冷却塔原理及计算

冷却塔原理及计算CoolingTowers:DesignandOperationConsiderations

Coolingtowersareaveryimportantpartofmanychemicalplants.

Theyrepresentarelativelyinexpensiveanddependablemeansofremovinglowgradeheatfromcoolingwater.Figure1:ClosedLoopCoolingTowerSystemThemake-upwatersourceisusedtoreplenishwaterlosttoevaporation.

Hotwaterfromheatexchangersissenttothecoolingtower.

Thewaterexitsthecoolingtowerandissentbacktotheexchangersortootherunitsforfurthercooling.TypesofCoolingTowers

Coolingtowersfallintotwomainsub-divisions:

naturaldraftandmechanicaldraft.

Naturaldraftdesignsuseverylargeconcretechimneystointroduceairthroughthemedia.

Duetothetremendoussizeofthesetowers(500fthighand400ftindiameteratthebase)theyaregenerallyusedforwaterflowratesabove200,000gal/min.

UsuallythesetypesoftowersareonlyusedbyutilitypowerstationsintheUnitedStates.

Mechanicaldraftcoolingtowersaremuchmorewidelyused.

Thesetowersutilizelargefanstoforceairthroughcirculatedwater.

Thewaterfallsdownwardoverfillsurfaceswhichhelpincreasethecontacttimebetweenthewaterandtheair.

Thishelpsmaximizeheattransferbetweenthetwo.TypesofMechanicalDraftTowersFigure2:MechanicalDraftCounterflowTower

Figure3:MechanicalDraftCrossflowTowerMechanicaldrafttowersoffercontrolofcoolingratesintheirfandiameterandspeedofoperation.

Thesetowersoftencontainseveralareas(eachwiththeirownfan)calledcells.CoolingTowerTheory

Heatistransferredfromwaterdropstothesurroundingairbythetransferofsensibleandlatentheat.Figure4:WaterDropwithInterfacialFilmThismovementofheatcanbemodeledwitharelationknownastheMerkelEquation:

(1)

where:

KaV/L=towercharacteristic

K=masstransfercoefficient(lbwater/hft2)

a=contactarea/towervolume

V=activecoolingvolume/planarea

L=waterrate(lb/hft2)

T1=hotwatertemperature(0For0C)

T2=coldwatertemperature(0For0C)

T=bulkwatertemperature(0For0C)

hw=enthalpyofair-watervapormixtureatbulkwatertemperature

(J/kgdryairorBtu/lbdryair)

ha=enthalpyofair-watervapormixtureatwetbulbtemperature

(J/kgdryairorBtu/lbdryair)

Thermodynamicsalsodictatethattheheatremovedfromthewatermustbeequaltotheheatabsorbedbythesurroundingair:

(2)

(3)where:

L/G=liquidtogasmassflowratio(lb/lborkg/kg)

T1=hotwatertemperature(0For0C)

T2=coldwatertemperature(0For0C)

h2=enthalpyofair-watervapormixtureatexhaustwet-bulbtemperature(sameunitsasabove)

h1=enthalpyofair-watervapormixtureatinletwet-bulbtemperature(sameunitsasabove)ThetowercharacteristicvaluecanbecalculatedbysolvingEquation1withtheChebyshevnumbericalmethod:

(4)Figure5:GraphicalRepresentationofTowerCharacteristicThefollowingrepresentsakeytoFigure5:C'=Enteringairenthalpyatwet-bulbtemperature,TwbBC=InitialenthalpydrivingforceCD=AiroperatinglinewithslopeL/GDEF=Projectingtheexitingairpointontothewateroperatinglineandthenontothe

temperatureaxisshowstheoutletairweb-bulbtemperature

AsshownbyEquation1,byfindingtheareabetweenABCDinFigure5,onecanfindthetowercharacteristic.

AnincreaseinheatloadwouldhavethefollowingeffectsonthediagraminFigure5:1.

IncreaseinthelengthoflineCD,andaCDlineshifttotheright2.

Increasesinhotandcoldwatertemperatures3.

Increasesinrangeandapproachareas

Theincreasedheatloadcausesthehotwatertemperaturetoincreaseconsiderablyfasterthandoesthecoldwatertemperature.

AlthoughtheareaABCDshouldremainconstant,itactuallydecreasesabout2%forevery100Fincreaseinhotwatertemperatureabove1000F.

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