外文文献及译文secret.docx

外文文献及译文secret.docx

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外文文献及译文secret

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.

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.Splashfilltendstomaximizethesurfaceareaofwateravailableforheattransferbyforcingwatertobreakapartintosmallerdropletsandremainentrainedintheairstreamforalongertime.Successivelayersofstaggeredsplashbarsarearrangedthroughwhichthewaterisdirected.Filmfillachievesthiseffectbyforcingwatertoflowinthinlayersoverdenselypackedfillsheetsthatarearrangedforverticalflow.Towersusingfilmtypefillareusuallymorecompactforagiventhermalload,anadvantageifspaceforthetowersiteislimited.Splashfillisnotassensitivetoairorwaterdistributionproblemsandperformsbetterwherewaterqualityissopoorthatexcessivedepositsinthefillmaterialareaproblem.

CounterflowInducedDraft

Airinacounterflowinduceddraftcoolingtowerisdrawnthroughthetowerbyafanorfanslocatedatthetopofthetower.Theairentersthetoweratlouversinthebaseandthencomesintocontactwithwaterthatisdistributedfrombasinsatthetopofthetower.Thus,therelativedirectionsarecounter（downforthewater,upfortheair）inthisconfiguration.ThisarrangementisshowninFigure4.2.15.Inthisconfiguration,thetemperatureofthewaterdecreasesasitfallsdownthroughthecounterflowingair,andtheairisheatedandhumidified.Dropletsofwaterthatmighthavebeenentrainedintheairstreamarecaughtatthedrifteliminatorsandreturnedtothesump.Airandsomecarryoverdropletsareejectedthroughthefansandoutthetopofthetower.Thewaterthathasbeencooledcollectsinthesumpandispumpedbacktothecondenser.

FIGURE4.2.15Counterflowinduceddraftcoolingtower.

Counterflowtowersgenerallyhavebetterperformancethancrossflowtypesbecauseoftheevenairdistributionthroughthetowerfillmaterial.Thesetowersalsoejectairathighervelocitieswhichreducesproblemswithexhaustairrecirculationintothetower.However,thesetowersarealsosomewhattallerthancrossflowtypesandthusrequiremorecondenserpumphead.

CrossflowInducedDraft

Asinthecounterflowcoolingtower,thefaninthecrossflowtowerislocatedatthetopoftheunit（Figure4.2.16）.Airentersthetoweratsideorendlouversandmoveshorizontallythroughthetowerfill.Waterisdistributedfromthetopofthetowerwhereitisdirectedintothefillandiscooledbydirectcontactheattransferwiththeairincrossflow（airhorizontalandwaterdown）.Watercollectedinthesumpispumpedbacktothechillercondenser.Theincreased

airflowpossiblewiththecrossflowtowerallowsthesetowerstohaveamuchloweroverallheight.Thisresultsinlowerpumpheadrequiredonthecondenserwaterpumpcomparedtothecounterflowtower.Thereducedheightalsoincreasesthepossibilityofrecirculatingtheexhaustairfromthetopofthetowerbackintothesideorendairintakeswhichcanreducethetower’seffectiveness.

CounterflowForcedDraft

Counterflowforceddraftcoolingtowershavethefanmountedatornearthebottomoftheunitneartheairintakes（Figure4.2.17）.Asintheothertowers,waterisdistributeddownthroughthetoweranditsfill,andthroughdirectcontactwithatmosphericairitiscooled.Thermaloperationofthistowerissimilartothecounterflowinduceddraftcoolingtower.Fanvibrationisnotassevereforthisarrangementcomparedtoinduceddrafttowers.Thereisalsosomeadditionalevaporativecoolingbenefitbecausethefandischargesairdirectlyacrossthesumpwhichfurthercoolsthewater.Therearesomedisadvantagestothistower.First,theairdistributionthroughthefillisuneven,whichreducestowereffectiveness.Second,thereisriskofexhaustairrecirculationbecauseofthehighsuctionvelocityatthefaninlets,whichcanreducetowereffectiveness.Thesetowersfindapplicationsinsmallandmedium-sizedsystems.

Materials

Coolingtowersoperateinacontinuouslywetconditionthatrequiresconstructionmaterialstomeetchallengingcriteria.Besidesthewetconditions,recirculatingwatercouldhaveahighconcentrationofmineralsaltsduetotheevaporationprocess.Coolingtowermanufacturersbuildtheirunitsfromacombinationofmaterialsthatprovidethebestcombinationofcorrosionresistanceandcost.Woodisatraditionalmaterialusedincoolingtowerconstruction.Redwoodorfirareoftenusedandareusuallypressuretreatedwithpreservativechemicals.Chemicalssuchaschromatedcopperarsenateoracidcopperchromatehelppreventdecayduetofungiordestructionbytermites.

FIGURE4.2.16Crossflowinduceddraftcoolingtower.

FIGURE4.2.17Counterflowforceddraftcoolingtower.

Galvanizedsteeliscommonlyusedforsmall-tomid-sizedcoolingtowerstructures.Hardwareisusuallymadeofbrassorbronze.Criticalcomponents,suchasdriveshafts,hardwaremountingpoints,etc.,maybemadefrom302or304stainlesssteel.Castironcanbefoundinbasecastings,motorhousings,andfanhubs.Metalscoatedwithplasticsarefindingapplicationforspecialcomponents.

Manymanufacturersmakeextensiveuseoffiberglass-reinforcedplastic（FRP）intheirstructure,pipe,fanblades,casing,inletlouvers,andconnectioncomponents.Polyvinylchloride（PVC）isusedforfillmedia,drifteliminators,andlouvers.Fillbarsandfloworificesarecommonlyinjectionmoldedfrompolypropyleneandacrylonitrilebutadienestyrene（ABS）.

Concreteisnormallyusedforthewaterbasinorsumpoffielderectedtowers.Tilesormasonryareusedinspecialtytowerswhenaestheticsareimportant.

Performance

Rejectionoftheheatloadproducedatthechillingequipmentistheprimarygoalofacoolingtowersystem.Thisheatrejectioncanbeaccomplishedwithanoptimizedsystemthatminimizesthetotalcompressorpowerrequirementsofthechillerandthetowerloadssuchasthefansandcondenserpumps.Severalcriteriamustbedeterminedbeforethedesignercancompleteathoroughcoolingtoweranalysis,includingselectionoftowerrange,water-to-airratio,approach,filltypeandconfiguration,andwaterdistributionsystem.Table4.2.6listssomeofthecommondesigncriteriaandnormallyacceptedrangesforcoolingtowers.

MostcommonHVACapplicationsrequiringacoolingtowerwillusean“off