in room temperature is allowed to measure changes in heating load so as to vary the timing pattern of the thermostat. 28. In the two-position "weatherstat system," the weatherstat, located outdoors, operates in essentially the same way by turning its own heat supply (and simultaneously that of the building or zone) on and off so as to maintain its own temperature within its differential. Here the ambient temperature variation which causes variation in the timing pattern is the full range of "effective" outdoor temperature (including the effects of sun and wind), which constitutes the heating load. 29. In electronic systems the ambient temperature at the timer or cycle is, in effect, held constant by means of an ambient temperature compensator, and the ON and OFF points are reset by remote temperature elements such as a room

thermostat, an outdoor compensator, or any other temperature controller suitable for this purpose either alone or in combination. Raising the operating range of the cycle is equivalent to reduction of the ambient temperature, and thus increases the percentage of ON time. 30. In whatever form the principle is applied, timed two-position control offers a great advantage over simple two-position control in that it greatly reduces the swings in the controlled variable resulting from a large total lag. Since the controller need not wait until it can detect cyclic changes in the controlled variable and then signal for corrective action, control system lags have no significant effect, and the lags in the heat source and distribution system serve only to smooth out the "humps" and "valleys" in heat delivery so as to approximate closely the results of a continuous-delivery system with proportional position control. 31. In timed two-position control, the addition of heat to the thermostat bimetal is a factor in offset. 32. In analyzing the cycle of a thermostat used in this type of control, you can see that the control point must vary if the bimetal is to heat and cool at different rates necessary to time the cycle for the various load conditions. As the outside air temperature decreases, the heat loss from the space increases, and the ON cycle of the burner must lengthen in order to replace heat lost at an increased rate. This means that the heating rate of the thermostat bimetal must be slower so that the burner will remain on longer. It also means that the cooling rate of the bimetal during the OFF part of the cycle must be faster so that the burner will come on sooner. Both of these demand that the difference between the bimetal temperature and the air temperature become greater. This difference is secured by a sustained deviation of the room temperature, which is called offset. 33. Quantitatively, in timed two-position control, offset is equal to the total added heat minus the manual differential of the thermostat. Total heat is equal to the difference between the maximum temperature of the bimetal and the temperature of the air surrounding it. Manual differential is the differential for which the thermostat is set. 34. Both offset and temperature swing can be reduced to negligible quantities by using a relatively narrow manual differential (such as 1 ½° or 2°) and a small amount of artificial heat applied directly to the sensing element. 35. Proportional control. If proportional control the final control element moves to a position proportional to the deviation of the value of the controlled variable from the set point. There is one and only one position of the

final control element for each value of the controlled variable within the proportional band of the controller. Thus, the position of the final control element is a continuous linear function of the value of the controlled variable. 36. Because there is but one position of the final control element for each value of the controlled variable, a sustained deviation is necessary to place the final control element in any position other than the middle of its range (assuming the set point to be in the middle of the proportional band). Offset therefore becomes a major problem in proportional position control. 37. As an example, suppose we have proportional control of a hot water coil used in heating a room. Under ideal load conditions, the thermostat is in the middle of its proportional band, the coil valve is half open, and there is no offset. Now suppose that the outside temperature drops, increasing the load on the heating coil. At once, more heat is required in steady supply to replace the heat which is being lost from the room at a greater rate. To deliver the required heat, the coil valve must open further and remain in that position as long as the increased load exits. To do this, the temperature must deviate from the set point and sustain that deviation because the position of the final control element is proportional to the amount of deviation. 38. As the load condition increases from the ideal, offset increases toward colder; and as the load condition decreases from the ideal, offset increases toward warmer. 39. Floating control. Floating control is a mode of control in which the final control element moves at a predetermined rate in a corrective direction until the controller is satisfied or until a movement in the other direction is desired. The direction of movement corresponds to the direction of deviation of the controlled variable. Floating control is further divided into several subclasses, two of which are of interest to us: a. Proportional-speed floating control in which the final control element is moved at a rate proportional to the deviation of the controlled variable. b. Single-speed floating control in which the final control element is moved at one speed throughout its entire range. 40. Either is adaptable to systems having a fast reaction rate, a slight transfer lag, and a slow load change. In general, proportional-speed control can be used in systems having somewhat faster load changes than those operating successfully with single-speed floating control. 41. Series 20 Control. The series 20 control circuit acts to make and break an electrical circuit which results in two-position response. This con-

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trol is designed for low voltage, two-position control of: a. Motorized valves. b. Motorized dampers. c. Relays. 42. Series 20 control circuits are not "fail safe" and should not be used where continued operation of the controlled equipment would be hazardous if the power failed. Series 20 motors (and equipment under their control) remain in whatever position they happen to occupy at power failure. 43. A series 20 control circuit consists of one holding and two starting circuits. The motor rotates in one direction only, making a half turn each time one of the starting circuits and the holding circuit are completed. The holding circuit is made-at the beginning and broken at the end of each half turn by a cam and switch arrangement on the moor. 44. Figure 34 illustrates a complete series 20 control loop. The equipment includes a thermostat, an actuator, a valve, and a control transformer. 45. Let's assume that the temperature in the water tower drops to the set point on the thermostat. The bellows will contract, causing the R and B leads to contact. The starting circuit is now established. The motor is energized and starts to rotate clockwise. As the motor and cam rotate, the right blade of the maintaining switch makes contact with S-2, and the holding circuit is established. The holding circuit is independent of the starting circuit. Once it is completed, it furnishes current to the motor, regardless of the thermostat action. 46. When the motor shaft has rotated 180°, the cam opens contact S-1. All circuits are incomplete, the motor stops, and the steam valve is now completely open and will remain open until the red and white leads make contact in the thermostat. 47. You can see that this is going to cause a rise in temperature of the water. The rise in temperature will become sufficient to move the thermostat blade to contact the W lead. Now the starting circuit is reestablished. The motor is energized and begins to rotate once again in the clockwise position. As the motor and cam rotate, the left blade of the maintaining switch makes contact with S-l, and the holding circuit is established. Once again, the motor will rotate a 180° before the cam breaks the holding circuit. 48. The steam valve is closed and will remain there until the thermostat calls for it to open. The control of the valve in this manner will produce the two-position response which was pointed out before 49. Series 40 Control. The series 40 control circuit acts to make and break an electrical circuit which results

in two-position response. This circuit is a line voltage control circuit which is switched directly by the singlepole, single-throw switching action of a series 40 controller. Series 40 is a two-position control and requires two wires. It can be used to control fans, lights, electric motors, and other standard line voltage equipment, as well as a series 40 controlled device and line voltage. The series 40 control circuit depends on the equipment under control as to whether it is "fail safe" or not. 50. In operation the equipment under control is energized when the controller switch is closed and deenergized when it is open. Normally the series 40 controller makes and breaks the load directly, as shown in figure 35. It is possible for loads to exceed the controller rating. In this situation a simple control relay is used between the controller and the load circuit. 51. Figure 35 shows a complete series 40 control loop. It includes a series 40 thermostat series 40 controlled device, and line voltage. The thermostat is sensing the temperature of the water. A drop in temperature below the set point causes the mercury bulb to rotate, allowing the mercury to close the circuit to the solenoid valve. The solenoid opens the valve and allows the steam to enter and heat the water. This cycle continues as the temperature changes, and you can see that this is ON and OFF control or two-position. 52. Series 60 Control. The series 60 control circuits make and break electrical circuits which results in two kinds of response. Series 60 controls can be used as two-position and floating response. 53. The series 60 two-position control circuit is similar to the series 20 except that series 60 is a line voltage circuit. It can be used for industrial application, using line voltage equipment and installations where single-pole, double-throw control of line voltage is required. It is not a "fail safe" control circuit and should not be used where it would be hazardous. 54. The series 60 floating control produces another response that is different from two-position. Series 60 floating control is commonly applied to motorized valves on tank level control systems, motorized dampers for static pressure regulation, and specialized pressure and temperature control systems. 55. The series 60 floating control circuit uses either low voltage or line voltage, depending on the equipment selected. The basic pattern of the floating control circuit is like that of the two-position circuit except that the motor is reversible and limit switches are substituted for maintaining switches.

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Figure 34. Series 20 control loop. 56. In floating control there is no fixed number of positions for the final control element. The valve or damper can assume any position between its two extremes as long as the controlled variable remains within the values corresponding to the neutral zone of the controller. Furthermore, when the controlled variable 62 is outside the neutral one of the controller, the final control element travels toward the corrective position until the value of the controlled variable is brought back into the neutral zone of the controller or until the final control element reaches its extreme position.

Figure 35. Series 40 control loop. 57. Figure 36 shows a series 60 two-position and floating control circuit. You can see that the two are different in the voltage operation as well as in the response. 58. Because of the close similarity of the series 20 and series 60 circuits, we will not discuss its operation to a great extent. The main difference is that the 60 operates on line voltage, whereas 20 uses low voltage. 59. Figure 36, B, illustrates a complete series 60 floating control circuit. The equipment includes a temperature floating controller and a floating control motor. 60. Referring to figure 36,B, when the temperature drops, the controller blade completes a circuit from contacts R to B. This causes the motor to energize by line voltage. The capacitor in the circuit causes the motor to turn clockwise, which will establish a corrective action of the final control element. 61. The motor moves at a single speed toward opening the valve. It will stop if sufficient heat is added to raise the temperature on the thermostat to open R and B. If not, it runs until the limit breaks and stops the motor. 62. On a rise in temperature, the thermostat closes R to W. This allows line voltage to go into the motor through W and once again energizes the motor. The capacitor in the circuit now causes the motor to rotate in the counterclockwise direction. The motor rotates, 63 closing the valve, until the temperature is corrected or until the limit of its travel is reached by limit switch S2. The motor would remain here until heat causes a rise in temperature sufficient to cause the thermostat to float the blade back to the neutral position if the desired temperature is satisfied. 63. Series 90 Control. The series 90 control circuit acts to balance a bridge which results in modulating or proportional control response. 64. The series 90 control circuit is a low-voltage bridge circuit which operates to position the controlled device (usually a damper or motorized valve) at any point between full-open and full-closed. It can be used to operate motorized valves, motorized dampers, and sequence-switching mechanisms. 65. Figure 37 shows a typical application of a series 90 control circuit. The temperature of the equipment cooling space is being controlled by governing the amount of air that moves across the direct expansion (DX) coil. The thermostat modulates to control the modulating motor. The motor, in turn, proportionally controls the face and bypass dampers to control temperature. 66. Figure 38 shows how a balancing relay is made. The balancing relay is applied to the series 90 control circuit. The amount of current passing through coils 1 and 2 governs the position

Figure 36. Series 60 control loop. of the contact blade in respect to the two contacts of the motor. 67. When equal amount of current flow through both coils of the balancing relay, the contact blade Is in the center of the space between the two motor contacts, and the 24 volts cannot be applied to the motor. You see that there is current flow in each coil even though the motor isn't running. 68. In figure 38, if coil C1 receives more current flow, and thus becomes stronger, the contact blade 64 moves to the left and completes the circuit between motor winding W1 and the transformer. Current also passes through the capacitor and W2. The motor will rotate in the clockwise direction. When coil C2 receives more current flow, the contact blade will move to the right and a circuit is made once again to the motor. The capacitor is now in series with winding W1. You know that this causes the motor to rotate in the opposite direction now. 69. Figure 39 illustrates a bridge circuit which

is used in the series 90 control circuit. It consists of two potentiometers and the coils of the balancing relay. One potentiometer is located in the motor, and its wiper is moved by the rotation of the motor. The other potentiometer is in the controller, and its wiper is moved by the thermal system. 70. The thermostat is satisfied and the bridge is balanced. Power (24 volts) is applied to the bridge by the transformer. There is a path for current flow; in fact there are two paths for current flow. The left circuit has a total of 135 ohms resistance plus coil C1. The right circuit has a total of 135 ohms resistance plus coil C2. The amount of current flow is equal in both circuits. This is called a balanced bridge. 71. Figure 40 illustrates a complete series 90 control circuit. It consists of a modulating controller, modulating motor, and control transformer. 72. Referring to figure 40, you see by the dotted wipers that the temperature has increased. The wiper in the controller potentiometer is now at a new position on the resistance. The left circuit now has 97 ½- ohms resistance plus coil C1. The right circuit now has 162 ½ohms resistance plus coil C2. Current will flow in the left and right circuit, as indicated by the arrows. According to Ohm’s law (E = IR), 25 amps will flow through the left circuit and .15 amps will flow through the right circuit. As a result of the unbalance of the bridge, coil C1 has a stronger magnetic field and coil C2 has a weaker magnetic field. 73. Power is now applied to the motor windings, and the motor begins to rotate. The motor runs clockwise. As it turns clockwise, it moves the wiper on the motor potentiometer to the right, as shown. Now the circuits on the left and right are once again balanced in resistance. The balancing relay is balanced, and power is broken to the motor. The chilled water valve was opened during the change of the motor. The

Figure 38. Series 90 balancing relay. temperature rise indicates that more cooling is needed. The series 90 control circuit will continually reposition the valve to correspond to the changes in temperature. 74. Modulating control is a much better mode of control that two-position. As you have seen, any change with modulating control immediately causes a proportional change of the final control element. But we must remember that if we want to have more accurate control, it costs more. 75. Electrical Actuator Adjustment. We have discussed the electric controls that may be used to control temperature, pressure, flow, humidity, and any other variable. Those controls must be installed, adjusted, and calibrated properly before they are able to control those variables. 76. One of the important items that has to operate properly is the final control element, such as the damper, louver, valve, or any device that might be used to control the control agent.

Figure 37. Series 90 application.

Figure 39. Current flow at one instant in a balanced bridge circuit. 65

Figure 40. Complete series 90 control circuit. 77. For example, if temperature is being controlled, the control loop may be properly operating to maintain the temperature. But let us look at the control loop from the standpoint that the actuator isn't in adjustment with the chilled water valve. Now, the control loop cannot possibly maintain the temperature. Why can't the control loop be able to control the temperature? Well, look at figure 41 and we will see why. 78. Figure 41 illustrates a control loop in which the actuator is out of adjustment with the final control element. The temperature in the duct is below the set point. The controller sensed this and controlled the actuator (modulating motor) in a manner to compensate for the lower temperature. 79. The controller signaled the actuator to close the valve so that the temperature could increase. The actuator moved to this position, stopping at the extent of 66 its travel. The linkage between the actuator and the valve causes the valve to remain open, so chilled water continues to flow through the coil. 80. With the actuator out of adjustment, the control system will not function properly. So we see why it cannot control. Therefore the actuator must be adjusted to the device it operates. 81. Electrical Control Applications. Electrical controls can be applied any time that a measured variable is to be maintained. As you have seen in the previous discussions, the controller senses the measured variable, and the final control element regulates the control agent to maintain the measured variable at a set point. 82. Maintaining temperature in a space with series 90 control. In figure 42, a system controlling temperature is illustrated. You can see that

Figure 41. Actuator out of adjustment. a series 90 modulating control circuit is being used to maintain the temperature. 83. The thermostat, T-l, is set to maintain a desired temperature of 72° F. If, for instance, the temperature sensed by the bulb is 72° F., the controller operates the motor to position the face and bypass dampers at a halfway position. This would mean that the bridge circuit of the series 90 control is balanced because the temperature is at set point. 84. When the temperature drops below set point (72° F.), the bridge circuit is unbalanced and the motor will run clockwise to close the face dampers and open the bypass dampers. This allows more air to pass through the bypass dampers. The result is less cooling because less air is cooling in contact with the coil. The series 90 motor runs until the bridge is once again balanced. The motor will hold the dampers in this position until the thermostat senses a temperature change which will once again unbalance the bridge. 85. When the temperature increases above set point, the bridge becomes unbalanced in the opposite direction. The controller now causes the motor to rotate in the counterclockwise direction. The motor now opens the face dampers and closes the bypass dampers. Now more cooling is produced because of the greater amount of air coming in contact with the cooling coil. The motor will run (opening the face dampers) until the bridge is once again balanced. 67 86. Operating in this manner, the temperature is controlled by this modulating control system. 87. Maintaining relative humidity with series 90 control. If we are to maintain humidity, it first has to be measured by a controller. The humidity controllers usually employ hair, leather, wood, or some moisturesensitive element to sense humidity and to convert it into movement. This movement, in turn, operates the controller. 88. Looking at figure 43, you will see a modulating control system maintaining humidity. The humidity controlled senses the percent of humidity as the air moves through the duct. This control circuit operates in the same manner as the previous system which was maintaining temperature. The main difference between the two systems is the sensing device which operates the wiper in the controller. 89. As the humidity increases above set point (50 percent) the hair expands in length. This allows the spring tension to move the wiper arm on the potentiometer. The bridge is then unbalanced and the series 90 motor is started rotating counterclockwise. The motorized valve is modulated toward close due to the increased humidity. The motor will continue to run (closing the valve)

Figure 42. Maintaining temperature with a series 90 control. until the potentiometer in the motor balances the potentiometer in the humidity controller. 90. The opposite action occurs when the humidity drops below set point. The bridge is unbalanced in the left circuit now. The motor rotates clockwise and modulates the motorized valve toward open. The motor will run (opening the valve) until the bridge is balanced. The system will remain here until the humidity changes. 91. Maintaining temperature and relative humidity with high limit control. A control system which maintains temperature and humidity at a high limit is illustrated in figure 44. The system is composed of several units that we have discussed before, but in this case they are used in conjunction with each other. The devices are a series 90 motor, thermostat, and humidistat. 92. The control circuit of the temperature control system with high limit humidity is diagramed in figure 45. As you can see, the series 90 thermostat has two potentiometers. The wipers of these pots are moved by the temperature sensed by the bulb ("pots" is short for potentiometers). 93. The front pot forms a bridge circuit with the series 90 motor that operates the face and bypass dampers. The only thing abnormal in this part of the 68 temperature circuit is the pot of the humidistat being in the blue wire of the right circuit 94. The rear pot forms a bridge circuit with the series 90 motor which operates the reheat valve. The rear pot is somewhat out of line with the front pot. A factory calibration, the "dead" spot is about 7/64 inch. You can see in figure 45 that the wiper in the rear pot cannot start to operate the reheat valve at the instant the front pot wiper reaches B. The temperature must drop farther for the rear wiper to reach W, where it will begin to unbalance the bridge circuit. 95. If you will refer to figures 44 and 45, we will discuss the operation of the control system and its circuit as it functions to maintain the temperature and relative humidity at a high limit. The set point is 72° F., which the control system strives to maintain. We have seen that the face and bypass dampers will be at midposition when the temperature is at set 'point. It will modulate the face and bypass dampers to control the temperature. The system operates in this manner until the humidity reaches the high limit. 96. Let us go through the operation when the humidity increases above the high limit. This

Figure 43. Maintaining relative humidity with a series 90 control.

Figure 44. Maintaining temperature and humidity with a high limit humidity control. 69

Figure 45. Temperature and high limit humidity control circuit. causes more resistance to be placed in the right circuit. The motor runs clockwise, opens the face dampers, and closes the bypass dampers. Of course, this allows more of the air to come in contact with the coil. As a result, a greater amount of moisture is removed from the c.f.m. moved through the duct. In other words, the high limit humidistat overrode the thermostat and caused the face 70 damper to open more than the thermostat wanted it to open. Naturally, if the face damper is open more than the thermostat is calling for it to open, the room temperature will drop below the desired value. 97. The rear pot of the thermostat now comes into action. The drop in temperature causes the rear wiper to begin unbalancing the bridge circuit

Figure 46. Air compressor station. of the reheat valve. The reheat valve brings the temperature back up to the desired amount by adding heat through the steam coil. When the sensible heat is added, it also lowers the relative humidity. The temperature and humidity both have now been satisfied for the desired conditions. This system continually strives to maintain temperature and a maximum humidity within the limits of the controllers. 21. Air Supply 1. Almost all large air-conditioning systems require a supply of compressed air. It is used to operate valves, controllers, transmitters, thermostats, humidistats, receivers, etc. Such air can be furnished to an installation by two possible sources: a. Compressed air may be available in the building by a remote compressor system. b. Compressed air may be furnished by a compressor station installed within the building. Figure 46 illustrates a simple, typical air-supply system. It consists of a compressor, storage tank, filter, pressure gauges, safety valve, pressure reducing valve, etc. 2. A Compressor Components. Compressors are made in a number of different sizes and designs. They are driven by electric motors or gasoline engines. Some are the single-stage type, while others are of the multistage type. The multistage compressor is designed to develop higher pressures than the single-stage unit. Compressors are cooled either by air or by a liquid coolant. Compressor units are mounted in a variety of ways: stationary, on skids, and with single or several wheels which have rubber tires or steel rims. 71 3. Most of the air compressors installed in large buildings are anchored tightly to the floor and are driven by an electric motor. The compressor is connected to the motor by a belt-drive arrangement. Belt-driven compressors usually have more than one belt. If one belt needs changing, they all must be replaced. New belts should be adjusted to specifications and then checked regularly because they sometimes stretch. The air compressor is very similar in construction to the refrigeration compressor. Care and maintenance procedures are the same. 4. Air cleaners and filters. The air in an air compressor must be clean to protect the air compressor and other controls and equipment operated by the air pressure. A defective air cleaner will not filter the air. The minute particles in dirty air are apt to restrict the flow of air, reducing the efficiency of the compressor and operating air controls. Air cleaners may be constructed of screen mesh or may be a filter disc type. No matter what type cleaner is used, it must be serviced periodically. Operating conditions will determine the service requirements. Regardless of how frequently a cleaner is serviced, you must never use a volatile cleaner such as gasoline or diesel fuel for cleaning purposes. A nonvolatile cleaner is required because, as the air is compressed, it generates heat and the combination of compressed air and fuel plus generated heat leads to explosions. The higher the compressor pressures are, the higher the temperature and the greater the emphasis which must be placed on safety precautions. A bomb explodes because the internal pressures

are greater than the housing can stand. Therefore, several safety devices are built into the air compressor systems. 5. For efficiency and safety, air enters the filter and passes on to the low-pressure cylinder. As air leaves the low-pressure cylinder, it is cooled in the intercooler before being compressed in the high-pressure side. The air must be cooled again in some manner because heated air doesn't have the body that cold air has under pressure. 6. To clean a dry pad filter, shake out the dirt from the element and blow air through the filter in a reverse direction. Then clean the filter with a nonflammable cleaning fluid. 7. Controls. Safety controls are required for a compressed air unit. Pressure regulators, relief valves, safety valves, pressure switches, etc., are some of the devices for control of compressed air and compressor operation. 8. Relief valves' are installed in the receiver and intercooler to relieve excessive pressures. Relief valves and safety pop valves are usually set at the factory and the setting should not be changed, although some units may be disassembled for inspection. Some installations require special valves in addition to the standard relief valves. If a valve is disassembled for service, all parts should be thoroughly checked. Proper operation of safety valves is very important, as the name implies. Excessive internal pressures can cause the air compressor unit to explode, so be sure that only recommended procedures and parts are employed when servicing a valve. Cleaning is also important so that this valve will not stick open, thus causing the second-stage pressure to drop to zero. 9. Intercooler. In the compressor the air is compressed and then sent into an intercooler, where it is cooled. The intercooler consists of a tank with coils through which air or water is passed to cool the compressed air. Under normal operating conditions the air can be kept at a reasonable temperature by use of aftercoolers. The aftercooler is generally located between the air compressor and storage tank. Its function is to cool the air to a desirable temperature and to condense moisture out of the air. 10. Air tank. The air tank is a storage facility for the compressed air. This tank is a sealed unit and will require minor maintenance. All piping connections must be fit tight, and valves adjusted according to specifications. The air tank is generally located in a cool place for efficient unit operation. 11. Chemical drier. A means of removing moisture from the air is the use of a chemical drier for absorbing moisture. After the chemical in the drier has become saturated with moisture, it must be reactivated by heat or be replaced. The drier in the air compressor system is in

many ways similar in construction to the type of dehydrator used in a refrigeration system. The dehumidifying cartridge containing the chemical is generally placed in the pressure line. One type of chemical that has been successfully used is calcium chloride. Refer to the manufacturer's manual for recommended procedures for cartridge reactivation or replacement. 12. Motor. The motor size will vary with the compressor size. Refer to the motor nameplate data for specification. Maintenance such as cleaning and lubrication should be done periodically. 13. Traps and drains. Traps and drains are used to remove moisture that may have accumulated in the system. The size of the air control system will determine the number of traps and drains that are used. These components must be cleaned periodically to remove any moisture that may have collected in the system. Refer to control air system diagram for actual location of these components. Generally, a trap or separator is located near the aftercooler 14. Gauge. Gauge locations will vary with each control air system. Most gauge locations are visible to the operator so that he can make an accurate reading on the air pressure. The air pressure must remain constant for accurate control operation; therefore a close inspection must be maintained on gauge readings. If there is a variation in air pressure, the cause must be found immediately and corrected. 15. Maintenance. Before starting the compressor, make sure the crankcase is filled to the proper level with a recommended grade compressor oil. The crankcase is filled to the line on the oil indicator or oil level elbow located near the bottom of the compressor base. All compressor parts are oiled from this base reservoir. A close check must be periodically maintained on the oil level for proper lubrication. 16. If the compressor is new, it should be drained and refilled every 2 weeks of constant operation. When the compressor is "broken in," drain and refill after every 2 or 3 months of daily operation or the equivalent. 17. The unit must be kept clean, since dirt is responsible for most compressor troubles. The air filters should be checked periodically and cleaned weekly with a nonexplosive solvent or by blowing air through the filter media. Make sure dry filters are free of all moisture. The screen type filter should be dipped in oil for better filtering action. 18. Special care must be given to make sure that all components are free of moisture. The air storage tank must be drained of moisture at least once a week and, if necessary, more often. Chem-

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ical driers are used for extracting moisture from the air in excessively humid areas. 19. The belts should be tight enough to prevent slippage, but not so tight as to cause excessive strain on the motor shaft or bearings. V-belts require more slack than fiat belts. 20. Exhaust and intake valves may become dirty after a period of operation. This can result in valve leakage and cutting down on efficient compressor operation. Periodically, valves should be removed, inspected, and thoroughly cleaned. If they continue to leak after cleaning, they should be replaced. An indication of valve leakage is any dark spots on the valve seat or polished surface. Pop-off safety valves should be blown off every 6 months to insure against sticking. Reducing valves should be checked periodically to insure that they maintain the correct system pressure at all times. 21. Troubleshooting. If there is a pressure loss in the receiver, it can be caused by insufficient power unit operation, slipping drive belt, leaky pipe joints, obstructed air intake filter, obstructed or burned valves, or worn rings. Knocks usually result from insufficient or improper lubrication, too thin a cylinder head gasket, worn bearings, loose flywheel, or foreign materials on the top of the piston. If the compressor begins to knock, it should be shut down immediately and the trouble reported so that the necessary repairs or adjustments can be made. 22. Pneumatic Control System 1. The pneumatic control system, illustrated in figure 47, consists of five major parts. They are: a. Source of air supply. b. Lines leading from the source of supply to the controllers (thermostats, humidistats, etc.). These lines are referred to as supply pressure lines. c. The controllers, thermostats, humidistats, recorders, etc. d. The lines leading from controllers to the controlled devices such as valves, dampers, etc. These lines are referred to as control pressure lines. e. The controlled devices (dampers. valves, etc.). 2. Lines. In order for the controller or any pneumatic control device to operate successfully, the devices must be connected to a regulated air supply. This air must be clean and dry and supplied at a pressure from 15 to 20 p.s.i.g. The installation must be planned to prevent water, oil, or dirt being carried through the piping into the control or instruments. 3. All tubing, pipes, and fittings must be clean inside and free of burrs. Shellac or a recommended joint compound may be applied sparingly

Figure 47. -Typical air supply system. to the male threads. All joints should be checked under pressure with a soap and water solution. 4. In reference to figure 48, the supply header furnishes air to a series of instruments in a building area. Note that the supply header is pitched 1/4 inch to 1 foot to help in the drainage of entrained oil or moisture. Sumps and drains are located in the low points of the system and should be blown off daily. The sump can be constructed of pipe of sufficient volume to hold all the collected water until it is blown out. Clean brass or iron pipe and fittings ½2 inch or larger should be used for the header. 5. The tubing that supplies air to the instruments should be taken fi6m the top of the header. This is an added precaution against letting the moisture enter the instruments and other controlling devices. The connections can be made at the side of the header when necessary, but never at the bottom. 6. The air connections at the instruments are 1/4inch National Pipe Thread (N.P.T.), 3/8-inch copper tubing (not less than .300 inch inside diameter (I.D.), or 1/4-inch iron pipe standard (I.P.S.). Brass pipe is used for the air supply piping. Where corrosive conditions require it, 1/4-inch I.P.S. clean, new, black iron pipe can be used. Copper tubing is most practical and can be kept free from leaks. The output piping to control valves should be ¾3/- inch copper tubing with few exceptions. 7. The air filter and supply pressure regulator, shown in figure 48, are installed in the supply piping immediately before the instrument. These components must be firmly supported to prevent the sagging of tubing. Arrows on these devices indicate how they must be connected in the system. Shutoff valves are installed in the system to enable the repairman to remove devices without shutting off the whole air-supply system.

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Figure 48. Air distribution piping. 8. The air filter catches any moisture, dirt, oil, and other foreign materials that may pass through the system piping. Most filters have a rated capacity as to the amount of moisture they can hold; therefore, they must be checked and drained periodically. Normally, this operation should be done daily, but under severe conditions of heavy moistened air it must be done more frequently. Close attention must be given to this detail for efficient and successful instrument and control operation. 9. The filter may be serviced by removing the bottom cover and removing the filtering element. The filter may be cleaned with an approved cleaning solvent or compressed air. Whenever the filter element looks too dirty, it should be replaced with a new element. 10. Pneumatic piping for instrumentation in large installations becomes complicated. Many instruments are located at designated positions in the duct system and must have compressed air piped to them; therefore it is advisable to refer to the installation schematic drawings when determining the exact location of piping and associated controls. 11. Reducing Valve Station. The supply pressure for most single temperature controllers runs approximately 15 p.s.i. Figure 49 illustrates an air filter and a reducing station for a single pressure system. Where two or more controllers or temperature thermostats are required, a dual-pressure system is used. The supply pressure for a dual installation is approximately 15 to 20 p.s.i. Figure 50 shows a reducing 74 valve station for a dual pressure system and illustrates a valve and switch for selecting either of the two pressures. 12. In most installations the air piping for the control system is concealed internally into the building structure. Generally, very little servicing is required unless there is some possible damage due to building alterations. Piping in the fan and equipment rooms is often exposed. In most instances the exposed lines are run along out-of-the-way places with properly designed supports and hangers. Extra precautions must be taken so that the lines do not become damaged. 13. Instruments and Controls. Automatic controls are designed to do a specific job in an air-conditioning system. The controls may open

Figure 49. Single-pressure system.

Figure 50. Dual-pressure system. or close valves and dampers, and operate other equipment automatically whenever the need arises. Years of research have gone into the design of these controls; and if they are properly installed and maintained, they will function efficiently. The type and number of electric and pneumatic controls used will vary with the size of airconditioning installation and with equipment usage in the building. 14. In equipment cooling, many different types of instruments and controls are needed to control the conditioned air at a required temperature and humidity. All instruments, whether they are recording, indicating, or control type, must operate and record accurately. You learned earlier in this chapter that special care h given to the compressed air supplied to the controls. It must be a clean, dry air and supplied at approximately 20 p.s.i.g. This air pressure initiates control operation. 15. Location. The location of controls and instruments will vary with each air-conditioning installation. Generally, a control is mounted near the device it operates. For example, a damper motor is usually located near the damper it operates. To find the exact location of controls, refer to your installation airconditioning drawings. 16. All controls and instruments must be installed in a clean, dry location. They must be mounted securely to prevent sagging or vibration and must be accessible for cleaning, adjusting, and repair. 17. Terminology. Before you can understand the operating principles of controls, you must know the terms that are applied to instrumentation. The following is a list of some of the most common terms and their meanings: a. Direct-acting controller is a control that is adjusted to give an increasing air output pressure with an increase in the variable, whether it is temperature, pressure, flow, vacuum, or liquid level.

b. Reverse-acting controller is a control that is adjusted to give a decreasing air output pressure with an increase in the variable, whether it is temperature, pressure, flow, vacuum, or liquid level. c. Direct-acting diaphragm valve is a valve that closes when the air pressure is applied to its diaphragm motor. It may be referred to as an air-to-close valve. d. Reverse-acting diaphragm valve is a valve which opens when the air is applied to its diaphragm motor. It may be referred to as an air-to-open valve. e. Set point is the value of the controlled variable that is asked of the controller by setting the indicator to that value. f. Control point is the actual temperature, pressure, flow, vacuum or liquid level at any given instant regardless of what the set point may be. g. Proportional control is the type of control action where the control signal varies in proportion to changes in the controlled variable, and may be any value from minimum to maximum. h. Sensitivity of a controller is the ratio of output pressure change to the movement of the pen or indicating pointer. i. High sensitivity results in a large output pressure change for a given pen or pointer movement. j. Low sensitivity gives a small output pressure change for a given pen or pointer movement. k. Throttling range is used to designate the sensitivity of a controller and is expressed as the movement of the pen or pointer in percent of chart, or scale, width necessary to cause a full opening or closing of the control valve. l. Automatic reset response is only used when automatic reset is adopted to the controller. It is an additional output pressure change resulting from a control point change and provides at a rate dependent upon the proportional response. It acts in the same direction as the proportional response and continues until the set point and control point are together. m. Reset action is the control action in which the corrections are made in proportion to the time a condition has been off and the amount of deviation. n. Controlling medium is the liquid, vapor, or gas, the flow of which through the diaphragm valve is varied in accordance with the demands of the process. o. Processed or controlled medium is the liquid, vapor, gas, or solid which is to be maintained at a constant value by varying the flow of the controlled medium. p. Load change is any factor which requires a change in the flow of the controlling medium in order to maintain the control point of the process.

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Figure 52. Remote bulb thermostat. Figure 51. Spiral bimetallic thermostat. These factors may include a change in the temperature, pressure, rate of flow, or composition of either the controlling or the controlled medium. q. Hunting is the changing or variation of the controlled variable about the control point, generally caused by excessive diaphragm valve movement. r. Wandering is an irregular shift of the controlled variable about the control point resulting from frequent load changes. 18. Thermostat. The thermostat is a nerve center of heating and cooling control centers and operates either pneumatically or electrically. The thermostat is a sensitive unit that responds to changes in room temperature and indicates where more or less heat is required. It transmits its indicating signal to the primary control for action. On an electric thermostat this is done by the making and breaking of electrical contact within the thermostat itself; within the pneumatic thermostat a pressure relay regulates the air to the controlled unit. 19. Thermostats usually differ in construction according to the type of primary control with which they are used. Probably the most common type of thermostat is the spiral bimetallic type shown in figure 51. 20. Figure 51 illustrates a remote bulb type thermostat. This type of thermostat is used in installations where severe vibration may exist at the point of measurement, or where it is desirable to have an instrument at a central location. The capillary tube shown in figure 52 is usually a liquid-filled element. It is sensitive to temperature changes and will control temperature accordingly. 21. Another type of thermostat frequently used is a bellows type shown in figure 53. 22. Location. The location for a thermostat should be representative of that part of the building where a required temperature is to be maintained. It should be installed where it will be exposed to free circulation of air, free from drafts, and away from the direct rays of the sun or any type of radiant heat. 23. Maintenance. The internal mechanism of a thermostat should be cleaned of dust and dirt. The contacts should be cleaned by drawing a piece of hardfinish paper (such as a common post card with a hard smooth finish) between the contacts. Never use emery cloth or other abrasives to clean the contacts. For recommended procedures or part replacement. refer to the manufacturer's maintenance manual. 24. Humidistat. Figures 54 and 55 illustrate

Figure 53. Bellows type thermostat. 76

Figure 54. Room humidistat. Figure 56. Hygrometer with motor-driven fan. room and insertion type humidistats. The humidistat is designed for the accurate control of the addition to or removal of moisture from air in a system or space. Room humidistats are available with various elements consisting of wood, hair, or animal membrane with adjustable sensitivity. Insertion humidistats are designed for accurate control of the relative amounts of moisture in heating, ventilating, and air-conditioning ducts. 25. Operation. Under normal operating conditions, the humidistat will control the humidity within 1 percent relative humidity. Most humidity controls operate electrically to regulate dampers, valves, or other regulating devices. For example, when a humidifying device having a spray nozzle is used, a solenoid valve is ordinarily inserted ahead of the nozzle. A humidistat in the conditioned space energizes the solenoid when the relative humidity drops below the humidistat setting. As soon as the humidity in the conditioned space is brought up to that required to satisfy the humidistat, the circuit is opened and the solenoid shuts off automatically. 26. Maintenance. The humidistat is a very delicate instrument and must be handled with care. The instrument must be encased at all times and kept free of dust and other foreign materials. It must be mounted securely and located where there is a good circulation of air through its mechanism. All adjustments must be made with special precautions since they are very sensitive devices. Refer to the manufacturer's manuals for recommended maintenance and adjustment procedures. 27. Hygrometers. The hygrometer is a device used to measure, record, and control humidity. There are many types and designs of these instruments made by various manufacturers, but their principles of operation are similar. 28. The hygrometer gives instantaneous readings of a measured area and will regulate valves or other controls to maintain a necessary humidity. 29. There are two types of hygrometer instruments. They are referred to as recording and recordingcontrolling types. 30. Figure 56 shows a recording-controlling type hygrometer. This instrument is installed in the area in which the humidity is to be measured. When the instrument is installed within an area, the air to be measured is circulated through the wet- and dry-bulb housing by a suction fan, as shown in figure 56. The fan draws the air through the bulb housing by use of an intake and exhaust port, usually located in back of the panel housing, creating conditions similar to those which psychrometric tables are obtained. In applications where bulbs of hygrometers must be located inside an apparatus, room, or duct, and where a continuous source of water supply is not available, a water feed instrument, as illustrated in figure 57, is used. The water supplied to the instrument must be cleaned and constant.

Figure 55. Insertion type humidistat. 77

Figure 58. Damper control air movement. and, as air is applied to the diaphragm, the piston is forced to move outward, causing the stem to move in the same direction. This forces a tension on the spring. The air that is fed from the controller to the damper operator usually ranges from 0 to 15 p.s.i., different spring ranges are available for different applications. Generally, 5-to l0 p.s.i. spring range is the most commonly used spring design tension; and with such a spring tension, the operator is in normal position when the control pressure is 5 p.s.i., a illustrated in figure 59. It is in its opposite-tonormal position when the control pressure is above or below 5 p.s.i. under normal load conditions. At

Figure 57. Water feed hygrometer. 31. Operation. The operating principles of a hygrometer are similar to the operating principles of a sling psychrometer, while on a hygrometer these temperatures are transmitted through a Bourdon tube to an instrument mechanism which records the temperature and humidity. Refer to the manufacturer's manual for specific hygrometer instrument action. 32. Maintenance. The instrument mechanism is similar in construction to the type used in transmitters and recorders, a explained later in this chapter; therefore its components can be maintained in the same manner. The major difference in hygrometer construction is the addition of water to the wick. The water and wick must be kept clean r accurate instrument operation. Periodic cleaning of the wick is required. Refer to manufacturer's maintenance manuals for specific instructions for maintenance and adjustment procedures. 33. Controlled Operator. Controlled operators require position changing according to variations of a controlled medium. For example, damper operators position dampers in many ways to regulate airflow, some of which are illustrated in figure 58. Blades may be used in parallel or opposed operation, depending on their use in duct system. 34. Damper operators. The damper operator is generally of the piston type, a shown in figure 59. The piston is attached to an operating stem

Figure 59. Piston damper operation.

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Figure 62. Damper operator with positioner. of valves to meet various requirements. Each type will have its own specific performance rating. 37. Positioner for operators. Figures 61 and 62 illustrate how a positioner can be applied to a valve or damper operator. The positioner provides a means of getting greater accuracy in positioning an operator and also increases the repositioning power. 38. In reference to figures 61 and 62, note that the positioner has a supply-air connection. Internally it has supply and exhaust valves like a pneumatic relay. The valves are operated jointly by the pressure from the controller and by the spring attached to the operator stem. A small change in control pressure can produce a large change in pressure on the operator until the stem moves sufficiently to cause the spring to stop the operation. 39. When positioners are used, the spring determines the operating range of the valve or damper operator. The range can be adjusted over a wide limit. 40. Maintenance of controlled operators. One of the most important things to remember when inspecting a damper operator is to make sure the stems or levers are clean. They must be lubricated as required. Keeping the unit clean is very important. If the diaphragm needs replacing, the following procedure is recommended: a. Remove the cylinder head and throw away old diaphragm. b. Place new diaphragm in its proper position. c. Roll back flange and insert the piston in the diaphragm. d. Place the assembly on the upper end of the cylinder with the loop of the diaphragm between the piston and the cylinder wall. Make sure that the diaphragm does not wrinkle. e. Put the cylinder in place with the air connection in the desired position and tighten the screws uniformly.

Figure 60. Diaphragm valve. 5 p.s.i., the operator assumes a midposition which is proportional to the air pressure. 35. Operators are generally mounted on the damper frame wherever possible and are connected directly to a damper louver. They can be mounted externally on the duct and operate through a crank arm on a shaft extension to the damper louver. 36. Pneumatic valves. Pneumatic valves consist of a diaphragm or bellows and a spring. Figure 60 illustrates a typical diaphragm valve. Its operation is very similar to that of the damper operator. The valve spring acts to either open or close the valve in accordance with the applied air pressure. The bellows type valve is generally used on convector, unit ventilators, and radiators where space is more restricted. Diaphragm valves are generally used on larger cooling and heating coil installation. There are many types and designs

Figure 61. Valve with positioner. 79

Figure 63. Exploded view of diaphragm valve. 41. The damper must be checked periodically for rust and corrosion. If corrosion deposits are found, they should be removed immediately with a steel brush, and the surface should be repainted. All pivots, linkage, and levers should be cleaned to remove dirt and other foreign matter is that they may operate freely. 42. The following components of a valve should be checked periodically and replaced if found unserviceable: a. Leaky or worn diaphragm. b. Leaky packing nut. c. Worn or pitted valve or valve seats and disks. d. Weak or broken spring. e. Corroded or dirty valve stem. 43. Figure 63 shows the exploded view of a diaphragm valve. Note the positions of each component in this valve. Care must be taken when replacing the rubber diaphragm. A kinked diaphragm will cause erratic operation. 44. Controllers. Controllers are used to regulate valve, dampers, and other devices by me of pressure or temperature. Because there are so many different designs of controllers, it is impossible to cover each controller difference in this memorandum. To understand the specific operation, maintenance, and calibration of any instrument or control, always refer to the manufacturer's manual in this section a general discussion will be given on controller operation, maintenance, and calibration. Figure 64 illustrates a typical controller. This type of controller records graphically the variations in temperature or pressure of a measured process.

45. Operation. Figure 64 illustrates a typical controller installation. The purpose of the controller in this system is to control the temperature of a process by operating a direct-acting diaphragm valve on a steam supply line. To understand its operation, let us assume that the temperature of the process is below that for which the controller is set. Because the temperature is low, the air regulating mechanism in the controller allows the valve to remain open, ,allowing more steam to flow into the process. As the temperature increases toward the control point setting, the bulb measures this increase. As soon as the control point is reached, the Bourdon tube uncoils. This action forces a change in the internal pressure regulating mechanism in the controller and forces air pressure down on the regulation valve, forcing it to close. 46. Maintenance. The chart on the controller must be replaced periodically. The chart on most controllers can be removed easily by disengaging the pen from the chart and removing the chart from the hub. Place a fresh chart on the clock hub and rotate it until the correct time line is opposite the reference arc, which is usually indi-

Figure 64. A typical controller installation.

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cated on the control face. The clock face generally has small pins or clamps mounted in it, the purpose of which is to hold the paper and serve as a means of driving the chart. 47. You should fill the pen with manufacturer's recommended grade and type of ink. General instructions as to how to fill the pen are supplied with the ink. Occasionally it is necessary to wash the pens with water or alcohol. 48. If the pen fails to touch the chart paper because of insufficient tension on the pen arm, bend the pen arm slightly toward the chart so that the pen touches the chart lightly. If the pen fails to follow the time line, adjust the chart so that a time line corresponds to the reference arc. Bend the pen so that it rests on the time line matching the reference arc. 49. The following precautions should be followed to improve controller efficiency and operation: a. Do not allow water or steam to come into direct contact with the instrument. If it is necessary to wash out pipes, tanks, or other apparatus with steam or hot water, remove instrument bulb first. b. Do not subject the pressure element of instrument to a higher pressure than maximum range of the chart unless the instrument is designed to take care of it. c. Do not allow the controller door to remain open longer than is necessary. d. Blow out the compressor receivers periodically to remove moisture and other foreign material. e. Blow out moisture traps at regular intervals. 50. Calibration. The procedures you will study here are considered as basic control calibration for most types of controls-pneumatic, electric, and electronic. The control manufacturer furnishes pamphlets with his control to guide you in servicing and calibrating a specific control. The basic procedures are: a. Set the controller set point to the sensed variable-temperature, pressure, humidity, etc. b. Adjust the controller to the midrange of the controlled device. c. Set the dial to the desired value. 51. Transmitters. The transmission system consists of a transmitter and a receiver. The transmitter and receiver with their connecting tubes and accessories form a system that measures the magnitude (temperature or pressure) of a process change and indicates this value at the receiver. 52. The transmitter or receiver may be either an indicating or recording type instrument. These instruments can be used as temperature or pressure transmitters, depending on the type of variable that needs to be measured.

53. Temperature transmitter. In using the temperature transmitter, the bulb of the tube system is placed in the apparatus to be measured at the point where the temperature is to be controlled and where the circulation is a maximum. It should not be too close to a radiating coil or an open steam inlet. 54. If the bulb is to be placed into a separable bulb, well, or stem, these units should be fitted into the apparatus first. Then insert the bulb and tighten the coupling nut. 55. Installations where a well is-furnished with a thermospeed sleeve must be given special consideration. To install a bulb with thermospeed sleeve, first separate the bulb from the well. Then screw the well tightly into the apparatus. Start the bulb into the well carefully to avoid any damaging. Force the bulb as far as it will go into the well, then tighten sufficiently to hold the bulb in place. 56. If the tube system is of a vapor pressure type, make certain the elevation of the bulb with respect to the instrument case is the same as that for which the controller is designed. Should the elevation be slightly different, it will be necessary to reset the pen to agree with the reading of an accurate test thermometer. Bulb elevation data will be given on the data plate of the instrument or in the manufacturer's maintenance manuals. 57. Pressure transmitter. Where pressure transmitters are used to measure the pressure of hot, moist atmosphere, a condensate loop should be installed beneath the instrument. The added pipe length protects the instrument from the effect of high temperature, and the loop retains condensate when the apparatus is shut down. 58. If the medium being measured is corrosive, the pressure element should be protected by use of a purge system or suitable sealing liquid. 59. Operation. Figure 65 illustrates schematically a transmission system which measures the temperature of a process. 60. The transmitter shown in figure 65 is actuated by a Bourdon tube. With an increase in process temperature, the Bourdon tube tends to uncoil and actuates components in a pressure mechanism. The mechanism, in turn, establishes an equilibrium at a new output air pressure, proportional to the pointer movement. The output line from the transmitter is connected to a bellows of a receiver, as shown in figure 65. The air pressure within the bellows actuates the pen of the instrument. The receiver pen then records values which are identical to those indicated to the transmitter pointer. 61. The dotted portion of figure 65 illustrates the receiver controller. This part of the instrument

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Figure 65. A typical transmission system. regulates a valve which controls a fluid or gas entering a process. 62. Maintenance. The transmitter or receiver must be mounted on a wall or panel where it will be free from vibration. It should not be installed where extreme temperatures may damage the delicate components. 63. The chart must be replaced at designated periods. Replacement procedures will vary with instrument design. Some transmitters are designed to use a type of disc chart similar to the one shown in figure 65, while others use a chart in the form of a roll. When a chart is replaced, care should be exercised to set it at the reference point so that the chart will record accurately for the particular time. 64. The pen must be filled with a recommended instrument ink. If the ink does not flow, start it flowing by touching the pen with the filler. Dried ink may be removed by washing the pen with warm water. If the pen fails to touch the chart, bend the pen arm inward so that it will hear lightly on the chart. If the pen does not follow the reference arc, adjust the length of the pen arm by bending the pen point to a length indicated by the time reference arc on the chart plate. 65. Temperature and Pressure Recorders. The temperature or pressure recorders operate on the same general principles as other type controls. The Bourdon tube principle is adapted to the recorder operation. The temperature or pressure recorder is used to record graphically the temperature or pressure of a process or apparatus operation. The thermal bulb attached to the recorder is placed in the process that is to be measured. Any change in the process temperature is transmitted through the Bourdon tube to the recorder mechanism and is shown graphically on the recorder chart. Figure 66 shows a typical type temperature or pressure recorder. 66. Figure 67 illustrates a typical temperature recorder installation. The connecting tubing from the recorder is placed in such a position that it will not receive additional heat from heat surfaces such as boilers radiators, pipes, etc. The bulb of the instrument is placed at the point where circulation is best. This is necessary for an accurate measurement and recording. 67. Figure 68 illustrates a typical pressure recorder installation. On liquid line installations excessive pulsations may occur; in such a condition, a needle valve, as shown in figure 68, is installed in the line. The oil seals are installed in the line to prevent excessive pressures and corrosive liquids from damaging the instrument. 68. The components of a temperature or pressure recorder are similar to other types of controllers and transmitters; therefore maintenance procedures are similar. 69. Chan replacement is done periodically. When replacing a chart, make sure the chart is inserted properly into the pins or clamps mounted on the clock hub. Rotate the chart until the correct time line is opposite the reference arc inscribed on the control face. Special guides attached to the recorder door generally hold the chart flat against the plate.

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Figure 66. A typical recorder.

Figure 67. Temperature recorder installation. 83

Figure 68. Pressure recorder installation. 70. Adjusting and cleaning of the pen is done as discussed previously in this chapter. 71. Recorders are calibrated and adjusted to testing conditions at the factory. Do not make any adjustments unless it is certain that the instrument is out of adjustment. Refer to the manufacturer's maintenance manuals on procedures for calibration of their instrument. 72. The following precautions must be observed when using a temperature or pressure recorder: a. Never allow a stream of water or jet of steam to come into direct contact with the instrument mechanism. b. Never wash or flush out pipes, tanks, or apparatus with steam or hot water, as it might allow the bulb of the instrument to reach a higher temperature than maximum range on the chart unless the instrument is designed to take care of these high temperatures. It is recommended that the bulb be removed when cleaning the apparatus. c. Never subject the Bourdon spring of a pressure recorder to a higher pressure than the maximum range of the chart unless the instrument is designed for higher pressures. d. Never allow the door of the instrument to remain open any longer than necessary. Keep the instrument clean and free of dust and other foreign materials. 73. Electric Fire Protection Control. The purpose of the fire protection control is to protect the installation against the spread of fire by automatically switching off air-conditioning fans and closing dampen. If a fire starts, fans become quite a hazard by increasing the intensity of the fire and helping it circulate through fire walls and from room to room. Most air-conditioning systems that use fan installations use some type of fire protection device in connection with these fans. 74. Operation and construction. The back plate and helix tube of a fire protection control is made of steel to

form a strong incasement. The sensitive bimetal helix reacts instantly to a temperature change, the rotation of the helix being transmitted by a cam or roller follower to a switch. The electrical contacts are enclosed in a dustproof case, and the contacts are opened by the cam or roller follower in the event the air temperature around the helix exceeds the cutout setting. 75. The cutout point is adjustable, approximately from 75° F. to 160° F. It is provided with a dial stop to prevent the adjustment from exceeding a maximum of 125° F. 76. The cam has both a high and low limit stop and will not rotate forward to the ON position even if the fire comes in contact with the bimetal helix. If the temperature becomes high enough to reach the cutout setting, the control may lock out and will require manual reset before the unit may be placed back into operation. The manual reset lever is exposed for easy setting ;and is located on the control housing. Removal of the cover exposes the instrument terminals and wiring. 77. Maintenance. Field repairs are not recommended by the manufacturer. If the control is not functioning properly, it is recommended that the unit be replaced. 78. Airflow Detector and Control. The air-flow detector type instrument is used to control and detect airflow movements. 79. Operation. The airflow instrument contains a heat source within its flow-sensing leg and operates on the principle of heat transfer. The electrical mechanism in the ambient compensating leg is actuated whenever there is a change in rate of flow beyond the set point of the unit. If there is a greater transfer of heat away from the flow-sensing leg containing the heat source than that for which the unit is set, the contacts in the electrical mechanism remain closed. If the heat transfer is less than the set value, the contacts of the electrical mechanism will open. 80. Since this device actually controls an electrical circuit by a switch action, it can be used to, control fans, alarm systems, air circulators, and other air-conditioning and dust collecting systems. 81. Maintenance. The airflow measuring instrument should be placed in a duct area with the two legs side by side in the airflow stream. The electrical connector should face either downstream or upstream. It can be placed in other directions, but this results in some calibration changes. The set point is generally set by changes in turbulence, but the instrument may be located in a turbulent region if it is calibrated in position. Airflow measuring devices require very little maintenance since they are hermetically sealed. It is recommended that the instrument be replaced if you find that it is not functioning properly.

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Figure 69. Schematic of an air-conditioner system. 82. Calibration. With the instrument installed, energize the heater for approximately 5 minutes to obtain an equilibrium. The airflow through the instrument should be maintained at the desired point for approximately 1 minute. Proceed to adjust the instrument to the point where electrical contacts just operate. Simulating decreased air-flow conditions or dirty filters may be accomplished by blocking off a section of the clean filter media. It is recommended that you refer to the manufacturers' maintenance manuals for specific instructions on calibration for their instrument. 83. Control System Operation. So far in this chapter you have not been shown how control instruments are used in a control system. Figure 69 is a simple sketch of a control system and illustrates operation of a heating and ventilating fan air-conditioning system. 84. In reference to figure 69, insertion thermostat T2 measures the temperature of return air and regulates modulating motor valve V2 in accordance with the heat measured in the conditioned area. The insertion thermostat T3 measures the temperature of the discharge air into the conditioned area and adjusts V2 to keep the air from entering the conditioned area at too hot or too cool a temperature. As the return air rises in temperature, T2 will close valve V2; and if the air continues to rise in temperature, T2 will shift the control of damper motor M1 to insertion thermostat T1 to take 85 in a volume of outdoor air for cooling. This quantity of air will vary with the outdoor air temperature to keep the air that is entering the coil at-the setting of T3. Humidity controller HI operates a solenoid valve V1 to add moisture to the air when required. A control panel is used to monitor all operations and control the positioning of the dampers and the closing of the dampers when the fan stops. 85. Special procedures must be followed when replacing an instrument or control that is not operating properly. Since every air-conditioning control system has its design differences, it is impossible to give specific information or replacement procedures. It is recommended that you refer to your installation SOP, and ask your supervisor for information regarding instrument or control replacement procedures. CAUTION: Check all system components for proper operation before adjusting, repairing, or replacing a control. Many times, controls are functioning properly but the equipment they control needs servicing. 23. Graphic Panel 1. The graphic panel, as the name implies, is a graphic illustration showing flow diagrams, recording and indicating devices, switches, and controlled equipment used in a building. The graphic panel in an airconditioning installation shows

Figure 70. Sectional view of a graphic panel. graphically the complete installation with indicating, recording, and control instruments. These instruments arc mounted in such a position, with appropriate symbols and flow lines that they represent schematically the systems they monitor and control. The graphic illustration will vary with the size and necessary instrumentation required in the building air-conditioning system. The panel is assembled from one or more sections and is so arranged that the operator monitoring the controls can easily see the lights, controls, and instruments mounted on the graphic panel. Some of the control instruments on the panel that can be used are: fan and pump control switches; damper controls; pressure, temperature, and humidity indicator and recorders; and associated controllers with manual, automatic, and cascade controls. Pilot lights on the panel give a continuous information concerning the operation of fans, pumps, and other equipment. 2. The graphic panel illustrated in figure 70 provides a graphic representation of refrigeration equipment, pumps, fans, and flow lines. The pilot lights indicate the operation of the cooling tower fans, chilled water pumps, and condenser water. The chilled water temperature is always indicated and recorded. Complete monitoring of the controls is done by. means of selector 86 switches, pushbutton switches, and recording controllers to provide remote control of equipment as represented on the panel. 3. The graphic panel shown in figure 70 is only a sectional view of an air-conditioning system; if the other sections were shown, the complete air-conditioning system would be graphically illustrated. 4. Most graphic illustrated panels use colors to identify components and flow lines. The following colors can identify most components, but this color code system may not be standard on all graphic panels. Steam flow lines and heating coil..........................Red Cooling water now lines and cooling coil ............Blue Condenser water line..........................................Green Duck work............................................................Black Control wiring and piping ................................Yellow Background ..........................Tan, gray, blue, or green 5. The sections of the panel generally illustrate the following systems and controls: (1) The refrigeration system. (2) Water circulating systems. (3) Air systems. (4) Room induction system and exhaust fans. (5) Interior zones.

(6) Temperature and flow recorders and indicators (7) Building air-conditioning system. 6. The graphic panel can be installed in any available space a building. It is generally installed inside a room, away from the operating refrigeration equipment. The advantages of having the panel installed in a separate room are cleanliness, less noise, and better lighting facilities. In some installations the panel is mounted in the same room as the operating equipment.. This arrangement allows the monitoring operator to be in direct contact with the refrigeration equipment operator in case trouble develops in the air-conditioning system. In either case, the monitoring operator must be in contact with tie equipment operator at all times by voice or through an annunciator system. Close cooperation between all operating personnel is very important for efficient and effective equipment operation. In some installations, more than one graphic panel is installed in the building for a more complete monitoring system. Review Exercises NOTE: The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the test. Do not submit your answers for grading. 1. What principle of operation does a thermostatic expansion valve use? (Sec. 18, Par. 6)

5. You have received a complaint from the messhall. The newly installed walk-in, used for fresh vegetables, is too cold. The system uses an automatic expansion valve and a low-pressure motor control. How would you correct the below-design temperature in the walk-in? (Sec. 19, Par. 10)

6. An ice cube maker is out of order. You find the storage bin is nearly empty. After troubleshooting the system you locate the trouble in the bin thermostat. What malfunction most probably caused the thermostat to become inoperative? (Sec. 19, Par. 12-15)

7. You find the air conditioner is not operating and that someone has inked the feeler bulb of the TMC. What has caused the air conditioner to become inoperative, and how would you correct the malfunction? (Sec. 19, Par. 20)

8. Why are snap action and mercury switches used in electric controls? (Sec. 20, Par. 2)

2. The control response a motor control uses is _____________. (Sec. 19, Par. 2)

9. What type of short exists when the ohmmeter connected to a terminal of the TMC and the TMC case indicates zero ohms? (Sec. 20, Par. 7)

3. How would you set a LPC for a wider differential? (Sec. 19, Par. 5)

10. Which mode of electric control would you use to operate a refrigeration unit? Why? (Sec. 20, Par. 13)

4. At what pressure will the compressor cutoff if the LPC settings are 40 p.s.i. and 15 p.s.i.? (Sec. 19, Par. 9)

11. A simple two-position control is used to open a set of louvers when the temperature is 80° F. and to close them when it is 70° F. What is the control point temperature? (Sec. 20, Par. 18)

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12. How does the timed two-position control differ in response from the simple two-position control? (Sec. 20, Par. 24)

20. How does the amount of current flowing through a series 90 balancing relay affect the operation of a series 90 control? (Sec. 20, Pars. 66-68)

13. How is the heating rate of a bimetal element in a thermostat dampened? (Sec. 20, Pars. 31 and 32)

21. When will the series 90 motor stop running? (Sec. 20, Par. 73)

14. You are installing a cooling coil that is used to cool an area within close tolerances. What type of control would you install? Why? (Sec. 20, Pars. 35-37)

22. The damper in a duct system is dosed, but the control is calling for e airflow. What has most probably malfunctioned? (Sec. 20, Pars. 79 and 80)

15. The controls for a system used for heating and cooling must be replaced. The new control system must operate gas valves for heating and relays for cooling. Lag time is not a problem. What type of control would you install? (Sec. 20, Par. 41)

23. What is the main difference between a series 90 humidity control system and a series 90 temperature control system? (Sec. 20, Par. 88)

24. Which side of the bridge would be the proper place to connect a humidistat for high limit control in a temperature control circuit? (Sec. 20, Par. 93)

16. How is the rotation of a series 20 motor controlled? (Sec. 20, Pars. 43-46) 25. One belt of a three-belt set driving an sir compressor is broken. How many belts would you install? Why? (Sec. 21, Par. 3)

17. Which type of electric controls acts similar to a single-pole, single-throw switch? (Sec. 20, Par. 49)

26. What would you suspect if the air compressor begins to lose efficiency? (Sec. 21, Par. 4) 18. The series 60 motor used on a motorized valve to maintain a liquid level in a tank is burned out. Can you substitute it with any series 60 motor? Why? (Sec. 20, Par. 55)

19. Can you substitute a series 60 two-position motor for a series 20 motor? Why? (Sec. 20, Par. 58)

27. The first stage of a two-stage compressor is operating normally but the output of the second stage s zero. The compressor has an intercooler between stages. What has caused the secondstage pressure to drop to zero? (Sec. 21, Par. 8)

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28. You have just finished installing an air compressor. What should you check before you start the compressor? (Sec. 21, Par. 15)

37. The piston type damper operator is at its normal position. The control line pressure is 3 p.s.i.g. Why hasn't the operator begun to open the damper? (Sec. 22, Par. 34)

29. What will probably occur if you replace a standard air compressor head gasket with a thin head gasket? (Sec. 21, Par. 21)

38. What determines the operating range of a positioner used for damper operation? (Sec. 22, Par. 39)

30. What are supply-air lines? Control air lines? (Sec. 22, Par. 1) 39. After overhauling a damper operator, you notice that it is operating erratically. What has caused this erratic operation? (Sec. 22, Par. 43)

31. How much pitch must you allow for a supply air header 12-feet long? (Sec. 22, Par. 4)

32. What factor determines the frequency of draining moisture from the compressor air filter? (Sec. 22, Par. 8)

40. The pen on a recorder chart is skipping on the chart. What should you do to correct this fault? (Sec. 22, Par. 48)

33. What type of controller would you install if you wanted a steam valve to open on a decrease in temperature? (Sec. 22, Par. 17)

41. When is it necessary to install a condensate loop on a pressure transmitter? (Sec. 22, Par. 57)

34. How do you clean the contact points on a thermostat? (Sec. 22, Par. 23)

42. The recorder you are going to install has been removed from a cannibalized building. The pen is full of dried ink. How should you clean the pen? (Sec. 22, Par. 64)

35. Under normal conditions, how dose to the set point will a humidistat control the humidity of the conditioned air? (Sec. 22, Par. 25) 43. The system fan is off and the dampers are dosed. The fan switch is in the ON position. What caused the system to shut down and how would you restart it? (Sec. 22, Pars. 73, 75, and 76)

36. Which type of controller is used to measure, record, and control humidity? (Sec. 22, Par. 27)

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44. How can you check the operation of an air-flow detector? (Sec. 22, Par. 82)

46. On graphic panels, temperature is always indicated and recorded. (Sec. 23, Par. 2)

45. Why is a graphic panel an asset in an airconditioning system? (Sec. 23, Par. 1)

47. A malfunction is shown on a graphic panel. The component indicated on the panel is color coded green. What system is malfunctioning? (Sec. 23, Par. 4)

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CHAPTER 7

Evaporative Cooling
CAN YOU RECALL your days at the local swimming pool or perhaps at the beach? If a slight wind was blowing, you will remember that you were more comfortable in the water than out of it. When you climbed out of the pool, the water in your wet bathing suit evaporated rapidly to leave your skin chilled from the loss of heat. Though you didn't give it much thought at the time, you were really experiencing evaporative cooling. 2. Applying the same principle, our older surgeons used an ether spray to freeze those portions of the skin in which incisions were to be made. Rapid evaporation of the ether reduced the temperature of the skin and underlying flesh to the freezing point. You can demonstrate this principle for yourself by wetting your hand with alcohol or water and holding it in the airstream from a fan. 24. Principle and Application of Evaporative Cooling 1. In an evaporative cooler the air is drawn through a finely divided water spray or a wet pad so that a portion of the water is being continually evaporated. The latent heat of evaporation, which must be passed on to the water to evaporate it, is supplied from the heat of the incoming air, thus reducing the dry-bulb temperature, an increase in the relative humidity and dewpoint temperature, and an unchanged wet-bulb temperature. 2. The water which is recirculated continually through an evaporative cooler assumes the wet-bulb temperature of the entering air after a short period of operation. The recirculated water will remain at the air wet-bulb temperature with no external heating or cooling. Makeup water is added to replace the evaporated water. 3. The temperature reduction, which can be made in the air passed through an evaporative cooler, depends entirely on the wet-bulb temperature of the air which is to be cooled. The wet-bulb temperature of the air entering the evaporative cooler is at the lowest temperature to which the circulating air may be cooled. 4. Evaporative cooling should not be used to cool air for spaces requiring constant temperature and humidity control, such as hospital operating rooms and certain types of highly technical electronic equipment. Evaporative cooling is best suited and chiefly used for cooling the space for the comfort of personnel. 5. Application. As we have shown, evaporative cooling depends on the evaporation of water; thus, it can be successful only under atmospheric conditions of a low relative humidity. It can be used only where the difference between the outdoor we-bulb and dry-bulb temperature is relatively high. In the arid regions of the southwestern United States, where there is low relative humidity, properly installed and operated evaporative cooling units cool comfortably. This type of system brings in 100 percent outside air. It may be equipped with a humidistat so that when the inside humidity is high and the cooler cannot function properly as an evaporative cooler, the water is cut off and the unit can operate as a straight mechanical ventilating unit. Whenever the outdoor wt-bulb temperature is 73° F. or lower, effective cooling and indoor comfort can be maintained by evaporative cooling. 6. The leaving air temperature of an evaporative cooler usually is just short of saturation; that is, the drybulb temperature of the air leaving a cooler does not quite reach the wet-bulb temperature of the air entering the cooler. An evaporative cooler operating at 90 percent efficiency will cool the air a number of degrees equal to 90 percent of its original wet-bulb depression. The measurement of the approach to the entering wet-bulb temperature is the saturation efficiency of the cooler. Air entering at 95° dry-bulb and 75° wet-bulb will be cooled to 77° if the cooler is operating at90 percent efficiency. Computation: The depression amounts to 950 (dry-bulb temperature) minus 75° (wet-bulb temperature), or 20°. Ninety percent of 20 is 18. Subtract 18 from 95° (the dry-bulb temperature) and you have 77?, which is the temperature to which the air is cooled.

