Optimal detailed design for integral cooling water systems

A cooling water system is used in industrial processes to cool down hot process streams from their supply conditions to target temperatures. A cooling water system is comprised of three major components, namely a cooling tower, a heat exchanger network (HEN), and a pumping system; these elements have strong interactions with each other and therefore must be optimized simultaneously to yield an optimal cooling water system. This paper presents a mixed-integer non linear programming model for the optimal design of integral cooling water systems. The model includes a superstructure for the HEN component of the system that allows arrangements of coolers in series and/or in parallel, and also allows the bypass of fresh water and/or previously-used water to improve the performance of the cooling network. The optimization of HEN coolers is made simultaneously to take into consideration from the synthesis stage the interactions between the pumping effects and the heat transfer coefficients that determine their transfer areas. The model considers as objective function the minimization of the capital costs for the cooling tower, HEN coolers and pumps, as well as the operating costs due to the fan of the tower, pumping requirements and water makeup. One example problem is presented to show the application of the proposed method.