In this work, an optimization model for a cooling water system which supplies a heat exchanger network is developed. The model considers the thermal and hydraulic interactions in the process, and is applied to the study and analysis of typical operational cases. The objective is to minimize the total operating cost of the system, which includes cooling water make-up and energy costs. A base case with fixed configuration is optimized for a given set of climatic conditions. Optimal operating conditions are obtained for several thermal specifications on the water that leaves the tower under different cost coefficients for water and energy. Additionally, constraints on the tower performance are imposed on the system as well as climatic changes. Results indicate that forced water withdrawal is an optimal operational practice to relieve the cooling tower load whenever additional heat must be removed from the cooling tower.