1 Heat Transfer
Content
Heat is a form of energy associated with the disordered movement of molecules. Temperature of material is an indication of its internal energy, the greater the molecular motion, the greater the internal energy and the higher the temperature. Heat transfer is the transfer of heat from a region of high temperature to a region of low temperature. Heat transfer is involved in many pharmaceutical processes like drying, evaporation, distillation, heating of material or solvent and sterilization.
Mechanisms of heat transfer: 1) Conduction: the main mechanism of heat transfer in solids is direct contact with each other. When heat is added to objects, molecules begin to vibrate faster and move further apart. The heat energy is transferred from molecule to molecule. 2) Convection: it is the transfer of energy from region to another by motion of fluid. 3) Radiation: all bodies with temperature above absolute zero radiate heat in the form of electromagnetic waves which travel in straight line at the speed of light. When this radiation falls on another surface, it may be; transmitted, reflected or absorbed. The absorbed fraction is transformed into heat e.g. sun, IR lamps.
Sources of heat energy: They are; 1) Direct sources: flame, electrical heater, sunlight. 2) Indirect sources: i.e. energy pass through 2 steps 1
a. Through heated element. b. Through source of energy. E.g. oils, water, brine water (saline), gases and Steam Steam • Its raw material is water which is vaporized under normal or reduced pressure. • Steam is cheap, not pumped, used to give both sensible heat and latent heat. Sensible heat: the amount of heat transferred which can be measured by temperature measure. Latent heat of vaporization: the amount of heat responsible for the conversion of liquid into gas. • Steam is generated in boiler by fitting a valve that will not open to allow the escape of steam until a certain pressure is reached. • N.B. As the pressure increase, the boiling point of water will increase and steam temperature increase. Pressure (Psig) Temperature of steam○C 0 100 15 121 30 134
Types of steam: Saturated (wet) steam: Rise direct1y from boiling water. The Steam carries water droplets which condense upon cooling. Never used in heat exchanger. Unsaturated (dry) steam: Its pressure is less than that of wet steam at the same temperature. No water content, so not condensed by cooling Used in heat exchanger. Super heated steam: Saturated steam may become superheated if it is isolated from contact with excess liquid water and either heated further or 2
caused to expand by reducing the pressure. In using steam in heat exchanger, the steam may be condensed to term a film of water droplet that hinders heat exchange and enhance more condensation of more steam so water droplets must be removed by using steam dryers and steam traps.
I.Steam dryers (water separators): • Used to separate water from steam. • depends on the fact that steam rapidly changes its direction when it become in contact with a surface, while water is left behind or deposited on the surface. a. Simple type: It consists of a vessel with a vertical plate reaching almost the bottom. The wet steam enters the vessel at the top, passes down one side and up on the other side, leaving the water in the bottom of the vessel b. Screw Steam dryers
The wet steam passes into the upper part of a cylindrical vessel containing a central pipe; its outside wall is screw so that wet steam takes spiral movement round the pipe. Water drips down into the bottom while dry steam which changes its direction rapidly passes up and out the pipe.
II. Steam Traps
Widely used for removing condensate of water from the steam. Steam trap is supplied with vent constructed to allow the drainage
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of water droplets and prevent steam loss, this process occurs automatically. Types of steam traps: I. Mechanical types: These types depend on the physical difference between condensate and steam. Floating steam trap: Wet steam is pushed through inlet and then water will condensate and collected under the floating head. When condensate level rises, the floating head rises & the outlet is opened till the condensate is discharged, the floating head falls and the outlet is closed and so on.
Floating steam trap
II. Thermostatic steam trap: These types depend on the temperature difference between condensate and steam. The condensate has lower temperature than steam 1. Simple thermostatic trap: • A pipe contains central sheet or tube of metal sensitive to temperature change. • The higher temperature of steam will cause expansion of the metal to close the vent and the flow of steam contrary to its inlet and if water droplets are present will decrease the temperature of the metallic tube or sheet and leads to returning to its position i.e. opening of the vent for water condensate to go out. Disadvantage: • Not sensitive to small temperature difference.
