外文资料翻译空冷热交换器和空冷塔.docx

外文资料翻译空冷热交换器和空冷塔.docx

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外文资料翻译空冷热交换器和空冷塔

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空冷热交换器和空冷塔

二〇一三年五月

Air-cooledHeatExchangersandCoolingTowers

D.G.KROGERSc.D.（MIT）

（ThistextisapartofMRKROGER'sbook.include8.4,9.3,9.4）

8.4RECIRCULATION

Heatedplumeairmayrecirculateinanair-cooledheatexchanger,therebyreducingthecoolingeffectivenessofthesystem.Figure8.4.1depicts,schematically,across-sectionofanair-cooledheatexchanger.Intheabsenceofwind,thebuoyantjetorplumerisesverticallyabovetheheatexchanger.Apartofthewarmplumeairmayhoweverbedrawnbackintotheinletofthetower.Thisphenomenonisknownas"recirculation".Plumerecirculationisusuallyavariablephenomenoninfluencedbymanyfactors,includingheatexchangerconfigurationandorientation,surroundingstructuresandprevailingweatherconditions.Becauseofhigherdischargevelocities,recirculationisusuallylessininduceddraftthaninforceddraftdesigns.

Figure8.4.1:

Air-flowpatternaboutforceddraftair-cooledheatexchanger.

Lichtenstein[51LI1]definesarecirculationfactoras

（8.4.1）

wheremristherecirculatingairmassflowrate,whilemaistheambientairflowrateintotheheatexchanger.

Althoughtheresultsofnumerousstudiesonrecirculationdoappearintheliterature,mostareexperimentalinvestigationsperformedonheatexchangershavingspecificgeometriesandoperatingunderprescribedconditionse.g.[74KE1,81SL1].GunterandShipes[72GUlldefinecertainrecirculationflowlimitsandpresenttheresultsoffieldtestsperformedonair-cooledheatexchangers.Problemsassociatedwithsolvingrecirculatingflowpatternsnumericallyhavebeenreported[81EP1].Krogeretal.investigatedtheproblemanalytically,experimentallyandnumericallyandrecommendaspecificequationwithwhichtheperformanceeffectivenessofessentiallytwo-dimensionalmechanicaldraftheatexchangersexperiencingrecirculation,canbepredicted[88KR1,89KR1,91DU1,93DU1,95DU1].

8.4.1RECIRCULATIONANALYSIS

Consideronehalfofatwo-dimensionalmechanicaldraftair-cooledheatexchangerinwhichrecirculationoccurs.Forpurposesofanalysis,theheatexchangerisrepresentedbyastraightlineatanelevationHiabovegroundlevelasshowninfigure8.4.2（a）.

Figure8.4.2:

Flowpatternaboutheatexchanger.

Itisassumedthatthevelocityoftheairenteringtheheatexchangeralongitsperipheryisinthehorizontaldirectionandhasameanvalue,vi（theactualinletvelocityishighestattheedgeofthefanplatformanddecreasestowardsgroundlevel）.Theoutletvelocity,vo,isassumedtobeuniformandintheverticaldirection.

Considertheparticularstreamlineattheoutletoftheheatexchangerthatdivergesfromtheplumeat1andformstheouter"boundary"oftherecirculatingairstream.Thisstreamlinewillentertheplatformat2,somedistanceHrbelowtheheatexchanger.Forpurposesofanalysisitwillbeassumedthattheelevationof1isapproximatelyHrabovetheheatexchanger.Ifviscouseffects,mixingandheattransfertotheambientairareneglected,Bernoulli'sequationcanbeappliedbetween1and2togive

（8.4.2）

ItisreasonabletoassumethatthetotalpressureatIisapproximatelyequaltothestagnationpressureoftheambientairatthatelevationi.e.

（8.4.3）

At2thestaticpressurecanbeexpressedas

（8.4.4）

Furthermore,fortheambientairfarfromtheheatexchanger

（8.4.5）

Substituteequations（8.4.3）,（8.4.4）and（8.4.5）intoequation（8.4.2）andfind

（8.4.6）

DuetoviscouseffectsthevelocityattheinletatelevationHiisinpracticeequaltozero.TheVelocitygradientinthisimmediateregionishoweververysteepandthevelocitypeaksatavaluethatishigherthanthemeaninletvelocity.Examplesofnumericallydeterminedinletvelocitydistributionsfordifferentoutletvelocitiesandheatexchangergeometriesareshowninfigure8.4.3[95DU1].Sincemostoftherecirculationoccursinthisregionthevelocityv2isofimportancebutdifficulttoquantifyanalytically.For

itwillbeassumedthatv2canbereplacedapproximatelybythemeaninletvelocity,vi,inequation（8.4.6）.Thus

（8.4.7）

Figure8.4.3:

Two-dimensionalinletvelocitydistributionforWi/2=5.1m.

Accordingtotheequationofmassconservation,theflowperunitdepthofthetowercanbeexpressedas

（8.4.8）

iftheamountofrecirculationissmall.

Accordingtoequations（8.4.1）and（8.4.8）therecirculationfactoris

（8.4.9）

Substituteequations（8.4.7）and（8.4.8）intoequation（8.4.9）andfind

（8.4.10）

where

istheFroudenumberbasedonthewidthoftheheatexchanger.

