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发布时间：2023-11-04 02:00:45 来源：文档文库

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冷却塔原理及计算CoolingTowers:DesignandOperationConsiderationsCoolingtowersareaveryimportantpartofmanychemicalplants.Theyrepresentarelativelyinexpensiveanddependablemeansofremovinglowgradeheatfromcoolingwater.Figure1:ClosedLoopCoolingTowerSystemThemake-upwatersourceisusedtoreplenishwaterlosttoevaporation.Hotwaterfromheatexchangersissenttothecoolingtower.Thewaterexitsthecoolingtowerandissentbacktotheexchangersortootherunitsforfurthercooling.TypesofCoolingTowersCoolingtowersfallintotwomainsub-divisions:naturaldraftandmechanicaldraft.Naturaldraftdesignsuseverylargeconcretechimneystointroduceairthroughthemedia.Duetothetremendoussizeofthesetowers(500fthighand400ftindiameteratthebasetheyaregenerallyusedforwaterflowratesabove200,000gal/min.UsuallythesetypesoftowersareonlyusedbyutilitypowerstationsintheUnitedStates.Mechanicaldraftcoolingtowersaremuchmorewidelyused.Thesetowersutilizelargefanstoforceairthroughcirculatedwater.Thewaterfallsdownwardoverfillsurfaceswhichhelpincreasethecontacttimebetweenthewaterandtheair.Thishelpsmaximizeheattransferbetweenthetwo.TypesofMechanicalDraftTowers
Figure2:MechanicalDraftCounterflowTowerFigure3:MechanicalDraftCrossflowTowerMechanicaldrafttowersoffercontrolofcoolingratesintheirfandiameterandspeedofoperation.Thesetowersoftencontainseveralareas(eachwiththeirownfancalledcells.CoolingTowerTheoryHeatistransferredfromwaterdropstothesurroundingairbythetransferofsensibleandlatentheat.Figure4:WaterDropwithInterfacialFilmThismovementofheatcanbemodeledwitharelationknownastheMerkelEquation:(1where:KaV/L=towercharacteristicK=masstransfercoefficient(lbwater/hft2a=contactarea/towervolumeV=activecoolingvolume/planareaL=waterrate(lb/hft2T1=hotwatertemperature(0For0CT2=coldwatertemperature(0For0CT=bulkwatertemperature(0For0Chw=enthalpyofair-watervapormixtureatbulkwatertemperature(J/kgdryairorBtu/lbdryairha=enthalpyofair-watervapormixtureatwetbulbtemperature(J/kgdryairorBtu/lbdryairThermodynamicsalsodictatethattheheatremovedfromthewatermustbeequaltotheheatabsorbedbythesurroundingair:(2(3

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