(679c) Minimization of Capital and Operating Costs in Cooling Water Distribution Systems

Cooling water is the fluid most commonly used in heat exchangers for cooling processes. The most widely used concept for cooling water semi closed loops is the one that a pumping system supplies water from the cooling tower basin through a header to branches where heat exchangers are located, and after heat absorption in these exchangers, the water returns from these branches through another header to the cooling tower top to reject the absorbed heat. The use of this kind of concept results in a reasonably simple design procedure for the cooling water system, but it can yield high electrical energy demands. As good engineering practice, the pressure drops in heat exchangers must not exceed 0.5 to 1.0 bar, but there are cases that this pressure drop may be higher than the recommended values. Cooling water consumers distant, above 500 m, from the cooling tower also impose high pressure drops in the control valves of the branches closest to the main pumping system. To avoid a short circuit, branches with heat exchangers with low pressure drops or near the cooling tower may have control valves that impose additional pressure drops and, therefore, mechanical energy losses. As a result, the pumping system must compensate these energy losses by increasing the consumed electrical energy.

Alternatives are analyzed to reduce the system total electrical energy consumed. Among these alternatives are the use of auxiliary (booster) pumps in the branches with the highest head losses, the installation of secondary headers, and the use of separate cooling towers. On the other hand, these alternatives result in an increase of capital costs. The mathematical modeling of the pumping system allows the estimate of system mechanical energy demands as a function of the presence of booster bumps and secondary headers. The synthesis of the cooling water distribution system must minimize the overall costs.

The optimal synthesis of the cooling water system shall determine the values for the decision variables of the system model that minimizes the capital and operating costs. Among the model decision variables are: capacity of the cooling water main pumping system; number of pumps in the cooling water main pumping system; capacity and location of the cooling water booster pumps; layout of the cooling water supply and return headers; required minimum pressure drop through control valves. An existing cooling water distribution system is selected as study case, so the optimal synthesis solutions can be compared to the existing one.

A sensitivity analysis is performed varying the design parameters such as cooling water total and individuals demands, cooling water consumers’ locations, heat exchangers pressure drop and cooling tower elevation. Evidently, these project parameters have a direct impact in the cooling water distribution system and the resulting cost. As a result, a methodology is established so it can be applied to the detailed design of the cooling water distribution system.