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化工原理
PrinciplesofChemicalIndustry化工原理Capacityandeconomyof
multiple-effectevaporatorsCapacityandeconomyof
multiCapacityandeconomyof
multiple-effectevaporatorsTheincreaseineconomythroughtheuseofmultiple-effectevaporationisobtainedatthecostofreducedcapacity.Capacityandeconomyof
multi
Itmaybethoughtthatbyprovidingseveraltimesasmuchheatingsurfacetheevaporatingwouldbeincreased,butitisnotthecase.Itmaybethoughtthatbypr
Thetotalcapacityofamultiple-effectevaporatorisusuallynotgreaterthanthatofasingle-effectevaporatorhavingaheatingsurfaceequaltooneoftheeffectsandoperatingunderthesameterminalcondition.ThetotalcapacityofamulIftheheatingloadandtheheatofdilutionareneglected,thecapacityofanevaporatoris
directlyproportionaltotherateofheattransfer.
(16-13)IftheheatingloadandtheThetotalcapacityisproportionaltothetotalrateofhearttransferqT
(16-14)Thetotalcapacityisproporti
AssumethatthesurfaceareaisAineacheffectandthattheoverallcoefficientUisalsothesameineacheffect.Then(16-15)Assumethatthesurfaceare
Δtisthetotaltemperaturedropbetweenthesteaminthefirsteffectandthevaporinthelasteffect.Δtisthetotaltemperature
Supposenowthatasingle-effectevaporatorwithasurfaceareaAisoperatingwiththesametotaltemperaturedrop.Supposenowthatasingle-efIftheoverallcoefficientisthesameasineacheffectofthetriple-effectevaporator.ForthesingleeffectIftheoverallcoefficientisThisisexactlythesameequationasthatforthemultiple-effectevaporatorTheboiling-pointelevationtendstomakethecapacityofthemultiple-effectevaporatorlessthanthatofthecorrespondingsingleeffect.Thisisexactlythesameequat
Offsettingthisarethechangesinoverallcoefficientsinamultiple-effectevaporator.Offsettingthisarethechan
Theaveragecoefficientforthemultiple-effectevaporatorwouldbegreaterthanthatforthesingle-effect.TheaveragecoefficientfEffectofliquidheadandboiling-pointelevation
Theliquidheadandtheboiling-pointelevationinfluencethecapacityofamultiple-effectevaporatorevenmorethantheydothatofasingleeffectEffectofliquidheadandThereductionincapacitycausedbytheliquidhead,asbefore,cannotbeestimatedquantitatively.Theliquidheadreducesthetemperaturedropavailableineacheffectofamultiple-effectofamultiple-effectevaporator.ThereductionincapacitycausThetemperaturedropinanyeffectiscalculatedfromthetemperatureofsaturatedsteamatthepressureofthesteamchest,andnotfromthetemperatureoftheboilingliquidinthepreviouseffect.ThetemperaturedropinanyefThismeansthattheboiling-pointelevationinanyeffectislostfromthetotalavailabletemperaturedrop.Thismeansthattheboiling-poThislossoccursineveryeffectofamultiple-effectevaporator,andtheresultinglossofcapacity.ThislossoccursineveryeConsiderthesingle-effectevaporator.Ofthetotaltemperaturedropof181℃,theshadedpartrepresentsthelossintemperaturedrop105℃Theactualdrivingforceforheattransferisrepresentedbytheunshadedpart.Considerthesingle-effecteva105ºtemperature100º176º281º105ºtemperature100º176º281ºThediagramforthedouble-effectevaporatorshowstwoshadedportionsbecausethereisaboiling-pointelevationinthetwoeffect.Theresidualunshadedpart,85º,issmallerthaninthediagramforthesingleeffect.Thediagramforthedouble-eff50º100º281º176º35º226º246º105ºtemperature100º176º281º50º100º281º176º35º226º246º105ºInthetriple-effectevaporatorthereareshadedportionssincethereisalosstemperaturedropineachofthreeeffects,andthetotalnetavailabletemperaturedrop,79℃Inthetriple-effectevapor105ºtemperature100º176º281º50º100º281º176º35º226º246º105ºtemperature100º176º281º50º
SubstitutionfromEq.(16-2)intoEq.(16-8)gives
(16-16)SubstitutionfromEq.(16-2Theeconomyofamultiple-effectevaporatorisnotinfluencedbyboiling-pointelevationsifminorfactors,suchasthetemperatureofthefeedandchangesinheatsofevaporization,areneglected.ThenbyEqs.(16-16)Theeconomyofamultiple-effeAkilogramofsteamcondensinginthefirsteffectgeneratesaboutakilogramofvapor,whichcondensesinthesecondeffect,generatinganotherkilogramthere,andsoon.AkilogramofsteamcondensTheeconomyofamultiple-effect,evaporatordependingonheat-balanceconsiderationsandnotontherateheattransfer.Thecapacity,isreducedbytheboiling-pointelevationTheeconomyofamultiple-effeOptimumnumberofeffectsThecostofeacheffectofanevaporatorpersquaremeterofsurfaceisafunctionofitstotalarea.OptimumnumberofeffectsThecTheinvestmentrequiredforanN-effectevaporatorisaboutNtimesthatforasingle-effectevaporatorofthesamecapacity.TheinvestmentrequiredforTheoptimumnumberofeffectsmustbefoundfromaneconomicbalancebetweenthesavingsinsteamobtainedbymultiple-effectoperationandtheaddedinvestmentrequired.Theoptimumnumberofeffect化工原理英文教材多效蒸发能力与经济性Capacityandeconomyofmultipleeffectevaporators课件化工原理英文教材多效蒸发能力与经济性Capacityandeconomyofmultipleeffectevaporators课件化工原理
