Shell-and-Tube Heat Exchangers - Cheresources.com

them were considered too cumbersome to use. Therefore graphs were prepared plottingt2 − t1T1−T2F( PR , ) , where P = and R = are parameters on which F depends. Figures C4.adin Appendix C of the textbook by Mills display such graphs. Nowadays, one can compute theseT1−t1t2 − t1factors quickly with a pocket calculator. Given next are the two common factors.F1−2F2−4⎡ 2R + 1⎤⎛ 1−P⎞⎢ ⎥ln⎜ ⎟⎢ R −1 ⎥ ⎝1−PR⎠=⎣ ⎦⎡2A+ R + 1⎤ln ⎢ ⎥2⎢⎣A− R + 1⎥⎦⎡ 2R + 1⎤⎛ 1−P⎞⎢ ⎥ln⎜ ⎟⎢2( R −1)⎥ ⎝1−PR⎠=⎣ ⎦⎡2A+ B+ R + 1⎤ln ⎢⎥2⎢⎣A+ B− R + 1⎥⎦2 2PPwhere A = −1 − R , B = ( 1−P)( 1−PR)The first and second subscripts on the factor F correspond to the number of shell and tubepasses, respectively. The simplifying assumptions mentioned in the previous paragraph, givenin Perry’s Handbook, are as follows.1. The heat exchanger is at steady state.2. The specific heat of each stream remains constant throughout the exchanger.3. The overall heat transfer coefficient U is constant.4. All elements of a given fluid stream experience the same thermal history as they pass throughthe heat exchanger (see footnote in Perry for a discussion regarding the violation of thisassumption in shell-and-tube heat exchangers).5. Heat losses are negligible.The formula given above for F1 − 2also applies for one shell pass and 2, 4, (or any multiple of 2)tube passes. Likewise, the formula for F2 − 4also applies for two shell passes and 4, 8, (or anymultiple of 4) tube passes.In designing heat exchangers, one should avoid the steep portion of the curves of F versus P ,because small errors in estimating P can cause large changes in the value of F . A misleadingrule of thumb is that F ≥ 0.8, but the correct idea is that the region of steep fall-off in the curvesshould be avoided.4