In the previous section, the need for a preliminary estimate of the heat exchanger size and configuration was indicated. This estimate is essential for a hand design method and useful for a computer design. Additionally, there are occasions - preliminary estimates of plant cost, layout, and space requirements, for example - when a good approximate size estimate is sufficient and more quickly available than a detailed design. Eventually, if the plant is to be built, the detailed calculations will have to be done, but that task can profitably wait until the preliminary studies have ascertained if the plant looks economically promising and have given a definite idea of how the components are to be arranged.

In this section, a preliminary design method is presented in some detail. This method covers many of the usual process applications for shell-and-tube exchangers, and it can be extended with care and insight to other cases. The accuracy of the method is strongly dependent on the skill of the user in estimating or selecting appropriate heat transfer coefficient values; in this there is no substitute for experience, although Table 2 may be helpful.

A. The basic design equation

The basic design equation to be used in this section is

\[\label{eq3.1.1(8)} A_{0}=\dfrac{Q_{T}}{U_{0}F\Delta T_{LM}}=\dfrac{Q_{T}}{U_{0}\Delta T_{M}} \tag{3.1.1(8)}\]