HEAT TRANSFER HEAT EXCHANGERS

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Simon Abner Brooks
7 years ago
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1 HET EXCHNGER Types f Heat Exchangers Heat exchangers are classfed accrdng t flw arrangement and type f cnstructn.. Duble-ppe heat exchanger ne flud flws thrugh the smaller ppe whle the ther flud flws thrugh the annular space between the tw ppes. (a) Parallel flw the ht and cld fluds enter the same end, flw n the same drectn and leave at the same end. Fgure (a): Duble-ppe heat exchanger parallel flw (b)cunter flw the ht and cld fluds enter at ppste ends, flw n ppste drectn and leave at ppste ends. Fgure (b): Duble-ppe heat exchanger cunter flw

(a) Parallel flw the ht and cld fluds enter the same end, flw n the same drectn and leave at the same end.

2 2. Crssflw heat exchanger the flw f ne flud s perpendcular t the ther flud. (a) Unmxed flud s sad t be unmxed because the fns prevent mtn n the y drectn. Fgure 2(a): Crss-flw heat exchanger fnned bth flud unmxed x y (b)mxed Fgure 2(b): Crss-flw heat exchanger unfnned wth ne flud mxed the ther flud unmxed x y 2

Fgure 2(a): Crss-flw heat exchanger fnned bth flud unmxed x y (b)mxed Fgure

3 3. Shell-and-tube heat exchanger baffles are ncluded t ncrease the cnvectn ceffcent f the shell-sde flud by nducng turbulence Fgure 3(a): Shell-and-tube heat exchanger wth ne shell and ne tube pass Fgure 3(b): Mult-pass flw arrangement ne-shell pass and tw tube-passes Fgure 3(c): Mult-pass flw arrangement ne-shell pass and tw tube-passes 3

ne shell and ne tube pass Fgure 3(b): Mult-pass flw arrangement ne-shell pass and tw

4 4. Cmpact heat exchanger these devce have dense arrays f fnned tubes r plates and are typcally used when at least ne f the fluds s a gas. 4

5 The Overall Heat Transfer Ceffcent heat exchanger typcally nvlves tw flwng fluds separated by a sld wall. Heat s frst transferred frm the ht flud t the wall by cnvectn, thrugh the wall by cnductn, and frm the wall t the cld flud agan by cnvectn. ny radatn effects are usually ncluded n the cnvectn heat transfer ceffcents. The thermal resstance netwrk asscated wth ths heat transfer prcess s gven belw. R R ttal R R wall R ln( r / r ) 2kL = nner & = uter R U U U s U ln( r / r ) 2kL 5

flud agan by cnvectn. ny radatn effects are usually ncluded n the cnvectn heat transfer ceffcents.

6 When the wall thckness f the tube s small and the thermal cnductvty f the tube materal s hgh, as s usually the case, the thermal resstance f the tube s neglgble (R wall = 0) and the nner and uter surfaces f the tube are almst dentcal ( = = s ). U h h Fulng Factr The perfrmance f heat exchangers usually deterrates wth tme as a result f accumulatn f depsts n heat transfer surfaces. The layer f depsts represents addtnal resstance t heat transfer and causes the rate f heat transfer n a heat exchanger t decrease. The net effect f these accumulatns n heat transfer s represented by a fulng factr R f, whch s a measure f the thermal resstance ntrduced by fulng. The unts f R f s (K/W). When cnsderng fulng factr s cnsdered, the verall heat transfer ceffcent has t be mdfed as gven belw. f, f, U s U U R ln( r / r ) R 2kL Questn : duble-ppe heat exchanger s cnstructed f a stanless steel (k = 5. W/m K) nner tube f nner dameter D =.5 cm and uter dameter D =.9 cm and an uter shell f nner dameter 3.2 cm. The cnvectn heat transfer ceffcent s gven t be h = 800 W/m2 K n the nner surface f the tube and h = 200 W/m 2 K n the uter surface. Fr a fulng factr f R f, = m 2 K/ W n the tube sde and R f, = m 2 K/ W n the shell sde, determne, (a) the thermal resstance f the heat exchanger per unt length and (b) the verall heat transfer ceffcents, U and U based n the nner and uter surface areas f the tube, respectvely. 6

The layer f depsts represents addtnal resstance t heat transfer and causes the rate f heat transfer n a heat exchanger t decrease.

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SOLID MECHANICS TUTORIAL FRICTION CLUTCHES

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