HEAT EXCHANGERS. Associate Professor. IIT Delhi Mech/IITD

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1 HEAT EXHANGES Prabal Talukdar Assoate Professor Departent of Meanal Engneerng IIT Del E-al:

2 Heat Exangers Heat exangers are deves tat faltate te exange of eat between two fluds tat are at dfferent teperatures wle keepng te fro xng wt ea oter Heat exangers are oonly used n prate n a wde range of applatons, fro eatng and ar ondtonng systes n a ouseold, to eal proessng and power produton n large plants

3 Types of Heat Exangers Double ppe Heat exanger

4 opat Heat Exanger Te rato of te eat transfer surfae area of a eat exanger to ts volue s alled te area densty β A eat exanger wt β / 3 (or 200 ft 2 /ft 3 ) s lassfed as beng opat Exaples of opat eat exangers are ar radators ( / 3 ), glass era gas turbne eat exangers ( / 3 ), te regenerator of a Strlng engne ( 5,000 2 / 3 ), and te uan lung ( 20, / 3 ) opat eat exangers are oonly used n gas-to-gas and gas-to-lqud d (or lqud-to-gas) d )eat exangers to ounterat te low eat transfer oeffent assoated wt gas flow wt nreased surfae area

5 ross Flow Heat Exanger In opat eat exangers, te two fluds usually ove perpendular to ea oter, and su flow onfguraton s alled ross-flow Tere are two types of ross flow eat exangers (a) Unxed and (b) Mxed flow

6 Sell-and-Tube Heat Ex Sell-and-tube eat exangers ontan a large nuber of tubes (soetes several undred) paked n a sell wt ter axes parallel to tat of te sell Baffles are oonly plaed n te sell to fore te sell-sde flud to flow aross te sell to enane eat transfer and to antan unfor spang between te tubes

7 Multpass flow arrangeent n sell-and-tube eat exangers

8 Overall Heat Transfer oeff Overall Heat Transfer oeff A πd ) D D ln( A o πd o kl 2 ) D ln(d o wall π o D D ) ln( o o o wall total A kl 2 A \ π T A U T A U T UA T Q o o s Δ Δ Δ Δ o o wall o o s A A A U A U UA Wen wall tkness s very sall and k s large wall beoes 0 and A A o o U o U U U

9 Iportant earks o Te overall eat transfer oeffent U s donated by te saller onveton oeffent, sne te nverse of a large nuber s sall Wen one of te onveton oeffents s u saller tan te oter (say, << o), we ave / >> /o, and tus U Terefore, te saller eat transfer oeffent reates a bottlenek on te pat of eat flow and serously pedes eat transfer Ts stuaton arses frequently wen one of te fluds s a gas and te oter s a lqud In su ases, fns are oonly used on te gas sde to enane te produt UA s and tus te eat transfer on tat sde U

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11 Foulng Fator Te perforane of eat exangers usually deterorates wt te as a result of auulaton of deposts on eat transfer surfaes Te layer of deposts represents addtonal resstane to eat transfer and auses te rate of eat transfer n a eat exanger to derease Te net effet of tese auulatons on eat transfer s represented by a foulng fator f, w s a easure of te teral resstane ntrodued d by foulng Preptaton foulng of as partles on supereater tubes (fro Stea, Its Generaton, and Use, Babok and Wlox o, 978)

12 Foulng esstanes (Foulng Fators) Te deposton of sale on eat transfer surfae redues te eat ttransfer rate and nrease te pressure drop and pupng power Te overall eat transfer oeffent onsderng foulng resstane on te nsde and outsde U 0 A0 A A A A f, f, o w o o o

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14 Analyss of eat exanger Log ean teperature dfferene (LMTD) Effetveness NTU Q p (T,out T, n ) Q p (T,n T, out )

15 Heat apaty Heat apaty rate p p Te flud wt a large eat apaty rate wll experene a sall teperature ange, and te flud wt a sall eat apaty rate wll experene a large teperature ange Wen te eat apaty rates of te two fluds are equal to ea oter, ten te teperature rse of a old flud s equal to te teperature drop of te ot flud s

16 ondenser/boler Q fg Te eat apaty rate of a flud durng a pase-ange proess ust approa nfnty sne te teperature ange s pratally zero Tat s, p wen T 0, so tat te eat transfer rate s a fnte quantty Terefore, n eat exanger analyss, a ondensng or bolng flud s onvenently odeled as a flud wose eat apaty rate s nfnty

17 Mean teperature Q UA s Δ T U average an be alulated A s an be alulated How to alulate ean teperature? One way s to alulate te Log ean teperature dfferene In order to develop a relaton for te equvalent average teperature dfferene between te two fluds, onsder a parallel-flow double-ppe eat exanger (next slde)

18 LMTD parallel-flow double-ppe eat exanger δ Q dt δq p p dt δqq dt δq Takng ter dfferene, we get dt dt d(t T ) δq p p dt Te rate of eat transfer n te dfferental seton of te eat exanger an also be expressed as δq U(T T ) das d(t T ) UdAs T T p p p p

19 d(t T T T ) UdA s p p Integratng fro te nlet of te eat exanger to ts outlet, we obtan T ln T,out,n ΔT ln ΔT Q 2 ΔT T T,out,n UA l UA s Q p (T UAs T p p p T,n T,out T,out T,n Q p(t Q& T Δ s T l ΔT ΔT ln( ΔT / ΔT 2 2 ),out,n,out,out T Q T T T,n,out,n ),n Here T ( T,n T,n ) and T 2 ( T,out T,out ) represent te teperature dfferene between te two fluds at te two ends (nlet and outlet) of te eat exanger p Q )

20 LMTD Q UA Δ s T l ΔT l ΔT ΔT ln( ΔT / ΔT 2 2 ) Te log ean teperature dfferene etod s easy to use n eat exanger analyss wen te nlet and te outlet teperatures of te ot and old fluds are known or an be deterned fro an energy balane One T l, te ass flow rates, and te overall eat transfer oeffent are avalable, te eat transfer surfae area of te eat exanger an be deterned fro Q UA Δ s T l

21 For spefed nlet and outlet teperatures, te log ean teperature dfferene for a ounter-flow eat exanger s always greater tan tat for a parallel-flow eat exanger ΔT T ΔTT T Δ 2 l ln( ΔT / ΔT ) 2

22 In a ounter-flow eat exanger, te teperature dfferene between te ot and te old fluds wll rean onstant along te eat exanger wen te eat apaty rates of te two fluds are equal ΔT onstant wen ΔT l Δ T ΔT 2