Heat exchanger effectiveness(Contd) Define: Heat exchangereffectiveness, 8 max Actual heat transfer, g, can be determined from simple energy balance g=qn=m,p,hIh,inIh, out g=mc c p, c (c,out SO Ch (Thi-T,ou) ccout C in Thus min OR= min If the heat exchanger effectiveness were known then the actual heat transferred could be found from max Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 6 Heat exchanger effectiveness (Cont’d) Define: Heat exchanger effectiveness, • Actual heat transfer, q, can be determined from simple energy balance • Thus: • If the heat exchanger effectiveness were known, then the actual heat transferred could be found from: ( ) ( ) ( ) ( ) h h i n h out c c out c i n h h p h h i n h out c c p c c out c i n so q C T T C T T q q m c T T q m c T T , , , , , , , , , , : = − = − = = − = = − ( ) ( ) ( ) ( ) min , , , , min , , , , max h i c i c c o c i h i c i h h i h o C T T C T T OR C T T C T T q q − − = − − = = max q q q qmax =
Number of Transfer units Define: Numberof Transfer Units, NTU NTu UA min nTU depends on both the heat exchanger design (UA) and the operating conditions(cmin) Define: Capacity Ratio, Cr r min max Effectiveness is a function of capacity ratio and the ntu E=f(NTU, Cr) Relationships between E, nTU and Cr can be computed Tables 11.3 and 11.4 and Figures 11.14-l1.9 in textbook Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 7 Number of Transfer Units Define: Number of Transfer Units, NTU • NTU depends on both the heat exchanger design (UA) and the operating conditions (Cmin). Define: Capacity Ratio, Cr • Effectiveness is a function of capacity ratio and the NTU • Relationships between , NTU and Cr can be computed. Tables 11.3 and 11.4 and Figures 11.14 – 11.19 in textbook C C /C (C 1) r = min max r ( ) NTU Cr = f , Cmin UA NTU