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What’s Up Cooling

Towersater-cooled HVAC and industrial cooling sys-

tems offer many significant benefits over air-

cooled cooling systems—most notably the ability to

achieve lower process fluid temperatures and cor-

responding pressures, resulting in:

About the Author

WBy Frank T. Morrison, Member ASHRAE

Large closed-circuit cooling tower. It combines a heat exchanger and a cooling tower into one relatively compact design.

Frank T. Morrison is product marketing manager,comfort cooling and light industrial systems, withBaltimore Aircoil Company, Baltimore. He is amember of ASHRAE TC 8.6, Cooling Towers.

1. Lower system energy usage;2. Smaller equipment size and cost; and3. Reduced maintenance and extended

life of mechanical equipment.The advantages of evaporative cooling

stem from several key factors. First, cool-ing towers use the ambient wet-bulb tem-perature of the entering air as the heatsink, which is typically 10°F to 30°F(5.5°C to 16.7°C) lower than the drybulb, depending on the local climate. Thelower the temperature of the heat sink,the more efficient the process.

Second, the evaporative cooling processinvolves both latent and sensible heattransfer (primarily latent) where a smallportion of the recirculating flow is evapo-rated to cool the remaining water. For ev-ery pound of water evaporated into theairstream, approximately 1,050 Btu of heatis rejected.* In contrast, a pound of air atstandard conditions has a heat content ofonly 0.24 Btu/lb-°F (1.0 kJ/kg-°C), mean-ing that much greater air volume is re-quired to reject the same heat load in aircooled (sensible only) cooling systems as

with

Copyright 2004, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. This posting is by permission from ASHRAEJournal. This article may not be copied nor distributed in either paper or digital form without ASHRAE's permission. Contact ASHRAE atwww.ashrae.org.

J u l y 2 0 0 4 A S H R A E J o u r n a l 1 7

water falling downward and the air traversing across the fillsheet horizontally, or counterflow, with water flowing down-ward and air traversing vertically across the fill sheet. Crossflowdesigns, typically with relatively large fan plenums and exter-nally accessible gravity water distribution systems, offer ex-cellent features in terms of maintenance and access to criticaltower components relative to other designs. Counterflow filltowers can offer a smaller footprint than crossflow fill modelsin the lower tonnage ranges, but tend to be taller than theircrossflow counterparts, resulting in increased pump head, whichtranslates to higher pump energy as well as the requirementfor taller architectural screens. Counterflow towers also usepressurized spray systems typically not accessible for inspec-tion without shutting down the tower. Larger cell sizes coupledwith improved fill designs can produce capacities as high as1,350 nominal tons (5935 kW) in a single factory-assembledcell. These larger cells shorten tower installation schedules andreduce the uncertainty due to weather, labor issues, etc. associ-ated with field-erected towers.

Closed-circuit cooling towers, typically used on closed-loopsystems for water-source heat pumps or air compressor cool-

*Every pound of water evaporated into the airstream allows the air to carryaway approximately 1,050 Btu (1108 kJ) of energy from the process to becooled. This value varies slightly with climate.

Crossflow cooling tower installation on sports arena.

Hybrid wet-dry closed-circuit cooling towers.

compared to evaporatively cooled systems. For instance, a typi-cal open cooling tower requires 250 cfm per ton (97 m3/h perkW) of heat rejected while air-cooled condensers, operating at ahigher condensing temperature, require 600 to as much as 900cfm per ton (233 to 349 m3/h per kW) with correspondinglyhigher fan horsepower (kW).

Third, cooling towers allow direct contact between the wa-ter and the air, which is a highly efficient process. This mixingoccurs in the fill, sometimes called the wet deck, which is typi-cally comprised of sheets of thermoformed plastic. The fillprovides a large amount of low-cost surface area for air andwater to contact each other.