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Figure 71. Evaporator cooler. 7. Components of Evaporative Cooling Units. Components of evaporative cooling units are very much the same in each type except that some have a different method of supplying the water to the evaporating pads. 8. Drip units. Typical drip type evaporative cooler components consist of the following: a motor-driven fan, or blower; a circulating water pump, piping, and water distributors; a water collecting pan with water makeup float valve and drain; and water evaporating surfaces. All these components are assembled in a weatherproof cabinet, the complete assembly being known as a selfcontained unit. It will range in size from the small, oneroom cooler with a capacity of about 700 cubic feet per minute (c.f.m.) to the large industrial coolers with capacities up to about 30,000 c.f.m. 9. The small, one-room cooler normally uses a propeller fan; the larger coolers use a squirrel-cage or centrifugal blower. The propeller fan cooler discharges the cooled air directly into the conditioned space from the vaned air-discharge outlet of the unit. Duct work for air distribution is never attached to this type of unit. 10. Coolers of 2000-c.f.m. capacity and above use the squirrel-cage blowers which are driven by a motor with a V-pulley and a V-belt. Figure 71 illustrates this type. Electric motors vary in size according to the size of the fan blade or blower: 11. The recirculating water pump is a vertical-shaft, directly connected unit of light construction. The pump

impeller is suspended in the pump housing from a ballbearing motor shaft, which eliminates pump bearings and packing. The pump housing has a wire-mesh screen through which the water passes as it is drawn in by the pump impeller and forced through the discharge tube to the distributor. Pumps sit in the water of the collecting pan or are sometimes mounted on a special frame. In either case, they must be insulated to prevent vibration and transmission of noise. Figure 72 illustrates this type of water pump and its breakdown. 12. A water distributor is a trough which receives the water from the recirculating water pump through the distributor head and distributes it evenly over the top of the evaporating surface pads. One distributor is provided for each pad in the cooler. The water flows through weirs (triangular openings) of the trough onto the pads which are on the air inlet sides of the cooler. (See fig. 73.) 13. The piping system consists of a T-fitting or a water distributor head with one inlet and three outlets which are connected by pipe or tubing to the water distributors. The inlet connection from the pump is usually a rubber hose. In figure 74 you can see that the quantity of water which flows to the branch piping system is regulated by an adjustable hose clamp which throttles the flow of water in the hose connecting the water pump to the water branch tubes. The quantity of water which flows through the branch tubes to the troughs is equalized by rubber or metal metering rings placed in each branch tube. 14. The water collecting pan forms the bottom of the cooler and contains the water, the water float makeup valve, and water drain standpipe, as shown in figure 75. The makeup valve controls the level of the water in the collecting pan and automatically admits water to replace any that is lost through evaporation and bleedoff. 15. Water in the collecting pan should be kept at a depth sufficient to keep the recirculating pump primed. The overflow pipe consists of a removable length of pipe, the top of which is slightly below the top edge of the collecting pan. When the water level in the collecting pan rises to the top of the overflow pipe, the excess water flows into the pipe and is carried away to a drain. 16. Some drip type evaporative coolers (the older models and the small, one-room type) are not equipped with a recirculating water pump or troughs. In these coolers the water is supplied directly to the distributors. In this case the distributors are usually copper tubes with small holes drilled about I inch apart in order to give an even distribution of water over the top edge of the evaporating pads. The flow of water is controlled by a water valve mounted on the inside of the front panel. Other models, in order to control

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Figure 72. Breakdown of water pump. the flow of water, use an electric solenoid water valve which opens and closes when the cooler is turned on and off. In this case the quantity of water also is regulated by an adjustable hose clamp on the hose feeding water to the distributor header and by metering rings in the branch pipes or tubes. Many coolers of this type have been installed in the past on military bases; however, present military design criteria do not permit installation of a drip type cooler that does not have a recirculating water pump. 17. The water evaporative surface of drip type evaporative coolers consists of one or more pads of aspen wood excelsior, redwood excelsior, a mixture of redwood and aspen wood excelsior, glass wool, or a fiber made of 93 other materials. The bulb excelsior is inserted in either cheesecloth, hardware cloth, or a metal frame to bind and hold the material together in pads. The pads are held in place by a barbed rack or other suitable means. Each pad is placed in a louvered frame, as you can see in figure 76. The louvered frames are fitted into openings on two sides and on the back of the cooler through which the air is drawn by the fan. The louvers serve a double purpose: they help to distribute the air uniformly over the entire area of the water evaporating pads, and they prevent water from wetting the area surrounding the cooler. 18. Spray units. Spray units (sometimes called

Figure 73. Weirs. air washer units) are made in sizes ranging from 3500 c.f.m. through 12,000 c.f.m. They are larger in dimension and weigh considerably more than the drip type unit. They are designed to keep the pads free of excess dust and water solids for a longer period of operating time than are the drip type units. They use sprays to wet and contin-

Figure 75. Water makeup valve and standpipe. uously wash down the inlet side of the filter pads and partially saturate the incoming air by direct contact as the air passes through the sprays. 19. There are two main types of spray designs used by manufacturers, with essentially the same results. These are the "spray nozzle" and the "rotating disk" (sometimes called slinger), shown in figures 77 and 78. Both spray type units consist of the following principal components: piping; water collecting pan with makeup float valve and drain; recirculating water pump; evaporative filter and eliminator pads; sprays, either nozzle or spray disk; and a motor-driven blower-all assembled in a self-contained weatherproof cabinet. 20. The piping system consists of the supply line and piping to the spray nozzle. The collecting pan forms a part of the bottom of the evaporative cooler cabinet, which contains the float-actuated water makeup valve and drain. Since the float controls the water valve, it also controls the level of the water in the collecting pan. The spray disk, or wheel, assembly of the evaporative cooler supplies a continuous sheet of atomized water over the air inlet face of the evaporative filter pad. The spray disk collecting pan is located below and in front of the center of the filter pad. The water in the collecting pan is supplied to the disk, or wheel, by a water pump similar to those used on drip type units. The centrifugal action of the rotating disk distributes the water evenly in all directions in a vertical plane so that the resulting curtain of spray water falling over the entire inlet face of the evaporative filter pads washes down the air inlet side of the pads continuously. 21. The automatic flush valve consists of an electrically operated solenoid valve and an electric timer combined in one unit. This valve flushes the water from the water collecting pan regularly and automatically. This action helps to keep the

Figure 74. Waterflow adjustment.

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Figure 76. Breakdown of side panel.

Figure 77. Spray nozzle evaporative cooler. 95

Figure 78. Rotating disc evaporative cooler. collecting pan clean. The solenoid valve is operated by means of a coil of wire wound on a soft iron spool which forms an electric magnet. When the current flows through the coil, the valve stem is lifted to open the valve. When current is cut off, the valve is closed either by gravity or by a spring. The operating solenoid part of the valve is completely enclosed. The water drain is controlled by a plunger, and a diaphragm isolates the working mechanism of the valve from the water. The valve assembly is also provided with an overflow connection and a manual operating lever to completely drain the water collecting pan. 22. The electric timer regulates the flushing action of the solenoid valve. It consists of a 1-hour electric timeclock which is adjustable from 0 to 2 1/2 minutes drain time. The settling of the drain time will depend on the dirt content of the air entering the cooler, the salt content of the water, and the resistance in the waste plumbing. 23. The spray nozzle does the same things as a spray disk. Nozzles are usually mounted in both a vertical and a horizontal position: the vertical nozzles are used for washing (down the evaporative filter pads, and the horizontal nozzles for forming a curtain of spray water over the entire air inlet area. Water is supplied to the nozzles through the piping system by a centrifugal pump of large capacity and heavy-duty construction, mounted in the water collecting pan. Figure 79 illustrates this type of 96

water pump which is completely sealed. All recirculating water pumps have a screen of sonic type to prevent particles of dirt and foreign matter from getting into the water piping system. 24. Evaporative filter pads and eliminator pads are usually made of fibers of various materials and are supported by angle frames and heavy wire mesh. Normally, spray type evaporative coolers use a specially treated hygroscopic spun glass for filter pads. This material assists in holding the fibers in place and helps the water adhere to the surface. The evaporative filter pads form the water evaporating surface. The eliminator pads remove the moisture from the cooler air after it leaves the evaporative filter pads and prevent waterdrops from being carried over into the fan and motor of the cooler unit. 25. Blower fans, known also as centrifugal fans, are used on spray type units. The fan and the motor which drives it are both equipped with grooved type pulleys and are connected with V-belts. The fan has a self-aligning, self-oiling bearing on each side of the impeller wheel. Blower fans are designed and rated for air delivery against 1/4 -inch water gauge static pressure resistance at the discharge outlet of the unit. If the static pressure is greater than 1/4 inch, water gauge efficiency will be lost. Blower fans, depending on size, are normally used to supply air through a duct system. 26. Rotary-Drum Evaporative Units. As the name implies, the rotary-drum evaporative cooling unit uses a rotating drum, powered by a reduction gear and motor. Figure 80 illustrates a partly dissembled rotary-drum unit. Other principal components are the exterior air-filter unit, the rotor housing, the water tank and the float-actuated water makeup valve, an automatic flush valve, and a motor-driven fan or blower. All of these components are mounted in a metal weatherproof cabinet. The rotarydrum type evaporative coolers are usually made in sizes ranging from 2500

Figure 70. Sealed water pump.

Figure 80. Breakdown of rotary-drum evaporative cooler. through 6000 c.f.m. Blowers and motors are designed the same as for the drip type evaporative cooler units. 27. The rotor, which is driven by an electric motor at an approximate speed of 1 1/2 r.p.m., is cylindrical and consists of alternate layers of corrugated and flat screen wound on a drum. It revolves in the rotor housing, the lower portion being constantly wet by its immersion in the water tank of the housing. When in operation, the rotor continuously exposes an evenly wetted surface to the incoming air. The rotor housing contains the revolving rotor and supports the rotor bearings and the electric gear motor. The lower portion of the rotor housing forms the water tank. The float-actuated water makeup valve controls the water level in the tank. 28. The automatic flush valve empties the water from the tank automatically. This action, which helps to keep the rotor and water tank clean, is repeated regularly by an electrically operated solenoid valve. The operating solenoid of the valve is completely enclosed. The water drain is controlled by a plunger, and a diaphragm isolates the working mechanism of the valve from the water. The valve assembly is provided with an overflow connection. A manually operated valve is also provided to completely drain the water tank. 29. The action of the solenoid valve is regulated by an electric timer, a 1-hour electric timeclock with contacts that open and close according to the time limit for which the timer is designed. The setting of the drain time, usually adjustable

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from 0 to 2 1/2 minutes, is dependent on the amount of dirt in the air that enters the cooler, the salt content of the water, and the resistance in the waste or drain plumbing. 30. The filters of the rotary-drum type evaporative cooler, called air filters, are usually of the impregnated, washable, metal type. The filter unit is located on the air intake side, where the air can be cleaned as it enters the cooler. 31. Slinger Type Evaporative Units. The slinger type unit may be constructed as a double unit. The metal weatherproof cabinet may contain two sets of the principal components, such as two sets of spray wheel assemblies. The blower, or fan, of the slinger type evaporative cooler is the same as on the rotary-drum unita motor-driven centrifugal blower. It is driven by a Vbelt connecting the blower and motor by means of grooved pulleys. The blower which is used to supply air through the duct system is designed and rated for air delivery against V4 -inch water gauge static pressure resistance at the discharge outlet. It has two self-aligning, self-oiling bearings, one on each side of the blower wheel. The electric motor operating the blower is mounted high in the metal cabinet to protect it from too much moisture. 32. The electric motor operating the spray wheel is scaled in a waterproof assembly with water-seal packing around the shaft to the spray wheel. The spray wheel picks up water from the collecting pan and distributes it in all directions in a vertical plane by the centrifugal action of the rotating spray wheel. This action supplies a continuous sheet of atomized water which is sprayed over the air inlet side of the evaporative filter pad. As the air passes through the water, dust and dirt are removed and the air is partially cooled. This continuous sheet of atomized water also continually washes the dirt from the pad. The water collecting pan, which is located below and in front of the evaporative filter pad, forms a part of the bottom of the evaporative cooler cabinet. A floatactuated water makeup valve, which controls the level of water, is contained in the collecting pan. 33. The automatic flush valve assembly consists of the flush valve, which flushes the water from the water collecting pan regularly and automatically; solenoid valve; and timer. The timer is electric and has an adjustable drain-time setting which regulates the flow of current to the solenoid valve. The solenoid valve operates the drain valve. This assembly is very similar to the flush valve assembly used with the rotary-drum type evaporative cooler. 34. The evaporative and eliminator pads are usually made of various washable materials supported by angle

frames and heavy wire mesh. The fibers are impregnated with a hygroscopic material which not only helps to hold the fibers in place but also treats the fibers so that water will adhere to their surface. The evaporative filter pads form the water filter and evaporating surface, and the eliminator pads remove moisture from the cooled air. The eliminator pads also prevent small drops of water trapped in the air from being carried over into the fan and motor of the blower compartment. 35. The cabinets for slinger type evaporative coolers are usually constructed the same as those for other large type evaporative coolers. Heavy-gauge galvanized steel is used to enclose the entire unit. The sides are removable to provide access to the interior. 25. Installation Procedures 1. The size and style of the unit determine the location and the type of structure required to support it. 2. Locations. Units mounted in building windows are of the small, propeller fan and blower types. These are light in weight and will not do damage to the building structure. 3. The large, heavy, blower type units must be mounted on self-supporting platforms adjacent to, but independent of, the building. In some cases--the air washer type unit, for instance--it may be more economical to mount the unit on a concrete platform. 4. The dimensions and operating weight of the cooler should be determined before starting construction of the supporting structure or mounting platform. A walkway with guard rails round the cooler will give you a safe working area. Each platform should also have a ladder, built as part of the structure. 5. Never mount cooler units on the building roof. Each unit must be mounted on the platform so that it is rigid and level. In some cases this may require shims and the bolting down of the unit. Every cooler manufacturer furnishes mounting and installation instructions with each type of unit. You should understand these instructions before installing the cooler. 6. Connections. The various connections required for evaporative coolers must be installed as specified for each cooler. Check the instructions and see that the proper size pipe, valves, switches, wire, and fuse boxes are installed. Only in this way can you be sure that the equipment will give the expected service. 7. Water supply and drain. The small, window type evaporative cooler normally uses 1/4-inch copper tubing to carry the water supply. A 1/4-inch fitting is located on the side of the sillcock valve which is installed on any ordinary 1/4-inch outside water valve (garden hose).

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8. Large units must have a water supply line of at least 3/4-inch pipe or tubing. A globe shutoff valve is installed in the supply line on the inlet side to the unit. Coolers using a water solenoid valve instead of a recirculating pump should have a water strainer installed on the inlet side of the solenoid valve. A water faucet with a hose bibb should be installed in the supply line near each cooler. This is to be used in washing down the interior of evaporative coolers immediately after heavy duststorms or when maintenance service is being done. 9. The water drain or was system for evaporative coolers should be at least 1 1/4 inches in diameter to reduce drain stoppage. The drain system should be connected to the sewer or to a street drainage system. In freezing areas the water supply system should be insulated against freezing, or the unit may be installed to permit the complete draining of the system. 10. Electric connection. Small propeller units (window type) are usually connected by inserting the electric cord plug into a convenient outlet. Thus they can be placed in or out of operation by a toggle switch on the front of the unit. 11. The larger units should be equipped with their own fuses. Sometimes this may require a separate main switch and fuse box, depending on the power requirements of the unit. Pushbutton stations or toggle switches are used to start and stop equipment operation. Other units may require two separate switches, one for the water recirculating pump motor and one for the blower motor, hooked in series with one of the motor leads. This allows the unit to be used for ventilation. The larger units usually require magnetic starters and sometimes have pilot lights or other devices to indicate when and what part of the cooling unit is in operation. All switches, controls, pilot lights, and Indicators should be mounted on a control panel located in a convenient place. Each large evaporative cooler should have a disconnect switch mounted inside the unit to permit maintenance personnel to control the unit operation while performing maintenance service. 12. Air Distribution and Supply Ducts. Duct work for evaporative cooling systems is designed and installed in the same manner as that refrigerative air-conditioning systems. There must be a properly sized supply duct and adequate exhaust outlets. 13. Supply ducts. The supply duct enters the building below the roof bearing plate direct from a unit which is mounted on a structure platform. The duct must be installed without cutting any structural members of the building. A slide damper of air stop should be provided for winter closure of the duct system.

14. When practical, corridor space should be used as a plenum for air distribution ducts. The plenum must be airtight except for supply outlet grilles. Air supply branch ducts to rooms or spaces a located to provide an even distribution of air. Branch ducts and multiple air outlets from the main duct usually have dampers or splitters to balance the flow of air. When the system is checked and adjustments are made, the dampers or splitter should be locked in place so that unauthorized persons cannot readjust them. Directional flow vanes are usually installed on the supply outlet so that air may be directed where required. Do not use wire mesh screens on supply outlets; they provide no means of directing the flow of air. 15. Exhaust outlets. To have proper cool air circulation throughout the various spaces and rooms of a building, each room must have a proper size exhaust outlet leading to the outside of the building. Exhaust outlets should have louvers or adjustable vents for regulating the circulation of air. Exhaust fans may be installed to insure positive air circulation through spaces with high temperatures, such as messhalls and areas around ranges. 16. If windows are used as exhaust outlets, they should be raised to a fixed position. Window stops should be installed to prevent personnel from raising windows in excess of 4 inches. Not less than 2 square feet of louvered exhaust openings should be provided for each 1000 cubic feet of air delivered to a room or space when such exhaust openings are used in lieu of raised windows. All doors should be kept dosed to maintain a balance of the airflow throughout the conditioned space. 17. Automatic Controls. Automatic controls are not essential but are convenient. When automatic thermostat and humidity controls are used, they should be checked for proper operation and also should be adjusted, set, and locked in position to prevent readjustment by the using organization. 18. A thermostat is used to prevent the temperature from going below a predetermined value. The thermostat controls the operation of the blower and water pump by starting and stopping them when the temperature in the cooled area is above or below the predetermined thermostat setting (usually 80° F.). 19. In some areas the evaporative cooler will have a humidistat to control the humidity of the space being cooled. The humidistat when used, controls the starting and stopping of the water pump whenever the relative humidity in the space being cooled is below or above the predetermined humidistat setting (usually 55 percent). Usually these controls are not used together. In case they

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are, the thermostat is usually connected to operate the blower and the humidistat to control the water pump. In any case, they are connected to permit a constant supply of air by continuous operation of the blower, thus turning the unit into a straight mechanical ventilation system when atmospheric conditions prevent the unit from functioning as an evaporative cooler. 20. Startup Checks and Adjustments. The successful functioning of evaporative air coolers depends directly upon the manner in which they are operated. Normally, the using organization is responsible only for starting and stopping evaporative air cooling units and systems. The personnel of the using organization must be instructed thoroughly in the operation of electrical switches, water valves, and other controls. They are cautioned not to start the blowers prior to starting the water pumps after long shutdown periods. 21. Startup Procedures. When a new or inactive evaporative air cooling unit is to be placed in service, you should perform the startup services to prepare the equipment for operation. Before starting the equipment, you must inspect all parts, accessories, and units to see that they are secure and correctly adjusted. Seasonal startup is scheduled well ahead of the time the equipment is to be used. This allows ample time for inspection and startup services. During initial startup procedures, all supply outlets, vanes, dampers, and splitters should be opened for normal airflow. Moist rags should be placed over the supply air outlets into each space being airconditioned to catch the dust and construction dirt before it is discharged into the space where occupants are on duty. 22. The ratings of motor overload protection devices should be checked against the motor nameplate ampere ratings. If the devices are oversize or undersize, thermal elements of the proper size should be installed. 23. An ammeter should be connected to the blower motor circuit prior to starting the motor. Starting and running currents should be recorded when the blower is first operated, with all pads and filters dry and in place. If the running current is equal to or less than the overload rating of the motor, then the motor will not be overloaded under final load conditions. 24. Testing the unit with clean, dry pads assures you that the unit can be operated subsequently without water for ventilating purposes, since pads are always in place when the unit is operating. If the running current is in excess of the motor overload rating, you must determine the cause. In many cases, overload is due to excessive fan speed. Correct this before continuing the operation of the motor. The fan speed should be reduced by adjusting the variable-pitch motor pulley or by reducing

the size of the motor pulley. Where this is impractical, the blower pulley size should be increased. You should use pulleys of the correct size rather than attempt to cut down the original ones. An ammeter should be used to check the starting and running currents of the pump motor after the water collecting tank has been filled and the pump first operated. If the running current is in excess of the motor overload rating, determine the cause and correct it before continuing the operation of the pump. 25. During the initial operation of the unit with water on the pads or sprays, air delivery in the supply ducts should be determined by a velometer or other velocity-indicating instrument. Take readings at a sufficient number of cross-sectional spots in the same section of the supply duct so that you can arrive at an average velocity reading. Multiply the average velocity by the square-foot, cross-sectional area of the duct. This computation will give the quantity of airflow in cubic feet per minute. If the rate of air delivery approximates the designed capacity of the system, the unit should be continued in operation. If the rate of air delivery is considerably in excess of design capacity, reduce the fan speed. In such a case, the fan motor is usually overloaded and the velocity of the air through the water evaporating pads is excessive. If this is the case, drops of water may be carried over into the fan compartment and cause shorting of electrical circuits, motor "burnouts," rapid deterioration of belts, and excessive rust and corrosion. After balancing air distribution to all spaces by the use of velocity indicating instruments, lock all dampers, splitters, and directional flow vanes in their final position in such a manner that tampering or readjustment is impossible except by the maintenance personnel. Units which operate at efficiencies below 80 percent require adjustment to improve their performance. 26. Shutdown Services. When evaporative air cooling equipment is to remain idle for long periods of time, you must perform the preventive maintenance shutdown services. This service protects the equipment, conserves critical materials, and prepares the equipment for minimum startup service. All parts, accessories, and equipment are inspected to insure proper servicing for seasonal shutdown or standby condition. 27. The shutdown service for evaporative air cooling equipment depends upon its condition and the method of storage. Usually the coolers are drained and washed out with water under pressure. If they are to remain attached to the building, they should be protected from the weather by some type of cover. Normally these coolers should be removed from the building, stored in a dry place, and overhauled during the winter season. This procedure puts the coolers in good

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condition for the next season's run. Having been overhauled, they should give very little trouble during the cooling season. 26. Preventive Maintenance and Inspections 1. Definite procedures for the preventive maintenance of refrigerating and air-conditioning equipment are necessary for efficient operation. The objectives of preventive maintenance are as follows: to prevent breakdown, to insure proper maintenance, to provide immediate and adequate minor repairs and avoid major repairs, to control maintenance costs, to establish specific personnel assignments, and to develop minimum but adequate maintenance records and data. 2. Well-planned inspections and up-to-date and correct records are required for a successful preventive

maintenance program. Inspection is a key phase of preventive maintenance. It is a simple fact that when minor deficiencies are overlooked, they can cause major breakdowns in the future. This eventually defeats any preventive maintenance program. The responsibility of detection is the duty of all personnel assigned to preventive maintenance. 3. Servicing Components. Table 1 contains instructions which will serve as guide procedures for inspecting and servicing the components of evaporative air cooling equipment. It may be necessary to supplement these instructions and procedures with the manufacturer's instructions where the equipment is not standardized in design.

TABLE 1

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TABLE 1 Cont'd

4. Major Repairs. Major repairs and renovation of evaporative air cooling units should be done during the winter every year. At post where there are a great many units to maintain, the shop space should be sufficient to permit proper repair of the units. Smaller units can be removed from their platforms and taken into the shop as self-contained units. Since large units are not readily movable, their component parts should be removed to the shop. All units should be dismantled every year, thoroughly overhauled, cleaned, and painted to prevent rust. It is best that fans be dismantled and wheels and scrolls cleaned and painted. Both inside and outside casings, pad frames, eliminators, water makeup pans, and metal structural parts should be cleaned and painted. Pads should be replaced as required. The best practice is to see that the water distributing system and the spray pump are dismantled and inspected for cleanliness or excessive impeller wear. Badly worn impellers should be replaced. All bearings on the fans, pumps, and motors must be cleaned, checked for wear, and replaced when necessary. All the ball bearings should be repacked with grease and the sleeve bearings lubricated with oil or grease as necessary. It is approved practice that units, when repaired, be replaced on their stands and their air intake louvers suitably covered to prevent the pads and the equipment from becoming dust laden. Review Exercises NOTE: The following exercise are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other note on the chapter content. Immediately check your answers with the key at the end of the text. Do not submit your answers for grading.

1. Agree. Disagree. Evaporative cooling removes heat from the air to evaporate the water. (Sec. 24, Par. 1)

2. With an evaporative cooler, the air can be cooled to its __________________ temperature. (Sec. 24, Par. 3)

3. List the following cities in order of the best environment for evaporative cooling: New York, New York. New Orleans, Louisiana. Dallas, Texas. Phoenix, Arizona. (Sec. 24, Par. 5)

4. What would be the most probable cause of low water supply to the distributor in an evaporative cooler? (Sec. 24, Par. 11)

5. Which type of evaporative cooler would you install in a dusty area? (Sec. 24, Par. 18)

6. On spray type evaporative coolers that utilize a flush valve, what controls the frequency of operation? (Sec. 24, Par. 22)

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7. What maintenance Is required when water droplets are being blown into the room from a spray type evaporative cooler? (Sec. 24, Par. 24)

13. A complaint is submitted to the refrigeration shop about excessive noise from the evaporative cooler. You find that the window opposite the cooler is open 2" and the cooler is rated at 4500 c.f.m. The window measures 2 1/2' x 5'. How can you correct the complaint? (Sec. 25, Par. 16)

8. A small evaporative cooler is connected to a building using duct work. The duct work contains four elbows and a diffuser. What may happen to the evaporative cooler? (Sec. 24, Par. 25)

14. What precaution should you give to the user after you have checked the cooler for season operation? (Sec. 25, Par. 20)

9. Which evaporative cooler would require a heavy structure to support it, the 3000 c.f.m. drip type or 3000 c.f.m. rotary-drum type? (Sec. 25, Par. 1)

15. The blower motor on an evaporative cooler has burned out. How could this have been prevented? (Sec. 25, Par. 22)

16. How can you reduce the speed of the blower in an evaporative cooler? (Sec. 25, Par. 24) 10. The drain on an evaporative cooler is plugging up regularly. How can you correct this condition? (Sec. 25, Par. 9) 17. How many c.f.m. is being delivered from a 12" x 24" duct when the average velometer reading is 50 f.p.m.? (Sec. 25, Par. 25) 11. A new evaporative cooler, drip type, is installed in the messhall. Two switches are included with the unit. What function could these two switches serve and how are they connected? (Sec. 25, Par. 11)

18. What service must you perform on the troughs and weirs of a drip type evaporative cooler? (Sec. 26, Par. 3)

19. How is the water distribution system cleaned? (Sec. 26, Par. 3) 12. You have just installed an evaporative cooler in a room. What size exhaust opening should be provided to allow proper cool air circulation? (Sec. 25, Par. 15)

20. What size feeler gauge is used to adjust the axial clearance of the blower wheel? (Sec. 26, Par. 3)

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CHAPTER 8

Mechanical Ventilation
HAVE YOU EVER waved a paper in front of your face on a warm day? This is one form of ventilation system. The air moving across your face helped the moisture (sweat) evaporate. This evaporation process removed heat from your body so that you felt a cooling sensation. 2. You will study various ventilation systems and their application. There are many factors which must be considered when you install a ventilation system. These factors are discussed in this chapter. 27. Ventilation and Distribution 1. When ventilating a room or building, the factors to consider are the tightness of construction, the number of occupants, and the kind of work being done. Whenever human beings work in close quarters, the gaseous products from respiration, the odors from perspiration, and the heat radiated from the body should be removed. All of these byproducts of human activity tend to reduce human efficiency. 2. Proper air distribution is essential in a ventilating system. The system not only should deliver a definite amount of air to a room but also should distribute it evenly. If this is not done, the occupants will be uncomfortable from drafts, stuffiness, and temperature differences between the floor and ceiling. 3. For instance, in comfort ventilation, a corner or spot near a window will be noticeably hotter or cooler than the rest of the room. Since individual comfort is, in this case, the main purpose of ventilation, you are really interested only in distributing the air over the floor area of a room to a height of about 7 feet. Complete air distribution is more important when removing fumes and vapors from a building or room. Poor air distribution causes gas fumes and vapors to remain in various areas of the building and creates an explosion hazard. In all cases where air is exhausted from a space, replacement air must be supplied. 4. Air Distribution Standards. Air from fans and duets is delivered to a room through grilles. The purpose of a grille is to distribute the air evenly and silently, without creating drafts. 5. So that the people will not have a feeling of stuffiness, there should be a slight movement of air at all times. An air movement of 25 to 35 feet per minute (f.p.m.) is most satisfactory, but air motions of 20 to 50 f.p.m. will usually prove acceptable. 6. Grille manufacturers normally rate their grilles as follows: a. Quantity of air in cubic feet per minute (c.f.m.). b. Outlet velocity in f.p.m. c. Nominal grille size. d. Blow of air in feet. (By "blow" we mean the horizontal and vertical distance a stream of air travels from the grille until it slows to a maximum velocity of 50 f.p.m.; see fig. 81.) e. Drop of air in feet. (By "drop" we mean the vertical distance the lower edge of a horizontal projected airstream drops between the grille and the end of the blow.) 7. Air Distribution Limiting Factors. A few of the air distribution limiting factors are discussed in the following paragraphs. 8. Blow. The blow at a grille must be sufficient to produce satisfactory conditions throughout the conditioned area. Overblowing results in drafts and underblowing may cause improper air mixing. If cooling air is being supplied, the cool air may drop too fast to permit proper mixing with the warmer room air. Experience has shown that the blow should be about three-fourths of the distance toward an outside wall or window. This distance should be modified if the room height or height of a beam is such that the drop will cause a draft over the people inside. 9. Drop. Place the grilles so that the airstream at the end of the blow is not less than 6 feet above the floor level. However, the airstream should not touch the ceiling as this may cause a dirt streak on the ceiling. 10. Air motion. To achieve desirable air motion in a building or a room without exceeding velocity limits is one of the more critical air

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Figure 81. Blow and drop. distribution problems. Some of the factors which are apt to cause the motion of the air to exceed desirable limits are: (1) excessive air discharge velocities, (2) high number of air changes per hour, (3) premature drop of cold air into the conditioned space, and (4) overblowing of the air. 11. Temperature differential. Temperature differential is an important factor affecting grille performance. An air differential of 5° requires little concern, but a differential of 50° requires considerable care in the proper location and selection of grilles. 12. Dirt. Although the supply air may have been carefully filtered, it still contains small dust particles which will settle on ceilings and walls. A good rule is to locate the grille at least two widths below the ceiling and make sure that the air does not shoot up. and hit the ceiling during the blow. 13. Noise. Grille noise is usually caused by the air discharge velocity and the grille size. To insure quiet operation, make sure that the manufacturer's recommendations are not exceeded and that air distribution over the grille is fairly even. 14. The next subject we will discuss is ventilating fans. We will learn how to select a specific fan for a particular application. 28. Ventilation Fans 1. The devices used to produce airflow are referred to as fans, blowers, exhausters, and propellers. 2. Types of Fans. The fans used with ventilating systems are divided into two groups, radial-flow and axialflow. 3. Radial-flow fans. Radial-flow fans-more popularly called centrifugal, blower, or squirrel-cage-are used when a considerable amount of duct work is involved. The principal feature which distinguishes one type of centrifugal fan from another is the curvature of the blades. The main types are the forward curved blade,

the radial blade, and the backward curved blade. The tip of the forward curved blade inclines in the direction of rotation, while the radial blade is straight, and the tip of the backward curved blade inclines in a direction opposite to the rotation. The performance characteristics of each fan depends, of course, on its type. For a given output, the forward curved blade is used for relatively low speed operation. The radial blade is used for average speed operation, while the backward curved blade is used for relatively, high speed operation. 4. Axial-flow fans. The axial-flow fan is one in which the air flows in line with the impeller axis, within a cylinder or ring. These fans are divided into various types. Among the more popular types are: the propeller, tube-axial, and vane-axial. 5. The old design of propeller fan, which consists of a propeller wheel within a mounted ring or plate, will not handle air against high resistance. It is not suited for a system with ducts, grilles, filters, etc. However, the propeller fan can be used to remove air from areas to the outside atmosphere without a duct. 6. Recently, fan manufacturers have developed a special type of propeller fan which can move large volumes of air against considerable frictional resistance. This fan has a large hub and short adjustable blades. It is highly efficient, but operates at high speeds. This high speed operation causes noise. therefore it should be used only in applications 'where noise presents no problem. 7. The tube-axial fan consists of an axial-flow wheel within a cylinder. It is normally used against appreciable frictional resistance. 8. The vane-axial fan is similar to the tube axial type. It consists of an axial-flow fan within a cylinder, with guide vanes before or after the fan. The purpose of the guide vanes is to increase its efficiency. A fan of this type is more often used against moderate frictional resistance. 9. Fan Capacity. The most important factor to consider when selecting a fan for a specific job is the proper capacity. Too often the fan is selected on the basis of diameter only. However, to determine the fan capacity you must calculate the c.f.m. and the static pressure. After you calculate the c.f.m. and static pressure, select the fan on the basis of efficiency, noise, cost, and physical size. You can estimate fan efficiency by dividing the power output by the power input. Total efficiencies range from 50 to 65 percent in small propeller fans to nearly 80 percent in centrifugal fans. Fan manufacturers publish data which will aid you in selecting the correct fan size. The noise level of the fan is an important factor which must be considered before selection. Office

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buildings require quieter operating fans than do industrial shops. 10. Computing fan capacity is quite simple. You must know what size fan will make a given number of air changes in a room in an hour. Once you know the c.f.m. of air needed, you can select the fan capable of delivering that amount. Multiplying the dimensions (length X width X height) of a room by the air changes required per hour will give you the total quantity of air to be moved per hour. Stating this as an equation, we can say: Q = CV 60 where Q = quantity of air per minute (c.f.m.) C = air changes per hour V = volume of room in cubic feet 60 = minutes/hour 11. Now suppose you want to find the fan capacity for a room requiring 12 air changes per hour. Suppose the room is 100 feet by 25 feet by 14 feet. Substituting in the formula, Q = 12 X 100 X 25 X 14 = 420,000 = 7,000 c.f.m. 60 60 Therefore the fan capacity must be 7,000 c.f.m. 12. Fan Motors. Ventilating fans are usually driven by electric motors. Small fans, especially those which are operated at high speeds, are normally connected directly to the motor shaft. Large fans and those which operate at lower speeds are connected to motors through pulleys. 13. When you select a fan motor, it should be one size larger than is required for normal load conditions. This is necessary because larger volumes of air may be required. 14. Some type of thermal switch should be provided in the air inlet duct to break the circuit to the motor in the event of a fire. Thermal switches of this type are usually set to open the motor circuit whenever the inlet air exceeds 135° F. Electric fan motors should also have manual electric switches so that you can control the operation of the motors when servicing them. 29. Air Ducts 1. Air ducts are pipes used to carry and distribute fresh air or exhaust air from a building or room. Ducts are usually constructed of galvanized sheet steel. Two types of ducts are round and rectangular. Round ducts require less metal to carry the same amount of air, but rectangular ducts are used in most installations because of space considerations. 2. The correct size of ducts to be installed may be determined by using various charts and formulas procured from manufacturers of air-conditioning equipment. However, for all practical purposes, it should be the same size as the outlet opening of the fan assembly.

3. Friction Losses in Ducts. When air flows through a duct, it loses some of its pressure because of friction. The greater the amount of air flowing through a duct of a given size, the greater is the friction loss. Furthermore, the power needed to deliver a certain amount of air increases rapidly as the size of the duct decreases. For this reason, ducts should be of sufficient size to keep friction losses to a minimum. Friction losses are usually computed by the use of formulas; however, charts procured from manufacturers may be used. 4. To measure the air velocity through a duct or grille, the ventilating system must be in operation. Several different instruments may be used to measure this velocity. These include the "Alnor" velometer and the anemometer. The "Alnor" velometer is the one most commonly used. It is a convenient instrument for spot readings and is adaptable to many uses. For example, it measures velocities within an enclosure or duct, and at grilles. It is sufficiently accurate for all practical purposes. The anemometer is a propeller or revolving vane instrument which' is connected through a gear train to a set of recording dials. The dials indicate the linear feet of air passing the instrument in a unit length of time (feet per minute). 5. Duct Fire Dampers. Fire dampen are used as safety devices to shut off the airflow in supply and exhaust ducts in case of fire. To automatically shut off the air, ducts may be equipped with dampers and fuse links. These should be provided when recommended by the National Board of Fire Underwriters. 6. Now that we have the air flowing through the duct, we must furnish some sort of outlet for it. This will be the subject for our next discussion. 30. Air Outlets 1. Air supply outlets are either of the wall or ceiling types. A number of different kinds of each have been developed. The type and kind required will depend upon the air distribution system you are using and the physical layout of the room or building. 2. Wall Outlets. Wall outlets are classified according to the type of openings. They are as follows: (1) perforated grilles, (2) vaned grilles, (3) registers, (4) slotted outlets, (5) ejector nozzles, and (6) wall diffusers. 3. Perforated grilles. Perforated grilles have a small vane ratio and are not adjustable. They are generally used where .the direction of the airflow is not controlled. Perforated grilles may also be used as return grilles. 4. Vaned grilles. Vaned grilles are either of the fixed or adjustable types. They may be used for wall, floor, and baseboard applications. The fixed type is used where the direction of the airflow

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is not controlled, while the adjustable type is used where the proper control of the air is essential. Vaned grilles are widely used, since they may be installed either vertically or horizontally. This permits a wide selection of grilles to meet particular requirements. 5. Registers. Perforated grilles designed with dampers or an arrangement of adjustable louvers are called registers. These units have a poor air outlet distribution and, for this reason, have only a limited use. 6. Slotted outlets. These outlets may be procured in a number of different designs, perforated, slotted, or a combination of both. They are used frequently in a long narrow room with a low ceiling. However, the air quantity and distribution must be carefully planned; changes after installation are difficult. The blow for these outlets is less than for other types, therefore they can be used where obstructions might prevent proper distribution by other grilles. 7. Ejector nozzles. Ejector nozzles consist of a pressure reduction box, sound reduction box, and diffuser. Ejector nozzles give a long blow. They are used in places where cooking, drying, freezing, etc., are in process. Because of noise limitations, they are not used where comfort is the primary objective. 8. Wall diffusers. The design of wall diffusers is similar to ceiling outlet diffusers, which we will discuss next. 9. Ceiling Outlets. Ceiling outlets most commonly used are of three types. These are as follows: (1) plaques, (2) ceiling diffusers, and (3) perforated ceilings and panels. 10. Plaques. Plaque outlets are of simple design. The plaque is a flat surface, such as a thin piece of board or metal, constructed an inch or two below the opening. The air hits the board or plate and flows through the opening between the plate and the ceiling and outward into the room. The plaque outlet is not widely used because the flow of air is difficult to control. 11. Ceiling diffusers. Ceiling diffusers are either round or rectangular shaped and are installed flush with the ceiling, or parallel and below the ceiling. Performance of the different types varies according to the principle used. Some have no internal induction, but hasten external induction by supplying air in multiple layers. Others have internal induction and distribute air over an entire half sphere. 12. Perforated ceiling and panels. In these types of outlets, the air is diffused through perforations. These panels are neat in appearance and maintain a low rate of air movement. 13. We'll discuss the location of these outlets and grilles in the next section.

31. Location of Supply and Return Grilles 1. A room having air supply grilles without return grilles must have some type of opening into a corridor or adjoining room. This opening is required so that air can leave the conditioned room. If 3000 c.f.m. of air is continually supplied to a room, 3000 c.f.m. must somehow leave the room. Some of the air passes out through cracks or around the windows or doors if the room has no return air register. This leakage, however, is normally not sufficient; and a relief opening is needed to insure that the air has a free exit path. The relief opening acts in much the same way as any opening in a recirculation air duct, except that no fan is moving the air through it. 2. The term "envelope" is defined as the outer boundary of an airstream. The envelope of a supply grille is a sharp beam similar to a beam from a searchlight. Some air from the stream discharged by the grille leaves the envelope and mixes with the adjacent room air, causing eddy currents and air motion in the air next to the envelope. The location of the return outlets may affect the pattern of the supply-air envelope in the plan and elevation pattern. 3. Supply-air envelopes, as they appear in a plan view, are determined by the type of grille selected. Different manufacturers offer grilles having different plan view standard envelope patterns. The plan view envelope is ordinarily called the deflection of the grille. Standard deflections are available from manufacturers. Grilles are also available with vertical vanes or bars adjustable either individually or in groups of five or six vanes. With such grilles, adjustments may be made in the field after installation to obtain any deflection desired. Fixed deflection grilles cannot be tampered with and consequently lack the flexibility of the adjustable type. Figure 82 illustrates an elevation (side view) of a room having a high air-supply outlet and a low return opening. This arrangement insures that the conditioned air will be blown across the room above the breathing level, will drop to the floor at the opposite wall, and will be slowly drawn across the room at the breathing level to the return register. Figure 83 illustrates an incorrect location for a return outlet. It shows that the air in half the room would have little motion. If occupants were near the return grille, they would be covered with a blanket of cold air. The cold air would not have an opportunity to mix with the warm room air. 4. Sometimes fresh air must be supplied from outlets located in the ceiling. Figure 84 illustrates an arrangement where the air supply grille is located in the middle of the ceiling with a low return opening located in the wall. This layout is satisfactory because the air will he blown

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Figure 82. High supply outlet and low return. across the room above the breathing level of the occupant. 5. Figure 85 shows a common method of supplying and returning air from one grille. This arrangement with the air supply grille and return opening built into' the same supply duct is unusual and not often used. It is interesting, however, to note the airflow pattern. The apparent "dead spot" near the floor is of little importance. 32. Ventilating Equipment Components and Installation 1. When installing a ventilating system, you must consider several factors. First of all, for comfort, a certain number of air changes an hour are needed. This number varies according to the temperature and humidity of the region as well as the purpose for which the building or room is intended. For instance, in setting up a ventilating system for a large messhall, you must remember that the ventilating problems for the kitchen will differ from those for the main dining room, even though the two are part of the same building. Because of the purpose of the room, removing heat, moisture, and odors are your main concern in the kitchen, while in the dining room your biggest item is supplying the proper amount of air for the number of occupants. Moreover, in planning you must consider the temperature extremes Figure 84. Ceiling supply outlet and return at floor level. and the humidity of the region. For example, generally speaking you can set up fairly comfortable surroundings by supplying an air change of 8 to 10 times an hour. However, during hot weather, 20 to 30 complete air changes are desirable in northern climates; and as many as 60 may be needed in southern regions. 2. Obviously you cannot rigidly follow a set pattern, since each installation will carry its own particular problems. However, each manufacturer has certain specifications for the installation of his particular ventilating equipment. These specifications may vary from those of other manufacturers. Also, the installation of identical equipment may vary due to location, source of power, local installation procedures, codes, regulations, etc. 3. When installing a ventilating system, you should consult text books, ventilation guides and manuals, and manufacturer's data and catalogs. These will be especially helpful when you are determining the quantity of air that must be removed to carry away gases, fumes, dirt, heat, vapors, and other undesirable foreign matter. Re-

Figure 83. Incorrect installation of high supply outlet and low return.

Figure 85. Supplying and returning air with one fixture.

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member, also, that you must comply with the safety practices of the National Safety Council and the Fire Underwriter's Board. 4. Removal of Heat, Odors, and Gases. The most common method used to remove heat, odors, and gases is by operation of propeller fans. The installation of fans in the attic will draw outside air into the building through the windows, doors, and other openings and discharge the contaminated air through the attic to the atmosphere. Satisfactory ventilating effects are usually obtained by providing approximately 60 air changes per hour. The accepted procedure is to install one or two large capacity fans instead of many small ones. Where a single fan is used, it should be installed as near to the center of the room as possible. In cases where many fans are used, the total volume of air to be moved should be divided among the fans in relation to their capacity. For instance, if four exhaust fans are required to ventilate a rectangular building, they should be installed in a manner to permit each fan to ventilate one-fourth of the building. 5. The following paragraphs discuss various standards used for different applications. 6. Attics. Vertical and horizontal fans are frequently used to ventilate attics. These units draw outside air into the building through windows, doors, and other openings and discharge it through the attic to the outside. 7. Dishwashing spaces. Dishwashing spaces, when constructed as a separate room, should be provided with an exhaust ventilation capable of producing 90 air changes per hour. The capacity of the fan should be based on the floor space measured to the bottom of the hood. If the dishwashing machine without a hood is installed in a separate room, not less than 60 air changes per hour should be provided for the entire room. 8. Kitchens. The ventilating equipment installed in large kitchens should be capable of supplying 20 air changes per hour. Horizontal exhaust fans are generally used in kitchens and are normally placed at ceiling level. Exhaust openings at the outside of the wall should be provided with louvers to keep out the weather. A bird screen (an ordinary window screen) should be placed between the fan and louvers to keep out birds and insects. 9. Kitchen ranges or deep fryers should be equipped with ventilating hoods. The effectiveness of these hoods depends upon exhausting large quantities of air in order to remove the vapors. These hoods should extend approximately 1 foot beyond the outside edges of the equipment they serve. The bottom tip of the hood is usually 6h feet to 7 feet above the floor. Hoods and exhaust ducts are usually constructed of galvanized sheet

or cement asbestos board. A double canopy hood is constructed with an inner shell which forms an air passage between the shell and the hood. The air inlet extends completely around the perimeter of the hood and has a 2- to 3-inch space between the bottom tip of the hood and the inner shell. Inner shells have openings at the top for exhaust air from the center of the hood area. The fan selected for a single canopy hood should have a capacity of 200 c.f.m. per linear foot of hood perimeter, while the fan for a double canopy hood should have a capacity of 150 c.f.m. per linear foot. 10. Round ducts are normally used with hoods. They extend from the hood, through the ceiling and roof, and terminate with a weatherproof cap. The vertical propeller fan is normally used in this application. 11. Fan guards must be installed on all fans to protect personnel from accidental contact. Kitchen exhaust hoods and ducts should be installed at least 18 inches away from the stove. Most kitchen range hoods have grease filters. These filters should be cleaned weekly to reduce fire hazard. Fans and duct work should have a sufficient number of access doors to permit easy cleaning. 12. Dining area. The ventilating equipment installed in messhalls should be capable of 10 air changes per hour. The fans generally used are of the propeller type. They are installed in the wall at ceiling level. The exhaust openings should be equipped with bird screens. Either wood or metal louvers should be installed over the openings to keep out the weather. Automatic louvers are preferable. 13. Laundries. The ventilating equipment installed in laundries should be capable of 30 air changes per hour. 14. Barracks. The ventilating equipment installed in barracks should be capable of supplying fresh air at a rate of 15 c.f.m. for each occupant. 15. Offices. The ventilating equipment installed in offices and other similar spaces should be. capable of delivering 10 c.f.m. per person. 16. Theaters and chapels. In windowless or crowded enclosures (theaters and chapels) 10 c.f.m. per occupant is required. Another method of calculating this requirement is to provide 2 c.f.m. for each square foot of floor area. 17. Removal of Hazardous Fumes and Vapors. The removal of hazardous fumes and vapors from buildings or spaces is accomplished by the use of special explosion proof or spark proof ventilating fans and motors. Fans and motors of this type are entirely enclosed so that any electrical sparks from either unit cannot cause an explosion. The maximum conveying velocities for hazardous fumes and vapors are approximately 2000 f.p.m.

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18. When installing a system for removing hazardous fumes and gases you must take care to locate the exhaust fans so that the fumes and vapors are positively removed and do not create a dangerous situation by remaining in the building. Some hazardous fumes and vapors are lighter than air, while others are heavier. Consequently, the specific gravity of each hazardous gas determines the location of the fan. Following are a number of gases and their specific gravity: Type Gas Formula Specific Gravity Acetylene C2H2 0.90 Ammonia NH3 0.59 Butane C4H10 2.01 Carbon dioxide CO2 1.527 Carbon monoxide CO 0.967 Chlorine C12 2.49 Hydrochloric acid HC1 1.26 Nitric oxide NO 0939 Sulphur dioxide SO2 2.263 19. If the specific gravity of the gas is less than one, the gas is lighter than air and will rise to the ceiling. If the specific gravity is more than one, the gas is heavier than air and sinks to the floor. The exhaust fan should always be placed so that it removes the air as fast as the vapor is released. Some of the buildings and rooms in which hazardous fumes and vapors are generated are garages, paint shops, refrigeration shops, battery shops, etc. 20. Garages. In garages where toxic and explosive gases cannot be ventilated by gravity, forced ventilation must be used. The system should be capable of supplying or exhausting a minimum of 1 c.f.m./square feet of floor space. Carbon monoxide produced from the incomplete burning of gasoline is lighter than air. For this reason, the exhaust fans used to remove this gas must be installed at least 7 feet above the floor. Gasoline vapors must also be removed. Since these vapors are heavier than air, they must be exhausted at floor level. 21. Paint shops. Paint shops should be provided with both supply and exhaust ventilation that are capable of producing 20 air changes per hour. Horizontal propeller fans are generally used and installed in the wall at a height of approximately 7 feet. 22. In paint shops and paint spray booths the electrical circuits which operate the exhaust fan and the air compressor should be interconnected so that the compressor can run only when the exhaust fan is operating. Such a safety measure lessens the probability of personnel being overcome by paint fumes. It is also desirable for the ventilating system to operate for a short time after painting operations have ceased.

23. When installing an exhaust system for a paint shop or paint booth, some means must be provided to filter the particles of paint out of the air as it is forced through the exhaust grille and out of the building. 24. To filter out paint particles from the exhaust air, various types of filters and louvers may be installed at the grille. If the paint shop is located in an isolated area, less rigid precautions need be taken. 25. Removal of Foreign Particles. Many shops need an exhaust system with & collector to gather up and hold material that might clutter up the area. With this system, the airflow must be sufficient to catch the dust, chips, metal filings, etc., as they are produced. Air ducts or exhaust pipes carry these materials through the exhauster to the collector. If the collector is not used, the system must be designed so that the exhaust will not contaminate the fresh air which is reentering the building. 26. A local exhaust system consists essentially of four parts: (1) hoods or partial enclosures, (2) air ducts, (3) a collector, and (4) an exhauster. Data pertaining to the quantity of air that must be removed in order to control hazards or nuisances is available, as well as information on the necessary entrance velocities to hoods having different sizes and shapes. 27. The size of air ducts depends on the amount of air to be moved and on minimum and maximum air velocities. The minimum velocity must be strong enough to keep particles from settling in the ducts, while the maximum velocity is limited by noise. If quiet operation is necessary, the velocity should not exceed 1200 f.p.m. In carpentry or machine shops the system must pull air over the article being worked on and must carry off the dust or chips. The air velocity for this application depends on the weight and size of the dust particles or chips. Fine, dry dust requires a velocity of approximately 3000 f.p.m., while larger particles, heavy loads, and moist materials require air velocities up to 6000 f.p.m. 28. Fans are the most common type of exhauster. Propeller type fans prove satisfactory in low velocity systems; but the centrifugal type fan is necessary for a high resistance, high velocity system. 29. Vertical Discharge Ventilating System. A vertical discharge ventilating system, shown in figure 86, is designed with a vertical discharge propeller fan. The complete system is mounted on the ceiling joists in the attic of the building to be ventilated. With this arrangement, air is drawn into the building through the doors, windows, and other openings, then circulated through the building and finally exhausted through the fan into the attic. From the attic the air is forced out into the atmosphere through a ridge ventilator. Instead of ridge ventilators, gables, roof monitors, cupolas, and other similar openings may be used.