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2. Balanced pressure expansion steam trap: It consists of a closed capsule made of Cu-alloy and fixed at the top of the trap. The capsule contains a liquid with boiling point lower than that of water. When the capsule is surrounded by steam the liquid inside it will boil and the capsule expand and the outlet is closed, in presence of condensate the capsule becomes cold and contracts and opens the outlet and discharges the condensate. Advantage: 1. Rapid response to small temperature difference. 2. Increase the steam pressure raises the boiling point of water, but the same pressure acts on the surface of the capsule and elevates the boiling points of liquid inside the capsule by a similar amount. The title applied to this type “Balanced pressure expansion trap”.
Fig. Simple thermostatic trap
Fig. Balanced pressure expansion steam trap
Heat exchange equipment Heat exchanger A device, used to transfer heat from a fluid on one side of a barrier to a fluid on the other side without bringing the fluids into direct contact. Classification of heat exchangers A. Tubular heat exchanger; Single passes tubular heat exchanger Fixed head heat exchanger 5
Floating head heat exchanger B. Special heat exchanger; Spiral heat exchanger A) Tubular Heat Exchanger: 1. Single Pass Tubular Heat Exchanger (1:1 Heat Exchanger)
It consists of a bundle of parallel tubes, the ends of which expanded into plate B1 and B2. The tube bundle is inside a cylinder shell (C) and is provided with 2 distributing chambers D1 and D2 and the two chambers cover El and E2. Steam is introduced from opening F into the shell-side space surrounding the tubes; condensate is withdrawn through connection G. The fluid to be heated is pumped into one distributing head then flow through the tubes to the other distributing chamber, finally discharged from the outlet. The heat flow occurs through the tubes from the surrounding steam to the cooler fluid in the tubes. It’s called 1:1 heat exchanger because, it has one shell side pass (i.e. steam flow in one direction only). The first number (1) denotes the number of horizontal passage inside the shell; the second number denotes number of horizontal passage inside the tubes.
Fig. Single pass tubular heat exchanger 2. Fixed Head Heat Exchanger (1:4 Heat Exchangers) The same as single pass but modified so as to give multi pass heat exchange and this is done by: 6
a. Introduction of a group of baffles in the case containing the tubes, these perpendicular baffles will inverse the steam or hot liquid flow several times which leads increase the velocity of flow outside the tubes and increase the path of the flow. b. Introduction of transverse septa in the distribution chambers which inverses the product flow in the tubes and elongates their path before discharge. The apparatus has I shell pass and 4 tube pass so it is called 1,4 Fixed head heat exchanger and it is called fixed head because the tubes are fixed by metallic septa on their heads
Fig. Fixed head heat exchanger Disadvantage 1. Tubes can not expand freely so, deformation of tubes as a result of increasing temperature may occur. 2. Tubes can not be removed for cleaning. 3. Floating head heat exchangers: • The same as single pass exchanger, but one tube plate end is fixed to the chamber with septa and the second is attached to a floating head cover so that the tube bundle can he removed relative to the shell. Advantage: 1. Tube can expand freely due to the presence of floating head. 2. Tubes can be cleaned easily. 3. Used for liquid-liquid or gas-liquid heat transfer.
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Fig. Floating head heat exchangers B.
Special heat exchangers: Spiral heat exchanger: Two liquids (cold and hot) flow in a spiral manner counter currently to each other in 2 adjacent spiral pipes. used for heating and cooling products which are: I. Highly viscous materials (cream, ointment etc) 2. Products containing solid matter. Advantages: Compared to a traditional heat exchanger a spiral heat exchanger: I. Takes up only one sixth of the space 2. Means lower costs for buildings, pumps valves and piping 3. Uses 75% less pumping energy 4. Provides higher K- value and a close temperature approach 5. Ensures continual self-cleaning and effect for maximum operating efficiency.
