STG-2 Mean Temperature Difference - A Reappraisal
The derivation of the mean temperature difference in heat exchangers is based on a number of assumptions or idealizations, the most important ones being (a) constant heat transfer coefficient throughout the exchanger; (b) the temperature of either fluid is constant over any cross-section of its nominal path, i.e., complete mixing, no stratification or bypassing; and (c) an additional assumption for shell-and-tube exchangers, which has not always been fully recognized, is that within one baffle crossing the shell fluid temperature change is small with respect to its overall change, i.e., the number of baffles is large. In actual exchangers, any of the above assumptions are frequently subject to various degrees of invalidation. This paper examines the effects of deviating from assumptions (a) and (b) and presents a new solution to case (c).