Cooling tower

engineers newsletter
volume34–1 ●
selecting cooling towers for efficiency: 
Range or approach? 
from the editor … Whenwasthelasttimeyourevised
It’s tempting to rely on ARI standard yourspecificationsorselection
rating conditions for flow rates and parametersforcoolingtowers?Ordo
temperature differences when youspecifyuniqueparametersfor
designing chilled water systems. coolingtowerselectiononeveryjob?
But as valuable as these benchmarks SomeHVACdesignersspecify3gpm
are for verifying performance, they are ofcoolingwaterpertonofchiller
unlikely to reflect optimal conditions for capacity;othersspecifyless.Still
the entire system … especially as
mechanical efficiencies improve and
customer requirements change.
The same caveat applies to current
rules of thumb, such as a 10°F ∆T
across the cooling tower or a
condenser water flow rate of 3 gpm/
ton. Basing the design of the
condenser water loop on either of
these parameters may short-change
the performance potential of the
system and overlook opportunities to
reduce costs.
In this issue, veteran applications
engineer, Don Eppelheimer, explores
the chiller–tower relationship by
demonstrating that a wider cooling
tower range not only delivers cost
savings but may also improve
the efficiency of the entire chilled
water system.
.
CTI STD-201. Just as the Air-
Conditioning and Refrigeration Institute
(ARI) develops performance standards to
certify chillers, the Cooling Technology
Institute (CTI) provides a certification
program to validate the performance of
cooling towers. Unlike chillers, however,
there is no standard set of selection
conditions for cooling towers. Towers that
receive certification under CTI STD-201
will provide predictable performance
within the operating limits illustrated
above. For more information, visit the
CTI web site at www.cti.org
providinginsightsfortoday’shvacsystemdesigner
Tower selection 101 
Thethermodynamicrealmofcooling
towerscanbedefinedbyjustthree
temperatures…
…the“hot”waterenteringthecooling
othersbasetheirselectionson
somethingotherthanflow,suchasa
tower,the“cold”waterleavingthe
condenser∆Tof85/95inhumid
tower,andthedesignambientwet
climatesor80/90inlesssultrylocales.
bulbofthegeographicregionwhere
thetowerwillbeused. 
99/85/78 95/85/78 
Approachisthetemperature
90/80/71 102/83/78
differencebetweenwhatisbeing
Areyourtowerselectionguidelines
producedandthe“powersource”that
listedabove?Doyouknowwhateach
createstheproduct.Inthecaseofa
numberrepresentsandwhythose
coolingtower,the“product”iscold
particularvaluesaresignificant?
waterleavingthetowerandambient
wetbulbisthedrivingforcethat
createsthecoldwater.Ifacooling
towerproduces85°Fcoldwaterwhen
theambientwetbulbis78°F,thenthe
coolingtowerapproachis7°F.
Theeffectivenessofaheatexchange
processcanbegaugedbyexamining
theapproachtemperature.For
example,acoolingcoilthatcan
produce48°Fleavingairwith42°F
enteringwater(anapproachof6°F)is
moreeffectivethanacoolingcoilthat
onlycanproduce50°Fleavingairwith
thesame42°Fenteringwater(an
approachof8°F).Thesamewillhold
trueforcoolingtowers.Foragiven
typeofcoolingtower,acloser(smaller)
©2005AmericanStandardInc.Allrightsreserved ● 1 
approachtemperatureindicatesamore
effectivetower.
1
Selectingacooling
towerwithacloseapproachwillsupply
thechillercondenserwithcoolerwater
…butthecapitalcostandenergy
consumptionofthetowerwillbe
higher,too.
Still,thecoolingtowerisn’tthe
mostgrievousenergyconsumerina
chilledwatersystem.Differenttower
selectionscanaffordopportunities
toincreasetheoverallefficiencyof
thesystem.
Mechanical efficiencyreferstothe
fanpowerthat’srequiredtocirculate
ambientairoverthecoolingtowerfill.
Differenttypesofcoolingtowersdiffer
intheirmechanicalefficiencies.
