The term fouling refers to the deposition of material on a heat transfer surface, usually resulting in an increase in the resistance to heat transfer and a subsequent loss of thermal exchange capacity of the heat transfer equipment. Furthermore, the deposits restrict flow to a greater or lesser extent which results in increased pumping energy requirements. In practice, some fouling usually occurs in most heat exchangers, and in some it becomes the predominant resistance. It is therefore necessary for the designer to estimate the probable magnitude of the fouling resistance as well as means by which it may be reduced.

Once fouling is accepted as unavoidable, steps must be taken to provide for periodic shutdown and cleaning of the equipment or proper treatment of the streams involved so that fouling is mitigated. In some cases, provisions for cleaning may be the dominating factors in heal exchanger design. Since 1960 considerable progress has been made in understanding the fouling process; however, this has not resulted in significant improvement in the ability of the designer to predict fouling resistances. Evidence of increased interest is noted in the rapidly expanding literature on fouling and the number of conferences and reviews on the subject.

Sizing of heat exchangers is based on the heat duty and the temperatures, properties, and flow rates of the hot and cold streams. This information is used to determine the individual heat transfer coefficients in the equation for the overall heat transfer coefficient

\[\frac{1}{U_{1}} = \frac{1}{\alpha_{1}} + R_{fl} + \frac{x A_{w}}{\lambda_{w} A_{2}} + \left(\frac{1}{\alpha_{2}} + R_{f2}\right)\frac{A_{1}}{A_{2}}\notag\]

Where U₁ is the overall heat transfer coefficient based on the area A₁