Predicting operational service time of heat exchangers between forced maintenance outages is a subject, which has been studied intensively for many years. Numerous technical and economic advantages would occur if such predictive data were available, to permit appropriate preventive actions to control potential fouling problems developing on either the cooling water side or the process fluid side. The calculation methodology developed by Kern and other researchers can be applied to determine when critical fouling values will be reached, once the fouling factors for specific unit have been determined. However, such methodology is limited by its ability to express only total fouling, which may be understood as the sum of fouling on exchanger process side, plus that on the waterside. This does not provide for in-service isolation and measurement of fouling factors on both sides of exchangers, nor permit necessary corrective actions, when possible, in the more critical area. Deducting these values from those verified in the plant for the total fouling, fouling which occurs on the process side is determined more precisely. This defines the intensity of the fouling problem on each interface where thermic exchange takes place.