A nonlinear optimization algorithm is applied to the design of air-cooled hear exchangers. In such equipment. the cold fluid (air) is impelled across hanks of finned tubes by means of fans in forced or induced draft. The hot stream flows inside the tubes in one or more passes, and the process that rakes place may be cooling of either a gas or a liquid. or condensation of either a pure vapor or a mixture. The objective function is the minimum cost of the unit (investment and operation), subject to certain geometric and thermohydraulic constraints. The optimization algorithm used is that developed by Biegler and Cuthrell [1], and programmed by them in the OPT package. The problem posed in this case is made of 10 optimization variables, subject to five constraints related to geometric and operational parameters of the hear exchanger