corse on cooling

8/13/2019 Corse on cooling

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Process Cooling

OverviewThe cooling of equipment and products is an integral part of many manufacturingprocesses. Following the inside out method, opportunities for improving the energy

efficiency of process cooling systems exist by reducing the cooling load, improving theenergy efficiency of the distribution system and improving the energy efficiency of theprimary cooling units. This chapter discusses typical cooling systems, energy usecharacteristics or primary cooling units, and opportunities for improving energyefficiency.

Process Cooling SystemsMost industrial processes use water to transport heat from the process or equipment backto the primary cooling unit. The most common types of primary cooling units are coolingtowers, 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 thendischarged to sewer. Diagrams of these systems, and the approximate costs of coolingare shown below. In all cases, the cost of electricity is assumed to be $0.10 /kWh, thecost of natural gas is $10 /mmBtu and the cost of water is $6.00 per 1,000 gallons.

Cooling TowerCooling towers provide cooling by evaporating water. A typical cooling tower coolingsystem is shown below. The system uses an open tank as a well for return water from theprocess and cooling tower. In the cooling tower loop, water is pumped from the chilledwater tank to the top of the cooling tower, where it gravity feeds back to the chilled watertank. The process loop shown below includes a bypass loop to accommodate flow from aconstant speed pump if the water required by the process loads varies.

Process

Load 1

Process

Load 2

Chilled Water Tank

Cooling Tower

Bypass

Valve

Process PumpCooling Tower Pump

The approximate cost of cooling with a cooling tower can be estimated by considering acooling tower with a nominal rating of 500 tons. The cooling capacity of cooling towersis 15,000 Btu/nominal ton. Water flow through most cooling towers is 3 gpm pernominal 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-hpcooling 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/hrP / Q = (18 kW + 20 kW) / 7.5 mmBtu/hr = 5 kWh/mmBtuPC = 5 kWh/mmBtu x $0.10 /kWh = $0.50 /mmBtu

In addition, cooling towers evaporate about 1% of water flow. Assuming the total of thewater and sewer charges for water is $6.00 per 1,000 gallons, the quantity and cost ofmakeup water is about:

W = (1,500 gal/min x 60 min/hr x 1%) / 7.5 mmBtu/hr = 120 gal/mmBtuWC = 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 ChillerA cooling system with a water-cooled chiller is shown below. Water-cooled chillers areslightly 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 coolingtower fan and pump. Thus, the energy use and cost to provide 1 mmBtu of cooling areabout:

0.8 kW/ton / 12,000 Btu/ton x 1,000,000 Btu/mmBtu = 67 kWh/mmBtu67 kWh/mmBtu x $0.10 /kWh = $6.70 /mmBtu

Process Pump

Process

Load 1

Process

Load 2

Chiller

Cooling Tower

Cooling Tower Pump

Bypass

Valve


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Air-Cooled ChillerA cooling system with an air-cooled chiller is shown below. Air cooled chillers areslightly less energy efficient than water cooled chillers, but are generally less expensiveto 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/mmBtu83 kWh/mmBtu x $0.10 /kWh = $8.30 /mmBtu

Process Pump

Process

Load 1

Process

Load 2

Chiller

Bypass

Valve

Air

Absorption ChillerA cooling system with an absorption chiller is shown below. Absorption chillers use heatrather than electricity as the primary source of energy. Thus, absorption chillers can bepowered with waste heat from other processes, or with a dedicated source of heat such asa boiler.

Process Pump

Process

Load 1

Process

Load 2

Absorption

Chiller

Bypass

Valve

Boiler

Steam

The efficiency of the absorption chillers increases with increasing temperature heat. Thecoefficient of performance for a single-effect absorption chiller powered with steam isabout 1. Assuming the steam is generated by an 80% efficient boiler, the energy use andcost 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/mmBtu1.25 mmBtu/mmBtu x $10.00 /mmBtu = $12.50 /mmBtu

Open-Loop Water CoolingAn open-oop cooling system is shown below.