Theinfluenceofawindwallordeepplenumcanbedeterminedapproximatelybyconsideringflowconditionsbetweenthetopofthewindwall,（Hi+Hw）,asshowninfigure8.4.2（b）andelevationHi.ConsidertheextremecasewhenHwissolarge（Hw=Hwo）thatnorecirculationtakesplaceandtheambientairvelocitynearthetopofthewindwalliszero.Inthisparticularcasethestaticpressureatthetowerexitisessentiallyequaltotheambientstagnationpressure.Withtheseassumptions,applyBernoulli'sequationbetweenthetoweroutletatthetopofthewindwallandtheelevationHi.

（8.4.11）

But

（8.4.12）

Substituteequation（8.4.12）intoequation（8.4.11）andfind

（8.4.13）

Ifitisassumedthattherecirculationdecreasesapproximatelylinearlywithincreasingwindwallheight,equation（8,4.10）maybeextendedasfollows:

（8.4.14）

SincetherecirculationisassumedtobeessentiallyzeroatHw=Hwo,finda=1.

Substituteequation（8.4.13）intoequation（8.4.14）andfind

（8.4.15）

where

isthedensimetricFroudenumberbasedonthewindwallheight.

Itisimportanttodeterminetheeffectivenessofthesystemwhenrecirculationoccurs.Effectivenessinthiscase,isdefinedas

（8.4.16）

Theinterrelationbetweentherecirculationandtheeffectivenessiscomplexinarealheatexchanger.Twoextremescanhoweverbeevaluatedanalyticallyi.e.

1.Nomixing

Thewarmrecirculatingairdoesnotmixatallwiththecoldambientinflow,resultinginatemperaturedistributionasshowninfigure8.4.4（a）.Therecirculatingstreamassumesthetemperatureoftheheatexchangerfluid

.

Figure8.4.4:

Recirculationflowpatterns.

Thisineffectmeansthatthepartoftheheatexchangerwhererecirculationoccurs,transfersnoheat.Theactualheattransferrateisthusgivenby

（8.4.17）

resultinginaneffectivenessduetorecirculationof

（8.4.18）

Substituteequation（8.4.15）intoequation（8.4.18）andfind

（8.4.19）

2.Perfectmixing

Therecirculatingairmixesperfectlywiththeinflowingambientair,resultinginauniformincreaseinboththeeffectiveinletairtemperatureandtheoutletairtemperatureasshowninfigure8.4.4（b）.

Ifforpurposeofillustration,itisassumedthatthetemperatureoftheheatexchanger,

isconstant,itfollowsfromequation（3.5.22）thattheeffectivenessundercross-flowconditionsis

（8.4.20）

or

（8.4.21）

Furthermoretheenthalpyenteringtheheatexchangeris

or

（8.4.22）

Substituteequation（8.4.22）intoequation（8.4.21）andfind

（8.4.23）

Inthiscasetheeffectivenessduetorecircuiationisgivenby

Fromequation（8.4.22）and（8.4.23）,substitutethevaluesofTirandTorintothisequation,tofindtheeffectivenessoftheheatexchanger.

（8.4.24）

Inpracticetheeffectivenesswillbesomevaluebetweenthatgivenbyequation（8.4.18）andequation（8.4.24）.Actualmeasurementsconductedonair-cooledheatexchangersappeartosuggestthatrelativelylittlemixingoccurs.Thistendencyisconfirmedbynumericalanalysisoftheproblem[89KR1,95DU1].

Figure8.4.5:

Heatexchangereffectiveness.

DuvenhageandKroger[95DU1]solvedtherecirculationproblemnumericallyandcorrelatedtheirresultsoverawiderangeofoperatingconditionsandheatexchangergeometriesbymeansofthefollowingempiricalequation:

（8.4.25）

Thisequationisvalidinthe

and

where

.Inthisequation

representstheeffectiveheightabovetheinlettothefanplatformandincludestheplenumheightinadditiontoanywindwallheight.

Equation（8.4.25）isshowngraphicallyinfigure8.4.5.Forvaluesof

equation（8.4.19）isingoodagreementwithequation（8.4.25）.

8.4.2MEASURINGRECIRCULATION

Intheabsenceofwindwalls,recirculationcanbesignificantresultinginacorrespondingreductioninheattransfereffectiveness.Asshowninfigure8.4.6,smokegeneratedatthelowerendoutletofanA-frametypeforceddraftair-cooledheatexchangerwithoutwindwalls,isdrawndirectlydownwardsintothelowpressureregioncreatedbythefans.TheresultsofrecirculationtestsconductedattheMarimbapowerplantarereportedbyConradieandKroger[89CO1].Theyactuallymeasuredtheverticaltemperaturedistributionoftheairenteringtheheatexchangerandobservedarelativelyhighertemperatureinthevicinityofthefanplatform.Asshownbythesmoketrailinfigure8.4.7recirculationoftheplumeairoccursinthisregionBecauseoftheapproximately10mhighwindwallsurroundingthearrayofA-frameheatexchangerbundles,areductionineffectivenessoflessthatonepercentisexperiencedundernormaloperatingconditionsintheabsenceofwind.Theeffectivenesscanbedeterminedaccordingtoequation（8.4.25）.

Figure8.4.6:

Plumeairrecirculatinginair-cooledsteamcondenser.

Figure8.4.7:

VisualizationofrecirculationwithsmokeattheMatimbapowerplant.

Generallylessrecirculationoccursininduceddraftcoolingsystemsduetotherelativelyhighfanoutletvelocityandheightofd