Cooling water systems are generally designed with a set of heat exchangers arranged in parallel. This arrangement results in higher cooling water flowrate and low cooling water return temperature thus reducing cooling tower efficiency. Previous research on cooling water systems has focused mainly on heat exchanger network thus excluding the interaction between heat exchanger network and the cooling towers. The studies completed on cooling water system in which the interaction between the cooling tower and the heat exchanger network was taken into consideration were limited to systems with single cooling tower. The main aim of this study was to develop a design methodology for synthesis and optimization of cooling water systems with multiple cooling towers. The design intends to debottleneck the cooling towers by reducing the circulating water flowrate. The study focuses mainly on cooling systems consisting of multiple cooling towers that supply a common set of heat exchangers. In this work the mathematical optimization technique was developed for optimization and synthesis of cooling water system. The heat exchanger network was synthesized using the mathematical optimization technique. This technique is based on superstructure in which all opportunities for cooling water reuse are explored. The cooling tower model was used to predict the thermal performance of the cooling towers while taking the thermal conditions of the associated heat exchanger network into account. The propose technique debotlleneck the cooling towers by decreasing the circulating water flowrate. This implies that a given set of cooling towers can manage an increased heat load. From the case studies, 22% decrease in circulating water flowrate was realized. The blowdown and makeup were also decreased by 7%. Furthermore, the cooling tower effectiveness was also improved by 4%. A decrease in the overall circulation water has an added benefit of decreasing the overall power consumption of the circulating pumps. There is also a potential for the reduction of makeup and blowdown water flowrate.
Dissertation (MEng)--University of Pretoria, 2011.