Fig. Spiral heat exchanger 8
Mechanisms of heat transfer: 1) Conduction: the main mechanism of heat transfer in solids is direct contact with each other. When heat is added to objects, molecules begin to vibrate faster and move further apart. The heat energy is transferred from molecule to molecule. 2) Convection: it is the transfer of energy from region to another by motion of fluid. 3) Radiation: all bodies with temperature above absolute zero radiate heat in the form of electromagnetic waves which travel in straight line at the speed of light. When this radiation falls on another surface, it may be; transmitted, reflected or absorbed. The absorbed fraction is transformed into heat e.g. sun, IR lamps.
Sources of heat energy: They are; 1) Direct sources: flame, electrical heater, sunlight. 2) Indirect sources: i.e. energy pass through 2 steps 1
a. Through heated element. b. Through source of energy. E.g. oils, water, brine water (saline), gases and Steam Steam • Its raw material is water which is vaporized under normal or reduced pressure. • Steam is cheap, not pumped, used to give both sensible heat and latent heat. Sensible heat: the amount of heat transferred which can be measured by temperature measure. Latent heat of vaporization: the amount of heat responsible for the conversion of liquid into gas. • Steam is generated in boiler by fitting a valve that will not open to allow the escape of steam until a certain pressure is reached. • N.B. As the pressure increase, the boiling point of water will increase and steam temperature increase. Pressure (Psig) Temperature of steam○C 0 100 15 121 30 134
Types of steam: Saturated (wet) steam: Rise direct1y from boiling water. The Steam carries water droplets which condense upon cooling. Never used in heat exchanger. Unsaturated (dry) steam: Its pressure is less than that of wet steam at the same temperature. No water content, so not condensed by cooling Used in heat exchanger. Super heated steam: Saturated steam may become superheated if it is isolated from contact with excess liquid water and either heated further or 2
caused to expand by reducing the pressure. In using steam in heat exchanger, the steam may be condensed to term a film of water droplet that hinders heat exchange and enhance more condensation of more steam so water droplets must be removed by using steam dryers and steam traps.
I.Steam dryers (water separators): • Used to separate water from steam. • depends on the fact that steam rapidly changes its direction when it become in contact with a surface, while water is left behind or deposited on the surface. a. Simple type: It consists of a vessel with a vertical plate reaching almost the bottom. The wet steam enters the vessel at the top, passes down one side and up on the other side, leaving the water in the bottom of the vessel b. Screw Steam dryers
The wet steam passes into the upper part of a cylindrical vessel containing a central pipe; its outside wall is screw so that wet steam takes spiral movement round the pipe. Water drips down into the bottom while dry steam which changes its direction rapidly passes up and out the pipe.
II. Steam Traps
Widely used for removing condensate of water from the steam. Steam trap is supplied with vent constructed to allow the drainage
3
of water droplets and prevent steam loss, this process occurs automatically. Types of steam traps: I. Mechanical types: These types depend on the physical difference between condensate and steam. Floating steam trap: Wet steam is pushed through inlet and then water will condensate and collected under the floating head. When condensate level rises, the floating head rises & the outlet is opened till the condensate is discharged, the floating head falls and the outlet is closed and so on.
Floating steam trap
II. Thermostatic steam trap: These types depend on the temperature difference between condensate and steam. The condensate has lower temperature than steam 1. Simple thermostatic trap: • A pipe contains central sheet or tube of metal sensitive to temperature change. • The higher temperature of steam will cause expansion of the metal to close the vent and the flow of steam contrary to its inlet and if water droplets are present will decrease the temperature of the metallic tube or sheet and leads to returning to its position i.e. opening of the vent for water condensate to go out. Disadvantage: • Not sensitive to small temperature difference.
