制冷技术英文版Ch1109064.docx
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制冷技术英文版Ch1109064.docx
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制冷技术英文版Ch1109064
Chapter11CondensingEquipment
11-1)FunctionandClassificationofCondensers
(1)FunctionofCondensers
Theheat-transferprocessinacondenseroccursinthreestagesasimplicatedinFig.11-1.
1.Desuperheatingofthehotvapor.
2.Condensingofthevaportoliquidstateandreleaseofthelatentheat.
3.Subcoolingoftheliquidrefrigerant.
Fig.11-1,Thestateandtemperaturevariationinsideacondenser
(2)ClassificationofCondensers
Fig.11-2Classificationofcondensers[1]
11-2)Water-CooledCondensers
Typesofwater-cooledcondenserscanbeclassifiedaccordingtotheirconstruction(Fig.11-2).Eachhasfeaturesthatmakeitsuitableforcertainapplication.
(1)Double-pipecondenser
Fig.11-3,Double-pipewater-cooledcondenser
Fig.11-4Circularandtrombonedouble-tubecondenser[11]
Fig.11-5Doublepipecondenserwithcleanabletubes
(2)Shell-and-coilcondenser壳盘管冷凝器
Fig.11-6Shell-and-coilcondenser(verticaltype)[1]
(3)Shell-and-tubecondenser壳管冷凝器
Theshell-and-tubecondenserconsistsofacylindricalmetalshellandanumberofstraighttubeswhicharearrangedinparallelandheldinplaceattheendsbytubesheets.Therearetwomaintypesofshell-and-tubecondensers:
horizontalandvertical.
a)Horizontalshell-and-tubecondenser
Fig.11-7Two-passshellandtubecondenser
b)Verticalshell-and-tubecondensers
Fig.11-10Verticalshell-and-tubecondenser
(4)Heattransferinwatercooledcondenser
Thefollowingequationisusedtocalculateheattransferinwatercooledcondenser.
(11-1)
where
:
Heattransferrate,kW.
:
overallheattransfercoefficient,kW/m2K
:
Heattransferarea,m2
:
meaneffectivetemperaturedifference(METD),K
Theoverallheattransfercoefficient:
(11-2)
where
:
thermalresistanceinrefrigerantside
:
thermalresistanceoftubewall
:
thermalresistanceofwaterfouling
:
thermalresistanceinwaterside
:
ratioofoutsidetoinsidetubesurfacearea
Sincethetemperatureofcoolingmediaisconstantlychanginginacondenser,thetemperaturedifferencebetweenthetherefrigerantandthecoolingmediaisnotconstant,asseeninFig.11-11.AmeantemperaturedifferencebetweenthemmustbedeterminedforEquation11-1.IthasbeenfoundthattheMeanEffectiveTemperatureDifference(METD)isthebestwaytorepresentthismeantemperaturedifference,whichcanbeobtainedfromthefollowingequation:
(11-3)
where
:
temperaturedifferenceatoneendofthecondenser,K
:
temperaturedifferenceatotherendofthecondenser,K
Fig.11-11Temperatureprofileinacondenser
AswecanfindfromEquation11-1,increasing
i.e.,METD,isoneofthewaystoenhanceheattransferforthecondensers.Themeantemperature,
canbeincreasedbytheflowarrangementinthecondensers.Counterflowandparallelflowsaretwocommonarrangementsappliedtoshell-and-tubeheatexchangers.Foragivensetofrequiredconditions,thecounterflowarrangmentgivesagreaterMETDthantheparallelflowarrangementdoes.However,theMETDisaffectedbytheflowarrangementwhentherefrigerantiscondensedfromsaturatedvaportosaturatedliquidbecausethethetemperatureoftherefrigerantisconstantinthisprocess(Fig.11-11).
Inordertoincreasetheheattransferperformanceofcondensers,theoverallthermalresistance,
shouldbeassmallaspossible.OfthefourelementsinvolvedinEquation11-2,thethermalresistanceofmetaltubewallresistanceRwistheleastsignificantandcanbeignored.usually.Thus,thekofacondenserisdeterminedprimarilybytheotherthreefactorsandusuallythefoulingresistanceandtheliquidfilmresistancearethemostsignificant.Table11-1comparesthethermalresistancesinawatercooledcondenserwiththeammoniaasrefrigerant.
Table11-1Thermalresistancesinawatercooledcondenser
Thermalresistancetype
Heattransfercoefficient(
)
Thermalresistance
(
)
Condensationofrefrigerant
10602
9.4×10-5
Lubricationoilfilm*
2500~3333
3~4×10-4
Thermalconduction(steeltube)
18182
5.5×10-5
Foulingofwater
2674
3.74×10-4
Convectionofwater
4759
2.10×10-4
Total
923.5
10.83×10-4
*:
Thisisspecialforammonia.Someoftherefrigerantsdonothavethistypeofresistancebecausethelubricationoilcandissolveintheserefrigerants.
11-3)CoolingTower冷却塔
(1)TypesofCoolingTowers
Fig.11-13MechanicalDraftCoolingTower[13]
(2)CoolingTowerPerformance
Fig.11-14Rangeandapproachofcoolingtowers
(11-4)
(3)Make-upwaterandpurging(补充水和排污)
Incoolingtowers,evaporationandwaterdropletscarriedawaybytheairflowarethetwocausesofwaterloss.Also,waterinthetowerbasinshouldberegularlyremovedtopreventtheaccumulationofthedissolvedsolids.Forthesereasons,itisessentialtosupplyenoughmake-upwatertothecoolingtower.
Theevaporatedwaterdoesnotcarryawaythesolids,sothattheconcentrationofthesolidswouldreachalevelatwhichtheywouldprecipitateoutofthewateronsurfacesasacoating,orscale.Thiswillreducetheeffectivenessofthecondenser.
Fig.11-15Make-upandpurgingofacoolingtower
(11-5)
Andthemake-upwaterhasthustobe:
(11-9)
11-4)Air-CooledCondensers
Fig11-16,Air-cooledcondenser(naturaldrafttypewithwirefins)[15]
Fig.11-17Typicalforceddraftaircondenserarrangement
11-5)EvaporativeCondensers蒸发式冷凝器
Fig.11-18Schematicdiagramofevaporativecondenser
References
1.DossatR.J.,PrinciplesofRefrigeration,ThirdEdition,Prentice-Hall,Inc.1991.
2.WangS.K.HandbookofAirconditioning&Refrigeration,McGraw-Hill,1994.
3.GosneyM.W.,PrinciplesofRefrigeration,CambridgeUniversityPress,1982
4.Source:
Perry’sChemicalEngineersHandbook(Page:
12-17),McGraw-Hill,1997
5.BureauofEnergyEfficiency,MinistryofPower,India.CoolingTowers.In:
Energy
6.EfficiencyinElectricalUtilities.Chapter7,pg135-151.Edward,2004
7.ZhuP.G.Theprincipleandcalculationofheatexchangers.TsinghuaPub.Comp.,Beijing.1987.7
8.WuY.Z.Theprincipleandequipmentofrefrigeration.XianJiaotongPress.,Xian,1998.1
9.WuY.Z.Designofrefrigerationsystem.MechanicalPress.Beijing,1988
10.ZhengX.D.Theprincipleandequipmentofrefrigeration.MechanicalPress.Beijing,1988
11.
12.
13.http:
//www.geo4va.vt.edu/A2/A2.htm
14.
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