Facts at finger tips

Heat transfer

"Y2EBEKKAH-ARSHALL 

Modesofheattransfer L Cuc

nmclu

Bc

Toevaluatetemperatures:

(Internalcoil,isothermalheatingmedium)

Asingleplanewall: 

k 

T 1

T 2 q 

Bccl (Internalcoil,nonisothermalcoolingmedium)

L  k 2 T 2

k 1

T 1 Compositewalls:



ovllcc

Cylindricalwall:



q  T 1 T 2

Cvc

c 



L 2

L 1

q  heatexChangers T 3

T 

k 

Lmmpuc

r 1 r 2

Countercurrentflow: 

T b 

h

Cocurrentflow: 



Fora straightfin:



 A 

L 

hccv (σ)

or

r(mbyby) σ

=5.67 5.6710 –8W/(m2ŠK4) 0.173 10 –8Btu/(hŠft2Š°R4)

Forabody(1)thatissmallin comparisontoitssurroundings(2):

heattransferinaBatCh hl(bl hl( bl) )

heattransferfLUids h pmc • Highsp Highspecifi ecificheat cheat • Highthermal conductivity • Highde Highdensity nsity • Lowvis Lowviscosit cosity y

A surfacearea surfacearea A c ccross-sectionalarea   A avg  avg  averageofinsideandoutsidearea A iiinsideareaoftubes A o  o outsideareaoftubes c c c coolantspecificheat    coolantspecificheat  C C   heatcapacityrateofthecoldfluid C  heatcapacityrateofthecoldfluid C H  heatcapacityrateofthehotfluid H heatcapacityrateofthehotfluid C min minsmallerofCCandCH c p  p heatcapacityatconstantpressure F  configurationcorrectionfactor  configurationcorrectionfactor  hi heattransfercoefficient,insidetubes heattransfercoefficient,insidetubes ho heattransfercoefficient,outsidetubes heattransfercoefficient,outsidetubes k  thermalconductivity  thermalconductivity  L wallthickness(orcylinderlength) wallthickness(orcylinderlength) mmass m coolantflowrate p exposedperimeter  exposedperimeter  Q heattransferred heattransferred R resistance resistance R fi  foulingresistance,insidetube fi foulingresistance,insidetube R fo   foulingresistance,outsidetube fo  foulingresistance,outsidetube t tube-wallthickness tube-wallthickness T Absolutetemperature(Kor°R) Absolutetemperature(Kor°R) T b  b temperatureatbaseoffin T b1 b1initialbatchtemperature T b2  b2 finalbatchtemperature T Ci  inlettemperatureofthecold Ci inlettemperatureofthecold fluid(orcoolant) T Co   outlettemperatureofthecoldfluid Co  outlettemperatureofthecoldfluid T Hi   inlet(orinitial)temperatureofthehot Hi  inlet(orinitial)temperatureofthehot fluid T Ho   outlettemperatureofthehotfluid Ho  outlettemperatureofthehotfluid T Mheating-mediumtemperature heating-mediumtemperature T w  walltemperature w  T ∞bulkfluidtemperature U  overallheattransfercoefficientbased overallheattransfercoefficientbased onarea A   A andthelogmeantemandthelogmeantemperaturedifference logmeantemperaturedifference ∆T lm lmlogmeantemperaturedifference emissivityofthebody  εemissivityofthebody  ∈heatexchangereffectiveness heatexchangereffectiveness λ  heatofphasechange(vaporization, heatofphasechange(vaporization, condensation,melting,fusionasthe casemaybe) θ ti time me

Herearedesirablepropertiestolookforwhen makingyourheattransferfluidselection:

sy

op

• Lowvap Lowvaporpress orpressure ure • Highboi Highboilingpo lingpoint int • Highfl Highflashpoi ashpoint nt • Highautoignition temperature • Lowreactivitywiththe productbeingheated • Lowtox Lowtoxicity icity

• Abilitytobepumpedat ambienttemperatures • Goodthermalstability • Reasonablefluidprice • Lowflu Lowfluiddegra iddegradatio dationrate nrate • Noncorrosivetomaterials ofconstruction • Lowsensitivitytooxidation • Lowpres Lowpressuredro suredrop p