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Figure 86. Vertical discharge ventilating tan. Doors between the rooms on this system must remain open or be louvered so that the air can circulate throughout the building. By looking at figure 86, you will notice that a bird screen is installed at the, exhaust opening of the ridge ventilator and another screen is installed over the air intake opening in the ceiling. 30. This ventilating system is also equipped with a batten door which, when closed, stops airflow through the attic. The construction of the fan crate is also shown in this installation. It is designed with a fan shroud, which makes the operation of the fan more efficient. 111 31. Sleeve bearing fans and motors without thrust bearings should not be used in conjunction with vertic2a discharge ventilating systems. Fans and motors used in vertical discharge systems must be equipped with thrust type ball bearings. 32. Vertical Discharge Ventilating Supply System. Buildings or rooms with high internal heat loads, such as auditoriums, classrooms, and laundries, frequently use a vertical discharge ventilating supply system. In this system, large propeller type supply fans mounted from the ceiling blow the air down over the occupants. Air should be drawn in from the outside through louvered

Figure 87. Horizontal discharge fan. monitors to the fan. Then, after passing over the occupants, the air can be vented to the outside through windows and doors. 33. Horizontal Discharge Ventilating System. Horizontal discharge ventilating systems are usually installed in the outside wall of a building or in a roof monitor, as shown in figure 87. Louvers which open and close automatically are generally installed in the outside wall. In a ventilating system of this type the air is drawn through the building in the same manner as with vertical discharge fans previously discussed. However, the air is not pushed through the attic openings as in the case of vertical discharge fans. Instead, it is pulled through by suction and then forced through the opening into the atmosphere. 34. Louvers. Louvers are generally used when installing horizontal discharge exhaust fans in roof monitors or roof gables. They can also be used to weatherproof horizontal exhaust openings. Fixed wooden louvers, unless properly made, may restrict air movement or give insufficient protection against bad weather. Wood louvers set in the frame at a 60° angle and spaced so that 2 inches of the opening is left between the crosspieces will keep out the rain. Figure 88 shows the major dimensions and capacities of louver panels constructed from l-inch wood stock. 35. For structural reasons, do not construct louver lengths to exceed 5 feet. If the capacity requires greater length, use multiple sections. 36. Automatic louvers for use with horizontal discharge ventilating systems can be procured from fan manufacturers. Various methods are used to open and close them. They may be actuated by an electric solenoid or a motor. When the fan is installed next to the louver panel, the louvers are actuated by air pressure. Air 112 pressure produced by the fan forces the louvers open, which are hinged at the top of the louver frame. When the fan stops, the force of gravity closes them. The installation of metal louvers is simple. They are attached to the exhaust opening by wood or sheet metal screws. 37. Fans. Fans installed in ceilings or roof must be installed without cutting any structural members of the building. Ceiling and roof construction must be strengthened to support the additional weight. Access doors to attics or fan enclosures must be provided -for inspection and maintenance purposes. You must use unpainted canvas or other flexible material to connect ducts to fan outlets or inlets. 38. Fan drive motors should be protected by either built-in or external thermal overload devices. A thermal switch may be installed in the inlet airstream of a fan for the purpose of stopping the fan in case of fire. These switches are usually set to open the circuit to the fan drive motor when the inlet air exceeds 135° F. 39. Filters. Air filters should be installed in all supply ventilating systems where dust or other foreign matter may be harmful to the activities conducted in the ventilated space. Special conditions may require the use of absorbents or electrical precipitators. Filters or dust collectors may be required on exhaust systems in special uses where the fan discharge would create an objectionable condition in the immediate vicinity or area. 40. Ducts. Dusts used for ventilation are usually constructed of galvanized sheet steel. Their construction should be as smooth as possible where the air passes over the inner surfaces. Ducts should be airtight and rigidly attached to reduce vibration. 41. When installing ducts for ventilating purposes, follow these suggested rules:

Figure 88. Wood louver details. a. Use smooth duct materials to decrease air friction. b. Avoid sharp turns. c. Pipe the air as directly as possible to the required location. d. When a duct must be altered to go through an opening or between structural members of a building, make the change as slight as possible. 113 33. Inspection and Maintenance of Mechanical Ventilating Equipment 1. The primary reason for inspections and maintenance is to let us determine the operating

condition of an item of equipment and :to correct any discrepancy which may be found. -These services should be performed on equipment at periodic intervals according to an inspection and maintenance schedule. You must maintain inspection and maintenance records for all of the ventilating equipment. Inspections and maintenance services should be scheduled to permit only a minimum of interference with the using organization. 2. Major repairs of ventilating equipment should be done during the winter season or when the equipment is not in use. Regular inspections show us when certain items of equipment are in need of major repairs. 3. Inspections and Maintenance Services. Definite procedures for inspection and maintenance services of ventilating systems are necessary if we are to have efficient and safe operation. The following paragraphs discuss these services in general. The instructions given here should serve as a guide for inspecting and maintaining all mechanical ventilating equipment. It may be necessary to supplement these procedures with the manufacturer's instructions, since the equipment is not standardized in design and may require slightly different maintenance. Whenever warranted, tile frequencies of inspections and maintenance services should be adjusted to meet local operating conditions. 4. Fan Assembly. Fans should be inspected periodically for proper operation, lubrication, and cleanliness. Fan blades must be properly aligned and free to rotate within their housings. The pulleys on the fan shaft and the motor shaft can be checked for alignment by using a straightedge. Loose pulleys must be tightened. 5. You must replace all defective fan blades. Dirty fan blades will cause vibration; therefore they must be cleaned. Fan blades may be cleaned by using a suitable cleaning solvent or detergent and clean rags. After you do the cleaning, you must tighten all bolts, nuts, and screws on the fan assembly. 6. The axial clearance of each centrifugal fan must be checked to insure that the fan wheel is not binding in the scroll. The axial clearance may be adjusted by relocating the position of the shaft thrust collar. After final adjustment, the total axial motion should be approximately 1/32 of an inch. Also after final adjustment, lock the thrust collar in place with the thrust collar setscrew. Worn thrust washers must be replaced. 7. Inspect and lubricate fan drives in accordance with manufacturer's instructions. If the drive unit for the fan has a direct flexible connection, inspect the couplings periodically for wear and alignment. 8. The inspection and lubrication of fan bearings, especially of continuously operated fans, should be

performed at regular intervals. The shaft sleeve bearings of fans are lubricated with oil, while ball bearings are packed with grease. The fan grease cups require filling once each year. Over lubrication will cause oil to drip from the bearing, which will result in unsightly collections of oil and dirt. 9. Each fan should be disassembled and inspected for defects yearly. Clean the fan shaft bearings and check each bearing for wear. Replace any bearings which are unserviceable. Clean and paint the interior and exterior of the fan housing, the fan wheel, and similar items with rust resistive paint. Care should be taken when working on fan wheels, as they are statically and dynamically balanced. 10. Lubrication of motor sleeve bearings. Lubricate motor sleeve type bearings to the proper level, preferably when the motors are at a stand-still rather than when they are running. This procedure will prevent any false oil level indication. The oil level should be observed for a few seconds to determine that it is at the proper level. Use a good grade of oil to lubricate sleeve type bearings. 11. Cleaning ball bearings. Motors equipped with ball bearings and which operate at 1800 revolutions per minute and lower should be cleaned and lubricated yearly. Motors operating above 1800 r.p.m. should be disassembled every 6 months. The bearings should be cleaned with approved solvent and repacked with new grease. You must be sure that no dirt and grit enter the bearing chambers. If the motor is provided with selfsealed prelubricated ball bearings, the manufacturer's recommendations should be followed for cleaning and relubricating procedures. 12. Cleaning sleeve type bearings. Before cleaning the bearings, you must drain the oil from the bearing chambers. Then flush the bearing chamber with an approved solvent, allow sufficient time for the bearing to dry, and refill the chamber with clean oil. 13. Duct Maintenance. Inspect duct periodically for air leaks, cleanliness, and structural condition. Repair or replace the defective ducts or duct connections. Remove all accumulations of foreign matter on the interior of the ducts. If applicable, inspect sound absorbing and insulating material on the interior of the ducts to determine that the materials are insulated securely and adequately. Inspect the duct hangers and supports and repair or replace as necessary. 14. All ducts should be cleaned annually. Protective paints should be applied to the air ducts to protect them from corrosion. One of the more effective protective coatings is red lead paint. Apply three coats of paint. The first coat should be a rust resistive type such as red lead paint;

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the second coat should also be a rust resistive paint but tinted to the desired color. Finally a chlorinated rubberbase paint is used for the finish coat, particularly where the presence of highly corrosive gases would injure standard paints. 15. Hood Maintenance. Inspect all the hoods for the following conditions: broken or cracked surfaces, poor connections to exhaust ducts, and accumulations of material such as dust, dirt, or grease. Repair or replace any defective hoods. Remove all accumulations of foreign matter by washing the hoods with hot water, steam, or an approved solvent You must include some measurement of relative airflow through the hoods. Static pressure or hood suction measurements will prove useful during this check if data is available on air volumes and pressures at the time the exhaust system was installed. A marked reduction in air suction can be traced to one or more of the following conditions: (1) reduced performance of the exhaust fan due to belt slippage or an accumulation of material on the fan wheel or in the fan housing, (2) incorrect direction of rotation of the exhaust fan, (3) reduced airflow caused by defective exhaust piping, and (4) losses in suction due to additional exhaust points being added to the system. Clean and/or paint hoods as necessary. 16. Filter Maintenance. When filters become dirty and clogged, they increase the resistance to the passage of the airstream and thus reduce the efficiency of the system. Therefore, filters should be inspected and cleaned or replaced periodically. The frequency of inspection and cleaning will depend on the- type of system in which the filter is installed and on the type of filter. Usually, filters should be cleaned or renewed at least every 2 or 3 months. However, if the ventilating system is used moderately, the cleaning or renewal operation may be reduced to once during a full season. Under dusty conditions the filters may require cleaning or renewal weekly. 17. Viscous impingement filters. Viscous impingement filters, throw-away type, should be discarded when they become dirty. However, certain types of viscous impingement filters are designed to be cleaned and reused. Cleaning may be accomplished by using hot water, steam, or a cleaning solution that will remove the adhesive coating. After the filters are washed, they should be dried. To recoat the filters, dip them in an adhesive bath long enough to coat all of the surfaces. Then remove the filters and allow them to drain for approximately 10 to 12 hours. You should use the adhesive coating recommended by the manufacturer. 18. Special filter cleaning equipment, such as washers and oilers, may be used to recondition these

filters. Dirty filter are placed into a three-stage washing machine. The first operation removes loose dust and dirt by ordinary washing. Next, the filters are washed by a hot alkaline solution under pressure. After the filters are washed in the alkaline solution they are rinsed with clear water. The filters are now drained, dried, and immersed in a high-temperature filter adhesive bath. After the filters are removed from the adhesive bath, they are placed in a centrifuge where excessive adhesive liquids are removed. The reconditioned filters are then installed in a ventilating system or stored in special storage racks. 19. Grease filters. Grease filters are cleaned in hot soapy water, or by spraying them with hot water. Use a pressure of approximately 20 pounds per square inch and direct the spray at the outlet side of the filter. After the filter is washed, place it in a vertical position and allow it to drain. 20. Electrical precipitators. Electrical precipitators may be cleaned in place manually with a brush or automatically by washing the plates with hot water sprayed from fixed or moving nozzles. Precipitators which may be washed in place are provided with drains to carry away the waste water. Before any cleaning operation is performed, you must turn off the electricity. It is best to follow the manufacturer's instructions to determine the exact method of cleaning and the frequency of cleaning electrical precipitators. 21. V-Belts. Inspect V-belts for breaks, evidence of wear, and proper tension. A belt is tensioned properly, if it has a deflection of 1/2 inch midway between the pulleys. If a belt is too loose it will slip; while an excessively tight belt will cause increased loads and premature bearing wear. Multiple V-belts must have the same tension; otherwise the tighter belt will carry most of the load and wear out sooner. 22. The recommended procedure for V-belt replacement is (1) loosen the motor at its base and shift it closer to the fan, (2) place the belt on the motor pulley, (3) slip it over the fan pulley, (4) align the pulleys and belt, (5) adjust the belt tension, and (6) tighten the motor mounting bolts in the V-groove of pulleys. V-belts must fit; otherwise rapid wear, noise, and slipping will result. 23. Multiple V-belts are furnished in matched sets by manufacturers to insure uniformity of length and tension. If a V-belt in a multiple V-belt set needs to be replaced, be sure to replace the entire set, even when some of the old belts seem to be in good condition. 24. Louver Maintenance. Inspect louver assemblies periodically to determine that they are intact and that they control the airflow properly. Replace or repair any loose or defective louvers. Wooden louver assemblies should be painted ap-

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proximately once each year. Check the freedom of movement of automatic louvers and correct any deficiencies. Place winter enclosures in position at the beginning of the winter season and remove them at the beginning of the summer season. Review Exercises NOTE: The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the test. Do not submit your answers for grading. 1. When is it important that complete air distribution be accomplished? (Sec. 27, Par. 3)

7. What type of grille should you select for an air outlet located in the floor and the airflow must be controlled ? (Sec. 30, Pars. 2-7)

8. Which wall outlet is used for long narrow rooms? (Sec. 30, Par. 6)

9. What determines the pattern of the supply air envelopes? (Sec. 31, Par. 3)

10. List several factors that must be considered when preparing to install a ventilating system. (Sec. 32, Pars. 1-3)

2. What are two ways you could reduce excessive grille noise? (Sec. 27, Par. 13)

11. Where should you install a fan in a room that contains carbon dioxide? (Sec. 32, Pars. 18 and 19)

3. Which type of fan is normally used in a ventilating system with considerable duct work? (Sec. 28, Par. 3)

12. What safety measure could you add when installing an exhaust fan in a paint spray booth? (Sec. 32, Par. 22)

4. You are to select a fan for a room requiring 30 air changes per hour. The size of the room is 14 feet by 60 feet by 60 feet. What must the fan capacity be? (Sec. 28, Pars. 10 and 11)

13. What factors determine the desired air velocity in a ventilating exhaust system? (Sec. 32, Pars. 26-28)

14. What would result from poorly constructed fixed wooden louvers? (Sec. 32, Pars. 34) 5. If you needed to reduce the friction loss of an air duct, would you increase or decrease the size of the duct? (Sec. 29, Par. 3) 15. When is it necessary to install filters in the exhaust system? (Sec. 32, Par. 39) 6. Why are duct fire dampers used in mechanical ventilation systems? (Sec. 28, Par. 5)

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16. Dirt on the fan blades will have what effect on the fan operation ? (Sec. 33, Par. 5)

18. What determines the frequency of cleaning an air filter? (Sec. 33, Par. 16)

17. What will result from the overlubrication of fan motors and other ventilating equipment? (Sec. 33, Par. 8)

19. What will excessively tight fan belts cause? (Sec. 33, Par. 21)

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CHAPTER 9

Heat Pumps
AT THE conclusion of this chapter you will know what heat pumps are and will understand their operating principles. We will also discuss the different types of heat pumps, their sources and sinks, heat storage, and pump components. 34. Performance 1. The operating principle of the heat pump is that of the heat-power thermodynamic cycle governing the conversion of mechanical energy to heat. It is derived from the Second Law of Thermodynamics (Carnot's Principle). The law states that the efficiency of a thermodynamic engine is proportional to the amount of heat transferred from the source of heat to the condenser; and that heat passes only from a warmer to a colder body. However, Carnot's formula for figuring the coefficient of performance (COP) cannot be applied to an actual system. It is used only on an ideal compressor, with an ideal refrigerant. 2. "Coefficient of performance (COP) " is a term used to express the ratio of output to input. We use the term "coefficient" because in a refrigeration system the performance will be greater than 100 percent. A simple formula to express this is: COP = output input The actual coefficient of performance is the ratio of refrigeration effect to the actual work input. The work input is figured in brake horsepower of the compressor. Actual COP = refrigeration effect b.hp. X 2545 Using this formula on a refrigeration system, we must know the rated capacity of the compressor in B.t.u.'s (refrigeration effect) and the brake horsepower. The 2545 is a constant used to convert b.hp. into B.t.u.'s. For example, you have a compressor rated at 500,000 B.t.u.'s/hr. It is using R-12 in a 46° F. evaporator that has a 12° F. superheat. The compressor’s discharge pressure corresponds to 110° F. Under these conditions the brake horsepower is 42. You would figure the actual COP as follows: Actual COP = 500,000 = 500,000 = 4.67 to 1 42 X 2545 106,890 In this actual situation we have put in 106,890 B.t.u.'s and received 500,000 B.t.u.'s of refrigeration effect, a ratio of more than 4 1/2 to 1. 3. In figuring the coefficient of performance of a heat pump, we must include the heat equivalent of the compressor. Thus we use the following formula: Heat pump COP = refrigeration effect + work input work input = refrigeration effect + b.hp. X 2545 b.hp. x 2545 We shall use the same rating of 500,000 B.t.u.'s/hr. If all the other conditions are the same as in the previous problem, the b.hp. will be 42. We can find the ratio by the following method: Heat pump COP = 500,000 + 42 X 2545 42 X 2545 = 500,000 + 106,890 106,890 = 606,890 106,890 = 5.67 to 1 The input of 1 B.t.u. has given us a higher output on the heat pump cycle. 4. The heating coefficient of performance (COP) of an installed heat pump is the ratio of useful heating effect to the heat equivalent of the total energy required to operate the system. If total energy input of all auxiliaries such as fans and pumps is not included, it should be so stated. 5. Now we'll compare the economy of a heat pump to an electrical resistance heater. We will use a 50,000B.u/hr. electrical resistance heater in our example. 3410 B.t.u./hr. = 1kw.-hr. (Kw.-hr. = kilowatt-hour) 50,00 B.t.u./hr. = 14.6 kw.-hr. 3410 B.t.u./hr. 6. At 3 cents per kw.-hr., the heat load would cost 14.6 X .03 = $.438/hr. = $.438 X 24 or

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$10.50/day. The monthly electric bill would be $315.00. 7. Actually, the cost is much less than this, as the heat load of a house averages much less than 50,000 B.t.u./hr. At this point we will introduce a new term--'"degree day." Degree day is a unit based upon temperature difference and time. It is used in estimating fuel consumption and in specifying the nominal heating load of a building during winter months. For any 1 day, when the mean temperature is less than 65° F., there exists as many degree days as there are Fahrenheit degrees difference in temperature between the mean temperature for the day and 65° F. The average or mean temperature in our example is 35° F. over a 9-month period. We will assume that we have 10,000 degree days. The heat load for 50,000 B.t.u./hr./70° F. temperature difference house would be: 50,000 or 714 B.t.u./ 70 degree F./hr. The heat load for 24 hours would be: 714 X 24 = 17.136 B.t.u./degree day; 17,136 3410 = 5.02 kw.-hr./degree day. 5.02 X .03 X 10,000 = $1506/seasonal cost for heating by electrical resistance. 8. The heat pump reduces this cost considerably because it uses electricity only to drive the compressor. The refrigeration cycle permits the condenser to release three or four times as much heat as it takes in electrical energy to drive the compressor. This coefficient of performance means that 1 kw.-hr. of electrical energy driving the compressor can release not 3410 B.t.u./hr., but 3410 X 3 or 10,230 B.t.u./hr. 9. You can increase the efficiency further by using a warmer heat source than the outside air. Some heat sources you might use are well water, lake water, or earth. If you could find a well furnishing 60° F. water, the coefficient of performance would be 4 or 5 and this factor would bring the cost of operation close to that of the oil or gas heating system. 10. The term "performance factor" is similar to coefficient of performance but is used when referring to values based on an extended period of performance. The period of time covered would be given when using this term. If supplemental heat is involved, its effect should also be specified. 35. Types of Heat Pumps 1. Heat pumps a classified according to the type of heat source and sink, heating and cooling distribution fluids, thermodynamic cycle, building structure, and size and configuration. We will discuss the more common types (shown in fig. 89) in the following paragraphs. 2. Air-to-Air (Refrigerant Changeover). This is the more common type heat pump system.

Figure 89. Types of heat pumps. Figure 89,A, shows the refrigerant flow. You will notice that two expansion valves, two check valves, and a changeover valve are used to control the direction of refrigerant flow. The changeover valve (A), which receives a signal from a room thermostat, controls the function of the two heat exchanger coils.' The flow, when cooling is desired, is indicated with the plain arrow. Expansion valve C will act as the metering device for coil F and check valve D will allow the hot liquid refrigerant to pass into the receiver. When heating is desired (determined by the thermostat), the indoor coil F must take on the function of a condenser 119

and the outdoor coil G, the evaporator. Expansion valve B will act as the metering device and check valve E will allow flow to the receiver. 3. In small units, the expansion and check valves may be replaced with a capillary tube. A few installations have been made in which the forced convection indoor coil has been replaced by a radiant panel. 4. Air-to-Air (Air Changeover). The heat pump circuit shown in figure 89,B, is the air-to-air (air changeover) type. Changeover is accomplished with dampers which control the flow of air across the two heat exchanger coils. Figure 89,B, shows the system when heating is desired. The indoor air is passing through damper A, over coil I, and out damper E, while the outdoor air is passing through damper C, over coil J, and out damper G. During the cooling cycle, dampers A, C, E, and G are closed and dampers B, D, F, and H are open? This arrangement permits outdoor air to pass through damper B, over coil I, and out damper F. The indoor air will now pass through damper D, over coil J, and out damper H. 5. The dampers may be electrically or pneumatically operated. 6. Water-to-Air (Refrigerant Changeover). This heat pump is illustrated in figure 89,C. The water-to-air heat pump uses water as a heat source and sink, and uses air to transmit heat to or from the conditioned space. The operation is similar to the air-to-air type (refrigerant changeover). 7. During the cooling cycle, the refrigerant passes through the changeover valve A to heat exchanger G. Check valve D will permit flow to the receiver, and expansion valve C will meter the flow to the coil F. When heating is desired, the changeover valve A will divert the refrigerant flow to coil F. Check valve E will allow refrigerant to pass to the receiver, and expansion valve B will meter the flow of refrigerant to heat exchanger G. 8. The coefficient of performance for this type heat pump is higher than the air-to-air types. 9. Earth-to-Air (Refrigerant Changeover). Earthto-air heat pumps employ direct expansion of the refrigerant in an embedded coil, as illustrated in figure 89,D. They may also be of the indirect type which we've discussed under the water-to-air type. 10. The operation of this system is identical to the air-to-air (refrigerant changeover) type except that the outdoor coil is embedded in the ground. 11. Water-to-Water (Water Changeover). This type heat pump uses water for the heat source and sink for both heating and cooling operation. Changeover may be accomplished in the refrigerant circuit, but in many

installations it is more convenient to perform the changeover with valves, as illustrated in figure 89,E. 12. Valves A, B, C, and D are controlled by a room or space thermostat. When the thermostat senses that cooling is needed, valve (A) will allow the water to pass through the condenser and discharge it out valve D. The return water will flow through valve (B) to the chiller and back to the supply through valve (D). 13. During the heat cycle, the valves will be positioned to permit water to pass through valve A to the chiller and then discharge through valve C. The return water will flow through valve B to the condenser. From the condenser it will flow through valve D to the supply inlet of the coil. 14. An earth-to-air heat pump (not shown in fig. 89) may be like the earth-to-air type shown, except for the substitution of a refrigerant-water heat exchanger for the finned coil shown on the indoor side. It may also take a form similar to the water-to-water system when a secondary-fluid ground coil is used. 15. Some heat pumps which use earth as the heat source and sink are essentially of the water-to-air type. An antifreeze solution is pumped through a loop comprised of a pipe coil embedded in the earth and the chiller-condenser. 16. Other types of heat pumps, other than those listed, are possible. An example is one which uses solar energy as a source of heat. Its refrigerant circuit may resemble the water-to-air, air-to-air, or other types, depending on the form of solar collector and the means of heating and cooling distribution which is employed. 17. Another variation is the use of more than one heat source. Some heat pumps have utilized air as the primary heat source, but are changed over to extract heat from another source (water earth, etc.) during peak load periods. When solar energy is used, another source must be used during periods of insufficient solar radiation. 36. Heat Sources and Sinks 1. The more practical choice of heat source and sink for a particular application will be influenced primarily by geographic location, climatic conditions, initial cost, availability, and type of structure. A more detailed discussion of design and selection factors for each heat source and sink follows. 2. Air. Outdoor air offers a universal heat source and heat sink medium for the heat pump. Extendedsurface forced convection heat exchanger coils are employed to transfer the heat between the refrigerant and air. These surfaces are as much as twice the size of the indoor coil surface. The volume of outdoor air handled is also greater in about the same proportion. The temperature dif-

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Figure 90. Heat Pump component performance characteristics. ference, during heating, between the outdoor air and the refrigerant is approximately 10°-25° F. 3. Selection. The two factors that you must consider when selecting a heat pump are the variation in outdoor air temperature and the formation of frost. As the outdoor temperature decreases, the capacity of the heat pump (during heating operation). also decreases. Selecting a heat pump for a specific air temperature is more critical than for a fuel-fired system. Care must be exercised to size the equipment for as low a balance point as is practically possible for heating without having excessive and unnecessary cooling capacity during the summer periods. 4. The procedure for finding this balance point (outdoor temperature at which the capacity matches the heating requirements) will be discussed in the following paragraphs. 5. The performance characteristics of a heat pump system can be estimated by evaluating and individual components. Figure 90 illustrates the data that manufacturers make available with their heat pump. 6. The conditions of system balance can be established by the following procedure: a. Choose a combination of evaporator refrigerant temperature Tr and condensing temperature Tc. b. Determine the compressor refrigerating effect from performance curves similar to those shown in figure 90,A. c. Determine the compressor power input (Pc) in kilowatts, as illustrated in figure 90,B. d. Determine the condenser capacity from Qe = Qe + 3413 Pc - Qce where Qc = condenser capacity (B.t.u./hr.) Qc = compressor refrigeration effect (evaporator capacity)(B.t.u./hr.) Qce = heat loss from compressor to surrounding air (B.t.u./hr.) Pc = power input (kw)

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Figure 91. Heat pump system balance. e. Plot Qc obtained from step d on a chart similar to 90,C. f. Select other condensing temperatures in combination with the original evaporator temperature from step a and repeat steps b-e as necessary to determine the condenser capacity at which the system balances. Points A and B on figure 91 represent the results of these calculations. Two points will normally be sufficient to determine the balancing condenser capacity. g. Select other evaporator temperatures and repeat steps a-f. h. For each evaporator find the corresponding heat source temperature (Ta) from a chart similar to 90,D. 7. Once the conditions of system balance are known, it is relatively easy to establish the heating performance characteristics. The net heating effect may consist of condenser heat, or depending upon the system design, may also include heat losses from the compressor, motor, and refrigerant subcooler coil (if used). 8. For a heat pump which employs a constant temperature heat source, a few computations will generally establish the balancing conditions for Tr and Tc. A heat pump which uses a variable heat source such as air requires a wide range of Tr to establish balancing conditions. 9. Figure 92 shows the performance characteristics of a typical heat pump determined either from actual system tests or from an analytical procedure such as we've discussed. Heating and cooling loads for typical residence are also shown in figure 92. If the balance point is above the heating design temperature (Td), then supplemental heat will be required, as. shown by the shaded area in figure 92.

10. We've just. discussed one of the factors that you must consider when selecting a heat pump; now we'll discuss the other-frost formation. 11. When the surface temperature of an outdoor air coil is 32° F. or lower, frost will form. The accumulation of frost will tend to reduce heat transfer, which reduces the capacity of the system. Research has shown that with a nominal amount of frost deposit, the heat transfer capacity of the coil is not substantially affected. The nominal amount is 2.5 pounds/square feet of coil face surface. The number of defrosting operations required will be influenced by the (1) climate, (2) air-coil design, and (3) hours of operation. 12. Experience has shown that little or no defrosting is required with temperatures below 20° F. and below 60 percent relative humidity. However, under very humid conditions; when small suspended water droplets may be present in the air, the rate of defrost may be three times as great as you would predict, using psychrometric theory. 13. Coil construction. The air-source heat pump uses the extended or fin type coil. The external surface of the tube is known as the primary, and the fin surface is called the secondary. The primary surface consists of tubes which may be staggered, or placed in line with respect to the heat flow. The staggered arrangement is preferred because it obtains a higher heat transfer value. 14. A more important factor in the performance of extended surface coil is the bond between the tube and fin A firm contact between the tube

Figure 92. Heat pump operating characteristics.

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Figure 93. Condensing media flow. and fin will insure free heat transfer from the fin to the tube. 15. Most coils are constructed of aluminum fins and copper tubes, but copper fins on copper tubes are also used. Fin spacing varies from 8 to 14 per inch. The fin spacing will be determined by the (1) duty to be performed, (2) possibility of lint accumulation, and (3) consideration of frost accumulation. 16. Coil flow arrangements. In air-cooling coils the air usually flows at right angles to the tubes. In a onerow coil the direction of airflow would be at right angles to the tube, but in multiple-row coils the airflow may be circuited, as shown in figure 93. Most dry expansion coils use the counterflow circuit to secure the advantage of the highest possible mean temperature difference. Crossflow is also used, but is difficult to control because of the problem of equal parallel circuit loading. 17. Coil selection. The various factors you must consider when selection a coil are:

a. The duty requPb4 and de capacity needed to maintain balance with other system components. b. Temperature of entering air (D.B. and W.B.). c. Available cooling media and operating temperatures. d. Space and size limitations. e. C.f.m. limitations. f. Allowable frictional resistances in air circuit and cooling media piping system. g. Characteristics of individual coil designs. h. Installation requirements.-type of automatic control etc. i. Coil air face velocity. 18. Coil ratings are based on a uniform face velocity. Airflow interference, caused by air entrance at odd angles or by blocking a portion of the coil face, will affect performance. To maintain the rated performance of the coil, it is necessary that the air quantity (c.f.m.) be adjusted while the system is operated and kept at this value. 19. You'll find that the more common causes of airflow reduction are (1) dirty filters, (2) dirty coils, and (3) frost accumulation on the coil. You will avoid these difficulties if you implement a good preventive maintenance program. 20. In the selection of coils, sufficient surface area must be installed to transfer the total heat load from the air to the cooling media-refrigerant. This transfer must occur under the required temperature conditions and maximum flow rates of both air and refrigerant. The coil total heat capacity must be in balance with the capacity of related equipment, such as the compressor. Therefore, in making coil selections you will have to consult manufacturer's rating tables or the manufacturer's local representative. 21. Heat transfer and airflow resistance. The rate of heat transfer from the air to the refrigerant is affected by three resistances. These three resistances are: (1) From the air to the surface of the tube usually external surface or air-film resistance. (2) The resistance to the conduction of heat through the fin and tube metal. (3) The resistance to the flow of heat between the internal surface of the metal and the fluid in the tube. 22. The metal to heat conduction and the internal tube surface resistances are comparably low. The resistance that you would be more interested in is the external surface or air-film resistance. You may overcome this resistance by extending the coil surface by means of fins. 23. The transfer of sensible heat between the cooling medium and the airstream is influenced by

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(1) The temperature difference. (2) The design and surface arrangement of the coil. (3) The velocity and character of the airstream. (4) The velocity and character of the medium in the tubes. 24. Water. Water is considered to be an ideal heat source subject to the considerations listed below: (1) City water--availability, high operating expense, scale formation on coils, and low temperature during the winter season. (2) Well water-availability, original cost of drilling the well, composition of water (calcium, magnesium), and the life of the well (dry up). (3) Surface water-availability, and it may contain chlorides and micro-organisms (algae). (4) Waste water-availability temperature, and it is very difficult to mass produce this type heat pump. 25. City water is not a good heat source because of its nonavailability and its high operating cost. Well water is particularly attractive from the standpoint of its relatively high and nearly constant temperature (50° F. in northern areas and 60° F. or higher in the south). You can obtain information on well water availability, temperature, and chemical and physical analysis from local U.S. Geological Survey offices. These offices are located in most major cities. 26. Utilization of water during cooling operation follows the conventional practice with water-cooled condensers. Water-refrigerant heat exchangers generally take the form of either shell-and-coil or shell-and-tube type direct-expansion water coolers. These heat exchangers are circuited to permit usage of the shell-andcoil or shell-and-tube as a refrigerant condenser during the heating cycle and as a refrigerant evaporator during the cooling cycle. 27. Earth. Heat transfer through buried coils has not been used extensively because of high installation cost, ground area requirements, and the uncertainty of predicting performance. 28. Compositions of soil vary quite widely (wet clay to sand) and affect the thermal properties and overall performance. 29. Earth coils, usually arranged horizontally, are submerged 3 to 6 feet below the surface. A lower depth may be preferred but excavation cost requires a compromise. The mean ground temperature for a specific area generally follows the mean annual climatic temperature. 37. Heat Storage

1. The use of heat storage can improve the performance of a heat pump. Installations of heat pumps with heat storage have been made in large buildings. 2. We'll all have to agree that all materials possess the property of heat storage. The structural materials of a building are always in the process either of absorbing heat from or delivering heat to the interior space. This effect is more pronounced in cooling operation where greater air temperature variation is tolerated. Heat storage tends to reduce the rate of temperature change and helps in some measure to reduce the peak load requirements. 3. A heat pump, with heat storage capabilities, can serve not only to reduce the size of a heat pump necessary for a given load but also to provide a more desirable electric load. This may be done by shifting part of the load to the time of day when the cost of power is least. Power is the cheapest during off-peak time. The electric hot water heater is a common example of such a heat storage application. 4. There are two types of heat storage systems that have been employed: (1) sensible heat storage systems, and (2) latent heat storage systems. The latter is actually a combination of the two. Heat storage, in the heat pump system, may be utilized on the high side when heat is available at a temperature suitable for direct heating. It is used on the low side as an intermittent heat source at temperatures lower than the heated space. 38. Heat Pump Components 1. The components used in heat pumps and the practices followed bear a direct relationship to the air conditioner discussed earlier in this volume. In this section we will discuss the component peculiar to this system-the reversing or change-over valve. Our discussion will cover the operation and application of the valve. 2. Operation. The 4-way reversing valve is operated by a solenoid pilot 3-way valve which actuates the piston-operated main valve (reversing valve). The pilot valve may be a separate component or an internal part of the main valve. The pilot valve directs the actuating pressures-compressor discharge and suction-to the top of the main valve piston. Figures 94,A, and 94,B, illustrates a heat pump system, during both heating (B) and cooling (A) cycles, using a typical 4-way reversing valve. You can see that the main valve is externally operated by a solenoid pilot 3-way valve: There are also two thermostatic expansion valves and two check valves used in the system. 3. Now we'll cover the cooling cycle (fig. 94,A). The pilot valve is energized, thus allowing compressor suction pressure to the top of the main valve piston. This causes the main piston

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Figure 94. Heat pump system with 4-way reversing valve, solenoid pilot 3-way valve, two thermostatic expansion valves, and two check valves. 125

to rise. With the main valve piston in this position, the compressor discharge follows path D to C, while the suction gas (from the evaporator to the compressor) follows path E to S. 4. During the heating cycle, shown in figure 94,B, the pilot valve is deenergized. This allows the compressor discharge pressure to be admitted to the top of the main valve piston and to move the piston down. In this position the compressor discharge gas follows path D to E, and the evaporator return pressure from C to S. 5. We can summarize our discussion of the two cycles by stating that by energizing and deenergizing the pilot valve, the direction of refrigerant flow is reversed. We can also conclude that the main valve piston is held in position by the pressure drop across the closed valve poppets. 6. Application. We've already covered one application of the reversing valve; now we'll discuss two more. They are shown in figures 95 and 96. Figure 95 shows a system with one 4-way reversing valve, one solenoid pilot 3-way valve, one thermostatic expansion valve, and four check valves. The system shown in figure 96 uses a 3-way reversing valve, a 4-way reversing valve, a solenoid pilot 3-way valve, and two thermostatic expansion valves. This system allows refrigerant to flow in one direction in the heat exchanger coils, while the other systems allow flow in either direction. 7. It is imperative in the system shown in figure 96 to provide positive free draining of the liquid refrigerant into the top of the receiver from the bottom of the coil when the coil is used as a condenser. In addition, it may be necessary to allow for drainage of liquid refrigerant from the condenser before reversing the 4-way valve. If caution is not taken, the liquid refrigerant can enter the compressor and cause serious damage.

8. When a water cooled condenser is used in the system, it is necessary to add additional control devices to protect it against freezeup. Freezeup may occur during the heating cycle because the condenser is used as a heat source (evaporator). Two methods of protection are shown in figures 97 and 98. 9. Figure 97 shows a system using an evaporator pressure regulator (EPR), connected to the condenser. The EPR is used to prevent freezeup of the water flowing through the condenser during the heat cycle. When the system is returned to the cooling cycle, the EPR valve must be bypassed by a check valve which will allow the hot gas to flow to the condenser. A solenoid water valve must be used to bypass the condenser water regulating valve during the heat cycle. This will permit a full flow of water through the condenser. 10. A constant pressure liquid expansion valve is used in the system shown in figure 98. It feeds liquid refrigerant to the condenser when it is used as an evaporator during the heat cycle. The valve must be adjusted to prevent the suction pressure from falling below the pressure corresponding to the refrigerant saturation temperature 33° F. during the heat cycle. This valve must be bypassed with a check valve which will permit the condensed liquid refrigerant to flow to the receiver during the cooling cycle. In addition, the connection to the receiver, to which the valve is connected, must have a dip tube (quill) to insure an adequate supply of liquid refrigerant to the valve. A solenoid water valve must be- used to bypass the condenser water regulating valve when the condenser serves as an evaporator. This is done to insure complete evaporation of all of the refrigerant being fed by the constant pressure liquid expansion valve. Liquid refrigerant must never be allowed to return to the compressor.

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Figure 93. Heat pump system with 4-way reversing valve, solenoid pilot 3-way valve, thermostatic expansion valve, and four check valves. 127

Figure 96. Heat pump system with 3-way reversing valve, 4-way reversing valve, solenoid pilot 3-way valve, and two thermostatic expansion valves. 128

Figure 97. Reverse cycle for defrosting using a 4-way reversing valve and an evaporator pressure regulating valve. 129

Figure 98. Reverse cycle for defrosting using a 4-way reversing valve and a constant pressure liquid expansion valve. 130

Review Exercises NOTE: The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the test. Do not submit your answers for grading. 1. What is the COP of a refrigeration cycle when the refrigeration effect is 300,000 B.t.u.’s and the brake horsepower is 40? (Sec. 34, Par. 2)

7. What is the maximum temperature of the refrigerant during the heating cycle when the outside temperature is 50 F.? (Sec. 36, Par. 2)

8. Why is outdoor temperature an important factor in selecting a heat pump? (Sec. 36, Pars. 3-9)

9. Will 40 pounds of frost substantially affect a 5' x 4' coil? (Sec. 36, Par. 11)

2. How much would it cost to operate a 100,000 B.t.u./hr. electrical resistance hear for 1 day? The cost of electricity is 2 a kilowatt-hour. (Sec. 34, Pars. 5 and 6)

10. How many fins does a 4-foot coil contain? (Sec. 36, Par. 15)

11. What are the most common causes of airflow reduction? (Sec. 36, Par. 19) 3. How many degree days would you have if the average temperature for a 90-day period is 5° F.? (Sec. 34, Par. 7)

12. Why isn't city water considered a good heat source? (Sec. 36, Par. 24)

4. Why is a hat pump less expensive to operate than an electric resistance heater? (Sec. 34, Par. 8)

13. Which water source is considered the best heat source? Why? (Sec. 36, Par. 25)

14. Why is heat storage beneficial to a heat pump? (Sec. 37, Par. 2) 5. Which type of heating system is the cheapest to operate, the air-to-air heat pump or an oil fired heating system? (Sec. 34, Par. 9) 15. The heat pump is operating as an air conditioner. The room temperature falls below the thermostat setting, but the unit will not reverse its cycle. Which component has most likely malfunctioned? (Sec. 38, Par. 2)

6. How many expansion devices does an air-to-air (refrigerant changeover) heat pump have? (Sec. 35, Par. 2)

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16. What occurs when the pilot valve is energized? (Sec. 38, Par. 3)

18. How can you prevent freezeup of a water-cooled condenser during the heat cycle? (Sec. 38, Pars. 9 and 10)

17. An open in the pilot valve solenoid will cause the heat pump to operate on the cycle. (Sec. 38, Par. 4)

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Answers to Review Exercises
CHAPTER 1 1. The factor that determines the type of filter design you would use is the degree of cleanliness required for the conditioned area. (Sec. 1, Par. 6) 2. The filter arrangement used in a duct system having a velocity of 500 f.p.m. is with the filtering medium placed on edge. (Sec. 1, Par. 10) 3. When the pressure drop through a duct system is 2 p.s.i.g. the filters are dirty. (Sec. 1, Par. 13) 4. The traveling media filter requires the least amount of attention because the media roll usually lasts 3 months. (Sec. 1, Par. 17) 5. The type of filter you should install is a moving curtain filter because it is considered to be fail safe. When the media runs out, an indication it given and the circuit to the filter motor opens. (Sec. 1, Par. 19) 6. The surface area of a dry filter may be increased by pleating the filtering medium. (Sec. 1, Par. 22) 7. The initial resistance of the filter is higher than the resistance at which the fan will operate. This condition will cause the motor to overheat. (Sec. 1, Par. 26) 8. An ionizing filter handling 3800 c.f.m. of air will consume 97 watts. (Sec. 1, Par. 31) 9. The cost of filter operation for 1 hour is $.17 97 X 60 = 5820 watts 5820 watts = 5.82 kilowatts 5.82 X $ .03 = $.175 (Sec. 1, Par. 31 and Question 8) 10. A dry-bulb temperature of 50° F. and a dewpoint temperature of 50° F. is 100 percent relative humidity. This high humidity will impair the dielectric properties of the filter. (Sec. 1, Par. 36) 11. The most probable cause of an odor in an airconditioning system is a wet, dirty cooling coil. (Sec. 2, Par. 2) 12. Air at 70° F. and 100 percent humidity is saturated. (Sec. 3, Par. 4) 13. 2000 c.f.m. can be handled effectively by a 5-ton cooling coil. (Sec. 3, Par. 7) 14. The quality of a liquid absorbent is controlled by the automatic regulation of cooling waterflow through a cooling coil in the absorbent sump. (Sec. 3, Par. 12) 15. The air temperature should be within 1° to 5° of the absorbent temperature. To lower the temperature differential, more contact surface should be added or the absorbent temperature should be lowered. (Sec. 3, Par. 14) 16. The adsorption efficiency of a dynamic dehumidifier with an entering moisture content of 25 grains and an adsorbed moisture content of 20 grains is 80 percent, or 20 = 4 = .8 = 80 25 5 percent. (Sec. 3, Par. 21) 17. The economy of desorption is 1200 watts and the cost it $ .30. 400 X 3 = 1200 watts 1200 watts = 12 kilowatts 12 X .025 = .30 (Sec. 3, Par. 22) To evaporate 9 pounds of water, 9450 B.t.u.'s must be added to the water. 1050 X 9 = 9450 B.t.u.'s. (Sec. 3, Par. 28) Adding moisture to the air with an atomizer humidifier will not affect the wet-bulb temperature. (Sec. 3, Par. 28) A humidistat can be used to control a valve in the compressed air line. As more air is allowed to pass through the line, more moisture will escape into the air. (Sec. 3, Par. 32). The maximum efficiency of the impact humidifier as compared to the atomizer type is 50 percent. The atomizer uses 100 percent of the water supplied to it, while the impact uses 20 to 50 percent. (Sec. 3, Par. 37) The rate of airflow is important because more evaporation will occur in a given period of time with an increase in c.f.m. (Sec. 3, Par. 38) 100 B.t.u.'s added to a forced-evaporation humidifier per hour with an airflow rate of 20 pounds of dry air per hour will add 5 B.t.u.'s to each pound of dry air. (Sec. 3, Par. 40) To correct this condition-water droplets leaving the washer-you could install a bypass duct and allow a velocity of 500 f.p.m. to pass through the washer. (Sec. 3, Par. 44) The pressure has increased because the eliminator plates have become plugged. This condition can be prevented by installing flooding nozzles in the air washer. (Sec. 3, Par. 44) CHAPTER 2 1. On a centigrade the thermometer 15.5° is equivalent to 60° on a Fahrenheit thermometer: C = 5 (60 - 32);C = 5 x 28 ; C = 140; 9 9 1 9 C = 15.55° (Sec. 4, Par. 4) 2. On a Fahrenheit thermometer 104° is equivalent to 40° on a centigrade thermometer. F= 9 X 40 + 32; F = 360 5 5 + 32; F = 72 + 32; F = 104°. (Sec. 4, Par. 4) 3. It would require 3.8 B.t.u.’s to raise the temperature of 8 pounds of cast iron 4°. (B.t.u. = 0.119 X 8 X 4. B.t.u. = 0.119 X 32, B.t.u. = 3.8) (Sec. 4, Par. 6) 4. The term applied to the sum of sensible heat and latent heat is "total heat" (Sec. 4 Par. 8)

18. 19. 20.

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5. The dry-bulb thermometer will always indicate a higher temperature than the wet-bulb thermometer except when the air is saturated; then they will both indicate the same. (Sec. 4, Pars. 10 and 11) 6. After whirling a sling psychrometer with the wet-bulb thermometer wick dry, the psychrometer thermometers would read the same. (Sec. 4, Par. 11; Sec. 5, Par. 3) 7. The difference in the dry-bulb thermometer reading and wet-bulb thermometer reading will become greater as the relative humidity decreased. (Sec. 4, Par. 11; and Sec. 5, Par. 3) 8. In order to determine the relative humidity, the dry-bulb and wet-bulb temperatures must be known. (Sec. 5, Par. 3) 9. Distilled water should be used to set the wick of a wetbulb thermometer to help prevent the clogging of the wick. (Sec. 5, Par. 6) 10. If the total pressure of an air-conditioning system remains constant and the air ducts become partially clogged, the static pressure will increase to over-come the added resistance and the velocity pressure will decrease. (Sec. 6, Pars. 8-11) 11. If the total airflow pressure is equal to 20 inches of water and the static pressure is equal to 4 inches of water, the velocity pressure is equal to the total pressure minus the static pressure or 16 inches of water. (Sec. 6, Par. 11) 12. Yes, it is possible to determine static pressure with a velometer. The velometer indicates the velocity pressure which you would subtract from the total pressure to get the static pressure. (Sec. 6, Pars. 11 and 19) CHAPTER 3 1. In calculating the wall area you must subtract 16” from the length to find the inside wall area. The ceiling sits on top of the wall so that the height measurement is not affected. The wall area is 126.67 square feet. 10' = 120" height 14'= 168" length 168"- 16"= 152" 120" X 152" = 18240 square inches 18240 ÷ 144 = 126.67 square feet (Sec. 7, Par. 2) 2. You should tell the user to draw the drapes to help eliminate solar heat gain and to start the unit earlier so that it wouldn't work against a peak load condition. (Sec. 7, Pars. 5-8) 3. The heat load from occupants will affect humidity the most. (Sec. 7, Par. 11) 4. To remove the heat which is causing abnormal unit operation, you should ventilate the area. (Sec. 8, Pars. 4 and 6) 5. The efficiency that would be lost is 85 - 75 or 10 percent. (Sec. 8. Par. 9) 6. Cork should be used to insulate a 40° F. storage room, because this particular application is not considered a fire hazard area. (Sec. 9, Par. 5) 7. You should insulate the strainer with an asbestos pad or blanket to facilitate the cleaning of the strainer. (Sec. 9, Par. 8) 8. You should use fibrous glass dabs, because they have a low moisture-absorbing quality and offer no attraction to insects, vermin, fungus growth, or fire. (Sec. 9, Par. 14)

9. The most probable cause of a 55° F. temperature reduction is moisture in the insulation around the pipe. The evaporation of the moisture will cause a heat loss. (Sec. 9, Par. 18) 10. When you insulate a valve in a 2-inch pipeline the insulation should be the same thickness as the pipe. The insulation usually consists entirely of insulating cement. (Sec. 9, Par. 20) 11. The solar radiation through a 20' x 40' brick Wall with a 30° F. differential is Q = UA (t1 – T0) Q = .34 x 800 (30) Q = 272 x 30 Q = 8160 B.t.u./hr. (Sec. 10, Pars. 10 and 13) 12. The gross area of the wall is 120 square feet. The window area is 16 square feet. Glass = 1.13 X 16 X 22 = 397.76 B.t.u./hr. Brick = 120 - 16 = 104 sq. ft. 104 X .34 X 22 = 777.48 B.t.u./hr. Total heat gain = 397.76 + 777.48 = 1175.24 B.t.u./hr. (Sec. 10, Par. 13) 13. Human load will give off the most latent heat gain. (Sec. 10, Par. 13) 14. To find the total cooling load, you must add 10 percent to the sensible load. Total load = 42,156 + 4,215.6 + 8,750 = 55,121.6 B.t.u. (Sec. 10, Par. 14) 15. 57,150 B.t.u. (sensible) 5,715 B.t.u. (safety factor) 9,170 B.t.u. (latent) 72,035 72,035 total heat load. 12,000 B.t.u. per ton of refrigeration. 72,035 12,000 = approximately 6 tons. (Sec. 10, Par. 14) CHAPTER 4 1. Before you plug in an air-conditioning unit you should read the nameplate to check the power requirements of the air conditioner. (Sec. 11 , Par. 5) 2. When the round third prong is removed from an airconditioning unit plug an ungrounded condition will exist. When the air-conditioning unit is not grounded, a possible electrical shock hazard also exists. (Sec. 11, Pars. 9, 10, and 16) 3. It is not permissible to connect a 9.5-ampere rated airconditioner to a 15-ampere circuit when other equipment are using the same circuit. The total load of the air conditioner shall not exceed 50 percent of the current rating of the circuit if the circuit feeds other equipment. (Sec. II, Par. 12) 4. If you are to replace an air-conditioner compressor motor that has burned out due to excessive overload, you should also replace the motor overload protector. If the overload protector was operating correctly, the motor would not have burned out from an overload. (Sec. 11, Pars. 24 and 25)

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5. As the room air passes through the evaporator its heat is absorbed by the refrigerant. (Sec. 11, Par. 28) 6. The refrigerant gas temperature is raised at the compressor above the outside air temperature. (Sec. 11, Par. 29) 7. The air filter, evaporator coils, and condenser coils will collect dirt and thus restrict airflow which will result in reduced air-conditioning unit output. (Sec. 11 Par. 32) 8. Before you check a capacitor with an ohmmeter you should discharge the capacitor. (Sec. 11, Par. 44) 9. If the ohmmeter indicates zero (no continuity) when you check an overload protector. the protector is defective and should be replaced. (Sec. 11, Par. 46) 10. A low wattage draw is an indication of a low refrigerant charge. (Sec. 11, Par. 51) 11. The two major causes of poor performance of an air conditioner are dirty filters and low voltage. (Sec. 11, Par. 62) 12. When using superheated steam to clean a condenser, be sure that the temperature of the seam is not above the melting point of any of the materials from which the condenser is constructed. (Sec. 12, Par. 7) 13. When mixing water and acid, always add the acid to the water. If water is added to the acid, rapid heating will occur which will cause the acid to spew from the container. (Sec. 12, Par. 12) 14. If the water bleed tube of the evaporative condenser should become clogged, the formation of scale will increase. The bleeding off of some of the recirculated water and replenishing it with makeup water will decrease the amount of solids suspended in the cooling water. (Sec. 12, Pars. 18 and 19) 15. Two of the conditions that would prevent the compressor from unloading are; a broken spring in the hydraulic cylinder which moves the floating piston when the oil pressure is relieved or the oil pressure is not released from the valving mechanism hydraulic cylinder. There are several causes that would prevent the release of the oil pressure. Some of these causes are: defective pressure-sensing device, broken mechanism that opens the bleed orifice (item 9 in fig. 18), and a clogged bleed orifice (items 9 and 10 in fig. 18). (Sec. 12, Pars. 3139) 16. The pan of the capacity control actuator that regulates the oil pressure to the compressor cylinder unloader mechanisms is the valving mechanism. (Sec. 12, Par. 31) 17. Spring pressure in the cylinder unloader mechanism will hold the compressor suction valves open. (Sec. 12, Par. 39) 18. The compressor must be loaded before adjusting the unloader system. (Sec. 12, Par. 41) 19. Before you install a solenoid valve, check the valve data plate for the power requirements and the arrow on the valve body for direction of liquid flow thru the valve. (Sec. 12, Par. 49) 20. Before you install a new solenoid valve in place of a burned out one, you should find the cause for the burned out coil. You should check the voltage of the power source and the power requirements of the valve. Another possible cause could be high ambient temperatures. Sec. 12. Par. 50

21. The two methods of varying the volume of the air handled by an air conditioning system are by the use of dampen or by varying the speed of the fans. (Sec. 12, Par. 56) CHAPTER 5 1. Bypass dampeners are used to regulate airflow from return ducts. (Sec. 13, Par. 2) 2. One probable cause of erratic damper operation is binding blades. (Sec. 13, Par. 7) 3. The forward blade fan is most commonly used in a duct system. (Sec. 14, Par. 2) 4. The propeller, or disc, type fan should be installed in an area requiring large amounts of exhaust air. (Sec. 14, Par. 3) 5. The axial adjustment of the blower wheel is accomplished by relocating the shaft thrust collar. (Sec. 14, Par. 9) 6. Cooling coils are made of copper or aluminum because these metals readily conduct heat. (Sec. 15, Par. 1) 7. A 2-foot coil would contain 144 fins-24 X 6 = 144. (Sec. 15, Par. 2) 8. You would straighten the fins with a special fin comb. (Sec. 15, Par. 4) 9. Brine solution is used in a system that requires a low temperature for dehumidification purposes. (Sec. 16, Par. 1) 10. The type of pressure loss caused by an elbow in the duct is dynamic loss. (Sec. 17, Par. 2) 11. The velocity reduction method of duct sizing is not used because it does not take any account of the relative pressure losses in various branches. (Sec. 17, Par. 4) 12. A system with a velocity rating of 2400 f.p.m. is considered a high-velocity system. (Sec. 17, Par. 6) 13. Duct joints are sealed with compound, tape, or by welding or soldering. (Sec. 17, Par. 9) 14. The type of duct materials you would use when corrosive fumes are to be handled are copper, stain less steel, monel lead-coated or lead. (Sec. 17, Par. 12) 15. When air flows from a small chamber toward a large area, the air tends to flow in a straight line. (Sec. 17, Par. 16) 16. The loss of cooling effect of a 12-sq. ft. duct having a differential of 10° and a U-factor of 1.14 is 136.8 B.t.u/hr. Q = UA (t1 – t0) Q = 1.14 x 12 x 10. Q = 136 B.t.u./hr. (Sec. 17, Par. 18) 17. Most duct air leakage occurs at transverse seams located against a wall or ceiling. (Sec. 17, Par. 22) 18. The amount of air required when the sensible hit load is 49000 B.t.u./hr. and the temperature change is 15° F. is 3025 c. f. m. c.f.m. = 49000 1.08 X 15 c.f.m. = 49000 16.2 c.f.m. = 3025 (Sec. 17, Par. 25)

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19. You can check the vertical flow from a grille by using a lighted match, a warm thermometer, or alcohol on your arm. (Sec. 17, Par. 34) 20. The horizontal airflow pattern is controlled by the rear frets of the grille. (Sec. 17. Par. 34) 21. The baseboard type of diffuser is the hardest to use for balancing because it has few adjustments. (Sec. 17, Par. 38) CHAPTER 6 1. The thermostatic expansion valve uses the vapor-tension principle in its operation. (Sec. 18, Par. 6) 2. The control response a motor control uses is twoposition. (Sec. 19, Par. 2) 3. To set a LPC for a wider differential you would turn the adjusting screw so that more force is exerted upon the bar. (Sec. 19, Par. 5) 4. The compressor will cut off at 25 p.s.i. 4 0 p.s.i. - 15 p.s.i. = 25 p.s.i. (Sec. 19, Par. 9) 5. Since the system uses an automatic expansion valve, a low-pressure motor control cannot be used to control compressor cycling. You must install a thermostatic motor control and adjust it to the desired cutout and cutin temperatures. (Sec. 19, Par. 10) 6. A broken feeler bulb on a thermostat will give the sale indication as a filed ice bin. (Sec. 19, Pars. 12-15) 7. The air conditioner is not running because the kinked feeler bulb acts as if a loss .of power element charge and will not close the contacts in the TMC. To correct this condition, you must replace the power element or the entire TMC. (Sec. 19. Par. 20) 8. Snap action and mercury switches are used to prevent control failure due to arcing when the circuit is open or closed. (Sec. 20, Par. 2) 9. A direct short is indicated when the ohmmeter reads zero ohms resistance. (Sec. 20, Par. 7) 10. The mode of electric control you would use to operate a refrigeration unit is two-position because the unit requires on-off operation. (Sec. 20, Par. 13) 11. The control point would be at any point between the two extremes because the control cycles the louvers between the extremes and is never satisfied. (Sec. 20, Par. 18) 12. The timed two-position control responds to gradual changes in the controlled variable, while the simple twoposition control responds to one of two extremes. (Sec. 20, Par. 24) 13. A heater is used to slow down the action of the bimetal element. (Sec. 20, Pars. 31 and 32) 14. You should install a proportional response control because system offset is minimized. (Sec. 20, Pars. 3537) 15. Since lag time is not a problem, a simple two-position control, series 20, can be used. It is cheaper, easier to maintain and calibrate, and safer because it operates at low voltages. (Sec. 20, Par. 41) 16. The change in variable causes a bellows to expand and make a circuit to the starting winding of the motor. The

17. 18. 19. 20.

21. 22.

23.

24. 25.

26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

motor is energized and begins to rotate clockwise. After it has rotated 180°, a cam operated switch will break the circuit and stop the motor (Sec. 20, Pars. 43-46) The series 40 control action is similar to a single-pole single-throw switch. (Sec. 20, Par. 49) No, you can't substitute it with anything but a series 60 floating motor because it is revertible and the two position is not. (Sec. 20, Par. 55) You cannot substitute a series 20 motor with a series 60 because the 20 operates on low voltage, while the 60 uses line voltage. (Sec. 20, Par. 58) The amount of current flowing through the relay will affect the position of the contact blade between the two motor contacts which, in turn, controls the position of the controlled device. (Sec. 20, Pars. 66-68) The series 90 motor will stop running when the balancing relay is balanced. (Sec. 20, Par. 73) The most probable cause of a damper remaining closed when the control calls for it to be open is loose locknut on the linkage to the damper shaft (Sec. 20, Pars. 79 and 80) The main difference between a series 90 humidity control system and a series 90 temperature control system is the sensing device which operates the controller wiper. (Sec. 20, Par. 18) The humidistat is wired into the blue wire of the right circuit. (Sec. 20, Par. 93) When one belt in a set breaks. you must replace the complete set because the remainder of the belts are stretched and the new belt will not have the proper tension. (Sec. 21, Par. 3) A compressor losing efficiency is usually caused by defective air cleaner. (Sec. 21, Par. 4) When the first stage is operating at normal and the second-stage pressure is zero, the pressure relief valve is stuck open. (Sec. 21, Par. 8) Before you start a newly installed compressor, you must check the oil level in the compressor crankcase (Sec. 21, Par, 15) If you replace the standard head gasket with a thin head gasket, the compressor will probably knock (Sec. 21, Par. 21) Supply-air lines are lines connecting the controllers to the air source, and the control air lines connect the controllers to the controlled device. (Sec. 22, Par. 1) You must allow a 1 1/2-inch pitch for a 12-foot supplyair header, 1/8 inch per foot of header. 3 inches. 1/4 inch per foot. (Sec. 22, Par. 4) The amount of moisture present in the air determine the frequency of draining the filters. (Sec. 22, Par. 8) You would install a reverse acting controller so that a decrease in temperature will cause an increase in air pressure to the valve. (Sec. 22. Par. 17) You clean the contact points on a thermostat by drawing a piece of hard-finish paper between then (Sec. 22, Par. 23) Under normal conditions, a humidistat will control the humidity within 1 percent R.H. of the set point (Sec. 22, Pat. 25) Hygrometers are the controllers used to measure, record, and control humidity. (Sec. 22, Par. 27)

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37. The spring in the piston type damper operator usually functions between 5 and 10 p.s.i.g. At 3 p.s.i.g. the damper should be at its normal position. (Sec. 22, Par. 34) 38. The spring attached to the operator stem determines the operating range of the positioner. (Sec. 22, Par. 39) 39. When you overhauled the operator you probably kinked the diaphragm, which would cause erratic operation of the damper operator (Sec. 22, Par. 43) 40. To correct a skipping pen, you must bend the pen arm slightly toward the chart. The pen should rest on the chart lightly. (Sec. 22, Par. 48) 41. A condensate loop should be installed on a pressure transmitter when it is used to measure the pressure of a hot, moist atmosphere. (Sec. 22, Par. 57) 42. The dried ink may be cleaned from the pen by washing it in warm water. (Sec. 22. Par. 64) 43. The fire protection control has malfunctioned or was activated and shut the system down. You should check the fire protection control, then reset it. (Sec. 22, Pars. 73, 75, and 76) 44. You can check the operation of an airflow detector by blocking off a section of the filters or by closing a damper before the air reaches the instrument. (Sec. 22, Par. 82) 45. A graphic panel is an asset because you can monitor and control the entire system from one central location. (Sec. 23, Par. 1) 46. On graphic panels, chill water temperature is always indicated and recorded. (Sec. 23, Par. 2) 47. When a green coded component on the graphic panel is malfunctioning you are having trouble with the condensing water system. (Sec. 23, Par. 4) CHAPTER 7 1. Disagree. Evaporative cooling changes sensible heat to latent heat but doesn't affect the wet-bulb temperature (total heat). (Sec. 24, Par. 1) 2. With an evaporative cooler, the air can be cooled to its wet-bulb temperature. (Sec. 24, Par. 3) 3. Phoenix, Arizona, is first because it has a high average dry-bulb temperature and low wet-bulb temperature. Dallas, Texas follows second and New York is third. New Orleans is fourth because its average dry-bulb temperature is in the middle 90's and the wet-bulb temperature in the mid 80,s. (Sec. 24, Par. 5) 4. The most probable cause of low water supply to the distributor in an evaporative cooler is a plugged pump intake screen. (Sec. 24, Par. 11) 5. The spray type evaporative cooler should be-installed in a dusty area because it keeps the pads free of dust for a longer period of time. (Sec. 24, Par. 18) 6. An electric timer controls the frequency of operation of the flush valve on spray type evaporative coolers. (Sec. 24, Par. 22) 7. The eliminator pads must be placed when water droplets are carried in the air to the conditioned area. (Sec. 24, Par. 24) 8. Since centrifugal fans are rated for a delivery against 1/4-inch water gauge static pressure, nothing would

9. 10.

11.

12.

13.

14. 15. 16. 17.

18. 19.

happen unless the pressure exceeded ¼ inch. If it exceeded 1/4 inch the cooler would lose efficiency. (Sec. 24, Par. 25) The 3,000 c.f.m. rotary drum would require a heavy structure because of its sis and weight. (Sec. 25, Par. 1) The drain should be 1 1/4 inch in diameter to reduce stoppage. If stoppage occurs with a larger drain you must flush, the cooler sump more often. (Sec. 25, Par. 9) The function of the two switches is to cont the operation of the recirculating pump motor and the blower or fan motor. They are connected in series with one of the motor leads. This procedure allows the cooler to be used as a ventilation system. (Sec. 25, Par. 11) You must provide an opening large enough to exhaust all the air brought into the area by the evaporative cooler. The size of the opening is obtained from the cooler manufacturer or data books. (Sec. 25, Par. 15) The exhaust opening is not sufficient (less than 1 square foot) and is causing noise. You must allow 9 sq. ft. of louvered exhaust for a 4500 c.f.m. evaporative cooler. (Sec. 25, Par. 16) You should caution the user not to start the blower before the water pump. (Sec. 25, Par. 20) The burned-out motor could have been prevented by installing a motor overload protective device in series with the motor lead. (Sec. 25, Par. 22) You can reduce the spied by adjusting the motor pulley or by reducing the size of the motor pulley? (Sec. 25, Par. 24) 100 c.f.m. is delivered from a 12” X 24” duct with a velocity reading of 50 f.p.m. 12” X 24” = 288 sq. in. 288 sq. in. = 2 sq. ft. 50 X 2 = 100 c.f.m. (Sec. 25, Par. 25) The service that you must accomplish on troughs and weirs of a drip type evaporative cooler is cleaning, painting, or replacement. (Sec. 26, Par. 3) The water distribution system is cleaned by flushing it with a 10 percent solution of muriatic acid. (Sec. 26, Par. 3) 20. The axial clearance of the blower wheel is 1/32” To adjust the clearance you would use a .030 feeter gauge because 1/32” = .0313. (Sec. 26, Par. 3) CHAPTER 8

1. Complete air distribution is important when hazardous vapors and fumes may exist in an area such as a battery shop. (Sec. 27, Par. 3) 2. The two ways you could reduce grille noise are: change the size of the grille or reduce the air discharge velocity. (Sec. 27, Par. 13) 3. The radial-flow fan is normally used in a ventilating system which has considerable duct work. (Sec. 28, Par. 3) 4. The fan capacity must be at least 1,260 c.f.m. for a room 14 feet by 60 feet requiring 30

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12.

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14. 15. 16. 17.

18.

19.

air changes per hour. To find what capacity fan is needed use the formula Q = CV ; Q = 30 X 60 X 60 X 14 ; Q = 75600 60 60 60 Q= 1,260 c.f.m. (Sec. 28, Pars. 10 and 11) To reduce air duct friction loss you would increase the duct size. (Sec. 29, Par. 3) Duct fire dampers are used to automatically shut off fans and ducts in event of a fire. (Sec. 29, Par. 5) The type of grille that should be used in a flood air outlet which requires controlled airflow is a vaned grille. (Sec. 30, Pars. 2-7) Slotted outlets are used for long narrow rooms. (Sec. 30, Par. 6) The pattern of the supply air envelope is determined by the air outlet grille. (Sec. 31, Par. 3) Several of the factors that must be considered when preparing to install a ventilating system are: the purpose of the building or room to be ventilated, the temperature and humidity of the region; the size of the building or room, the number of occupants, and local and national codes and regulations. (Sec. 32, Pars. 1-3) In a room that contains carbon dioxide the fan should be located close to the floor. The specific gravity of carbon dioxide is 1.527 which is heavier than air; so it will settle to the floor. (Sec. 32, Pars. 18 and 19) When installing an exhaust fan in a paint shop or paint spray booth, the electrical circuit for the fan and air compressor should be interconnected. This will insure that the fan is operating when spray painting is being done. (Sec. 32, Par. 22) The factors that determine the desired exhaust air velocity are: what is to be exhausted, amount to be exhausted, and the desired noise level. (Sec. 32, Pars. 26-28) Poorly constructed fixed wooden louvers would result in restriction of airflow and insufficient protection against bad weather. (Sec. 32, Par. 34) Filters should be installed in the exhaust system when the discharge would create an objectionable condition in the immediate area. (Sec. 32, Par. 39) Dirt on the fan blades will unbalance the fan and cause fan vibration during operation. (Sec. 33, Par. 5) Overlubrication of fan motors and other ventilating equipment will result in collections of oil and dirt which could restrict airflow, cause motors to overheat, and present a fire hazard. (Sec. 33, Par. 8) The frequency of cleaning an air filter depends on the following: type of system in which the filter is installed, how much the system is used, and weather conditions. (See. 33, Par. 16) Excessively tight fan belts will cause an increase in the fan motor load and premature bearing wear. (Sec. 33, Par. 21) CHAPTER 9

1. COP =

2.

3.

4.

5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16.

17. 18.

300,000 40 X 2545 = 300,000 101,800 = 2.94 to 1 (Sec. 34, Par. 2) 100,000 B.t.u./hr. = 29.2 kw.-hr 29.2 X .02 = $.584 per hour .584 X 24= $14.02 per day. (Sec. 34, Pars. 5 and 6) You would have 5400 degree days. 65 - 5 = 60 degree days per day. 60 X 90 = 5400 degree days. (Sec. 34, Par. 7) The heat pump is cheaper to operate because it uses electricity only to drive the compressor. The refrigeration releases more heat per watt consumed. (Sec. 34, Par. 8) The oil-fired heating system is cheaper than the air-to-air heat pump. (Sec. 34, Par. 9) The air-to-air (refrigerant changeover) heat pump has two expansion devices. (Sec. 35, Par. 2) The maximum temperature of the refrigerant when the outside temperature is 50° F. is 75° F. (Sec. 36, Par. 2) Outdoor temperature is an important factor because a balance point above the design temperature would require supplemental heating which is not economical. (Sec. 36, Pars. 3-9) 40 pounds of frost on a 5-feet x 4-feet coil will not substantially affect the coil because the nominal amount it could hold is 50 pounds. (Sec. 36, Par. 11) A 4-foot coil can contain 384 to 672 fins. (Sec. 36, Par. 15) The most common causes of airflow reduction are dirty filter, dirty coils, and frost accumulation on the coil. (Sec. 36, Par. 19) City water is not considered a good heat source because of its poor availability and high operating cost. (Sec. 36, Par. 24) Well water is considered the best heat source because of its relatively constant temperature. (Sec. 36, Par. 25) Heat storage is beneficial to a heat pump because it tends to reduce the rate of temperature change and helps to reduce peak load requirements. (Sec. 37, Par. 2) The solenoid pilot 3-way valve has probably malfunctioned when the unit will not reverse its cycle. (Sec. 38, Par. 2) When the pilot valve is energized, suction pressure is allowed to pass to the top of the main valve piston. This will cause the piston to rise, allowing the compressor discharge to pas to the condenser. (Sec. 38, Par. 3) An open in the pilot valve solenoid will cause the heat pump to operate on the heat cycle. (Sec. 38, Par. 4) To prevent freeze-up of a water-cooled condenser during the heat cycle, you should install an evaporator pressure regulator or a constant pressure liquid expansion valve on the condenser. (Sec. 38, Pars. 9 and 10)

138

SUBCOURSE OD1750

EDITION A

REFRIGERATION AND AIR CONDITIONING IV (EQUIPMENT COOLING)

REFRIGERATION AND AIR CONDITIONING IV (EQUIPMENT COOLING) Subcourse OD1750 Edition A United States Army Combined Arms Support Command Fort Lee, VA 23801-1809
14 Credit Hours INTRODUCTION This subcourse is the last of four subcourses devoted to basic instruction in refrigeration and air conditioning. The scope of this subcourse takes in unit components of the absorption system, including their functions and maintenance; water treatment methods and their relationship to centrifugal systems; centrifugal water pumps and electronic control systems, including the relationship of amplifier, bridge and discriminator circuits to electronic controls. The subcourse consists of three lessons. Lesson 1. 2. 3. Direct Expansion and Absorption System. Centrifugal Systems and Water Treatment. Centrifugal Water Pumps and Electronic Control Systems.

Unless otherwise stated, whenever the masculine gender is used, both men and women are included.

