New methodologies have recently been reported for the synthesis of cooling networks. Some of them have been based on the pinch point method, with the minimum cold utility consumption as the main target (Kim and Smith (2001) and (2003)), and some others on mathematical programming models (Feng et al. (2005); Majozi and Moodley (2007)). However, these works have ignored the trade offs between the capital costs for the coolers, the utility consumption and the pumping costs. In this work, an algorithm for the synthesis of cooling networks is presented. The new method is based on a MINLP formulation to minimize the total annual cost of the network, which includes the capital investment for the coolers, the utility costs and the pumping costs. Within this formulation, the outlet temperature for the cold utility is treated as an optimization variable. The proposed superstructure is based on a stage-wise representation (Yee and Grossmann, 1990), and allows the bypass of either fresh or previously-used cold utility as part of the solution strategy. Also, the model treats as optimization variables the outlet temperatures for each stage of the superstructure independently. Relationships between the pressure drops and the film heat transfer coefficients are used to provide better estimation of the exchangers' geometries and pumping costs. Operational and geometric constraints are considered as part of the formulation. To avoid operational problems due to fouling conditions, an upper limit is imposed for the outlet temperature of the cold utility for each cooler. Several examples that show the application of the proposed methodology are presented.

References Feng, X., Shen, R., Wang, B., (2005). Recirculating cooling-water network with an intermediate cooling-water main. Energy & Fuels, 19, 1723-1728. Kim, J., Smith, R., (2001). Cooling Water System Design. Chemical Engineering Science, 56, 3641-3658. Kim, J., Smith, R., (2003). Automated retrofit design of cooling-water systems. American Institute of Chemical Engineering Journal, 49 (7), 1712-1730. Majozi T., Moodley A., (2007). Simultaneous targeting design for cooling water systems with multiple cooling water supplies. In press Computers and Chemical Engineering, 27-March-2007. Yee, T.F., Grossmann, I.E. (1990). Simultaneous optimization model for heat integration –II Heat exchanger network synthesis. Computers and Chemical Engineering. 14, 1165-1184.

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