A methodology is proposed to determine the feasible region for shell and tube heat exchanger designs on a pressure drop diagram. By accounting for operating as well as geometrical constraints, the feasible region is defined so as to eliminate trial-and-error during the design activity. Every point on this plot of shellside versus tubeside pressure drop corresponds to a unique design in terms of tube length, shell diameter and baffle spacing. Furthermore, curves may be plotted for designs corresponding to a specified heat exchange area or a given total annual cost. Such curves permit screening of various design options prior to detailed rating of the exchangers, and allow heat exchanger design targets to be established for minimum area or cost. The area target ensures an exchanger of the smallest size with minimum capital cost, whereas the cost target yields the optimum pressure drops accounting for the tradeoff between power consumption and heat exchanger area. The methodology is equation-based and can be conveniently implemented on a computer.