传热基本原理

ThermalCalculationsOverview Contents Conduction convectionandradiationStreamcoefficientsOverallcoefficientAveragevaluesforwholeexchangerHowTASCworksTASCmodes Q UA T Heatflow HeatgoesfromhottocoldbyConductionConvectionRadiation Conduction Heattransferratepredictedbywhere wisthethermalconductivityofthewall Metalwall say Thot Tcold yw Typicalvaluesofthermalconductivity Convection withconduction Inthemodelthebulkandwalltemps arethesametheslopeatthewallisthesameTheheatfluxisgivenbyor Streamheattransfercoefficient isthestreamcoefficientsometimesreferredtoasthefilmcoefficient typicalvaluesFluidState W m2 KWaterSinglePhase5000 7500WaterBoiling 5Bar3000 10000SteamCondensing1Bar10000 15000OrganicSinglePhase0 5 2 5cp750 1500OrganicBoiling0 5 2 5cp1000 3500OrganicCondensing0 5 2 5cp1500 4000Gas1Bar80 125Gas10Bar250 400 Radiation TransferofheatbetweenobjectswithouttheuseofanintermediatefluidStefan Boltzmann sLaw HeattransferQ A T4 To4 Stefan Boltzmannconstant5 67x10 8W m2K4 Whendesigningshellandtubeheatexchangersradiationisnotnormallyconsidered Overallcoefficient U Wehavethermalresistancesinserieswhere Thot Tcold Fouling Wemustaddtheresistancesduetothefouling oftencalledfoulingfactors Thot Tcold Typicalfoulingresistance Fluidstreamrm2K WSeawaterbelow50oC0 0001Treatedcoolingwaterbelow50oC0 0002Treatedcoolingwaterabove50oC0 0003Untreatedwater0 0005 0 0008Fueloil0 0008Crudeoil0 0003 0 0010Refrigerantliquid0 0002Steam oilfree 0 0001Compressedair0 0003Naturalgas0 0002 Overallcoefficientfortube UdefinedonthebasisoftheareaoftheoutsideofthetubeMustcorrectforsmallerinsidearea hencewhere di do Localandmeanvalues Overall meansfromthehotsidetothecoldsideincludingallresistancesHoweveritisstillataparticularpointintheexchanger i e itislocalHenceyoucanhavealocal overallcoefficientLOCALLYFORWHOLEEXCHANGER Integratingovertheexchangerarea LocalequationRearrangingandintegrating dQ dA TotalareaAT Definitionsofmeanvalues FrompreviousslidesComparingthetwosides Meanstreamcoefficients Whenrequired theseshouldbeaveragedasfollows Alternativeform SpecialcasewhereTsarelinearwithQ Typicalofsingle phaseduties withconstantspecificheats Parallelflow usuallycountercurrentEqn integratestogivelog meantemperaturedifference LMTD Ta Tb Q Temperature DerivationofLMTD ThedefinitionofMTDChangingvariablesIntegratingRe arranging Whenthetemp linesarenotstraight Divideintostraight linezonesandcalc LMTDforeachzone Then Q Temp 2 3 4 5 1 Calculatingmeanoverallcoefficient DivideintozonesoverwhichUdoesnotvarygreatly usuallysamezonesasforMTDcalc TakeUforzoneas Ua Ub 2whereUaandUbarethevaluesattheendsofthezoneCalculatetheareaofeachzonefromA Q U TLMHenceforwholeexchanger Multipassexchangers Forsingle phaseduties theoreticalcorrectionfactors FT havebeenderivedFTvaluesarelessthan1DonotdesignforFTlessthan0 8 Q Temp T1 T2 t1 t2 Note modernsoftwaredoesnotusethesechartsbutdoesthedetailedintegration TypicalFTcorrectionfactorcurves T t Shell tubeside1 2 inlet outlet FTproblems TASCdoesnotusethesecorrectionfactorsItusesarigorousstep by stepcalculationHowever youshouldnotethatthecurvesaresteepforlowFT makingthedesignsensitivetoprocessconditions HowTASChandles2passes WehaveforthetwopassesdhI UI T tI dA 2dhII UII T tII dA 2Dividing Hin tout hout A dA tin hin Hin Meantube sitetemperature WecanaveragethetemperaturesbetweenthepassesThetwopassexchangerthenappearslikeaone passexchangerHenceuseexistingzonemethod A Temp T tII tI tav Morethan2passes Guessinter passtemp IntegratetobackendofexchangerTemps mustmeetatbackendRefineguesstilltheydoThencalc tube sideaveragetemp asbefore A Temp Mustguesstemp heretostartcalc Linesmustmeethere ZoninginTASC TASCusesintelligentmethodstodividetheexchangerintotherequiredzoneswithwell chosenboundaries Temp Cross flowcalculation TASCincludessimilarrigorousmethodsforsolvingcross flowexchangers TEMAXtype CHECKING thebasicTASCcalc Startingpointfullyspecifiedgeometry hencetheAactualknowntherequiredheatduty Qreqtheprocessinletandoutletconditions implicitinabove theallowablepressuredropsonthetwosides DpallowMainresultsofcalculationthecalculatedheattransferareatodotheduty Acalcthecalculatedstreampressuredrops Dpcalc TheCHECKINGresults Arearatio Aactual AcalcThepressuredropratioforeachstream Dpcalc DpallowDetailedperformanceinformationincludingthe resistancediagram Stream Fouling Wall Resistancediagram continued Alsoshowstheareaattributabletoeachresistanceandthedistributionoftemperaturesfromhottocoldstream Heattransferarea Temperature Ts Tt Tw CHECKING or RATING SomepeoplesoftwarecompaniesusethewordRATINGwherewesayCHECKINGTheyarethesamethingCHECKINGisansweringthequestionswillthisexchangerhandlethespecifieddutyandhowmuchunderorover surfacedoesithave SIMULATION ForexchangerwithfullyspecifiedgeometryCalculatestheoutletconditionsofthetwostreamsfromtheirinletconditionsThereforeanswersthequestion Howwillthisexchangerperform Set Set Geometryset Calc Calc DESIGN SetRequiredduty inletandoutletconditions MaximumallowablepressuredropSomeotheroptionalandpracticalconstraintsCalculates whatisthebestexchangergeometrytoachievetheduty YouhaveCHECKINGtyperesultsforthebestandalternativedesigns THERMOSIPHONOftenspeltTHERMOSYPHON Handlesthenatural circulationcalculationsinreboilers THERMOSYPHON cont UserfullyspecifiesreboilergeometryincludingtheinletandoutletpipingUserspecifiesthepressureinthecolumn theheightoftheliquidinthecolumn andtheinletandoutletconditionoftheheatingfluidTASCcalculatestheactualdutyQandthecoldstreamflowrateduetonaturalcirculationdrivenbytheliquidheadinthecolumnTASCalsocalculatesthehotstreamflow ratetomeetspecifiedoutletconditionsNote Fixedflowoptionalsoavailable Commentsonmodes CHECKINGQuickandeasytousebutResultsmaynotbeveryaccurateiftheexchangerisgreatlyunder oversurfacedSIMULATIONResultsareatr