Optimization of cooling water system with compression refrigeration cycle

Abstract

Abstract Cooling water system and chilled water system are wildly used in industry to reject waste heat. When cooling water is not capable to cool down the hot stream to target temperature, chilled water is required. So far, cooling water system and chilled water system are designed separately because the two systems use water with different qualities. Desalted water is used in chilled water system but it cannot be used in cooling water system, because water losing is huge in open cooling tower. Meanwhile, fresh water cannot be used in chilled water system because of high concentration of impurities. In this work, closed cooling tower is applied, because there is no lose of recirculating water in closed cooling tower, desalted water can be used in cooling water system. A large amount of water was saved by using closed cooling tower. Cooling water system and chilled water system was integrated into one, cooling duty was redistributed between the two systems. Stage-wise heat exchanger network was employed where water can be reused. For returned hot water, they are firstly sent to closed cooling tower to be cooled down to a certain temperature, and then part of the water is further cooled down by refrigeration cycle. The model is formulated as mixed-integer nonlinear programming (MINLP) problem. The objective is to minimize the cost of integrated cooling and chilled water system, and determine the optimal heat load distribution between closed cooling tower and refrigeration cycle. Results show that optimization model yields significant reduction on total annual cost and energy consumption, in comparison with system where cooling water system and refrigeration cycle are separated.