Cooling

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Process Cooling
Overview
The cooling of equipment and products is an integral part of many manufacturing processes. Following the inside out method, opportunities for improving the energy efficiency of process cooling systems exist by reducing the cooling load, improving the energy efficiency of the distribution system and improving the energy efficiency of the primary cooling units. This chapter discusses typical cooling systems, energy use characteristics or primary cooling units, and opportunities for improving energy efficiency.

Process Cooling Systems
Most industrial processes use water to transport heat from the process or equipment back to the primary cooling unit. The most common types of primary cooling units are cooling towers, water-cooled chillers, air-cooled chillers and absorption chillers. In addition, water is sometimes used in an open loop to cool processes or equipment and then discharged to sewer. Diagrams of these systems, and the approximate costs of cooling are shown below. In all cases, the cost of electricity is assumed to be $0.10 /kWh, the cost of natural gas is $10 /mmBtu and the cost of water is $6.00 per 1,000 gallons. Cooling Tower Cooling towers provide cooling by evaporating water. A typical cooling tower cooling system is shown below. The system uses an open tank as a well for return water from the process and cooling tower. In the cooling tower loop, water is pumped from the chilled water tank to the top of the cooling tower, where it gravity feeds back to the chilled water tank. The process loop shown below includes a bypass loop to accommodate flow from a constant speed pump if the water required by the process loads varies.
Cooling Tower Process Load 1 Process Load 2 Bypass Valve

Chilled Water Tank

Cooling Tower Pump

Process Pump

The approximate cost of cooling with a cooling tower can be estimated by considering a cooling tower with a nominal rating of 500 tons. The cooling capacity of cooling towers is 15,000 Btu/nominal ton. Water flow through most cooling towers is 3 gpm per nominal ton. Total pressure rise through a cooling tower pump is frequently about 40 ft-

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H20, and pumps are about 70% efficient. A 500 nominal ton cooling tower uses a 30-hp cooling tower fan, that is about 80% loaded. Both pump and fan motors are 90% efficient. Thus, the pump and fan power use are about: Pp = 1,500 gpm x 40 ft-H20 / 3,960 gpm-ft-H20/hp x 70% x 90%) x 0.75 kW/hp = 18 kW Pf = 30 hp x 80% / 90% x 0.75 kW/hp = 20 kW The cooling provided and the power and cost per unit cooling are : Q = 500 ntons x 15,000 Btu/nton = 7.5 mmBtu/hr P / Q = (18 kW + 20 kW) / 7.5 mmBtu/hr = 5 kWh/mmBtu PC = 5 kWh/mmBtu x $0.10 /kWh = $0.50 /mmBtu In addition, cooling towers evaporate about 1% of water flow. Assuming the total of the water and sewer charges for water is $6.00 per 1,000 gallons, the quantity and cost of makeup water is about: W = (1,500 gal/min x 60 min/hr x 1%) / 7.5 mmBtu/hr = 120 gal/mmBtu WC = 120 gal/mmBtu x $6.00 / 1,000 gallons = $0.72 /mmBtu The total cost is of cooling with a cooling tower is about: $0.50 /mmBtu + $0.72 /mmBtu = $1.22 /mmBtu Water-Cooled Chiller A cooling system with a water-cooled chiller is shown below. Water-cooled chillers are slightly more energy efficient than water cooled chillers, but require a cooling tower. Water-cooled chillers require about 0.8 kW per ton of cooling, including the cooling tower fan and pump. Thus, the energy use and cost to provide 1 mmBtu of cooling are about: 0.8 kW/ton / 12,000 Btu/ton x 1,000,000 Btu/mmBtu = 67 kWh/mmBtu 67 kWh/mmBtu x $0.10 /kWh = $6.70 /mmBtu
Cooling Tower Process Load 1 Process Load 2 Bypass Valve

Chiller

Cooling Tower Pump

Process Pump

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Air-Cooled Chiller A cooling system with an air-cooled chiller is shown below. Air cooled chillers are slightly less energy efficient than water cooled chillers, but are generally less expensive to purchase and easier to maintain. Air-cooled chillers require about 1 kW per ton of cooling. Thus, the energy use and cost to provide 1 mmBtu of cooling are about: 1 kW/ton / 12,000 Btu/ton x 1,000,000 Btu/mmBtu = 83 kWh/mmBtu 83 kWh/mmBtu x $0.10 /kWh = $8.30 /mmBtu

Process Load 1

Process Load 2

Bypass Valve

Chiller Air

Process Pump

Absorption Chiller A cooling system with an absorption chiller is shown below. Absorption chillers use heat rather than electricity as the primary source of energy. Thus, absorption chillers can be powered with waste heat from other processes, or with a dedicated source of heat such as a boiler.