These reasons combine to explain why evaporative coolingtowers are smaller and require much less fan energy than air-cooled equipment. On the system side, water-cooled heat ex-changers, such as refrigerant condensers or process units, alsohave higher overall heat transfer coefficients than their air-cooled counterparts. The system benefits from the lower cool-ing fluid temperature, which results in lower condensingtemperatures and pressures, increasing process output with lessenergy input. Due to water’s ability to efficiently transport largequantities of heat over relatively long distances, water-cooledsystems allow the economical separation of the compressionand heat rejection equipment. For example, the chiller can belocated in a basement machine room and the cooling tower onthe roof, many floors above. This typically is not an optionwith air-cooled systems. Multiple air-cooled systems on theroof, with their duct penetrations, also increase the chance ofcompromising the roofing system.

Cooling Tower ConfigurationsA variety of highly efficient cooling tower fills are used to

achieve high capacities in the smallest unit footprint possible.Factory-assembled tower designs can be either crossflow, with

Due to water’s ability to effi-

ciently transport large quan-

tities of heat over relatively

long distances, water-cooled

systems allow the economi-

cal separation of the com-

pression and heat rejection

equipment.

‘

’

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ing, have evolved significantly. A closedcircuit tower, also known as a “fluidcooler,” combines a heat exchanger anda cooling tower into one relatively com-pact design. The fluid to be cooled, usu-ally water or an aqueous glycol solution,is kept clean and contaminant-free in aclosed loop, separate from the open loopspray water and airflow on the outside ofthe coil, or tube bundle. Newer designs,known as “combined-flow” units, signifi-cantly improve performance by using anopen fill section to lower the spray watertemperature compared to coil-only de-signs. The size of the coil section can bereduced for a given duty, lowering theunit weight and reducing the cost of thesupporting steel. Additionally, most of theevaporation occurs in the fill section ofthese combined-flow towers, reducingthe potential for scale on the coil heattransfer surface.

These advances have improved thefunctionality of closed circuit towers aswell as lowered the point where they canbe economically justified over open tow-ers in many applications. For instance,by closing the condenser water loop, con-denser tube bundle cleaning can be nearlyeliminated while the chiller operates atpeak performance at all times. If any foul-ing and scaling occurs, it does so on theopen side of the tower, where it can beeasily controlled through a proper watertreatment program. The chiller energysavings and reduced maintenance ex-penses often can offset the higher cost ofthe closed circuit tower.

Closed circuit towers can offer a hydrau-lic advantage on certain projects. Opentowers must be installed above the heatsource, while closed circuit towers can belocated below since the process fluid iscontained in a closed loop and the spraywater recirculates within the tower itself.This flexibility can help solve site loca-tion problems for architects and engineers.

Thermal PerformanceIndependent certification of thermal

performance is increasingly required,with most manufacturers participating inthe Cooling Technology Institute (CTI)

certification program under CTI STD 201(note that the CTI performance test codeis CTI STD 105 for open cooling towersand CTI 105S for closed circuit coolingtowers). The California Energy Code,known as Title 24, mandates the use ofCTI certified open cooling towers in its2005 edition (note that no requirementsexist for field-erected towers, which arenot often used in HVAC applications).This same requirement is being consid-ered for the 2004 edition of the ANSI/ASHRAE/IESNA Standard 90.1, EnergyStandard for Buildings Except Low-RiseResidential Buildings, adopted by manybuilding codes in the United States.

As most other components of the sys-tem are certified, specifying CTI certifi-cation on cooling towers reduces thelikelihood, as well as the resultant liabil-ity, for deficient towers on a project forthe consulting engineer, contractor, andthe owner, while also eliminating theneed for costly field performance tests.

Thermally deficient cooling towers resultin higher energy usage from two perspec-tives: first, the tower must operate longerat maximum fan horsepower while try-ing to attain the design cold water tem-perature, and second, the system (chiller)must consume more energy because itmust operate at all times at a higher thannecessary condensing pressure (head) tosatisfy the load due to higher fluid tem-peratures provided by the cooling tower.

Certification is important because tem-perature matters, since even small devia-tions from the expected design have asubstantial impact on the system over time.For instance, a cooling tower that is 20%deficient elevates the leaving water tem-perature by approximately 2.5°F (1.4°C).Typically, this higher water temperaturewill result in 6% more energy being con-sumed by the chiller. On a 500-ton (2197kW) system during peak conditions, this6% penalty translates into approximately17 kW of additional energy usage, result-

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water treatment program are crucial tomaximizing tower life.