PrinciplesofChemicalIndustry化工原理Capacityandeconomyof
multiple-effectevaporatorsCapacityandeconomyof
multiCapacityandeconomyof
multiple-effectevaporatorsTheincreaseineconomythroughtheuseofmultiple-effectevaporationisobtainedatthecostofreducedcapacity.Capacityandeconomyof
multi
Itmaybethoughtthatbyprovidingseveraltimesasmuchheatingsurfacetheevaporatingwouldbeincreased,butitisnotthecase.Itmaybethoughtthatbypr
Thetotalcapacityofamultiple-effectevaporatorisusuallynotgreaterthanthatofasingle-effectevaporatorhavingaheatingsurfaceequaltooneoftheeffectsandoperatingunderthesameterminalcondition.ThetotalcapacityofamulIftheheatingloadandtheheatofdilutionareneglected,thecapacityofanevaporatoris
directlyproportionaltotherateofheattransfer.
(16-13)IftheheatingloadandtheThetotalcapacityisproportionaltothetotalrateofhearttransferqT
(16-14)Thetotalcapacityisproporti
AssumethatthesurfaceareaisAineacheffectandthattheoverallcoefficientUisalsothesameineacheffect.Then(16-15)Assumethatthesurfaceare
Δtisthetotaltemperaturedropbetweenthesteaminthefirsteffectandthevaporinthelasteffect.Δtisthetotaltemperature
Supposenowthatasingle-effectevaporatorwithasurfaceareaAisoperatingwiththesametotaltemperaturedrop.Supposenowthatasingle-efIftheoverallcoefficientisthesameasineacheffectofthetriple-effectevaporator.ForthesingleeffectIftheoverallcoefficientisThisisexactlythesameequationasthatforthemultiple-effectevaporatorTheboiling-pointelevationtendstomakethecapacityofthemultiple-effectevaporatorlessthanthatofthecorrespondingsingleeffect.Thisisexactlythesameequat
Offsettingthisarethechangesinoverallcoefficientsinamultiple-effectevaporator.Offsettingthisarethechan
Theaveragecoefficientforthemultiple-effectevaporatorwouldbegreaterthanthatforthesingle-effect.TheaveragecoefficientfEffectofliquidheadandboiling-pointelevation
Theliquidheadandtheboiling-pointelevationinfluencethecapacityofamultiple-effectevaporatorevenmorethantheydothatofasingleeffectEffectofliquidheadandThereductionincapacitycausedbytheliquidhead,asbefore,cannotbeestimatedquantitatively.Theliquidheadreducesthetemperaturedropavailableineacheffectofamultiple-effectofamultiple-effectevaporator.ThereductionincapacitycausThetemperaturedropinanyeffectiscalculatedfromthetemperatureofsaturatedsteamatthepressureofthesteamchest,andnotfromthetemperatureoftheboilingliquidinthepreviouseffect.ThetemperaturedropinanyefThismeansthattheboiling-pointelevationinanyeffectislostfromthetotalavailabletemperaturedrop.Thismeansthattheboiling-poThislossoccursineveryeffectofamultiple-effectevaporator,andtheresultinglossofcapacity.ThislossoccursineveryeConsiderthesingle-effectevaporator.Ofthetotaltemperaturedropof181℃,theshadedpartrepresentsthelossintemperaturedrop105℃Theactualdrivingforceforheattransferisrepresentedbytheunshadedpart.Considerthesingle-effecteva105ºtemperature100º176º281º105ºtemperature100º176º281ºThediagramforthedouble-effectevaporatorshowstwoshadedportionsbecausethereisaboiling-pointelevationinthetwoeffect.Theresidualunshadedpart,85º,issmallerthaninthediagramforthesingleeffect.Thediagramforthedouble-eff50º100º281º176º35º226º246º105ºtemperature100º176º281º50º100º281º176º35º226º246º105ºInthetriple-effectevaporatorthereareshadedportionssincethereisalosstemperaturedropineachofthreeeffects,andthetotalnetavailabletemperaturedrop,79℃Inthetriple-effectevapor105ºtemperature100º176º281º50º100º281º176º35º226º246º105ºtemperature100º176º281º50º
SubstitutionfromEq.(16-2)intoEq.(16-8)gives
(16-16)SubstitutionfromEq.(16-2Theeconomyofamultiple-effectevaporatorisnotinfluencedbyboiling-pointelevationsifminorfactors,suchasthetemperatureofthefeedandchangesi
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