流体力学与传热课件evaporation

流体力学与传热课件evaporationEvaporationdiffersfromdistillationinthatthevaporusuallyisasinglecomponent,andevenwhenthevaporisamixture,noattemptismadeintheevaporationsteptoseparatethevaporintofractions.华南理工大学化工学院伍钦ceqwuvaporationdiffersfromcrystallizationinthatemphasisisplacedonconcentratingasolutionratherthanformingandbuildingcrystals.Normally,inevaporationthethickliquoristhevaluableproductandthevaporiscondensedanddiscarded.华南理工大学化工学院伍钦ceqwuiquidcharacteristicsThepracticalsolutionofanevaporationproblemisprofoundlyaffectedbythecharacteroftheliquortobeconcentrated.Someofthemostimportantpropertiesofevaporatingliquidsareasfollows.华南理工大学化工学院伍钦ceqwuoncentrationAstheconcentrationincreases,thesolutionbecomesmoreandmoreindividualistic.Thedensityandviscosityincreasewithsolidcontent.Continuedboilingofasaturatedsolutioncausescrystalstoform;thesemustberemovedorthetubeclog.华南理工大学化工学院伍钦ceqwuheboilingpointofthesolutionmayalsoriseconsiderablyasthesolidcontentincreases,sothattheboilingtemperatureofaconcentratedsolutionmaybemuchhigherthanthatofwateratthesamepressure.华南理工大学化工学院伍钦ceqwuoamingAstablefoamaccompaniesthevaporoutoftheevaporator,causingheavyentrainment.Inextremecasestheentiremassofliquidmayboiloverintothevaporoutletandbelost.华南理工大学化工学院伍钦ceqwuemperaturesensitivityManyfinechemicals,pharmaceuticalproducts,andfoodsaredamagedwhenheatedtomoderatetemperaturesforrelativelyshorttime.华南理工大学化工学院伍钦ceqwucaleSomesolutionsdepositscaleontheheatingsurfaces.Theover-allcoefficientthensteadilydiminishes,untiltheevaporatormustbeshutdownandthetubescleaned.华南理工大学化工学院伍钦ceqwuingle-andmultiple-effectoperationMostevaporatorsareheatedbysteamcondensingonmetaltubes.Nearlyalwaysthematerialtobeevaporatedflowsinsidethetubes.Reducingtheboilingtemperatureoftheliquidincreasingthetemperaturedifferencebetweenthestreamandtheboilingliquidandtherebyincreasestheheat-transferrateintheevaporator华南理工大学化工学院伍钦ceqwuhenasingleevaporatorisused,thevaporfromtheboilingliquidiscondensedanddiscarded,themethodiscalledsingle-effectevaporation.Ifthevaporfromoneevaporatorisfedintothestreamchestofasecondevaporatorandvaporfromthesecondisthensenttoacondenser,theoperationbecomedouble-effect.华南理工大学化工学院伍钦ceqwudditionaleffectscanbeaddedinthesamemanner.Aseriesofevaporatorsbetweenthestreamsupplyandthecondenseriscalledmultiple-effectevaporator.华南理工大学化工学院伍钦ceqwuypesofevaporatorsThechieftypesofstream-heatedtubularevaporatorinusestodayare:Short-tubeevaporatorsLong-tubeverticalevaporatorsforced-circulationUpward-flow(climbing-film)down-ward-flow(falling-film)华南理工大学化工学院伍钦ceqwunce-throughandcirculationevaporatorsEvaporatorsmaybeoperatedeitherasonce-throughorcirculationunits.Inonce-throughoperationthefeedliquorpassesthroughthetubesonlyonce,releasesthevapor,andleavestheunitasthickerliquor.Alltheevaporationisaccomplishedinasinglepass.华南理工大学化工学院伍钦ceqwuncirculationevaporatorsapoolofliquidisheldwithintheequipment.Incomingfeedmixeswiththeliquidfromthepool,andmixturepassesthroughthetubes.UnevaporatedliquiddischargedfromthetubesreturnstothepoolAllshort-tubeandforced-circulationevaporatorsareoperatedinthisway.华南理工大学化工学院伍钦ceqwuhort-tubeevaporatorsIntheoldertypesofevaporatorsthetubesare“short”.Intheshort-tubeverticalevaporatorshowninFigvaporfeedStreaminterconcentratecondensateCentraldowncomer华南理工大学化工学院伍钦ceqwuhestreamcondensesoutsidethetubes.Thetubebundlecontainsalargecentraldowncomer,thecross-sectionalareaofwhichis25to40percentofthetotalcross-sectionalareaofthetubesMostoftheboilingtakesplaceinthesmallertubes,sothattheliquidrisesthroughthesetubesandreturnsthroughthedowncomer.华南理工大学化工学院伍钦ceqwunthisevaporatorthedrivingforceforflowofliquidthroughthetubesisthedifferenceindensitybetweentheliquidinthedowncomerandthemixtureofliquidandvaporinthetubes.Circulationisbynaturalconvectionbutatmuchlessrapidratethaninlong-tubenatural-circulationevaporators;theheat-transfercoefficients,therefore,arefairlyhighwiththinliquidsbutlowwhentheliquidisviscous.华南理工大学化工学院伍钦ceqwuong-tubeevaporatorswithupwardflowAtypicallong-tubeverticalevaporatorwithupwardflowoftheliquidisshowninFig.StreaminfeedVaporoutcondensateConcentrateout华南理工大学化工学院伍钦ceqwuorced-circulationevaporatorsHighercoefficientsareobtainedinforced-circulationevaporators,anexampleofwhichisshowninfig.DeflectorplateVaporoutStreaminCondensate