CONTENTS Page Preface.................................................................................................................................................................... Acknowledgment.................................................................................................................................................... Lesson 1 Chapter 1 2 Lesson 2 Chapter 3 4 Lesson 3 Chapter 5 6 7 Centrifugal Water Pumps)...................................................................................................................................... 96 Fundamentals of Electronic Controls..................................................................................................................... 103 Electronic Control Systems ................................................................................................................................... 132 Answers to Review Exercises................................................................................................................................. 139 The passing score for ACCP material is 70%. Centrifugal Systems ............................................................................................................................................... 46 Water Treatment ................................................................................................................................................... 77 Direct Expansion Systems ..................................................................................................................................... 1 ii iii

Absorption Systems ............................................................................................................................................... 26

Preface YOU HAVE studied the fundamentals and commercial refrigeration and air-conditioning systems. This final volume deals with another phase of your career ladder-equipment cooling. Since the principles of equipment cooling are common to all refrigeration systems, your mastery of the subject should be easy. All of the systems covered in this volume can be applied to commercial refrigeration and air conditioning. To qualify you in equipment cooling, we will present the following systems in this volume: (1) (2) (3) (4) (5) (6) (7) Direct expansion Absorption Centrifugal Water treatment Centrifugal water pumps Fundamentals of electronic controls Electronic control

Keep this memorandum for your own use.

ii

ACKNOWLEDGMENT Acknowledgment is made to the following companies for the use of copyright material in this CDC: Honeywell, Incorporated, Minneapolis, Minnesota; Carrier Air Conditioning Company, Carrier Parkway, Syracuse, New York; Terry Steam Turbine Company, Hartford, Connecticut; Koppers Company, Incorporated, Baltimore, Maryland

iii

CHAPTER 1

Direct Expansion Systems JUST WHAT DO we mean when we say "direct expansion"? In the dictionary we find that the word "direct" means an unbroken connection or a straight bearing of one upon or toward another; "expansion" relates to the act or process of expanding or growing (in size or volume). Now we can see that a direct expansion system for equipment cooling is one in which the controlled variable comes in direct contact with the single refrigerant source, thereby causing the liquid refrigerant to boil and expand. The centrifugal and absorption systems differ in that that they us a secondary refrigerantwater or brine-to cool the variable. 2. We will cover various components peculiar to large direct expansion systems, normally of 20 tons or more in capacity. Remember, the window- and floormounted air-conditioning units are also considered direct expansion systems. Before we discuss the installation of a semihermetic condensing unit-the most commonly used unit for direct expansion systems-we will cover the various coils that are used in a direct expansion system. The application of the water-cooled semihermetic condensing unit will concern us in the second section, and we will conclude the chapter with system servicing and troubleshooting. 1. Coil Operation 1. There are three coils used in the typical system. From the outside in, the coil sequence is: (1) preheat, (2) direct expansion (D/X), and (3) reheat. We will discuss the application of these coils, their use and control, and the valves and dampers which control the flow of water and air. 2. Preheat Coil. You must consider three things before installing a preheat coil in an equipment cooling system. These are: (1) Is preheat necessary? (2) Will the coil be subjected to subfreezing temperature? (3) What size preheat coils are needed? 3. After you have determined a need, provided for freezing temperatures, and correctly sized the coil, you are ready to install the coil. The next problem is where to install it. The preheat coil is installed in the outside air duct, before the mixing of outside and return air. Now we are ready to discuss a few applications of a preheat coil. 4. Thermostatically controlled water or steam valve. Figure 1 shows a system that uses a narrow range temperature controller. The temperature of the incoming air is sensed by the thermostat feeler bulb. The thermostat is calibrated to modulate the valve open when the temperature is 35° F. 5. The damper on the face of the preheat coil closes when the fan is turned off and opens when it is turned on. This damper is normally closed when the fan is off or if the fan fails to operate. This prevents preheat coil freezeup. 6. Thermostatically controlled face and bypass dampers. The mixed air temperature remains relatively constant until the outside air temperature exceeds the desired mixed air temperature. The use of the face and bypass damper, illustrated in figure 2, makes it possible to control mixed air temperature without endangering the preheat coil. The damper is controlled by a temperature controller in the mixed air duct while the preheat coil is controlled by a valve which is modulated by a narrow range temperature controller in the outside air duct. The face and bypass damper will close and the return air opens when the supply fan is turned off. 7. D/X Coil. In equipment cooling systems, the D/X coil is located after the preheat coil. It serves two primary functions-cooling and dehumidification. 8. Simple on-off control. The compressor is controlled by a space thermostat in an on-off manner. Figure 3 shows a system using this type of control. This system is best suited for use on small compressors and where large variations in temperature and humidity are not objectionable.

1

Figure 1. Control of preheat with outdoor air thermostat. 9. The differential adjustment on the thermostat should be set relatively wide to prevent short cycling under light load conditions. The control circuit is connected to the load side of the fan starter so that turning on the fan energizes the control systems. 10. Two-speed compressor. Figure 4 shows a typical two-speed compressor installation. A two-stage thermostat (space) cycles the compressor between low speed and off during light load conditions and cycles the unit between high and low speed during heavier loads. The thermostat also shuts off the compressor if the space temperature falls below the set point. 11. The humidistat cycles the compressor from low to high speed when space humidity rises above the high limit set point. It can do this when the compressor is on low speed. This system is best suited for use on reasonably small compressors where large swings in temperature and relative humidity can be tolerated. 12. Solenoid valve installation. Figure 5 shows a system which uses a space thermostat to operate a solenoid valve and a nonrestarting relay. The

Figure 3. On-off compressor control. two-position thermostat opens the refrigerant solenoid valve when the space temperature rises and closes it when the temperature drops below the set point. This control action will cause large swings in temperature and relative humidity. The nonrestarting relay prevents short cycling of the compressor during the off cycle. It allows the compressor to pump down before it cycles "off." 13. Multiple D/X coil solenoid valves. The system shown in figure 6 is similar to that previously discussed (fig. 5) except that it now has two D/X coils and two solenoid valves. The two-stage space thermostat operates D/X coil 1 in an on-off manner when the cooling load is light. It also holds the valve to coil 1 open and operates the valve to coil 2 in an on-off manner during heavy load conditions. The nonrestarting relay functions the same as the one in figure 5. 14. The supply fan starter circuit must be energized, in both applications, before the control circuit to the solenoid valves can be completed. 15. Two-position control and modulating control of a face and bypass damper. This system uses a face and bypass damper (shown in fig. 7) to bypass air around the D/X coil during light load conditions. The space thermostat opens the refrigerant solenoid valve when the face damper opens to a position representing a minimum cooling

Figure 2. Preheat control with bypass and return air dampers. 2

Figure 4. Two-speed compressor control.

Figure 5. On-off control with a solenoid valve. load. It also modulates the face and bypass dampers to mix the cooled air with the bypassed air as necessary to maintain the correct space temperature. A capacity controlled compressor must be used if short cycling, under light load conditions, is to be avoided. 16. It is necessary to adjust the face damper so that it does not close completely. This will help prevent coil frosting under light load conditions. The control circuit to the solenoid valve is wired in series with the supply fan motor. When the fan is shut off, the solenoid valve will close. 17. Two-position control and modulating control of a return air bypass damper. This system, shown in figure 8, is similar to the system we have just discussed. The only difference is that we bypass return air instead of mixed air under light load conditions. 18. Reheat Coil. The reheat coil is used to heat the air after it has passed through the D/X coil. It expands the air, thus lowering the relative humidity. A D/X coil and reheat coil are used to control humidity. 19. Simple two-position control. Figure 9 shows a system which uses a space thermostat to control a reheat coil and a D/X coil. It opens the solenoid valve to the heating coil when the

Figure 7. Two-position control of a D/X coil solenoid valve and modulating control of a face and bypass damper. space temperature falls below the set point temperature, and opens the D/X coil solenoid valve when the temperature is above the set point. A two-position humidistat is provided to open the cooling coil solenoid valve when the space relative humidity exceeds the set point of the controller. When a humid condition exists, the humidistat will override the thermostat. The thermostat senses the reduced air temperature and opens the reheat coil solenoid valve which will lower the relative humidity. The D/X coil solenoid valve will close when the supply fan is shut off. 20. Control of dehumidification with a face and bypass damper. We discussed the use of face and bypass dampers when we discussed D/X coils. Now we will apply this damper system to humidity control, as shown in figure 10. A space humidity controller is used to open the D/X coil valve when a predetermined minimum dehumidification load is reached. It also modulates the face and bypass damper to provide the mixture of dehumidified and bypass air necessary to maintain space relative humidity. 21. The space thermostat modulates the reheat coil valve as needed to maintain space temperature. If the space humidity drops below the set

Figure 6. On-off control of multiple D/X coil solenoid valves. 3

Figure 8. Two-position control of a D/X coil solenoid valve and modulating control of a return air bypass damper.

Figure 9. Dehumidification control in a two-position D/X system. point of the humidity controller, and the space temperature rises because the discharge air is too warm to cool the space, the thermostat will open the D/X coil valve and modulate the face and bypass damper to lower the space temperature. The reheat coil must be controlled by a modulated valve so that the thermostat can position the valve within its range. This will prevent large swings in temperature and relative humidity. This system also provides a method of closing the D/X coil valve when the supply fan is shut off. 22. Control of dehumidification with a return air bypass system. Figure 11 shows a system which uses a return air bypass damper to control airflow across the D/X coil for dehumidification. The space humidistat opens the D/X coil valve when a predetermined minimum cooling load is reached and positions the bypass damper to maintain space relative humidity. 23. The space thermostat acts in a way that is similar to that of the thermostat in figure 10. The control circuit to the D/X coil valve is connected to the supply fan so that the valve will close when the fan is shut off. This arrangement helps prevent coil frosting and reheat coil freezeup.

Figure 11. Dehumidification control in a D/X return air bypass system. 24. We have discussed the three coils that you will find in a typical equipment cooling system. Now we will discuss a complete system which maintains temperature, relative humidity, and air changes. 25. Typical D/X Equipment Cooling System. Figure 12 shows a system which may be used to condition air for electronic equipment operation. Thermostat T1 senses outdoor (incoming) air and modulates the preheat coil valve to the full open position when the temperature falls below the controller set point. A further drop in temperature will cause the thermostat T1 to modulate the outside and exhaust air dampers shut and the return air damper open. 26. The space thermostat (T2) operates the reheat coil valve as necessary to maintain a predetermined space temperature. The space thermostat (T2) will modulate the cooling coil valve when the space humidity is within the tolerance of the humidistat. The space humidistat opens the cooling coil valve when a minimum cooling load is sensed. It has prime control of this valve. The outside and exhaust air dampers are fitted with a stop so that they will not completely close. This procedure allows for the correct amount of air changes per hour. 27. There are many other direct expansion systems. The blueprints for your installation will help you to better understand the operation of your system. Most of the system components are similar to those previously discussed. 2. Application of Water-Cooled Condensing Units 1. Water-cooled semihermetic condensing units are rated in accordance with ARI Standards with water entering the condenser at 75°F. 2. Condensing units are available for different temperature ranges. We are interested in the "high temperature" unit, as it is used for air conditioning

Figure 10. Dehumidification control in a D/X face and bypass system. 4

Figure 12. Typical D/X equipment cooling system. or other applications requiring a +25°F. to +50°F suction temperature. 3. A medium temperature unit (-10° F. to +25° F.) should not be selected or equipment cooling applications where the compressor would be subjected to high suction pressure over extended shutdown periods. This would result in motor overload and stopping when the cooling load is peak. To prevent this possibility, the proper unit must be selected considering the highest suction pressure the unit will be subjected to for more than a brief period of time. 4. Compressor Protection. During shutdown, refrigerant may condense in the compressor crank-case and be absorbed by the lubricating oil. The best protection against excessive accumulation of liquid refrigerant is the automatic pump-down control. The compressor must start from a low-pressure switch (suction pressure) at all times. Figure 19 (in Section 3) shows a recommended control wiring diagram that incorporates an automatic pump-down control. When the pressure in the crankcase rises, the compressor will cycle on. It will run until the pressure drops to the lowpressure switch cutoff setting. 5. In systems where the refrigerant-oil ratio is 2:1 or less, automatic pump-down control may be omitted. It may also be omitted on systems where the evaporator is always 40° or more below the compressor ambient temperature. However, the use of an automatic pumpdown control is definitely preferable whenever possible. 6. Water Supply. Water-cooled condensing units should have adequate water supply and disposal facilities. Selection of water-cooled units must be based on the 5 maximum water temperature and the quantity of water which is available to the unit. Now that you have selected the proper equipment, let's discuss the installation of equipment. 3. Installation 1. Before you start installing the unit you must consider space requirements, equipment ventilation, vibration, and the electrical requirements. 2. The dimensions for the condensing unit are given in the manufacturer's tables. You must allow additional room for component removal, such as the compressor or dehydrator. The suction and discharge compressor service valves, along with the compressor oil sight glass, must be readily accessible to facilitate maintenance and troubleshooting. The space must be warmer than the refrigerated space to prevent refrigerant from condensing in the compressor crankcase during extended shutdown periods. Water-cooled units must be adequately protected from freezeup. Some method of drainage must be provided if the unit is to be shut down during the winter months. 3. Install the unit where the floor is strong enough to support it. It is not necessary to install it on a special foundation, because most of the vibration is absorbed by the compressor mounting springs. On critical installations (e.g., hospitals and communication centers) it may be desirable to inclose the unit in an equipment room to prevent direct transmission of sound to occupied spaces. Place the unit where it will not be damaged by traffic or flooding. It may be necessary to cage or elevate the unit. 4. The next step in installing a unit is to

Figure 13. Three phase wiring diagram for a semihermetic condensing unit. inspect the shipment for loss or damage. You must report any loss or damage to your supervisor immediately. Refer to ASA-B9.1-1953, American Standards Association's "Mechanical Refrigeration Safety Code" when you install the unit. 5. Before installing the unit, check the electric service to insure that it is adequate. The voltage at the motor terminals must not vary more than plus or minus 10 percent of the rated nameplate voltage requirement. Phase unbalance for three-phase units must not exceed 2 percent. Where an unbalance exists, you must connect the two lines with the higher amperages through the switch heater elements. Figure 13 shows a typical wiring diagram for a semihermetic condensing unit. 6. A table of wire size requirements is provided with the manufacturer's installation handbook. For instance a 220-volt three-phase condensing unit requiring 8 amperes at full load must be wired with number 8 wire 6 if the length the run is 300 feet. However, number 14 wire can be used if the run is limited to 10 feet. 7. Piping and Accessories. The liquid and suction lines are usually constructed of soft copper tubing. To help absorb vibrations, loop or sweep the two lines near the condensing unit. Use a vibration isolation type hanger, show in figure 14, to fasten the tubing on walls or supports. 8. Shutoff valves. The suction and discharge shutoff valves (service valves) are of the back-seating type and have gauge ports. Frontseating the valve closes the refrigerant line and opens the gauge port to the pressure in the compressor. 9. Backseating the valve shuts off pressure to the gauge port. To attach a gauge or charging line to the gauge port, backseat the valve to prevent escape of refrigerant.

Figure 14. Vibration isolation type hanger. 10. Use a square ratchet or box-end wrench (1/4inch) to open and close the valve. Do not use pliers or an adjustable wrench since they are likely to round the valve stem. Do not use excessive force to turn the stem. If it turns hard, loosen the packing gland nut. If the valve sticks on its seat, a sharp rap on the wrench will usually break it free. 11. Liquid line solenoid valve. Many manufacturers use this type of valve on their units to prevent damage to the compressor which would result from flooding of the crankcase with refrigerant during shutdown. This type of valve also provides a compressor pump-down feature on many units. The valve is installed in the liquid refrigerant line directly ahead of the expansion valve. It must be installed in a vertical position and wired as shown in the wiring diagram (fig. 13). 12. Liquid line sight glass. The liquid line sight glass is installed between the dehydrator and expansion valve. You should locate the sight glass so that it is convenient to place a light behind the glass when you are observing the liquid for a proper charge. 13. Water regulating valves. Install the water regulating valve with the capillary down and the arrow on the valve body in the direction of water-flow. Backseat the liquid line shutoff valve and connect the capillary of the water regulating valve of the 1/4-inch flare connection on the liquid line shutoff valve. Open the shutoff valve one turn from the backseated position. This allows refrigerant pressure to reach the water regulating valve and still leaves the liquid line open. 14. Water-cooled condenser connections. When city water is used as the condensing media, the condenser circuits are normally connected in series. When cooling tower water is used for condensing, the condenser circuits are connected in parallel. See figure 15 for correct condenser water connections.

15. Leak Testing the System. After all the components have been installed, you are ready to leak test the system. Charge the system with dry nitrogen or carbon dioxide (40 p.s.i.g.) and check all the joints with a soap solution. Release the pressure and repair any leaks that may have been found. After the leaks have been repaired, charge the system with the recommended refrigerant to 10 p.s.i.g. Add enough dry nitrogen or carbon dioxide to build the pressure to 150 p.s.i.g. and leak test with a halide leak detector. Purge the system and repair all leaky joints that you may have found. Do not allow the compressor to build up pressure since overheating and damage may result. Do not use oxygen to build up pressure! 16. Dehydrating the System. Moisture in the system causes oil sludge and corrosion. It is likely to freeze up the expansion valve during operation. The best means of dehydration is evacuation with a pump especially built for this purpose. The condensing unit is dehydrated at the factory and is given a partial or holding charge. Leave all the service valves on the condensing unit closed until the piping and accessories have been dehydrated. Do not install a strainer-dehydrator until the piping is complete and the system is ready for evacuation.

Figure 15. Condenser connections. 7

Figure 16. Vacuum indicator. 17. Make the following preparations before dehydrating the system: (1) Obtain a vacuum pump that will produce a vacuum of 2 inches Hg absolute. Do not use the compressor as a vacuum pump since this may cause serious damage to the compressor. (2) Obtain a vacuum indicator similar to that shown in figure 16. These indicators are available through manufacturers' service departments. (3) Keep the ambient temperature above 60° F. to speed the evaporation of moisture. 18. Description and use of the vacuum indicator. The vacuum indicator consists of a wet bulb thermometer in an insulated glass tube containing distilled water. Part of the tube is exposed so that the thermometer can be read and the water level checked. When the indicator is connected to the vacuum pump suction line, the thermometer reads the temperature of the water in the tube. The temperature is related to the absolute pressure in the tube. Figure 17 gives the absolute pressures corresponding to various temperatures. To determine the 8

vacuum in inches of mercury, subtract the absolute pressure from the barometer reading. 19. Handle the vacuum indicator with care. It must be vacuum-tight to give a true reading. The top seal of the indicator is not designed to support a long run of connecting tubes. Fasten the tubes to supports to prevent damage to the indicator. Use only distilled water in the indicator and be sure the wick is clean. Oil or dirt on the wick causes erroneous readings. 20. To prevent loss of oil from the vacuum pump and contamination of the indicator, you must install shutoff valves in the suction line at the vacuum pump and the vacuum indicator. When shutting off the pump, close the indicator valve and pump valve, and then turn off the pump. Now we are ready to dehydrate the system. 21. Procedure for dehydrating the system. Connect the pump and vacuum indicator to the system. Put a jumper line between the high and low side so that the pump will draw a vacuum on all portions of the system. Open the compressor shutoff valves and start the vacuum pump. Open the indicator shutoff valve occasionally and take a reading. Keep the valve open for at least 3 minutes for each reading. You must keep the indicator valve closed at all other times to decrease the amount of water the pump must handle and to hasten dehydration. When the pressure drops to a value corresponding to the vapor pressure of the water in the indicator, the temperature will start to drop. 22. In the example illustrated in figure 18, the ambient temperature and the temperature of the water in the indicator is 60° F. Starting at 60° F., and 0 time, the temperature of the indicator water remains at 60° F. until the pressure in the system is pulled down to the pressure corresponding to the saturation temperature of the water (60° F.). Point A in figure 18 shows the temperature saturation point. At this point the moisture in the system begins to boil. The temperature drops slowly until the free moisture is removed. Point A to Point B illustrates the time required for free moisture evaporization. After the free moisture is removed, the

Figure 17. Temperature-pressure relationship.

Figure 18. Dehydration pulldown curve. absorbed moisture is removed, point B to point C. Dehydration is completed at point C, provided the ambient temperature stays at 60° F. or higher. If the ambient temperature falls below 60° F., the moisture will form ice before moisture removal is complete. 23. You should continue the dehydrating procedure until the vacuum indicator shows a reading of 35° F. Looking back at figure 17, you will find that a 35° F. reading corresponds to a pressure of 0.204 inch Hg absolute. This procedure may take several hours, and many times it is advantageous to run the vacuum pump all night. After evacuation, turn off the indicator valve (if open) and the pump suction shutoff valve, and break the vacuum with the recommended refrigerant. Disconnect the pump and vacuum indicator. 24. Charging the System. The refrigerant may be charged into the low side of the system as a gas or into the high side as a liquid. We will discuss both methods of charging in this section. 25. To charge into the low side as a gas, backseat the compressor suction and discharge valves and connect your gauge and manifold to the appropriate compressor gauge connections The next step is to connect a refrigerant drum to the middle manifold hose. Open the drum valve and purge the hoses, gauges, and manifold. Then tighten all the hose connection. Turn the suction shutoff valves a couple of turns from the backseat position and open the drum valve as far as possible. 9 Remember, keep the refrigerant drum in an upright position to prevent liquid refrigerant from entering the compressor. You can now turn the compressor discharge shutoff valve about one-fourth to one-half turn from the backseat position so that compressor discharge pressure can be read at the manifold discharge pressure gauge. 26. Before you start the compressor you must check the following items: (1) Proper oil level in the compressor sight glass (one-third to two-thirds full). (2) Main water supply valve (water-cooled condenser). (3) Liquid line valve. Valve stem should be positioned two turns from its backseat to allow pressure to be applied to the water regulating valve. (4) Main power disconnect switch (ON position). 27. After you have started the compressor you must check the following items: (1) Correct oil pressure. (2) Water regulating valve adjustment. (3) Control settings. (4) Oil level in the compressor crankcase. 28. Check the refrigerant charge frequently while charging by observing the liquid line sight glass. The refrigerant charge is sufficient when flashing (bubbles) disappears. If the pressure within the drum, during charging, drops to the level of the suction pressure, all the remaining refrigerant in the drum may be removed by frontseating the compressor suction shutoff valve.

This procedure will cause a vacuum to be pulled on the refrigerant drum. 29. When the system is sufficiently charged, close the refrigerant drum valve and backseat the compressor suction and discharge shutoff valves. Disconnect the charging lines from the compressor gauge ports and connect the lines from the dual pressurestat to the charging lines and "crack" the valves off their backseat. 30. Liquid charging into the high side can be done by either of two methods. One method is to charge into the liquid line with the compressor running. The other method is to charge directly into the systems liquid receiver. Since charging liquid into the receiver is much faster, systems containing more than 100 pounds of refrigerant are usually charged this way. Let us discuss both methods in detail. 31. Systems to be charged into the liquid line first must have a charging port installed in the liquid line. Then use the following procedure: (1) Close king valve. (2) Connect inverted drum to charging port. (3) Open drum service valve. (4) Purge air from charging lines. (5) Operate unit until fully charged. (6) Reopen king valve; this system is now in operation. 32. Charging liquid into the receiver is performed according to the following general procedure: (1) Turn off electrical power to unit. (2) Connect the inverted and elevated refrigerant drum to the receiver charging valve. (3) Open drum service valve. (4) Purge air from charging line. (5) Open the charging valve. (6) Several minutes are required to transfer a drum of refrigerant in this manner; the transfer time can be shortened by heating the drum (do not use flame). (7) When sufficient charge has been transferred into the system, power can be turned on. (8) By checking the pressure gauges and the sight glass, you can determine when the system is fully charged. To maintain the efficiency of the machinery you have installed, you must service and troubleshoot it. 33. Checking Operation. When you are starting a newly installed compressor, be on the alert for any sign of trouble. 34. The high-pressure setting of the dual pressurestat, shown in figure 19, should not require a change; however, the low-pressure setting will probably require adjustment, depending upon the evaporator temperature. Check the high-pressure cutout by

throttling the condenser water. This will allow the head pressure to rise gradually. The cut-out and cut-in pressures should be within 10 to 15 pounds of the values outlined in the manufacturer’s handbooks. If they are not, the pressurestat would be readjusted. You can check the low-pressure settings by frontseating the compressor shutoff valve or the liquid line shutoff valve. The cut-in and cut-out point may be adjusted if it is necessary. 35. The units are shipped with "full" oil charges. Do not assume that the charge is sufficient. Stop the unit, without pump-down, after 15 or 20 minutes of operating time and immediately recheck the oil level in the compressor sight glass. The oil level must be onethird to two-thirds of the way up on the sight glass. You can check oil pump pressure by looking at the oil pressure relief valve through the sight glass during compressor operation. Pressure is adequate if oil is being discharged from the relief valve. 36. Adjust the water regulating valve to the most economical head pressure for the locality. Normally, this is 120 to 140 p.s.i.g. for R-12 and 200 to 230 for R-22. 4. Servicing and Troubleshooting 1. We have covered several service techniques in the previous section that relate to installation, including leak testing, dehydrating, and charging into the low side as a gas and into the high side with liquid. We shall now go further into servicing as it relates to disassembly, inspection, and reassembly of individual components. By means of tables at the end of this chapter, you will then focus on troubleshooting techniques. 2. Servicing. Servicing direct expansion systems embodies a wide range of related topics, from removing the refrigerant charge and testing for leaking valves to terminal assembly and testing capacitors and relays. 3. Removing Refrigerant. The refrigerant charge can be removed by connecting a refrigerant drum to the gauge port of the liquid line shutoff valve. Turn the stem two turns off its backseat and run the unit. Most of the refrigerant can be removed in this manner. The remainder may be removed by placing the drum in a bucket of ice or by slowly releasing it to the atmosphere. 4. Pump-down procedure. If possible, you should allow the compressor to run until it is warm before pumping it down. Then pump the system down as follows: (1) Close (frontseat) the liquid line shutoff valve on the condenser. (2) Hold the pressurestat switch closed so that the unit will not trip off on low pressure. (3) Run the compressor until the compound

10

Figure 19. Single-phase wiring diagram for a semihermetic condensing unit. gauge (registering low side pressure) registers 2 p.s.i.g. (4) Stop the compressor and watch the gauge. If the pressure rises, pump down again. Repeat the operation until the pressure remains at 2 p.s.i.g. (5) Frontseat the compressor discharge and suction shutoff valves. (6) If the compressor is to be left pumped down for any period, tag the disconnect switch to prevent accidental starting of the unit. 5. If the compressor is the only component to be removed, pumping down the crankcase will be sufficient. This may be done by front-seating the suction shutoff valve and completing steps (1)-(5) listed under pumpdown procedure. 11

You must stop the compressor several times during pump-down to prevent excessive foaming of the oil as the refrigerant boils out since the foaming oil may be pumped from the crankcase. 6. Breaking refrigerant connections. When it becomes necessary to open a charged system, the component or line to be removed or opened should be pumped down or evacuated to 2 p.s.i.g. You must allow enough time for all adjacent parts to warm to room temperature before you break the connection. This prevents moisture from condensing on the inside of the system. 7. After the component has warmed to room temperature, you are ready to break the connection and make the necessary repairs. 8. Cleaning the expansion valve strainer. To clean the expansion valve strainer, you must close the liquid line shutoff valve and pump down the system to 2 p.s.i.g. Disconnect the valve and plug the tube ends. Remove the screen and clean it with a recommended cleaning solvent. After the screen is clean and dry, reinstall it in the valve and connect the valve in the system. Purge the lines and valves; then open (two turns off the backseat) the liquid line shutoff valve. 9. Cleaning suction strainers. Most suction strainers are located in the suction manifold on the compressor. Pump down the compressor to 2 p.s.i.g. and frontseat the discharge shutoff valve. At this point, you must check the manufacturer’s handbook to locate the strainer. Remove and clean it with solvent. After the strainer drys, replace it, purge the compressor, and start the unit. Figure 20 shows two different types of strainers, basket and disc, and their location in the compressor motor. 10. Purging noncondensable gases. Noncondensable gases (air) collect in the condenser (water-cooled) above the refrigerant. The presence of these gases cause excessive power consumption, a rise in leaving water temperature, and high compressor discharge pressure. 11. To purge these gases from the system, stop the compressor for 15 to 20 minutes. Then open the purge cock (if available) or loosen a connection at the highest point of the condenser for a few seconds. After purging is completed, close the purge cock (or tighten the connection) and run the compressor. If the discharge pressure is still high, repeat the procedure until the discharge pressure returns to normal. 12. Adding oil. Add only the recommended oil listed in the manufacturer's handbook. The oil should be taken directly from a sealed container. Do not use oil that has been exposed to the atmosphere because it may contain some absorbed moisture.

13. To add oil, pump down the compressor to 2 p.s.i.g. Remove the oil filter plug (if available) or disconnect the pressurestat connection on the suction manifold. Insert a funnel and pour in the oil. Hold the oil container close to the funnel to minimize contact with the air. The correct amount of oil needed can be estimated by observing the oil sight glass (one-third to two-thirds full). After sufficient oil is added, connect the pressurestat or replace the oil filler plug, purge the compressor, and start the unit. 14. Removing oil. To remove excess oil from the crankcase, pump down the compressor to 2 p.s.i.g. Loosen the oil plug (if available), allowing the pressure to escape slowly. Then use a hand suction pump to remove the desired amount of oil. If a filler plug is not available, loosen the bottom plate or drain plug. Retighten the plate or plug when the oil assumes a safe level in the crankcase one-third to two-thirds full. Purge and start the compressor. 15. Testing for leaking valves. Leaky compressor valves will cause a serious reduction in the capacity of the system. Install a manifold and gauge set. Start the compressor and allow it to run until it is warm; then frontseat the suction shutoff valve. Pump down the compressor to 2 p.s.i.g. Stop the compressor and quickly frontseat the discharge shutoff valve. Observe the suction and discharge gauges. If a discharge valve is leaking, the pressures will equalize rapidly. The maximum allowable discharge pressure drop is 3 p.s.i.g. per minute. 16. There is no simple method of testing suction valves. If there is an indicated loss of capacity and the discharge valves check properly, you must remove the head and valve plate and check the valves physically. 17. Disassembly, inspection, and reassembly of valve plates. Pump down the compressor to 2 p.s.i.g. and remove the compressor head capscrews. Tap the head with a wooden or plastic mallet to free it if it is stuck and remove the cylinder head. 18. Remove the discharge valves and valve stops as shown in figure 21. Free the valve plate from the dowel pins and cylinder deck. Many valve plates have tapped holes. The capscrews are screwed into them and function as jacking screws. Now you can remove the suction valves from the dowel pin. Figure 22 shows the suction valve and suction valve positioning spring. Inspect the valve seats and valves. If the valve seats look worn or damaged, replace the valve plate assembly (fig. 21). 19. It is preferable to install new valves with a new valve plate. If new valves are not available, turn the old valves over and install them

12

Figure 20. Suction strainers with the unworn seat toward the valve seat. If the valve seats and valves are not noticeably worn, it is still good practice to turn the discharge valves; otherwise they may not seat properly. 20. The suction valves are doweled and may be reinstalled as they were originally. You must never interchange valves. Be careful when replacing the suction valves. The positioning springs must be placed on the dowels first. Place them with their ends toward the cylinder deck and the middle bowed upward. 21. Worn valves may be reconditioned by lapping them, using a fine scouring powder and a piece of glass. 13 Mix refrigerant oil with the powder to form a liquid paste. Then move the valve in a figure 8 motion over the paste and glass. After the valve is reconditioned, clean and reinstall it. 22. Use new valve plate and cylinder head gasket when you install the valve plate and cylinder head. 23. Disassembly, inspection and assembly of the oil pump and bearing head. Remove the oil pump cover, shown in figure 23. This will free the oil feed guide retainer spring and the oil feed guide. Then remove the oil pump drive segment.

Figure 21. Valve plate assembly. 24. After you remove the bearing head you can remove the plunger snaprings which hold the plunger, plunger spring, and guide spring in the pump plunger cylinder. Snapring or jeweler's needle-nose pliers are recommended for removing the shapings.

25. Push the pump rotor out of the bearing head by pressing against the bearing side of the rotor. The rotor retaining ring will come out with the rotor. Installing a new pump and bearing head is the only positive way of eliminating oil pump trouble. However, if the cause of the trouble is determined, replacement parts are available for almost all compressors. 26. The first step in installing the oil pump and bearing head is to install the rotor retaining ring in the ring groove of the rotor, with the chamfered edge toward the compressor. Compress the retaining spring and insert the pump rotor into the bearing head. 27. The plungers (flat ends in), plunger springs, spring guides, and snaprings are installed in the plunger cylinders. Compress the snaprings and force them into their grooves. Place a new bearing head gasket and the bearing head into position and bolt them to the crankcase. Install the drive segment. Be careful not to forget the lockwashers (shown in fig. 23). Insert the oil feed guide with the large diameter inward. Place the guide spring so that it fits over the

Figure 22. Suction valve positioning spring. 14

Figure 23. Compressor breakdown. small diameter of the oil feed guide; then install a new pump cover gasket and pump cover. 28. Disassembly, inspection, and assembly of the eccentric shaft and pistons. Remove the oil pump and bearing head previously described. Remove the motor end cover, being careful not to damage the motor windings. Do not allow the cover to drop off. You must support it and lift it off horizontally until it clears the motor windings. Remove the bottom plate and block the eccentric so that it will not turn. Remove the equalizer tube and lock screw assembly from the motor end of the 15 shaft. Look at figure 23 for the location of these components. 29. Pull the rotor out, using a hook through the holes on the rotor. Do not hammer on the motor end of the shaft or rotor since this may cause the eccentric straps or connecting rods to bend. 30. Remove the bolts holding the counterweights and eccentric strap shields onto the eccentric shaft. (Refer to fig. 24 during these procedures.) Remove the eccentric strap side shields and the pump end counterweight through the

Figure 24. Removing counterweights and eccentric strap shields. bearing head opening. The motor end counterweight will hang on the eccentric shaft until the shaft is removed. Pull the eccentric shaft through the bearing head opening. Rotate the shaft, tapping it lightly to prevent the eccentric straps from jamming. Guide the straps off the shaft by hand. The eccentric straps and pistons are removed through the bottom plate opening. 31. The piston pin is locked in place with a lockring. The pin can be removed by tapping lightly on the 16 chamfered end of the pin (the end not having a lockring). 32. Examine the parts to see that they are not worn beyond the limits given in the manufacturer's handbook. To reassemble, follow the disassembly instructions in reverse order. 33. Terminal assembly. Refer to figure 25 for the relative positions of the parts. The washers

Figure 25. Terminal block breakdown. are usually color coded and slightly different in size. Assemble them as shown. 34. The terminal mounting plate assembly is originally installed with a small space left between the outer terminal block and the surface of the mounting plate. This provides further tightening of the terminal bushing in case of a leak. To stop a leak, tighten the terminal block capscrews only enough to stop the leakage 17 of gas. Do not tighten the capscrews so that the terminal block is flush with the mounting plate. If further tightening will cause this situation, the terminal assembly must be replaced. 35. To replace the assembly, pump down the compressor to 2 p.s.i.g. and remove the assembly. Install the new assembly, using the recommended

torque on the capscrews (1.5 ft. lbs.); purge and start the compressor. Avoid excess torque since terminal block and components are generally constructed of plastic or bakelite. 36. Testing capacitors and relay. The starting capacitor used in single-phase units is wired as shown in figure 19. Capacitors are connected in series with one power lead to the motor starting winding. These capacitors may fail because of a short or open circuit. If they are short circuited, the starting current draw will be excessive. The compressor may not start and will cause fuses to blow because of the increased load. If it is connected in a circuit feeding lights, the lights will dim. A humming sound from the compressor motor indicates improper phasing between the starting and running windings caused by an open-circuited capacitor. To check starting capacitors, replace them with good capacitors and observe the operation of the unit. 37. The running capacitors are connected across the running and starting terminals of the compressor. If short circuited, they will allow an excessive current to pass to the start winding continuously. The compressor may not start. If it does, it will be cut off by the motor over-load switch. If they are open, the compressor will operate, but will draw more power than normal when running and will stall on heavy loads. To test for opencircuited capacitors, an ammeter should be connected in series with one power lead. With good running capacitors, the current requirement will be less than it is when the capacitor is disconnected. An open capacitor will cause no change in current draw when it is disconnected. 38. The relay is the potential or voltage type. The contacts are normally closed when there is no power to the unit and open approximately one-fifth of a second after power is applied. The operation of the relay magnetic coil is governed by the voltage through its windings. Upon starting, the counter EMF of the motor builds up, causing a rise in voltage through the relay coil. As the voltage across the coil rises, the magnetic attraction of the relay arm overcomes the spring tension. This causes the arm to move and force the relay contacts open. The starting capacitors, which are in series with the starting winding when the relay contacts are closed, are disconnected from the circuit. 39. If the relay fails with the contacts open, the starting capacitors will not be energized. The compressor motor will hum but will not start. After the power has been on for 5 to 20 seconds, the overload relay will cut off the power to the compressor motor. 40. To check the relay for contacts that fail to close, put a jumper across the relay contacts and turn on the

power. If the unit starts with the jumper, but will not start without it, you must replace the relay. 41. When the relay fails with the contacts closed, the starting capacitors will continue to be energized after the compressor has come up to speed. The compressor will start but will run with a loud grinding hum. The overload relay will shut the compressor off after the compressor has run for a short time due to the extra load of the start winding. This type of relay failure can cause damage to the motor windings and the running capacitor. 42. A visual inspection will determine if relay contacts fail to open. Remove the relay cover and observe its operation. If it does not open after the power has been applied for a few moments, you must replace the relay. 43. Oil safety switch. Many units have oil safety switches which protect the compressor from low or no oil pressure. This control has two circuits-heater and control. 44. This switch measures the difference between oil pump discharge pressure and crankcase pressure. If the net oil pressure drops below the permissible limits, the differential pressure switch energizes the heater circuit which will cause the bimetal switch in the control circuit to open in approximately 1 minute. Low oil pressure may result from the loss of oil, oil pump failure, worn bearings, or excessive refrigerant in the oil. Figure 26 shows a typical oil pressure safety switch. 45. The differential pressure switch is factory calibrated to open when the oil pump discharge pressure is 18 p.s.i.g. greater than the crankcase pressure. It will close when the difference is 11 p.s.i.g. Its adjustment should not be attempted

Figure 26. Oil pressure safety switch.

18

in the field. If the differential pressure switch functions properly and the compressor continues to run after 1 minute, the time-delay heater circuit is defective and the oil pressure safety switch should be replaced. The switch should be checked monthly for correct operation. 46. Troubleshooting. One of your most important responsibilities is the troubleshooting and correction of

malfunctions of these systems. Throughout this chapter we have given basic principles of D/X systems. Using this knowledge and the information that we have provided in tables 1 through 10, you should have little trouble in achieving the desired skill levels.

TABLE 1

TABLE 2

TABLE 3

19

TABLE 4

TABLE 5

TABLE 6

TABLE 7

20

TABLE 8

TABLE 9

TABLE 10

Review Exercises The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the text. Do not submit your answers for grading. 1. There are three things which must be considered before installing a preheat coil. Name them. (Sec. 1, Par. 2) 4. 2. After you have inspected a thermostatically controlled steam preheat coil, you find that the

valve is closed and the outside temperature is 33° F. What is the most probable malfunction, if any? (Sec. 1, Par. 4)

3.

What two functions does a D/X coil serve? (Sec. 1, Par. 7)

What has occurred when a compressor using simple on-off control short cycles? (Sec. 1, Par. 9)

21

5.

What function does the humidistat serve on a two-speed compressor installation? (Sec. 1, Par. 11)

12. Answering a service call, what conclusion would you make from these symptoms? (1) The suction pressure is high.

6.

Why is a nonrestarting relay installed in a solenoid (D/X coil) valve installation? (Sec. 1, Par. 12)

(2) The cooling load is at its peak.

(3) The motor is short cycling on its over load protector. (Sec. 2, Par. 3) 7. A service call is received from Building 1020 with a complaint of no air conditioning. The system uses two D/X coils and two solenoid valves. Which component should you check before troubleshooting the solenoid valve control circuit? (Sec. 1, Par. 14)

13. What would occur if you installed a medium temperature unit for a 40° F suction temperature application? (Sec. 2, Par. 3)

8.

What type compressor must be used when twoposition control of a D/X coil and modulating control of a face and bypass damper are employed to control air temperature? (Sec. 1, Par. 15)

14. What could cause the compressor on an air conditioner to start when the thermostat controlling the liquid solenoid valve is satisfied? Why? (Sec. 2, Par. 4, and fig. 19)

9.

The most probable cause of low supply air temperature and high humidity in an equipment cooling system ____________. (Sec. 1, Par. 18)

15. When may the automatic pump-down feature be omitted? (Sec. 2, Par. 5)

16. Name the four factors you should consider before you install a D/X system. (Sec. 3, Par. 1) 10. How are large swings in relative humidity prevented when face and bypass dampers are used to control dehumidification? (Sec. 1, Par. 20)

17. How can you correct the following situation? Refrigerant is condensing in the compressor crankcase. (Sec. 3, Par. 2)

11. Which control has prime control of the D/X coil if a space thermostat and humidistat are installed in the system? (Sec. 1, Par. 26)

18. Is it necessary to install a condensing unit on a special foundation? Why? (Sec. 3, Par. 3)

22

19. What is the minimum and maximum voltages that can be supplied to a 220-volt unit? (Sec. 3, Par. 5)

27. Why is it temperature dehydrating (Sec. 3, Par.

important to keep the ambient above 60° F. when you are a system with a vacuum pump? 17)

20. How much phase unbalance is tolerable between phases of a three-phase installation? (Sec. 3, Par. 5)

28. What pressure corresponds to a vacuum indicator reading of 45° F.? (Sec. 3, Par. 18, and fig. 17)

21. During gauge installation, in which position is the shutoff valve set and why? (Sec. 3, Par. 9)

29. Why are shutoff valves installed in the vacuum pump suction line? (Sec. 3, Par. 20)

22. Where would you install a liquid line sight glass in the system? (Sec. 3, Par. 12)

30. The type of moisture that is first removed from a refrigeration system is _____________ moisture. (Sec. 3, Par. 22)

23. When city water is used as the condensing medium, the condenser circuits are connected in ______________. (Sec. 3, Par. 14)

31. Why do you have to backseat the suction and discharge shutoff valves before you connect the gauge manifold? (Sec. 3, Par. 25)

24. When cooling tower water is used, condenser circuits are connected _________________. (Sec. 3, Par. 14)

the in

32. What four items must be checked before you start a newly installed compressor? (Sec. 3, Par. 26)

33. How does frontseating the suction shutoff valve affect the low-pressure control? (Sec. 3, Par. 34) 25. Which types of gases may be used to pressurize the system for leak testing? (Sec. 3, Par. 15) 34. Why do you place the refrigerant cylinder in ice when you want to evacuate all the refrigerant from a system? (Sec. 4, Par. 3) 26. After you have disassembled a compressor, you find an excessive amount of sludge in the crankcase. What caused this sludge? (Sec 3, Par. 16)

35. Why is a partial pressure, 2 p.s.i.g., allowed to remain in the system after pumpdown? (Sec. 4, Par. 4)

23

36. Why should you allow sufficient time for a component to warm to room temperature before removing it from the system? (Sec. 4, Par. 6)

42. What is the emergency procedure that you can use to recondition worn compressor valves? (Sec. 4, Par. 21)

37. The two types of suction strainers are _______________ and ________________ (Sec. 4, Par. 9)

43. How is the oil feed guide installed? (Sec. 4, Par. 27)

38. Where do noncondensable gases collect in a water-cooled refrigerating system? (Sec. 4, Par. 10)

44. Why should you use a hook device rather than a hammer to remove the rotor? (Sec. 4, Par. 29)

39. What condition most probably exists when the following symptoms are indicated? (1) Excessive amperage draw.

45. (Agree)(Disagree) The terminal block is tightened flush with the mounting plate. (Sec. 4, Par. 34)

(2) The condenser water temperature is normal.

46. The amount of torque required when tightening the capscrews on a terminal block is _______________. (Sec. 4, Par. 35)

(3) The discharge temperature, felt by hand at the compressor discharge line, is above normal. (Sec. 4, Par. 10)

40. What would a discharge pressure drop of 10 p.s.i.g. per minute with the discharge shutoff valve frontseated indicate? (Sec. 4, Par. 15)

47. The following complaint concerning an inoperative air conditioner is submitted to the shop: the air conditioner keeps blowing fuses when it tries to start. After troubleshooting the unit you find that the starting current draw is above normal. Which component should you check and what should you check it for (Sec. 4, Par. 36)

41. How are valve plates removed from cylinder decks? (Sec. 4, Par. 18)

48. What will cause a humming sound from the compressor motor? (Sec. 4, Par. 36)

49. The contacts of the starting relay are normally _________________. (Sec. 4, Par. 38)

24

50. What causes the contacts of the starting relay to open? (Sec. 4, Par. 38)

57. A loose feeler bulb for a thermostatic expansion valve will cause an abnormally cold suction line. Why is the line cold? (Sec. 4, table 5)

51. Which type of relay failure can cause damage to the motor windings? (Sec. 4, Par. 41) 58. A hissing expansion valve indicates _____________________. (Sec. 4, table 6) 52. The two circuits that make up the oil safety switch are _______________ and ______________. (Sec. 4, Par. 43) 59. Too much superheat will cause ____________________. (Sec. 4, table 6) 53. The pressure which cause the oil safety switch to operate are ________________ and ________________ (Sec. 4, Par. 44) 60. During a routine inspection, you find the watercooled condenser exceptionally hot. What are the most probable faults and how should you correct them? (Sec. 4, table 7)

54. (Agree)(Disagree) The differential pressure switch in the oil safety switch will open when the pressure differential drops. (Sec. 4, Par. 45)

55. What can cause an inoperative motor starter? (Sec. 4, table 1)

61. A low suction pressure and loss of system capacity indicates __________________. (Sec. 4, table 10)

56. What should you suspect when the dehydrator is frosted and the suction pressure is below normal? (Sec. 4, table 2)

62. How would you correct this fault: A capacity controlled compressor short cycling? (Sec. 4, table 10)

25

CHAPTER 2

Absorption Systems HOW ABSURD IT is to use water as a refrigerant; yet absorption systems do. You know that this can be done only under specific conditions. Within a deep vacuum, water will boil (vaporize) at a very low temperature. For example, when a vacuum of 29.99 inches is obtained, the water will boil at approximately 40° Fahrenheit. Hence, vacuum is the key to absorption air conditioning. 2. The absorption system is one of the simplest of all types of automatic air-conditioning systems. Though this machine has few moving parts, it has an immense cooling capacity. We shall discuss in this chapter terminology, identification, and function of unit components; starting and operating procedures; and maintenance of the absorption system. 5. Terminology, Identification, and Function of Units 1. The complete absorption refrigeration unit contains a generator, a condenser, an absorber, and an evaporator. The condenser and generator are combined in the upper shell of the machine, while the evaporator and absorber are combined in the lower shell, as shown in figure 27. 2. The heat exchanger, purge unit, solution pump, and evaporating pump are mounted between the support legs of the unit. The purge unit is used to remove noncondensables from the machine. The capacity control valve controls the water leaving the condenser. This valve is controlled thermostatically by a remote bulb placed in the chilled water line. 3. Figure 28 is a simple block diagram of the absorption refrigeration cycle. The refrigerant used is common tap water and the absorbent is a special salt, lithium bromide. 4. To understand the operation of the refrigeration cycle, consider two self-contained vessels: one containing the salt solution (absorber) and the other (evaporator) containing water, joined together as shown in item 1 of figure 28. Ordinary table salt absorbs water vapor when it is exposed to damp weather. The salt solution in the absorber has a much greater ability to absorb the water vapor from the evaporator. The water in the evaporator boiling at a low temperature does the same job as refrigerants R-12, R-13, and R-22. As the water vaporizes, the water vapor travels from the evaporator to the absorber, where it is absorbed into the salt solution. The evaporator pump, shown in item 2 of figure 28, circulates water from the evaporator tank to a spray header to wet the surface of the coil. The cooling effect of the spray boiling at approximately 40° F. on the coil surface chills the water inside the coil, and this chilled water is

Figure 27. Absorption unit components.

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Figure 28. Absorption refrigeration cycle. circulated in a closed cycle to the cooling coils. This refrigeration effect is known as flash cooling. 5. In reference to item 3 of figure 28, note the addition of the generator and accessory equipment. These components are necessary for continuous and efficient operation. The salt solution would become diluted and the action stopped if it were not for the regeneration of the salt solution. To keep the salt solution in the absorber at its proper strength so that it will have the ability to absorb water, the salt solution is pumped to a generator where heat is used to raise its temperature and boil off the excess water. The salt concentrate is then returned to the absorber to continue its cycle. The water that is boiled off from the salt solution in the generator is condensed in the condenser and returned to the evaporator as shown in item 4 of

figure 28. The heat exchanger uses a hot solution from the generator to preheat the diluted solution. This raises the overall efficiency because less heat will be required to bring the diluted solution to a boil. Condensing water, which is circulated through the coils of the absorber and the condenser, removes waste heat from the unit. By comparing figure 29 with figure 27, you will get a better understanding of the relation between basic operating principles and an actual installation. 6. Controls. Figure 30 illustrates a typical control panel for an absorption refrigeration unit. The purpose of each control listed in this figure is described in the following paragraphs. Turning the off-run-start switch (1) the START position energizes the electric pneumatic switch (2), which activates the control system of the absorption machine. Supply air pressure of 15 p.s.i.g. (3) passes to the chilled water thermostat (4), then to the concentration limit thermostat (5), and finally to the capacity control valve (7). 7. The chilled water thermostat (4) is a direct acting control with a 7° F. differential. For every degree change in the chilled water temperature, there is approximately a 2-pound change in its branch line air pressure. Its thermal element is located in the leaving chilled water line. As the leaving chilled water temperature drops below the control setting of the thermostat, the supply air pressure (3) is throttled, causing the capacity control valve (7) to throttle the condenser water quantity. With a constant load on the machine, the capacity control valve throttles just enough condensing water to balance the load. 8. The concentration limit thermostat (5) is a direct acting bleed type control, with the thermal element located in the vapor condensate well. Its purpose is to prevent the solution from concentrating beyond the point where solidification results. At startup, the capacity control valve (7) is closed and remains closed until the vapor condensate well temperature rises above the control point of the concentration limit thermostat. As it does, the thermostat begins to throttle the air bleeding to the atmosphere, thus raising the branch line pressure (6) and opening the capacity control valve. This control valve on some absorption models may be controlled electrically instead of pneumatically. 9. Safety controls. Two safety controls are usually used in the control systems. They are the chilled water safety thermostat and the solution pressurestat. In moist instances, any malfunction occurring during operation is immediately reflected by a rise in the chilled water temperature. The thermal element of the chilled water safety

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Figure 29. Absorption refrigeration cycle. thermostat is located in the chilled water line leaving the machine. The control point is set approximately 10° F. above the design leaving chilled water temperature. A temperature rise above the control point shuts off the air supply. All control lines are then bled and the system is shut down. When the off-run-start switch is in the START position, this control is bypassed. The switch should not be placed in the RUN position until after you obtain a chilled water temperature below the control setting. 10. The solution pressurestat located in the 28

Figure 30. Control panel. discharge line of the solution pump is set to cut in on a rising pressure at 40 p.s.i.g. and cut out on a falling pressure at 30 p.s.i.g. If for any reason the discharge pressure falls below the control point, the system will be shut down in the same manner as described above. 11. Special control. Special chilled water controllers may be installed in the field for special applications. These controls are used to maintain the chilled water temperatures within a plus or minus 2° F. Explosionproof controls and motor are installed for special applications. Refer to the manufacturer's manual on the operation and maintenance of these controls and motors. 12. Thermometers. Thermometers are installed in several locations in the system. Below is a general listing of thermometer locations and their purposes: (1) Chilled water piping to indicate the entering chilled water temperature. (2) Chilled water pump suction piping to indicate leaving chilled water temperature. 29 (3) Condensing water piping entering the absorber section. (4) Condensing water piping leaving the absorber section. For proper temperature measurements, the thermometer is located in the generator bypass line. (5) Condensing water piping leaving the condenser section. (6) Condensing water piping to indicate the total condensing water temperature to the cooling tower or drain. 13. Pressure Gauges. Pressure gauges are installed in several locations in the system. The following is a general listing of gauge locations: (1) Purge water line after the strainer and before the purge water jet. (2) Purge water line after the jet. (3) Steam line before the generator section. (4) Discharge line from the chilled water pump.

(5) Discharge line from the condenser water pump. 14. Water Seals. Older models of absorption machines require mechanical seals on the solution and evaporator pumps. However, the newer machines have hermetically sealed pumps that eliminate the need for mechanical seals. The older models require external water seals; therefore, it is necessary to supply a water seal tank to maintain water on the seals for lubrication purposes and so that water rather than air leaks into the machine in case the seals break or leak. 15. The water seal tank has a float control to limit the quantity of water to the seals when the machine is in operation. The operator must open the manual valve supplying the seal water tank before startup and must close the manual valve on shutdown. This is the standard method of control. The alternate method is one where a check valve is installed in the supply line to the tank, as well as an antisyphon vacuum breaker. When the machine is shut down a visual check can be made to determine the condition of the seal and to prevent a large quantity of water from leaking into the machine if the seal is worn or cracked. If mechanical seals have to be replaced, the manufacturer's instructions must be carefully followed in order to do the job correctly and prevent the new seals from leaking. During operation, the evaporator pump makes up for the water lost by a seal; but during shutdown, it is possible to lose a large amount of water from the tank if a large leak exists. Therefore, leaky seals must be replaced immediately. Having learned the importance of water seals in the absorption system, we can now discuss the starting procedures. 6. Starting Procedures 1. Some absorption systems are completely automatic and can be started by simply pushing a start button, while in other systems the machine is automatic but the auxiliary equipment is manually operated. The type of startup determines the starting procedure. Therefore, each starting procedure is outlined separately, and the machine operator can perform the starting operations applicable to the type of startup required. Even though some systems are automatic, it would be advisable to check the system as described below before starting the unit. 2. Daily Startup. Use the following steps in performing a normal startup. (1) Check vacuum in machine (see Maintenance, Section 8). (2) Check mechanical seals for leakage (see Maintenance, Section 8).

(3) Check water level in evaporator sight glass. (4) Check absorber section for presence of water. (5) Start condensing water pump. (6) Check temperature of condensing water going to machine. Do not start cooling tower fan until the condenser water it has warmed up to the recommended setting. (7) Start the purge unit. • Push start button on the purge control panel. • Open purge steam supply valve. • Check the standpipe for water seal circulation before starting the pumps. (8) Start the chilled water pump and open the valves to insure circulation through the evaporator tubes and airconditioning equipment. (9) Start the refrigerant pump and open the valve in the refrigerant pump discharge line. (10) Start the purging machine. Open the absorber purge valve located in the purge line to the absorber. The generator purge valve located in the purge line between the absorber and generator must be open. (11) Wait until the machine is completely purged. There will be a substantial drop in the leaving chilled water temperature when the machine is completely purged. If the leaving chilled water temperature does not drop and there are no leaks in the machine, then the steam jets should be cleaned. (12) Open the main steam valve to the machine. (13) Check steam pressure supply to see that it is within the proper range. (14) Place the control panel switch in the START position. (15) Check the main air supply pressure gauge to insure that 15 p.s.i.g. is supplied to the control panel. (16) Start solution pump. Be sure the strong solution return valve is open at all times. (17) When the leaving chilled water temperature has dropped below the safety thermostat setting, move the control panel switch from START to RUN. 3. Startup After Standby Shutdown. This procedure is basically the same as for daily startup. There are, however, additional preparation steps that must first be performed in order to put the machine in operational condition for startup. In order to prepare the machine for startup, the nitrogen with which the machine has been charged must be removed and a vacuum pulled on the machine. This is done by operating the purge unit until the machine has been purged of nitrogen and a satisfactory vacuum reading attained.

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4. Startup After Extended Shutdown: This procedure is basically the same as for daily startup except for the additional preparation steps that must first be performed to put the machine in operational condition for startup. The preparations necessary after extended shutdown are similar to an initial startup of a new machine. The complete system must be prepared for operation in these steps: (1) Check all drains that should be closed in the chilled water and condensing water circuits. (2) Fill the condensing water circuit. (3) Start the purge unit to remove all air and nitrogen from the machine. (4) Fill the primary and secondary chilled water circuits. (5) Purge the chilled water circuit of air. • Start the chilled water pump. • Open the diaphragm valve in the chilled water pump discharge line. • Open the diaphragm valve in the chilled water return line to the machine and continue purging until the recommended vacuum is obtained. (6) Purge the refrigerant circuit. Do not start the refrigerant pump until chilled water is circulating through the evaporator tubes. • Start the refrigerant pump. • Open the valve in the refrigerant pump discharge line and allow the refrigerant to circulate until the recommended vacuum is obtained on the machine. (7) Shut down the purge unit. (8) Shut down the primary chilled water circuit. • Close the diaphragm valve in the primary chilled water pump discharge line. • Shut off the primary chilled water pump. The machine is now in operational order and ready for instant startup. The procedures for daily startup should now be followed to place the machine in operation. 7. Operating Procedures 1. You must make periodic checks on the machine while it is in operation and keep a daily operating log. Compare observations with the following recommended operating conditions and make any necessary adjustments. 2. Evaporator, Absorber, and Generator Levels. As an operator you will have to visually check the sight glasses on the evaporator, absorber, and generator. 3. Evaporator sight glass water level. The normal operating evaporator tank water level is approximately 1 inch above the horizontal centerline. At a high level, the chilled water may spill over the evaporator tank into the

solution in the absorber, causing a loss of operating efficiency. A low level will cause the chilled water pump to cavitate (surge). 4. Solution level in absorber. Normal operating level is approximately one-third of the absorber sight glass at full load operation. At partial load operation, the solution level will vary between one-third and two-thirds of the sight glass. The solution level may require adjustment when the leaving chilled water temperature is changed, which is done by manually adjusting the chilled water thermostat. If the setting is lowered, the solution level will drop and solution must be added. If the setting is raised, the solution level will rise and solution must be removed from the machine. Operating instructions for the specific machine should be followed in adjusting the solution level. 5. Solution boiling level in generator. The solution boiling level is set at initial startup of the machine and should not vary during operation. The boiling level can be checked by looking into the mirror near the generator bull's-eye. A light should be visible at all times. If the light is obscured, the boiling level is too high and should be adjusted. A temporary measure is to adjust the solution flow by throttling the generator flow valve in the line to the generator. For more detailed procedures, consult the service bulletin for your machine on how to check high boiling. 6. Purging. Proper purging is necessary to obtain and maintain a vacuum on an absorption system. 7. Purge operation. Water pressure, steam pressure, and water temperature must be within recommended limits to insure satisfactory operation. The steam supplied to the jets must be dry. Operate the jets with the bleed petcock open at all times. When jets are operating properly, the first stage will run hot, the second stage warm or cool. When air is being handled, the second stage will tend to get hot. Wet steam will cause the first stage diffuser to run cold. If too wet, the purge system will not operate. Check the circulation of seal water through the seal chambers. If water is circulating through the seal chambers, there will be an overflow of water from the standpipe. If the purge unit stops because of salinity indicator operation, you must immediately close the machine purge valve. Shut off the steam supply to the steam jets and open the reset switch to shut off the alarm. If lithium bromide should pass into the purge water tank, the water should be drained and the tank flushed; also flush the steam jets and condenser. Clean water can be introduced in the pressure tap between the purge valve and the first stage of the purge unit. Resume normal operation by filling

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Figure 31. Jet purge unit. the tank, bleeding the pump, and closing the reset switch. 8. Jet purge. On some systems, the jet purge, shown in figure 31, has been adapted to the unit. It is entirely automatic and provides a source of very low pressure which is capable of removing noncondensables from the machine when required. Since noncondensables travel from high-pressure regions to low-pressure regions-generator, condenser, evaporator, absorber--the purge suction tube is located in the lower 32 section of the absorber. The jet purge system is made up of the following components: (1) Purge tank (12-gallon capacity). (2) Purge pump (submersible). (3) Jet evacuator (operates on the venturi principle). (4) Purge valve (usually operated by a hydromotor). (5) Adjustable drip tube (keeps solution in purge tank at 53 percent).

(6) Purge cooling coil (keeps purge solution at a low temperature). (7) Four-probe level controller (shortest probe and longest probe are safety controls). (8) Generator purge line (allows purging of the generator during operation). (9) Purge control switches (auto-manual, auto-off located in the control panel or center). (10) Purge alarm light (in control panel or center to indicate high or low level). Proper purging of the system is useless unless you maintain the recommended maximum steam pressure. 9. Machine Supply Steam Pressure. The maximum steam pressure at the generator should never exceed the manufacturer's specifications. Excessive steam pressures may cause the solution to solidify and make it necessary to shut down the machine. 10. Solution Solidification. Excessive steam pressure is not the only possible cause of solution solidification. Entering condensing water at too low a temperature, an excessive air leak, improperly adjusted controls, or power failure shutting the machine off so that it cannot go through a dilution cycle may also cause this difficulty. Solidification will cause the machine to stop, but there will be no permanent damage to the machine. After the solution is desolidified, the machine may be placed back in operation, but the cause of the difficulty should be corrected. 11. A steam desolidification line is encased in the solution heat exchanger of the machine. The procedure for desolidification outlined below should be followed step by step: (1) Close the absorber purge valve and the purge steam supply valve. This will isolate the machine from the purge unit and prevent air from entering the machine. (2) Shut off the condensing water pump but leave the main steam supply valve open. This allows the solution to heat without vapor being condensed in the condenser. (3) Open the manual dilution valve which will allow chilled water to enter the solution circuit and dilute the solution. (4) Open the steam supply valve and steam condensate return valve in the desolidification line. (5) Start the solution pump and pump the solution up to the generator; close the generator flow valve. Allow the solution to heat up in the generator; then open the generator flow valve and allow the solution to drain back to the absorber. As it begins to liquefy, the solution will start to flow. This process may have to be repeated several times before the solution has liquefied enough to permit the circulation.

(6) Put the machine back into operation by starting the condensing water pump and purge unit. (7) The reason for solidification should be determined and corrected. You have completed desolidification and have the absorption system operating properly. Let us now discuss shutdown procedures. 12. Shutdowns. Each shutdown--daily, standby, and extended-requires proper “off” sequencing of the system components to avoid damage to the machine and to keep the lithium bromide from solidifying. 13. Daily shutdowns. To stop a completely automatic system you must push the stop button. This will automatically close the capacity control valve and purge valve. All other components will operate for approximately 7 minutes after this short period, the machine will shut down automatically. The following procedure is recommended for daily shutdown on automatic machines with manual auxiliaries: (1) Move the start switch to the OFF position. (2) Shut down the purge unit. • Close the absorber purge valve. • Close the purge steam supply valve. • Push the stop button on the purge control panel to stop the purge pump. (3) Dilute the solution sufficiently to prevent solidification during shutdown. • Open the manual dilution valve for the proper length of time. The time will range from approximately 2 to 5 minutes and must be determined by experience for each machine. • Close the manual dilution valve after the proper interval. This valve must not be left unattended during the dilution period since too long an interval will weaken the solution and lengthen the recovery period when the machine is placed back in operation. (4) Shut down the refrigerant and chilled water circuit • Shut down the refrigerant water pump. • Close the valve in the refrigerant pump discharge line. • Shut down the secondary chilled water pump. (5) Shut down the condensing water circuit. • Shut down the condensing water pump. • Shut down other auxiliaries in this system such as cooling tower, cooling tower fan, and auxiliary valves. (6) Close the main steam supply valve to shut off the steam to the machine. (7) Shut down the solution pump. After the solution has drained from the generator back to

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the absorber, the solution circuit will be ready for startup. It is not necessary to close either of the solution valves. 14. Standby shutdown. This type of shutdown is used at an installation where it is not necessary to use the machine for cooling at irregular intervals during the winter or off-cooling seasons. This procedure does not apply if freeing temperatures are expected in the machine room. The procedure is the same for daily shutdown except for the following two steps: (1) Dilution should be sufficient to insure that solidification of the solution will not take place at the lowest temperatures expected in the machine room. (2) The final step in the procedure is to charge the machine with nitrogen. • Connect the nitrogen tank to the nitrogen charging valve. On some systems, the alcohol charging valve is used as the connection for charging nitrogen into the system. • Set the pressure-reducing valve on the nitrogen tank to 18 p.s.i.g. This is the maximum allowable pressure that may be used on the machine. Higher pressures will cause leakage at the pump seals. • Open the nitrogen valve on the nitrogen tank and allow the nitrogen to enter the machine. Observe the pressure on the solution pump discharge gauge. When this gauge reads 3 to 5 p.s.i.g., close the nitrogen valve and remove the nitrogen charging line. 15. Extended shutdown. When the machine is to be placed out of service for an extended length of time, as during the winter, there are many special services which may be required to protect the equipment from freezing temperatures. The procedures are the same as for daily shutdown except for the following additional services: (1) The solution must be diluted enough to insure against solidification at the lowest expected temperatures in the machine room. To do this, put the machine through three dilution cycles before it is shut down. (2) Store the solution in the generator by closing the strong valve and running the solution pump until the solution is pumped from the absorber into the generator. Then close the diluted solution valve before shutting off the solution pump. (3) The machine is charged with nitrogen to prevent air from getting into the machine as outlined in the procedure for standby shutdown. (4) Drain all the chilled water from the machine and other equipment. Leave all the drains open: except the one from the machine proper. (5) Drain all the condensing water from the machine and other equipment and leave the drains open.