The most cost-effective material ofconstruction for towers today is G-235**hot dip galvanized steel, from both astructural and corrosion resistance stand-point. G-235 is the heaviest mill galva-nizing commercially available, and offersa substantial amount of protection ascompared to the lighter zinc thicknessesin use several decades ago, providing re-liable corrosion protection for mostHVAC and industrial system water chem-istries. The most common upgrade fromG-235 galvanized steel is Type 304 stain-less steel. Parts that are submerged dur-ing operation and/or at shutdown canbenefit the most by upgrading to stain-less steel.

Critical components, such as cold-wa-ter basins, often use either stainless steel,plastic, or coated metals to add to lon-gevity and/or guard against upsets incooling water chemistry. Plastic basinsgenerally are limited to small towers forstructural reasons, while stainless steelbasins can be used on all sizes. Somemanufacturers weld the seams on stain-less basins for improved leak resistance.Corrosion-resistant plastic or compositesare used in the spray water distributionsystems where possible on both open andclosed circuit towers. Light-weight, cor-rosion-resistant fiberglass reinforcedpolyester (FRP) also is popular for cas-ing panels for corrosion resistance andlighter weight.

Energy-Efficient Fan SystemsThe market has moved from centrifu-

gal fans in favor of lower energy axialfans, reducing horsepower by 50% ormore for the same capacity. This push forefficiency has been driven by the risingcost of energy and the impact of energycodes. For instance, Title 24 in Califor-nia is restricting centrifugal fan coolingtowers more than 300 cooling tower tons(1319 kW heat rejection) in the prescrip-tive approach to compliance, allowing

ing in higher electricity bills for the owner.Furthermore, the cooling tower has towork harder, at all times, not just at peakconditions, to meet the load, adding to theoverall energy penalty. A tower that is 20%deficient can cost an owner from three toeight times the original purchase cost ofthe cooling tower over its operating life-time in terms of higher total energy costs.1

Besides the substantial energy penalty,this additional energy consumption im-poses an unnecessarily greater load onthe electrical grid during the critical peakperiods and results in higher demandcharges to the owner. Society also pays aprice in terms of higher pollution levelsresulting from this wasted energy.

CTI certification of closed circuit cool-ing tower performance often is over-looked, but is equally as important.Because closed-circuit towers often areused on distributed loads such as water-source heat pump loops, the effect of adeficient unit can be masked to the equip-ment owner. However, the same perfor-mance penalties and increased operationalcosts mentioned earlier exist for thermallydeficient closed circuit towers. For in-stance, when a water-source heat pumptrips due to high head pressure during peakloading, a deficient tower often is to blame,not the heat pump. In addition to the en-ergy advantages, supplying the propercooling fluid temperatures at all times ex-tends equipment life, in this case by al-lowing the heat pump compressor tooperate at a lower temperature againstlower head pressure.

Materials of ConstructionBased on historical data, the average life

of a cooling tower is estimated at approxi-mately 20 years. Well-maintained towersoften can operate well beyond that. Mosttowers are designed such that air-movingcomponents and heat transfer media canbe replaced when necessary, often result-ing in higher unit performance as techno-logical advances occur in the industry. Thekey to longevity is keeping the base struc-ture of the tower usable, especially the coldwater basin, so both the original choice ofmaterials and a regular maintenance and

** Note that the G-235 designation refers to 2.35ounces of zinc per square foot (717 g per m2) ofthe steel sheet.

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their use only where ducted installationsare required, sound requirements pre-dominate, or when oversized to the pointwhere they meet the energy requirementsof axial fan towers.

These new codes require that any fanmotor greater than 7.5 hp (5.6 kW) use ameans of reducing power consumption atoff peak loads and/or lower ambient tem-peratures. Variable frequency drives(VFDs) are typically used to meet this re-quirement in cooling towers. VFDs alsoeliminate the need for motor starters, help-ing to offset the cost of the drives. Lowerhorsepower pony motors, usually sized forone-third of the horsepower of the primarymotor, also can be used. This method isan advantage when motor redundancy isdesired to increase system reliability.