Process Load 1

Process Load 2

Bypass Valve

Boiler Steam

Absorption Chiller

Process Pump

The efficiency of the absorption chillers increases with increasing temperature heat. The coefficient of performance for a single-effect absorption chiller powered with steam is about 1. Assuming the steam is generated by an 80% efficient boiler, the energy use and cost to generate 1 mmBtu of cooling are about:

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1 Btu / Btu / 80% x 1,000,000 Btu/mmBtu = 1.25 mmBtu/mmBtu 1.25 mmBtu/mmBtu x $10.00 /mmBtu = $12.50 /mmBtu Open-Loop Water Cooling An open-oop cooling system is shown below.
From City Water Supply

Process Load 1

Process Load 2

To Sewer

Assuming the temperature of the water increases by 10 F during the cooling process, the quantity of water needed to provide 1 mmBtu of cooling is about: V = 1 mmBtu / (8.32 lb/gal x 1 Btu/lb-F x 10 F) = 12,000 gallons Assuming the total water and sewer charge for water is $6.00 / 1,000 gallons, the cost providing 1 mmBtu of cooling is about : C = 12,000 gallons/mmBtu x $6.00 / 1,000 gallons = $72 / mmBtu Choice of Process Cooling System As demonstrated above, the cost of cooling varies from about $1 per mmBtu for cooling towers, to about $10 per mmBtu for chillers, to about $70 per mmBtu for open-loop cooling. Thus, it is wise to use cooling towers instead of chillers or open-loop cooling whenever possible. In many cases, the choice of primary cooling is determined by the required temperature of cooling water at the process. Cooling towers can generate cooling water at a few degrees above the outdoor air wet-bulb temperature. Since wetbulb temperature is a few degrees below the dry bulb temperature, this means that cooling towers can generate cooling water at close to ambient dry bulb temperature. Thus, whenever the outdoor air temperature is at or below the required cooling water temperature, a cooling tower may be able to deliver the required cooling.

Cooling Towers
A cooling tower is a counter-flow or cross-flow heat exchanger that removes heat from water and transfers it to air. Cooling towers come in many configurations. An induced-

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draft cooling tower, which is common in HVAC and industrial applications because of its energy efficiency, is shown below.

Source: Modification of original from ASHRAE Handbook: HVAC Systems and Equipment, 2000 The temperature difference of water through a tower, dT = Tw1-Tw2, is determined by the load, Ql, and the mass flow rate of water, mw. Neither the size of the tower nor the state of the outside air influences the temperature difference; however, larger towers or lower outdoor air wet-bulb temperatures will decrease the exit water temperature, Tw2. Sensible and Latent Cooling Depending on the entering air and water temperatures, the water may be cooled by sensible and latent cooling of the air, or simply by latent cooling of the air. In either case, latent, i.e. evaporative, cooling is dominant. For example, consider the case in which the air enters at a lower temperature than the water (Figure 3a). The air will leave completely saturated and the cooling is part sensible and part latent. The sensible portion occurs as the air temperature increases by absorbing heat from the water. The latent portion occurs as some of the water evaporates, which draws energy out of the water.

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If the air enters at the same wet bulb temperature as before, but at a higher dry-bulb temperature than the water, then the air will cool as it saturates (Figure 3b). Thus, the sensible cooling component is negative, and the all the cooling is due to evaporation. In general, cooling is dominated by latent cooling.

Figure 2. Psychrometric process lines for air through a cooling tower, if the entering air temperature is a) less than the entering water temperature, and b) greater than the entering water temperature. The total cooling, ma (ha2 – ha1) is the same for both cases since enthalpy is a function of wet-bulb temperature alone. However, the dry-bulb temperature significantly influences the evaporation rate, mwe = ma (wa2-wa1). The rate of evaporation increases as the dry-bulb temperature increases for a given wet-bulb temperature.