(6) Drain the water from the purge condenser shell by opening the drain connection on the bottom of the purge condenser. (7) Drain all the water from the purge condenser coil by removing the tubing between the water jet piping and purge condenser coil. (8) Drain all the water out of the seal tank by opening the drain connection in the bottom of the water seal tank. (9) Drain all the water out of the water sea lines and the pump seal chambers by opening the petcock located in the line in the bottom of the pump seal chambers. (10) Drain all the steam traps and steam drop legs. 16. Most maintenance is performed while the system is shut down. Let us now discuss maintenance of absorption air-conditioning systems. 8. Maintenance 1. The maintenance procedures listed in this section are carried out at time intervals listed in the manufacturers' service manuals. We will not set any time interval because it varies with equipment models, and your particular SOP will outline this information. We will discuss annual maintenance because most manufacturers' handbooks list the same tasks to be performed at that time. 2. Checking Vacuum. Before starting the machine, you should check it to see if air has leaked into the unit while it was shut down. Open the valve in the line from the absorber to the manometer and determine the pressure in the machine. Figure 32 illustrates a manometer reading. Take the temperature of the machine room and locate the corresponding pressure on the chart in figure 33. If the pressure reading in the machine is more than 0.1 inch of mercury higher than the pressure located on the curve, then there is air in the machine. This should be noted on the daily log sheet. If the condition recurs on the next two or three startups, the machine should be shut down as soon as possible and tested for leaks. Air leakage will cause corrosion inside the machine, and over a period of time will result in serious trouble and shorten the life of the equipment. 3. Checking Mechanical Pump Seals. The mechanical pump seals, as shown in figure 34, should be checked for leakage before starting the machine. Close the petcocks in the water lines to the pump seal chambers. Observe the readings of the compound pressure gauges in the water lines between the petcock and the pump seal chambers. If the gauge shows a vacuum, this is

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Figure 32. Absorber manometer. an indication of a leaking seal. If only a small amount of seal water has been lost, the leak is small and the machine may be placed in operation; but the seal should be replaced at the first opportunity. If a large amount of seal water has been lost, then the seal should be replaced before the unit is put into operation. 4. Flushing Seal Chamber. Flushing the seal chamber is recommended for lengthening the life of the seals. Approximately 15 minutes after the machine is stated and the solution has concentrated, drain approximately 1 quart of water out of each seal chamber by use of drain petcocks located on each chamber. This is necessary to prevent the buildup of solution concentration in the chamber by the solution that may leak past the seal faces. Make sure that the drain water is replaced, since continually draining water would result in a loss of evaporator water. 5. Checking Water in Evaporator Sight Glass. Before starting the machine, the water level in the evaporator sight glass should be checked. If the water level is the same as when the machine was shut down, the condition indicates that there is no leakage. If the level is higher, then chilled water has leaked back into the machine. The machine should not be started under these conditions, since it is possible to lose the solution charge. Consult the instructions for the machine to cover this situation. 6. Checking Absorber for Presence of Water. Turn on the light at the absorber bull's-eye. Look into the absorber section through the inspection hole opposite

the light. No water should be visible. If water is visible it has leaked into the section from the chilled water or seal water system. Under these conditions, the machine should not be started since it is possible to lose the solution charge. Consult the instructions for the machine to cover this situation. 7. Adding Octyl Alcohol to Solution. Once a week, about 6 ounces of octyl alcohol should be added to the solution circuit while the machine is running. This cleans the outside of the tubes in the generator and absorber and improves their efficiency in transferring heat. The procedure is as follows: (1) Pour about 8 ounces of octyl alcohol in a glass container. (2) Hold the container under the alcohol charging connection as shown in figure 35. The end of the charging connection must be kept close to the bottom to prevent air from entering the machine. (3) Slowly open the charging valve and observe the alcohol level as it is drawn into the machine. Close the valve quickly so that the level of liquid remains above the end of intake tube to prevent air from entering the machine. 8. If the alcohol is drawn rapidly into the charging connection, it indicates that the conical strainer and solution spray header are clean. A progressive decrease in the rate at which alcohol is drawn shows that these units are becoming clogged. If alcohol is not drawn into the charging connection, it is an indication that the conical strainer is clogged. In this case, the conical strainer should be removed and cleaned at the next shutdown. If the condition still persists, it will be necessary to remove and clean the solution spray header. 9. Cleaning Purge Steam Jet. This is an important part of the maintenance since the purge unit must be kept in good operating condition to maintain efficiency of the machine. The following procedures will apply to both single- and two-stage steam jets: (1) Check to be sure that the absorber purge valve (item 1 in fig. 36) is closed. (2) Close the purge steam supply valve (item 2). (3) Remove the steam jet cap. (4) Use a piece of thin wire through the top of the steam jet to loosen any dirt in the nozzle. (5) Open the purge steam supply valve to blow out loosened dirt and then close the valve. (6) Replace the steam jet cap. 10. Checking Evaporator Water Circuit for Lithium Bromide. While the quantity of solution does not formally change, a high boiling level in the generator may force solution into the evaporator water circuit. A solution test kit must be

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Figure 33. Pressure and temperature curve.

Figure 34. Seal water system. 36

Figure 35. Octyl alcohol charging.

used to detect and measure the percentage of lithium bromide in the evaporator water. The test kit contains three bottles labeled No. 1, No. 2, and No. 3. No. 1 contains an indicator solution, No. 2 contains silver nitrate, and No. 3 is a standard solution of lithium bromide. The test kit is used as follows: (1) Place ten drops of evaporator water sample from the system into a clean bottle. (2) Add three drops of No. 1 to the sample. (3) Count the number of drops of No. 2 solution necessary to turn the sample a permanent red. Record the number of drops of No. 2 used. (4) Place ten drops of the standard solution, No. 3, in a clean bottle. (5) Add three drops of No. 1 solution. (6) Count the number of drops of No. 2 necessary to give a permanent red. Record the number of drops. 11. The standard sample of lithium bromide is a 1-percent solution. By comparing the number of drops of solution No. 2 required to turn the

Figure 36. Steam jet purge. 37

Figure 37. Manual dilution. evaporator water sample red with the number of drops required to turn the standard red, the percentage of lithium bromide can be determined. If the evaporator water sample requires less of No. 2 than the standard, then it contains less than 1 percent of lithium bromide. If the test shows the lithium bromide content of the evaporator water to be greater than 1 percent, it should be reclaimed. 12. Reclaiming Solution. The lithium bromide is reclaimed by passing evaporator water into the solution circuit while the machine is in operation. The length of time required for reclamation will be determined by the amount of salt in the evaporator water circuit. The capacity of the machine will be partly reduced during this period. The process should be continued until the test shows less than one-half of 1 percent. The procedure for reclaiming solution is as follows: (1) Crack the manual dilution valve, item A in figure 37, and feed water slowly into the solution circuit. (2) Check the boiling level through the generator bull's-eye sight glass. If the light cannot be seen, the boiling level is too high. Bring the boiling level down by slightly closing the dilution valve until the light can be seen. (3) Continue the process until the test shows less than one-half of 1 percent. This may take anywhere from a few hours to several days, depending on the amount of salt in the evaporator water circuit. 13. Annual Maintenance. Before annual maintenance is started, the machine should be shut down and charged with nitrogen as outlined in the procedure for extended shutdown. The following paragraphs are arranged in the same sequence as the work would normally be performed.

14. Cleaning lithium bromide solution. To clean the lithium bromide solution, it must be removed from the machine as follows: (1) Open the valves in the solution line to and from the generator; this will drain the solution into the absorber section. (2) Attach a suitable rubber hose to the discharge connection of the solution pump. (3) Close the valves in the solution to and from the generator and close the valve in the vapor condensate return line. This isolates the generator from the absorber. (4) Start the solution pump and pump the solution into drums. The pump should shut off automatically when the absorber is empty. (5) Remove the plug in the solution inductor to drain the piping below the absorber. NOTE: The solution in the drums should be allowed to stand for 2 or 3 days to allow the dirt to settle. 15. Cleaning absorber sight glass. The bull's-eye sight, evaporator tank sight and absorber reflex glasses should be carefully removed and cleaned. Cracked glasses or those with collected foreign matter that cannot be cleaned should be replaced. New gaskets should be used when the glasses are reinstalled. 16. Cleaning solution strainer. The procedure for removing and cleaning the conical solution strainer is as follows: (1) Remove the nuts holding the solution supply header. (2) Remove the nuts and bolts in the flange connection to the solution piping. (3) Remove the solution supply header. Figure 38 illustrates the solution supply header. (4) Remove the bolts holding the blank flange on the solution supply header. (5) Carefully remove the strainer and clean it by flushing it with water. (6) Replace the strainer and use a new gasket under the blank flange. Be sure that the flange faces are clean so that the flange will seal properly when bolted back in place. Do not replace the supply header until the spray header has been removed and cleaned. 17. Cleaning solution spray header. If the supply header has not been removed, proceed with

Figure 38. Solution supply header.

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Figure 39. Solution sprayheader. steps (1), (2), and (3) in the preceding paragraph. When this has been done, remove the solution spray header, being extremely careful that the spray nozzles do not strike the sides of the opening. Clean it by flushing. Any nozzles that are not clear should have the nozzle cap removed and cleaned individually. The old gasket material should be thoroughly removed from the spray header and a new gasket used when it is ready for reassembly. A solution spray header is shown in figure 39. 18. To install the spray header, slide it back through the opening until it is 2 inches from the far end. Remove the plug from the absorber at the end opposite to the header opening. Insert a rod through this hole and lift the end of the spray header so as to guide it through the last couple of inches into its proper position. Install the supply header, using a new gasket. Replace the plug in the far end of the absorber. 19. Cleaning chilled water spray header. The chilled water spray header is removed and cleaned by the same procedure as just outlined. However, even more care must be exercised in handling this header since it is possible not only to damage the nozzles but, if the header is allowed to tip, the eliminator plates may be bent. These are thin plates like the fins in an automobile radiator which when bent will lose their effectiveness. Clean off the old gasket material and install a new gasket before replacing the header on the machine. A plug must be removed from the opposite end as before so that a guide rod can be used on the far end of the header. 20. Cleaning primary purge connections on absorber. Clean the primary purge connections on the absorber by removing the plugs in the T connections at the primary purge line and cleaning the line with a wire or nylon brush. Only a small amount of water should be used to wash out this area. Replace the plugs after the cleaning operation is completed. 21. Inspection of vacuum type valves. All vacuum type valves should have their bonnets removed and the diaphragms checked for cracks or signs of wear which might indicate a future failure. Following is a list of the different vacuum type valves used in an absorption system:

Purge valves Solution valve Manual dilution valve Chilled water makeup valve Vapor condensate return valve Absorber manometer valve Solution charging valve Chilled water valves 22. Although proper service will cause some diaphragms to last longer, it is considered good maintenance practice to replace all diaphragms every 2 years. This helps to prevent a breakdown or an interruption of service during the cooling seasons. 23. Checking generator sight glasses. The generator overflow sight glass and the generator bull's-eye sight glasses should be removed and cleaned. Glasses that are damaged should be replaced. New gaskets should always be used when the glasses are reinstalled. 24. Cleaning water seal system. The entire water seal system should be inspected and cleaned according to the following procedure: (1) Open the drain connections on the bottom of the water seal tank and drain the water. (2) Open the petcocks on the bottom of the pump seal chambers and drain the water from the lines and chambers. (3) Disconnect the water seal lines between the water seal tank and the pump seal chambers and clean them by reverse flushing with water. Use compressed air to blow out the lines after flushing. (4) Inspect and clean all the pipe connections. (5) Clean the purge tank and flush it to remove the loosened dirt. (6) Reinstall the water seal lines. 25. Cleaning absorber and condenser tubes. The absorber and condenser tubes should be cleaned at least once a year. More frequent cleaning may be necessary as indicated by a steady rise in vapor condensate temperature during the season. A steady decrease in temperature of the condensing water leaving the machine may also indicate scaling. This condition may be further confirmed by inspection of the thermometer well in the condenser water line leaving the machine. The presence of scale here is associated with scaling in the tubes. Cleaning should be done as follows: (1) Remove both headers from the absorber and condenser. (2) Inspect the tubes to determine the type of scale. (3) Soft scale may be removed by cleaning with a nylon bristle brush. Metal brushes of any kind which might scratch the surface must never be used. Hard scale which cannot be removed

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with a brush will require treatment with suitable chemicals. 26. Cleaning condensing water system. The procedures for cleaning the condensing water system of an absorption refrigeration system are similar to the procedures used to clean the condensing water systems on compression refrigeration systems. 27. Cleaning salinity indicator. The salinity indicator should be removed and the electrodes cleaned of accumulated deposits. 28. Vacuum Testing Machine for Leaks. After completing the maintenance work, the machine should next be tested for leaks according to the following procedures. (1) Close all valves except the vapor condensate valve which must be left open. (2) Start the water jet on the purge unit and open the absorber purge valve and the evacuation valve. Operate the purge unit until a vacuum of at least 25 inches of mercury is read on the manometer. Record this reading. (3) Shut down water jet and close the valves. (4) Check the manometer vacuum 24 hours later. The maximum allowable loss is one-tenth of an inch of mercury in 24 hours. If the loss is within limits, then charge the machine with solution. A machine that does not meet the vacuum requirements must be tested for leaks with a halide leak detector. 29. Halide Leak Detector Test. The procedures for testing with the halide leak detector are as follows: (1) Make sure that all valves are closed except the vapor condensate valve. (2) Charge the machine with Refrigerant-12 to a pressure of 5 p.s.i.g. Use the refrigerant type charging valve on the absorber. Read the charging pressure in the machine on the solution pump pressure gauge. (3) After charging with Refrigerant-12, the machine should be further charged to 18 p.s.i.g. with nitrogen, using the procedure previously given under Extended shutdown. (4) Test the machine for leaks with a halide leak detector. Stop all the leaks that are found. (5) Perform another vacuum test to determine that the machine is now satisfactory. 30. Charging the Machine. After maintenance is completed and the machine passes a satisfactory vacuum test, the machine should be charged with solution. The machine must be kept charged with solution at all times except while maintenance is being done. Storage drums used to hold the solution should be moved as little as possible so as not to disturb dirt which

Figure 40. Solution charging. has settled to the bottom. Figure 40 illustrates the method of solution charging. (1) Start the water jet on the purge unit and open the purge valve on the absorber. (2) Continue purging until a vacuum reading of 25 inches of mercury is obtained on the manometer. Close the valves and shut off the jet when the vacuum is satisfactory. (3) Connect the hose to the solution charging valve and place the other end of the hose into the drum of solution. Do not let the hose touch the bottom of the drum since this would draw up dirt that had settled there. (4) Open the solution charging valve and allow the solution to enter the machine. (5) After all of the solution has been transferred into the machine, close the solution valve in the line from the generator and open the solution valve in the line to the generator. (6) Start the solution pump which will pump all the solution up to the generator. When the pump shuts off, close the valve in the line to the generator. This last step is necessary because all of the solution should be stored in the generator. (7) If the machine is to remain out of service, then it should be charged with nitrogen as previously outlined. 31. Troubleshooting. Troubleshooting and correction are two of your most important duties. We have discussed the operation and service that you perform on absorption systems. This information, coupled with that in tables 1-18, should give you the knowledge you need to carry out your assigned tasks.

40

TABLE 11

TABLE 12

TABLE 13

41

TABLE 14

TABLE 15

TABLE 16

TABLE 17

TABLE 18

42

Review Exercises The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the text. Do not submit your answers for grading. 1. While you are performing your hourly check of the absorption system, you notice that the condenser waterflow has dropped off and that the system is operating at a reduced capacity. What component should you troubleshoot and where is the component located? (Sec. 5, Par. 2) 7. What will occur if the feeler bulb of the concentration limit thermostat is broken? (Sec. 5, Par. 8)

8.

The plant operator submits the following complaint: (1) The chill water temperature is 57° F. (The design temperature is 45° F.) (2) The off-run-start switch is in the RUN position. (3) The solution pump is off and the last discharge pressure reading was 36 p.s.i.g. What has occurred within the system to cause a shutdown? How do you restart the unit? (Sec. 5, Pars. 9 and 10)

2.

The refrigerant used in this system is _____________________ and the absorbent is ______________. (Sec. 5, Par. 3)

3.

What will occur within the system when heat is not supplied to the generator? (Sec. 5, Par. 5)

9.

Why are the solution and chilled water pumps equipped with mechanical seals? (Sec. 5, Par. 14)

4.

(Agree)(Disagree) The heat exchanger heats the strong solution. (Sec. 5, Par. 5) 10. (Agree)(Disagree) The float control in the solution pump water seal tank controls makeup water to the tank automatically. (Sec. 5, Par. 15)

5.

During a routine inspection you find that the supply air pressure to the chill water thermostat is 3 p.s.i.g. What component is affected? How does this component affect the operation of the system? (Sec. 5, Pars. 6 and 7)

11. The primary difference between daily startup and startup after standby shutdown is that __________________. (Sec. 6, Par. 3)

6.

A 2° chilled water temperature change will cause the branch line pressure to change _____________ p.s.i.g. (Sec. 5, Par. 7)

12. The evaporator pump is surging. What caused this surging? (Sec. 7, Par. 3)

43

13. A solution level in the absorber of two-thirds (Sec. 7, Par. 4)

22. Why should you flush the seal chamber after startup? (Sec. 8, Par. 4)

14. When is the solution boiling level in the generator set? (Sec. 7, Par. 5)

23. An increased water level in the evaporator after shutdown indicates that __________________. (Sec. 8, Par. 5)

15. What will occur when air is being handled by the purge unit? (Sec. 7, Par. 7)

24. Why is octyl alcohol added to the solution? (Sec. 8, Par. 7)

16. Excessive steam pressure will cause _____________________. (Sec. 7, Par. 9)

17. Where would you connect the nitrogen tank if the system did not have a nitrogen charging valve? (Sec. 7, Par. 14)

25. How would you correct the following malfunction? The octyl alcohol charging valve is open but the alcohol is not being drawn into the machine. What would you do if this situation occurred frequently? (Sec. 8, Par. 8)

18. To dilute the solution for extended shutdown, you must put the system through ______________ dilution cycles. (Sec. 7, Par. 15)

26. The following complaint has been received at your shop. The steam jet purge unit on an absorption system is operating but is not purging air that is present in the absorber. What is the most probable cause and how would you correct it? (Sec. 8, Par. 9)

19. How can you determine whether air has leaked in the machine during shutdown? (Sec. 8, Par. 2)

27. Bottle number 2 in the solution test set contains _______________. (Sec. 8, Par. 10)

20. Air will cause the insides of the machine to (Sec. 8, Par. 2)

28. How many drops of indicator solution do you add to the solution sample? (Sec. 8, Par. 10)

21. How do you check a mechanical pump seal for leaks? (Sec. 8, Par. 3)

29. The standard sample (bottle No. 3) is a ________________ percent solution. (Sec. 8, Par 11)

44

30. During an evaporator water solution test, more silver nitrate was needed to turn the sample red than the standard solution. What does this indicate? How is this situation remedied? (Sec. 8, Pars. 10 and 11)

36. The operating log shows a steady increase in vapor condensate temperature. What maintenance is required? (Sec. 8, Par. 25)

31. What determines the length of time needed to reclaim the evaporator water? (Sec. 8, Par. 12)

37. How is soft scale removed from condenser tubes? (Sec. 8, Par. 25)

32. How long does it take for the dirt in the solution to settle out when the solution is placed in drums? (Sec. 8, Par. 14)

38. What is the maximum allowable vacuum loss during a vacuum leak test? (Sec. 8, Par. 28)

33. How is the conical strainer cleaned? Par. 16)

(Sec. 8,

39. Which refrigerant is added to the system to perform a halide leak test? (Sec. 8, Par. 29)

34. How is the purge connection on the absorber cleaned? (Sec. 8, Par. 20)

40. List at least three possible causes of lithium bromide solidification at startup. (Sec. 8, table 11)

35. (Agree)(Disagree) The diaphragm is a vacuum type valve should be replaced yearly. (Sec. 8, Par. 22) 41. How can you make sure that a seal is leaking? (Sec. 8, table 12)

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CHAPTER 3

Centrifugal Systems

FEW PEOPLE realize the importance of the refrigeration specialist in this age of aerospace weapons systems. For them, refrigeration has nothing to do with launching a missile and reaching the moon. However, we know that without control of the environment of a launch complex the military goals of defense and space conquest would never be achieved. 2. The centrifugal refrigeration system is often used in such weapons systems as Titan, Bomarc, and SAGE. In this chapter we will discuss the operation of this system, the complete refrigeration cycle, each component of the unit, and the general maintenance requirements. 9. Refrigeration Cycle 1. The centrifugal system uses the same general type of compression refrigeration cycle used on other mechanical systems. Its features are: • A centrifugal compressor of two or more stages. • A low-pressure refrigerant known as Refrigerant11. Approximately 1200 pounds of refrigerant are required for fully charging a centrifugal machine. 2. An economizer in the liquid return from the condenser to the evaporator acts as the expansion device. You can compare the economizer to the high side float (metering device) used on older model refrigerators. The use of this piece of equipment reduces the horsepower required per ton of refrigeration cycle. This increase in efficiency is made possible by using a multistage turbocompressor and piping the flash gas to the second stage. 3. A schematic of the centrifugal cycle is shown in figure 41. We will begin the cycle at the evaporator. The chilled water flowing through the tubes is warmer than the refrigerant in the shell surrounding the tubes, and heat flows from the chilled water to the refrigerant. This heat evaporates the refrigerant at a temperature corresponding to the pressure in the evaporator. 4. The refrigerant vapors are drawn from the evaporator shell into the suction inlet of the compressor.

The suction vapors are partially compressed by the firststage impeller and join the flash gas vapor coming from the economizer at the second-stage impeller inlet. The refrigerant gas discharged by the compressor condenses on the outside of the condenser tubes by giving up heat through the condenser tubes to the cooler condenser water. The condensing temperature corresponds to the operating pressure in the condenser. 5. The liquefied refrigerant drains from the condenser shell down through an inside conduit into the condenser float chamber. The rising refrigerant level in this chamber opens the float valve and allows the liquid to pass into the economizer chamber. The pressure in the economizer chamber is approximately halfway between the condensing and evaporating pressures: consequently, enough of the warm liquid refrigerant evaporates to cool the remainder to the lower temperature corresponding to the lower pressure in the economizer chamber. This evaporation takes place by rapid "flashing" into gas as the liquid refrigerant passes through the float valve and the conduit leading into the economizer chamber. The flashed vapors pass through eliminator baffles and a conduit to the suction side of the second stage of the compressor. 6. The cooled liquid then flows into the economizer float chamber located below the condenser float chamber. The rising level in the economizer float chamber opens the float valve and allows the liquid refrigerant to pass into the bottom of the cooler. Since the evaporator pressure is lower than the economizer pressure, some of the liquid is evaporated (flashed) to cool the remainder to the operating temperature of the evaporator. These vapors pass up through the liquid refrigerant to the compressor suction. The remaining liquid serves as a reserve for the refrigerant continually being evaporated by the chilled water. The cycle is thus complete. 7. Now that you understand the complete refrigeration

46

Figure 41. Centrifugal cycle. cycle, let us study the compressor in more detail. 10. Centrifugal Compressor 1. A cutaway view of the compressor is shown in figure 42. The easiest way to understand centrifugal compressor operation is to think of a centrifugal fan. Like the fan, the compressor takes in gas at the end (in line with the shaft) and whirls it at a high speed. The high-velocity gas leaving the impellers is converted to a pressure greater than the inlet. At normal speed, with R-11, the suction temperature is 65° F. below the temperature of condensation. At maximum speed, the compressor will produce a suction temperature of approximately 85° F. below the condensing temperature of R-11. Changing the speed of the compressor varies the suction temperature. 2. The compressor casing and the various stationary passages inside the compressor shaft are made of hard steel with keyways provided for each impeller. The impellers are of the built-up type. The hub disc and cover are machined steel forgings. The blading is sheet steel formed to curve backward with respect to the direction of rotation and is riveted to the hubs and covers. After assembly, the wheels are given a hotdipped lead coating to reduce corrosion damage. The rotor 47

Figure 42. Compressor cutaway.

Figure 43. Bearing assembly. assembly, consisting of the shaft and impellers, runs in two sleeve type bearings. 3. In figure 43 a thermometer is inserted in top of each bearing cover (1) for indicating temperature. Each bearing also has two large oil rings (2) to insure lubrication. The upper and lower bearing liners (3) are held in place by the upper and lower bearing retainers (4). 4. Brass labyrinths (5) between stages and at the ends of the casing restrict the flow of gas between stages and between the compressor casing and bearing chambers. 5. In operation, the pressure differential across each impeller produces an axial thrust toward the suction end of the compressor. This thrust is supported by a "kingsbury" thrust bearing at the suction end of the shaft. 6. Compressor Lubricating System. The entire oiling system is housed within the compressor casing and the oil is circulated through cored opening, drilled pages, and fixed copper fines. This eliminates all of the usual external lines and their danger of possible rupture, damage, or leakage. All of the oil for the lubricating system is circulated by a helical gear pump, shown in figure 44, which is submerged in the oil reservoir. The simple, positive drive insures ample oil for pressure lubricating and cooling all journal bearings, thrust bearings, and seal surfaces. The reservoir which houses the oil pump is an integral part of the compressor casing and is accessible through a cover plate on the end of the compressor. Circulating water cooling coils are fitted to the cover plate to maintain proper oil temperature. 7. In general, the lubricating system (shown schematically in fig. 45) consists of the gear type oil 48

pump, driven from the main compressor shaft and supplying oil through various connections and passages for the thrust bearing, the two shaft bearings, the oil pump worm gear drive, and for the shaft seal-with the necessary gauges and control valves to permit the system to operate automatically. 8. The oil pressure or feed circuits are as follows, according to figure 45: • When the compressor starts, the pump (1) starts to circulate oil, which is supplied first entirely to the thrust bearing (3). • After passing through the thrust bearing, the oiling system divides into two paths known as "A" circuit and "B" circuit. 9. In the first path, the oil flows through the strainer (29) and the proper orifices to the pump gear (2) and to the rear shaft journal bearing (4). Since the thrust, rear journal bearing, and worm drive for the oil pump are all located above the oil pump chamber, the return oil merely drops back into the pump chamber from these parts. 10. In the second path, oil flows through the check valve (5) and filter (7) to actuate the shaft seal (8) and supply the front shaft journal bearing (9). Since part of the oil passes out through the front of the seal to atmospheric pressure, various valves are required in the supply lines as well as in the lines returning oil to the pump chamber. The check valve (5) does not open during compressor startup until the pump pressure reaches 8 p.s.i.g. After the valve (5) opens, the flow of oil is as described previously. If the seal oil reservoir (6) is not full, a small part of the oil passes through the orifice (28) to fill the reservoir. Oil under pressure to the seal

Figure 44. Compressor oil pump.

Figure 45. Compressor oil system schematic. expands the seal bellows to move the stationary seal back against its stop, allowing the oil to pass through the seal in two directions: (1) inside the compressor and (2) to the atmospheric side of the shaft seal. 11. The oil passing to the compressor (vacuum) side of the seal flows to the front journal bearing (9), through two small holes in the inner floating seal ring (12) -which is located in the seal housing--to prevent unnecessary flow of oil from the vacuum side of the seal. The bearing overflow drops to the bottom of the bearing chamber (10), draining back to the oil pump chamber through the proper passage in the manifold (18). 12. The oil passing to the atmosphere is restricted by floating rings between the stationary seal and rotating seal hubs and between the housing cover and the rotating seal hub. Most of it passes directly to the atmospheric float chamber (13). The water-jacketed seal housing cover (11) cools this oil and minimizes the refrigerant loss from it. A small amount of oil passes the seal rings and is returned to the atmospheric float chamber (13) through a connection (30). From the float chamber, the oil goes 49 through the automatic oil stop valve (16), up to the bearing chamber (10), and returns through the manifold to the oil pump chamber along with the oil overflow from the front bearing. Oil returns from the atmospheric float chamber since the pressure in the bearing chamber is always below atmospheric. This pressure, being equalized with the compressor suction through the rear shaft labyrinth, is always a vacuum during operation. From the bearing chamber, the oil flows by gravity through the manifold (18), to the oil pump chamber. The automatic stop valve (16) is provided to prevent flow of refrigerant vapor from the machine in case the pressure inside the machine during shutdown rises above atmospheric. The valve is set to open at approximately 8 pounds and is actuated by an oil pressure line taken from the oil pump discharge and, therefore, opens immediately after the compressor is started. Valve 16 also prevents outside air from entering the machine when the machine pressure is below atmospheric. This valve is necessary because the atmospheric float valve (14) is designed for level control only and is not a stop valve. Valve 17 is

Figure 46. Compressor oil heater. the oil pressure regulator. It is actuated by pressure "back of seal" through line 15 and controls oil pressure by returning excess oil back to the oil pump chamber. 13. Oil pressure gauges 22 and 23 on the control panel indicate the seal oil reservoir pressure and the pressure back of seal respectively. When the seal oil reservoir is full, gauge 22 indicates the pressure on the seal bellows. Gauge 23 indicates the pressure in the space between the seal and the inner floating ring or back of seal pressure which controls valve 17. 14. The air vent and vacuum breaker (27) admits atmospheric pressure during shutdown to the seal oil reservoir to maintain a head of oil on the seal. It operates as a gravity check valve. The oil heater (31) heats the oil during shutdown to prevent excessive absorption of refrigerant by the oil. A flow switch located in the water supply to the oil cooler manifold automatically turns the heater on when the water supply is shut off by hand, and cuts the heater off when the water is turned on. A schematic diagram of the oil heater is shown in figure 46. The oil cooler (19) cools the oil as it is returned to the pump chamber during operation. Bearing thermometers 24 and 25 indicate the temperature of the shaft bearings. Oil rings 20 and 21-also shown in figure 45-bring additional oil from the bearing wells to the shaft. Relief valve 26 in the oil pump discharge line relieves any unusually high pressure that may occur accidentally, and thus protects the system against any damage. 15. Compressor Shaft Seal. A shaft seal is provided where the shaft extends through the compressor casing. The seal assembly is shown in figure 47. 16. The seal is formed between a ring, called the rotating scaling seat which is fitted against a shoulder on the shaft, and stationary sealing seat which is attached to the seal housing through a flexible member or bellows assembly. The contact faces on these seal seats are carefully machined and ground to make a vacuum-tight joint when in contact. A spring called the seal spring moves the stationary seal seat into contact with the rotating seal seat to make the proper seal when the compressor is shut down. A floating ring is located between the hub of the stationary sealing seat and the hub of the rotating sealing seat. A seal oil reservoir and filter chamber is attached to the compressor housing above the seal to provide oil to maintain a head of oil to the seal surfaces

Figure 47. Shaft seal assembly. 50

Figure 48. Diagram of compressor seal end. during shutdown periods. The shaft seal consists of two highly polished metal surfaces which are held tightly together by a spring during shutdown, but are separated by a film of oil under pressure during operation. The positive supply of oil from the oil pump during operation and from the seal reservoir during shutdown prevents any inward leakage of air or outward leakage of refrigerant. In addition, the low oil pressure safety control will automatically stop the compressor if the oil pressure to the seal falls below a safe minimum. Figure 48 shows a cutaway diagram of the seal installed on the seal end of the compressor. 17. Lubricant. A high-grade turbine oil, such as DTE heavy medium or approved equal, is the type of oil recommended for centrifugal compressor usage. To be sure of specifications on grade and type of oil to use, it is 51 advisable to refer to the manufacturer's maintenance manual. 18. If a machine is to be started for the first time or if all the oil has been drained from the unit, the following lubrication procedures are recommended: • The machine pressure must be atmospheric. • Remove the cover on the front bearing at the coupling end of the compressor and pour 1 gallon of oil into the front bearing level. • Fill the seal oil pressure chamber by removing the cover. • Remove the cover from the rear bearing and pour oil into the chamber until the indicated height is reached as recommended on the pump chamber plate. • Fill the atmospheric float chamber through

the connection on the side of the chamber until oil shows in the sight glass. • Pour a small amount of oil into the thrust bearing housing by removing the strainer cap and pouring oil into the strainer. Under normal operating conditions, the following lubrication procedures are recommended: • Replace the oil filter regularly, depending on the length of operation and the condition of the filter. • If at any time some oil is withdrawn from the machine, replace with new oil. • Clean and inspect the strainer in the thrust bearing at least once a year. Replace the complete oil charge at least once a year. • After shutdown periods of more than a month, remove the bearing covers and add 1 quart of oil to each bearing well before starting. 19. To drain the oil system, allow the machine to warm up until the temperature is approximately 75° F. The machine must be at atmospheric pressure. Drain the pump chamber by removing the drain plug. Replace the plug, then drain the atmospheric float chamber in the same manner. By draining these two chambers, practically all of the oil is removed. The oil left in the bearing wells and seal reservoir is useful for keeping the bearing in satisfactory condition and as a sealing oil. 20. CAUTIONS: To keep the machine in the best operating condition, the following cautions must be observed: • The electric heater in the oil pump chamber must be turned on during shutdown periods and must be turned off when the cooling water is turned on. • Do not overcharge the system with oil. The oil level will fall as the oil is circulated through the system; but under normal operation, the oil level will increase approximately 7 percent in volume as the refrigerant becomes absorbed in it. The oil level in the machine will be approximately one-half glass. • Oil can be added to the filling connection on the side of the atmospheric float chamber only while the machine is in operation and the atmospheric return valve is open. 21. Now that you have a proper knowledge of compressor operation, let's discuss the type of drive for the compressor. 11. Compressor Gear Drive 1. The gear drive is a separate component mounted between the compressor and electric motor. The gears are speed increasers required to obtain the proper compressor speed through the use of standard speed 52

motors. The gears are of the double helical type, properly balanced for smooth operation, and pressure lubricated. The gear wheel and pinion are inclosed in an oiltight case, split at the horizontal centerline. Lubrication is from the gear type oil pump. The unit has an oil level sight glass, a pressure gauge, and an externally mounted oil strainer and oil cooler. A diagram illustrating the gear parts is shown in figure 49. 2. Lubrication. A good gear oil must be used for the lubrication of high-speed gears. The oil must be kept clean by filtering, and filters changed as often as possible. The temperature of the oil should be kept within the range of 130° F. to 180° F. Water cooling should be used whenever necessary to keep the temperature within these limits. 3. Type of Oil. The best grade of oil to use on a gear depends on journal speeds, tooth speeds, and clearances. In general, it is better to use an oil too heavy than one too light. The gears will be somewhat warmer, but the heavier oil will take care of higher temperature if it is not more than a few degrees. The heavier oil is rated at 400 to 580 seconds Saybolt viscosity at 100° F. 4. Water Cooling of Gears. The gears are water cooled by circulating water through water jackets cast in the ends of the gear casing or by means of either an internal or an external oil cooler. This system is connected to a supply of cool, clean water, at a minimum pressure of 5 pounds. A regulating device must be installed in the water supply line. The discharge line should have free outlet without valves to avoid possibility of excessive pressures on the system. Piping must be arranged so that all the water can be drained or blown out of the water jackets or cooler if the unit is to be subjected to freezing temperatures. 5. Inspection. Inspect to see that both the driving and driven machines are in line. If you are not sure that alignment is correct, check this point with gauges. Try out the water cooling system to see if it is functioning properly. When starting, see that you have sufficient oil in the gear casing and that the oil pump gives required pressure (4 to 8 pounds). When the temperature of the oil in the casing reaches 100° F. to 110° F., turn on the water cooling system. Add sufficient oil from time to time in order to maintain the proper oil level. Never allow the gear wheel to dip into the oil. 6. Regular cleaning of the lubrication system and tests of the lubricant are essential. Clean the strainer at least once a week and more often if necessary. The manufacturer recommends that the gear case should be drained and be completely cleaned out every 2 to 3 months. Refill with new filtered oil. Between oil changes, samples of oil

Figure 49. Gear drive components. should be drawn off and the oil checked. If water is present, the water should be drawn off. If there is a considerable amount of water in the oil, remove all oil and separate the water from the oil before it is used again. 7. Repair. All working parts of the gear drive are easily accessible for inspection and repair except the oil pump. If you should have to dismantle the gears, you must take precautions to prevent any damage to gear teeth. The slightest bruise will result in noisy operation. When the gears are removed, place them on a clean cloth placed on a board and block them so that they cannot roll off. Cover the gears with a cloth to protect them from dust and dirt. 53 8. Bearing shells, oil slingers, etc., are marked and should be returned to their proper places. Gaskets are used between the oil pump bracket and oil pump and under handhole covers. All parts must be clean before reassembly. Make sure that no metal burrs or cloth lint is present on any part of the unit. Coat faces of flanges with shellac before bolting them together. A thin coat of shellac on the bearing supports will prevent oil leaks at these points. Before final replacement of the cover, make a careful inspection to see that all parts are properly placed and secured. 9. Worn bearings must be replaced immediately because they will cause the gears to wear. Bearings are interchangeable, and when new bearings

are installed the gears are restored to their original center distance and alignment. It is not recommended to rebabbit bearings, for the heat required to rebabbit the bearings will cause some distortion of the bearing shell. Do not renew or scrape one bearing alone, but always renew or scrape in pairs; this will help eliminate tooth misalignment. Do not adjust bearing clearances by planing the joint, thereby bringing the halves closer together, since trouble will result. 10. The oil pump is a geared type. During assembly, care must be taken to see that the paper gasket between the pump body and bracket is of the proper thickness. A gasket that is too thick will reduce the capacity and cause failure in oil pressure, while a too thin gasket will cause an excessive load to be thrown on the gears, resulting in wear and destruction of the gears. Writing paper makes a good gasket when shellacked in place. Never use a rubber gasket on any oil joint. "Cinch" fittings are used on all pipes connected to the oil pump bracket; use this type on all replacements. Threaded fittings may cause the bracket to be pulled out of line, causing noisy operation and wear on gears. Couplings should not be driven on or off the gear or pinion shafts, since hammering is liable to injure both surfaces. Provisions have been made for using a jacking device for putting on or removing couplings from shafts. 11. Gear tooth contact and wear should be uniformly distributed over the entire length of both gear and pinion helixes. If heavier wear is noted on any portion of the helixes or any part of the tooth face, it may indicate improper setting of the gear casing, misalignment of connecting shafts, vibration, excessive or irregular wear on the bearings, or poor lubricant. Should gear teeth become damaged during inspection or operation, remove burrs by use of a fine file or oil stone. Never use these tools to correct the tooth contour. Misalignment, poor lubrication, and vibration can cause pitting of tooth surfaces or flaking of metal in certain areas of the gear. If this happens, check alignment and remove all steel particles. Check the manufacturer's maintenance manual for specific maintenance procedures and instructions. 12. You now understand the drive system for the compressor, but we must learn how the drive is coupled to the motor and the compressor. 12. Couplings 1. The couplings used to connect the motor to the speed-increasing gears and from the gears to the compressors are self-alining coupling. They are of the flexible geared type, consisting of two externally geared hubs that are pressed on and

Figure 50. Mounting coupling on shaft. geared to the shaft. These hubs are inclosed by a twopiece externally geared floating cover which functions as a single unit when the halves are bolted together. The cover is supported on the hub teeth during operation. A spacer or spool piece is used with the cover for the compressor coupling. The hub teeth and cover teeth are engaged around the complete circumference, and the cover and shafts revolve as one unit. The cover and each shaft is free to move independently of each other within the limits of the coupling, thus providing for reasonable angular and parallel misalignment as well as end float. The amount of misalignment that the coupling will handle without excessive stressing varies with the size of the coupling. In all cases, the coupling should be treated as a joint that will take care of only small misalignments. 2. Installation and Alignment Procedures or Coupling. Figure 50 illustrates the method used to mount each half coupling on the shaft. In reference to figure 50, place the sleeve over the shaft end and lubricate the surface of the shaft. Expand the hub with heat, using hot oil, steam, or open flame. When using a flame, do not apply the flame to the hub teeth. Use two long bolts in the puller holes to handle the war coupling. Locate the hub on the shaft with the face of the hub flush with the shaft end. Install the key with a tight fit on the sides and a slight clearance between the top of the key and the hub. 3. Check the angular alignment, as shown in figures 51 and 52. For normal hub separation, as shown in figure 51, use a feeler gauge at five points 90° apart. Recheck the angular alignment as discussed above. Figure 53 shows how to check the offset alignment by the sight method. Figure 54 illustrates the method for checking alignment by the instrument method. This method

54

Figure 51. Checking angular alignment (normal separation). is recommended by the manufacturer. Fasten or clamp the indicator bracket on one hub with the dial indicator button contacting the alignment surface of the opposite hub. Rotate the shaft on which the indicator is attached to the hub, and take readings at four point, 90° apart. Move either machine until readings are identical. Reverse the indicator to the opposite hub and check. Recheck the angular alignment as discussed before. 4. Figure 55 illustrates the method for checking offset alignment with wide hub separation. Use the dial indicator as discussed in checking offset alignment by the instrument method, then check the angular alignment as discussed before. 5. In checking for angular and offset alignment

Figure 53. Checking offset alignment (sight method). on the floating shaft arrangement, it is possible to correct both angular and offset misalignment in one operation. In reference to figure 56, position units to be coupled with the correct shaft separation. Install and assemble the coupling. Clamp the indicator bar to the flange of one coupling with the indicator button resting on the floating shaft approximately 12 inches from the teeth centerline of this coupling. Rotate the units, taking readings at four points, 90° apart. Move either machine until the readings are identical. 6. After checking and setting the offset and angular alignment, insert the gasket as shown in figure 57. Inspect to insure the gasket is not torn or damaged. Clean the coupling flanges and insert the gasket between the flanges, making sure to position the O-ring in the groove. Figure 58

Figure 52. Checking angular alignment (wide separation). 55

Figure 54. Checking offset alignment (instrument method).

Figure 57. Gasket insert. Figure 55. Checking offset alignment (wide hub separation). illustrates the method of positioning gaskets between each set of flanges for spacer and floating shaft type coupling. Assemble the coupling as shown in figure 59. Keep the bolt holes in both flanges and gasket in line. Insert the body fitting bolts and nuts and tighten the bolts and nuts with wrenches no larger than the one furnished with the coupling until the flanges are drawn together. Using an oversize wrench on the heads of nuts and bolts may round their heads or strip the threads. 7. Lubricate the coupling as illustrated in figure 60. Remove both lubricating plugs and apply the quantity and type of lubricant as specified by the manufacturer's instruction data sheet. If grease is used, positioning of the lubrication holes is not necessary. When a fluid lubricant is used, it is recommended that the lubricating holes be positioned approximately 45° from the vertical to prevent loss of lubricant. A good oil lubricant no lighter than 150 seconds Saybolt Universal (SSU) or heavier than 1000 SSU at 210° F. can be used. Before replacing the lubrication plugs, check the copper ring gaskets to make sure they are in position and are undamaged. Tighten plugs with the wrench furnished with coupling as shown in figure 61. 8. The coupling must be well lubricated at all times. The couplings that use oil collector rings in the end of the cover can be lubricated while stopped or running. The compressor should not be started until the coupling has been checked for proper amount of oil. Oil will overflow the oiling ring with the coupling at rest when enough oil has been added. Other types of couplings may have sleeves attached by a gasket to the hubs with no oiling ring. The manufacturer will give specifications as to the amount of oil required to fill this unit. Unless a large amount of oil is lost from the gasketed type, it is only necessary to check the amount of oil in the coupling twice a

Figure 56. Checking angular and offset alignment. 56

Figure 58. Insertion of both gaskets.

Figure 59. Assembling the coupling. year by draining and refilling with the correct amount. 9. Check of Coupling Alignment on Operating Machine. In checking the alignment of an operating centrifugal unit, proceed as follows: Make sure the machine has operated long enough to bring the compressor gear and motor up to operating temperatures. Then stop the machine and disconnect both couplings, and with straightedge and feelers check the hubs. Check the compressor coupling for parallelism, vertically and horizontally, noticing how much it will be necessary to move the gear, vertically or horizontally, to bring the coupling within 0.002 inch tolerance for alignment. Then check the coupling for angularity by use of feelers to insure that the faces of the hubs are spaced equally apart at the top and bottom. To secure this alignment for angularity, it is necessary to shift the gear at one end either

Figure 61. Tightening the coupling plug. vertically or horizontally. Caution must be used so that the parallel alignment is not disturbed. Recheck the parallel alignment to make sure that it is within its tolerance. After the coupling has been aligned, assemble the coupling. Now that we have reassembled the coupling, we shall study the drive motor and controls. 13. Drive Motor and Controls 1. The motor furnished with a centrifugal machine is an a.c. electric motor, three-phase, 60 cycle. The motor will be a general-purpose type with a normal starting torque, adjustable speed wound rotor and sleeve bearings. For wound rotor motors, the controller consists of three component parts: • Primary circuit breaker panel • Secondary drum control panel • Secondary resistor grids 2. The primary circuit breaker is the main starting device used to connect the motor to the power supply. Air breakers are supplied for the lower voltages and oil breakers for 1000 volts. This breaker is a part of the control for the motor and should be preceded by an isolating switch. The breaker provides line protection (short circuit and ground fault) according to the rating of the size of breaker and is equipped with thermal overload relays for motor running protection set at 115 percent of motor rating. Undervoltage protection and line ammeter also form a part of the primary panel. 3. The secondary drum control is used to adjust the amount of resistance in the slipring circuit of the motor and is used to accelerate and regulate the speed of the motor. A resistor, which is an energy dissipating unit, is used with the drum to provide speed regulation. The maximum amount

Figure 60. Coupling lubrication. 57

Figure 62. Cross section of the condenser. of energy turned into heat in the resistor amounts to 15 percent of the motor rating. In mounting the resistor, allow for free air circulation by clearance on all sides and at the top. 4. Manual starting of the machine at the motor location assures you complete supervision of the unit. Interlocking wiring connections between drum controller and circuit breaker makes it necessary to return the drum to full low-speed position (all resistance in) before the breaker can be closed. The oil pressure switch is bypassed when holding the start button closed. Releasing the start button before the oil pressure switch closes will cause the breaker to trip out-hence a false start. Very large size air breakers are electrically operated but manually controlled by start-stop pushbuttons on the panel. The drum controller lever must always be moved to the OFF position before pressing the start button. 58 5. The motor, controlled by various automatic and manual controls propels the compressor. The compressor in turn pumps the refrigerant through the system's condenser, cooler, and economizer. 14. Condenser, Cooler, and Economizer 1. The condenser is a shell and tube type similar in construction to the cooler. The primary function of the condenser is to receive the hot refrigerant gas from the compressor and condense it to a liquid. A secondary function of the condenser is to collect and concentrate noncondensable gases so that they may be removed by the purge recovery system. The top portion of the condenser is baffled, as shown in figure 62. This baffle incloses a portion of the first water pass. The noncondensables rise to the top portion of the condenser because they are lighter than

Figure 63. Condenser diagram. refrigerant vapors and because it is the coolest portion of the condenser. 2. A perforated baffle or distribution plate, as shown in figure 62, is installed along the tube bundle to prevent direct impact of the compressor discharge on the tubes. The baffle also serves to distribute the gas throughout the length of the condenser. The condensed refrigerant leaves the condenser through a bottom connection at one end and flows it the condenser float trap chamber into the economizer chamber. The water boxes of all condensers are designed for a maximum working pressure of 200 p.s.i.g. The water box, item 1 in figure 63, is provided with the necessary division plates to give the required flow. Water box covers, items 2 and 3 in figure 63, may be removed without disturbing any refrigerant joint since the tube sheets are welded into the condenser and flange. Vent and drain openings are provided in the water circuit. The condenser is connected to the compressor and the cooler shell with expansion joints to allow for differences in expansion between them. Figure 63 is a side view of the condenser. 3. Condenser. The following procedures should be followed in cleaning condenser tubes: (1) Shut off the main line inlet and outlet valves. (2) Drain water from condenser through the water box drain valve. Open the vent cock in the gauge line or remove the gauge to help draining. (3) Remove all nuts from the water box covers, leaving two on loosely for safety. (4) Using special threaded jacking bolts, force the cover away from the flanges. As soon as the covers are loose from the gaskets, secure a rope to the rigging bolt in the cover and suspend from overhead. Remove the last two nuts and place on the floor. 59 (5) Scrape both the cover and the matching flange free of any gasket material, items 4, 5, and 6 in figure 63. (6) Remove the water box division plate by sliding it out from its grooves. Caution should be used in removing this plate; it is made of cast iron. Penetrating oil may be used to help remove the plate. (7) Use a nylon brush or equal type on the end of a long rod. Clean each tube with a scrubbing motion and flush each tube after the brushing has been completed. CAUTION: Do not permit tubes to be exposed to air long enough to dry before cleaning since dry sludge is more difficult to remove. (8) Replace the division plate after first shellacking the required round rubber gasket in the two grooves. (9) Replace the water box covers after first putting graphite on both sides of each gasket, since this prevents sticking of the gaskets to the flanges. CAUTION: Care must be taken with the water box cover on the water box end to see that the division plate matches up the rib to the flanges. (10) Tighten all nuts evenly. (11) Close the drain and gauge cock. (12) Open the main line water valve and fill the tubes with water. Operate the pump, if possible, to check for leaktight joints. 4. Cooler. The cooler is of horizontal shell and tube construction with fixed tube sheets. The shell is low carbon steel plate rolled to shape and electrically welded. The cooler and condenser both have corrosion-resistant cast iron water boxes. They are designed to permit complete inspection without breaking the main pipe joints. Full-size separate cover plates give access to all tubes for easy cleaning. The cooler water boxes are designed for maximum 200 pounds working pressure. They are provided with cast iron division plates

Figure 64. Cross section of cooler. to give the required water pass flow. Both the cooler and condenser have tube sheets of cupro-nickel, welded to the shell flange. Cupronickel is highly resistant to corrosion. 5. The tubes in the cooler are copper tubes with an extended surface. The belled ends are rolled into concentric grooves in the holes of the tube sheets. Tube ends are rolled into the tube sheets and expanded into internal support sheets. The normal refrigerant charge in the cooler covers only about 50 percent of the tube bundle. However, during operation, the violent boiling of the refrigerant usually covers the tube bundle. The cooler is equipped with multibend, nonferrous eliminator plates above the tube bundle which remove the liquid droplets from the vapor stream and prevent carryover of liquid refrigerant particles into the compressor suction. Inspection covers are provided in the ends of the cooler to permit access to the eliminators. Figure 64 is a crosssection diagram of the cooler. 6. A rupture valve with a 15-pound bunting disc is provided on the cooler, and a 15-p.s.i.g. pop safety valve is screwed into a flange above the rupture disc. These items are strictly for safety, because it is highly improbable that a pressure greater than 5 to 8 p.s.i.g. will ever be attained without purposely blocking off the compressor suction opening. 7. An expansion thermometer indicates the temperature of the refrigerant within the cooler during operation. A sight glass is provided to observe the charging and operating refrigerant level. A charging valve with connections is located on the side of the cooler for adding or removing refrigerant. The connection is piped to the bottom of the cooler so that complete drainage of refrigerant is possible. A refrigerant drain to the atmosphere is also located near the charging connection and expansion thermometer. 8. A small chamber is welded to the cooler shell at a point opposite the economizer and above 60

the tube bundle. A continuous supply of liquid from the condenser float chamber is brought to the expansion chamber while the machine is running. The bulb of the refrigerant thermometer and the refrigerant safety thermostat bulb are inserted in this expansion chamber for measuring refrigerant temperature. 9. Cleaning. Depending on local operating conditions, the tubes of the evaporator should be cleaned at least once a year. Cleaning schedules should be outlined in the standard operating procedures. You will be required to make frequent checks of the chilled water temperatures in the evaporator. If these temperature readings at full load operation begin to vary from the designed temperatures, fouling of the tube surfaces is beginning. Cleaning is required if leaving chilled water temperature cannot be maintained. 10. Repair. Retubing is about the only major repair that is done on the evaporator (cooler). This work should be done by a manufacturer's representative. 11. Cooler and Condenser Checkpoints. You must check the cooler and condenser for proper refrigerant level and make sure that the tubes in the cooler and condenser are in efficient operating condition. The correct refrigerant charging level is indicated by a cross wire on the sight glass. The machine must be shut down to get an accurate reading on the sight glass. For efficient operation, the refrigerant level must not be lower than one-half of an inch below the cross wire; a refrigerant level above this reference line indicates an over-charge. Overcharging is caused by the addition of too much refrigerant. When this condition exists, the overcharged refrigerant must be removed. 12. If the machine has been in operation for long periods of time, the refrigerant level will drop due to refrigerant loss. When this condition exists, additional refrigerant must be added to the system to bring the refrigerant level up to its proper height as indicated on the cross wire. Observe all cautions and do not overcharge the cooler. 13. A method of determining if the tube bundle of either the cooler or condenser is operating efficiently is to observe the relation between the change in temperature of the condenser water or brine and the refrigerant temperature. In most cases, the brine or condenser waterflow is held constant. Under such conditions, the temperature change of chilled and condenser water is a direct indication of the load. As the load increases, the temperature difference between the leaving chilled water or condenser cooling water and the refrigerant increases. A close check should be made of the temperature differences at full load when the machine is first operated, and a comparison made from time to time

during operation. During constant operation over long periods of time, the cooler and condenser tubes may become dirty or scaled and the temperature difference between leaving water or brine will increase. If the increase in temperature is approximately 2° or 3° at full load, the tubes should be cleaned. 14. Read the condenser pressure gauge when taking readings of the temperature difference between leaving condenser water and condensing temperature. Before taking readings, make sure the condenser is completely free of air. The purge unit should be operated for at least 24 hours before readings are taken. 15. Economizer. A complete explanation of the function of the economizer was given under the refrigeration cycle. The economizer is located in the cooler shell at the opposite end from the compressor suction connection and above the tube bundle. 16. The economizer is a chamber with the necessary passages and float valves, connected by an internal conduit passing longitudinally through the cooler gas space to the compressor second-stage inlet. This connection maintains a pressure in the economizer chamber that is intermediate (about 0 p.s.i.g.) between the cooler and condenser pressures and carries away the vapors generated in the chamber. Before entering the conduit, the economizer vapors pass through eliminator baffles to extract any free liquid refrigerant and drain it back into the chamber. (Item 9 of fig. 64 is a front view of the economizer chamber.) 17. There are two floats in separate chambers on the front end of the economizer. The top or condenser float valve keeps the condenser drained of refrigerant and admits the refrigerant from the condenser into the economizer chamber. The bottom, or economizer, float valve returns the liquid to the cooler. 18. This system is also equipped with another fine feature to assure smoother operation. Let's discuss the hot gas bypass system. 15. Hot Gas Bypass 1. The automatic hot gas bypass is used to prevent the compressor from surging at low loads. In case of low load conditions, hot gas is bypassed directly from the condenser through the cooler to the suction side of the compressor. The hot gas supplements the small volume of gas that is being evaporated in the evaporator due to low load conditions. Surging generally occurs at light load, and the actual surge point will vary with different compressors. In most instances, it usually develops at some point well below 50 percent capacity. If the leaving chilled water is held at a constant

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Figure 65. Hot gas bypass. temperature, the returning chilled water temperature becomes an indication of the load. This temperature is used to control the hot gas bypass. A thermostat, set in the returning chilled water, operates to bleed air off the branch line serving the hot gas bypass valve. The thermostat is set to start opening the bypass valve slightly before the compressor hits its surge point. Figure 65 illustrates components and location of the hot gas bypass line. 2. A liquid line injection system is provided in the hot gas bypass system to desuperheat the gas by vaporization in the bypass line before it enters the compressor suction. If the gas is not desuperheated, the compressor will overheat. The automatic liquid injection system components consist of a pair of flanges in the hot gas line, an orifice, a liquid line from the condenser to one of the flanges, and a liquid line strainer with two shutoff valves. 3. The automatic valve shown in figure 65 is normally closed. When this valve is closed, there is no flow of gas through the orifice. The pressure at point M, just below the orifice, is the same as the condenser pressure; therefore, no liquid will flow through the liquid line. When the occasion arises for the need of hot gas, the valve is opened automatically and a pressure drop will exist across the orifice. The amount of pressure drop is a direct function in determining the rate of gasflow through the orifice. The larger the flow of hot gas through the bypass and orifice, the lower the pressure at point M will become in relation to the condenser pressure, and the greater will be the pressure differential to force desuperheating liquid through the liquid line. As the amount of hot bypass gas is increased or decreased by 62

the opening or closing of the valve, the amount of desuperheating liquid forced through the liquid line is automatically increased or decreased. 4. The two shutoff valves in the liquid line are normally left wide open and are closed only to service the liquid line components. The special flange (located near the orifice) is installed at a slightly higher level than the surface of the liquid lying in the bottom of the condenser. When no hot gas is flowing through the bypass, no unbalance will exist in the liquid line. Therefore, the liquid will not flow and collect in the gas pipe above the automatic valve. This prevents the danger of getting a “slug” of liquid through the hot gas bypass line whenever the valve is opened. It also provides a means of distributing the liquid into the hot gas stream as evenly and as finely as possible. The flange is constructed with a deep concentric groove in one face for even distribution of the liquid. 5. How are undesirables such as water and air expelled from this system? The purge unit will do this important task for us. 16. Purge Unit 1. The presence of even a small amount of water in a refrigeration system must be avoided at all times; otherwise excessive corrosion of various parts of the system may occur. Any appreciable amount of water is caused by a leak from one of the water circuits. Since the pressure within a portion of the centrifugal refrigeration system is less than atmospheric, the possibility exists that air may enter the system. Since air contains water vapor; a small amount of water will enter whenever air enters. 2. The function of the purge system is to remove water vapor and air from the refrigeration system and to recover refrigerant vapors which are mixed with these gases. The air is automatically purged to the atmosphere. The refrigerant is condensed and automatically returned to the cooler as a liquid. Water, if present, is trapped in a compartment of the purge separator unit from which it can be drained manually. Thus the purge and recovery system maintains the highest possible refrigerating efficiency. 3. Components. The following discussion of the component items of the purge system is referenced to figure 66. • Stop valve--on main condenser, item 1. This valve is always open except during repairs. • Pressure-reducing valve--in suction line, item 2, to regulate the compressor suction pressure. • Stop valve--in suction line, item 3, located in the end of the purge unit casing. This valve is to be open when the purge unit is in operation and closed at all other times. • Pressure gauge--this gauge, item 4, indicates

Figure 66. Purge unit schematic. 63

the pressure on the oil reservoir. NOTE: Before adding oil, at item 23, be sure the pressure is at zero. • Compressor, item 5--to be operated continuously when the centrifugal compressor is operating, and before starting the machine as required by the presence of air. • High-pressure cutout switch, item 7--connected to the compressor discharge. Adjusted to stop the compressor if the purge condenser pressure increases to about 110 p.s.i.g. because of some abnormal condition. The switch closes again automatically on the reduction of pressure to about 75 p.s.i.g. • Auxiliary oil reservoir, item 8--this reservoir serves as a chamber to relieve the refrigerant from the compressor crankcase and also to contain extra oil for the compressor. The refrigerant vapor, which flashes from the compressor crankcase, passes up through the reservoir and into the compressor suction line. The free space above the oil level separates the oil from the refrigerant vapor before the vapor goes into the suction side of the purge compressor. The oil storage capacity of the reservoir is slightly larger than the operating charge of oil required by the compressor. • Sight glass, item 9--for oil level in the compressor and auxiliary oil reservoir, located in front of casing. • Compressor discharge line, item 10. • Condenser, item 11--cooled by air from a fan on compressor motor. It liquefies most of the refrigerant and water vapor contained in the mixture delivered by the compressor. • Evacuator chamber, item 12--for separation of air, refrigerant, and water. Chamber can be easily taken apart for inspection and repairs. • Baffle, item 13--allows the condensate to settle and air to separate for purging. This is the delivery point for the mixture of air, water (if any), and liquid refrigerant from condenser. • Weir and trap, item 14--located in the center of evacuation chamber. Since the water is lighter than liquid refrigerant the water is trapped above the liquid refrigerant in the upper compartment. Only refrigerant liquid can pass to the lower compartment. • Float valve, item 15--a high-pressure float valve, opening when the liquid level rises, allows the gas pressure to force the liquid refrigerant into the economizer. • Equalizer tube, item 16--to equalize the vapor pressure between the upper and lower compartments. • Two sight glasses, items 17 and 17A--on lower liquid compartment, visible at the end of the casing. These glasses show refrigerant level in the separator.

• Sight glass, item 18--on upper compartment to indicate the presence of water. • Stop valve at the end of casing, item 19--permits water to be drained from the upper compartment. The valve is marked "Water Drain" and is closed except when draining water. • Automatic relief valve, item 20--to purge air to the atmosphere. • Stop valve marked “Refrigerant Return" in the return liquid refrigerant line, item 21-located at the end of the casing. Open only when purge is operating. • Stop valve, item 22--on economizer in the return refrigerant connection. Open at all times except when machine is shut down for a long period or being tested. • Plug in oil filling connection of reservoir, item 23--pressure in the system must be balanced with the atmospheric pressure to add oil through this fitting. • Cap, item 24--or draining oil from the compressor crankcase and oil reservoir. Oil may also be added through this connection (not shown in fig. 66) if (1) a packless refrigerant valve is installed in place of cap at the connection and (2) the purge compressor is operated in a vacuum. • Connections between auxiliary reservoir and compressor crankcase, item 25. • Motor and belt--not shown in figure 66. • Wiring diagram inside the casing. • Casing that completely incloses the purge recovery unit and is removable to provide a means to work on components. • Plugged tee after pressure-reducing valve on line from condenser, item 26. • Capped tee on line leading to cooler, item 27. • Temporary connector pipe from water drain from separator to liquid refrigerant line to cooler, item 28. 4. Purge Recovery Operation. The purge recovery operation is automatic once the purge switch is turned on and the four valves listed below and referred to in figure 66 are opened: (1) Stop valve on main condenser (2) Stop valve in suction line (3) Stop valve in the return liquid refrigerant line (4) Stop valve on economizer in return refrigerant connection 5. If there should be an air leakage in the system, operation of the purge unit will remove this air. It is recommended that you stop the purge unit at intervals and shut off valves (1) an (4) listed above to check for leaks in the system. A tight machine will not collect air no matter how long the purge unit is shut off. Presence of air in the system is shown by an increase in head 64

Figure 67. Suction and relief pressure. pressure in the condenser. The pressure can develop suddenly or gradually during machine operation. By checking the difference between leaving condenser water temperature and the temperature on the condenser gauge, you can determine the presence of air. A sudden increase between these temperatures may be caused by air. In some instances, a sudden increase in cooler pressure over the pressure corresponding to cooler temperatures during operation may be caused by air leakage. 6. Small air leakages are very difficult to determine. It may take one or more days to detect an air leakage in the machine. A leak that shows up immediately or within a few hours is large and must be found and repaired immediately. Air pressure built up in the condenser is released to the atmosphere by the purge air relief valve. Excessive air leakage into the machine will cause the relief valve to pop off continuously, resulting in a large amount of refrigerant discharged to the atmosphere. 7. Refrigerant loss depends on operational conditions; therefore, these conditions have a determining effect on the amount of refrigerant lost. You should maintain a careful log on refrigerant charged and the shutdown level in the cooler. In this manner, you can determine the time a leak develops and the amount of refrigerant lost, find the cause, and correct the trouble. 8. Moisture removal by the purge recovery unit is just as important as air removal. The moisture may enter the machine by humidity in the air that can leak into the machine or by a brine or water leak in the cooler or condenser. If there are no water leaks, the amount of water collected by the purge unit will be small (1 ounce per day) under normal operating conditions. If large amounts of water are collected by the purge unit (onehalf pint per day), the machine must be checked for leaky tubes. Water can be removed more rapidly when the machine is stopped than when operating. If the machine is collecting a large amount of moisture. It is advisable to run the purge unit a short time after the machine is stopped and before it is started. Running the purge unit before the machine is started will help to reduce purging time after the machine is started. 9. The pressure-reducing valve (2), shown in figure 66, is adjusted to produce a suction pressure on the purge recovery unit and will not allow condensation in the suction line. If condensation does occur, the condensate will collect in the crankcase of the purge unit compressor, causing a foaming and excessive oil loss. The table in figure 67 can be used as a guide for setting the pressurereducing valve. If the pressure-reducing valve is wide open, there will be a pressure drop of a few pounds across the valve and the suction pressure cannot be adjusted higher than a few pounds below the machine condensing pressure. 10. Purge Unit Maintenance. After repairs or before charging, it is necessary to remove large quantities of air from the machine. This can be done by discharging the air from the water removal valve (item 19, fig. 66). Caution must be observed in the removal of air, since there is some danger of refrigerant being discharged with the air and being wasted to atmosphere. 11. If the normal delivery of refrigerant is interrupted, it is usually caused by the stop valve (item 21, fig. 66) being closed or because the float valve is not operating. This malfunction is indicated by a liquid rise in the upper sight glass. Immediate action must be taken to correct this trouble. If the liquid is not visible in the lower glass, the float valve is failing to close properly. 12. Water or moisture in the system will collect on the top of the refrigerant in the evacuation chamber. If any water does collect, it can be seen through the upper sight glass and should be drained. In most normal operating machines, the water collection is small; but if a large amount of water collects quite regularly, a leak in the condenser or cooler has most likely occurred and must be located and corrected immediately.