Experienceleadsustothebest
thermalefficiencyforcoolingtowers
usedinaparticularmarketor
geographiclocation.It’squitelikelythat
thesamecoldwatertemperaturehas
beenusedtoselectcoolingtowersin
yourareaforyears.However,approach
temperatureonlyrepresentsthe
efficiencyofthecoolingtower’s
evaporationprocess.Itnotonlysays
littleabouttheefficiencyofthechilled
watersystem,buttheeffectoftower
approachonchilledwatersystem
efficiencyalsoislimited.Whatdrives
1
Notethateffectiveness referstothethermal
efficiencyofthecoolingtowerfillandthe
evaporativeprocess;donotconfuseitwiththe
mechanicalefficiencyofthecoolingtowerfan.
Precepts of tower sizing. Four
fundamental factors affect tower
size: heat load, range, approach, and
ambient wet-bulb temperature. If three
of these factors remain constant, then
changing the fourth factor will affect
tower size in this way:
•  Tower size varies directly and linearly
with the heat rejection load.
•  Tower size varies inversely with
range.
•  Tower size varies inversely with
approach.
•  Tower size varies inversely with wet-
bulb temperature.
[From Cooling Tower Performance: Basic
Theory and Practice, a June 1986 paper
published by Marley Cooling Technologies and
available online at http://www.marleyct.com/
pdf_forms/CTII-1.pdf]
theefficiencyofthechilledwater
systemisthecoolingtowerrange.
Rangeisthetemperaturedifference
betweenthehotandcoldwateratthe
tower.Increasing the rangewillreduce
thecapitalcostandenergycostofthe
tower;italsowillreducethecapital
costandenergyconsumptionofthe
condenserwatersystem.However,
increasingthecoolingtowerrangeis
onlypossibleifthechilleriscapableof
producingwarmerleavingcondenser
water.Selectingchillersforwarmer
leavingcondenserwaterwillincrease
chillerenergyconsumptionandmay
alsoincreasethedollar-per-toncostof
thechiller.
Thisbegsthequestion: Whatcooling
towerrangeresultsinthelowest
capitalcostforthechilledwater
system?Further,whatcoolingtower
rangeresultsinthelowestannual
energycostforthechilledwater
system?Thisauthorfirmlybelieves
thatincreasing cooling tower range
from9.4°Fto14°Formorewill reduce
capital cost AND annual energy cost.
2
Opportunity to engineer 
Now,wecometothefunpartofthe
designprocess…theopportunityto
exerciseabitofengineeringjudgment.
Thereisathermodynamicpricetopay
whenthecoolingtowerrangeis
increased.Thatpenaltyoccursatthe
chiller.Wecanpaythatpricenowby
specifyingamoreefficientchiller,or
wecanpayitlaterbyallowingthe
increasedcoolingtowerrangeto
diminishchillerCOP.Thefollowing
exampleillustratesthisconcept.
Alternative 1:  Base design.  Amiddle
schoolinTennesseerequiresachilled
watersystemwith800tonsofcooling
capacity(Alternative1schematic).To
meetthespecification,theengineer
hasproposedan800-toncentrifugal
2
TuminChanechoesthissentimentinhis
Engineered Systemsarticle,“AChiller
Challenge.”Youcanfinditat<http://
www.esmagazine.com/CDA/ArticleInformation/
features/BNP__Features__Item/
0,2503,76249,00.html>.
Alternative 1:  Base design 
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providinginsightsfortoday’sHVACsystemdesigner TraneEngineers Newslettervolume34–1 ● 3
Alternative 2: Wider range, smaller tower Alternative 3: Wider range, optimized system
chiller;theunitunderconsideration
wasselectedatARIconditionsandis
thelowestcostcentrifugalmachine
thatcomplieswithASHRAEStandard
90.1’sminimumefficiencies.Pressure
dropsthroughtheevaporatorand
condenserdonotexceed25ft.