feedConcentrateout华南理工大学化工学院伍钦ceqwuereacentrifugalpumpforcesliquidthroughthetubesatanenteringvelocityof2to5.5m/sThetubesareundersufficientstaticheadtoensurethatthereisnoboilinginthetubes;theliquidbecomessuperheatedasthestaticheadisreducedduringflowfromtheheatertothevaporspace,andit“flashes”intoamixtureofvaporandsprayinthere华南理工大学化工学院伍钦ceqwuilmevaporatorsConcentrationofhighlyheat-sensitivematerialssuchasorangejuicerequiresaminimumtimeofexposuretoaheatedsurfaceVaporoutStreaminCondensate

FeedinConcentrateoutThiscanbedoneinonce-throughfilmevaporators华南理工大学化工学院伍钦ceqwuherearetwotypesoffilmevaporators:

climbing-filmevaporatorfalling-filmevaporatorStreaminVaporoutFeedinConcentrateoutcondensateThechiefprobleminafall-filmevaporatoristhatofdistributingtheliquiduniformlyasafilminsidetubes.华南理工大学化工学院伍钦ceqwuhisisdonebyinsertsthemetalplateinthetubeendstocausetheliquidtoflowevenlyintoeachtube,orbysprayingthefeedontheinsidesurfaceofeachtubewiththe“spider”distributorsStillanotherwayistouseanindividualspraynozzleinsideeachtube华南理工大学化工学院伍钦ceqwuorgoodheattransfertheReynoldsnumberofthefallingfilmshouldbegreaterthan2000atallpointinthetube.Duringevaporationtheamountofliquidiscontinuouslyreducedasitflowsdownward.Toogreatareductioncanleadtodryspotsnearthebottomofthetube.华南理工大学化工学院伍钦ceqwualling-filmevaporatorscanbeusedinconcentratingsensitiveproducts.Theyarealsowelladaptedtoconcentratingviscousliquids.华南理工大学化工学院伍钦ceqwuerformanceof

