冷却塔原理及计算p

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1、冷却塔原理及计算CoolingTowers:DesignandOperationConsiderations   Coolingtowersareaveryimportantpartofmanychemicalplants. Theyrepresentarelativelyinexpensiveanddependablemeansofremovinglowgradeheatfromcoolingwater.       Figure1:ClosedLoopCoolingTowerSystem  Themake-upwatersourceisusedtoreplenishwaterlost

2、toevaporation. Hotwaterfromheatexchangersissenttothecoolingtower. Thewaterexitsthecoolingtowerandissentbacktotheexchangersortootherunitsforfurthercooling.TypesofCoolingTowers   Coolingtowersfallintotwomainsub-divisions: naturaldraftandmechanicaldraft. Naturaldraftdesignsuseverylargeconcretechimne

3、ystointroduceairthroughthemedia. Duetothetremendoussizeofthesetowers(500fthighand400ftindiameteratthebase)theyaregenerallyusedforwaterflowratesabove200,000gal/min. UsuallythesetypesoftowersareonlyusedbyutilitypowerstationsintheUnitedStates. Mechanicaldraftcoolingtowersaremuchmorewidelyused. These

4、towersutilizelargefanstoforceairthroughcirculatedwater. Thewaterfallsdownwardoverfillsurfaceswhichhelpincreasethecontacttimebetweenthewaterandtheair. Thishelpsmaximizeheattransferbetweenthetwo.TypesofMechanicalDraftTowers  Figure2:MechanicalDraftCounterflowTower               Figure3:MechanicalDr

5、aftCrossflowTower  Mechanicaldrafttowersoffercontrolofcoolingratesintheirfandiameterandspeedofoperation. Thesetowersoftencontainseveralareas(eachwiththeirownfan)calledcells.CoolingTowerTheory  Heatistransferredfromwaterdropstothesurroundingairbythetransferofsensibleandlatentheat.Figure4:WaterDrop

6、withInterfacialFilm  ThismovementofheatcanbemodeledwitharelationknownastheMerkelEquation:                                                                       (1)        where:               KaV/L=towercharacteristic                   K=masstransfercoefficient(lbwater/hft2)                   a=c

7、ontactarea/towervolume                   V=activecoolingvolume/planarea                   L=waterrate(lb/hft2)               T1=hotwatertemperature(0For0C)               T2=coldwatertemperature(0For0C)               T=