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Figure 68. Control panel electrical diagram. 13. The purge unit compressor and centrifugal compressor use the same type and grade of oil. Oil can be added to purge the compressor by closing stop valves (items 3 and 21, fig. 66), removing plug (23) in the top of the oil sight glass, and adding oil. Oil can be drained by removing the oil plug (24, fig. 66). The oil level can be checked by a showing of oil at any point in the oil sight glass while the compressor is running or shut down. The level of oil will fluctuate accordingly. The oil level should be checked daily. 14. Other components that must be closely checked in the purge recovery unit are as follows: • Belt tension. • Relief valve for rightness when closed to prevent loss of refrigerant. • Condenser clean and free from air obstruction • High-pressure cutout which shuts down if condenser pressure reaches 110 pounds. 15. CAUTION: The high-pressure cutout remakes contact automatically to startoff the purge recovery unit on 75 pounds. Single-phase motors have a built-in thermal overload to stop the motor on overload. It automatically resets itself to start the motor in a few minutes. 16. The system is running and purged. Let us now study our safety controls: 66 17. Safety Controls 1. Safety controls are provided to stop the centrifugal machine under any hazardous condition. Figure 68 illustrates the electrical wiring diagram. All the controls are mounted on a control panel. The safety controls are as follows: • Low water temperature cutout • High condenser pressure cutout • Low refrigerant temperature cutout • Low oil pressure cutout 2. All of the safety controls except the low oil pressure cutout are manual reset instruments. Each safety instrument operates a relay switch which has one normally open and one normally closed contactor. When a safety instrument is in the safe position, the corresponding relay is energized and the current is passed through the closed contactor to a pilot light which lights to indicate a safe operating condition. Should an unsafe condition exist, a safety control will deenergize the corresponding relay and the normally open contactor will open to deenergize the pilot light; the normally closed contactor will then close to energize the circuit breaker trip circuit.

When the circuit breaker trip circuit is energized, the circuit breaker trips open and stops the compressor motor. The pilot light will not go back on until a safe operating condition exists and the safety cutout has been manually reset. The oil safety switch operates somewhat differently. Since the oil pressure is not up to design conditions until the compressor comes up to speed, the relay for the oil pressure switch must be bypassed when the machine is started. The relay for the oil safety switch is bypassed by a time-delay relay, which keeps the trip circuit open until the compressor is up to speed. After a predetermined time interval, the time-delay relay closes the trip circuit at the circuit breaker and the oil safety switch serves its function. If the oil pressure does not build up before the time-delay relay closes, the trip circuit will be energized and the machine will stop. 3. The low oil pressure cuts out at 6 pounds and in at 12 pounds. The high condenser pressure cuts out at 15 pounds and in at 8 pounds. The low refrigerant and temperature cutout is set after operation in accordance to the job requirement. Generally, these controls should be set to cut out at 32° F. and to cut in at approximately 35° F. The low water temperature cutout should be set to cut out at 38° F. and to cut in at 43° F. 4. There are other safety controls built into the circuit breaker which are not part of the control panel, and reference should be made to the circuit breaker operating instructions for details of these controls. Such items as overload protection and undervoltage protection will be covered therein. 5. In addition to the pilot lights mentioned, a pilot light for the purge high-pressure cutout is on the safety control panel. The high-pressure cutout, which serves to protect the purge recovery compressor from high head pressure, is located in the purge recovery unit. When the high-pressure cutout functions on high head pressure, the pilot light on the control panel is lighted. 6. One or more machines at each installation are provided with two sets of starting equipment. One set is an operating controller and the other a standby controller. In order that the machine safety controls can operate the controlling breaker, a rotary selector switch is provided on the safety control panel. By means of the rotary selector switch, the machine safety controls can operate either of the controlling circuit breakers. Safety controls are used for safe operation of the system, but operating controls affect the capacity.

18. Operating Controls 1. The three methods of controlling the capacity output of a centrifugal machine are listed below: • Controlling the speed of the compressor • Throttling the suction of the compressor • Increasing the discharge pressure of the compressor. 2. The three methods given are listed in order of their efficiency. At partial loads, the power requirements will be least if the compressor speed is reduced, not quite as low if the suction is throttled, and highest if the condenser water is throttled to increase the discharge pressure. 3. Where the compressor is driven by a variablespeed motor, motor speed and compressor speed are controlled by varying the resistance in the rotor circuit of the motor by means of a secondary controller. 4. Damper Control. Throttling the suction of the compressor is obtained by means of a throttling damper built into the cooler suction flange. By throttling the compressor suction, the pressure differential through which the compressor must handle the refrigerant vapor is increased. Suction damper control requires somewhat more power at partial loads than at variable-speed control. The increase in power consumption is overbalanced by the increased effectiveness in maintaining a nonsurging operation at lower loads. For this reason, the machines are equipped with dampers, even though the main control is variable speed. Suction damper control modulation is effected by means of a temperature controller that sends air pressure signals to the suction damper motor in response to temperature changes of chilled water leaving the cooler. 5. Condenser Water Control. By throttling the condenser water, the condenser pressure is increased, thereby increasing the pressure differential on the compressor and reducing its capacity. The occasion may arise where the variable-speed control cannot be adjusted low enough to meet operating conditions. In such a case, the condenser water may be throttled and the compressor speed requirement brought up into the range of speed control. 6. Speed control and suction damper control are combined to control the temperature of the chilled water leaving the cooler. The suction damper modulates to control the leaving chilled water temperature on each balanced speed step. As the refrigeration load decreases, the suction damper will gradually close in response to decreasing air pressure in the branch line from the suction damper controller. As the suction damper approaches the closed position, a light on the

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control panel will indicate that the motor speed should be decreased to the next balanced step. The converse is true if the refrigeration load increases. 7. The lights for indicating a speed change are energized by mercury type pressure controls that sense branch air pressure from the suction chamber controller. The controller that energizes the "speed decrease" light also closes the light circuit on decreasing branch air pressure; the controller that energizes the "speed increase" light also closes the light circuit on increasing branch air pressure. The control system drawings give actual settings for pressure controllers; the final settings should be determined under actual operating conditions. You must determine what pressure change corresponds to a speed change and then adjust the pressure controller accordingly. Refer to the manufacturer's manual on details of adjustments. This information on operating controls will help you better understand the operation of the entire system. 19. System Operation 1. It is very difficult to give definite instructions in this text on the operating procedures for a given installation. Various design factors change the location of controls, types of controls used, and equipment location, and will have a definite effect on operational procedures. Listed below is a general description of startup and shutdown instruction. It is recommended that you follow your installation standard operating procedures for definite operating instructions. 2. Seasonal Starting. Listed below are the recommended steps that can be used in normal starting: (1) Check oil levels for motor, gear, coupling, compressor, and bearing wells. (2) Allow condenser water to circulate through the condenser. Be sure to vent air and allow the water to flow through slowly. This precaution must be observed to avoid water hammer. (3) Allow water or brine to circulate through the cooler. Be sure to vent air and allow the liquid to flow through slowly. As explained above, this will help in preventing water hammer. (4) Make sure that air pressure is present at all airoperated controls. (5) Start the purge unit before starting the machine; this helps in removing air from the machine. Then move the switch on the front of the casing to the ON position. The purge recovery unit should be operated at all times while the machine is operating.

(6) Make sure all safety controls have been reset and that the control lever is in position No. 1 (all resistance in). (7) Close the circuit breaker for all safety controls by pushing the starting switch or button in. (8) Bring the machine up to 75 percent full load with all resistance in. Check oil gauges to make sure proper oil pressure is being developed. If proper oil pressure is not developed in approximately 10 seconds, the machine will cut out on low oil pressure. (9) Open the valve to allow the cooling water to circulate to the compressor oil cooler, gear or turbine oil cooler, and seal jacket. The water circulating to the compressor oil cooler must be kept low enough in temperature to prevent the highest bearing temperature from exceeding a temperature of 130° F. Then adjust to give a temperature from 140° F. to 180° F. The seal bearing temperature should run approximately 160° F., while the thrust bearing temperature is running at approximately 145° F. under normal operating conditions. These temperatures should be checked closely until they maintain a satisfactory point. (10) After starting, the machine may surge until the air in the condenser has been removed. During this surging period, the machine should be run at a high speed; this helps in the process of purging. The condenser pressure should not exceed 15 p.s.i.g., and the input current to motor-driven machines should not run over 100 percent of the full load motor rating. The machine will steady itself out as soon as all the air has been purged. After leveling out the motor speed, the damper maybe adjusted to give the desired coolant temperature. The motor should be increased slowly, point to point. Do not proceed to the next speed point until the motor has obtained a steady speed. Keep a close observation on the ammeter to make sure that the motor does not become overloaded. 3. Normal and Emergency Shutdown. Normal shutdown procedures are performed in the same manner as emergency shutdown procedures. The following steps are used in shutting down the centrifugal machine: (1) Stop the motor by throwing the switch on the controller. (2) After the machine has stopped, turn off the water valve which supplies water to the compressor oil, gear oil cooler, and seal housing. (3) Shut down all pumps as required. 4. Shutdown periods may be broken down into two classes. The two classes are standby and extended shutdown. Standby shutdown may be machine must be available for immediate use;

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extended shutdown is defined as that period of time during which the machine is out of service. 5. Standby shutdown. The following checks must be made during standby shutdown and corrective action taken: (1) Maintain proper oil level in the oil reservoir and in the suction damper stuffing box. (2) Room temperature must be above freezing. (3) Machine must be kept free of leaks. (4) Purge unit must be operated as necessary to keep the machine pressure below atmospheric pressure. (5) If the machine pressure builds up in the unit due to room temperature rather than leakage of air into the machine, a small quantity of water circulated through the condenser or cooler will hold the machine pressure below atmospheric. Periodic operation of the purge unit will accomplish the same result. (6) The machine should be operated a few minutes each week to circulate oil and lower the refrigerant temperature. 6. Extended shutdown. If the system is free of leaks and the purge unit holds down the machine pressure, the following instructions and corrective actions must be taken in long shutdown periods: (1) Drain all water from the compressor, gear and turbine oil cooler, condenser, cooler, seal jacket, pumps, and piping if freezing temperatures are likely to develop in the machine room. (2) It is possible for the oil to become excessively diluted with refrigerant, causing the oil level in the pump chamber to rise. This level should not be allowed to rise into the rear bearing chamber; if this occurs, remove the entire charge of oil. 7. Logs and Records. A daily operating log is maintained at each attended plant for a record of observed temperature readings, waterflow, maintenance performed, and any unusual conditions which affect an installation operation. You are held responsible for keeping an accurate log while on duty. A good log will help you spot trouble fast. A typical log sheet has spaces for all important entries, and a carefully kept log will help to make troubleshooting easier. 8. A master chart of preventive maintenance duties, each component identified, is usually prepared by the supervisor and includes daily, weekly, and monthly maintenance services. The preventive maintenance items included on the chart are applicable to a specific installation. The items on the chart must be checked accordingly. Proper sustained operation is the result of good maintenance.

20. Systems Maintenance 1. It is very difficult to set up a definite maintenance schedule since so many operational factors must be considered. You must familiarize yourself with the operating procedures at your installation and follow recommendations. We shall discuss the proper procedures for replacing oil, charging the unit, removing refrigerant, and troubleshooting. 2. Replacing Oil. The following procedure is used in the renewal of the oil: (1) Pressure in the machine should be approximately 1 p.s.i.g. (2) Drain oil from the bottom of the main oil reservoir cover. (3) Remove the main oil reservoir cover and clean the chamber to remove all impurities. (4) Replace the main oil reservoir cover and secure tightly. (5) Remove the bearing access cover plates. (6) Lift up the shaft bearing caps by reaching through the bearing access hole and removing the two large capscrews. (7) Fill the bearing approximately three-fourths of the full charge, allowing the excess oil to flow into the main oil reservoir. (8) Replace the bearing cap and secure with capscrews. (9) Remove the brass plug from the thrust housing, and remove the strainer; clean and replace. (10) Replace the plug and secure. (11) Drain oil through the plug in back of the seal oil reservoir. (12) Remove the cover from the seal oil reservoir. (13) Remove the filter from the chamber; replace with a new filter. (14) Refill the reservoir with oil. (15) Replace the cover and secure tightly. (16) Drain the oil through the plug at the bottom of the atmospheric oil reservoir. (17) Remove the atmospheric oil filling plug and pour in fresh oil until the level is halfway in the atmospheric reservoir sight glass. (18) Replace the plug and secure tightly. (19) Operate the purge unit to remove as much air as possible. (20) Add oil to the atmospheric float chamber, if main oil reservoir indicates under-charge after short operation. 3. Charging the Unit. The manufacturer ships the refrigerant (R-11) in large metal drums which weigh approximately 200 pounds. At temperatures above 74° F., the drum will be under pressure. To prevent injury or loss of refrigerant, never open the drums to the atmosphere when they are above this temperature.

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It is possible to charge refrigerant from an open drum a 60° temperatures, although it is recommended that leaktight connections be made to the charging valve. The charging valve is located on the side of the cooler. To help in the charging procedure, each refrigerant drum has a special type plug installed on the side of the drum. This plug is specially engineered for charging purposes. The charging connection on the drum consists of a 2inch plug in which is inserted a smaller 3/4-inch plug. The 3/4-inch opening inside the drum is covered with a friction cap. The cap prevents leakage into or out of the drum when the 3/4-inch plug is unscrewed. 4. Refrigerant charging. To charge the machine with refrigerant, proceed as follows: (1) The machine must be under a vacuum. (2) Fit a 3/4-inch nipple into the standard globe valve and close the valve. (3) Remove the 3/4-inch plug inserted in the 2-inch plug from the drum. (4) Place the valve with the nipple into the opening, making sure that it is far enough in to push off the cap inside the drum. (5) Place the drum in a horizontal position near the cooler charging valve with the use of a hoist. The drum should be high enough to allow the refrigerant to flow as a liquid, by gravity, from the drum into the charging line. Rotate the drum so that the valve is at the bottom. (6) Connect the two valves (drum and cooler) with a copper tube and fittings, making sure all the joints are leakproof. (7) Open both valves and allow the refrigerant to flow into the cooler. Operate the machine to maintain a vacuum after the initial reduction to zero.

(8) When the drum is empty, close the valve on the cooler and disconnect the drum. Remove the valve for use with the next drum. Complete charging of the machine requires 1200 pounds of refrigerant. 5. Adding refrigerant to bring refrigerant to standard level. When adding refrigerant, use the same procedures that we have just discussed. Another method that can be used to add refrigerant is simply to allow the refrigerant to be drawn in as a gas. Let the drum rest on the floor and let the gas escape into the cooler while the machine is in operation or idle. 6. Removing refrigerant. In removing refrigerant from the cooler, the following procedure is recommended: (1) By use of the purge recovery unit, inject air into the machine until the pressure is 5 pounds gauge. (2) Connect tubing to the charging valve on the cooler and allow the refrigerant to discharge into the refrigerant drum. (3) Less loss of refrigerant will take place if the refrigerant is cold. Always allow space in the drum for refrigerant expansion. 7. Troubleshooting. The steps to be taken in detecting and correcting improper operation of the centrifugal machine are outlined in table 19. Use the proper methods for making these service adjustments, repairs, and corrections as outlined in this chapter. All settings, clearances, and adjustments must be made to manufacture’s specifications. The manufacturer’s maintenance catalog gives definite clearances, temperatures, pressure, and positions for adjustment of component parts. These tolerances must be set as recommended for efficient operation; carelessness in these settings can cause extensive damage to the machine.

TABLE 19

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TABLE 19-continued

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TABLE 19-Continued

Review Exercises The following exercises are study aids. Write your answer in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the text. 72

1.

The refrigerant charge is approximately ___________ pounds. (Sec. 9, Par. 1)

2.

Which component reduces the horsepower requirement per ton of refrigeration? (Sec. 9, Par. 2)

11. The oil pump is driven from the _____________________. (Sec. 10, Par. 7)

3.

(Agree)(Disagree) The refrigerant flows through the tubes in the cooler. (Sec. 9, Par. 3)

12. Which component does the pump lubricate first? (Sec. 10, Par. 8)

4.

The liquid refrigerant, from the condenser, enters the _______________. (Sec. 9, Par. 5)

13. How is oil returned from the oil pump drive gear? (Sec. 10, Par. 9)

14. How is the shaft seal actuated? (Sec. 1, Par. 10) 5. How much pressure is there within economizer chamber? (Sec. 9, Par. 5) the 15. What purpose do the two holes in the inner floating seal ring serve? (Sec. 10, Par. 11) 6. The suction gas is taken in by the compressor in _____________ the shaft. (Sec. 10, Par. 1) 16. The automatic stop valve is set to open at approximately ________________ pounds. (Sec. 10, Par. 12) 7. How are the wheels (impellers) protected from corrosion? (Sec. 1, Par. 2) 17. Which oil pressure gauges are mounted on the control panel? (Sec. 10, Par. 13) 8. Each bearing has ______________ large oil rings. (Sec. 10, Par. 3) 18. How is the oil heater energized shutdown? (Sec. 10; Par. 14) 9. What prevents interstage leakage of gas? (Sec. 10, Par. 4) during

19. (Agree)(Disagree) During operation the two polished surfaces of the shaft seal are held together with a spring. (Sec. 10, Par. 16) 20. What type oil is used in centrifugal compressors? (Sec. 10, Par. 17)

10. Which end of the compressor will axial thrust affect? (Sec. 10, Par. 5)

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21. The compressor gear drive (increases, decreases) the motor to compressor speed. (Sec. 11, Par. 1)

30. The motor furnished with the centrifugal machine is __________ phase, _________________ cycle, and has an ________________ rotor. (Sec. 13, Par. 1)

22. The grade of oil to use on a gear depends on __________, ___________, and ______________.(Sec. 11, Par. 3) 31. The secondary drum control is used to adjust the amount of resistance in the ___________________ of the motor which regulates motor ____________________ (Sec. 13, Par. 3)

23. When would you turn on the gear drive cooling water? (Sec. 11, Par. 5)

24. Worn bearings in the gear drive will cause ___________________. (Sec. 11, Par. 9)

32. Which switch is bypassed when the start button is held closed? (Sec. 13, Par. 4)

25. Which coupling uses a spool piece? (Sec. 12, Par. 1)

33. What is the secondary function condenser? (Sec. 14, Par. 1)

of

the

26. How is the hub expanded when it is to be installed on the shaft? (Sec. 12, Par. 2)

34. What prevents the discharge gas from directly hitting the condenser tubes? (Sec. 14, Par. 2)

27. The angular alignment of a coupling is checked with a _________________. (Sec. 12, Par. 3)

35. What precaution would you observe while removing the water box cover? (Sec. 14, Par. 3)

28. Which instrument is used to check the offset alignment of a coupling? (Sec. 12, Par. 4)

36. A burst rupture disc is caused __________________ (Sec. 14, Par. 6)

by

29. Which type of coupling can be lubricated while the compressor is running? (Sec. 12, Par. 8)

37. How can you determine the refrigerant charge of the system? (Sec. 14, Par. 11)

38. What is indicated when the temperature differential of the refrigerant and chilled water increases? (Sec. 14, Par. 13)

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39. ________________ is prevented by the hot gas bypass. (Sec. 15, Par. 1)

48. When are large quantities of air normally purged from the centrifugal refrigeration system? (Sec. 16, Par. 10)

40. Why is the liquid injector used in the hot gas bypass? (Sec. 15, Par. 2)

49. When is water drained from the separator unit? (Sec. 16, Par. 12)

41. What controls the amount of liquid refrigerant flowing to the hot gas bypass? (Sec. 15, Par. 3)

50. The four safety controls that will stop the centrifugal are _______________, ________, ___________, and _______________. (Sec. 17, Par. 1)

42. (Agree) (Disagree) The high-pressure control on the purge unit must be reset manually. (Sec. 16, Par. 3) 51. Which safety control does not require manual resetting? (Sec. 17, Par. 2) 43. Where is the weir and trap located on the purge unit? (Sec. 16, Par. 3) 52. What is the differential for the high condenser pressure control? (Sec. 17, Par. 3) 44. High head pressure indicates that ___________________. (Sec. 16, Par. 5) 53. How can you change (switch over) controllers? (Sec. 17, Par. 6) 45. How is the air pressure in the condenser released to the atmosphere? (Sec. 16, Par. 6) 54. The most efficient method of controlling the capacity of the centrifugal is to ____________________. (Sec. 18, Pars. 1 and 2)

46. What amount of water collected by the purge unit is an indication of leaky tubes? (Sec. 16, Par. 8)

47. When will a pressure drop exist across the pressure-regulating valve? (Sec. 16, Par. 9)

55. What will occur if you add more resistance to the rotor circuit of the drive motor? (Sec. 18, Par. 3)

56. When is suction damper control more effective than speed control? (Sec. 18, Par. 4)

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57. What is the position of the drum controller lever during startup? (Sec. 19, Par. 2)

63. What is one of the most probable causes of high condenser pressure? (Sec. 20, table 19)

58. What will cause the oil level to rise in the pump chamber during an extended shutdown? (Sec. 19, Par. 6)

64. Surging is caused by _________________, ________________, or ________________. (Sec. 20, table 19)

59. The pressure within the machine during an oil replacement operation should be approximately _______________ p.s.i.g. (Sec. 20, Par. 2)

65. What would occur if the economizer float valve stuck? (Sec. 20, table 19)

60. (Agree)(Disagree) The 2-inch plug in the refrigerant drum prevents leakage when the 3/4inch plug is removed. (Sec. 20, Par. 3)

66. What will cause a low "back of seal" oil pressure and a high seal oil pressure? (Sec. 20, table 19)

61. How is refrigerant charged into the system as a gas? (Sec. 20, Par. 5)

67. Noisy couplings are caused by ___________________, ________________, or _________________. (Sec. 20, table 19)

62. How do you pressurize the system to remove refrigerant? (Sec. 20, Par. 6)

68. (Agree)(Disagree) A high oil level in the speed gear will cause the gear to overheat. (Sec. 20, table 19)

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CHAPTER 4

Water Treatment
WATER USED IN air-conditioning systems may create problems with equipment, such as scale, corrosion, and organic growths. Scale formation is one of the greatest problems in air-conditioning systems that have watercooled condensers and cooling towers. Corrosion is always a problem in an open water recirculating system in which water sprays come in contact with air. The organic growth we are greatly concerned with is algae or slime. Since algae thrive on heat and sunlight they will be a problem in cooling towers. As a refrigeration specialist or technician you will save the military great sums of money if you test and treat your equipment water. For example, if you allowed scale to reach the thickness of a dime in a water-cooled condenser, it would cut the efficiency of the machine more than 50 percent. 21. Scale 1. When water is heated or evaporated, insolubles are deposited on metal surfaces. These deposits usually occur on the metal in the cooling towers, evaporative condensers, or inside the pipes and tubes of the condenser water system which have a recirculating water system. What causes scale? We can explain it in a simple formula: Ca (HCO3) + heat = CaCO3, + CO2 + H2O Calcium calcium carbon bicarbonate + heat = carbonate + dioxide + water In this formula the calcium carbonate is the villain. Calcium carbonate is the chief scale-forming deposit found in air-conditioning systems, but magnesium carbonate and calcium sulfate can also cause some degree of scaling. 2. Causes of Scale. A rising temperature decreases the solubility of calcium carbonate and calcium sulfate. This is known as reverse solubility. Sodium compounds such as table salt (sodium chloride), on the other hand, have a direct solubility. Suppose you take a glass of water 80° F. and dissolve table salt into the water. Soon you will saturate the water and no amount of stirring would cause any more salt to go into solution. But if you heat the water to 100° F., more salt can be dissolved into the solution. This dissolving action is known as direct solubility. But if you reaccomplish these steps using calcium saturates instead of table salt, you would see more solids precipitate out of the solution as the heat is increased. This action is suitably called reverse solubility and occurs in a water-cooled condenser cooling tower. 3. You will find that scale will form on heat transfer surfaces when you use water containing even a small amount of hardness. The pH value of the water determines if the hard water will cause scale or corrosion. The pH scale is from 0 to 14. Neutral water has a pH value of 7.0. Any reading under 7.0 is acid, while a reading above 7.0 is base or alkaline. 4. Let us compare pH to temperature. A thermometer measures the temperature of a solution, while pH measures the intensity of acid or base in a solution. As you know, pH means potential hydrogen. When a hydrogen atom has lost its electron (H+ ), it becomes a positive hydrogen ion. When a great many of these hydrogen atoms make this change, the solution will become highly acid and attack metals. When the hydrogen atom gains electrons, the solution will be base and have a pH value from 7.1 to 14. A base solution contains more hydroxyl ions (OH-). Scale will form when a base solution is exposed to a temperature rise, providing the hardness is 200 parts per million or higher. Notice the recommended pH for cooling towers in figure 69. 5. You will find that it is very important to test for solids in the water because solid content (hardness) determines the amount of scale formation. Hardness is the amount of calcium and magnesium compounds in solution in the water. Water containing 200 p.p.m. hardness and a pH indication of 9 or above will enhance the formation of scale. To avoid scale in cooling towers, you must control hardness. The maximum p.p.m. standards for cooling towers are

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titration directly measures the soap-consuming capacity of a water. You will study this test in the following paragraphs. 9. To begin the soap hardness test, measure 50 milliliters of the sample water into the hardness testing bottle. Add the standard soap solution to the water, 0.5 ml. at a time, from the soap burette, shown in figure 70. Shake bottle vigorously after each application and place it on its side. If no lather forms, continue adding 0.5-ml. portions of soap solution to a maximum of 6 ml and place the bottle on its side. Now you must use the formula below if you have a permanent lather to complete the test. If a permanent lather does not appear, see para 10. Hardness (p.p.m.) (total number or ml. of standard = 20 X soap solution required for permanent lather) 10. If a permanent lather does not appear after adding 6 ml. of the standard soap solution,

Figure 69. pH scale. 100 p.p.m. for makeup water and 200 p.p.m. for bleedoff water. 6. In cooling towers and evaporative condensers the water becomes harder due to evaporation. The term used to compare hardness to the circulating water to the makeup water is cycles of concentration. For example, 2 cycles of concentration indicate that the circulating water is twice as hard as the makeup water. If the makeup water contained 100 p.p.m., the circulating water would contain 200 p.p.m. To avoid this damaging concentration, you will find it is necessary to limit the cycles of concentration. Bleedoff is an effective method used for this purpose. The amount of bleedoff can be calculated by using the following formula: Cycles of concentration = bleedoff hardness (circulating water) makeup hardness For example: if the bleedoff (circulating water) is 150 p.p.m. and the makeup is 50 p.p.m., the cycles of concentration are 3. 7. There are many methods of treating water to prevent scale. A few of these are: • Bleedoff-regulate the amount of bleedoff water to keep the cycles of concentration within tolerance. • pH adjustment-maintain the pH of the water between 7 and 9, as near 8 as possible. • Add polyphosphates-keeps scale forming compounds in solution. • Zeolite water softening-exchanges a nonscale forming element for calcium and magnesium compounds. Before we discuss water softening, we will introduce the soap hardness test. 8. Soap Hardness Test. The soap hardness test is used to measure total hardness. The presence of calcium and magnesium salts, and to a lesser degree other dissolved minerals, constitutes hardness in water. Hardness can be best determined by soap titration. Soap 78

Figure 70. Soap hardness test equipment.

Figure 71. Accelator. repeat the test with a new water sample. This time dilute 25 ml. of the sample water with an equal quantity of zero-hardness water (distilled water). Conduct the test as you studied previously. When a permanent lather has been obtained, calculate the hardness as follows: (total number of ml. of standard soap solution required for permanent lather) 11. Water Softening. Hard waters are potable but are objectionable because they form scale inside of plumbing and on metal system components. A temporary hardness can be caused by magnesium bicarbonate. Hard water can be softened by two different methods. The first is the lime-soda process which changes calcium and magnesium compounds from soluble to insoluble forms and then removes these insolubles by sedimentation and filtration. The second and most common is zeolite or base-exchange process. This process replaces soluble calcium and magnesium compounds with soluble sodium compounds. 12. Lime-soda process. Lime-soda process plants are essentially the same as water filtration plants. Lime and soda ash are added to raw water; the softening reaction occurs during mixing and flocculation. The precipitated calcium and magnesium a removed by sedimentation and filtration. An additional process, called recarbonation, which is the introduction of carbon dioxide gas, is frequently applied immediately prior to filtration. If the raw water has high turbidity, the turbidity is partial removed by sedimentation prior to the adding of the lime and soda. = 40 X 79 13. Zeolite process. The zeolite process is usually used for water which has low turbidity and does not require filtration. Treatment may be given to the entire supply at one point. This system is commonly used to soften water for special uses, such as for the control of scale. In such cases, the treatment units are located at points near the equipment requiring treated water. 14. Turbidity is a muddy or unclear condition of water which is caused by suspended silt, clay, sand, or organic materials such a decaying vegetation or animal waste. Turbidity can be corrected by sedimentation, filtration, or traps. In most cases the water supply and sanitation personnel will supply you with usable, potable water. 15. Softening devices. Softening devices include patented equipment such as the Accelator and Spiractor. The Accelator is also used as a combined flocculation and sedimentation unit without softening. When this unit is operated before filtration to treat water with low suspended solids and low alkalinity, it may be necessary to add lime or clay to add weight and prevent rising floc. 16. The Accelator, shown in figure 71, is a suspended solid clarifier. Precipitates which are formed are kept in motion by a combination of mechanical agitation and hydraulic flow. Velocity of waterflow through the system is controlled to keep precipitates in suspension at a level where water passes through them. The accumulated

with a swirling motion. The upward velocity keeps the granular material in suspension. As the water rises, velocity decreases to a point where material is no longer in suspension. The contact time, 8 to 10 minutes, is enough to complete softening actions. Softened water is drawn off from the top of the cone. The size of calcium carbonate granules increases during the process, increasing the bulk of granules in the unit. The water level of the cone is kept down to the desired point by withdrawing the largest particles from the bottom. New material must be added, which can be produced by regrinding and screening the discharged material. Softened water is usually filtered through a sand filter to move turbidity. Advantages of the equipment are its small size, low installation cost, rapid treatment lack of moving parts and pumping equipment, and elimination of sludge disposal problems. The unit is most effective when hardness is predominantly calcium, there is less than 17 p.p.m. magnesium hardness (expressed as calcium carbonate), water temperature is about 50° F., and turbidity is less than 5 p.p.m. 18. Zeolite (ion exchange). Ion exchange is a chemical operation by which certain minerals that are ionized or dissociated in solution are exchanged (and thus removed) for other ions that are contained in a solid exchange medium, such as a zeolite sandbed. An example is the exchange of calcium and magnesium, in solution as hardness in water, for sodium contained in a sodium zeolite bed. The zeolites used in the process of ion exchange are insoluble, granular materials. A zeolite may be classified as follows: glauconite (or green sand), precipitated synthetic, organic (carbonaceus), synthetic resin, and clay. Various zeolites are used, depending on the type of water treatment required. Most zeolites possess the property cation, or base exchange, but anion exchangers are also available and may be used when demineralization of water is required. In the course of treating water, the capacity of the zeolite bed to exchange ions is depleted. This depletion requires the bed to be regenerated by the use of some chemical that contains the specific ion needed for the exchange. For instance, when a sodium zeolite is used to soften water by exchanging the sodium ion for the calcium and magnesium ions of hard water, the zeolite gradually becomes depleted of the sodium ion. Thus, it will not take up the calcium and magnesium ions from the water passing through the bed. The sodium ion is restored to the zeolite by uniformly distributing a salt or brine solution on top of the bed and permitting it to pass evenly down through the bed. The salt removes the calcium and magnesium taken up by the bed as soluble chlorides and restores the zeolite to its original condition. Beds may also be regenerated with acid, sodium carbonate,

Figure 72. Spiractor precipitate is called the sludge blanket. When the Accelator is operating properly, the water above the sludge blanket and flowing over the weirs is clear. Operation depends on balancing the lift of particles by the velocity of upward flowing water against the pull of gravity. When the velocity of the water is gradually decreased, a point is reached at which the particles are too heavy to be supported by the velocity of the water. Continuous treatment builds up the sludge blanket which is drawn off as required. Operation of the equipment is covered in detail in the manufacturer’s instruction manuals. 17. The Spiractor, shown in figure 72, consists of an inverted conical tank in which the lime-soda softening reactions take place in the presence of a suspended bed of granular calcium carbonate. In operation, the tank is slightly more than half filled with 0.1 to 0.2 millimeter granules. Hardwater and chemicals enter the bottom of the unit close to each other. They mix immediately as the treated stream of water rises through the granular bed

80

sodium hydroxide, or potassium permanganate, depending on the type of zeolite being used. 19. In addition to the problem of scale, the refrigeration man knows that corrosion is a constant problem. Let us now study corrosion, its causes, its effects, and its control. 22. Corrosion 1. In the refrigeration/air-conditioning field, corrosion has long been a problem. Even in the modern missile complexes, corrosion is prevalent. Corrosion is very difficult to prevent, but it can be controlled. Before we can control corrosion, we first must understand what causes it. 2. The effects of corrosion differ as to the type of corrosion, such as uniform, pitting, galvanic, erosioncorrosion, and electrochemical. We must understand various ways of treating the system to control these types of corrosion. Corrosion is generally more rapid in liquids with a low pH factor than in alkaline solutions. 3. Types of Corrosion. An air-conditioning system may have several types of corrosion in the water system. Many of these types are undoubtedly familiar to you. 4. Uniform corrosion. One of the most common types of corrosion encountered in acid environments is known as uniform corrosion. This is caused by acids, such as carbonic, which cause a uniform loss of metal throughout the condensating water system. 5. Pitting corrosion. Pitting corrosion is a nonuniform type, the result of a local cell action produced when a particle, flake, or bubble of gas deposited on a metal surface. The pitting is a local accelerated attack, which causes a cavity in the metal but does not affect the surrounding metal. Oxygen deficiency under such a deposit sets up an anodic action. This area keeps producing such action until the penetration finally weakens the structure and it falls, developing a pinhole leak. 6. Galvanic corrosion. When dissimilar metals which are capable of carrying electric current are present in a solution, galvanic corrosion occurs. This action is similar to the electroplating process used in industry to bond or plate dissimilar metals. When two metals similar to each other are joined together, there is little reaction. But the coupling of two metals from different groups causes accelerated corrosion in one of the two metals. When using large amounts of copper in a system and a few unions of steel, the steel will corrode at a rapid rate. In such cases you should install nonferrous metal instead of steel. Corrosion inhibitors reduce the corrosion rate but will not eliminate galvanic corrosion.

7. Erosion-corrosion. Erosion-corrosion is caused by suspended matter or air bubbles in a rapidly moving water. The matter can be fine to coarse sand, depending on the velocity of the water. Usually the greatest amount of erosion-corrosion will take place at elbows and Ubends. Another place where erosion-corrosion takes place is on the impellers of centrifugal pumps. 8. Good filtration installations will remove grains of sand and other matter that are large enough to cause erosion-corrosion. To get rid of air tapped in a system, it is recommended that hand- or spring-operated bleed valves be installed in the highest point of the water system. Purging the water system gets rid of the air bubbles that enter the system in the makeup water. 9. Electrochemical corrosion. Electrochemical corrosion occurs when a difference in electrical potential exists between two parts of a metal in contact with an electrolyte (water). The difference in potential will cause electric current to flow. The difference in potential may be set up by two dissimilar metals, by a difference in temperature or amount of oxygen, or by the concentration of the electrolyte at the two points of contact with the metal. The anode is the point at which the current flow is from the metal to the electrolyte; it is here that corrosion occurs. The cathode, which is usually not attached, is the point of current flow from the electrolyte to the metal. This action is shown in figure 73. 10. Corrosion Inhibitor. The most common chemicals used as inhibitors are chromates and polyphosphates. These inhibitors alone serve only to decease the rate of corrosion, but if other water treatments are used in conjunction with them, corrosion may be nearly stopped. 11. Chromates. Chromates are seldom present n untreated water; however, they may occur as a result of industrial waste contamination. The chromates are used extensively to inhibit corrosion and are effective in the water air-conditioning systems in concentrations of 200500 p.p.m. at a pH of 7.0 to 8.5. Chromates are the most commonly used corrosion inhibitors in chilled water systems. For corrosion prevention the most favorable range is with the pH from 7.5 to 9.5, but scaling becomes a problem at the higher pH range. Consequently, the pH should be held near the lower range where corrosion protection is excellent. Because it is more economical, sodium bichromate (Na2Cr2O72H2O) is the most commonly used chromate compound. Sodium chromate (Na2CrO4) is also used widely. 12. Chromate concentration is stated in p.p.m. 81

Figure 73. Causes and effects of corrosion. Chromates are anodic inhibitors but can intensify pitting if they are used in insufficient amounts. Field tests must be performed to be sure the required amount of chromate is in the water, and to check the pH. Corrosion is greatest when the pH is between 0 to 4.5. 13. Chromate concentration is tested by color comparison. The color of the treated water is matched against a known chromate disc. For example, if the sample of treated water matches a tube known to contain 200 p.p.m. of chromate, the sample would also contain 200 p.p.m. of chromate. 14. Polyphosphates. Phosphates, particularly the polyphosphates, are used in cooling water treatment. The ability to prevent metal loss with polyphosphate treatment is inferior to the chromate treatment previously discussed. In addition, pitting is more extensive with polyphosphates. Unlike chromate, high polyphosphate concentrations are not practical because of the precipitation of calcium phosphate. 15. One advantage of using polyphosphates is that there is no yellow residue such as produced by chromates. This highly undesirable residue is often deposited on buildings, automobiles, and surrounding vegetation by the wind through cooling towers or evaporative condensers, when the system is treated by chromates. Also, polyphosphate treatment reduces corrosion products (sludge and rust) known as tuberculation. 16. A factor limiting the use of polyphosphates in 82 cooling water systems is the reversion of polyphosphates to orthophosphates. Orthophosphates provide less protection than polyphosphates, and orthophosphates react with the calcium content of the water and precipitate calcium phosphate. This precipitation forms deposits on heat exchanger surfaces. The reversion of polyphosphates is increased by long-time retention and high water temperatures. Bleedoff must be adjusted on the condenser water system to avid exceeding the solubility of calcium phosphate. 17. The test used to determine the amount of polyphosphates in the system is similar the chromate color comparison test. 18. Corrosion inhibitor feeders. Many times a simple bag will be used to feed the chemicals into the water. The chemicals, in pellet or crystal form, are placed in nylon net bags and hung in the cooling tower sump. However, chilled water and brine systems require the use of a pot type feeder similar to the feeder shown in figure 74. 19. The chemical charge is prepared by dissolving the chemicals in a bucket and then filling the pressure tank (F) with the solution. Valves B and C are closed, and valve A is opened to drain the water out of the tank. After the water is drained, close valve A and open valves D and E. Then fill tank (F) with the dissolved chemical solution. Opening valves B and C after you have closed valves D and E will place the feeder in operation. The feedwater from the discharge

winds. Algae thrive in cooling towers and evaporative condensers, where there is abundance of sunlight and high temperatures to carry on their life’s processes. Algae formations will plug nozzles and prevent proper distribution of water, thus causing high condensing pressures and reduced system efficiency. In relation to the larger subject of algae, we will study residual chlorine tests, chlorine demand tests, pH determination, pH adjustment, chlorine disinfectants, hypochlorination, and chlorination control. 2. Residual Chlorine Test. The growth of algae is controlled by chlorination. The residual chlorine test is the test that we make to determine the quantity of available chlorine remaining in the water after satisfaction of the chlorine demand has occurred. Orthotolidine is the solution used in making the residual chlorine test. This solution reacts with the residual chlorine, taking on a color which is matched against a standard color in the comparator disc. Readings up to 5 p.p.m. may be read from the comparator disc. One p.p.m. will control algae and 1.5 p.p.m. will kill algae. 3. The time required for full development of color by orthotolidine depends on the temperature and kind of residual chlorine present. You will find that the color will develop several times faster when water is at 70° F. than when it is near the freezing point. For this reason, you must warm up cold samples quickly after mixing the sample with orthotolidine. Simply holding the sample tube in your hand is sufficient. 4. For samples containing only free chlorine, maximum color appears almost instantly and begins to fade in a minute. You must take the reading at maximum color intensity. However, a longer period is required for full color development of chloramines which may be present. Since samples containing combined chlorine develop their color at a rate primarily dependent upon temperature and to a lesser extent on the quantity of nitrogenous material present, observe the samples frequently and use their maximum value. 5. At 70° F. the maximum color develops in about 3 minutes, while at 32° F. it requires 6 minutes. The maximum color starts to fade after about 1½ minutes. Therefore, in the orthotolidine-arsenite (OTA) test, the water temperature should be about 70° F and the sample read at maximum color and in less than 5 minutes. Preferably, permit the color to develop in the dark. Read the sample frequently to insure observation of maximum color. 6. Use enough chlorine so that the residual

Figure 74. Pot type feeder. side of the pump with force the solution into tie suction side of the pump. Within a few minutes, the solution will be washed out of the tank. This feeder is nonadjustable. 20. Another type of feeder you may use is the pot type proportional feeder. This type, similar to the one shown in figure 74, has an opening to permit charging with chemicals in briquette or lump form. A portion of the water to be treated is passed through the tank, gradually dissolving the chemicals. 21. The degree of proportionality is questionable at times, because there is little control over the solution rate of the briquettes or the chemical incorporated in them. Although this system is classified as proportional, it cannot be used where accuracy of feed is required. It is used successfully in our application because we have a large range in p.p.m. to control-for example, 250-300 p.p.m. chromate. 22. Now that we have studied corrosion and corrosion control, let’s discuss algae. 23. Algae 1. Algae are slimy living growth of one-celled animals and plants. They may be brought by birds or high 83

in the finished water after 30 minutes of contact time will be as follows:

present, the pH value, and the temperature of the water. Remember that the high pH and low temperature retard disinfection by chlorination. For comparative purposes, it is imperative that all test conditions be stated, such as water sample temperature or room temperature. 11. The smallest amount of residual chlorine considered to be significant is 0.1 mg/1 Cl. Some of the chlorine-consuming agents in the water are nonpathogenic, but they contribute to the total chlorine demand of the water just as other agents do. 12. Chlorine demand in most water is satisfied 10 minutes after the chlorine is added. After the first 10 minutes of chlorination, disinfection continues but at a diminishing rate. A standard period of 30 minutes of contact time is used to insure that highly resistant organisms have been destroyed, provided that a high enough dosage has been applied. 13. The chlorine demand test is used as a guide in determining how much chlorine is needed to treat a given water. Briefly, the test consists of preparing a measured test dosage of chlorine, adding it to a sample of the water to be treated, and adding the resultant residual after 30 minutes of contact time. The required dosage is then computed; it is the chlorine needed to equal the sum of the demand plus the minimum contact residual. 14. To determine the chlorine demand, calcium hypochlorite, containing 70 percent available chlorine, is used for the test. Mix 7.14 grams of calcium hypochlorite (Ca(OCL)2) with 1000 cc. of the best water available to produce 5000 p.p.m. chlorine solution. One milliliter of this standard solution (reagent), when added to 1000 cc. of the water to be tested, equals 5 p.p.m. chlorine test dosage. Thus, with 1 milliliter of the reagent equaling 5 p.p.m., any proportionate test dosage may be arrived at by using one-fifth, 0.2 ml., of the reagent in 1000 cc. of the water for each p.p.m. of chlorine dosage desired. After adding a test dosage of a known strength of a 1000-cc. sample of the water to be tested (5 p.p.m., or 1 ml. of the reagent is normally used), wait 30 minutes and run a chlorine residual test. You subtract the chlorine residual from the test dosage to obtain the chlorine demand. 15. If you do not obtain a residual after a 30-minute period, the test is invalid and must be repeated. You increase the reagent by 5 p.p.m. each time until a residual is obtained. If, for example, the test were repeated two times, the results would be recorded as follows:

These residuals are effective for water temperatures ranging from 32° to 77° F. Bactericidal efficiency of chlorine increases with an increase in water temperature. 7. Two types of residual chlorine have been mentioned. The first is the free available chlorine which can be measured by the OTA test. It is valuable because it kills algae quickly. The second is the combined available chlorine, produced by the chloramines, a slower acting type and therefore one which requires a higher concentration to achieve an equivalent bactericidal effect in the same contact time. 8. The orthotolidine-arsenite (OTA) test is the preferable one in determining chlorine residuals since it permits the measurement of the relative amounts of free available chlorine, combined available chlorine, and color caused by interfering substances. The test is best performed in a laboratory because the accuracy of the results is dependent upon the quantity of available chlorine preset, the adherence to time intervals between the addition of reagents and the temperature of the sample. With water temperatures above 68° F, the accuracy decreases, whereas below this temperature, it increases. 9. The free available chlorine residual subtracted from the total residual chlorine would equal the combined available residual. You recall that the combined available residual is actually that slower acting residual created by the chloramines which have formed in the water. Since the OT test measures only the total available chlorine residual, it impossible to determine the combined available chlorine residual with this test. With the orthotolidine test, both the free and combined available chlorine are measured. If it is desired to determine whether the residual is present in either the free or combined form, it is necessary to employ the orthotolidine-arsenite test. 10. Chlorine Demand Test. The chorine demand of water is the difference between the quantity of chlorine applied in water treatment and the total available residual chlorine present at the end of a specified contact period. The chlorine demand is dependent upon the amount of chlorine applied (amount applied is dependent upon the free available and combined available chlorine), the nature and the quantity of chlorine-consuming agents

16 pH Determination. The pH determination

84 and residual chlorine tests are both made with the color comparator. Knowing the pH value of water is important for several reasons. First, the pH value influences the amounts of chemicals used for coagulation. Second, the disinfecting action of chlorine (to control algae) is retarded by a high pH. If pH is above 8.4, the rate of disinfection decreases sharply. Third, the corrosion rate is lowest at a pH of 14, increases to a pH of 10, and remains essentially uniform until a pH of 4.3 is reached, when it increases rapidly. 17. But, how do you determine the pH value of water with the comparator? Three indicator solutions are supplied for making pH determinations with the comparator. Bromcresol purple green is used for the pH range from 4.4 to 6.0. Bromthymol blue is used for pH values from 6.0 to 7.6. Cresol red-thymol blue is used for pH values from 7.6 t 9.2. Standard color discs covering each range are supplied with the comparator. Generally, the bromthymol blue indicator is used first since most pH values fall within its range. The readings for pH are made immediately after adding the indicator. You should keep in mind that clorimetric indicators provide sharp changes in readings over a short span of the pH range, but once the end of the range has been reached, little change in color is noted even though a considerable change in pH takes place. For this reason readings of 5.8 to 6.0, obtained when using the bromcresol purple green indicator, should be checked by taking a reading with bromthymol blue. Similarly, pH readings of 7.6 to 7.8 on the cresol red-thymol blue disc should be checked on the bromthymol blue disc. 18. To determine the pH value, fill the tubes to the mark with the water sample. Add the indicator solution to one tube in the amount specified by the manufacturer, usually 0.5 ml. (10 drops) for a 10-ml. sample tube and proportionally more for larger tubes. Mix the water and indicator and place the tube in the comparator. 19. After you place the tube in the comparator, you match for color and read pH directly. If the color is at either the upper or lower range of the indicator selected, repeat the test with the next higher or lower indicator. 20. If a color comparator is not available, methyl orange and phenolphthalein indicators may be used to make an approximate pH determination. These indicators are used primarily for alkalinity determinations, but they can be used for a rough check of pH values. 21. To determine a low pH that is around 4.3, fill a test bottle to the 50-ml. mark with a sample of the water to be tested and add 2 drops of methyl orange indicator. Observe the test bottle against a white background and interpret the color thus: pinkish red, pH below 4.3; yellow, pH above 4.3. 85 22. To determine a high pH that is around 8.3, fill a test bottle to the 50-ml. mark and add 2 drops of phenolphthalein indicator. Observe the test bottle against a white background and interpret thus: pink, pH above 8.3; colorless, pH below 8.3. 23. pH Adjustment. Caustic soda, soda ash, and sodium hydroxide can be added to water to increase the pH. The caustic soda or sodium hydroxide treatment uses a solution feeder to add the chemical. This is the type of feeder used to chlorinate water for algae control. Soda ash is added by means of a proportioning pot type feeder. The amount you would add depends upon the pH of the water. Test the water frequently while adding these chemicals and stop the treatment when the desired pH level is reached. 24. Acids are added to lower the pH. The types used are sulphuric, phosphoric, and sodium sulfate. They are added through solution feeders. Add only enough acid to reduce the pH (alkalinity) to the proper zone. The zone is usually 7-9 pH, preferably a pH of 8. 25. Chlorine Disinfectants. Chlorine disinfectants are available in a number of different forms. The two forms that we will use are calcium and sodium hypochlorite. 26. Calcium hypochlorite. Calcium hypochlorite, Ca (OCl)2, is a relatively stable, dry granule or powder in which the chlorine is readily soluble. It is prepared under a number of trade names, including HTH, Perchloron, and Hoodchlor. It is furnished in 3- to 100-pound containers and has 65 to 70 percent of available chlorine by weight. Because of its concentrated form and ease of handling, calcium hypochlorite is preferred over other hypochlorites. 27. Sodium hypochlorite. Sodium hypochlorite, NaOCl, is generally furnished as a solution that is highly alkaline and therefore reasonably stable. Federal specifications call for solutions having 5 and 10 percent available chlorine by weight. Shipping costs limit its use to areas where it is available locally. It is so furnished as powder under various names, such as Lobax and HTH-I5. The powder generally consists of calcium hypochlorite and soda ash, which react in water to form sodium hypochlorite. 28. Hypochlorinators. Hypochlorinators, or solution feeders, introduce chlorine into the water supply in the form of hypochlorite solution. They are usually modified positive-displacement piston or diaphragm mechanical pumps. However, hydraulic displacement hypochlorinators are also used. Selection of a feeder depends on local

conditions, space requirements, water pressure conditions, and supervision available. Fully automatic types are actuated by pressure differentials produced by orifices, venturis, valves, meters, or similar devices. They can also be used to feed chemicals for scale and corrosion control. Common types of hypochlorinators are described below. 29. Proportioneers Chlor-O-Feeder. The Proportioneers Chlor-O-Feeder is a positive-displacement diaphragm type pump with electric drive (fig. 75) or hydraulic operating head (fig. 76). Maximum capacity of the most popular type, the heavy-duty midget Chlor-OFeeder, is 95 gallons of solution in 24 hours. 30. a. Semiautomatic control. The motor-driven type may be cross connected with a pump motor for semiautomatic control. The hydraulic type can be synchronized with pump operation by means of a solenoid valve. 31. b. Fully automatic control. Motor-driven types are made fully automatic by use of a secondary electrical control circuit actuated by a switch inserted in a disc or compound-meter gearbox. This switch closes momentarily each time a definite volume of water passes through the meter, thus starting the feeder. A timing element in the secondary circuit shuts off the feeder after a predetermined number of feeder strokes; the number of strokes is adjustable. In the hydraulic type, shown in figure 77, the meter actuates gears in a Treet-O-Control gearbox which in turn controls operation of a pilot valve in the water or air supply operating the feeder. The dosage rate is controlled by waterflow through the meter, thus automatically proportioning the treatment chemical. Opening and closing frequency of the valve thus determines frequency of operation of the Chlor-O-Feeder. 32. Wilson type DES hypochlorinator. The Wilson type DES hypochlorinator is a constant-rate, manually adjusted, electric-motor-driven, positive-displacement reciprocating pump for corrosive liquids, and is shown in figure 78. Maximum capacity is 120 gallons of solution per day. This unit is a piston pump with a diaphragm and oil chamber separating the pumped solution from the piston to prevent corrosion of working parts. 33. Model S hypochlorinator (manufactured by Precision Chemical Pump Corporation). The Model S hypochlorinator, shown in figure 79, is a positivedisplacement diaphragm pump with a manually adjustable feeding capacity of 3 to 60 gallons per day. A motordriven eccentric cam reciprocates the diaphragm, injecting the solution into the main supply. Use of chemically resistant plastic and synthetic rubber in critical parts contributes to long operating life.

34. Chlorination Control. To estimate dosage when no prior record of chlorination exists or where chlorine demand changes frequently: (1) Determine chlorine demand, or start chlorine feed at a low rate and raise feed by small steps; at the same time make repeated residual tests until a trace is found. Observe rate of flow treated and rate of chlorine feed at this point. Chlorine demand then equals dosage and is determined from the following equation:

(2) Add the minimum p.p.m. required residual to the p.p.m. demand in order to estimate the p.p.m. dosage required to obtain a satisfactory residual. Then set chlorinator rate of feed in accordance with the above estimated p.p.m dosage. Further upward adjustment after making residual tests is usually required because the demand increases as the residual is increased. 35. Rate of feed of hypochlorinators is found from the loss in volume of gallons of solution by determining change in depth of solution in its container. Knowing the solution strength, the pounds of chlorine used can be calculated:

36. Available chlorine content of the chlorine compound used must be known in order to calculate the rate of hypochlorite-solution feed. Available chlorine is usually marked on the container as a percentage of weight. Values generally are as follows: Calcium hypochlorite .........................70 percent Sodium hypochlorite (liquid) ..............10 percent (varies) (1) To find the actual weight of chlorine compound to be added, use the equation:

(2) To find the amount of 1-percent dosing solution needed to treat a given quantity of water with desired dosage, use the equation:

(3) To prepare various quantities of 1-percent dosing solution, use the amounts given table 20. (4) To find the rate of feed of chlorine in gallons per day, use the equation:

86

TABLE 20

(5) To feed the pounds of chlorine compound needed to prepare dosing solution of a desired strength, use the equation:

(6) To find the gallons of hypochlorite stock solution needed to prepare dosing solution of a required strength, use the equation:

37. CAUTION: Make dosing solutions strong enough so that the hypochlorinator can be adjusted to feed one-half its capacity per day or less. Avoid using a calcium hypochlorite dosing solution stronger than 2 percent, even if it is necessary to set the machine to feed its full day capacity. If calcium hypochlorite solution stronger than 2 percent is required when the feed is set a maximum, small amounts of sodium hexametaphsphate in the solution will permit maximum concentrations up to 5 percent. Solutions of sodium hypochlorite may be fed in greater concentrations. 38. Another problem area besides algae is turbid water, so let’s now study turbidity. 24. Turbidity 1. Turbidity in water is caused by suspended matter in a finely divided state. Clay, silt, organic matter, microscopic organisms, and similar materials are contributing causes of turbidity. 2. While the terms “turbidity” and “suspended matter” are related, they are not synonymous. Suspended matter is the amount of material in a water that can be removed by filtration. Turbidity is a measurement of the optical obstruction of light that is passed through a water sample. 3. Turbid makeup water to cooling systems may

cause plugging and overheating where solids settle out on heat exchanger surfaces. Corrosive action is increased because the deposits hinder the penetration of corrosion inhibitors. We will cover the Jackson turbidity test and turbidity treatment. 4. Turbidity Test. The Jackson candle turbidimeter is the standard instrument used for making turbidity measurements. It consists of a graduated glass tube, a standard candle, and a support for the candle and tube. The glass tube and the candle must be placed in a vertical position on the support so that the centerline of the glass tube passes through the centerline of the candle. The top of the support for the candle should be 7.6 centimeters (3 inches) below the bottom of the tube. The glass tube must be graduated, preferably to read direct in turbidities (p.p.m.), and the bottom must be flat and polished. Most of the tube should be enclosed in a metal or other suitable case when observations are being made. The candle support will have a spring or other device to keep the top of the candle pressed against the top the support. The candle will be made of beeswax and spermaceti, gauged to burn within the limits of 114 to 126 grains per hour. 5. Turbidity measurements are based on the depth of suspension required for the image of the candle flame to disappear when observed through the suspension. To insure uniform results, the flame should be kept a constant size and the same distance below the glass tube. This requires frequent trimming of the charred portion of the candle wick and frequent observations to see that the candle is pushed to the top of its support. Each time before lighting the candle, remove the charred part of the wick. Do not keep the candle lit for more than a few minutes at a time, for the flame has a tendency to increase in size. 6. The observation is made by pouring the suspension into the glass tube until the image of the candle flame just disappears from view. Pour slowly when the candle becomes only faintly visible. After the image disappears, remove 1 percent of the suspension from the tube; this should make the image visible again. Care should be taken to keep the glass tube clean on both 87

Figure 75. Proportioneers heavy-duty midget Chlor-O-Feeder. 88

Figure 76. Hydraulically driven hypochlorinator.

Figure 77. Motor-driven hypochlorinator. 89

Figure 78. Wilson type DES hypochlorinator. the inside and the outside. The accumulation of soot or moisture on the bottom of the tube may interfere with the accuracy of the results. The depth of the liquid is read in centimeters on the glass tube, and the corresponding turbidity measurement is recorded in parts per million. 7. Turbidity Treatment. Filtration is the most common method for removing suspended matter that you will encounter. Coagulants, flocculators, and sedimentation basins are also used but are more common to large water treatment facilities. 8. Sand and anthracite coal are the materials commonly used as filter media. The depth of the filter bed can range up to 30 inches, depending upon the type of filter you will be using. You will find that quartz sand, silica sand, and anthracite coal are used in most gravity and pressure type filters. 9. Gravity filters. As the name implies, the flow of water through the filter is obtained through gravity. These filters are not common to our career field because coagulants and flocculation are required before effective filtration can occur. 10. Pressure filers. Pressure filers are more widely used because they may be placed in the line under pressure and thus eliminate double piping. 11. Pressure filters may be of the vertical or horizontal type. The filter shells are steel, cylindrical in shape; with dished heads. Vertical filters range in diameter from 1 to 10 feet, with capacities from 2.4 g.p.m. to 235 g.p.m. at a filtering rate of 3 gals/sq.ft/min. Horizontal filters, 8 feet in diameter, may be 10 to 25 feet long, with capacities from 210 g.p.m. to 570 g.p.m. 12. Filter operation. When you initially operate, or operate the filter after backwashing it, you should allow the filtered water to waste for a few minutes. This procedure rids the system of possible suspended solids remaining in the underdrain system after backwashing and also permits a small amount of suspended matter to accumulate on the filter bed. As soon as the filter produces clear water, the unit is placed in normal service. 13. During operation, the suspended matter removed by the filter accumulates on the surface of the filter. A loss-of-head gauge indicates when backwashing is necessary. Backwashing is necessary when the gauge reads 5 p.s.i.g. 14. Backwashing rates are much higher than filtration rates because the bed must be expanded and the suspended matter washed away. This backwashing is continued for 5 to 10 minutes; then the filter is returned to service. 15. We have discussed the testing and treatment of water to be used in our systems. To make

Figure 79. Model S hypochlorinator. 90

valid tests and prescribe proper treatment, you must understand the proper methods of water sampling. 25. Sampling 1. Frequent chemical and bacteriological analyses or tests of raw and treated water are required to plan and control treatment and to insure a safe and potable water. Facilities needed for water analysis depend on the type of supply and treatment. They vary from a simple chlorine residual and pH comparator to a fully equipped laboratory. Our discussions here are not concerned with analysis as such, since the term “analysis” implies that we completely disassemble water into its elementary composition. In complete water analysis your required task is to obtain valid samples to be forwarded to the proper laboratories. The sampling and testing with which you personally are concerned are simple and consist only of routine type tests that can be made in the field or in a base laboratory with simple chemicals and comparator equipment. 2. Sampling Methods. Sampling is an extremely important operation in maintaining quality of water supply. Unless the water sample is representative, test results cannot be accurate. You must be very careful to obtain a sample that is not contaminated by any outside source, such as dirty hands, dirty faucets, dirty or unsterilized containers. Do not sabotage the entire operation before it gets a good start. Follow approved, correct sampling methods like those outlined here and use only chemically clean sample containers. 3. Chemical analysis. The following precautions and actions are necessary when samples for chemical analysis are taken: a. Wells. Pump the well until normal draw-down is reached. Rinse the chemically clean sample container with the water to be tested and then fill it. b. Surface supplies. Fill chemically clean raw water sample containers with water from the pump discharge only after the pump has operated long enough to flush the discharge line. Take the water sample from the pond, lake, or stream with a submerged sampler at the intake depth and location. c. Plant. Take samples inside a treatment plant from channels, pipe taps, or other points where good mixing is obtained. d. Tap or distribution system. Let tap water run long enough to draw the water from the main before taking samples. e. Sample for dissolved gas test. Take care to prevent change in dissolved gas content during sampling. Flush the line; then attach a rubber hose to the tap and let

the water flow until all air is removed from the hose. Drop the end of the hose to the bottom of a chemically clean sample bottle and fill gently, withdrawing the hose as the water rises. Test for dissolved gas immediately. 4. Bacteriological analysis. In obtaining samples for bacteriological analysis, contamination of the bottle, stopper, or sample often causes a potable water supply to be reported as nonpotable. Full compliance with all precautions listed in the paragraphs below is necessary to assure a correct analysis. a. Bottles. Use only sterilized bottles with glass stoppers. Cover the stopper and the neck of the bottle with a square of wrapping paper or other guard to protect against dust and handling. Before sterilizing the sample bottle to be used to test chlorinated water, place 0.02 to 0.05 gram of sodium thiosulfate, powdered or in solution, in each bottle to neutralize chlorine residual in sample. Keep the sterilization temperature under 392° F. to prevent decomposition of the thiosulfate. b. Sampling from a tap. After testing for chlorine residual, close the tap and heat the outlet with an alcohol or gasoline torch to destroy any contaminating material that may be on the lip of the faucet. Occasionally, extra samples may be collected without flaming the faucet to determine whether certain faucet outlets are contaminated. Flush the tap long enough to draw water from the main. Never use a rubber hose or other temporary attachment when drawing a sample from the tap. Without removing the protective cover, remove the bottle stopper and hold both cover and stopper in one hand. Do not touch the mouth of the bottle or sides of the stopper. Fill the bottle three-quarters full. Do not rinse the bottle, since thiosulfate will be lost. Replace the stopper and fasten the protective cover with the same care. c. Sampling from tanks, ponds, lakes, and streams. When collecting samples from standing water, remove the stopper as previously described and plunge the bottle, with the mouth down and hold at about a 45° angle, at least 3 inches beneath the surface. Tilt the bottle to allow the air to escape and to fill the bottle. When filling the bottle, move it in a direction away from the hand holding it so water that has contacted the hand does not enter the bottle. After filling, discard a quarter of the water and replace the stopper. d. Transporting and storing samples. Biological changes occur rapidly. Therefore, if the test is to be made at the installation, perform the test within an hour if possible or refrigerate it and test within 48 hours. If the sample is to be tested at a laboratory away from the installation,

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use the fastest means of transportation to get to the laboratory. e. Sample data. You must identify each sample. Note the sampling point, including building number and street location for sample of distribution system; source of water, such as installation water supply; and the date of collection. 5. Laboratory Methods and Procedures for Testing. As you were told earlier in this section, analysis is an involved process beyond the scope of your responsibility. However, nonstandard testing, either in a laboratory or in the field, may comprise a part of your daily work. Since you are probably going to be working in a base laboratory part of the time, laboratory technique are required knowledge. Some of the basic rules are outlined in the following paragraphs. 6 Cleanliness. Chemical and bacteriological tests can easily be invalidated by impurities introduced into the test by dirty hands, clothing, or equipment. Set up a regular daily schedule for cleaning laboratory equipment, furniture, and floors. 7. Personal safety. Keep hands away from your mouth or eyes, especially when working with poisonous chemicals or bacteriological cultures. Keep a diluted solution of lysol or mercuric chloride and a bicarbonate of soda solution at or near the laboratory sink at all times. Rinse hands with this solution immediately after washing any bacteriological-culture glassware or acid containers. Then wash thoroughly with soap and water. Never smoke or eat in the laboratory. Drinking from laboratory glassware may result in serious illness if a contaminated beaker is used. Do not use laboratory to prepare food or use incubators or refrigerators to store food. Review Exercises The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the text. Do not submit your answers for grading.

