Synthesis of cooling water systems with multiple cooling towers
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This paper presents a systematic approach for the synthesis of re-circulating cooling water systems consisting of a cooler network and a cooling tower network (i.e. a general arrangement of multiple interconnected cooling towers with different supply temperatures). An overall superstructure is developed, which contains all the different alternatives for the cooler and cooling tower networks, as well as all their potential interconnections for fixed data of hot process streams. The synthesis problem is formulated as a mixed integer nonlinear programming problem. The continuous variables include the flow rates and temperatures of the circulating cooling water in the superstructure while the integer variables describe the existence of the coolers and the cooling towers. The objective function is to minimize the total annual cost, which includes the investment costs of cooler units and cooling towers, as well as the operating costs due to make-up water and power consumption of the circulating-water pumps and cooling tower fans. The solution of the proposed MINLP formulation provides simultaneously both the optimal configuration and operating conditions of the cooler and cooling tower network to comply with given process cooling demand at minimum total annual cost. One of the most important design variables is the number of cooling water sources (i.e. cooling water streams at different temperatures originated from the outlets of cooling towers) for removing heat from several hot process streams at different temperature ranges. Results show that cooling systems consisting of multiple cooling towers with different supply temperature yield significant better results than traditional systems with a cooler network in parallel arrangement that is supplied with cooling water from a single cooling tower. 2012 Elsevier Ltd. All rights reserved.