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2. Balanced pressure expansion steam trap: It consists of a closed capsule made of Cu-alloy and fixed at the top of the trap. The capsule contains a liquid with boiling point lower than that of water. When the capsule is surrounded by steam the liquid inside it will boil and the capsule expand and the outlet is closed, in presence of condensate the capsule becomes cold and contracts and opens the outlet and discharges the condensate. Advantage: 1. Rapid response to small temperature difference. 2. Increase the steam pressure raises the boiling point of water, but the same pressure acts on the surface of the capsule and elevates the boiling points of liquid inside the capsule by a similar amount. The title applied to this type “Balanced pressure expansion trap”.
Fig. Simple thermostatic trap
Fig. Balanced pressure expansion steam trap
Heat exchange equipment Heat exchanger A device, used to transfer heat from a fluid on one side of a barrier to a fluid on the other side without bringing the fluids into direct contact. Classification of heat exchangers A. Tubular heat exchanger; Single passes tubular heat exchanger Fixed head heat exchanger 5
Floating head heat exchanger B. Special heat exchanger; Spiral heat exchanger A) Tubular Heat Exchanger: 1. Single Pass Tubular Heat Exchanger (1:1 Heat Exchanger)
It consists of a bundle of parallel tubes, the ends of which expanded into plate B1 and B2. The tube bundle is inside a cylinder shell (C) and is provided with 2 distributing chambers D1 and D2 and the two chambers cover El and E2. Steam is introduced from opening F into the shell-side space surrounding the tubes; condensate is withdrawn through connection G. The fluid to be heated is pumped into one distributing head then flow through the tubes to the other distributing chamber, finally discharged from the outlet. The heat flow occurs through the tubes from the surrounding steam to the cooler fluid in the tubes. It’s called 1:1 heat exchanger because, it has one shell side pass (i.e. steam flow in one direction only). The first number (1) denotes the number of horizontal passage inside the shell; the second number denotes number of horizontal passage inside the tubes.
Fig. Single pass tubular heat exchanger 2. Fixed Head Heat Exchanger (1:4 Heat Exchangers) The same as single pass but modified so as to give multi pass heat exchange and this is done by: 6
a. Introduction of a group of baffles in the case containing the tubes, these perpendicular baffles will inverse the steam or hot liquid flow several times which leads increase the velocity of flow outside the tubes and increase the path of the flow. b. Introduction of transverse septa in the distribution chambers which inverses the product flow in the tubes and elongates their path before discharge. The apparatus has I shell pass and 4 tube pass so it is called 1,4 Fixed head heat exchanger and it is called fixed head because the tubes are fixed by metallic septa on their heads
Fig. Fixed head heat exchanger Disadvantage 1. Tubes can not expand freely so, deformation of tubes as a result of increasing temperature may occur. 2. Tubes can not be removed for cleaning. 3. Floating head heat exchangers: • The same as single pass exchanger, but one tube plate end is fixed to the chamber with septa and the second is attached to a floating head cover so that the tube bundle can he removed relative to the shell. Advantage: 1. Tube can expand freely due to the presence of floating head. 2. Tubes can be cleaned easily. 3. Used for liquid-liquid or gas-liquid heat transfer.
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Fig. Floating head heat exchangers B.
Special heat exchangers: Spiral heat exchanger: Two liquids (cold and hot) flow in a spiral manner counter currently to each other in 2 adjacent spiral pipes. used for heating and cooling products which are: I. Highly viscous materials (cream, ointment etc) 2. Products containing solid matter. Advantages: Compared to a traditional heat exchanger a spiral heat exchanger: I. Takes up only one sixth of the space 2. Means lower costs for buildings, pumps valves and piping 3. Uses 75% less pumping energy 4. Provides higher K- value and a close temperature approach 5. Ensures continual self-cleaning and effect for maximum operating efficiency.
Fig. Spiral heat exchanger 8