Process

Load 1

Process

Load 2

From City Water Supply

To Sewer

Assuming the temperature of the water increases by 10 F during the cooling process, thequantity 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 costproviding 1 mmBtu of cooling is about :

C = 12,000 gallons/mmBtu x $6.00 / 1,000 gallons = $72 / mmBtu

Choice of Process Cooling SystemAs demonstrated above, the cost of cooling varies from about $1 per mmBtu for coolingtowers, to about $10 per mmBtu for chillers, to about $70 per mmBtu for open-loopcooling. Thus, it is wise to use cooling towers instead of chillers or open-loop coolingwhenever possible. In many cases, the choice of primary cooling is determined by therequired temperature of cooling water at the process. Cooling towers can generatecooling water at a few degrees above the outdoor air wet-bulb temperature. Since wet-bulb temperature is a few degrees below the dry bulb temperature, this means thatcooling 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 watertemperature, a cooling tower may be able to deliver the required cooling.

Cooling TowersA cooling tower is a counter-flow or cross-flow heat exchanger that removes heat fromwater 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 itsenergy 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 bythe load, Ql, and the mass flow rate of water, mw. Neither the size of the tower nor thestate of the outside air influences the temperature difference; however, larger towers orlower 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 bysensible 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 theair enters at a lower temperature than the water (Figure 3a). The air will leavecompletely saturated and the cooling is part sensible and part latent. The sensible portionoccurs as the air temperature increases by absorbing heat from the water. The latentportion 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-bulbtemperature than the water, then the air will cool as it saturates (Figure 3b). Thus, thesensible cooling component is negative, and the all the cooling is due to evaporation. Ingeneral, cooling is dominated by latent cooling.

Figure 2. Psychrometric process lines for air through a cooling tower, if the entering air

temperature is a) lessthan the entering water temperature, and b) greaterthan theentering water temperature.

The total cooling, ma (ha2ha1) is the same for both cases since enthalpy is a functionof wet-bulb temperature alone. However, the dry-bulb temperature significantlyinfluences the evaporation rate, mwe = ma (wa2-wa1). The rate of evaporation increasesas the dry-bulb temperature increases for a given wet-bulb temperature.


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Cooling Towers as Heat ExchangersBased on the previous discussion, it is clear that cooling tower performance is a functionof the wet-bulb temperature of the entering air. In an infinite cooling tower, the leavingair wet-bulb temperature would approach the entering water temperature, and the leavingwater temperature would approach the web-bulb temperature of the entering air. The

difference between the leaving water temperature and the entering air wet-bulbtemperature is called the approach. The relationship between air wet-bulb and watertemperature is shown in the figure below. In an infinite cooling tower, the approachwould be zero.

Source: ASHRAE Handbook, HVAC Systems and Equipment, 2004.

Neglecting fan power and assuming steady state operation, an energy balance on acooling tower gives:

mw1cpw Tw1mw2cpwTw2+ ma(ha1ha2) = 0

Assuming steady state operation, a mass balance on water flow gives:

mw1mw2 + ma(wa1wa2) = 0m

w2= m

w1+ m

a(w

a1w

a2)

Substituting mw2into the energy balance gives:

mw1cpw Tw1[mw1+ ma(wa1wa2)] cpwTw2+ ma(ha1ha2) = 0mw1cpw Tw1mw1cpwTw2 - ma(wa1wa2) cpwTw2+ ma(ha1ha2) = 0


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The fraction of incoming water that is evaporated, ma(wa2-wa1) / mw1, is typically lessthan 1%. Thus, ma(wa1wa2) is much less than mw1,and the term ma(wa1wa2) cpwTw2can be neglected with negligible error to give:

mw1cpw(Tw1Tw2) = 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 heattransfer.

E = Qactual / Qmax

For a heat exchanger, Qmax occurs if the air leaves the cooling tower completelysaturated at the temperature of the incoming water. Thus, effectiveness is

E = Qactual / Qmax = [mw1cpw(Tw1Tw2)] / [ ma(ha,sat,tw1- ha1)]

Energy Efficiency of Counterflow and Crossflow TowersThe two most common tower designs for HVAC applications are forced-air counterflowand induced air cross-flow. Cooling tower energy use is a function of fan and pumppower. To generate the same quantity of cooling, forced-air counterflow towers requiremore fan and more pump energy then induced-air crossflow towers. Thus, induced-aircrossflow towers are almost always more energy efficient.


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Forced-air counterflow towers require more fan energy because centrifugal fans are madeto generate low flow against high pressure, but cooling towers generally need high flowat low pressure. In comparison, induced air crossflow towers use propeller fans, whichgenerate high flow against low pressure, which is more suited to cooling towers.

