Estimating Number of Shells in Shell and Tube Heat Exchangers: A New Approach Based on Temperature Cross

Multipass heat exchangers are often designed by using the rule of thumb $FT⩾0.75,$ which is rather arbitrary. $FT$ falls sharply with the increase in temperature cross. Hence, only a limited temperature cross can be allowed. The ability to accommodate temperature cross increases rapidly as the number of shell passes is increased. Though many investigators have emphasized the importance of temperature cross in exchanger design, it has as yet not been explicitly accounted for in the design. This paper introduces a new approach for estimating the number shells in a shell and tube exchanger which directly accounts for temperature cross, rather than routing this effect through $FT$ or $XP$ (Ahmad et al.’s parameter, which is again a correction factor not directly related to temperature cross). The approach is compatible with the established design procedures and bypasses the $FT.$ It generates better designs by defining maximum permissible temperature cross, than the traditional designs based on specifying minimum permissible $FT.$ Expressions have also been provided which correlate the present formulation with that of Ahmad et al. [S0022-1481(00)00803-3]