文档介绍：第二节换热器的传热计算 ——传热计算方法(2) Xiamen University Dept. of Chem. & Biochem. Eng.
(二)传热单元数法(e-NTU法) (以p422例6-6为例) 1. 定义 (1)最小值流体 Cmin = (W ×cp )min W ×c p ——流体的热容量流率 本例中 Cmin = Wc ×c pc Cmax =Wh ×c ph 实际的传热量Q (2)传热效率ε= 最大可能的传热量Qmax t ­t 本例中ε= 2 1 (若热流体为最小值流体, T ­t T ­T 1 1 则ε= 1 2 ) T1 ­ t1 Xiamen University Dept. of Chem. & Biochem. Eng. 1 PDF created with pdfFactory trial version (3)传热单元数(number of transfer units) t2 dt KS KS (NTU ) = = = c òt 1 T ­ t (W ×c p )min Wc ×c pc Q Wcc pc (t2 ­ t1 ) = KSDtm 若 KS t2 ­ t1 \ (NTU )c = = 1 Wc ×c pc Dtm KS T1 ­T2 同理(NTU )h = = Wh ×c ph Dtm 则 L = H c (NTU )c 或 L = H h (NTU )h Xiamen University Dept. of Chem. & Biochem. Eng.
逆流时 T1 ­ t2 ­t1 + t1 T ­ t T ­t 1­ε 1­ε 1 2 = 1 1 = c = c T t T T W c C T2 ­t1 2 ­ 1 ­ 1 + 1 c pc min (t ­t ) 1­ εc T ­t W c 2 1 C 1 1 1­ h ph max T1 ­t1 (T ­t )­(T ­t ) 而 Q= KS 1 2 2 1 =W c (t ­t ) =W c (T ­T ) T ­t c pc 2 1 h ph 1 2 ln 1 2 T2 ­t1 Xiamen University Dept. of Chem. & Biochem. Eng. 2 PDF created with pdfFactory trial version T1 ­t2 KS (T1 ­T2 ) ­ (t2 ­ t1 ) \ ln = = (NTU)c (CR ­1) T2 ­t1 Wccpc t2 ­t1 T1 ­ t 2 令 \ = exp [­ (NTU ) c (1 ­ C R )] = A T2 ­ t1 1­ε c 则= A Þ 1­ε c = A­ CRεc A 1­ CRεc 1­ A 1 ­ exp [­ (NTU ) c (1 ­ C R )] \ εc = = 1­CR A 1 ­ C R × exp [­ (NTU ) c (1