空気线図三角.docx
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空気线図三角.docx
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空気线図三角
EnergyEfficientBuildings
CoolingTowers
Introduction
Acoolingtowerisacounter-floworcross-flowheatexchangerthatremovesheatfromwaterandtransfersittoair.Coolingtowerscomeinmanyconfigurations.Aninduced-draftcoolingtower,whichiscommoninHVACandindustrialapplications,isshowninFigure1a.Aswarmwaterfromtheprocessfallsthroughthetower,someofitevaporates,whichcoolstheremainingwater.Thecooledwatercollectsatthebottomofthecoolingtowerandisreturnedtotheplantwhereitisusedforcooling.Figure1bshowsanevaporativecondenser,whichiscommoninindustrialrefrigerationapplications.Water,whichiscooledbyevaporation,fallsoveraclosedheatexchanger(usuallycarryingrefrigerant)inthetoppartofthetower.Itthenfallsovermorefilltoenhanceevaporationinthelowerpartofthetower.Asmallpumpcirculateswaterfromthebottomtothetopofthetower.
Figure1a)opencircuitcoolingtower
1b)closedcircuitevaporativecoolingtower
Thetemperaturedifferenceofwaterthroughatower,dT=Tw1-Tw2,isdeterminedbytheload,Ql,andthemassflowrateofwater,mw.Neitherthesizeofthetowernorthestateoftheoutsideairinfluencesthetemperaturedifference;however,largertowersorloweroutdoorairwet-bulbtemperatureswilldecreasetheexitwatertemperature,Tw2.
SensibleandLatentCooling
Dependingontheenteringairandwatertemperatures,thewatermaybecooledbysensibleandlatentcoolingoftheair,orsimplybylatentcoolingoftheair.Ineithercase,latent,i.e.evaporative,coolingisdominant.Forexample,considerthecaseinwhichtheairentersatalowertemperaturethanthewater(Figure3a).Theairwillleavecompletelysaturatedandthecoolingispartsensibleandpartlatent.Thesensibleportionoccursastheairtemperatureincreasesbyabsorbingheatfromthewater.Thelatentportionoccursassomeofthewaterevaporates,whichdrawsenergyoutofthewater.
Iftheairentersatthesamewetbulbtemperatureasbefore,butatahigherdry-bulbtemperaturethanthewater,thentheairwillcoolasitsaturates(Figure3b).Thus,thesensiblecoolingcomponentisnegative,andtheallthecoolingisduetoevaporation.Ingeneral,coolingisdominatedbylatentcooling.
Figure2.Psychrometricprocesslinesforairthroughacoolingtower,iftheenteringairtemperatureisa)lessthantheenteringwatertemperature,andb)greaterthantheenteringwatertemperature.
Thetotalcooling,ma(ha2–ha1)isthesameforbothcasessinceenthalpyisafunctionofwet-bulbtemperaturealone.However,thedry-bulbtemperaturesignificantlyinfluencestheevaporationrate,mwe=ma(wa2-wa1).Therateofevaporationincreasesasthedry-bulbtemperatureincreasesforagivenwet-bulbtemperature.
CoolingTowersasHeatExchangers
Basedonthepreviousdiscussion,itisclearthatcoolingtowerperformanceisafunctionofthewet-bulbtemperatureoftheenteringair.Inaninfinitecoolingtower,theleavingairwet-bulbtemperaturewouldapproachtheenteringwatertemperature,andtheleavingwatertemperaturewouldapproachtheweb-bulbtemperatureoftheenteringair.Thedifferencebetweentheleavingwatertemperatureandtheenteringairwet-bulbtemperatureiscalledtheapproach.Therelationshipbetweenairwet-bulbandwatertemperatureisshowninthefigurebelow.Inaninfinitecoolingtower,theapproachwouldbezero.
Source:
ASHRAEHandbook,HVACSystemsandEquipment,2004.
Neglectingfanpowerandassumingsteadystateoperation,anenergybalanceonacoolingtowergives:
mw1cpwTw1–mw2cpwTw2+ma(ha1–ha2)=0
Assumingsteadystateoperation,amassbalanceonwaterflowgives:
mw1–mw2+ma(wa1–wa2)=0
mw2=mw1+ma(wa1–wa2)
Substitutingmw2intotheenergybalancegives:
mw1cpwTw1–[mw1+ma(wa1–wa2)]cpwTw2+ma(ha1–ha2)=0
mw1cpwTw1–mw1cpwTw2-ma(wa1–wa2)cpwTw2+ma(ha1–ha2)=0
Thefractionofincomingwaterthatisevaporated,ma(wa2-wa1)/mw1,istypicallylessthan1%.Thus,ma(wa1–wa2)ismuchlessthanmw1,andthetermma(wa1–wa2)cpwTw2canbeneglectedwithnegligibleerrortogive:
mw1cpw(Tw1–Tw2)=ma(ha2-ha1)
Bothsidesofthisequationrepresentthetotalcoolingcapacityofthetower.
Theeffectiveness,E,ofaheatexchangeristheratiooftheactualtomaximumheattransfer.
