In a large, complex cooling water heat exchangers network, cooling water flow rates are manipulated by adjusting the valves located at the rear of each heat exchanger. Since the heat exchangers are connected to a network, the manipulation of the cooling water flow rate in a heat exchanger by adjusting the valve results in the changes of the cooling water flow rates in the other heat exchangers. Therefore, the manipulation techniques of cooling water flow rates in a cooling water heat exchangers network are required in order to operate the cooling water networks efficiently. This paper presents the analysis and the manipulation method of the cooling water flow rates in all heat exchangers in a network simultaneously. A nonlinear programming( NLP) formulation is proposed to determine the optimal adjustment of the valves for the required flow rates of the cooling water in the heat exchangers. Solution of this formulation is obtained with a commercial NLP solver (GAMS/MINOS).