The performance of the Carnot cycle and a vapor-compression refrigeration cycle can be optimized by the choice of a heat-exchanger area between the evaporator and the condenser. In this paper, the analytical expressions for the optimal allocation of heat-exchanger area for the endoreversible and irreversible Carnot cycles with constant- and variable-temperature heat-reservoirs are derived. The optimal values for real vapor-compression cycles are obtained by numerical analysis. The results can guide the evaluation of existing refrigeration and air-conditioning plants or influence design practice for future plants. (C) 2003 Elsevier Ltd. All rights reserved.