Theengineeralsoproposedatwo-cell,
coolingtowerwithtwo20-hp,variable-
speedfanmotors.Thetower’scross-
flowdesignwasselectedforits
reliability,easeofmaintenance,and
lowheight.Thetowerselectionis
basedonarangeof9.4°Fandaflow
rateof2400gpm,whichisprovidedby
a40-hpcondenserwaterpump.With
thehelpofenergymodelingsoftware,
theengineerestimatesannualenergy
consumptionasfollows:
Alternative 2: Wider range, 
smaller tower.  Increasingthecooling
towerrangecanprovideseveral
benefits,includingquieteroperation,a
smallerfootprint,lowercapital
investment,andlessenergyuse.
Thedesignteamfirstinvestigated
the capitalcostsavingsofincreasing
thecoolingtowerrangeto14°F
(Alternative 2schematic).Inadditionto
reducingtheinitialcostofthecooling
towerby13 percent,italsoreduced
thetowerfootprintby25 percentand
itsweightby23 percent.
Anotherbenefitofincreasingthetower
rangefrom9.4°Fto14°Fisthedropin
condenserflowratefrom2400gpmto
1600gpm.Thecorresponding
reductionsinpressuredropdecreased
therequiredpumppowerfrom
40.16 bhpto15.89bhp,eventhough
ANNUALENERGYUSE
cooling tower range 9.4°F
centrifugalchiller
coolingtower
condenserwaterpump
259,776 kWh
66,911
85,769
TOTALCONSUMPTION 412,456 kWh
thecondenserwaterpipingwasn’t
resized:
Reselectingthecentrifugalchiller
basedon99°Fwaterleavingthe
condenser(duetothe14°Ftower
range)didn’taffectitscapitaland
installationcosts,butwarmer
condenserwaterincreasedthechiller’s
annualenergyconsumption.Anenergy
analysisconfirmed,however,thatthe
substantialcapitalcostreductionsfor
thecoolingtowerandcondenserwater
pumpwouldnotincreasetheoverall
operatingcostofthechilledwater
system.Powerreductionsatthe
PRESSUREDROPS
condenser water flow 2400 gpm 1600 gpm
condenser
coolingtower
condenserpiping
26.41 ft
12.23 ft
11.56 ft
12.34 ft
12.16 ft
5.32 ft
coolingtowerandcondenserwater
pumpexceededthechiller’sadditional
powerconsumption.Ultimately,the
projectedenergyconsumptionforthe
entirechilledwatersystemis8 percent
lessthanthebasedesign:
Alternative 3: Wider range, 
optimized system.  Theschool-district
administrationinourexamplewas
concernedaboutthecapitalcostsof
theirbuildingsandequipment,but
evenmoreattentivetoenergy/
operationandmaintenancecosts—the
totalcostofownership.
Sinceavailablespaceforthecooling
towerwasn’taselectionissue,the
designteamadoptedadifferenttack.
Couldthebenefitofawidercooling
towerrangebe“redirected”to
improvetheefficiencyofthechilled
watersystem?Whatwouldhappenif
therangewasincreasedwithout
downsizingthecoolingtowerfill?
Tofindout,thedesignengineerused
the14°Frangeandthedimensionsof
theoriginaltowertoreselectthe
tower forathirdtime(Alternative 3
schematic).Thiscombinationof
ANNUALENERGYUSE
cooling tower range 9.4°F 14°F
centrifugalchiller
coolingtower
condenserwaterpump
259,776
66,911
85,769
278,389 kWh
64,878
33,936
TOTALCONSUMPTION 412,456 377,203 kWh
“Having your cake and eating it,
too.” In most cases larger ∆Ts and the
associated lower flow rates will not only
save installation cost, but will usually
save energy over the course of the year.
This is especially true if a portion of the
first cost savings is reinvested in more
efficient chillers. With the same cost
chillers, at worst, the annual operating
cost with the lower flows will be about
equal to “standard” flows but still at a
lower first cost.
[From CoolingTools Chilled Water Plant Design
Guide, Pacific Gas and Electric (PG&E), <http://
www.hvacexchange.com/cooltools/>]
parametersreducedthefan
horsepowerrequirementfrom40hp
to20hp,whichyieldedfinancial
benefitsontwofronts:
• A5to6percentreductioninthe
projectedcapitalcostforthetower
duetosmallerfans,motors,and
drives
• A51percentreductioninthe
annualenergyconsumption
projectedforthetower
Ourengineerthenreselectedthe
centrifugalchiller,choosingheat-
transferoptionsthatwouldallowitto
operatemoreefficientlyatthehigher
towerrange.Theseenhancements
raisedthecostofthechiller,butby
lessthan5percentoftheoriginal
estimate.