tubularevaporatorsTheprincipalmeasuresoftheperformanceofasteam-heatedtubularevaporatorarecapacityandtheeconomyCapacityisdefinedasthenumberofkilogramsOfwatervaporizedperhour.华南理工大学化工学院伍钦ceqwuconomyisthenumberofkilogramsvaporizedperkilogramofstreamfedtotheunit.华南理工大学化工学院伍钦ceqwuvaporatorCapacityTherateofheattransferqthroughtheheatingsurfaceofanevaporatoristheproductofthreefactors:Theareaoftheheat-transfersurfaceATheoverallheat-transfercoefficientUTheoveralltemperaturedropΔt华南理工大学化工学院伍钦ceqwufthefeedtotheevaporatorisattheboilingtemperaturecorrespondingtotheabsolutepressureinthevaporspace.Alltheheattransferredthroughtheheatingsurfaceisavailableforevaporation.Thecapacityisproportionaltoq【16-1】华南理工大学化工学院伍钦ceqwufthefeediscold,theenergyisrequiredforheatingittoitsboilingpoint.Thecapacityforagivenvalueofqisreducedaccordingly,asheatusedtoheatthefeedisnotavailableforevaporation.华南理工大学化工学院伍钦ceqwufthefeedisatthetemperatureabovetheboilingpoint,aportionofthefeedevaporatesspontaneously.Thecapacityisgreaterthanthatcorrespondingtoq.thisprocessiscalled

flashevaporation.华南理工大学化工学院伍钦ceqwuemperaturedifferenceTheactualtemperaturedropacrosstheheatingsurfacedependson:ThesolutionbeingevaporatedThedifferenceinpressurebetweenthestreamchestandthevaporspaceabovetheboilingliquid,anddepthofliquidoverheatingsurface.Thefrictionlossinthetubes华南理工大学化工学院伍钦ceqwunactualevaporators,however,theboilingpointofasolutionisaffectedbytwofactors:TheboilingpointelevationAndliquidhead华南理工大学化工学院伍钦ceqwuoiling-pointelevationandDühring’sruleForagivenpressuretheboilingpointoftheaqueoussolutionsishigherthanthatofpurewater.Theincreaseinboilingpointoverthatofwaterisknownastheboiling-pointelevation(BPE)ofthesolution.华南理工大学化工学院伍钦ceqwuBPE)isbestfoundfromanempiricalruleknownasDühring’srule.Whichstatesthattheboilingpointofagivensolutionisalinearfunctionoftheboilingpointofpurewateratthesamepressure.华南理工大学化工学院伍钦ceqwuftheboilingpointofthesolutionisplottedagainstthatofwateratthesamepressure,astraightlineresults.Differentlinesareobtainedfordifferentconcentrations华南理工大学化工学院伍钦ceqwuxampleIfthepressureovera40%solutionofsodiumhydroxideissuchthatwaterboilsat200ºF(93.3ºC).FindtheBPEforthissolutionatthispressure.华南理工大学化工学院伍钦ceqwueadingupfromthexaxisat200200ºF(93.3ºC)tothelinefor40%solutionandthenhorizontallytotheyaxis,Itisfoundthattheboilingpointofthesolutionis250(121.1ºC)

BPEistherefore50ºF(27.8ºC)