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Cooling Towers as Heat Exchangers Based on the previous discussion, it is clear that cooling tower performance is a function of the wet-bulb temperature of the entering air. In an infinite cooling tower, the leaving air wet-bulb temperature would approach the entering water temperature, and the leaving water temperature would approach the web-bulb temperature of the entering air. The difference between the leaving water temperature and the entering air wet-bulb temperature is called the approach. The relationship between air wet-bulb and water temperature is shown in the figure below. In an infinite cooling tower, the approach would be zero.

Source: ASHRAE Handbook, HVAC Systems and Equipment, 2004. Neglecting fan power and assuming steady state operation, an energy balance on a cooling tower gives: mw1 cpw Tw1 – mw2 cpw Tw2 + ma (ha1 – ha2) = 0 Assuming steady state operation, a mass balance on water flow gives: mw1 – mw2 + ma (wa1 – wa2) = 0 mw2 = mw1 + ma (wa1 – wa2) Substituting mw2 into the energy balance gives: mw1 cpw Tw1 – [mw1 + ma (wa1 – wa2)] cpw Tw2 + ma (ha1 – ha2) = 0 mw1 cpw Tw1 – mw1 cpw Tw2 - ma (wa1 – wa2) cpw Tw2 + ma (ha1 – ha2) = 0

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The fraction of incoming water that is evaporated, ma (wa2-wa1) / mw1, is typically less than 1%. Thus, ma (wa1 – wa2) is much less than mw1, and the term ma (wa1 – wa2) cpw Tw2 can be neglected with negligible error to give: mw1 cpw (Tw1 – Tw2) = ma (ha2- ha1) Both sides of this equation represent the total cooling capacity of the tower. The effectiveness, E, of a heat exchanger is the ratio of the actual to maximum heat transfer. E = Qactual / Qmax For a heat exchanger, Qmax occurs if the air leaves the cooling tower completely saturated at the temperature of the incoming water. Thus, effectiveness is E = Qactual / Qmax = [mw1 cpw (Tw1 – Tw2)] / [ ma (ha,sat,tw1- ha1)] Energy Efficiency of Counterflow and Crossflow Towers The two most common tower designs for HVAC applications are forced-air counterflow and induced air cross-flow. Cooling tower energy use is a function of fan and pump power. To generate the same quantity of cooling, forced-air counterflow towers require more fan and more pump energy then induced-air crossflow towers. Thus, induced-air crossflow towers are almost always more energy efficient.

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Forced-air counterflow towers require more fan energy because centrifugal fans are made to generate low flow against high pressure, but cooling towers generally need high flow at low pressure. In comparison, induced air crossflow towers use propeller fans, which generate high flow against low pressure, which is more suited to cooling towers. Forced-air counterflow towers require more pump energy because these towers are taller in order to facilitate the counterflow heat transfer as the water falls through the tower. This height increases elevation head in the piping system. In addition, forced-air counterflow towers spray water through nozzles, which increases pressure drop. In comparision, induced-air crossflow towers are shorter and wider since the path of the air through the water is horizontal. In addition, the supply water simply drains from feeding pans into fill, which eliminates the need for nozzles. A comparison of cooling tower energy use for the same loads is shown below.

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Source: Marley Technical Report H-001A, “Cooling Tower Energy and Its Management”, October, 1982. Cooling Tower Control In HVAC applications, chiller evaporator loads vary depending on weather and building occupancy, and the quantity of heat rejected by the condenser varies accordingly. The cooling tower will always reject the all the heat from the condenser. However, the temperature of the cold water return to the condenser will decline at lower loads. Various methods are used to control cooling tower capacity to generate the desired cold water return temperature. The two control points for cooling towers are water flow and air flow. However, cooling tower manufacturers strongly recommend that water flow remain constant at all times. Thus, primary control methods generally rely on varying air flow. The common control methods are listed below. Run Fans Continuously This type of control results in the coldest possible return water temperature, which reduces chiller energy use. However, it also results in the highest cooling tower fan energy use. Because the improvement of chiller efficiency with lower condenser water temperature is asymptotical at some minimum temperature, this method of control rarely results in the best overall energy efficiency. Cycle Fans On and Off This type of control reduces excess fan energy use at cold outsider air temperatures, and is widely used. At relatively cold temperatures, however, the fan may cycle on and off too frequently. The maximum number of fan cycles is about 8 per hour. Thus, many cooling towers are equipped with water bypass loops. In most applications, water bypass control is only used at low temperatures when fan cycling could be a problem. Use Two-Speed Fan This method of control adds an intermediate level of cooling between full-on and full-off. This results in considerable fan energy savings, since fan energy varies with the cube of flow. Thus, fan energy at 50% air flow is only 12% of the fan energy at full air flow. This type of stepped control can be further extended with two cell towers with one fan in each cell. This leads to four possible steps of control. A typical relationship between cold water temperature and fan flow is shown below.