Cooling tower fan systems also haveevolved to meet the stringent sound require-ments found on many projects. This is dis-cussed in greater detail in the next section.

SoundSound is an important issue on many

projects. To meet the dual requirementsof low energy use and quiet operation, theindustry has responded with a variety ofsolutions. The simplest methods includeproper site selection and oversizing of thetower to reduce fan speed. For example, atower with a single-side air entry can beoriented such that the air entry side is di-rected away from the sound sensitive area.Physically oversizing the tower can reduce

the fan speed required for a given thermalduty, reducing the sound level, as soundis proportional to fan speed. The fan horse-power also is reduced, providing an en-ergy benefit to offset the higher first cost.

When these techniques are not suffi-cient to meet sound requirements, a vari-

DriftEliminators

Water Distribution Systems

Warm Air Out

FluidIn

FluidOut

Air InFan

Spray Pump

ety of low sound, high-efficiency axialfans are available. These fans use widerchord fan blades and/or more fan bladesto allow the fan to move the required airat a slower rotational speed, thus lower-ing the sound level. Attenuation packagesare available when very low sound lev-

Figure 1 (left): A typical crossflow cooling tower. Figure 2: A typical forced draft counterflow centrifugal fan cooling tower.

Hot Water In

Warm Air Out

Hot Water In

Warm

Air

Cold Water Basin

WarmAir

Sump

Air In Air InWaterWater

Air InletLouvers

Wet Deck SurfaceCooled Water Out

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els are required for a critical site.The cost of sound attenuation, in-cluding the effect on performance,must be evaluated vs. simpler meth-ods such as oversizing the tower tomeet the sound criteria for a project.Note that with either low-sound fansor attenuation, lower sound levelsoften come at the expense of lowerairflow. The system designer mustensure that the manufacturer’s rat-ings are adjusted to account for anydecrease in thermal performancefrom this reduction in airflow, andverify that the ratings with the lowsound fans and/or attenuation areCTI certified as may be required bythe applicable energy codes.

VFDs also can be used to providesound control. In most cases, thesteady sound of an efficient coolingtower fan is not objectionable, and tends to blend in with otherbackground noise. What people find objectionable is the abruptstopping and starting of the fan system. Properly setting the

tower control sequence to avoid ex-cessive cycling of fan motors is im-portant in this regard, as well as toprotect the motors from overheat-ing. VFDs solve this issue by allow-ing a soft start of the fans, followedby a gentle ramping up and downof the fan speed in line with the loadrequirement. Soft-starters also canbe used to eliminate startup noise.However, these devices do not pro-vide the close capacity control andenergy savings of VFDs, which canjustify the additional cost of a VFD.Overall, the energy savings coupledwith reduced stress on the motorand belts are an added bonus, whileproviding compliance with the en-ergy codes.

Sound requirements often can bemore stringent during evening

hours, especially for facilities near residential areas. As the wetbulb and heat load also are typically lower at that time, a towerholding a fixed leaving water temperature with a VFD is able

WaterDistribution

System

Air In

Warm Air Out

Warm

Air

Warm

Air

Water

WaterAir In

Wet DeckSurface

ColdWaterBasin

FluidOut

Coil

FluidOut

Air InletLouvers

SprayPump

Figure 3: A typical coil/fill (combined flow) closedcircuit cooling tower.

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to operate at reduced fan speeds, lower-ing the sound during these critical hours.This advantage is offset when the chillercan benefit by allowing the leaving wa-ter temperature to fall with wet bulb, re-ducing head pressure and thus chillerenergy consumption, but requiring thetower to operate at a higher fan speed.

