The importance of knowing how fouling develops and deteriorates the thermal-hydraulic performance of heat transfer processes becomes evident when the problem is already there and is difficult to eliminate. Fouling prediction is important in grassroot design and even more, in retrofit applications. The aim of the present work is to show the use of a fouling model to predict scaling in cooling systems in order to derive design guidelines. Fluid temperature and velocity are the two more important variables that determine the rate at which scaling takes places. This work focuses on retrofit of existing cooling systems for situations where new heat exchangers need to be incorporated into the network. It is shown how the pressure drop, fluid velocity and flow distribution are affected depending on the decision of where to place the new exchangers. All these factors are intimately related to the development of fouling. The model presented in this work includes the prediction with time of the increase of pressure drop and flow redistribution as fouling builds up. The use of the model is illustrated through a case study that shows that in the retrofit of cooling systems, the incorporation of new heat exchangers in parallel is recommended.