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Continuously Control Fan Speed with VSD This method results in the lowest fan energy use by continuously achieving savings, due to the fan law that fan energy varies with the cube of flow. Vary Air Flow Using Inlet Air Dampers Before VSDs, cooling towers were sometimes controlled by running the fan at full speed while varying the inlet air dampers to modulate air flow. This method of control results in intermediate energy savings between fan cycling and continuous VSD control. However, is rarely used now that the VSD control is now commonplace. Comparison of Energy Use with Various Methods of Cooling Tower Control Total chiller and cooling tower energy use for these control methods for a typical HVAC application are shown below. The data indicate that variable speed control of a propeller fan reduces fan energy use to 12% of the energy required for constant operation, 19% of the energy required for single-speed fan cycling operation, and 49% of the energy required for two-speed fan cycling operation. These results show that VSD retrofits of single-speed or two-speed cooling tower fans can be highly cost effective.

Source: Marley Technical Report H-001A, “Cooling Tower Energy and Its Management”, October, 1982. Variable Cold Water Set-Point Temperature The energy efficiency of all the control discussed above can be improved by varying the cold water set-point temperature with the outdoor air wet bulb temperature. This type of control takes into account the fact that towers can only produce water at a few degrees above the wet-bulb temperature (this temperature difference is called the “approach”); hence fan energy can be reduced when that temperature is achieved, since continued fan operation results in minimal further reductions in cold water temperature. Fan Motor Power with Fan Speed and Air Volume Flow Rate The figure below shows fan motor power draw as a function of input frequency for a cooling tower fan equipped with a VFD. The fan affinity laws would predict a relationship between fraction power (FP) and fraction speed (FS) of:

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FP = FS3 Regression of the data show a slightly better fit using the exponent 2.8: FP = FS2.8 Since fan speed is proportional to volume flow rate, this relation also hold for fraction volume flow rate, FV. FP = FV2.8 The slightly reduced exponent is caused by declining VFD, motor and fan efficiencies at reduced speed.

Source: “An Application of Adjustable Speed Drives for Cooling Tower Capacity Control”, Welch, W. and Beckman, J. Cooling Tower Bypass Plumbing Bypass control is typically used only at low outdoor air wetbulb temperatures in order to reduce fan cycling. Bypass should not be used in sub-freezing temperatures since this can lead to tower freeze up. The preferred tower bypass plumbing is shown below. The preferred valve is a single two-way butterfly valve placed in the bypass line.

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Source: BAC Product and Application Handbook, Volume 1, 2005. Cooling Tower Pumping Pressure Drop Typical cooling tower pressure drops are shown below. The Estimated Head Loss column is for a standard condenser and 15 year old piping. The Actual Head Loss column is for a low-pressure loss condenser and new piping.

Source: BAC Product and Application Handbook, Volume 1, 2005. Cooling Tower Selection In HVAC applications, the starting place for cooling towers selection is typically to match the “nominal cooling tower tons”, as supplied by the tower manufacturer, to the cooling capacity of the chiller or chiller plant. The water flow rate through the cooling tower is initially set at 3 gpm per “nominal cooling tower ton”. Subsequent design optimization may occur from this starting point. Engineering data for a typical model of induced-air crossflow cooling towers are shown below. Based on these data, fan motor hp is about 0.1 hp/ton and air flow rates are about 2,000 cfm/hp. A “nominal cooling tower ton” is defined as cooling 3 gpm of water from 95 F to 85 F at an air wetbulb temperature of 78 F. Thus, the actual cooling associated with a “nominal cooling tower ton” is:

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Qact = 3 gpm x 8.33 lb/gal x 60 min/hr x 1 Btu/lb F x (95 – 85) F = 15,000 Btu/hr This strange convention exists to make it easy for users to select cooling towers by matching the “nominal cooling capacity” of the chiller with the chiller cooling capacity. The convention works because most chillers have a COP of about 3, and total heat rejected by the condenser to the cooling tower is about 15,000 Btu/hr for every 12,000 Btu/hr through the evaporator.