Forced-air counterflow towers require more pump energy because these towers are tallerin 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-aircounterflow towers spray water through nozzles, which increases pressure drop. In

comparision, induced-air crossflow towers are shorter and wider since the path of the airthrough the water is horizontal. In addition, the supply water simply drains from feedingpans 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 ItsManagement, October, 1982.

Cooling Tower Control

In HVAC applications, chiller evaporator loads vary depending on weather and buildingoccupancy, 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, thetemperature 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 coldwater return temperature. The two control points for cooling towers are water flow andair flow. However, cooling tower manufacturers strongly recommend that water flowremain constant at all times. Thus, primary control methods generally rely on varying airflow. The common control methods are listed below.

Run Fans ContinuouslyThis 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 fanenergy use. Because the improvement of chiller efficiency with lower condenser watertemperature is asymptotical at some minimum temperature, this method of control rarelyresults in the best overall energy efficiency.

Cycle Fans On and OffThis type of control reduces excess fan energy use at cold outsider air temperatures, andis widely used. At relatively cold temperatures, however, the fan may cycle on and offtoo frequently. The maximum number of fan cycles is about 8 per hour. Thus, manycooling towers are equipped with water bypass loops. In most applications, water bypasscontrol is only used at low temperatures when fan cycling could be a problem.

Use Two-Speed FanThis 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 offlow. 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 ineach cell. This leads to four possible steps of control. A typical relationship betweencold water temperature and fan flow is shown below.


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Continuously Control Fan Speed with VSDThis method results in the lowest fan energy use by continuously achieving savings, dueto the fan law that fan energy varies with the cube of flow.

Vary Air Flow Using Inlet Air DampersBefore VSDs, cooling towers were sometimes controlled by running the fan at full speedwhile varying the inlet air dampers to modulate air flow. This method of control resultsin 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 ControlTotal chiller and cooling tower energy use for these control methods for a typical HVACapplication are shown below.

Source: Marley Technical Report H-001A, Cooling Tower Energy and Its

Management, October, 1982.

Variable Cold Water Set-Point TemperatureThe energy efficiency of all the control discussed above can be improved by varying thecold water set-point temperature with the outdoor air wet bulb temperature. This type ofcontrol takes into account the fact that towers can only produce water at a few degreesabove the wet-bulb temperature (this temperature difference is called the approach);hence fan energy can be reduced when that temperature is achieved, since continued fanoperation 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 acooling tower fan equipped with a VFD. The fan affinity laws would predict arelationship between fraction power (FP) and fraction speed (FS) of:

FP = FS3

Regression of the data show a slightly better fit using the exponent 2.8:


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

Cooling Tower Pumping Pressure DropTypical cooling tower pressure drops are shown below. The Estimated Head Losscolumn is for a standard condenser and 15 year old piping. The Actual Head Losscolumn is for a low-pressure loss condenser and new piping.


Cooling Tower Selection

In HVAC applications, the starting place for cooling towers selection is typically tomatch the nominal cooling tower tons, as supplied by the tower manufacturer, to thecooling capacity of the chiller or chiller plant. The water flow rate through the coolingtower 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 ofinduced-air crossflow cooling towers are shown below. Based on these data, fan motor hpis 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 atan 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 (9585) F = 15,000 Btu/hr

This strange convention exists to make it easy for users to select cooling towers bymatching 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 12,000Btu/hr through the evaporator.



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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 5gpm/ton is shown below. Similar performance data for specific cooling towers canusually be obtained from the manufacturer. These curves predict the temperature of thecold 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 beused as an input value in these charts, since the temperature range is set by the water flowrate and heat rejection rate of the condenser.


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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 dervivedfrom regressing data from the 3 gpm/ton and 5 gpm/ton curves shown above. Therelation and regression coefficients are shown below. The R2 for these relations exceeds0.995 and the average error, [abs(TcTc,pred)], is less than 0.8 F.

Tc = a + b Twb + c Tr + d Twb2+ e Tr2+ f Tr Twb

Coef 3 gpm/ton 5 gpm/ton

A 16.790751 24.6299229

B 0.6464308 0.45007792

C 2.2221763 3.32229591D 0.0016061 0.00261818

E -0.0159268 -0.0324886

F -0.015954 -0.0190476

These equations can be incorporated into software to predict cooling tower performancewith 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 atarget temperature, based on water temperature range Tr and TMY2 weather data. Thisinformation is useful in determining how often a cooling tower can replace a chiller incooling applications.

