One of the most important points in a heat exchanger is the overall heat transfer coefficient. This coefficient is obtained in terms of the conduction and convection resistances between the fluids separated by flat walls and cylindrical shells, respectively. The layer of scale on the surfaces may increase the resistance to heat transfer between the two fluids to a large extent. This effect can be taken into account by introducing an additional thermal resistance, called the fouling coefficient (Rf), and its value depends on the operating temperature, fluid velocity, and the operating time of the exchanger.