Source: BAC Product and Application Handbook, Volume 1, 2005.

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Source: Marley Cooling Towers, 2000. Cooling Tower Performance The performance of typical cooling towers at water flow rates of 3 gpm/ton and 5 gpm/ton is shown below. Similar performance data for specific cooling towers can usually be obtained from the manufacturer. These curves predict the temperature of the cold water leaving the cooling tower as a function of the water temperature range (Th-Tc) and entering air web bulb temperature. Temperature range is generally known and can be used as an input value in these charts, since the temperature range is set by the water flow rate and heat rejection rate of the condenser or process load.

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Source: ASHRAE Handbook, HVAC Systems and Equipment, 2004. A relation for the temperature of cooling water leaving the tower, Tc, can be dervived from regressing data from the 3 gpm/ton and 5 gpm/ton curves shown above. The relation and regression coefficients are shown below. The R2 for these relations exceeds 0.995 and the average error, [abs(Tc – Tc,pred)], is less than 1 F. Tc = a + b Twb + c Tr + d Twb2 + e Tr2 + f Tr Twb
Coef A B C D E F 3 gpm/ton 16.790751 0.6464308 2.2221763 0.0016061 -0.0159268 -0.015954 5 gpm/ton 24.6299229 0.45007792 3.32229591 0.00261818 -0.0324886 -0.0190476

These equations can be incorporated into software to predict cooling tower performance with varying ambient conditions. For example, CoolSim (Kissock, 1997) calculates exit water temperatures, and the fraction of time that a cooling tower can deliver water at a target temperature, based on water temperature range Tr and TMY2 weather data. This information is useful in determining how often a cooling tower can replace a chiller in cooling applications. Cooling Tower Performance at Reduced Air Flow Rates Comparison of the 3 gpm/ton and 5 gpm/ton performance maps can be used to estimate cooling tower performance at reduced air flow rates. To do so, note that the 5 gpm/ton

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chart shows tower performance for a higher water-to-air flow ratio, or, inversely, at a lower air-to-water flow rate ratio than the 3 gpm/ton curve. Thus, the 5 gpm/ton performance map indicates tower performance if water flow rate is held steady while the air flow rate is reduced to 3/5 = 60% of maximum airflow. The 3 gpm/ton performance map indicates tower performance when the air flow rate is at 100% of design air flow. Interpolating between the two curves, gives an acceptable estimate of tower performance at intermediate air flows.

Evaporation Rate
As discussed in the previous section, cooling in cooling towers is dominated by evaporation. The evaporation rate can be calculated from the pyschrometric relations in the previous section, if the inlet and exit conditions of the air are known. For example, consider the case in which the cooling load, Ql, mass flow rate of air, ma, (which can be calculated based on the fan cfm and specific volume of the inlet air), and inlet conditions of air are known. The enthalpy of the exit air, ha2, can be calculated from an energy balance. Ql = ma (ha2 – ha1) ha2 = ha1+ Ql / ma The state of the exit air can be fixed by assuming that it is 100% saturated with an enthalpy ha2. The evaporation rate, mwe, can be determined by a water mass balance on the air. mwe = ma (wa2- wa1) The fraction of water evaporated is: mwe / mw Using this method for entering air temperatures from 50 F to 90 F, we determined that the fraction of water evaporated typically ranges from about 0.5% to 1%, with an average value of about 0.75%. Another way to estimate the fraction of water evaporated is to assume that all cooling, Ql, is from evaporation, Qevap. The cooling load Ql, is the product of the water flow rate, mw, specific heat, cp, and temperature difference, dT. The evaporative cooling rate is the product of the water evaporated, mwe, and the latent heat of cooling, hfg. Ql = Qevap mw cp dT = mwe hfg Assuming the latent heat of evaporation of water, hfg, is 1,000 Btu/lb, and the temperature difference of water through the tower, dT, is 10 F, the fraction of water evaporated is:

