Original equipment manufacturers are looking for ways to reduce the product development cycle. A tool to reduce this time in the design of air conditioning systems is introduced in this paper. This is done by using a semitheoretical model of each of the components of a system. The semitheoretical model has the advantage of reducing the number of geometrical inputs that need to be specified when compared with a “theoretical” model. This paper focuses on the most complex of the components—the evaporator. A tube and fin evaporator was successfully modeled and validated. The model was tuned using supplier component data (Luvata, 2005, Heatcraft Evaporator—Version 3.07.05.122 B-DHLEX.EVP) for the evaporator. The tuned accuracy of heat transfer rate and pressure drop, both air- and refrigerant-side, was within 5% over the range of conditions representative of air conditioning applications.