Designing a 'Near Optimum' Cooling-Water System

Abstract

Cooling water is expensive to circulate. Reducing its flow - i.e., hiking exchanger outlet temperatures - can cut tower, pump and piping investment as much as one-third and operating cost almost in half. Heat-exchanger-network optimization has been accomplished in large integrated plants, such as petroleum refineries. In many of the chemical process industries, however, a plant contains several individual processes, and network optimization, except on a limited basis, is not feasible. So far, no one has developed similar procedures for designing and optimizing a cooling-water once through-exchanger system. This article attempts to fill the void by presenting a design basis that will produce a 'near optimum' system. A cooling-water system consists of four major components: heat exchangers, cooling towers, circulation piping and pumps. To optimize such a system, one must define the system interactions and apply these relationships to the simultaneous design of the aforementioned equipment. This article develops criteria that for most applications allow one to ignore system interactions, and still design a 'near optimum' system.

Cooling-water systems have long been designed by 'rules of thumb' that call for fixing the cool ant temperature-rise across all heat exchangers (usually 20 F) and setting the coolant inlet temperature to the heat exchanger at the site's wet-bulb temperature plus 8 F. These rules produce a workable cooling system; but, by taking the same coolant rise across all exchangers, regardless of the individual process outlet-temperatures, this cannot result in an optimized design.

The design method presented in this article replaces the 'rules of thumb' with criteria that are easy to apply and that take into account the effect that the individual exchanger process outlet- temperatures have on cooling-system economics. Economic analyses of actual process have shown that cooling-system investment can be reduced by one third, and cooling-system operating cost by one half, If the proposed design criteria are used instead of the 'rules of thumb.' It has been found that the controlling economic factor for a cooling system is the quantity of water being circulated. Reducing the flow (raising the coolant outlet temperature of heat exchangers) significantly reduces cooling tower, pump and piping investment, and operating cost, and only moderately increases the heat-exchanger investment. The overriding conclusion to be drawn is that cooling water is very expensive, and its conservation can result in significant savings.