1. What is the main scale-forming compound found in condensing water systems? (Sec 21, Par. 1)

2. Scale will form when the pH value is _________ ________to _________________ and the p.p.m. is __________________ or higher. (Sec. 21, Par. 4)

3. What are the cycles of concentration if the makeup water is 100 p.p.m. and the circulating water is 200 p.p.m.? (Sec. 21, Par. 6)

4. Give four methods of preventing scale. (Sec. 21, Par. 7)

5. During the soap hardness test you use 10 ml. of standard soap solution to obtain a permanent lather. What is the hardness of your sample? (Sec. 21, Par. 9)

6. Which softening process changes calcium and magnesium from a soluble to an insoluble state? (Sec. 21, Par. 11)

7. How does the zeolite process soften water? (Sec. 21, Par. 11)

8. Why is it necessary to add lime or clay to the Accelator? (Sec. 21, Par. 15)

9. What factors would limit the use of the Spiractor? (Sec. 21, Par. 17)

10 What is used to restore the sodium ions in a zeolite softener? (Sec. 21, Par. 18)

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11. In what type of liquid is corrosion more rapid? (Sec. 22, Par. 2)

19. How is the chromate concentration of treated water measured? (Sec. 22, Par. 13)

12. What is the most common type of corrosion in an acid liquid? (Sec. 22, Par. 4)

20. Why shouldn’t high concentrations polyphosphates be used? (Sec. 22, Par. 14)

of

13. Which type of corrosion is characterized by cavities and gradually develops into pinhole leaks? (Sec. 22, Par. 5)

21. Give two advantages using polyphosphates over chromates. (Sec. 22, Par. 15)

14. If a system contains an abundance of copper and a few unions of steel, and the steel unions are corroding at a very high rate, what type of corrosion is taking place? (Sec. 22, Par. 6)

22. Why must bleedoff be adjusted on condenser water systems when polyphosphates are used? (Sec. 22, Par. 16)

15. What causes erosion-corrosion and what is used to control this type of corrosion? (Sec. 22, Pars. 7 and 8)

23. In what two forms may chemical corrosion inhibitors be that are placed in a nylon net bag, which in turn is placed in a cooling tower? (Sec. 22, Par. 18)

16. What are the two most common chemical corrosion inhibitors? (Sec. 22, Par. 10)

24. What type of corrosion inhibitor feeders are required on chilled water and brine systems? (Sec. 22, Par. 18)

17. Chromates are most effective in air-conditioning water systems when the concentration is _____________ to ___________ and the pH is ____________________. (Sec. 22, Par. 11)

25. What are the effects of algae on the operation of an air-conditioning system? (Sec. 23, Par. 1)

26. How many p.p.m. of chlorine are needed to eliminate algae growth in a cooling tower? (Sec. 23, Par. 2) 18. What is the most common chromate used and why? (Sec. 22, Par. 11)

93 27. (Agree)(Disagree) During the performance of the residual chlorine test, you must heat the sample to 70° F. before adding the orthotolidine. (Sec. 23, Par. 3)

35. Why is calcium hypochlorite used more often than sodium hypochlorite? (Sec. 23, Pars. 26 and 27) 28. Why is chlorination an effective method of algae control in cooling towers and evaporative condensers? (Sec. 23, Par. 6) 36. Which hypochlorinator would you select if the water to be treated required 100 gallons of chlorine solution per day? Why? (Sec. 23, Par 32)

29. Why is the orthotolidine-arsenite test preferred to the orthotolidine test? (Sec. 23, Par. 8)

30. What is the combined available chlorine residual when the free available chlorine residual is 2.5 p.p.m. and the total residual chlorine is 3.25 p.p.m.? (Sec. 23, Par. 9)

37. The dosage of chlorine added to the 0.5 million gallons of water, when 20 pounds of chlorine is added per day, is approximately ______________ p.p.m. (solve to the nearest p.p.m.). (Sec. 23, Par. 34)

31. Describe the procedure used to perform the chlorine demand test. (Sec. 23, Pars. 13, 14, and 15)

38. How many pounds of HTH would you have to add to treat water which requires 30 pounds of chlorine? (Solve to the nearest pound). (Sec. 23, Pars. 35 and 36)

32. As the result of a pH determination with a color comparator, you have found the pH to be 7.7. How would you have reached this solution? (Sec. 23, Pars. 17, 18, and 19)

39. How many gallons of chlorine is added per day to treat 2 million gallons of water when the dosage is 1.5 p.p.m. and the strength of the dosing solution is 10 percent? (Sec. 23, Par. 36)

33. After you have added two drops of phenolphthalein indicator to the sample, the sample turned pink. The sample is (acid, alkaline). (Sec. 23, Par. 22)

40. What precautions must be followed while you are performing the Jackson turbidimeter test? (Sec. 24, Pars. 4, 5, and 6)

34. Which acids are used to lower the pH and how are they added to the water? (Sec. 23, Par. 24)

41. How many gallons of water can be filtered through a vertical type pressure filter in 1 hour? The diameter of the filter is 4 feet. (Sec. 24, Par. 11)

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42. What precautions for taking water samples is common to both chemical and bacteriological analysis? (Sec. 25, Pars. 3 and 4)

44. How far below the surface of the water in a tank should you hold the bottle when taking a sample? (Sec. 25, Par. 4,c)

43. How is a bottle sterilized when it is to be used for chlorine testing? (Sec. 25, Par. 4, a)

45. What type of solution should you wash your hands with after making water tests? (Sec. 25, Par. 7)

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CHAPTER 5

Centrifugal Water Pumps

IF YOU SWING a bucket of water around your head, the water does not spill out because centrifugal force presses it toward the bottom of the bucket. If a number of bottomless buckets were whirled around inside a pipe, and there were only one hole where water could leave the pipe, each pail would throw out some of its water as it passed this hole. It would also suck up more water at the center. This is exactly how the centrifugal pump works. Instead of buckets, however, a centrifugal pump has vertical ribs, or vanes, mounted on a revolving disc. The water takes up the space between the vanes, or ribs. The disc, as it revolves, forces water through the pump outlet to the various components the water serves. 2. In this chapter we will study installation, operation, and maintenance of centrifugal water pumps. 26. Installation 1. The installation of a centrifugal water pump includes laying a concrete foundation and aligning each component. The foundation should be sufficiently substantial to absorb any vibration and to form a permanent rigid support for the baseplate. Figure 80 shows a foundation and baseplate. This type of concrete foundation is important in maintaining the alignment of a directly driven unit. A mixture of 1 part cement, 3 parts sand, and 6 parts gravel or crushed rock is recommended. In building the foundation, you should leave the top approximately 1 inch low to allow for grouting. You should roughen and clean the top of the foundation before placing the unit on it. Foundation bolts of the proper size should be embedded in the concrete before it sets. Use a template or drawing to locate the bolts. A pipe sleeve about 2 diameters larger than the bolt is used to allow movement for the final positioning of the bolts. Place a washer between the bolthead and the inner surface of the pipe to hold the bolt in position. 2. Be sure the foundation bolts are long enough to project through the nuts one-fourth of a inch after allowance has been made for grouting, for the thickness of the bedplate, and for the thickness of the foundation bolt nut. We are now ready to install the pump unit.

3. Place wedges at four points, two below the approximate center of the pump and two below the approximate center of the motor. Some installations may require two additional wedges at the middle of the bedplate. By adjustment of the wedges you can bring the unit to an approximate level and provide for the proper distance above the foundation for grouting. By further adjustment of the wedges you can bring the coupling halves in reasonable alignment by tightening down the pump and motor holddown bolts. 4. Check the gap and angular misalignment on the coupling. The coupling shown in figure 81 is the “spider insert” type. The normal gap is one-sixteenth of an inch. The gap is the difference in the space between the coupling halves and the thickness of the spider insert. Angular misalignment may be checked by using calipers at four points on the circumference of the outer ends of the coupling hubs, at 90° intervals, as shown in figure 81. 5. The unit will be in angular alignment when the measurements show the ends of the coupling hubs to be the same distance apart at all four points. Gap and angular alignment is obtained by loosening the motor holddown bolts and shifting or shimming the motor as required. Tighten down the holddown bolts after adjustments have been made. 6. After the wedges have been adjusted, tighten the foundation bolts evenly but only finger-tight. Be sure you maintain the level of the bedplate. Final tightening of the foundation bolts is done after the grout has set 48 hours. 7. To grout the unit on the foundation, build a wooden dam around the foundation, as shown in figure 80, and wet the top surface of the concrete thoroughly. Now force the grout under the bedplate. The grout should be thin enough to level out under the bedplate, but not so wet that the cement will separate from the sand and float

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Figure 80. Pump foundation. to the surface. The recommended mixture for grout is 1 part of Portland cement to 3 parts of sharp sand. The grout should completely fill the space under the bedplate. Allow 48 hours for the grout to harden. 8. Alignment. Alignment of the pump and motor through the flexible coupling is of extreme importance for double-free mechanical operation. The following steps must be followed to establish the initial alignment of the pumping unit: (1) Tighten the foundation bolts. (2) Tighten the pump and motor holddown bolts. (3) Check the gap and angular adjustment as discussed previously. (4) Check parallel alignment by laying a straightedge across both coupling rims at the top, bottom, and both sides, as shown figure 82. The unit will be in horizontal parallel alignment when the straightedge rests evenly on both halves of the coupling at each side. In some special services a wide differential will prevail between the operating temperatures the pump and motor. Adjustment of alignment to satisfy such operating conditions must be governed by the specific application. The vertical difference of the shafts should be measured with a straightedge and feelers. To establish parallel alignment, thin shim stock is placed under the motor base. Occasionally, shims may be required under the pump base. (5) Remember, alignment in one direction may alter the alignment in another. Check through each alignment procedure after making an alignment alteration. 9. The unit should be checked periodically for alignment. If the unit does not stay in line

Figure 81. Checking angular alignment. 97

Figure 82. Checking parallel alignment.

after being properly installed, the following are possible causes: • Settling, seasoning, or springing of the foundation • Pipe strains distorting or shifting the pump • Shifting of the building structure • Spring of the baseplate 10. Piping. Connect the suction to the suction opening in the pump casing. Be sure that all suction connections are airtight. Use a good pipe joint compound on all threaded joints and airtight, packed unions. Suction piping smaller than the casing tapping may be used if necessary. Larger size suction piping than the casing tapping is not recommended. A strainer should be installed in suction line to protect the pump from foreign matter that may be present in the water. 11. The discharge piping is connected to the discharge threaded opening. This opening is larger than the suction opening. Smaller size discharge ping may be used, but the will be a loss of head and capacity. 12. Both pipes must be properly supported so that there will not be a strain set up. The strain could cause breakage of the pump casing or misalignment. 13. Now that you have installed the pump you are ready to check its operation. To check the operation, you must know the operating characteristics of the pump. 27. Operation 1. This centrifugal pump may be used as a cooling or chilled water pump. Whichever application it serves, the method of operation remains the same. The pump must be filled through the priming opening before it is started. Prime the pump by removing the priming plug on top of the pump casing and filling the pump with the liquid to be pumped. Be sure that all the plugs in the pump casing are screwed in tightly. Rotate the pump shaft by hand in the direction of the arrow on the casing to be sure that it moves freely. The pump is now ready to be started. Remember, after the pump is started, you must check to insure that the direction of rotation agrees with the arrow on the casing. 2. After the pump is up to speed, the priming time will depend on the size and length of the suction line. If for any reason the pump is stopped during the priming period, be sure to check the liquid level in the pump before restarting it. 3. If a newly installed pump fails to prime, you must be sure that the following conditions exist: (1) All the plugs on the pump casing are airtight. (2) The liquid level of the pump is at least to the priming level.

(3) All suction line joints are airtight. (4) The motor direction matches the arrow on the pump casing. (5) The motor reaches its rated nameplate speed. (6) Suction strainer is clean. 4. Insufficient pump discharge can be caused by improper priming, air leaks in the suction line or pump stuffing box, low motor speed, plugged impeller or suction opening, wrong direction of rotation, worn stuffing box packing, and mechanical pump defects. These faults can also be related to low pump pressure and excessive power consumption. Proper operation of the pump is the result of good maintenance policies. 28. Maintenance 1. If the internal components of the pump become worn, you should replace the entire pump with another of the same size to insure the same pumping capacity. After the new pump is installed, it must be aligned as previously discussed. 2. Stuffing Boxes. In repacking be sure that sufficient packing is placed back of the lantern ring, shown in figure 83, so that the liquid for sealing is brought in at the lantern ring and not at the packing. 3. The piping supplying the sealing liquid should be tightly fitted so that no air enters. On suction lifts, a small quantity of air entering the pump at this point may result in loss of suction. If the liquid being pumped is dirty, gritty, or acidic, the sealing liquid should be piped to the stuffing box from a clean source of water. This procedure will help prevent damage to the packing and shaft sleeve. 4. Packing should not be pressed too tight, since this may result in burning the packing and scoring the shaft sleeve. A stuffing box is not properly packed if friction in the box is so great that the shaft cannot be turned by hand. 5. Always remove and replace all of the old packing. Do not reuse any of the old packing rings. In placing the new packing each packing ring should be cut to the proper length so that the ends come together but do not overlap. The succeeding rings should be placed in the stuffing box so that the joints of the rings are staggered 180° apart for two-ring packing, 120° for threering, and so on. 6. If the pump is packed with metallic packing and stored for a great length of time, it may be necessary to apply leverage to free the rotor. When first starting the pump, the packing should be slightly loose, without causing an air leak. If the gland leaks, put some heavy oil in the stuffing

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Figure 83. Cutaway of bearing and stuffing box. box until the pump works properly. Then gradually tighten the gland. 7. When stuffing boxes are water sealed, you must be sure the water seal valves are opened sufficiently to allow a slight leakage of water. The leakage is piped away to a sump or sewer. Many pump failures occur because personnel observe liquid dripping from a gland and endeavor to stop it by tightening the gland bolts. Excessive tightening will cause the packing to burn and also may score the shaft. 8. All general-service pumps are shipped with the highest grade of soft, square asbestos packing, impregnated with oil and graphite. 9. Mechanical Seals. A mechanical seal is used in place of a stuffing box. This seal requires no

adjustments, but it may be necessary to replace certain items should they become scored or broken. Let us discuss dismantling and assembling the mechanical shaft seal assembly. 10. Dismantling. Back off the gland bolts to free the gland plates. Then remove the rotating element from the pump and take off the bearings and shaft nuts. Let us follow the remaining steps as illustrated in figure 84. 11. Remove the floating seat and sealing washer. Do not disturb the bellows unless it needs replacement. The bellows becomes adhered to the sleeve if the seal has been in use for any length of time and will be damaged if moved. If it requires replacement, it must be forced off the sleeve. After the bellows is removed, the remaining parts-spring, spring holder, retainer shell, and driving band-may be taken off. If the seal uses a set collar, you must measure its location on the shaft before removing it so as to correctly relocate it during assembly. 12. Assembly. In assembling a mechanical seal, clean up all the parts and lightly oil the surface of the floating seat and the shaft sleeve. Use light oil-not grease. 13. Make sure that the synthetic rubber seat is tight against the shoulder of the floating seat with the rounded outer edge to the rear to facilitate insertion. Push this assembly firmly into the cavity in the gland plate and seat it squarely. Do not push on the lapped face of the floating seat. 14. The next step is to put the spring holder or set collar in place. If a set collar is used you must locate the collar in a position on the shaft determined by the measurement taken during dismantling. 15. Place the remainder of the seal parts on the shaft as an assembly. When the extended length of the seal assembly is longer than the undercut portion of the sleeve or than the distance from the collar to the end of the sleeve, the spring must be compressed beforehand and tied

Figure 84. Cutaway of a mechanical seal.

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Figure 85. Flexible coupling. together with string. The string should be removed after installation and after partial tightening of the gland bolts. Be sure there are no burrs on the sleeve that would harm the bellows. The new bellow is pushed straight on the sleeve. 16. The casing joint gasket should be cut at least one-eighth of an inch oversized and trimmed after the upper half-casing is bolted down. 17. Bearings. The four types of bearings found in centrifugal pumps are grease-lubricated (1) ball and (2) roller bearings, (3) oil-lubricated sleeve bearing, and (4) oil-lubricated ball bearings. The importance of proper lubrication cannot be overemphasized. The frequency of lubrication depends upon the conditions of operation. Overlubrication is the primary cause of overheated bearings. For average operating conditions it is recommended that grease be added at intervals of 3 to 6 months. 18. The housing should be kept clean, for foreign matter will cause the bearing to wear prematurely. When you clean the bearing, use clean solvent and wipe it with a clean cloth. Do not use waste to wipe the bearing because it will leave lint. 19. A regular ball bearing grease must be used. A number 1 or 2 grease is satisfactory for most chill or cooling water pump applications. Mineral greases with a soda soap base are recommended. Greases made from animal or vegetable oil should not be used because of the danger of deterioration and the formation of acid. Most of the leading oil companies have special bearing greases that are satisfactory. For specific information of lubricant recommendations you should consult the manufacturer’s service bulletins. 20. The maximum operating temperature for ball bearings is 180° F. If the temperature rises above 180° F., the pump should be shut down and the cause determined. 21. The oil-lubricated ball bearing is filled with a good grade of filtered mineral oil (SAE 10) of approximately 150 Saybolt viscosity a 100° F. The oil should be changed when it becomes dirty, and the bearing should be cleaned at the same time. The bearing should be checked for wear frequently. Make sure that the oil rings are turning freely when the pump is first started. They are observed through the oil holes in the bearing caps. 22. The maximum operating temperature for babbitted sleeve bearings is 150° F. If the bearing temperature exceeds 150° F, shut down the pump until the cause is determined and corrected. Before the pump is started, the bearing should be flushed thoroughly with a light grade of oil to remove any dirt or foreign matter that may have accumulated during storage or installation. The bearing housing should then be filled to the indicated level with a good grade filtered mineral oil (SAE 10) of approximately 150 Saybolt viscosity at 100° F. 23. Couplings. We have already discussed the “spider insert” coupling. Another coupling you will come in contact with is the “Magic-Grip.” 24. The “Magic Grip” coupling, shown in figure 85, consists primarily of two cast iron discs and two bushings. The bushing is split, which allows it to slide easily on the shaft. The outer diameter of the bushing and the inside diameter of the coupling are tapered. There a four drilled recesses in the bushing which accommodate the OFF and ON positions of the setscrew holes of the coupling. The recesses in the bushings are offset so that when the setscrews are tightened the bushing will either draw in on the taper and

100 tighten on the shaft or push out of the taper and loosen on the shaft. 25. The coupling is not intended to be a universal joint. It is capable of taking care of minor angular misalignment, but you must be sure to carefully align the coupling during installation. 26. To install the coupling, slide the bushing on the pump or motor shaft with the recess holes away from the pump. Next place the coupling over the bushing. Insert both setscrews in the ON position and tighten them alternately until the coupling is tight on the shaft. 27. To remove the coupling, remove both setscrews from the ON position and insert them in the OFF position. Turn the setscrews until the coupling is free on the busing; then loosen the setscrews and remove the coupling from the bushing. The bushing will now slide off the shaft. Review Exercises The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the test. Do not submit your answers for grading. 1. How many pounds of cement would you have to mix with 12 pounds of sand and 24 pounds of crushed rock to form the concrete foundation for a pump? (Sec. 26, Par. 1) 9. How long should you allow the grout to harden? (Sec. 26, Par. 7) 5. How do you check the angular alignment of a “spider” coupling? (Sec. 26, Par. 4)

6. How is angular alignment accomplished? (Sec. 26, Par. 5)

7. Explain the procedure used to grout the pump unit on the foundation. (Sec. 26, Par. 7)

8. How many parts of Portland cement to sharp sand are used to make grout? (Sec. 26, Par. 7)

10. Explain the steps you must follow to establish the initial alignment of the pumping unit. (Sec. 26, Par. 8)

2. Why is a 1-inch space left between the concrete foundation and the baseplate? (Sec. 26, Par. 1)

11. Why would alignment be necessary after the unit has been operating for a period of time? (Sec. 26, Par. 9)

3. How large a pipe sleeve would you use with a baseplate bolt measuring three-fourths of an inch in diameter? (Sec. 26, Par. 1)

12. A _________________ is installed in the suction line to protect the pump from foreign matter. (Sec. 26, Par. 10)

4. Where do you place the wedges to level the baseplate? (Sec. 26, Par. 3)

13. What will occur if you install a smaller discharge pipe than the threaded discharge opening in the pump? (Sec. 26. Par. 11)

101

14. How is the pump primed? (Sec. 27, Par. 1)

23. Name the four types of bearings commonly found in centrifugal pumps. (Sec. 28, Par. 17)

15. Explain what you should do after the pump is primed and before it is stared. (Sec. 27, Par. 1)

24. What occurs when a bearing is lubricated too often? (Sec. 28, Par. 17)

16. List at least four causes for failure of a newly installed pump to prime. (Sec. 27, Par. 3)

25. What type of grease is recommended for greaselubricated bearings? (Sec. 28, Par. 19)

17. A pump that uses a stuffing box takes liquid in for sealing at ___________________. (Sec. 28, Par. 2)

26. Why aren’t vegetable and animal greases used to lubricate pump bearing? (Sec. 2, Par. 19)

18. When is it necessary to pipe water from a clean water source to the stuffing box? (Sec. 28, Par. 3)

27. The maximum operating temperature for greaselubricated bearings is __________________. (Sec. 28, Par. 20)

19. Why is exact packing tightening important? (Sec. 28, Par. 4)

28. The maximum operating temperature for an oillubricated babbitted sleeve bearing is ___________________. (Sec. 28, Par. 22)

20. How would you stagger the packing joints in the stuffing box that uses five rings? (Sec. 28, Par. 5)

29. What are the four drilled recesses in the bushing of a “Magic-Grip” coupling used for? (Sec. 28, Par. 24)

21. The first step to perform when dismantling a mechanical seal is to _________________. (Sec. 28, Par. 10)

30. (Agree)(Disagree) During installation of a “Magic-Grip” coupling, the recessed holes should be facing the pump. (Sec. 28, Par. 26)

22. Which item shouldn’t you disturb when dismantling a mechanical pump unless it is to be replaced? (Sec. 28, Par. 11)

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CHAPTER 6

Fundamentals of Electronic Controls

A MISSILE STREAKS across the sky. The missile’s flight is controlled electronically from a command post. The success of the launch and flight of the “bird” depends largely upon how well the electronic technicians performed their tasks. 2. Let us compare the missile launch to an electronic control system. The missile can be compared to the controlled variable-humidity, temperature, airflow, etc. The movable rocket motor is the controlled device. The controlled device is the component within the system that receives a signal from the control to compensate for a change in the variable. Last, but not least, we have the guidance system. Our controllers thermostats, humidistats, etc. -perform in much the same way as a guidance system. A change in the controlled variable will cause the controller to respond with a corrective signal. 3. In this chapter we will discuss vacuum tubes, amplification, semiconductors, transistor circuits, bridge circuits, and discriminator circuits. We will relate amplifier, bridge, and discriminator circuits to electronic controls. Electronic controls are becoming popular in the equipment cooling area of your career field because of their sensitivity and reaction time. 29. Vacuum Tubes 1. Electricity is based entirely upon the electron theory--that an electron is a minute, negatively charged particle. Atoms consist of a positively charged nucleus around which are grouped a number of electrons. The physical properties of any atom depend upon the number of electrons and the size of the nucleus; however, almost all matter has free electrons. The movement of these free electrons is known as a current of electricity. If the movement of electrons is in “one” direction only, this is direct current. If, however, the source of voltage is alternated between positive and negative, the movement of electrons will also alternate; this is alternating current.

2. The vacuum tube differs from other electrical devices in that the electric current does not flow through a conductor. Instead, it passed through a vacuum inside the tube. This flow of electrons is only possible if free electrons are somehow introduced into the vacuum. Electrons in the evacuated space will be attracted to a positively charged object within the same space because the electrons are negatively charged. Likewise, they will be repelled by another negatively charged object within the same space. Any movement of electrons under the influence of attraction or repulsion of charged objects is the current in a vacuum. The operation of all vacuum tubes depends upon an available supply of electrons. Electron emission can be accomplished by several methods--field, thermionic, photoelectric and bombardment-but the most important is thermionic emission. 3. Thermionic Emission. To get an idea of what occurs during thermionic emission you should visualize the Christmas sparkler. When you light the sparkler it burns and sparks in all directions. The filament in a vacuum tube reacts the same way when heated to a high temperature. Millions of electrons leave the filament in all directions and fly off into the surrounding space. The higher the temperature, within limits, the greater the number of electrons emitted. The filament in a directly heated vacuum tube is commonly referred to as a cathode. Refer to figure 86 for the symbol of a filament in a vacuum tube with heating sources. 4. The cathode must be heated to a high temperature before electrons will be given off. However this does not mean that the heating current must flow through the actual material that does the emitting. You can see in figure 87 that the part that does the heating can be electrically separate from the emitting element. A cathode that is separate from the filament is an indirectly heated cathode, whereas an emitting filament is a directly heated cathode. 5. Much greater electron emission can be

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Figure 86. Thermionic emission. obtained, at lower temperatures, by coating the cathode with special compounds. One of these is thoriated tungsten, or tungsten in which thorium is dissolved. However, much greater efficiency is achieved in the oxide-coated cathode, a cathode in which rare-earth oxides form a coating over a metal base. Usually this rare-earth oxide coating consists of barium or strontium oxide. Oxide-coated emitters have a long life and great emission efficiency. 6. The electrons emitted by the cathode stay in its immediate vicinity. These form a negatively charged cloud about the cathode. This cloud, which is called a space charge, will repel those electrons nearest the cathode and force them back in on it. In order to use these electrons, we must put a second element within the vacuum tube. This second element is called an anode (or plate), and it gives us our simplest type of vacuum tube, the diode. 7. Diode Vacuum Tube. Each vacuum tube must have at least two elements or electrodes: a cathode and an anode (commonly called a plate). The cathode is an emitter of electrons and the plate is a collector of electrons. Both elements are inclosed inside an envelope of glass or metal. This discussion centers around the vacuum tube diode from which the air as much possible has been removed. However, it should be understood that gaseous diodes do exist. The

Figure 87. Indirectly and directly heated cathodes. 104

Figure 88. Electron flow in a diode. term “diode” refers to the number of elements within the tube envelope (di meaning two) rather than to any specific application, as shown in figure 88. 8. The operation of the diode depends upon the fact that if a positive voltage is applied to the plate with respect to the heated cathode, current will flow through the tube. When the plate is negative with respect to the cathode, current will not flow through the tube. Since current will pass through a vacuum tube in only one direction, a diode can be used to change a.c. to d.c. 9. Diode as a half-wave rectifier. Experiments with diode vacuum tubes reveal that the amount of current which flows from cathode to plate depends upon two factors: the temperature of the cathode, and the potential (voltage) between the cathode and the plate. Refer to figure 89, a diagram of a simple diode rectifier circuit. 10. When an a.c. source is connected to the plate and cathode such a circuit, one-half of each a.c. cycle will be positive and the other half will be negative. Therefore, alternating voltage from the secondary of the transformer is applied to the diode tube in series with a load resistor, R. The voltage varies, as is usual with a.c., but current passes through the tube and R only when the plate is positive with respect to the cathode. In other words, current flows only during the half-cycle when the plate end of the transformer winding is positive. When the plate is negative, no current will pass. 11. Since the current through the diode flows in one direction only, it is direct current. This type of diode rectifier circuit is called a half-

Figure 89. Simple half-wave rectifier circuit. 105

Figure 90. Output of a half-wave rectifier. wave rectifier, because it rectifies only during one-half of the a.c. cycle. As a result, the rectified output will be pulses of d.c., as shown in figure 90. You can see from figure 90 that these pulses of direct current are quite different from pure direct current. It rises from zero to a maximum and returns to zero during the positive halfcycle of the alternating current, but does not flow at all during the negative half-cycle. This type of current is referred to as pulsating direct current to distinguish it from pure direct current. 12. In order to change this rectified alternating current into almost pure direct current, these fluctuations must be removed. In other words, it is necessary to cut off the humps at the tops of the half-cycles of current and fill in the gaps caused by the negative half-cycle of no current. This process is called “filtering” ‘ 13. Look at the complete electrical circuit of figure 91. Filtering is accomplished by connecting capacitors, choke coils (inductors), and resistors in the proper manner. If a filter circuit is added to the half-wave rectifier, a satisfactory degree of filtering can be obtained. Capacitors C1 and C2 have a small reactance at the a.c. frequency, and they are connected across the load resistor, R. These capacitors will become charged during the positive half-cycles as voltage is applied across the load resistor. The capacitors will discharge through R and L during the negative half-cycles, when the tube is not conducting, thus tending to smooth out, or filter out, the

Figure 91. Filter network added to a half-wave rectifier. 106

Figure 92. Full-wave rectifier. pulsating direct current. Such a capacitor is known as a filter capacitor. 14. Inductor L is a filter choke having high reactance at the a.c. frequency and a low value of d.c. resistance. It will oppose any current variations, but will allow direct current to flow almost unhindered through the circuit. In order use both alternations of a.c., this circuit must be converted to a full-wave rectifier. 15. Diode used or full-wave rectification. One disadvantage of the half-wave rectifier is that no current is available from the transformer during the negative half-cycle. Therefore, some of the voltage produced during the positive half cycle must be used to filter out the voltage variations. This filtering action reduces the average voltage output of the circuit. Since the circuit is conducting only half the time, it is not very efficient. Consequently, the full-wave rectifier, which rectifies both half-cycles, was developed for use in the power supply circuits of modern electronic equipment. 16. In a full-wave rectifier circuit, two diodes may be used. However, in many applications, the two diodes are included in one envelope and the tube is referred to as a duo-diode. A typical example of a full-wave rectifier circuit is shown in figure 92. In this circuit a duo-diode is used, and the transformer’s secondary winding has a center tap. Notice that the center tap current is turned to ground and then through R and inductor L to the cathode (filament) of V1. The voltage appearing across X and Y is 700 volts a.c. The center tap is at zero potential with 350 volts on each side. 17. Point X of the high-voltage winding is connected to plate P2, and Y is connected to P1. The plates conduct alternately, since at any given instant, one plate is positive and the other is negative. During one half-cycle, P1 will be positive with respect to the center tap of the transformer secondary winding while P2 will be negative. This means that P1 will be conducting while P2 is nonconducting. 18. During the other half-cycle, P1, will be negative and nonconducting while P2 will be positive and conducting. Therefore, since the two plates take turns in their operation, one plate is always conducting. Current flows through the load resistor in the same direction during both halves of the cycle, which is called full-wave rectification. The circuit shown in figure 92 is the basis for all a.c. operated power supplies that furnish d.c. voltages for electronic equipment. Notice that the heater voltage for the duo-diode is taken from a special secondary winding on the transformer. 19. The next tube you will study is the triode. The triode is used to amplify a signal. 30. Amplification 1. With the invention of the triode vacuum tube, the amplification of electrical power was introduced. Technically speaking, amplification means slaving a large d.c. voltage to a small varying signal voltage to make the large d.c. voltage have the same wave shape as the signal voltage. As a result, the wave-shaped d.c. voltage will do the same kind of work as the signal voltage will do, but in a larger quantity. After the triode came the tetrode, pentode, etc., to do a much better job of amplification than the triode. Amplification by use of the triode and other multi-element

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vacuum tubes will be discussed in this section. 2. Triode Vacuum Tube. In the diode tubes previously described, current in the plate circuit was determined by cathode temperature and by the voltage applied to the plate. A much more sensitive control of the plate current can be achieved by the use of a third electrode in the tube. The third electrode (or element), called a control grid, is usually made in the form of a spiral or screen of fine wire. It is physically located between the cathode and plate, and is in a separate electrical circuit. The term “grid” comes from its early physical form. 3. The control grid is placed much closer to the cathode than to the plate, in order to have a greater effect on the electrons that pass from the cathode to the plate. Because of its strategic location the grid can control plate current by variations in its voltage. The operation of a triode vacuum tube is explained in the following paragraphs. 4. If a small negative voltage (with respect to the cathode) is applied to the grid, there is a change in electron flow within the tube. Since the electrons are negative charges of electricity, the negative voltage on the grid will tend to repel the electrons emitted by the cathode, which tends to prevent them from passing through the grid on their way to the plate. However, the plate is highly positive with respect to the cathode and attracts many of the electrons through the grid. Thus, many electrons pass through the negative grid and reach the plate in spite of the opposition offered them by the negative grid voltage. 5. A small negative voltage on the grid of the vacuum tube will reduce the electron flow from the cathode to the plate. As the grid is made more and more negative, it repels the electrons from the cathode, and this in turn decreases plate current. When the grid bias reaches a certain negative value, the positive voltage on the plate is unable to attract any more electrons and the plate current decreases to zero. The point at which this negative voltage stops all plate current is referred to as cutoff bias for that particular tube. 6. Also, as the grid becomes less and less negative, the positive plate attracts more electrons and current increases. However, a point is reached where plate current does not increase even though the grid bias is made more positive. This point, which varies with different types of tubes, is called the saturation level of vacuum tubes. So you can see that the control grid acts as a valve controlling plate current. One other thing must be made clear at this point. If the positive plate voltage is

increased, the negative grid voltage must be increased if you need to limit current through the tube. 7. Control Grid Bias. Grid bias has been defined as the d.c. voltage (potential) on the grid with respect to the cathode. It is usually a negative voltage, but in some cases the grid is operated at a positive potential. Generally when the term “bias” is used, it is assumed to be negative. There are three general methods of providing this bias voltage. 8. The first is fixed bias. Figure 93 shows how the negative terminal of a battery could be connected to the control grid of a tube, and the cathode connected to ground to provide bias. If you say that the bias is 5 volts, you mean that the grid is 5 volts “negative” with respect to the cathode. Two methods of obtaining a bias of 5 volts are shown in figure 93. In diagram X the battery is connected with its negative terminal to the grid, while its positive terminal and the cathode are grounded. Diagram Y shows the positive terminal of the battery connected to the cathode, while its negative terminal and the grid are grounded. In either case, the grid is 5 volts negative with respect to the cathode. If the grid and the cathode are at the same potential, there is no difference in voltage and the tube is operating at zero bias (diagram Z). 9. The second method of obtaining grid bias is called cathode bias. The cathode bias method uses a resistor (Rk) connected in series with the cathode, as shown in figure 94. As the tube conducts, current is in such a direction that the end of the resistor nearest the cathode is positive. The voltage drop across Rk makes the grid negative with respect to the cathode. This negative grid bias is obtained from the steady d.c. across Rk. The amount of grid bias on the triode tube is determined by the voltage drop (IR) across Rk. 10. Any signal that is fed into the grid will change the amount of current through the tube, which in turn will change the grid bias, due to the fact that current also changes through the cathode resistor. To stabilize this bias voltage, the cathode resistor is bypassed by a condenser, C1, that has low resistance compared with the resistance of Rk. Here’s how this works. 11. As the triode conducts, condenser C1, will charge. If the tube, due to an input signal, tends to conduct less, C1, will discharge slightly across RR, and keep the voltage drop constant. The voltage drop across the cathode resistor is held almost constant, even though the signal is continually varying. 12. Our third method of getting grid bias is called contact potential, or grid-leak bias. This type of bias depends upon the input signal. Two circuits using contact potential or grid-leak bias,

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Figure 93. Using a battery to get fixed or zero bias. are shown figure 95. The action in each case is similarthat is, when an a.c. signal is applied to the grid, it draws current on the positive half-cycle. This current flows in the external circuit between the cathode and the grid. This current flow will charge condenser C1, as shown by the dark, heavy lines. One thing to keep in mind at this time is the ohmic value of the grid resistor. It is very high, in the order of several hundred thousand ohms. 13. As the signal voltage goes through the negative half-cycle, the condenser C1, starts discharging. The control grid cannot discharge through the tube since it is not an emitter of electrons. The only place to can start discharging is through the grid resistor, Rg,. This discharge path is flown by the dotted arrows. A negative voltage is developed across Rg, which biases the tube. Since the resistor, Rg, has a very high value (500,000 ohms to several megohms), the condenser only has time to discharge a small amount before a new cycle begins. This means that only a very small current flows, or leaks through. However, because of the large value of Rg, C1

Figure 94. Cathode biasing with a cathode resistor. 109

Figure 95. Connect potential bias. will remain continuously charged to some value as long as a signal is applied. 14. One of the main disadvantages of this type of bias is the fact that bias is developed only when a signal is applied to the grid. If the signal is removed for any reason, the tube conducts very heavily and may be damaged. This condition can be prevented by using “combination bias,” which uses both grid-leak bias and cathode bias. This combination provides the advantages needed with an added safety precaution in case the signal is removed. 15. Triode Tube Operation. Since a small voltage change on the grid causes a large change in plate current, the triode tube can be used as an amplifier. If a small a.c. voltage is applied between the cathode and the grid, it will cause a change in grid bias and thus vary plate current. This small a.c. voltage between cathode and grid is called a signal. 16. The large variations in plate current through the plate load resistor (RL) develops an a.c. voltage component across the resistor which is many times larger than the signal voltage. This process is called amplification and is illustrated in figure 96. 17. The one tube and its associated circuits (the input and output circuits) is called one stage of amplification or a one-stage amplifier. A single-stage amplifier might not produce enough amplification or gain to do a particular job. To increase the overall gain, the output of one stage may be coupled to the control grid of another stage and the output amplified again. Look at figure 97 for a twostage amplifier. There are various types of couplings. But generally the idea is to block the d.c. plate voltage of the preceding stage to keep it off the grid of the following stage because it would upset the bias of the following stage. A capacitor is used to couple one stage to another because a capacitor blocks d.c. or will not let it pass. 18. Tetrode Amplifiers. While a triode is a good amplifier at low frequencies, it has a fault when used in circuits having a high frequency. This fault results from the capacitance effect between the electrodes of the tube and is known as interelectrode capacitance. The capacitance which causes the most trouble is between the plate and the control grid. This capacitance couples the output circuit to the input circuit of the amplifier stage, which causes instability and unsatisfactory operation. 19. To correct this fault, another tube was built that has a grid similar to the control grid placed between the plate and the control grid as seen in figure 98. This new grid is connected to a positive potential somewhat lower than the plate potential. It is also connected to the cathode through a capacitor. The second grid serves as a screen between the plate and the control grid and is called a screen grid. The tube is called a tetrode. 20. Beam Power Tubes. Electron tubes which handle large amounts of current are known as beam power amplifiers. Let us compare a voltage

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Figure 96. Triode tube operation. amplifier with a power amplifier. A voltage amplifier may draw 10 milliamperes of plate current while a power amplifier can draw 250 milliamperes of plate current. The beam power amplifier is more rugged, with larger elements, and must dissipate heat faster due to the greater current. 21. In figure 99 a specially constructed tetrode which has a filament or cathode, control grid, screen grid, and plate is called a beam power tetrode. To eliminate secondary emission effect, the screen grid wires lie in the shadow of the control grid thus forming the space current into narrow beams. The resulting beams provide the effect of suppressor grid action, and thus permits the characteristic curves to be similar to those of a pentode. 22. Because of the amount of electrons in the negatively charged beam, any secondary electrons emitted by the plate are returned to the plate. By internally connecting the beam-forming plates to the cathode, the concentration of the electrons are

Figure 9. Two-stage amplifier.

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Figure 98. Tetrode amplifier circuit. even higher, causing the beam to act as a suppressor grid in the pentode. 23. Pentode Amplifiers. The tetrode tube is a better amplifier than the triode tube, but it also has a fault. A cold plate does not normally emit electrons. However, high-velocity electrons, produced by the positive potential on the screen grid, cause other electrons to be knocked from the plate. The liberation of these electrons is called secondary emission. The secondary electrons will be attracted to the positive screen grid and will reduce the plate current. To overcome this, a vacuum tube was designed that contains still another grid. This grid, shown in figure 100, is called a suppressor grid and is placed between the plate and the screen grid. A negative potential is applied to the suppressor grid, and the negative potential forces the secondary electrons back to the plate and prevents secondary electrons from reaching the screen grid. These five-element tubes, or pentodes, are the highest development of amplifier tubes. 24. Classes of Amplifiers. Amplifiers are divided into the following classes, based on tube operation or bias voltage: • A class A amplifier has plate current or conducts for 360° of the input signal. • A class B amplifier conducts for 180° of the input signal. • A class AB amplifier is a combination of both class A and B. • A class C amplifier has plate current flowing for approximately 120° of the input signal. 25. Vacuum tubes have several disadvantages -size, warming up period, etc. Transistors are rapidly replacing vacuum tubes in electronic controls. To understand transistors, you must have a good knowledge of semiconductors. 31. Semiconductors 1. The transistor was discovered in 1948 by the Bell Laboratories. The name comes from two words, “transfer” and “resistance.” The transistor is gradually replacing the vacuum tube and is playing a big part in the design of all types of electronic equipment. The main advantages

Figure 99. Construction of a beam-power tube.

Figure 100. Pentrode amplifier tube. 112

Figure 103. Atoms of semiconductors. ring. Another name for the outer ring or orbit is the valence ring. The helium atom and the hydrogen atom are both good conductors of electricity--the hydrogen atom being the better. 5. Atomic Number. Atoms of different elements are found to have a different number of protons and neutrons in their nucleus. The atomic numbers of some of the elements are listed in figure 101. Figure 102 shows the structure of a hydrogen atom and a helium atom, two examples of good conductors. Figure 103 shows the structure of a germanium atom and a silicon atom, which are examples of a semiconductor. 6. An atom that has only four electrons in its outer orbit or ring will combine with other atoms whose outer orbits are incomplete. If a number of germanium atoms are joined together into crystalline form, the process is called covalent bonding of germanium atoms. Figure 104 shows germanium atoms in covalent bonding. Figure 105 illustrates an atom of germanium and an atom of antimony. For simplification, only the nucleus and the outer rings are shown for each atom. The outer or valence ring for the germanium atom contains four electrons, while

Figure 101. Elements associated with transistors. of the transistor over the vacuum tube are that it smaller, lighter, and more rugged, and operates at lower voltages than the vacuum tube. 2. Atomic Structure. Essential to the understanding of semiconductor operation is the study of atomic characteristics and the basic structure of the atom. The atom contains a nucleus composed of protons and neutrons. Protons are positively charged particles, while neutrons are neutral particles. 3. The other component of the atom is the electron, which is a negatively charged particle. The electrons are arranged in orbits around the nucleus. The orbits, or rings, are numbered starting with the ring nearest the nucleus (which is No. 1) and progressing outward. 4. The maximum number of electrons permitted in each ring is as follows: Ring No. 1, 2 electrons; ring No. 2, 8 electrons; ring No. 3, 18 electrons; ring No. 4, 32 electrons. The atomic structure of germanium and silicon have 14 and 32 electrons respectively. The 3d ring in silicon and the 4th ring in germanium are incomplete, having only 4 electrons. These incomplete outer rings are important to the operation of semiconductor devices. A good conductor has less than 4 electrons in its outer ring. A good insulator has more than 4 electrons in its outer ring. A good semiconductor has 4 electrons in its outer

Figure 102. Structure of atoms.

Figure 104. Crystalline germanium.

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Figure 105. Typical atoms. the valence ring for the antimony atom contains five atoms. 7. If a small amount of antimony is added to crystalline germanium, the antimony atoms will distribute themselves throughout the structure of the germanium crystal. 8. Figure 106 shows that an antimony atom has gone into covalent bonding with germanium. The antimony atom in the material donates a free electron and these free electrons will support current flow through the material. The antimony is called a donor in that it donates free electrons. The germanium crystal now becomes an N-type (negative type) germanium. 9. P-type (positive type) germanium can be prepared by combining germanium and indium atoms. Figure 107 shows germanium and indium in covalent bonding. For every indium atom in the material, there will be a shortage of one electron that is needed to complete covalent bonding between the two elements. This shortage of an electron can be defined as a hole. This type of material will readily accept an electron to complete Figure 107. P-type germanium. its covalent bonding and is therefore called acceptor type material. 10. The hole can be looked upon as a positive type of current carrier, as compared to the electron which is a negative type current carrier. The hole can be moved from atom to atom the same as the electron can be moved from atom to atom. The hole moves in one direction and the electron moves in the opposite direction. 11. P-N Junctions. When N-type and P-type germanium are combined in a single crystal, an unusual but very important phenomenon occurs at the surface where contact is made between the two types of germanium. The contact surface is referred to as a P-N junction, shown in figure 108. 12. There will be a tendency for the electrons to gather at the junction in the N-type material and likewise an attraction for the holes gather at the junction of the P-type material. These current carriers will not completely neutralize themselves because movement of electrons and holes cause negative and positive ions to be produced, which means an electric field is set up in each type material that will tend to obstruct the movement of current carriers through the junction. This obstruction builds up a barrier that is referred to as a high resistance or potential hill. This electric field may be referred to as a potential hill battery since the two materials have acquired a polarity which opposes the normal movement of the current carries. 13. Reverse Bias. Figure 109 shows an external voltage applied to an N-P junction. The positive electrode of the battery is connected to the N-type material and the negative electrode is connected to the Ptype material. Since the N-type material has an excess of electrons, the positive voltage being applied to this material will

Figure 106. N-type germanium.

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Figure 108. P-N junction. attract these electrons toward that end of the germanium crystal. The negative voltage being applied to the P-type material, which has an excess of positive current-carrying holes, will attract these holes toward the other end of the crystal and away from the junction. The ammeter in figure 109 indicates no current flow. There is no possibility of recombination at the junction because the potential hill has been built up to a higher value by the application of an external voltage. This is called reversed bias condition or a high-resistance circuit. 14. Forward Bias. The battery can be connected with the opposite polarity and cause a different condition. In figure 110 the battery has been reversed, and now the negative electrode of the battery is connected to the Ntype material. This negative voltage will repel the electrons in the N-type material toward the junction. The positive electrode is connected to the P-type material which will repel the positive holes toward the junction. With this connection, recombination takes place at the junction, resulting in current toward the N-P junction. This method of connecting the battery is known as forward bias since it encourages current flow. 15. Diode Action. Combining P- and N-type germanium into a single crystal is the basis of both diode and transistor action. The P-N junction can be used as a rectifier because of its ability pass current in one direction and practically no current in the other. Applying an a.c. voltage to this junction results in a d.c. output similar to that produced by a vacuum tube diode. Figure 111 shows a semiconductor diode rectifying an alternating voltage. When this P-N junction is biased in the forward direction, current will flow across the load resistor, RL. When the junction is biased in the reverse direction, no current will flow across the load resistor, RL. Forward and reverse biasing is caused by the a.c. input. 16. Point-Contact Diode. Another type diode is the point-contact diode, shown in figure 112.

Figure 109. N-P junction with reverse bias. 115

Figure 110. P-N junction with forward bias.

Figure 111. Half-wave rectification. This diode operates similarly to the P-N junction type. It consists of a semiconductor (N-type germanium), a metal base, and a metallic point contact (cat whisker). A fine beryllium-copper or phosphor-bronze wire is pressed against the N-type germanium crystal. During the construction of the diode a relatively high current is passed through the metallic point contact into the N-type crystal. This high current causes a small P-type area to be formed around the point contact. Thus, a P-type and an N-type germanium are formed in the same crystal. The operation of this diode is similar to the P-N junction diode. 17. Transistor Triodes. A review of the operation of P-N germanium junctions reveals that a P-N junction biased in the forward direction is equivalent to the lowresistance element (high current for a given voltage). The P-N junction biased in the reverse direction is equivalent to a high-resistance element (low current for a given voltage). For a given current, the power developed in a high-resistance element is greater than that developed in a low-resistance element. (Power is equal to the current squared multiplied by the resistance value, or simply: P = I2R.) If a crystal containing two P-N junctions were prepared, a signal could be introduced into one P-N junction biased the forward direction (low resistance) and extracted from the other P-N junction biased in the reverse direction (high resistance). This biasing produces a power gain of the signal when developed in the external circuit. Such a device would transfer the signal current from a low-resistance circuit to a high-resistance circuit. 18. P-N-P and N-P-N Junction Transistors. The P-N-P transistor is constructed by placing a narrow strip of N-type germanium between two relatively long strips of P-type germanium. And, as the letters indicate, the NP-N transistor consists of a narrow strip of P-type germanium between two relatively long strips of N-type germanium.

19. To form two P-N junctions, three sections of germanium are required. Figure 113 shows the three sections separated. When the three sections are combined a P-N-P transistor is formed, and each section, like each element in a vacuum tube, has a specific name: emitter, base, and collector. The base is located between the emitter and collector, as the grid in a triode vacuum tube is located between the plate and cathode. 20. Note that when the three sections are combined, two space charge regions (barriers) occur at the junction even though there is no application of external voltages, or fields. This phenomenon is the same as that which occurs when two sections are combined so as to form a P-N junction diode. 21. Transistor action requires that one junction be biased in the forward direction and the second junction be biased in the reverse direction. Figure 114 shows the first junction biased in the forward direction. The second junction is not biased. Note that the space charge region (barrier) at the first junction is considerably reduced while the space charge region at the second junction is unchanged. The condition is identical to that of a P-N junction diode with forward bias. 22. Figure 114 shows the second junction biased in the reverse direction. The first junction is not biased. Note that the space charge region (barrier) at the second junction increases. Except for minority carriers (not shown), no current flows across the junction. This phenomenon is the same as that which occurs when two sections are combined to form a P-N junction diode with reverse bias. 23. Figure 115 shows what happens when junctions are biased simultaneously. Because of the simultaneous biasing, a large number of holes from the emitter do not combine with the electrons entering the base from the emitter-base battery. Many of the holes diffuse through the base and penetrate the base-collector space charge

Figure 112. Physical construction of a point-contact diode.

116

Figure 113. Two sections of P-type germanium and one section of N-type germanium. region. In the collector region the holes combine with electrons that enter the collector from the negative terminal of the base-collector battery. If holes that enter the base from the emitter-base junction avoid combination with electrons entering the base from the battery, the holes are attracted to the collector by the acceptor atoms (negative) in the collector and the negative potential of the base collector battery. 24. To obtain maximum power gain in a transistor, most of the holes from the emitter must diffuse through the base region into the collector region. This condition obtained in practice by making the base region very narrow compared the emitter and the collector regions. In practical transistors, approximately 95 percent of the current from the emitter reaches the collector. 25. By using forward bias on the emitter-to-base junction there is a relatively low resistance, whereas by using reverse bias on the collector-to-base junction there is a relatively high resistance. A typical value for the emitter-to-base resistance is around 500 ohms, and around 117 500,000 ohms for the collector-to-base resistance. By Ohms law, voltage is equal to current times resistance; thus, numerically stated:

26. Although the current gain (95 percent) in this particular transistor circuit is actually a loss, the ratio of resistance from emitter to collector more than makes up for this loss. Also, this same resistance ratio provides a power gain which makes the transistor adaptable to many electron circuits. 27. N-P-N Junction Transistors. The theory of operation of the N-P-N is similar to that of the P-N-P transistor. However, inspection and comparison of figures 115 and 116 will reveal two important differences: • The emitter-to-collector carrier in the P-N-P transistor is the hole. The emitter-to-collector carrier in the N-P-N transistor is the electron.

Figure 114. Forward bias between emitter and base (A) and reverse bias between base and collector (B) • The bias voltage polarities are reversed. This condition is necessitated by the different positional relationships of the two types of germanium as used in the two types of transistors. 28. Transistors and Electron Tubes. Some of the differences and similarities between electron tubes and transistors are discussed in the following paragraphs. 29. The main current flow in an electron tube is from cathode to plate (shown in fig. 117). In a junction transistor, the main current flow is from emitter to collector. The electron current in the electron tube passes through a grid. In the transistor, the electron current 118 passes through the base. The cathode, grid, and plate of the electron tube are comparable to the emitter, base, and collector, respectively, of the transistor. Plate current is determined mainly by grid to cathode voltage, and collector current is determined mainly by emitter-base voltage. The electron tube requires heater current to boil electrons from the cathode. The transistor has no heater. 30. For electron current flow in an electron tube, the plate is always positive with respect to the cathode. For current flow in a transistor, the collector may be positive or negative with respect to the emitter depending on whether the electrons or holes, respectively, are the emit-ter-to-collector carriers. For most electron tube

Figure 115. Simultaneous application of forward bias between emitter and base and reverse bias between base and collector of P-N-P transistor. applications, grid cathode current does not flow. For most transistor applications, current flows between emitter and base. Thus, in these cases, the input impedance of an electron tube is much higher than its output impedance and similarly the input impedance of a transistor is much lower than its output impedance. 31. Transistor Triode Symbols. Figure 118 shows the symbols used for transistor triodes. In the P-N-P transistor, the emitter-to-collector current carrier in the crystal is the hole. For holes to flow internally from emitter to collector, the collector must be negative with respect to the emitter. In the external circuit, electrons flow from emitter (opposite to direction of the emitter arrow) to collector. 32. In the N-P-N transistor, the emitter-to-collector current carrier in the crystal is the electron. For electrons to flow internally from emitter to collector, the collector must be positive with respect the emitter. In the external circuit, the electrons flow from the collector to the emitter (opposite to the direction of the emitter arrow). 33. Point-Contact Transistor. The point-contact transistor is similar to the point-contact diode except for a second metallic conductor (cat whisker). These cat whiskers are mounted relatively close together on the surface of a germanium crystal (either P- or N-type). A small area of P- or N-type is formed around these contact points. These two contacts are the emitter and collector. The base will be the N- or P-type of which the crystal was formed. The operation of the point-contact transistor is similar to the operation of the junction type. Now that you

Figure 116. Simultaneous application of forward bias between emitter and base and reverse bias between base and collector of N-P-N transistor. 119

Figure 117. Structure of a triode vacuum tube and a junction transistor. have studied transistors you must know how they are connected into the circuit. 32. Transistor Circuits 1. The circuit types in which transistors may be used are almost unlimited. However, regardless of the circuit variations, the transistor will be connected by one of three basic methods. These are: common base, common emitter, and common collector. These connections correspond to the grounded grid, grounded cathode, and grounded plate respectively. 2. Common Base Circuit. Figure 119 shows a common base circuit using a triode transistor. A thin layer of P-type material is sandwiched between two pellets of N-type material. The layer of P-type material is the base when the two pellets of N-type material are the collector and the emitter. The emitter is connected to the base through a small battery (B1). This battery is connected with its negative electrode to the N-type emitter and its positive electrode to the P-type base. Thus, the emitter-base junction has forward bias on it. Recombination of the electrons and holes causes base current (Ib) to flow. 3. Battery B2 is connected to produce reverse bias on the collector-base junction. However, current will flow in the collector-base circuit. Let’s see why this current will flow. In this emitter, electrons move toward the emitter-base junction due to the forward bias on that junction. Many of the electrons pass through the emitter-base junction into the base material. At this point the electrons are under the influence of the strong field produced by B2. Since the base material is very thin, the electrons are accelerated into the collector. This results in collector current (Ic), as shown in figure 119. About 95 percent of the electrons passing through the emitter-base junction enter the collector circuit. Thus, the base current (Ib), which is a result of recombination of electrons and holes, is only 5 percent of the emitter current. 4. Common Emitter Circuit. The circuit that will be encountered most often is the common emitter circuit shown in figure 120. Notice that the base is returned to the emitter and the collector is also returned the emitter. The base-emitter circuit is biased by a small battery whose negative electrode is connected to the N-type base and

Figure 118. Transistor symbols. 120

Figure 119. Common base circuit. The positive electrode to the P-type emitter. This forward bias results in a base-emitter current of 1 milliampere. In the collector circuit the battery is placed so as to put reverse bias on the collector-base junction. The collector current (Ic) is 20 milliamperes. Since the input is across the base emitter and the output is across the collector emitter, there is a current gain of 20. The positive voltage on the emitter repels its positive holes toward the base region. Because of their high velocity, and because of the strong negative field of the collector, the holes will pass right on through the base material and enter the collector. Only 5 percent or less of those carriers leaving the emitter will enter through the circuit. The other 95 percent or more will enter the collector and constitute collector current (Ic). 5. Common Collector Circuit. The common collector circuit in figure 121 operates in much the same manner as a cathode follower vacuum tube circuit. It has a high impedance and a low output impedance. It has a small power gain but no voltage gain in the circuit. The circuit is well suited for input and interstage coupling arrangements. 6. Transistor Amplifiers. Let’s put a signal voltage into the circuit of figure 122 and trace the electron flow. A coupling capacitor (C1) is used to couple the signal into the emitter-base circuit. Rg provides the right amount of forward bias. When the signal voltage rises in a positive direction, the emitter will be made less negative with respect to the base. This difference will result in a reduction of the forward bias on the emitter-base circuit and, therefore, a reduction in current flow through the emitter. Since the emitter current is reduced, the collector current will likewise be reduced at the same proportion. As the signal voltage starts increasing in a negative direction, the emitter will now become more negative with respect to the base, resulting in increased forward bias. Increased forward bias

Figure 120. Common emitter circuit. 121

Figure 121. Common collector circuit. 8. The electrical resistance of a semiconductor junction may vary considerably with its temperature. For this reason, the performance of a circuit will vary with the temperature unless the circuit is compensated for temperature variations. Compensating for temperature minimizes the effects of temperature on operating bias currents and will stabilize the d.c. operating conditions of the transistor. Now let us talk about the circuit that feeds the signal to the amplifier circuit-the bridge circuit. 33. Bridge Circuits Figure 122. Common base amplifier. will result in increased current flow in the emitter and collector circuits. 7. The signal being applied to the emitter-base circuit has now been reproduced in the collector circuit. The signal has been greatly amplified because the current flowing in the collector circuit is through a high impedance network. It is also possible to use a P-N-P type transistor, as shown in figure 123. 1. The brain of most electronic controls is a modified Wheatstone bridge. To understand the bridge circuit will review the operation of a variable resistor (potentiometer) first. One of the principal uses of the potentiometer is to take a voltage from one circuit to use in another. Figure 124 shows a potentiometer connected across a power source. The full 24 volts of the source is dropped between the two ends of the resistor; this means that 12 volts are being

Figure 123. Common emitter amplifier. 122

Figure 124. Potentiometer. dropped across each half, or 6 volts across each quarter (1/4). If a voltmeter is connected from one end, and to the movable wiper, it will read the voltage drop between that end and the wiper. Note that meter A is reading the voltage drop across ¼ of the resistance, or 6 volts. Meter B is reading the voltage drop across the remaining ¾ of the resistance, or 18 volts. As the wiper is moved clockwise, the voltage shown on meter A will increase and B will decrease. Later you will hear the word “pot.” This is short for potentiometer. 2. Figure 125 shows two resistances connected in parallel with their wipers connected to a voltmeter. Since

the two resistances are connected in parallel, the voltage applied by the battery is equally distributed along each of the two “pots.” Such a combination of “pots” is called a bridge. Notice that each wiper is at a positive potential with respect to point C of 6 volts, and consequently the voltmeter indication is zero volts. Since no current flows between the wipers, the bridge is said to be balanced. If wiper A is moved to the center of the top “pot,” detail A, it would take off 12 volts; however, wiper B is taking off 6 volts and the meter would read 6 volts, the difference between 6 and 12. Electrons would flow from B (negative) through the meter to A (positive in respect to B). The meter would be deflected to the left 6 volts, so we can say the bridge is unbalanced to the left. Moving wiper B toward the positive potential and A toward negative will cause the bridge to unbalance to the right because current would flow from A to B, deflecting the meter to the right, which is demonstrated in detail B of figure 125. 3. Look at figure 126, a Wheatstone bridge. The basic operation is the same as the common bridge shown in figure 125, but it uses only one variable resistor. 4. The variable resistor has a higher resistance value than the three fixed resistors. When the variable resistor is centered, it has the same value as the fixed resistors; the bridge is in balance, for no voltage is indicated by the meter. Each resistor drops 12 volts. Detail A of figure 126 shows R4 unbalanced to the left. Because of its higher resistance, it now drops 18 of the applied volts, and the remaining 6 volts are dropped by R1. The difference between 6 and 12 or 12

Figure 125. Simple bridge. 123

Figure 126. Wheatstone bridge. and 18 is across the meter (6 volts). Since current flows from negative to positive, the flow through the meter is toward the op of the page. Detail B of figure 126 shows R4 unbalanced to the right. This drops its value, causing most of the applied voltage to be dropped across R1 (18 volts). The difference between 12 and 18 (6 volts) is across the meter, but in this case flowing toward the bottom of the page (- to +). 5. The Wheatstone bridge can be used on a.c. or d.c., but if a.c. is used, it requires a phase detector, discussed later in this chapter. The a.c. Wheatstone bridge is used with most electronic controls. Note that in figure 127 the d.c. power source has been replaced with a transformer and the voltmeter has been replaced with an

amplifier. The amplifier simply “builds up” the small signal from the bridge to operate a relay. 6. T1 (thermostat) now takes the place of R3. The sensing element is a piece of resistance wire that changes in value as the temperature changes. An increase in temperature will cause a proportional increase in resistance. As you will note in figure 127, at set point of 74° F., the bridge is in balance. The voltage at points C and D is the same (7.5 volts), and the amplifier will keep the final control element in its present position until we have a temperature change. Now let’s assume the control point changes. 7. When the temperature at T1 is lower than set point, its resistance is less than 1000 ohms. This lower resistance causes more than 7.5 volts to be dropped by R2, which means that point C has a lower voltage than point D. The amplifier will then take the necessary action to correct the control point. 8. When the temperature at T1 is higher than set point, its resistance is more than 1000 ohms, causing less than 7.5 volts to be dropped across R2. Point C has a higher voltage than point D. The amplifier will once again take the necessary corrective action. 9. The resistance of T1 changes 2.2 ohms for each degree temperature change. This will cause only 0.0085volt change between points C and D. For this reason, to check the bridge circuit, one will have to use an electronic meter usually called a V.T.V.M. for vacuum tube voltmeter.