E=Qactual/Qmax
Foraheatexchanger,Qmaxoccursiftheairleavesthecoolingtowercompletelysaturatedatthetemperatureoftheincomingwater.Thus,effectivenessis
E=Qactual/Qmax=[mw1cpw(Tw1–Tw2)]/[ma(ha,sat,tw1-ha1)]
EnergyEfficiencyofCounterflowandCrossflowTowers
ThetwomostcommontowerdesignsforHVACapplicationsareforced-aircounterflowandinducedaircross-flow.Coolingtowerenergyuseisafunctionoffanandpumppower.Togeneratethesamequantityofcooling,forced-aircounterflowtowersrequiremorefanandmorepumpenergytheninduced-aircrossflowtowers.Thus,induced-aircrossflowtowersarealmostalwaysmoreenergyefficient.
Forced-aircounterflowtowersrequiremorefanenergybecausecentrifugalfansaremadetogeneratelowflowagainsthighpressure,butcoolingtowersgenerallyneedhighflowatlowpressure.Incomparison,inducedaircrossflowtowersusepropellerfans,whichgeneratehighflowagainstlowpressure,whichismoresuitedtocoolingtowers.
Forced-aircounterflowtowersrequiremorepumpenergybecausethesetowersaretallerinordertofacilitatethecounterflowheattransferasthewaterfallsthroughthetower.Thisheightincreaseselevationheadinthepipingsystem.Inaddition,forced-aircounterflowtowersspraywaterthroughnozzles,whichincreasespressuredrop.Incomparision,induced-aircrossflowtowersareshorterandwidersincethepathoftheairthroughthewaterishorizontal.Inaddition,thesupplywatersimplydrainsfromfeedingpansintofill,whicheliminatestheneedfornozzles.
Acomparisonofcoolingtowerenergyuseforthesameloadsisshownbelow.
ComparisonofF.D.BlowerTowervsI.D.PropellerTowerfor400Tons
Source:
MarleyTechnicalReportH-001A,“CoolingTowerEnergyandItsManagement”,October,1982.
CoolingTowerControl
InHVACapplications,chillerevaporatorloadsvarydependingonweatherandbuildingoccupancy,andthequantityofheatrejectedbythecondenservariesaccordingly.Thecoolingtowerwillalwaysrejectthealltheheatfromthecondenser.However,thetemperatureofthecoldwaterreturntothecondenserwilldeclineatlowerloads.
Variousmethodsareusedtocontrolcoolingtowercapacitytogeneratethedesiredcoldwaterreturntemperature.Thetwocontrolpointsforcoolingtowersarewaterflowandairflow.However,coolingtowermanufacturersstronglyrecommendthatwaterflowremainconstantatalltimes.Thus,primarycontrolmethodsgenerallyrelyonvaryingairflow.Thecommoncontrolmethodsarelistedbelow.
RunFansContinuously
Thistypeofcontrolresultsinthecoldestpossiblereturnwatertemperature,whichreduceschillerenergyuse.However,italsoresultsinthehighestcoolingtowerfanenergyuse.Becausetheimprovementofchillerefficiencywithlowercondenserwatertemperatureisasymptoticalatsomeminimumtemperature,thismethodofcontrolrarelyresultsinthebestoverallenergyefficiency.
CycleFansOnandOff
Thistypeofcontrolreducesexcessfanenergyuseatcoldoutsiderairtemperatures,andiswidelyused.Atrelativelycoldtemperatures,however,thefanmaycycleonandofftoofrequently.Themaximumnumberoffancyclesisabout8perhour.Thus,manycoolingtowersareequippedwithwaterbypassloops.Inmostapplications,waterbypasscontrolisonlyusedatlowtemperatureswhenfancyclingcouldbeaproblem.
UseTwo-SpeedFan
Thismethodofcontroladdsanintermediatelevelofcoolingbetweenfull-onandfull-off.Thisresultsinconsiderablefanenergysavings,sincefanenergyvarieswiththecubeofflow.Thus,fanenergyat50%airflowisonly12%ofthefanenergyatfullairflow.Thistypeofsteppedcontrolcanbefurtherextendedwithtwocelltowerswithonefanineachcell.Thisleadstofourpossiblestepsofcontrol.Atypicalrelationshipbetweencoldwatertemperatureandfanflowisshownbelow.
ContinuouslyControlFanSpeedwithVSD
Thismethodresultsinthelowestfanenergyusebycontinuouslyachievingsavings,duetothefanlawthatfanenergyvarieswiththecubeofflow.
VaryAirFlowUsingInletAirDampers
BeforeVSDs,coolingtowersweresometimescontrolledbyrunningthefanatfullspeedwhilevaryingtheinletairdamperstomodulateairflow.ThismethodofcontrolresultsinintermediateenergysavingsbetweenfancyclingandcontinuousVSDcontrol.However,israrelyusednowthattheVSDcontrolisnowcommonplace.
ComparisonofEnergyUsewithVariousMethodsofCoolingTowerControl
TotalchillerandcoolingtowerenergyuseforthesecontrolmethodsforatypicalHVACapplicationareshownbelow.
ComparativeEnergyUsagewithVariousMethodsofControl
Source:
MarleyTechnicalReportH-001A,“CoolingTowerEnergyandItsManagement”,October,1982.
VariableColdWaterSet-PointTemperature
Theenergyefficiencyofallthecontroldiscussedabovecanbeimprovedbyvaryingthecoldwaterset-pointtemperaturewiththeoutdoorairwetbulbtemperature.Thistypeofcontroltakesintoaccountthefactthattowerscanonlyproducewateratafewdegreesabovethewet-bulbtemperature(thistemperaturedifferenceiscalledthe“approach”);hencefanenergycanbereducedwhenthattemperatureisachieved,sincecontinuedfanoperationresultsinminimalfurtherreductionsincoldwatertemp
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