Optimal strategies for steady state heat exchanger operation

Two procedures for deriving the optimal steady state operation of heat exchangers are presented. The first procedure, which is based on calculus of variations, can be used in case the optimization criterion is minimum entropy generation. The second procedure can be used in principle for any optimization criterion. Here the minimum entropy generation and the minimum lost available work are envisaged as examples. Both procedures show that, whatever the optimization criterion, parallel flow heat exchangers cannot be optimal but counter-flow devices can be. The empirical wisdom embodied in conventional counter-flow heat exchangers is examined. The usual 'equilibrated' operation is in many cases rather close to the optimum.