Figure 127. A.c. Wheatstone bridge. 124

The vacuum tube voltmeter will usually have an ohms scale as well as ac. and d.c. voltage scales. 10. The V.T.V.M. must be plugged into the lower line for operation. Usually, there is no provision for current measurements. Its advantage, however, is an extremely high input resistance of 11 million ohms (11 meg) or more, as a d.c. voltmeter, resulting in negligible loading effect. Also resistance ranges up to R X 1000 allow measurements as high as 1000 megohms. The ohms scale reads from left to right like the volts scale and is linear without crowding at either end. The adjustments are as follows: (1) First, with the meter warmed up for several minutes on the d.c. volts position of the selector switch, set the zero adjust to line up the pointer on zero at the left edge of the scale. (2) With the leads apart and the selector on ohms, the ohms adjust is set to line the pointer with maximum resistance ( ) at the right of the scale. (3) Set the selector switch to the desired position and use. The ohms adjust should be set for each individual range. 11. CAUTION: When checking voltage on unfamiliar circuits, always start with the highest voltage scale for your safety as well as protection of the meter. 12. Another circuit that you could use in electronic controls is the discriminator circuit. It is used in conjunction with a bridge circuit. 34. Discriminator Circuits 1. The purpose of the discriminator circuit is to determine the direction in which the bridge is unbalanced

and take the necessary action to correct the condition. When the control point moves off set point, the bridge becomes unbalanced and sends a small signal to the control grid of the first-stage amplifier, as shown in figure 128. 2. The small a.c. signal imposed on the control grid of this triode causes it to conduct more when the signal is positive and less when it is negative. The sine wave in figure 128 shows the plate voltage at point A. Note that when the grid is more positive, the tube conducts more and most of the 300 v.d.c. is dropped across load resistor R7. When the grid is negative, most of the voltage is dropped across the tube. The sine wave has been inverted and is riding a fixed d.c. value of 150 volts. 3. The blocking capacitor C2 passes the amplified a.c. component to the second stage but blocks the high voltage d.c. R6 is the bias resistor for the control grid, and R5 is the bias bleeder to prevent self-bias. 4. Amplifier stages 2, 3, etc., as seen in figure 129, repeat the process until the signal is strong enough to drive a power tube or discriminator. At this point the signal voltage has been amplified to a sufficient level to drive a power tube. 5. The power amplifier require a higher voltage driving signal but controls a much larger current. This current is then used to energize a relay and operates the final control element. In the discriminator circuit shown in figure 130, when the signal goes negative, cutoff bias is reached on the control grid. Also, the tube will conduct only when the plate is positive. Plate current will therefore be similar to the output of a half-wave rectifier. 6. Since plate current flows in pulse, capacitor C5 is connected across the coil of the motor relay. The capacitor will charge while the plate

Figure 128. Bridge and amplifier circuit. 125

Figure 129. Second- and third-stage amplification. is conducting and discharge through the coil, holding it energized during the off cycle. This type control is two position, and the final control will either be in the fully open or fully closed position. 7. The bridge supply voltage must come from the same phase as the discriminator supply, shown in figure 131. Supplying voltage from the same phase insures a bridge signal that is either in phase or 180° out of phase with the discriminator supply. 8. The control grid of the discriminator is biased at cutoff; therefore, it will conduct only when the plate and the amplified bridge signal are both positive. With the temperature below set point, as in figure 131, point C will have the same polarity as point B (the resistance of T1 decreased); and will cause bridge signal to

Figure 130. Discriminator circuit. 126

Figure 131. Two-position control. be more positive at the same time the discriminator plate is positive (solid symbols, +). Current flows through the relay and also charges capacitor C5. During the next halfcycle (dotted symbols, +) the signal is negative and the discriminator plate is negative. No plate current can flow. Capacitor C5 discharges through the relay which holds it closed until the next alteration. 9. The valve controlling chill water or brine will remain closed until the temperature increases. If the temperature goes above the set point, the grid of the discriminator will be negative when the plate is positive and vice versa. No plate current can flow and the valve opens. 10. For modulating control, illustrated in figure 132, a modulating motor is used with a balancing potentiometer. The balancing potentiometer is wired in series with the thermostat resistor. Its purpose is to bring the bridge back into balance (no voltage between points C and D) when a deviation has been corrected. Assuming a rise in temperature at T1 and the polarity shown by the solid symbols, point C will be negative. Neither of the discriminator tubes will conduct because the control grids of both are negative beyond cutoff bias. During the next alternation (dotted symbols), when the signal is positive, discriminator number 2 will conduct because its plate is also positive. Capacitor C6 will charge and relay number 2 will energize, causing the motor to run counterclockwise; this moves the wiper of the balancing potentiometer to the right, adding resistance to R1, and removing resistance from T1 until no signal is applied to the amplifier. Cutoff bias is reached on the control grids of the discriminators, capacitor C6 discharges, relay 2 energizes, and the motor stops at its new position. 11. A decrease in temperature at T1, causes a 180° phase shift from the bridge. This phase shift places the grid of discriminator tube 1 positive at the same time as the plate. Relay 1 energizes and the motor runs clockwise until the bridge is once again balanced. 12. For control of relative humidity, the thermostat is replaced by a gold leaf humidistat. The principle of operation is the same; however, you must remember that moisture sensed by the gold leaf causes the resistance to change.

127

Figure 132. Modulating control. Review Exercises The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the text. Do not submit your answers. 1. Explain thermionic emission. (Sec. 29, Par. 3) 4. The electrons flow from the ________________ to the _________________in a vacuum tube. (Sec. 29, Par . 7)

5. Why does the diode rectify a.c.? (Sec. 29, Par. 8)

2. How does a directly heated cathode differ from an indirectly heated cathode? (Sec. 29, Par. 4)

6. What factors determine the amount of current flowing through a diode tube? (Sec. 29, Par. 9)

3. Name the elements of a diode vacuum tube. (Sec. 29, Par. 7)

7. The diode will conduct during the ___________ half-cycle of the alternating current. (Sec. 29, Par. 11)

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8. How can you filter half-wave rectification with a capacitor? (Sec. 29, Par. 13)

17. Why can a triode be used as an amplifier? (Sec. 30, Par. 15)

9. What is a duo-diode vacuum tube? (Sec. 2, Par. 16)

18. What is the potential of the screen grid with respect to the cathode in a tetrode vacuum tube? (Sec. 30, Par. 19)

10. What is the purpose of the control grid in a vacuum tube? (Sec. 30, Par. 2) 11. Where, inside the tube, is the control grid physically located? (Sec. 30, Par. 2)

19. How does a power amplifier differ from a triode amplifier? (Sec. 30, Par. 20)

20. What potential is applied to the suppressor grid of a pentode tube? (Sec 30, Par. 23) 12. The usual polarity of the grid with respect to the cathode is ________________. (Sec. 30, Par. 4)

21. What is a valence ring? (Sec. 31, Par. 4)

13. What will happen to the current through a triode if you make the control grid more negative? (Sec. 30, Par. 5)

22. A valence ring containing two electrons indicates a good ________________. (Sec. 31, Par. 4)

14. Define grid bias. Cutoff bias. (Sec. 30, Pars. 5 and 7)

23. How is N-type germanium made? (Sec. 31, Par. 8)

15. Name the types of grid bias used on vacuum tubes. (Sec. 30, Pars. 8, 9, and 12)

24. How does N-type germanium material differ from P-type germanium material? (Sec. 31, Pars. 8 and 9)

16. What is one disadvantage of contact potential bias? (Sec. 30, Par. 14)

25. To achieve reverse bias, the positive electrode of the battery is connected to the _______________ material and the negative to the ________________ material. (Sec. 31, Par. 13)

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26. Which type of bias encourages current flow? (Sec. 31, Par. 14)

35. When is a simple two-resistor bridge balanced? (Sec. 33, Par. 2)

27. How much power is developed in a circuit having 100 ohms resistance and an amperage draw of 5 amps? (Sec. 31, Par. 17)

36.

How is the Wheatstone bridge applied to electronic control? (Sec. 33, Par. 5)

37. 28. Where is the base of a P-N-P transistor located? (Sec. 31, Par. 19)

What will occur when the temperature at the thermostat, connected in a Wheatstone bridge, increases? (Sec. 33, Par. 8)

29.

How is maximum power gain obtained in a transistor? (Sec. 31, Par. 24)

38.

What type of meter is used to check out electronic controls? Why? (Sec. 33, Par. 9)

30.

What components of a vacuum tube are comparable to the emitter, base, and collector of a transistor? (Sec. 31, Par. 29)

39.

What is the first step you must take when using a V.T.V.M.? (Sec. 33, Par. 10)

31.

Name the three basic transistor circuits. (Sec. 32, Par. 1)

40.

What is the purpose of a discriminator circuit? (Sec. 34, Par. 1)

32.

Which transistor circuit has a high impedance input and a low impedance output? (Sec. 32, Par. 5)

41. Explain the function of the blocking capacitor. (Sec. 34, Par. 3)

42. 33. What is the purpose of a coupling capacitor between stages? (Sec. 32, Par. 6)

What has occurred when the signal in the discriminator circuit goes negative? (Sec. 34, Par. 5)

34. You are checking the voltage drop across a potentiometer. The applied voltage is 12 volts and three-fourths of the resistance is in the circuit. What is the voltage drop across the potentiometer? (Sec. 33, Par. 1)

43.

Why should the bridge supply voltage come from the same phase as the discriminator supply? (Sec. 34, Par. 7)

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44. When will the discriminator circuit conduct? (Sec. 34, Par. 8)

45. Why is a balancing potentiometer read with a modulating motor? (Sec. 34, Par. 10)

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CHAPTER 7

Electronic Control Systems

ELECTRONIC control is here to stay. It has been approximately 16 years since the control industry first showed how microvoltages, electronically amplified, could be used in controlling air-conditioning and equipment cooling systems. Despite an erroneous but perfectly human awe in the presence of a revolutionary form of power, engineers, designers, and building owners began to apply this new type of control to their systems. The ordinary serviceman shunned electronic control because the thought that it was a piece of hardware too technical to repair. By 1955, over 5000 electronic control systems were in use, and it had become evident that their adjustment and maintenance were not more difficult but actually simpler than those of the more traditional control systems--pneumatic and electric. 2. In this chapter you will study system components, applications, and the maintenance performed on electronic control systems. 35. Components 1. The components discussed in this section are the humidity sensing element, thermostats, and damper motor. The control panel will be discussed later in this chapter. It houses the bridge and amplifier circuits that we covered in Chapter 6. 2. Humidity Sensing Element. The sensing element should be located within the duct at a place where the air is thoroughly mixed and representative of average conditions. You must be careful not to locate the sensing element too close to sprays, washers, and heating or cooling coils. The location should be within 50 feet of the control panel. All wiring and mounting should be accomplished as specified by the manufacturer. 3. Thermostats. The thermostats you will study in this chapter are space, outdoor, and insertion. In addition, we will also cover thermostat maintenance. 4. Space thermostat. The thermostat should be mounted where it will be exposed only to typical or

average space temperature. You should avoid installing it on an outside wall or on a wall surface with hot or cold water pipes or air ducts behind it. 5. In general, try to keep the thermostat out of the way of traffic, but in a representative portion of the space being measured. The most desirable location is on an inside wall, 3 to 5 feet from the outside wall and about 54 inches above the floor. 6. Outdoor thermostat. The sensing element is a coil of fine wire wound on a plastic bobbin and coated for protection against dirt and moisture. The thermostat should be mounted out of the sun (on the north side of the building or in some other shaded location), above the snowline, and where it won’t be tampered with. 7. Insertion thermostat. When using this thermostat as a discharge air controller, you should mount it far enough downstream from the coil to insure thorough mixing of the air before its temperature is measured. When you use it as a return air controller, the thermostat is mounted where it will sense the average temperature of the return air from the conditioned space. If you mount it near a grille, it should be kept out of the airflow from open doors and windows. 8. To mount the thermostat, use the back of the box as a template. Mark the four holes to be drilled in the duct--the center hole and the three mounting holes. The center hole is used to insert the element. 9. Thermostat maintenance. To check the re-sistance of the sensing element, you must disconnect one of the leads at the panel. Place an ohmmeter across the leads. Remember, allow for the temperature of the element and accuracy of the meter. 10. A reading considerably less than the total resistance specified indicates a short, either in the element or in the leads to the element. If a short is indicated, take a resistance reading across the thermostat terminals. If the thermostat is shorted it must be replaced. If the meter reads more than the total resistance, there is an open

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Figure 133. Damper motor schematic. circuit. Again, a reading across the thermostat terminals will locate the trouble. 11. Excessive dirt accumulated on the element will reduce the sensitivity of the thermostat. Clean the element with a soft brush or cloth. Be careful not to damage the resistance element. 12. Damper Motor. The motor may be installed in any location except where excessive moisture, acid fumes, or other deteriorating vapors might attack the metal. The motor shaft should always be mounted horizontally. 13. The motor comes equipped with one crank arm. By loosening the screw and nut which clamp the crank arm to the motor shaft, the crank arm can be removed and repositioned in any one of the four 90° positions on the motor shaft. The adjustment screw on the face of the crank arm provides angular setting of the crank arm in steps of 22½° throughout any one of the four 90° angles. You can see by changing the position of the arm on the square crankshaft and through the means of the adjustment screw on the hub, the crank arm may be set in steps of 22½° for any position within a full circle. The crank arm may be placed on either end of the motor shat. 14. For instructions in the assembly of linkages you must refer to the instruction sheets packed in the carton with each linkage. 133 15. Motor Servicing. The only repairs that can be accomplished in the field are cleaning the potentiometer or limit switch contacts, repairing internal connecting wires, and replacing the internal wires. 16. If the motor will not run, check the transformer output first. Look for the transformer in figure 133. If it checks out good, use the transformer to check the motor. Disconnect the motor terminals (usually numbered 1, 2 and 3) and connect the transformer output leads to terminals 2 and 3. The motor should run clockwise, if it is not already at that end of its stroke. Similarly, connecting the transformer across terminals 1 and 3 should drive the motor counterclockwise. 17. If the motor responds to power from the transformer, the fault probably lies in the relay, wiring, or potentiometer. To check the potentiometer, disconnect terminals T, G, and Y from the outside leads. The resistance of the potentiometer windings can now be checked with an ohmmeter. The resistance across Y and G should be about 150 ohms. The resistance across T and either Y or G should change gradually from near 0 ohms about 135 ohms as the motor is driven through its stroke. 18. If the motor does not respond to direct power from the transformer, you must remove the motor cover and check for broken wires, defective limit switch, or a faulty condenser (capacitor).

Figure 134. Refrigerant solenoid valve control system. 36. Application 1. The electronic control system has definite characteristics-flexibility, sensitivity, simplicity, speed, and accuracy-that show to best advantage in an airconditioning system where signals from several controllers must be coordinated to actuate a series of control motors or valves. Each controller is a component of a modified Wheatstone bridge circuit. A change in the controlled variable will cause a change in the voltage across the bridge. This change in voltage is detected by an electronic relay which starts corrective controlled device action. The magnitude of the voltage change and the resulting device movement are a result of the amount of controlled variable change. 2. Authority “pots” in the control panel adjust the change in variable required at a controller to give a certain voltage change. For example, an outdoor thermostat might be adjusted to require a 10° temperature change to give the same voltage change as a 1° change at the space thermostat. For the remainder of this discussion, let us consider temperature as the controlled variable. 3. Voltages resulting from a rise in temperature differ in phase from voltages resulting from a drop in temperature and therefore can be distinguished. Voltages resulting from temperature changes at several thermostats are added in the bridge if they are of the same phase or subtracted if they differ in phase. The total voltage determines the position of the final controlled device. Each controller directly actuates the final controlled device. 4. All adjustments for setting up or changing a control sequence can be made from the control panel. The panel may be mounted in any readily accessible location. Selection of controls is simplified since one electronic control, with its broad range, replaces several conventional controls where each has a smaller range. 5. The following systems are typical examples of how electronics is applied to the control of airconditioning and equipment cooling systems. The control 134

sequence is given for each application. 6. Refrigerant Solenoid Valve Control. The electron control panel R1 in figure 134 will control space temperature by coordinating signals from the space thermostat T1 and the outdoor thermostat T4 to operate the refrigerant solenoid valve V1. T4 will raise the space temperature as the outdoor temperature rises to a predetermined schedule. T5 will remove T4 from the system when the outdoor temperature falls below the setting of T5 to prevent subcooling of the space at low outdoor temperature. 7. You will find that a nonstarting relay, R2, is wired into the compressor starting circuit. This relay will prevent the compressor from operating unless the solenoid valve is operating. 8. T1 is a space thermostat which may have an integral set point adjustment and a locking cover. T4 and T5 are insertion thermostats. 9. Summer-Water Compensation for a TwoPosition Heating or Cooling System. Controller T5 shown in figure 135 will select either the summer or winter compensation schedule. This selection depends upon the outdoor temperature. 10. On the winter compensation schedule, electronic relay panel R1 will control the space temperature by coordinating signals from space thermostat T1 and outdoor thermostat T3. The relay will operate either the heating or cooling equipment, depending upon the space temperature requirement. You can adjust the effect of T3 to overcome system offset or to elevate the space temperature as the outdoor temperature falls. 11. During the summer compensation schedule, the electronic panel will control temperature by coordinating the signals from T1 and the outdoor thermostat T4 to operate the appropriate equipment, depending upon space temperature requirements. T4 will elevate the space temperature

Figure 135. Two-position heating and cooling system.

as the outdoor temperature rises according to a predetermined schedule. 12. The last major topic that you will cover in this volume is maintenance of electronic controls. 37. Maintenance 1. In this section we shall discuss the adjustments, calibration, and calibration checks you will perform. After you have adjusted and calibrated the system, you will learn how it operates. This system differs from the systems previously discussed in that the electronic control panel controls a pneumatic relay. The section will be concluded with a troubleshooting chart. With the information given in this section, you should have very little trouble acquiring the skill to perform most types of maintenance performed on electronic control systems. 2. Adjustments. You will find that the throttling range adjustment determines the temperature change at the T1 thermostat. This adjustment will change the branch line air pressure from 3 to 13 p.s.i.g. An adjustable throttling range is commonly provided with a range from 1° to 50° F. 3. You should set the throttling range to as low a value as possible without causing instability or hunting of the branch line pressure. If the controlled variable varies continually and regularly reverses its direction, too low a setting of the throttling range is indicated. You must increase the throttling range until hunting stops. 4. Stable operation does not mean that the branch line pressure fails to change often; actually the control system is extremely sensitive, and small temperature changes are being detected continuously. It is important for you to learn to distinguish between “jumpiness” and “hunting.” Jumpiness is caused by sensitivity of the relay, while hunting is a definite periodic alternating action. You must not interpret small gauge pressure fluctuations as hunting. A condition of this type can be caused by resonance in the valve unit chambers. 5. The authority dials are graduated in percentages. These dials determine the respective authorities of discharge or outdoor thermostats with respect to the space thermostat. The space thermostat is commonly referred to as T1. The remaining thermostats, outdoor, duct, etc., are numbered T2, T3, and T4. With an authority of 25 percent, the outdoor thermostat is onequarter as effective as the space thermostat. When you set the authority dials at zero percent, you are eliminating all thermostats except T1 from the system. An authority setting of 5 percent means that a 20° change in outdoor temperature will have only as much effect as a 1° change at the space thermostat.

6. You may find that the control panel has a control point adjuster. This adjuster makes it possible to raise or lower the control point after the system is in operation. The control point adjuster is set at the time the system is calibrated. The control point adjuster dial contains as many as 60 divisions, each of which normally represents a 1° change at the space thermostat. 7. The factory calibration and the valve unit adjustment can be checked or corrected only when the throttling range knob is out. The factory calibration on most systems is properly adjusted when it is possible to obtain a branch line pressure within 1 pound of 8 p.s.i.g. with an amplifier output voltage of 1 ± ¼ volt d.c. If the calibration is not correct, you must turn the factory calibration potentiometer until 1 volt is read from a voltmeter connected at the (+) terminal of the relay and (-) terminal of the bridge panel. A voltmeter of no less than 20,000 ohms per volt resistance must be used. The next step is to turn the valve unit adjusting screw until the branch line pressure is between 7 and 9 p.s.i.g. Clockwise rotation of the valve unit adjustment screw decreases branch line pressure. The factory calibration is now correctly set. 8. Calibration. Before you calibrate an electronic control system you must determine the throttling range and the compensator authorities. Start your calibration with the adjustment knobs in the following positions: (1) Control point adjuster: FULL COOL (2) Throttling range: OUT (3) Authority dials: 0 9. After the knobs are set, you must check the factory calibration. The branch line pressure should be 8 p.s.i.g. (±1 p.s.i.g). The actual branch line pressure obtained will be referred to as control reference pressure (CRP). 10. Next, you must measure the temperature at T1. This temperature will be referred to as the control reference temperature (CRT). After you have obtained the two references, turn the throttling range to the desired setting. At the same time, turn the control point adjuster until the CRP is obtained (7-9 p.s.i.g.). 11. The authority dials are now set. This adjustment will change the branch pressure, so you must reset the control point adjuster to maintain a CRP of 7-9 p.s.i.g. The position of the control point adjuster represents the control reference temperature measured at T1. Increase or decease the temperature setting as desired. Remember, each scale division is equal to approximately 1° F. 12. If a space thermostat is not used, the

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calibration procedure will be the same, provided the discharge controller is connected to T1 (T2 is not used) and T3 authority is turned to the desired setting f the discharge controller is connected to the T3 position and T3 authority is tuned to the desired setting, the procedure is the same except that 70 F. is used as the CRT. The correction for the desired set point is made with the control point adjuster dial divisions representing approximately ½° F each. 13. Calibration Check. The calibration of any system should be checked after the system has been put in operation. First, we will check a winter system. 14. At the no-load condition, the control point (measured space temperature) should be equal to the set point. On compensated systems, the control point should be approximately equal to the set point, whereas on an uncompensated system, the control point will be slightly lower than the set point. On systems compensated to provide successively higher temperatures as the outdoor temperature falls, the control point can be expected to be higher than the set point. 15. For any summer system, at the no-load condition, the control point should equal the set point. If the outdoor temperature is above the no-load temperature on an uncompensated system, you may consider it normal because the control point will be slightly higher than the set point. However, on systems compensated to provide successively higher temperatures as the outdoor temperature rises, the control point can be expected to be higher than the set point. 16. To make a correction for a calibration error, simply rotate the control point adjuster the number of dial divisions equal to the calibration error. 17. Operation. The one electronic control discussed here is similar to those in other panels; that is, it contains a modified Wheatstone bridge circuit which provides the input voltage for the electronic amplifier. The amplified output voltage is then used to control a sensitive, high-capacity, piloted force-balance pneumatic valve unit. 18. A change in temperature at T1 will initiate control action by a signal from the bridge circuit.

Figure 136. Pneumatic valve unit. This signal change provides a voltage to be fed to the amplifier which operates the pneumatic valve unit. The system will then provide heating or cooling as required until the initial signal is balanced by a change in resistance at T1 and T2 (depending upon the system’s schedule). An outdoor thermostat, T3, is used to measure changes in outdoor temperature so that control action can be initiated immediately before outdoor weather changes can be detected at T1. This in effect compensates for system off. The authority of T3 may be selected so that in addition to compensating for offset, T3, will provide setup. For example, it will raise the system control point as outdoor temperature drops. 19. The output of the electronic amplifier controls the current through the magnetic coil. Look at figure 136 for the magnetic coil. As the voltage changes, the nozzle lever modulates over the nozzle. When the lever moves toward the nozzle, the branch line pressure will increase. The new branch line pressure, through the feedback bellows, opposes further movement of the nozzle lever. The forces which a upon the lever a now in balance. When the voltage decreases, the lever will move away from the nozzle. This movement will cause the branch line pressure to decrease until the forces are again in balance. 20. Troubleshooting. Troubleshooting a suspected defective device can be speeded up by relating apparent defects to possible causes. The troubleshooting guide, table 21, is broken up into portions related to the setup and calibration procedure given earlier.

TABLE 21

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TABLE 21-Continued

Review Exercises The following exercises are study aids. Write your answers in pencil in the space provided after each exercise. Use the blank pages to record other notes on the chapter content. Immediately check your answers with the key at the end of the text. Do not submit your answers. 1. What precaution should you observe when installing a humidity sensing element? (Sec. 35, Par. 2)

4. What factor will reduce the sensitivity of a thermostat? (Sec. 25, Par. 11)

5. Explain the procedure you would use to reposition the crank arm on a damper motor. (Sec. 35, Par. 13)

6. Name the repairs that can be made to the damper motor in the field. (Sec. 35, Par. 15) 2. Describe the outdoor thermostat element. (Sec. 35, Par. 5) sensing 7. How can you check the transformer output? (Sec. 35, Par. 16) 3. How do you check the resistance of a thermostat sensing element? (Sec. 35, Par. 9)

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8. What troubles may exist if the damper motor does not respond to direct transformer power? (Sec. 35, Par. 18)

16. How can you reset the control point after the system is in operation? (Sec. 37, Par. 6)

9. Which component in the control panel adjusts the change in variable required at a controller to give a certain voltage change? (Sec. 36, Par. 2)

17. A trouble call indicates that an electronic control system is not functioning properly. The following symptoms are present: (1) The amplifier output voltage is 1 volt. (2) The branch line pressure is 5 p.s.i.g. What is the most probable trouble? (Sec. 37, Par. 7)

10. What factor determines the position of the final control element? (Sec. 36, Par 3) 18. What is the control reference temperature? Control reference pressure? (Sec. 37, Pars. 9 and 10) 11. Where are the adjustments made for setting up or changing a control sequence? (Sec. 36, Par. 4) 19. When checking the calibration of a compensated system on winter schedule, what is the relationship of the control point to the set point? (Sec. 37, Par. 14)

12. Explain the function of the nonrestarting relay. Where is it connected? (Sec. 36, Par. 7)

13. How does the summer compensation schedule differ from the winter compensation schedule? (Sec. 36, Pars. 10 and 11)

20. How does a bridge signal affect the pneumatic relay? (Sec. 37, Pars. 18 and 19)

14. What has occurred when the controlled variable varies continually and reverses its direction regularly? (Sec. 37, Par. 3)

21. What will happen if a faulty connection exists between the amplifier and bridge? (Sec. 37, table 21)

15. With an authority setting of 10 percent, how much effect will t2 have when a 10° temperature change is felt? (Sec. 37, Par. 5)

22. The tubes in the control panel light up and burn out repeatedly. Which components would you check? (Sec. 37, table 21)

138 Answers to Review Exercises

1. The three things to consider before installing a preheat coil are necessity for preheat, entering air temperature, and size of coils needed. (Sec. 1, Par. 2) 2. The most probable malfunction when the stream valve is closed and the temperature is 33° F. is that the controller is out of calibration. (Sec. 1, Par. 4) 3. The two functions which the D/X coil serves are cooling and dehumidification. (Sec. 1, Par. 7) 4. When a compressor using simple on-off control short cycles, the differential adjustment on the thermostat is set too close. (Sec. 1, Par. 9) 5. On a two-speed compressor installation, the humidistat cycles the compressor from low to high speed when the space humidity exceeds the set point. (Sec. 1, Par. 11) 6. The nonrestarting relay prevents short cycling during the off cycle and allows the compressor to pump down before it cycles “off.” (Sec. 1, Par. 12) 7. When the solenoid valves are not operating, you should check the operation of the fan because the fan starter circuit has to be energized before the control circuit to the valve can be completed. (Sec. 1, Par. 14) 8. The type of compressor used when two-position control of a D/X coil and modulating control of a face and bypass damper are used is a capacity controlled compressor. (Sec. 1, Par. 15) 9. An inoperative reheat coil. (Sec. 1, Par. 18) 10. The humidistat positions the face and bypass dampers to provide a mixture of conditioned and recirculated air to limit large swings in relative humidity. (Sec. 1, Par. 20) 11. The space humidistat has prime control of the D/X coil during light loads when a space thermostat and humidistat are used to control coil operation. (Sec. 1, Par. 26) 12. The only conclusion you can make is that the unit is a “medium temperature unit.” Sec. 2. Par. 3) 13. If you installed a medium temperature unit for a 40° F. suction temperature application, the motor would overload and stop during peak load. (Sec. 2, Par. 3) 14. The low-pressure control will cycle the unit when the crankcase pressure exceeds the cut-in pressure setting of the control even though the thermostat has shut off the liquid line solenoid valve. (Sec. 2, Par. 4 and fig. 19) 15. The automatic pump-down feature may be omitted when the refrigerant-oil ratio is 2:1 or less or when the evaporator temperature is above 40° F. (Sec. 2, Par. 5) 16. Th four factors you must consider before installing a D/X system are space requirements, 139

17.

18.

19. 20. 21. 22. 23. 24. 25. 26. 27.

28. 29.

30. 31.

32.

33.

34.

equipment ventilation, vibration, and electrical requirements. (Sec. 3, Par. 1) To prevent refrigerant condensing in the compressor crankcase, warm the equipment area so the temperature will be higher than the refrigerated space. (Sec. 3, Par. 2) The compressor does not require a special foundation because most of the vibration is absorbed by the compressor mounting springs. (Sec. 3, Par. 3) The minimum and maximum voltage that can be supplied to a 220-volt unit is 198 volts to 242 volts. (Sec. 3, Par. 5) A 2-percent phase unbalance is allowable between any two phases of a three-phase installation. (Sec. 3. Par. 5) During gauge installation, the shutoff valve is back-seated to prevent the escape of refrigerant. (Sec. 3, Par. 9) The liquid line sight glass is located between the dehydrator and expansion valve. (Sec. 3, Par. 12) Series. (Sec. 3, Par. 14) Parallel. (Sec. 3, Par. 14) Dry nitrogen and carbon dioxide are used to pressurize the system for leak testing. (Sec. 3. Par. 15) Moisture in the system will cause sludge in the crankcase. (Sec. 3, Par. 16) The ambient temperature (60° F.) allows the moisture to boil in the system more readily. This reduces the amount of time required for dehydration. (Sec. 3, Par. 17) A vacuum indicator reading of 45° F. corresponds to a pressure of 0.3 inch Hg absolute. (Sec. 3, Par. 18, fig. 17) Shutoff valves are installed in the vacuum pump suction line to prevent loss of oil from the vacuum pump and contamination of the vacuum indictor. (Sec. 3, Par. 20) Free. (Sec. 3, Par. 22) The valves are backseated before installing the gauge manifold to isolate the gauge ports from the compressor ports to prevent the entrance of air or the loss of refrigerant. (Sec. 3, Par. 25) The four items that you must check before starting a new compressor are the oil level, main water supply valve, liquid line valve, and power disconnect switch. (Sec. 3, Par. 26) Frontseating the suction valve closes the suction line to the compressor port, which causes the pressure to drop and cut off the condensing unit on the low-pressure control. (Sec. 3, Par. 34) Placing a refrigerant cylinder in ice will cause the temperature and pressure of the refrigerant within the cylinder to fall below that which is still in the system. (Sec. 4, Par. 3)

35. A partial pressure is allowed to remain in the system to prevent moist air from entering the system when it is opened (Sec. 4, Par. 4) 36. To prevent moisture condensation, you must allow sufficient time for the component that is to be removed to warm to room temperature. (Sec. 4, Par. 6) 37. Basket; disc. (Sec. 4, Par. 9) 38. Noncondensable gases collect in the condenser, above the refrigerant. (Sec. 4, Par. 10) 39. Noncondensable gases are present in the condenser when the amperage draw is excessive, the condenser water temperature is normal, and the discharge temperature is above normal. (Sec. 4, Par. 10) 40. A discharge pressure drop of 10 p.s.i.g. per minute with the discharge shutoff valve frontseated would indicate a leaky compressor discharge valve. (Sec. 4, Par. 15) 41. Valve plates ere removed from cylinder decks with jacking screws. (Sec. 4, Par. 18) 42. The emergency procedure you can use to recondition a worn valve is to lap the valve with a mixture of fine scouring powder and refrigerant oil on a piece of glass in a figure 8 motion. (Sec 4, Par. 21) 43. The oil feed guide is installed with the large diameter inward. Sec. 4, Par. 27) 44. A hook is used to remove the rotor to prevent bending of the eccentric straps or connecting rods. (Sec. 4, Par. 29) 45. A small space is left to provide further tightening in case of a leak. (Sec. 4, Par. 34) 46. 1.5 foot-pounds. (Sec. 4, Par. 35) 47. Check the start capacitor for a short when the air conditioner keeps blowing fuses when it tries to start and the starting amperage draw is above normal. (Sec. 4, Par. 36) 48. A humming sound from the compressor motor indicates an open circuited capacitor. (Sec. 4 Par. 36) 49. Closed. (Sec. 4, Par. 38) 50. Counter EMF produced by the windings causes the contacts of the starting relay to open. (Sec. 4, Par. 38) 51. Relay failure with contacts closed can cause damage to the motor windings. (Sec. 4, Par. 41) 52. Heater (and) control. (Sec. 4, Par. 43) 53. Oil pump discharge pressure; crankcase pressure. (Sec. 4, Par. 44) 54. Disagree. The oil safety switch will close when the pressure differential drops. (Sec. 4, Par. 45) 55. A burned-out holding coil or broken contacts will cause an inoperative motor starter. (Sec. 4, table 1) 56. A restricted dehydrator is indicated when the dehydrator is frosted and the suction pressure is below normal. (Sec. 4, table 2) 57. The expansion valve is trying to maintain a constant superheat. To accomplish this with a loose bulb, the valve is full open, which causes liquid refrigerant to flood back to the compressor. (Sec. 4, table 5) 58. A low refrigerant charge (flash gas in the liquid line). (Sec. 4, table 6) 59. An excessive pressure drop in the evaporator. (Sec. 4, table 6) 140

60. The most probable causes for an exceptionally hot water-cooled condenser are an overcharge and noncondensable gases in the system. These conditions may be remedied by bleeding the non-condensables or excessive refrigerant from the condenser. (Sec. 4 , table 7) 61. An obstructed expansion valve. (Sec. 4, table 10) 62. When a capacity controlled compressor short cycles you must reset the compressor capacity control range. (Sec. 4, table 10) CHAPTER 2 1. The component that should be checked when the condenser waterflow has dropped off is the thermostat that controls the capacity control valve. The thermostat is located in the chill water line. (Sec. 5, Par. 2) 2. Tap water; lithium bromide. (Sec. 5, Par. 3) 3. When heat is not supplied to the generator, the salt solution in the absorber will become weak and the cooling action that takes place within the evaporator will stop. This will cause the chill water temperature to rise. (Sec. 5, Par. 5) 4. Disagree. It heats the weak solution. (Sec. 5, Par. 5) 5. The component is the capacity control valve. The reduced pressure will cause the thermostat to close the capacity control valve which reduces or stops the flow of water through the condenser. The capacity of the system will decrease without condenser waterflow. (Sec. 5, Pars. 6 and 7) 6. 4. (Sec. 5, Par. 7) 7. A broken concentration limit thermostat feeler bulb will cause the vapor condensate well temperature to rise because the capacity control valve will remain closed. (Sec. 5, Par. 8) 8. The chill water safety thermostat has shut the unit down because the leaving chill water temperature was 12° above the design temperature. To restart the unit, the off-runstart switch must be placed in the START position so that the chill water safety thermostat is bypassed. After the chill water temperature falls below the setting of the chill water safety control, the off-run-start switch placed in the RUN position. (Sec. 5, Pars. 9 and 10) 9. The pumps are equipped with mechanical seals because the system operates in a vacuum. (Sec. 5, Par. 14) 10. Disagree. It only controls the quantity of water in the tank. It does not open a makeup water line. (Sec. 5, Par. 14) 11. The nitrogen charge used during standby must be removed. (Sec. 6, Par. 3) 12. A low water level in the evaporator will cause the evaporator pump to surge. (Sec. 7, Par. 3) 13. A partial load. (Sec. 7, Par. 4) 14. The solution boiling level is set at initial startup of the machine. (Sec. 7, Par. 5) 15. When air is being handled, the second stage of the purge unit will tend to get hot. (Sec. 7, Par. 7)

16. Solution solidification. (Sec. 7, Par. 9) 17. You can connect the nitrogen tank to the alcohol charging valve to pressurize the system. (Sec. 7, Par. 14) 18. Three. (Sec. 7, Par. 15) 19. You can determine whether air has leaked in the machine during shutdown by observing the absorber manometer reading and checking it against the chart. (Sec. 8, Par. 2) 20. Corrode. (Sec. 8, Par. 2) 21. To check a mechanical pump for leaks, you must close the petcocks in the water line to the pump seal chamber and observe the compound pressure gauge. A vacuum indicates a leaky seal. (Sec. 8, Par. 3) 22. Flushing the seal chamber after startup will increase the life of the seal. (Sec. 8, Par. 4) 23. Chill water as leaked back into the machine. (Sec. 8, Par. 5) 24. Octyl alcohol is added to the solution to clean the outside of the tubes in the generator and absorber. (Sec. 8, Par. 7) 25. When actyl alcohol is not drawn into the system readily, the conical strainer is dirty and must be removed and cleaned. This is normally accomplished at the next scheduled shutdown. If this situation persists, the solution spray header must be removed and cleaned. (Sec. 8, Par. 8) 26. When the purge operates but does not purge, the steam jet nozzle is plugged. To correct this, you must close the absorber purge valve and the purge steam supply valve. Then remove the steam jet cap and clean the nozzle with a piece of wire. The steam supply valve can be opened to blow out the loosened dirt. After the nozzle is clean, replace the cap and open the valves. (Sec. 8, Par. 9) 27. Silver nitrate. (Sec. 8, Par. 10) 28. Three drops of indicator solution is added to the solution sample. (Sec. 8, Par. 10) 29. 1. (Sec. 8, Par. 11) 30. When more silver nitrate is needed to turn the sample red, the sample contains more than 1 percent of lithium bromide. The evaporator water must be reclaimed. (Sec. 8, Pars. 10 and 11) 31. The length of time needed to reclaim evaporator water depends upon the amount of salt (lithium bromide) in the evaporator water circuit. (Sec. 8, Par. 12) 32. It takes 2 or 3 days for the dirt to settle out when the solution is placed in drums. (Sec. Par. 14) 33. The conical strainer is cleaned by flushing it with water. (Sec. 8, Par. 16) 34. The purge is cleaned with a wire or nylon brush. (Sec. 8, Par. 20) 35. Disagree. The diaphragm in a vacuum type valve is replaced every 2 years. (Sec. 8, Par. 22) 36. A steady rise in vapor condensate temperature indicates that the absorber and condenser tubes must be cleaned. (Sec. 8, Par. 25) 37. Soft scale may be removed from the condenser

38. 39. 40.

41.

tubes with a nylon bristle brush. (Sec. 8, Par. 28) The maximum allowable vacuum loss during a vacuum leak test is one-tenth of an inch of Hg in 24 hours. (Sec. 8, Par. 28) The refrigerant used to perform a halide leak test is R-12. (Sec. 8, Par. 29) Three causes of lithium bromide solidification at startup are condenser water too old, air in machine, improper purging, or failure of strong solution valve. (Sec. 8, table 11) To check for a leaking seal, close the seal tank makeup valve and note the water level in the tank overnight (Sec. 8, table 12) CHAPTER 3

1. 1200 pounds. (Sec. 9, Par. 1) 2. The economizer reduces the horsepower requirement per ton of refrigeration. (Sec. 9, Par. 2) 3. Disagree. The chilled water flows through the tubes. (Sec. 9, Par. 3) 4. Condenser float chamber. (Sec. 9, Par. 5) 5. The pressure within the economizer chamber is approximately halfway between the condensing and evaporating pressures. (Sec. 9, Par. 5) 6. Line with the shaft. (Sec. 10, Par. 1) 7. The impellers are dipped in hot lead to protect them from corrosion. (Sec. 10, Par. 2) 8. Two. (Sec. 10 Par. 3) 9. Brass labyrinth packing prevents interstage leakage of gas. (Sec. 10, Par. 4) 10. Axial thrust will affect suction end of the compressor. (Sec. 10, Par. 5) 11. Main compressor shaft. (Sec. 10, Par. 7) 12. The pump lubricates the thrust bearing first. (Sec. 10, Par. 8) 13. Oil is returned from the oil pump drive gear by gravity. (Sec. 10, Par. 9) 14. Oil pressure actuates the shaft seal. (Sec. 10, Par. 10) 15. The two holes in the inner floating seal ring allow the passage of oil to the front journal bearing. (Sec. 10, Par. 11) 16. 8. (Sec. 10, Par. 12) 17. The oil pressure gauge located on the control panel are the seal oil reservoir and “back of seal.” (Sec. 3, Par. 13) 18. A flow switch in the water supply oil cooler line turns the oil heater on automatically when waterflow stops. (Sec. 10, Par. 14) 19. Disagree. They are held apart during operation. (Sec. 10, Par. 16) 20. A high-grade turbine oil is used in centrifugal compressors. (Sec. 10, Par. 17) 21. Increases. (Sec. 11, Par. 1) 22. Journal speed, tooth speeds, (and) clearances. (Sec. 11, Par. 3) 23. The gear drive cooling water is turned on when the oil temperature reaches 100° F. to 110° F. (Sec. 11, Par. 5) 24. Gear wear. (Sec. 11, Par. 9)

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25. The gear to compressor coupling uses a spool piece. (Sec. 12, Par. 1) 26. The hub is heated with oil, steam, or open flame to expand it: (Sec. 12, Par. 2) 27. Feeler gauge. (Sec. 12, Par. 3) 28. The offset alignment of a coupling is checked with a dial indicator. (Sec. 12, Par. 4) 29. The couplings that have collector rings in the end of the cover can be lubricated while running. (Sec. 12, Par. 8) 30. Three; 60; adjustable speed wound. (Sec. 13, Par. 3) 31. Slipring circuit; speed. (Sec. 13, Par. 3) 32. When the start button is held closed, the oil pressure switch is bypassed. (Sec. 13, Par. 4) 33. The secondary function of the condenser is to collect and concentrate noncondensable gases. (Sec. 14, Par. 1) 34. A perforated baffle is used to prevent the discharge gas from directly hitting the condenser tubes. (Sec. 14, Par. 2) 35. When you remove the water box cover you must leave two bolts in the cover until the cover is supported with a rope or chain. (Sec. 14, Par. 3) 36. A blocked compressor suction opening. (Sec. 14, Par. 6) 37. Check the sight glass on the cooler to determine the system refrigerant charge. (Sec. 4, Par. 11) 38. A load increase is indicated when the refrigerant and chill water temperature differential increases (Sec. 14, Par. 13) 39. Surging. (Sec. 15, Par. 1) 40. The liquid injector is used desuperheat the hot gas (Sec. 15, Par. 2) 41. The pressure drop across the orifice created by the flow of gas through the orifice controls the amount of liquid refrigerant flowing to the hot gas bypass. (Sec. 15, Par. 3) 42. Disagree. The high-pressure control resets automatically when the pressure falls to 75 p.s.i.g. (Sec. 16, Par. 3) 43. The weir and trap is located in the center of the evacuation chamber. (Sec. 16, Par. 3) 44. Air is in the system. (Sec. 16, Par. 5) 45. Air in the condenser is released through the purge air relief valve. (Sec. 16, Par. 6) 46. One-half pint of water per day collected by surge unit indicates leaky tubes. (Sec. 16, Par. 8) 47. A pressure drop will exist across the pressureregulating valve when it is wide open. (Sec. 16, Par. 9) 48. Large amounts of air are normally purged after repairs and before charging. (Sec. 16, Par. 10) 49. Water is drained from the separator unit when it can be seen in the upper sight glass. (Sec. 16, Par. 12) 50. Low oil pressure, high condenser pressure, low refrigerant temperature, (and) low water temperature. (Sec. 17, Par. 1) 51. The low oil pressure control does not require manual resetting. (Sec. 17, Par. 2)

52. 53. 54. 55. 56.

57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68.

The high condenser pressure control has a differ-ential of 7 pounds. (Sec. 17, Par. 3) You can change controllers with the rotary selecting switch on the safety control panel. (Sec. 17, Par. 6) Control the speed of the compressor. (Sec. 18, Pars. 1 and 2) When you add more resistance to the rotor circuit of the drive motor, the compressor speed will decrease. (Sec. 18, Par. 3) Suction damper control is more effective than speed control when it is necessary to maintain a non-surging operation at light loads. (Sec. 18, Par. 4) During startup the drum controller lever is in number 1 position, all resistance in the circuit to the rotor. (Sec. 19, Par. 2) Condensed refrigerant will cause the oil level to rise in the pump chamber during an extended shut-down. (Sec. 9,. Par 6) 1. (Sec. 20, Par. 2) Agree. The 2-inch plug does prevent leakage when the ¾- inch plug is removed. (Sec. 20, Par. 3) To charge refrigerant into the system as a gas, you must let the drum rest on the floor and open the drum charging valve. (Sec. 20, Par. 5) The system may be pressurized with the purge recovery unit. (Sec. 20, Par. 6) High condenser pressure is normally caused by air in the condenser. (Sec. 20, table 19) Light load, air leak, (or) high condenser pressure. (Sec. 20, table 19) When the economizer float valve is stuck, the compressor second stage will frost. (Sec. 20, table 19) Low “back of seal” oil pressure and a high seal oil pressure are caused by a dirty filter or a filter cartridge improperly installed. (Sec. 20, table 19) Misalignment, insufficient lubrication, (or) excessive wear. (Sec. 20, table 19) Agree. A high oil level will cause the gear to overheat. (Sec. 20, table 19) CHAPTER 4

1. 2. 3.

The main scale-forming compound found in con-densing water systems is calcium carbonate. (Sec. 21, Par. 1) 7.1 (to) 14; 200. (Sec. 21, Par. 4) Using the formula

4. 5.

(Sec. 21, Par. 6) Four methods of preventing scale are bleedoff, pH adjustment, adding polyphosphates, and using the zeolite softener. (Sec. 21, Par. 7) Using the formula Hardness p.p.m. = 20 X (total No. of ml. of std.

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6. 7. 8. 9.

10. 11. 12. 13. 14. 15.

16. 17. 18. 19.

20. 21.

22.

23.

24. 25.

soap solution required to obtain a permanent lather) p.p.m = 20 X 10 p.p.m = 200 (Sec. 21, Par. 9) The lime-soda process changes calcium and magnesium from a soluble to an insoluble state. (Sec. 21, Par. 11) The zeolite process replaces the calcium and magnesium compounds with soluble sodium compounds. (Sec. 21, Par. 11) It is necessary to add lime or clay to the Accelator to add weight which prevents rising floc. (Sec. 21, Par. 15) The factors that would limit the use of the Spiractor are excessive magnesium hardness, high water temperature, and turbidity over 5 p.p.m. (Sec. 21, Par. 17) A salt or brine solution is uniformly distributed on top of the zeolite bed, which passes evenly down through the bed. (Sec. 21, Par. 18) Corrosion is more rapid in a liquid with a low pH value. (Sec. 22, Par. 2) The most common type of corrosion in an acid liquid is uniform corrosion. (Sec. 22, Par. 4) Pitting corrosion is characterized by cavities and gradually develops into pinhole leaks. (Sec. 22, Par. 5) The type of corrosion that corrodes steel in a system that contains an abundance of copper is known as galvanic corrosion. (Sec. 22, Par. 6) Erosion-corrosion is caused by suspended matter or air bubbles; the best control for this type of corrosion is a good filtration system, and air purging valves installed in the highest point of the water system. (Sec. 22, Pars. 7 and 8) The two most common chemical corrosion inhibitors are chromates and polyphosphates. (Sec. 22, Par. 10) 200 (to) 500 p.p.m.; 7.5. (Sec. 22, Par. 11) The most common chromate used is sodium bichromate because it is more economical than others. (Sec. 22, Par. 11) The chromate concentration of treated water is measured by color comparison of the sample to that of a tube chromate water known to contain a certain p.p.m. of chromate. (Sec. 22, Par. 14) High concentration of polyphosphates precipitate out in the form of calcium phosphate. (Sec. 22, Par. 14) First of all, there is no yellow residue produced by polyphosphates, as there is by chromates. Secondly, polyphosphates reduce sludge and rust (tuberculation). (Sec. 22, Par. 15) Bleedoff must be adjusted on condenser water systems using polyphosphates to avoid exceeding the solubility of calcium phosphate. (Sec. 22, Par. 16) The chemical corrosion inhibitors that are in a nylon net bag which is placed in a cooling tower may be in pellet or crystal form. (Sec. 22, Par. 18) Chilled water and brine solution systems require the pot type corrosion inhibitor feeders. (Sec. 22, Par. 18) Algae formations will plug the nozzles in

26. 27. 28.

29.

30. 31.

32.

33. 34.

35. 36.

37.

cooling towers, thus causing high condensing temperatures and reducing the system’s capacity. (Sec. 23, Par. 1) The amount of chlorine needed to eliminate algae growth is 1.5 p.p.m. (Sec. 23, Par. 2) Disagree. The sample is heated after the orthotolidine is added. (Sec. 23, Par. 3) Chlorination is effective because the bactericidal efficiency of chlorine increases with the increase in the temperature of the water. (Sec. 23, Par. 6) The orthotolidine test measures only the total available chlorine residual, while the orthotolidine-arsenite test measures the relative amounts of free available chlorine, combined available chlorine, and color caused by interfering substances. (Sec. 23, Par. 8) The combined available chlorine residual is 3.25 – 2.5 = .75 p.p.m. (Sec. 23, Par. 9) To perform a chlorine demand test, you must first prepare a test sample by mixing 7.14 grams of calcium hypochlorite with 100 cc. Of water to produce a 5000 p.p.m. chlorine solution. Add 1 milliliter of this sample to the water to be tested. Wait 30 minutes and perform a chlorine residual test. You must then subtract the chlorine residual from the test dosage to obtain the chlorine demand. (Sec. 23, Pars. 13, 14, and 15) To perform the pH determination with a color comparator, you would fill the color comparator tube with the sample to be tested to the prescribed mark on the tube. The you would add 0.5 ml. mark on the tube. Then you would add 0.5 ml. of cresol red-thymol blue solution to the sample. After mixing the solution thoroughly in the sample, you would place the sample tube in the comparator and match the sample color with the cresol red-thymol blue disc. (Sec. 23, Pars. 17, 18, and 19) Alkaline, because a pink color indicates a pH above 8.3. (Sec. 23, Par. 22) Sulfuric, sodium sulfate, and phosphoric acids are added to adjust the pH. They are added to the water through a solution feeder. (Sec. 23, Par 24) Calcium hypochlorite contains more chlorine by weight; 65 to 70 percent available chlorine by weight. (Sec. 23, Pars. 26 and 27) To add 100 gallons of chlorine solution per day, you would select the Wilson type DES hypochlorinator because its capacity is 120 gallons per day. (Sec. 23, Par. 32) 4.

38. You would have to add 43 pounds of HTH to that water which requires 30 pounds of chlorine.

39. Thirty gallons of chlorine is added per day to treat 143

2 million gallons of water when the dosage is 1.5 p.p.m. and dosing solution is 10 percent. 40. The precautions that must be followed while per-forming the turbidimeter test are as follows: The glass tube must be placed in a vertical position with the centerlines matched. The top of the candle support should be 3 inches below the bottom of the tube. The candle must be made of beeswax and spermaceti, gauged to burn within 114 and 126 grains per hour. The flame must be a constant size and the same distance below the tube. The tube should be inclosed in a case when observations are made. Soot, moisture and impurities must not be accumulated on the bottom of the glass tube. (Sec. 24, Pars. 4, 5, and 6) 41. The number of gallons that a vertical type pressure filter, 4 feet in diameter, can treat in 1 hour is: Area = π2 Area = 3.146 X (1/2d) Area = 3.146 X (2 X 2) Area = 3.146 X 4 Area = 12.564 or 12.6 12.6 X 3 = 37.8 37.8 X 60 =2268 gallons. (Sec. 24, Par. 11) 42. The precaution for taking water samples that is common to both types of analysis is that the equipment (bottle, stopper, etc.) must be sterilized. (Sec. 25, Pars. 3 and 4) 43. To sterilize a bottle that is to be used for chlorine rating 0.2 to 0.5 grams of sodium thiosulfate is added to the sample in the bottle. Then it is sterilized at a temperature below 392° to prevent decomposition of the thiosulfate (Sec. 25, Par. 4, a) 44. You should hold the bottle least 3 inches below the surface of water in a tank when you take a sample. (Sec. 25, Par. 4, c) 45. A solution of lysol, mercuric chloride, or of bicarbonate of soda is used to rinse your hands after making water tests. (Sec. 25, Par. 7) CHAPTER 5 1. The amount of cement that you would mix with 12 pounds of sand and 24 pounds of crushed rock is 4 pounds. (Sec. 26, Par. 1) 2. A 1-inch space is left between the foundation and baseplate to allow enough room for grouting after the baseplate is level. (Sec. 26, Par. 1) 3. A ¾- inch baseplate bolt requires a sleeve made from 1.875-inch pipe. (Sec. 26, Par. 1) 4. To level the baseplate, you would place two wedges below the center of the pump and two a below the center of the motor. (Sec. 26, Par. 3) 5. The angular alignment of a “spider” is checked at four points on the circumference of the outer ends of the coupling hubs at 90° intervals. (Sec. 26, Par. 4) 6. Angular alignment is accomplished by loosening the motor holddown bolts and shifting or shimming the motor. (Sec. 26, Par. 5) 144

7. To grout the unit, you must build a wooden dam around the foundation and wet the top of the foundation. Then fill the space with grout. (Sec. 26, Par. 7) 8. One part of Portland cement to three parts of sharp sand is used to make grout. (Sec. 26, Par. 7) 9. You should allow 48 hours for the grout to harden. (Sec. 26, Par. 7) 10. To establish initial alignment of the pumping unit, you must tighten the foundation and holddown bolts. Check the gap, angular adjustment, and parallel alignment. Recheck alignment after each adjustment. (Sec. 26, Par. 9) 11. The unit may become misaligned because of foundation settling, seasoning, or springing; pipe strains; shifting of the building structure; or springing of the baseplate. (Sec. 26, Par. 9) 12. Strainer. (Sec. 26, Par. 10) 13. The pump will lose a and capacity if smaller discharge pipe is installed. (Sec. 26, Par. 11) 14. To prime the pump, fill it with the fluid to be pumped through the priming opening in the pump. (Sec. 27, Par. 1) 15. After the pump is primed and before it is started, make sure that all the pump connections are airtight and rotate the pump shaft by hand to be sure that it moves freely. (Sec. 27, Par. 1) 16. Loose pump connections, low liquid level in the pump, loose suction line joints, improper direction of rotation, motor not up to nameplate speed, and dirty suction strainer will cause the failure of a newly installed pump. (Sec. 27, Par. 3) 17. The lantern ring. (Sec. 28, Par. 2) 18. You must pipe clean water to the stuffing box when the water being pumped is dirty, gritty, or acidic. (Sec. 28, Par. 3) 19. Loose packing will leak excessively and tight packing will burn and score the shaft. (Sec. 28, Par. 4) 20. When five-ring packing is used, stagger the packing joints approximately 72°. (Sec. 28, Par. 5) 21. Back off the gland bolts. (Sec. 28, Par. 10) 22. The bellows should not be disturbed unless it is to be replaced. (Sec. 28, Par. 11) 23. The four types of bearings found in centrifugal pumps are grease-lubricated roller and ball bearings, oil-lubricated ball bearings, and oillubricated sleeve bearings. (Sec. 28, Par. 17) 24. Overlubrication causes overheated bearings. (Sec. 28, Par. 17) 25. Mineral greases with a soda soap base are recommended for grease lubricated bearings. (Sec. 28, Par. 19) 26. Vegetable and animal greases are not used to lubricate pump bearings because they may form acid and cause deterioration. (Sec. 28, Par. 19) 27. 180° F. (Sec. 28, Par. 20) 28. 150° F. (Sec. 28, Par. 22) 29. The four drilled recesses facilitate the removal and

installation of the coupling bushing. (Sec. 28, Par. 24) 30. Disagree. The recessed holes should face away from the pump. (Sec. 28, Par. 26) Chapter 6 1. Thermionic emission is a method of emitting electrons from the cathode with heat. (Sec. 29, Par. 3) 2. In a directly heated cathode, the material that heats also emits electrons, whereas the indirectly heated cathode has separate heating and emitter elements. (Sec. 29, Par. 4) 3. The elements of a diode vacuum tube are the cathode and plate. (Sec 29, Par. 7) 4. Cathode; plate. (Sec. 29, Par. 7) 5. The diode rectifies a.c. because current will pass through the tube in one direction. (Sec. 29, Par. 8) 6. The factors that determine the amount of current flowing through a diode tube are the temperature of the cathode and the potential difference between the cathode and plate. (Sec. 29, Par. 9 7. Positive. (Sec. 29, Par. 11) 8. The capacitors will filter half-wave rectification by charging during the positive half-cycle and discharging through the load resistance during the negative half-cycle. (Sec. 29, Par. 13) 9. A duo-diode is a tube containing two diode tubes. It may have one cathode and two plates. (Sec. 29, Par. 16) 10. The purpose of the control grid is to provide more sensitive control of the plate current. (Sec. 30, Par. 2) 11. The control grid is physically located between the cathode and plate. (Sec. 30, Par. 2 12. Negative. (Sec. 30, Par. 4) 13. When the grid is made more negative, the current through the tube will decrease. (Sec. 30, Par. 5) 14. Grid bias is the potential difference of the d.c. voltage on the grid with respect to the cathode. Cutoff bias is the point at which the negative grid voltage stops all current flow in the tube. (Sec. 30, Pars. 5 and 7) 15. The types of grid bias used on vacuum tubes are fixed, cathode, and contact potential. (Sec. 30, Pars. 8, 9, and 12) 16. A disadvantage of contact potential bias is that bias is developed only when a signal is applied to the grid. (Sec. 30, Par. 14) 17. The triode can be used as an amplifier because a small a.c. voltage applied between the cathode and grid will cause a change in at grid bias and vary the current passing through the tube. (Sec. 30, Par. 15) 18. The potential of the screen grid is positive with respect to the cathode. (Sec. 30, Par. 19) 19. The power amplifier handles larger values of current than triode amplifiers. (Sec. 30, Par. 20) 20. A negative potential is applied to the suppressor grid of a pentode tube. (Sec. 30, Par. 23) 21. The valence ring is the outer ring or orbit of an atom. (Sec. 31, Par. 4)

22. Conductor. (Sec. 31, Par. 4) 23. N-type germanium is made when an antimony atom has gone into covalent bonding with germanium. The antimony in the material donates a free electron. (Sec. 31, Par. 8) 24. N-type material has free electrons which support electron flow, whereas P-type material has a shortage of electrons. This shortage causes current to flow from the N-type material to the P-type material. (Sec. 31, Pars. 8 and 9) 25. N-type; P-type. (Sec. 31, Par. 13) 26. Forward bias encourages current flow. (Sec. 31, Par. 14) 27. 2500 watts is developed in a circuit having 100 ohms resistance and an amperage draw of 5 amps (P = I2R). (Sec 31, Par. 17) 28. The base is located between the emitter and collector. (Sect. 31, Par. 19) 29. Maximum power gain is obtained by making the base region very narrow compared to the emitter and collector regions. (Sec. 31, Par. 24) 30. The emitter is comparable to the cathode, the base to the grid, and the collector to the plate. (Sec. 31, Par. 29) 31. The three basic transistor circuits are the common base, common emitter, and common collector. (Sec. 32, Par. 1) 32. The common collector circuit has a high impedance input and a low impedance output. (Sec. 32, Par. 5) 33. The coupling capacitor is used to couple the signal into the emitter-base circuit of the transistor. (Sec. 32, Par. 6) 34. The voltage drop is 9 volts (3/4 X 12 = 9). (Sec. 33, Par. 1) 35. A simple two-resistor bridge is balanced when no current flows between the wipers. (Sec. 33, Par. 2) 36. The Wheatstone bridge sends a signal to the amplifier, which builds up the bridge signal to operate a relay. (Sec. 33, Par. 5) 37. The higher temperature will unbalance the bridge by increasing the resistance in one circuit. The signal from the bridge will be amplified and operate a relay. (Sec. 33, Par. 8) 38. A vacuum tube voltmeter is used because it has a high input resistance. (Sec. 33, Par. 9) 39. The first step to take when using a V.T.V.M. is to turn the meter on and allow it to warm up. (Sec. 33, Par. 10) 40. The purpose of the discriminator circuit is to determine in which direction the bridge is unbalanced and take the necessary action to correct the condition. (Sec. 34, Par. 1) 41. The function of the blocking capacitor is to pass a.c. to the second stage and block the highvoltage d.c. (Sec. 34, Par. 3) 42. When the signal in the discriminator circuit goes negative, the cutoff bias is reached on the control grid. (Sec. 34, Par. 5) 43. The bridge supply voltage should come from the same phase as the discriminator supply to insure a bridge signal that is either in phase of 180° out of

145

phase with the discriminator supply. (Sec. 34, Par. 7) 44. The discriminator circuit will conduct when the plate and the amplified bridge signal are both positive. (Sec. 34, Par. 8) 45. A balancing potentiometer is used with a modulating motor to bring the bridge back into balance when a deviation has been corrected. (Sec. 34, Par. 10) Chapter 7 1. The precaution you should observe when installing a humidity sensing element is to locate it not too close to sprayers, washers, and heating or cooling coils, but within 50 feet of the control panel. (Sec. 35, Par. 2) 2. The outdoor thermostat sensing element is a coil of fine wire wound on a plastic bobbin and coated for protection against dirt and moisture. (Sec. 35, Par. 5) 3. To check the resistance of the sensing element, disconnect the leads and connect an ohmmeter across them. (Sec. 35, Par. 9) 4. Dirt on the sensing element will reduce the sensitivity of a thermostat. (Sec. 35, Par. 11) 5. To reposition the crank arm on the damper motor shaft, loosen the screw and nut that hold the arm on the shaft. This will allow you to reposition the shaft in four different positions, 90° apart. The adjustment screw on the face of the crank arm provides angular setting of the crank arm in steps of 22 1/2° throughout any one of the four positions on the shaft. (Sec. 35, Par. 13) 6. The damper motor repairs that may be made in the field are cleaning the potentiometer or limit switch contacts, repairing internal connecting wires, and replacing the internal wires. (Sec. 35, Par. 15) 7. You can check the transformer output by connecting a voltmeter across its terminals. (Sec. 35, Par. 16) 8. If the damper motor does not respond to direct transformer power, the most probable faults are broken wires, defective limit switch, or faulty condenser. (Sec. 35, Par. 18) 9. The authority “pots” adjust the change in variable required to give a certain voltage change. (Sec. 36, Par. 2)

10. The total voltage across the bridge determines the position of the final control element. (Sec. 36, Par. 3) 11. The adjustments for setting up or changing a control sequence are made at the control panel. (Sec. 36, Par. 4) 12. The nonrestarting relay is connected in the compressor starting circuit. It will prevent the compressor from operating unless the solenoid valve is operating. (Sec. 36, Par. 7) 13. The summer compensation schedule differs from the winter compensation schedule in that outdoor thermostat T3 will be replaced by T1. (Sec. 36, Pars. 10 and 11) 14. When the controlled variable varies continually and reverses its direction regularly, the throttling range is set too low. (Sec. 37, Par. 3) 15. With a 10 percent authority and 10° temperature change T3 will have the same effect as a 1° change in temperature at T1. (Sec. 37, Par. 5) 16. The control point can be reset after the system is in operation by positioning the control point adjuster in the control panel. (Sec. 37, Par. 6) 17. When the amplifier output voltage is 1 volt and the branch line pressure is 5 p.s.i.g., the most probable trouble is that the valve unit is out of adjustment. (Sec. 37, Par. 7) 18. The control reference temperature is temperature measured at T1. The control reference pressure is the actual branch line pressure. (Sec. 37, Pars. 9 and 10) 19. The control point of a compensated system on winter schedule should be equal to the set point. (Sec. 37, Par. 14) 20. The bridge signal is amplified and fed to a magnetic coil in the pneumatic valve. The amount of current flowing through the coil positions nozzle lever over the nozzle. The position of this lever controls the amount of branch line pressure sent to the controlled device. (Sec. 37, Pars. 18 and 19) 21. A faulty connection between the amplifier and bridge will cause one or more of the tubes to remain cold. (Sec. 37, table 21) 22. The transformer output and the valve unit relay must be checked when the tubes light up and burn out repeatedly. (Sec. 37, table 21)

*U.S. Government Printing Office: 2001-628-075/40468

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