HEAT TRANSFER CHAPTER 11 Heat Exchangers 们au Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 1 HEAT TRANSFER CHAPTER 11 Heat Exchangers
Heat Exchangers. NTU-8 Method Where we’ ve been∴ Analysis of heat exchangers using log mean temperature difference (LMTD) q=y ATour -At i UA△TLMD Tr △T △T △D Where were going Computation of heat exchanger performance compared to the theoretical maximum possible for the flow conditions and hX type and size Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 2 Heat Exchangers, NTU- Method Where we’ve been …… • Analysis of heat exchangers using log mean temperature difference (LMTD) Where we’re going: • Computation of heat exchanger performance compared to the theoretical maximum possible for the flow conditions and HX type and size. LMTD i o out in UA T T T T T q UA = − = ln dq Ti To h dTc dT T
Heat Exchangers. NTU-8 Method KEY POINTS THIS LECTURE Concept of heat exchanger effectiveness,& based on the ratio of fluid heat capacity, C Concept of heat exchanger Numberof Transfer Units. NTU pplication of ntU-c method to predict the performance of a given heat exchanger ext book sections: $11.4-11.5 Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 3 Heat Exchangers, NTU- Method KEY POINTS THIS LECTURE • Concept of heat exchanger effectiveness, based on the ratio of fluid heat capacity, C. • Concept of heat exchanger Number of Transfer Units, NTU • Application of NTU- method to predict the performance of a given heat exchanger • Text book sections: §11.4 – 11.5
Recall earlier discussion For a condensing vapor or Ch Cond Out For an evaporating liquid (or Ch <<C Ev O What if Ch= Cc in a counterflow HX? △T1=△T Out Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 4 • For a condensing vapor • For an evaporating liquid • What if Ch = Cc in a counterflow HX? Recall earlier discussion ( ) or Ch Cc x T In Out x T In Out ( ) or Ch Cc x T In Out T1 = T2 TCond TEvap
Heat exchanger effectiveness Maximum possible heat transfer rate for any given inlet temperatures and flow rates occurs in a infinitely long counterflow HX c out out → C.7 Length of heat exchanger ·IfC<Ch,then:To=Th Cold fluid would reach hot fluid inlet t ano If Cs> Ch, then Tho=T Hot fluid would reach cold fuid inlet t and max Maximum△T max min(h. CI Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 5 Heat exchanger effectiveness • Maximum possible heat transfer rate for any given inlet temperatures and flow rates occurs in a infinitely long counterflow HX and : ( ) Cold fluid would reach hot fluid inlet T If max c h,i c,i c h c,o h,i q C T T C C , then: T T = − • = T Th,out Tc,in c out h in T T , , = Length of heat exchanger and : ( ) Hot fluid would reach cold fluid inlet T If max h h,i c,i c h h,o c,i q C T T C C , then: T T = − • = L → ( ) qmax = Cmin Th,i −Tc,i Maximum T