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CONDENSATION

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CONDENSATION

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  1. CONDENSATION UNIT -III

  2. UNIT 3-CONDENSATION HEAT AND MASS TRANSFER PREPARED BY K.MADHANKUMAR AP/ MECH TAGORE INSTITUTE OF ENGINEERING AND TECHNOLOGY

  3. CONTENTS INTRODUCTION DROPWISEAND FILMWISECONDENSATION PROCESSAPPLICATION VERTICALCONDENSATION HORIZONTALCONDENSATION REFERENCES 1/3/2017 2

  4. INTRODUCTION Processesofheattransfer havingphasechangearecomplexthansimple exchangebetweenfluids.Itusuallyinvolvesheatchangesatconstantornearly constanttemperature. When a vapor is exposed to a surface at a temperature below Tsat , condensationintheformofaliquidfilmorindividualdropletsoccursonthe surface. HEATTRANSFERFROMCONDENSINGVAPORS: Condensation of vapors on tube surfaces is cooler than condensing temperature of vapor for those such as water, hydrocarbon, and volatile substancesareprocessedwherelatentheatofvaporizationisremovedfor condensationtooccur.Heattransferoccursbetweenvaporandsurface condensationoccurs. and Condensingvaporconsistsofsinglesubstance,amixtureofcondensable noncondensable,ora mixtureoftwo ormorecondensablevapors. and 3

  5. Friction losses are very small hence condensation is a constant pressure process. Thecondensingtemperatureofasinglepuresubstancedependsonpressure andhence theprocessisisothermalandcondensateisapureliquid.Mixed vaporscondenseatconstantpressureprocessover widerangeoftemperature havingvariablecondensate. Examplesof condensationare: 1) 2) condensationofwaterfroma mixtureofsteamand air. Recovery of hydrocarbon solvents from air streams leaving extractions/dryingprocess. Condensationoccursintwo distinctmechanismatdifferentrates: 1) 2) Dropwisecondensation Film-typecondensation 4

  6. TYPESOFCONDENSATION: FILM-TYPECONDENSATION: Heretheliquidcondensateformsa filmora continuouslayer,ofliquidthatflows overthesurfaceoftubeundertheactionofgravity. Itis a layerofliquidinterposedbetweenthevaporandwall oftubethatservesas resistancetoheattransferandfixestheheattransfercoefficient. DROPWISECONDENSATION: Dropwisecondensation,characterizedby countlessdropletsofvaryingdiameters on thecondensingsurfaceinsteadofa continuousliquidfilm,isoneofthemost effectivemechanismsofheattransfer,andextremelylargeheattransfer coefficientscan beachievedwiththismechanism. 5

  7. I I I Liquid film ta>Film :b)Dropwise condensation coodenseison I I I I I ------_;._ --- 6

  8. The two mechanisms are different from each other and independent of the quantityofvaporcondensing: Filmwisecondensationhasgreaterrapidityatwhichthecondensateformson thetube.Duetotheresistanceofthecondensatefilmtoheatpassingthroughit theheattransfercoefficientfordropwisecondensationare4-8timesmorefor filmwise condensation. Drop wise condensation is seen in ethylene, glycol, glycerin, nitrobenzene, isoheptaneandsomeorganicvapors.Liquidmetalsusually wisemanner.Muchoftheexperimentalworkisbeenondrop ofsteamand few conclusionsare: condenseindrop wisecondensation •Filmtypecondensationofwateroccursontubesofcommon steamandtubeareclean,inpresenceorabsenceofairoron surface. metalsifboththe roughorpolished •Dropwisecondensationisonlyattainablewhencoolingsurfaceisnotwettedby theliquid.Usuallycondensationofsteamiscontaminatedbyoildroplets. 7

  9. •The quantity of contaminant required to contaminate vapor is minute and apparentlya monomolecularfilmisnecessary. •Effective drop promoters are strongly adsorbed by the surface, and the substances that merely prevent wetting are ineffective. Some promoters are effectiveonsomemetalssuchasmercaptansoncopperalloys. •Theaveragecoefficientobtainableinpurewisecondensation maybehighas 115kW/m20C(20000Btu/ft2. oF 8

  10. PROCESSAPPLICATIONS: •Mainlyinindustriesseparationofliquidmixturesarecarriedoutbydistillation where the compounds with lower boiling points are distilled off in pure conditionfromthosehavinghigherboilingpoints.Amixtureofsolutionhaving severalcompoundsexertsapartialpressurewherethemostvolatilecompounds cannot be boiled of from the rest without carrying some higher boiling compoundswith it. •Ifthevaporcomingoffiscondensed,havinglowerboilingpointthanoriginal solution will indicate increase in the proportion of volatile compounds. By successively and boiling quantity of distillation boilingoffpartofaliquid mixture,condensingthe it is possible to obtain vaporformed, a nearly pure separation by off the is a part of condensate, volatile compound by repetitions . Thus the accomplished by partial vaporization and subsequent condensation. •Continuousdistillationrequiresthepresenceofliquidatalltimesontheplates, sovaporsofthelessvolatilecompoundsinthefeedmaybecondensedand carrieddownward.Alwaysa volatilecompoundisused. 9