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mwe / mw = cp dT / hfg = 1 (Btu/lb-F) x 10 (F) / 1000 (Btu/lb) = 1% If on average, 75% of the cooling were from evaporation and 25% from sensible cooling, then the evaporation rate would be: 75% x 1% = 0.75% Thus, both methods suggest that 0.75% is a good estimate of the rate of evaporation; however, we have seen manufacturer data indicating average evaporation rates as low as 0.30%. Water lost to evaporation should not be subjected to sewer charges. Typical sewer charges are about $2.20 per hundred cubic feet. Some water may be lost as water droplets are blown from the tower by oversized fans or wind. This type of water loss is called “drift”. Drift rates are typically about 0.2% of flow (ASHRAE Handbook, HVAC Systems and Equipment, 2000); however, we generally assume that drift losses are included in the 0.75% evaporation rate.

Water Treatment and Blow Down Rate
Cooling tower water must be treated to prevent bacterial growth and maintain the concentration of dissolved solids at acceptable levels to prevent scale and corrosion. Bacterial Growth The typical method of controlling bacterial growth is to add biocides at prescribed intervals and to keep the cooling tower water circulating. If the tower will not be operated for a sustained period of time, then the cooling water should be drained. Dissolved Solids Water evaporated from a cooling tower does not contain dissolved solids. Thus, the concentration of dissolved solids will increase over time if only enough water is added to the tower to compensate for evaporation. To maintain the dissolved solids at acceptable levels, most towers periodically discharge some water and replace it with fresh water. This process is called blow down. It the level of dissolve solids increases too high, scale will be begin to form, and/or the water may become corrosive and damage piping, pumps, cooling tower surfaces and heat exchangers. Usually, the primary dissolved solid to control is calcium carbonate CaCO3. Blow down can be accomplished by continuously adding and removing a small quantity of water, periodically draining and refilling the cooling tower reservoir, or by metering the conductivity of water and adding fresh water only when needed. By far the most efficient method is to meter the conductivity of water, which increases in proportion to the level of dissolved solids, and add fresh water only when needed. The required quantity of blow down water depends on the acceptable quantity of dissolved solids in the tower water, PPMtarget, the quantity of dissolved solids in the

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makeup water, PPMmu, and the evaporation rate, mwe. The target level of dissolved solids is typically quantified in cycles, where: Cycles = PPMtarget / PPMmu For example, if the quantity of dissolved CaCO3 in the makeup water, PPMmu, is 77 ppm and the maximum level to prevent scaling, PPMtarget, is 231, then the cooling tower water must be maintained at three cycles: Cycles = PPMtarget / PPMmu = 231 ppm / 77 ppm = 3 By applying mass balances, it can be shown that the blow down water required to maintain a certain number of cycles is mwbd = mwe / (Cycles –1) The total makeup water required mwmu, is the sum of the water added for evaporation and blow down: mwmu = mwe + mwbd For example for a 1,000 gpm tower with a 0.75% evaporation rate and CaCO3 concentration at 3 Cycles, the quantity of makeup water required would be about: mwe = (mwe/mw) x mw = 0.75% x 1,000 gpm = 7.5 gpm mwbd = mwe / (Cycles –1) = 7.5 gpm / (3 – 1) = 3.75 gpm mwmu = mwe + mwbd = 7.5 gpm + 3.75 gpm = 11.25 gpm

Vapor Compression Chillers
Vapor compression chillers use two heat exchangers, a compressor and an expansion device to raise and lower the temperature of a refrigerant, so that the refrigerant can absorb heat from a low temperature reservoir and reject it to a high temperature reservoir. The basic equipment in vapor compression chillers is shown below.

The ideal cycle and actual cycles, as plotted on temperature versus entropy curves are shown below. The compressor raises the pressure of the refrigerant to a temperature 19

greater than the ambient temperature so that heat can be rejected from the condenser. The expansion device lowers the pressure of the refrigerant to a temperature lower than the temperature of the area or medium to be cooled so that heat can be added to the refrigerant in the evaporator.

The cycle is simplified in the figure below, which shows that chillers remove heat from a low temperature reservoir and push it into a high temperature reservoir. Pumping heat from a low to high temperature requires work to be added to the compressor.

