The basic design procedure for air-cooled heat exchangers is similar to that for shell-and tube exchangers. An initial configuration for the unit is obtained using an approximate
overall heat-transfer coefficient together with the design guidelines given above. An
important preliminary step in the design process is the selection of the outlet air
temperature. This parameter has a major effect on exchanger economics. Increasing the
outlet air temperature reduces the amount of air required, which reduces the fan power and, hence, the operating cost. However, it also reduces the air-side heat-transfer coefficient and the mean temperature difference in the exchanger, which increases the size of the unit and, hence, the capital investment. The same situation exists with water-cooled heat exchangers, but the feasible range of outlet temperatures tends to be significantly greater for air-cooled exchangers. Thus, optimization with respect to outlet air temperature (or equivalently, air flow rate) is an important aspect of air-cooled heat-exchanger design