Table1summarizestheresultsofall
threeselectionsinthisexample.The
lowesttotal owningandoperatingcost
resultedfromincreasingthetower
range,coupledwithcoolingtowerand
chillerselectionsaimedataffordable
efficiency.
Closing thoughts 
Whenitcomestoreducingboththe
capitalcostandoperatingexpenseofa
chilledwatersystem,coolingtower
rangecanbeaparticularlypotenttool.
Thegreatertherange,thegreaterthe
designteam’slatitudetofindcreative
andeffectivesolutionstoproject
constraints,suchasthebudgetsfor
capitalinvestmentandoperating
expense(asinthisexample),or
limitationsrelatedtonoiseoravailable
space.●
ByDonEppelheimer,applicationsengineer,and
BrendaBradley,informationdesigner,Trane.You
canfindthisandpreviousissuesoftheEngineers
Newsletterathttp://www.trane.com/commercial/
library/newsletters.asp.Tocomment,e-mailusat
[email protected].
Table 1. Summary of selection results for example chilled water system 
Alternative 1: Alternative 2: Alternative 3:
Base design Smaller tower Optimized system
Cooling tower range 9.4°F 14°F 14°F
References 
AmericanSocietyofHeating,Refrigeratingand
Air-ConditioningEngineers,Inc.(ASHRAE).2000.
2000 ASHRAE Handbook—HVAC Systems and
Equipment.Atlanta,GA:ASHRAE.
Grumman,D.(ed.).2003.ASHRAE Green Guide.
Atlanta,GA:ASHRAE.
MarleyCoolingTechnologies.Marley Publications
webpage[online].<http://www.marleyct.com/
publications.asp>[cited15December2004].
Taylor,S.,M.Hydeman,P.DuPont,T.Hartman,and
B.Jones.2000.Chilled Water Plant Design Guide.
SanFrancisco,CA:PacificGasandElectric
Company.
Condenser water flow 2400 gpm 1600 gpm 1600 gpm
Cooling tower parameters 
Developingtheenergydatashown
inTable1isn’tdifficult.Thechiller Footprint 18.75×22.08 ft 17.0×18.08 ft 18.75×22.08 ft
manufacturercaneasilyprovidefull-
Weight 38,050 lb 29,136 lb 37,726 lb
andpart-loadefficiencydataforthe
Cells 2 2 2
chillerofyourchoiceatvarious
40 hp 40 hp 20 hp
condenserflowrates,whileselection
Fanpower(total)
12.23 ft 12.16 ft 12.23 ft
softwarefromthecoolingtower
Staticlift
manufacturerwillprovidetherequired
Pressure drops 
towerperformancedata.Energy Condenser 26.41 ft
modelingtools,suchasTrane’sChiller
Coolingtower 12.23 ft
Plant Analyzer (whichwasusedto
Pipes,valvefittings 11.56 ft
generatethedatainthisnewsletter),
simplifycomparisonsofvariouschiller–
Pumppowerrequired 40.16 bhp
Chiller efficiency 6.18 COP
tower–pumpcombinations.
Annual energy consumption 
12.34 ft 20.68 ft
12.16 ft 12.23 ft
5.32 ft 5.32 ft
15.90 bhp 20.39 bhp
5.76 COP 6.09 COP
Centrifugalchiller 259,776 kWh 278,389 kWh 263,325 kWh
Coolingtower 66,911 kWh 64,878 kWh 32,437 kWh
Condenserwaterpump 85,769 kWh 33,936 kWh 43,547 kWh
Totalforsystem 412,456 kWh 377,203 kWh 339,309 kWh
Trane
A business of American Standard Companies Trane believes the facts and suggestions presented here to be accurate. However, final design and
www.trane.com
application decisions are your responsibility. Trane disclaims any responsibility for actions taken on
For more information, contact your local Trane
the material presented.
office or e-mail us at [email protected]
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