化学 化工专业 毕业设计论文 冷却塔 外文 英文 文献 翻译.docx
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外文资料
CoolingTowers
Ifachillerisusedtoprovidechilledwaterforbuildingairconditioning,thentheheatenergythatisabsorbedthroughthatprocessmustberejected.Thetwomostcommonwaystorejectthermalenergyfromthevaporcompressionprocessareeitherdirectlytotheairorthroughacoolingtower.Inacoolingtower,waterisrecirculatedandevaporativelycooledthroughdirectcontactheattransferwiththeambientair.Thiscooledwatercanthenbeusedtoabsorbandrejectthethermalenergyfromthecondenserofthechiller.ThemostcommoncoolingtowerusedforHVACapplicationsisthemechanicaldraftcoolingtower(Figure4.2.13).Themechanicaldrafttowerusesoneormorefanstoforceairthroughthetower,aheattransfermediaorfillthatbringstherecirculatedwaterintocontactwiththeair,awaterbasin(sump)tocollecttherecirculatedwater,andawaterdistributionsystemtoensureevendispersalofthewaterintothetowerfill.
Figure4.2.14showstherelationshipbetweentherecirculatingwaterandairastheyinteractinacounterflowcoolingtower.Theevaporativecoolingprocessinvolvessimultaneousheatandmasstransferasthewatercomesintocontactwiththeatmosphericair.Ideally,thewaterdistributionsystemcausesthewatertosplashoratomizeintosmallerdroplets,increasingthesurfaceareaofwateravailableforheattransfer.Theapproachtothewet-bulbisacommonlyusedindicatoroftowersizeandperformance.Itisdefinedasthetemperaturedifferencebetweenthecoolingwaterleavingthetowerandthewet-bulboftheairenteringthetower.Theoretically,thewaterbeingrecirculatedinatowercouldreachthewetbulbtemperature,butthisdoesnotoccurinactualtoweroperations.
山东XX大学外文文献及译文
FIGURE4.2.14Air/watertemperaturerelationshipinacounterflowcoolingtower.
Therangeforachiller/towercombinationisdeterminedbythecondenserthermalloadandthecoolingwaterflowrate,notbythecapacityofthecoolingtower.Therangeisdefinedasthetemperaturedifferencebetweenthewaterenteringthecoolingtowerandthatleaving.Thedriveroftowerperformanceistheambientwet-bulbtemperature.Thelowertheaveragewet-bulbtemperature,the“easier”itisforthetowertoattainthedesiredrange,typically6°C(10°F)forHVACapplications.Thus,inahot,dryclimatetowerscanbesizedsmallerthanthoseinahotandhumidareaforagivenheatload.
Coolingtowersarewidelyusedbecausetheyallowdesignerstoavoidsomecommonproblemswithrejectionofheatfromdifferentprocesses.Theprimaryadvantageofthemechanicaldraftcoolingtowerisitsabilitytocoolwatertowithin3–6°C(5–10°F)oftheambientwet-bulbtemperature.Thismeansmoreefficientoperationoftheconnectedchillingequipmentbecauseofimproved(lower)headpressureoperationwhichisaresultofthelowercondensingwatertemperaturessuppliedfromthetower.
CoolingTowerDesigns
TheASHRAESystemsandEquipmentHandbook(1996)describesover10typesofcoolingtowerdesigns.ThreebasiccoolingtowerdesignsareusedformostcommonHVACapplications.Baseduponairandwaterflowdirectionandlocationofthefans,thesetowerscanbeclassifiedascounterflowinduceddraft,crossflowinduceddraft,andcounterflowforceddraft.
Onecomponentcommontoallcoolingtowersistheheattransferpackingmaterial,orfill,installedbelowthewaterdistributionsystemandintheairpath.Thetwomostcommonfillsaresplashandfilm.Splashfilltendstomaximizethesurfaceareaofwateravailableforheattransfer
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byforcingwatertobreakapartintosmallerdropletsandremainentrainedintheairstreamforalongertime.Successivelayersofstaggeredsplashbarsarearrangedthroughwhichthewaterisdirected.Filmfillachievesthiseffectbyforcingwatertoflowinthinlayersoverdenselypackedfillsheetsthatarearrangedforverticalflow.Towersusingfilmtypefillareusuallymorecompactforagiventhermalload,anadvantageifspaceforthetowersiteislimited.Splashfillisnotassensitivetoairorwaterdistributionproblemsandperformsbetterwherewaterqualityissopoorthatexcessivedepositsinthefillmaterialareaproblem.
CounterflowInducedDraft
Airinacounterflowinduceddraftcoolingtowerisdrawnthroughthetowerbyafanorfanslocatedatthetopofthetower.Theairentersthetoweratlouversinthebaseandthencomesintocontactwithwaterthatisdistributedfrombasinsatthetopofthetower.Thus,therelativedirectionsarecounter(downforthewater,upfortheair)inthisconfiguration.ThisarrangementisshowninFigure4.2.15.Inthisconfiguration,thetemperatureofthewaterdecreasesasitfallsdownthroughthecounterflowingair,andtheairisheatedandhumidified.Dropletsofwaterthatmighthavebeenentrainedinthe