Although nowadays evaporators, like other forms of heat exchanger, are generally designed with the aid of sophisticated computer programs, it is worth considering the fundamentals of the process, and the influence of various factors on the design. Section A below considers in outline how a manual design might be performed. Section B discusses computer-aided design.

A. Manual design

The required surface area of a heat exchanger is:

\[\label{eq1} A = \int {\frac{d\dot {Q}}{U\left({T_h - T_c } \right)}}\tag{1}\]

where Q̇ is the heat load, U is the overall heat transfer coefficient (from Section 275), T_h is the temperature of the heating medium (for condensing fluids, the saturation temperature), and T_c is the temperature of the boiling liquid. In a steam heated evaporator, the steam condensing temperature T_h is almost constant throughout an evaporator. In a heat exchanger without phase change, it can usually be assumed that U is constant and that the specific heats of the fluids are approximately constant; thus, in a single-phase heater in which heat is supplied by isothermal condensation, the area required is