Every single heat exchanger in operation in modern industries is exposed to fouling to a greater or lesser extent depending on the surface temperature, surface condition, material of construction, fluid velocity, flow geometry and fluid composition. The fouling phenomenon is time dependent and will result in a decrease in thermal effectiveness of a heat exchanger. Once the thermal effectiveness decreases to a minimum acceptable level, cleaning of the equipment becomes necessary to restore its performance. This thesis investigates the effects of fouling in a double pipe heat exchanger. The first part consisted of the design and construction of a double pipe heat exchanger that corresponds to our budget and meets our demands. These demands are the possibility of the change in fluid velocity, the ability to use different fluid processes, to have an easy way to get th e inlet and outlet temperatures and to design a double pipe system easy to strip out in order to analyse fouling on the heat transfer surface. The main lines of research are carried out to establish a comparison between the normal development of fouling in a heat exchanger shown in theory books, and the development of fouling in a heat exchanger designed by us. The results of the research also indicate how fouling affects heat transfer, especially heat transfer coefficients and hot and cold fluid outlet temperatures. Dairy products were used as process fluid. Tests were carried out for periods of up to eight hours. The evolution o f temperature, heat transfer, overall h eat transfer coefficient and fouling resistance over time were investigated. The evolution of the temperatures with time and the overall h eat transfer coefficient values are affected by fouling. The evolution o f heat transfer with time occurred according to the different fouling mechanisms. Fouling deposits increased with time until reaching the point where they produced a blockage of the system.