ch11_supp - LMTD for multipass and crossflow

ch11_supp - LMTD for multipass and crossflow - Chapter 11...

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Unformatted text preview: Chapter 11 Supplemental Material 11S.1 Log Mean Temperature Difference Method for Multipass and Cross-Flow Heat Exchangers Although flow conditions are more complicated in multipass and cross-flow heat exchangers, Equations 11.6, 11.7, 11.14, and 11.15 may still be used if the follow- ing modification is made to the log mean temperature difference [1]: (11S.1) That is, the appropriate form of D T lm is obtained by applying a correction factor to the value of D T lm that would be computed under the assumption of counterflow con- ditions. Hence from Equation 11.17, D T 1 5 T h , i 2 T c , o and D T 2 5 T h , o 2 T c , i . Algebraic expressions for the correction factor F have been developed for vari- ous shell-and-tube and cross-flow heat exchanger configurations [13], and the results may be represented graphically. Selected results are shown in Figures 11S.1 through 11S.4 for common heat exchanger configurations. The notation ( T , t ) is used to specify the fluid temperatures, with the variable t always assigned to the tube-side D T lm 5 F D T lm,CF 0.2 0.6 0.4 1.5 1.0 0.8 3.0 2.0 6.0 4.0 R = T i T o ______ t o t i 1.0 0.9 0.8 0.7 0.6 0.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.0 0.9 F P = t o t i _____ T i t i T i T o t i t o F IGURE 11S.1 Correction factor for a shell-and-tube heat exchanger with one shell and any multiple of two tube passes (two, four, etc. tube passes). c11_supl.qxd 1/19/06 4:57 PM Page W-37 W-38 11S.1 j Log Mean Temperature Difference Method 0.2 1.0 0.8 0.6 0.4 3.0 2.0 1.5 6.0 4.0 1.0 0.9 0.8 0.7 0.6 0.5 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 R = T i T o ______ t o t i F P = t o t i _____ T i t i T i T o t i t o F IGURE...
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ch11_supp - LMTD for multipass and crossflow - Chapter 11...

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