Cooling Tower Performance at Reduced Air Flow RatesComparison of the 3 gpm/ton and 5 gpm/ton performance maps can be used to predictcooling tower performance at reduced air flow rates. For example, for a cooling tower


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operating with a water flow rate of 3 gpm/ton, the 3 gpm/ton performance map showstower performance at a set water-to-air flow rate ratio. The 5 gpm/ton chart shows towerperformance for a higher water-to-air flow ratio, or, inversely, at a lower air-to-waterflow rate ratio. Thus, the 5 gpm/ton performance map indicates tower performance ifwater flow rate is held steady while the air flow rate is reduced to 3/5 = 60% of maximum

airflow.

Regressing the data from the 3 gpm/ton and 5 gpm/ton performance curves, with fractionof air flow, FV, set to 1.0 for the 3 gpm/ton data and 0.6 for the 5 gpm/ton data gives thefollowing relation for the temperature of cooling water leaving the tower, Tc, at reducedair flow. The R2for this relation is R2= 0.978 and the average error [abs(TcTc,pred)]is 1.9 F. Theoretically, the fraction of air flow, FV, could vary between 0 and 1.0.However, this relation was generated using data that represent peak air flow at 0.6 and1.0. Thus, it is not recommended that this relationship be used outside of this range.

Tc = a + b Twb + c Tr + d Twb2+ e Tr2+ f Tr Twb + g FV

Coef Value

a 39.24367b 0.548254

c 2.772236d 0.002112

e -0.02421

f -0.0175

g -23.1667

Evaporation RateAs discussed in the previous section, cooling in cooling towers is dominated byevaporation. The evaporation rate can be calculated from the pyschrometric relations inthe 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 becalculated based on the fan cfm and specific volume of the inlet air), and inlet conditionsof air are known. The enthalpy of the exit air, ha2, can be calculated from an energybalance.

Ql = ma (ha2ha1)ha2 = ha1+ Ql / ma

The state of the exit air can be fixed by assuming that it is 100% saturated with anenthalpy ha2. The evaporation rate, mwe, can be determined by a water mass balance onthe air.

mwe = ma (wa2- wa1)

The fraction of water evaporated is:


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mwe / mw

Using this method for entering air temperatures from 50 F to 90 F, we determined that thefraction of water evaporated typically ranges from about 0.5% to 1%, with an averagevalue 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 theproduct of the water evaporated, mwe, and the latent heat of cooling, hfg.

Ql = Qevapmw cp dT = mwe hfg

Assuming the latent heat of evaporation of water, hfg, is 1,000 Btu/lb, and thetemperature difference of water through the tower, dT, is 10 F, the fraction of water

evaporated is:

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 as0.30%. Water lost to evaporation should not be subjected to sewer charges. Typicalsewer 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 orwind. This type of water loss is called drift. Drift rates are typically about0.2% offlow (ASHRAE Handbook, HVAC Systems and Equipment, 2000); however, wegenerally assume that drift losses are included in the 0.75% evaporation rate.

Water Treatment and Blow Down RateCooling tower water must be treated to prevent bacterial growth and maintain theconcentration of dissolved solids at acceptable levels to prevent scale and corrosion.

Bacterial GrowthThe typical method of controlling bacterial growth is to add biocides at prescribedintervals and to keep the cooling tower water circulating. If the tower will not beoperated for a sustained period of time, then the cooling water should be drained.


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Dissolved SolidsWater evaporated from a cooling tower does not contain dissolved solids. Thus, theconcentration of dissolved solids will increase over time if only enough water is added tothe tower to compensate for evaporation. To maintain the dissolved solids at acceptablelevels, 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, scalewill 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 solidto control is calcium carbonate CaCO3.

Blow down can be accomplished by continuously adding and removing a small quantityof water, periodically draining and refilling the cooling tower reservoir, or by meteringthe conductivity of water and adding fresh water only when needed. By far the mostefficient method is to meter the conductivity of water, which increases in proportion tothe level of dissolved solids, and add fresh water only when needed.

