Condensation and its type.pdf

Condensation-Definition, types of condensation, Laminar condensation over a flat vertical plate, Effect of non-condensable gases

1. Unit-5 Boiling and Condensation
Prepared by:
Ankur Sachdeva
Assistant Professor, ME

2. Condensation
•Thecondensationisaphasechangeprocessfrom
vapourtoliquid.
•Itoccurswhenthevapourstrikesasurfacewhich
isattemperature(T
s)belowthevapour
saturationtemperature(T
sat),thevapourreleases
itslatentheatandimmediatelyconvertsinto
liquidphase.
•Thecondensationmayoccurintwopossibleways
dependingontheconditionofthesurface:
–Filmwisecondensation,and
–Dropwisecondensation

3. Types of Condensation

4. Filmwise Condensation
•Ifthecondensationtakesplace
continuouslyoverasurfacecooled
bysomeprocessandthe
condensatefilmcoversentire
condensingsurfaceandfallsdown
undertheactionofgravity,the
situationiscalledfilmwise
condensation.
•Thepresenceofcondensatelayer
actsasaresistancetoheattransfer
betweenvapourandsurface.
•Thisresistanceincreaseswith
condensatethickness,which
increasesintheflowdirection,
henceitisdesirabletouseshort
verticalsurfacesofhorizontal
cylindersinsituationsinvolvingfilm
condensation.

5. Filmwise Condensation
•Theliquidfilmstartsformingatthetopofthe
plateandflowsdownwardundertheinfluenceof
gravity.
•Thethicknessofthefilmincreasesintheflow
directionxbecauseofcontinuedcondensationat
theliquid–vaporinterface.
•Heatintheamounth
fg(thelatentheatof
vaporization)isreleasedduringcondensationand
istransferredthroughthefilmtotheplate
surfaceattemperatureT
s

6. Velocity and Temperature Profiles in
Filmwise Condensation

7. Dropwise Condensation
•Indropwisecondensation,the
condensedvaporformsdropletson
thesurfaceinsteadofacontinuous
film.
•Dropwisecondensationcanoccur
whenthesurfaceisnon-wettingor
thesedropletsaretakenawayfrom
thesurfacebyexternalfloworby
gravity.
•Thevapourisindirectcontactwith
thesurfaceovermostoftheareaand
heattransferratesaremuchhigher
(morethan3–10timeshigher)as
thereisverylittleresistanceforheat
flowbetweenthevapourandthe
surface.
•Thedropletsdevelopatnucleation
sites(pointsofsurfaceimperfections
suchaspit,scratchandcavities),and
growinsizeasmorevapour
condensesonitsexposedsurface

8. Dropwise Condensation
•Dropwisecondensation,characterizedbycountless
dropletsofvaryingdiametersonthecondensing
surfaceinsteadofacontinuousliquidfilm,isoneof
themosteffectivemechanismsofheattransfer,and
extremelylargeheattransfercoefficientscanbe
achievedwiththismechanism.
•Indropwisecondensation,thesmalldropletsthat
formatthenucleationsitesonthesurfacegrowasa
resultofcontinuedcondensation,coalesceintolarge
droplets,andslidedownwhentheyreachacertain
size,clearingthesurfaceandexposingittovapor.
•Thereisnoliquidfilminthiscasetoresistheat
transfer.
•Asaresult,withdropwisecondensation,heat
transfercoefficientscanbeachievedthataremore
than10timeslargerthanthoseassociatedwithfilm
condensation.
•Largeheattransfercoefficientsenabledesignersto
achieveaspecifiedheattransferratewithasmaller
surfacearea

9. Laminar Condensation on a Vertical
Flat Plate
Governing Equation

10. Heat Transfer Relations for
Laminar Condensation
Vertical Plate
•Mass flow rate of the condensate,
•Thickness of condensate film,
•Local Heat Transfer Coefficient, =
•Average Heat Transfer Coefficient,

11. Heat Transfer Relations for
Laminar Condensation
Horizontal Tube (Outside surface):
•Average Heat Transfer Coefficient
Horizontal Tube (Inside surface):
•Average Heat Transfer Coefficient
–(Low Vapour Velocity)

12. Heat Transfer Relations for
Laminar Condensation
Horizontal Tube (Inside surface):
•Average Heat Transfer Coefficient
–(High Vapour Velocity)
Mass Velocity of Vapour and Liquid

13. Effect of Non-Condensable Gases
•Mostcondensersusedinsteampowerplants
operateatpressureswellbelowtheatmospheric
pressure(usuallyunder0.1atm)tomaximize
cyclethermalefficiency.
•Operationatsuchlowpressuresraisesthe
possibilityofair(anon-condensablegas)leaking
intothecondensers.
•Evensmallamountsofanon-condensablegasin
thevaporcausesignificantdropsinheattransfer
coefficientduringcondensation

14. Effect of Non-Condensable Gases
•Whenthevapormixedwitha
non-condensable gas
condenses,onlythenon-
condensablegasremainsinthe
vicinityofthesurface.
•Thisgaslayeractsasabarrier
betweenthevaporandthe
surface,andmakesitdifficult
forthevaportoreachthe
surface.
•Thevapornowmustdiffuse
throughthenon-condensable
gasfirstbeforereachingthe
surface,andthisreducesthe
effectivenessof the
condensationprocess