  11. In the power industry the term surface condenser is reserved for tubular equipmentwhichcondensessteamfromtheexhaustofturbinesandengines.The turbine is designed to obtain the mechanical work from heat; the maximum conversionisobtainedinthe turbineby maintaininga low-dischargetemperature. 10

  12. NUSSELTSTHEORY: Incondensationonaverticalsurfaceafilmofcondensateisformedandfurther condensationandheattransfertothesurfaceoccursbyconductionthrough filmwhich isassumedtobelaminarflowdownward. the Thethicknessofthisfilmgreatlyinfluencestherateofcondensation,since heataccompanyingtheremovalofvaporsfromthevaporphaseencounters condensatefilmasa resistance. the the Thethicknessofthefilmisthefunctionofvelocityofdrainagewhich varieswith thedeviationof thesurfacefromtheverticalposition. For verticalsurfacesthethicknessofthefilmincreasesfromtoptobottom.That iswhythecondensingcoefficientforavaporcondensingonverticalsurface decreasesfromtoptobottom,andtheheightofthecondensershouldn’t enoughto attainlargecondensingcoefficient. belarge Forallliquidstheviscositydecreasesasthetemperatureincreases, condensingcoefficientincreaseswiththecondensatetemperature. andthe 11

  13. FollowingassumptionsareassumedforNusselt’sequation: •The heatdeliveredbythe vaporislatentheatonly. •Theflowislaminar for condensatefilm,andtheheatistransferredthroughthe filmby conduction. •Thethicknessofthefilmatanypointisafunctionof meanvelocityofflowand ofthe amountofcondensatepassingat thatpoint. •Thevelocityoftheindividuallayersofthefilmisafunctionoftherelation betweenfrictionalshearingforceandthe weight ofthefilm. •Quantityofthecondensateisproportionaltothequantityofheattransferred, relatedtothethicknessofthefilm,temperaturedifferencebetweenvapor, surface. •The condensatefilmisso thinthattemperaturegradientislinear. •Thetemperatureofthesurfaceofthesolidisconstant.Thecurvatureofthe filmisneglected. 12

  14. C NDENSATIONONVERTICALSURFACE: O The heatisassumedtoflowthroughthecondensatefilmsolelybyconduction, andthelocalcoefficientHXisthereforegivenby: = kf 8 h Whereδ islocalfilmthickness. x Filmthicknessistypically2 to3 ordersofthemagnitudesmallerthanthetube diameter.Henceit canbe foundoutforflowinsideand outsidetube,fromthe equationfora flatplatewhichis: 0 = ( 23µIr P gcosp )1/3 Whereℾisthecondensateloading,themassrateperunitlengthofperiphery. Substitutingforδgivesthelocalheat-transfercoefficient,atadistanceLfromthe topofthe verticalsurface,theequation 1 3

  15. !J.T 0 = dL JI'1/3d!I' lrb r (p2g )3lLT The averagecoefficienth fortheentiretubeisdefinesas: Solvingfor∆Togives: Substituting∆Tointoh gives: Nowintegratingthisequationgives: 1 h ['t/3 d!I'= bkt t dL 1/3/2017 o Lr 311,II' o 1 4

  16. quationforh becomes: Ii(k:'t2 )~= 1.47(4II'b)-1/3 So e Thus the average coefficent for a vertical tube, provided that flow condensatefilmislaminar,is4/3timesthelocalcoefficientatthebottom tube. Now arrangingtheequationgives, 2 fPtD l't 1/3/2017 Thus the average coefficent for a vertical tube, provided that flow in the the condensate film is laminar, is 4/3 times the local coefficient at the bottom of 1 5

  17. C ONDENSATIONONHORIZONTALTUBES: For horizontal condensation we use equations corresponding tubes,thefollowingequationsapplytosinglehorizontaltube: for vertical (4['')-1/3 µ, 1 ( µ7 )3_ h 1. 51 k3p2g ff - And, 16 1/3/2017

  18. REFERENCES •ProcessheattransferbyDonaldkern •Unitoperationsofchemicalengineeringby McCabeandSmith •MasstransferoperationsbyTreybal •Condensationby PrabhalThakurIITDelhi,mechanicaldepartment 1/3/2017 17

  19. THANKYOU! 1/3/2017 18