华南理工大学化工学院伍钦ceqwuffectofliquidheadandfrictionontemperaturedropIfthedepthofliquidinanevaporatorisappreciable,theboilingpointcorrespondingtothepressureinthevaporspaceisthatofthesurfacelayerofliquidonly.AtthedistanceZmbelowthesurfaceisunderapressureofthevaporspaceplusaheadofZmofliquid.华南理工大学化工学院伍钦ceqwunanevaporator,therefore,theaverageboilingpointoftheliquidinthetubesishigherthantheboilingpointinthevaporspace.Thisincreaseinboilingpointlowerstheaveragetemperaturedropbetweenthesteamandtheliquidandreducesthecapacity.华南理工大学化工学院伍钦ceqwuhetruetemperaturedrop,correctedforbothboilingelevationandstatichead,isrepresentedbytheaveragetemperaturedropbetweenthesaturationtemperatureofstreamandthevariableliquidtemperature.华南理工大学化工学院伍钦ceqwueat-transfercoefficientTheoverallcoefficientisstronglyinfluencedbythedesignandmethodofoperationoftheevaporator.Inmostevaporatorsthefoulingfactorofthecondensingsteamandresistanceofthetubewallareverysmall,andtheyareusuallyneglected.华南理工大学化工学院伍钦ceqwuteam-filmcoefficientsThesteam-filmcoefficientishigh.Sincethepresenceofnon-condensablegasseriouslyreducesthefilmcoefficient.Provisionmustbemadetoventnoncondensablesfromthesteamchestandtopreventleakageofairinward.华南理工大学化工学院伍钦ceqwuiquid-sidecoefficientsTheliquid-sidecoefficientdependstoalargeextentonthevelocityoftheliquidovertheheatedsurface.Theresistanceoftheliquidsidecontrolstheoverallrateofheattransfertotheboilingliquidforviscousfluid.华南理工大学化工学院伍钦ceqwuorcedcirculationgiveshighliquid-sidecoefficient.Becauseofthedifficultyofmeasuringthehighindividualfilmcoefficientsinanevaporators,experimentalresultsareusuallyexpressedintermsofoverallcoefficients华南理工大学化工学院伍钦ceqwu/p>

Ifoneresistance(say,thatoftheliquidfilm)iscontrolling,largechangesintheotherresistanceshavealmostnoeffectontheoverallcoefficient.Typicaloverallcoefficientsforvarioustypesofevaporatorsaregivenintable华南理工大学化工学院伍钦ceqwuypeOverallcoefficientW/m2ºCLong-tubeverticalevaporatorNaturalcirculation1000-2500Forcecirculation2000-5000华南理工大学化工学院伍钦ceqwuvaporatorEconomyThechieffactorinfluencingtheeconomyofanevaporatorsystemisthenumberofeffects.Theeconomyalsoisinfluencedbythetemperatureofthefeed.Quantitatively,evaporatoreconomyisentirelyamatterofenthalpybalance.华南理工大学化工学院伍钦ceqwunthalpybalancesforsingle-effectevaporatorInsingle-effectevaporator,thelatentheatofcondensationoftheteamistransferredthroughaheatingsurfacetovaporizewaterfromaboilingsolution.Twoenthalpybalancesareneeded,onefortheteamandoneforthevapororliquidside.华南理工大学化工学院伍钦ceqwuig.showsasingle-effectEvaporator.VaporoutStreamincondensatefeedConcentrateoutTherateofsteamandofcondensateismsFeedis

mfand

thatoftheconcentrateismTherateofvaporflowtothecondenserismf-m华南理工大学化工学院伍钦ceqwutisassumedthatthereisnoleakageorentrainment,Thattheflowofnoncondensableisnegligible,andthatheatlossesfromtheevaporatorneednotbeconsidered.华南理工大学化工学院伍钦ceqwuoththesuperheatofthesteamandthesubcoolingofthecondensatearesmall,however,anditisacceptabletoneglecttheminmakinganenthalpybalance.Undertheseassumptionsthedifferencebetweentheenthalpyofthesteamandthatofthecondensateissimplyλs.华南理工大学化工学院伍钦ceqwuheenthalpybalanceforthesteamsideis【16-2】Theenthalpybalancefortheliquorsideis【16-3】Intheabsenceofheatlosses,theheattransferredfromthesteamtothetubesequalsthattransferredfromthetubestotheliquor.华南理工大学化工学院伍钦ceqwuhus,bycombingEqs.【16-2】and【16-3】【16-4】Theliquor-sideenthalpiesdependuponthecharacteristicsofthesolutionbeingconcentrated.mostofsolutionswhenmixedordiluteatconstanttemperaturedonotgivemuchheateffect.华南理工大学化工学院伍钦ceqwuomeofsolutionswhenmixedordiluteevolveconsiderableheateffect.Anequivalentamountofheatisrequired,inadditiontothelatentheatofvaporization,whendilutesolutionsofthesesubstancesareconcentratedtohighdensities.华南理工大学化工学院伍钦ceqwunthalpybalancewithnegligibleheatofdilutionForsolutionshavingnegligibleheatsofdilution,theenthalpybalancesoverasingle-effectevaporatorcanbecalculatedfromthespecificheatsandtemperaturesofthesolutions.华南理工大学化工学院伍钦ceqwuheheat-transferrateqontheliquorsideIncludes:qf,theheattransferredtothethinliquortochangeitstemperaturefromtftotheboilingtemperaturetqv,theheattoaccomplishtheevaporation华南理工大学化工学院伍钦ceqwufthespecificheatofthethinliquorisassumedconstantoverthetemperaturerange,thenand【16-5】【16-6】华南理工大学化工学院伍钦ceqwuftheboiling-pointelevationofthethickliquorisnegligible,λv=λ,thelatentheatofvaporizationofwateratthepressureinthevaporspace.Whentheboiling-pointelevationisappreciable,λv