Water IssuesCooling towers originally were used to

replace wasteful “once-through” systems,saving approximately 95% of the waterby recycling the water. Water use in a cool-ing tower is tied to two factors: the heatrejection load and the blowdown rate,which is the amount of water that is dis-charged to prevent the accumulation ofsolids in the cooling water. The evapora-tion due to the heat load is approximately1,050 Btu/lb (2442 kJ/kg) of water, asmentioned earlier. The blowdown rate de-termines the water chemistry, or cycles ofconcentration, of the recirculating water,and can vary depending on makeup waterquality, the treatment program, and thetower’s construction materials. A subtlepoint often overlooked is that water usedin evaporative cooling is eventually re-turned to the environment either throughevaporation or blowdown, vs. other natu-ral resources, such as oil or natural gas,that are consumed when used.

The cost and availability of water inNorth America is becoming an increas-ingly important issue, though one that isoften misinterpreted when it comes tocooling towers. Infrastructure upgrades tocomply with U.S. federal water standards,replace aging systems around the coun-try, and meet the growth in population areleading water utilities to recover these in-vestments through higher rates, thoughthese increases are to relatively low baserates. Recent droughts in some areas haveincreased awareness. The growing trendtowards privatization of water systemsprovides greater economies of scale and

more rapid application of new technolo-gies in the industry, helping to temper theneed for increases in water rates.

Even with recent increases, the addi-tional power cost associated with air-cooled equipment far outweighs the costof water in a typical commercial build-ing or process plant. Interestingly, on amacro-basis, air-cooled systems often canuse as much or more water than a water-cooled system. This is because air-cooledsystems consume 30% to 40% morepower than water-cooled systems, andone of the largest users of water in thecountry is power generation. Latest fig-ures from the United States GeologicalSurvey (USGS) show the average wateruse for power generation is 26 gallons(98 L) per kWh. Thus, society pays forwater use either at the power plant or thelocal level. Coupled with the environ-mental aspects of power generation andtransmission line losses, conserving en-ergy at the point of use makes sense.

Cooling tower water usage is a relativelysmall component of overall water usagein a typical commercial building, averag-ing between 20% and 30%, after domes-tic (sinks, lavatories, etc.) and landscapeuse. A cooling tower rejecting a 400-ton(1759 kW heat rejection) load at typicalHVAC conditions will evaporate approxi-mately 12 gpm, or 720 gph (0.75 L/s) atpeak load. Applying a typical load factorof 50% to the system (average load dividedby peak capacity) for 12 operating hoursper day, results in 4,320 gallons/day (16353 L/day) that is evaporated on an aver-age day. Adding in 1,440 gallons/day(5451 L/day) for blowdown at a typicalfour cycles of concentration gives a totalusage of 5,760 gallons/day (21 804 L/day).Based on a typical water rate of $1.50/1,000 gallons ($0.6/1000 L), this water

Note that this rate can vary widely from localityto locality. Local water (and often sewer) ratescan be found on the Web sites of most local wa-ter utilities.

Cycles of concentration (COC) is defined asthe ratio of the concentration of dissolved solids(i.e., chlorides, sulfates, etc.) in the recirculatingwater to the concentration found in the enteringmakeup water. The higher the COC, the lowerthe bleed rate required.

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costs the building owner about $9 per day, or a fraction of theelectricity costs associated with the cooling system, even afterincluding sewage and water treatment chemical costs.

Several low-cost methods can reduce cooling tower waterconsumption. The first is to set a proper bleed rate for the tower.As the evaporation rate is dependent on the load, which canvary widely, a constant bleed rate usually discharges more waterthan required. While this tends to improve the tower’s waterchemistry, higher water and sewage bills result. A properlyoperating conductivity meter can automatically control bleedto the proper amount required to maintain the desired towerchemistry in the system at all times. Because of their advan-tages, water utilities, such as the San Diego County Water Au-thority and the Contra Costa Water District in California, offerrebates of as much as $500 towards the installation of conduc-tivity meters to control bleed rates.

Also of importance to the owner is the sewage charge forthe tower bleed rate, which often equals or exceeds the watercharge. As shown in the example, only a portion of the water(25%) metered into the unit is actually discharged into thesewer as blowdown. To account for this, many utilities allowowners to meter their blowdown separately, rather than usethe full water reading as the sewer reading. The user only

pays for what actually goes down the sewer, significantly re-ducing the sewage charge, offset by any metering fee chargedby the utility.