Chiller efficiency, E, is frequently called the Coefficient of Performance, COP. Using the figure from above as a guide, the COP of the chiller is: COP = Useful output / Required Input = Qevap / Wc = Qevap / (Qcond-Qevap) Substituting dQ = T dS, and noting that in a reversible cycle, dSevap = dScond, gives the maximum (Carnot) efficiency, COPmax: COPmax = Tenv / (Troom-Tenv) For a chiller operating between room temperature of 70 F and outdoor air temperature of 90 F, the maximum COP is about 26.5. The COP of actual chillers is about 3. Thus, most of the useful work supplied to the compressor as electricity is lost as irreversibilities.

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Compressors Vapor compression chillers use three types of compressors: scroll, screw, reciprocating and centrifugal. Scroll compressors are positive displacement compressors used on small residential air conditioners (1-10 tons). Screw and reciprocating compressors are positive displacement compressors used in medium-sized chillers (10 – 300 tons). Centrifugal compressors are the most efficient at full load and are used in large chillers (> 300 tons). Reciprocating compressors can respond to variable cooling loads by turning on and off as needed. In processes with large variations, multiple reciprocating compressors are often staged so that they turn on and off as needed. Thus, multistage reciprocating chillers have excellent part-load efficiency. Screw and centrifugal compressors are often more energy efficient than reciprocating compressors when operating at full load. However, screw and centrifugal compressors must remain running even at part load. Thus, the efficiency of screw and centrifugal compressors at part load is often less than at full load. Part load losses can be minimized by staging multiple compressors or by installing a variable speed drive on the compressor motor. Air-Cooled Chiller Performance The energy efficiency of air-cooled chillers is frequently reported as the Energy Efficiency Ratio, EER. EER includes the electrical power for both the compressor and condenser fans. EER (Btu/Wh) = Qevap (Btu/hr) / [Wcomp (W) + Wcondfans (W)] Performance data from typical air-cooled chillers are shown below. The data show that energy efficiency decreases at part-loads, low leaving water temperatures, and high outdoor air temperatures.

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Water-Cooled Chiller Performance The energy efficiency of water-cooled chillers is typically reported in terms of kW of power to the compressor per ton of cooling generated. Note that kW/ton rating does not include the power to the required cooling tower fan and pump. Typically, cooling tower fan power adds an additional 0.05 kW/ton, and cooling tower pump power adds about 0.04 kW/ton. Performance data from a typical single-speed water-cooled chiller are shown below. The data show that compressor energy efficiency decreases at part-loads and high condenser water temperatures from the cooling tower.

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Absorption Chillers
Like vapor compression chillers, absorption chillers employ an evaporator, condenser and expansion valve. However, absorption chillers convert the refrigerant to a liquid before raising the pressure. Because the quantity of work required to raise the pressure of a liquid with a pump is so small compared the quantity of work required to raise the pressure of a vapor with a compressor, the quantity of pump work is typically neglected in energy efficiency calculations. However, heat is required to regenerate the refrigerant. Thus, absorption chillers use heat instead of work to generate cooling. A vapor compression cycle is shown below.

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The energy efficiency of absorption chillers is typically quantified as the coefficient of performance, COP. COP = Qevap / Qgenerator The COP depends primarily on the temperature of the supplied heat. The COP of doubleeffect chillers using steam as heat is about 1.2. The COP of single-effect chillers, which can use much lower temperature heat is typically about 0.8. A performance chart for a low-temperature single-effect chiller, capable of using waste heat from another process, is shown below.

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Energy Efficiency Opportunities: Minimize Cooling Load
• • • Improve process control to minimize cooling load Close open cooling loops and connect to cooling tower or chiller. Employ heat exchangers if both heating and cooling to reduce heating and cooling loads.

Energy Efficiency Opportunities: Distribution System
• • Install VFD on process cooling loop. Avoid mixing hot and cold streams.

Energy Efficiency Opportunities: Distribution System
• Use cooling towers rather than chillers whenever possible. 25

• • • •

Improve control of cooling tower fans. Apply for sewer exemption on cooling tower make-up water. Stage multiple chillers to improve part load performance Reclaim heat from air-cooled chillers/condensers

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