differsslightlyfromλ.Inpractice,however,itisnearlyalwayssufficientlyaccuratetouseλ.华南理工大学化工学院伍钦ceqwuhefinalequationfortheenthalpybalancecanbegottenfromEqs.【16-5】and【16-6】whentheheatofdilutionisnegligible.【16-7】Equation【16-7】

statesthattheheatfromthecondensingsteamisutilized华南理工大学化工学院伍钦ceqwuovaporizewaterfromthesolutionToheatthefeedtotheboilingpointIfthefeedentersabovetheboilingpointintheevaporator,partoftheevaporationisfromflash.华南理工大学化工学院伍钦ceqwunthalpybalancewithappreciableheatofdilutionIftheheatofdilutionoftheliquorbeingconcentratedistoolargetobeneglected,anenthalpy-concentrationdiagramisusedforthevaluesofHinEq.【16-4】华南理工大学化工学院伍钦ceqwuigureisanenthalpy-concentrationdiagramforsolutionofsodiumhydroxideandwater.华南理工大学化工学院伍钦ceqwuingle-effectcalculationsTheuseofmaterialbalances,enthalpybalances,andthecapacityequation【16-1】inthedesignofsingle-effectevaporationisshowninexample16.1华南理工大学化工学院伍钦ceqwuultiple-effectevaporatorsFigureshowsatriple-effectsystem.Steaminfeed华南理工大学化工学院伍钦ceqwuonnectionsaremadesothatthevaporfromoneeffectservesastheheatingmediumforthenext.Acondenserandairejectorestablishavacuuminthirdeffectintheseriesandwithdrawnoncondensablesfromthesystem.华南理工大学化工学院伍钦ceqwuhefirsteffectintheseriesistheeffecttowhichtherawsteamisfedandinwhichthepressureinthevaporspace.Thelasteffectisthatinwhichthevapor-spacepressureisminimum.Inthismannerthepressuredifferencebetweenthesteamandthecondenserisspreadacrosstwoormoreeffectsinthemultiple-effectsystem.华南理工大学化工学院伍钦ceqwuhepressureineacheffectislowerthanthatintheeffectfromwhichitreceivessteamandhigherthanthatoftheeffecttowhichitsuppliesvapor.Eacheffecthasatemperaturedropacrossitsheatingsurfacecorrespondingtothepressuredropinthateffect.华南理工大学化工学院伍钦ceqwunfigure,dilutefeedentersthefirsteffect,whereitispartlyconcentrated;Itflowstothesecondeffectforadditionalconcentrationandthentothethirdeffectforfinalconcentration.Thickliquorispumpedoutofthethirdeffect.华南理工大学化工学院伍钦ceqwunsteadyoperationallinternalconcentrations,flowrates,pressures,andtemperaturesarekeptconstant.Theheatingsurfaceinthefirsteffectwilltransmitperhouranamountofheatgivenbytheequation【16-8】华