Hybrid Wet-Dry Cooling TowersWhile properly controlling the bleed rate and installing a

separate blowdown meter are simple, high-return investments,other technologies exist for projects where water is expensiveor in short supply. These new designs combine wet and drycooling to reduce water use, some as much as 70% comparedto conventional towers. Typically, a dry finned coil section iscombined in series with an evaporative section in these units.The dry finned section handles as much of the load as pos-sible, with the unit able to operate completely dry at reducedambient. Both open and closed circuit versions are available.Manufacturers of these designs can assist in the payback analy-sis to determine if such units are justified for a particular project,comparing the higher energy requirements of air-cooled equip-ment vs. the cost of water and energy for the hybrid design.Rebates for hybrid equipment also are available from somewater utilities and can help offset the higher first cost.

These designs also have the added benefit of reducing or eveneliminating the visible, fog-like exhaust air discharge, also known

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as plume, from the tower under most op-erating conditions. This is accomplishedby reducing the amount of water evapo-rated into the discharge airstream whilesimultaneously reheating the discharge airwith the incoming warm fluid. Plume iseliminated completely in the dry mode,which is operational during colder weatherwhen plume is most likely to occur.

Water TreatmentThe large variety of alternative con-

struction materials allows users to matchunit construction to the water qualityavailable for their systems, while help-ing to protect the tower from temporaryupsets. Water treatment programs mustbe designed for three requirements:

1. Scale control;2. Protection of system components

against corrosion; and3. Control of biological contaminants,

such as Legionella pneumophilia, thebacterium that causes Legionnaires’ dis-ease.

The first two requirements help to en-sure energy efficiency and longevity ofthe cooling system, while the third en-sures safe operation.

Biological control is relatively easy toaccomplish and is essential to the safeoperation of the tower. ASHRAE haspublished Guideline 12-2000, Minimiz-ing the Risk of Legionellosis Associatedwith Building Water Systems.

Traditional chemical water treatmenthas adapted to changes in the industry,developing new environment-friendlytreatment chemicals and control solu-tions. Electronic water treatment systemshave grown in use as equipment ownersseek ways to reduce water treatmentchemical costs and storage issues, as wellas minimize the amount of maintenancerequired. However, the long-term effec-tiveness of these systems has not beenestablished. A 2002 ASHRAE researchstudy2 found that these devices can workunder the proper parameters for control-ling scale accumulation. Claims of bio-logical control of these devices have yetto be established by a formal study, andclose monitoring of both the tower water

chemistry and biological counts is pru-dent when using these devices. Ozonesystems, using modern technology tomaintain proper ozone level in the sys-tem, also are used to control biologicalactivity. However, their relatively highfirst cost limits their use.

Last, cooling towers can collect andconcentrate airborne dirt and debris overtime. To control this buildup, the coolingtower should be located so as to minimizecontaminant induction and a properblowdown rate should be maintained.Sidestream filters or separators haveproven valuable in this regard by effec-tively removing dirt and debris from thetower water. These devices are coupledwith a basin-sweeping nozzle package,which is available either as original equip-ment in the tower or as a field-installedaftermarket item. Cleaner tower watermakes water treatment regimens more ef-fective while keeping the cooling loop

cleaner, saving energy, reducing mainte-nance, and improving reliability of theentire cooling system.

ConclusionEvaporative water-cooled systems,

whether open or closed circuit, are the bestoverall heat rejection solution for most in-stallations. These systems offer designflexibility, save energy, and conserve re-sources while protecting and respectingthe environment. The cooling tower indus-try continues to develop innovative prod-ucts and services to meet the evolvingneeds of new and existing facilities.

References1. Facius, T. 1991. “Certified cooling tower

performance.” The Construction Specifier.October.

2. ASHRAE RP-1155 “Efficiency of non-chemical water treatments in controlling cal-cium scale accumulation in recirculating opencooling water systems.” Young Cho. DrexelUniversity.

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