The required quantity of blow down water depends on the acceptable quantity ofdissolved solids in the tower water, PPMtarget, the quantity of dissolved solids in themakeup water, PPMmu, and the evaporation rate, mwe. The target level of dissolvedsolids is typically quantified in cycles, where:

Cycles = PPMtarget / PPMmu

For example, if the quantity of dissolved CaCO3in the makeup water, PPMmu, is 77 ppmand the maximum level to prevent scaling, PPMtarget, is 231, then the cooling towerwater 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 tomaintain a certain number of cycles is

mwbd = mwe / (Cycles1)

The total makeup water required mwmu, is the sum of the water added for evaporationand blow down:

mwmu = mwe + mwbd

For example for a 1,000 gpm tower with a 0.75% evaporation rate and CaCO3concentration 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 gpmmwbd = mwe / (Cycles1) = 7.5 gpm / (31) = 3.75 gpmmwmu = mwe + mwbd = 7.5 gpm + 3.75 gpm = 11.25 gpm


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Vapor Compression ChillersVapor compression chillers use two heat exchangers, a compressor and an expansiondevice to raise and lower the temperature of a refrigerant, so that the refrigerant canabsorb 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 areshown below. The compressor raises the pressure of the refrigerant to a temperaturegreater 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 thanthe temperature of the area or medium to be cooled so that heat can be added to therefrigerant 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 heatfrom a low to high temperature requires work to be added to the compressor.


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Chiller efficiency, E, is frequently called the Coefficient of Performance, COP. Usingthe 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 themaximum (Carnot) efficiency, COPmax:

COPmax = Tenv / (Troom-Tenv)

For a chiller operating between room temperature of 70 F and outdoor air temperature of90 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 asirreversibilities.

Compressors

Vapor compression chillers use three types of compressors: scroll, screw, reciprocating

and centrifugal. Scroll compressors are positive displacement compressors used on smallresidential air conditioners (1-10 tons). Screw and reciprocating compressors are positivedisplacement compressors used in medium-sized chillers (10300 tons). Centrifugalcompressors 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 asneeded. In processes with large variations, multiple reciprocating compressors are oftenstaged so that they turn on and off as needed. Thus, multistage reciprocating chillershave 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 compressorsmust remain running even at part load. Thus, the efficiency of screw and centrifugalcompressors at part load is often less than at full load. Part load losses can be minimizedby staging multiple compressors or by installing a variable speed drive on the compressormotor.


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Air-Cooled Chiller Performance

The energy efficiency of air-cooled chillers is frequently reported as the EnergyEfficiency Ratio, EER. EER includes the electrical power for both the compressor andcondenser fans.

EER (Btu/Wh) = Qevap (Btu/hr) / [Wcomp (W) + Wcondfans (W)]

Performance data from typical air-cooled chillers are shown below. The data show thatenergy efficiency decreases at part-loads, low leaving water temperatures, and highoutdoor air temperatures.

Water-Cooled Chiller PerformanceThe energy efficiency of water-cooled chillers is typically reported in terms of kW ofpower to the compressor per ton of cooling generated. Note that kW/ton rating does notinclude the power to the required cooling tower fan and pump. Typically, cooling towerfan power adds an additional 0.05 kW/ton, and cooling tower pump power adds about


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0.04 kW/ton. Performance data from a typical single-speed water-cooled chiller areshown below. The data show that compressor energy efficiency decreases at part-loadsand high condenser water temperatures from the cooling tower.

Absorption Chillers

Like vapor compression chillers, absorption chillers employ an evaporator, condenser andexpansion valve. However, absorption chillers convert the refrigerant to a liquid beforeraising the pressure. Because the quantity of work required to raise the pressure of aliquid with a pump is so small compared the quantity of work required to raise thepressure of a vapor with a compressor, the quantity of pump work is typically neglectedin energy efficiency calculations. However, heat is required to regenerate the refrigerant.Thus, absorption chillers use heat instead of work to generate cooling. A vaporcompression cycle is shown below.


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The energy efficiency of absorption chillers is typically quantified as the coefficient ofperformance, COP.

COP = Qevap / Qgenerator

The COP depends primarily on the temperature of the supplied heat. The COP of double-effect chillers using steam as heat is about 1.2. The COP of single-effect chillers, whichcan use much lower temperature heat is typically about 0.8. A performance chart for alow-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 coolingloads.

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.


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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

corse on cooling

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