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. LMTD Method Where we’ ve been.. So far have focused on detailed heat transfer analysis of specific conditions, such as external heat transfer coefficient Where we’ re going: Investigate methods for larger system level anal ysis that combine all these modes of heat transfer in heat exchangers Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 2 Heat Exchangers, LMTD Method Where we’ve been …… • So far have focused on detailed heat transfer analysis of specific conditions, such as external heat transfer coefficient Where we’re going: • Investigate methods for larger system level analysis that combine all these modes of heat transfer in heat exchangers
Heat Exchangers. LMTD Method KEY POINTS THIS LECTURE Types of heat exchangers, advantages and di Isad vantages Overall heat transfer coefficient concept of fouling factor Log mean temperature difference Application of lmtd to heat exchanger anal ysis Text book sections: $11.1-11.3 Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 3 Heat Exchangers, LMTD Method KEY POINTS THIS LECTURE • Types of heat exchangers, advantages and disadvantages • Overall heat transfer coefficient, concept of fouling factor • Log mean temperature difference • Application of LMTD to heat exchanger analysis • Text book sections: §11.1 – 11.3
Heat Exchanger Types Aout B. Bout 个个个↑ Am山广00 ↑↑↑1 Example B in Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 4 Heat Exchanger Types TA,in TA,in TA,out TB,in TB,out TA,out TB,in TB,out TA,in TA,out TB,in TB,out Example:
Heat Exchanger Types(Contd Shell and tube: (common in chemical process industry) B. Shell side Shell and tube A,in(tube side) TBin(shell side) A, in(tube side) A out Shell and Tube out Bin(shell side) Aout Shell and tube A (tubeside) B out Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 5 Heat Exchanger Types (Cont’d) TB,out (shell side) TB,in (tube side) TA,in TA,out Shell and Tube: (tube side) TA,in TA,out (shell side) TB,in TB,out Shell and Tube: Shell and Tube: (shell side) TB,in TB,out (tubeside) TA,in TA,out Shell and Tube: (common in chemical process industry)
Heat Exchanger Types(Contd Shell and tube: oOn heard Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 6 Heat Exchanger Types (Cont’d) • Shell and Tube:
Heat Exchanger Types(Contd) Plate and frame Series of plates with flow channels embossed in them The two fluids are guided through alternating rows of the plates Advantages Application pictured Electrocoat paint in automotive assembly plant Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 7 Heat Exchanger Types (Cont’d) Plate and Frame • Series of plates with flow channels embossed in them. • The two fluids are guided through alternating rows of the plates • Advantages: __________________________ • Application pictured: Electrocoat paint in automotive assembly plant
Heat Exchanger Types(Contd) Plate and Fin Dense array of plates that guide alternating channels of fluids(typically air) Series of fins connect the plates and greatly increase the heat transfer area Advantage: very large heat transfer surface area per unit volume One common application: Aircraft environmental control systems HEIST Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 8 Heat Exchanger Types (Cont’d) Plate and Fin • Dense array of plates that guide alternating channels of fluids (typically air) • Series of fins connect the plates and greatly increase the heat transfer area • Advantage: very large heat transfer surface area per unit volume . • One common application: Aircraft environmental control systems
Overall heat transfer coefficient for hX Recall from earlier the overall thermal resistance concent UA, R A)2(+R+ n An(n hAh Types of resistances involved with heat exchangers(covered in previous sessions Cold side internal convection Cold side fouling factor Conduction through wall Hot side external convection(smooth wall or may involve fins) Hot side fouling factor Bout 个个个↑ Review how these were Ain calculated IIIIImIII ↑↑↑1 B in Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 9 TA,in TA,out TB,in TB,out Overall heat transfer coefficient for HX • Recall from earlier the overall thermal resistance concept: • Types of resistances involved with heat exchangers (covered in previous sessions) – Cold side internal convection – Cold side fouling factor – Conduction through wall – Hot side external convection (smooth wall or may involve fins) – Hot side fouling factor • Review how these were calculated o h o h f h w o c f c o c c c h h A hA R R A R hA U A U A U A ( ) 1 ( ) ( ) ( ) 1 1 1 1 , , + + + = + = =
Analysis of heat transfer Total heat transfer rate is found through energy balance, regardless of the hX type or flow path hot in hot. our old. out q For hot fluid - Energy balance ut ric, ricn(t+dT) m, p, hTh=m, Cp,h(/+dT)h+dqn Define h ph P h(Heat capacity rate for hot fluid) For the cold fluid d a=c dt (Note: no minus sign"- in this equation since heat flow in Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 10 Analysis of heat transfer • Total heat transfer rate is found through energy balance, regardless of the HX type or flow path For hot fluid: Energy balance: • Define For the cold fluid: (Note: no minus sign “-” in this equation, since heat flow in) ( ) h h p h h h p h h h p h h h i n out dq m c dT m c T m c T dT dq E E , , , = − = + + = Thot,in Tcold ,in Thot,out Tcold ,out q m c pT dq mc (T dT) p + (Heat capacity rate for hot fluid) dqc = Cc dTc dqh = −Ch dTh h p h Ch m c , =
