An experimental and theoretical study has been made of tubular heat exchangers in which phase change occurs. The efficiency of heat exchangers in which phase change occurs, depends strongly on the distribution of the vapour liquid mixture. This is the case in evaporators and condensers with phase change in the tubes. In view of the large number of applications of such heat exchangers (such as in industrial processes, refrigerators, heat pumps), improvements could lead to large energy savings. The programme consists of experimental studies and the development of a model, based on the Computational Fluid Dynamics TRIO Code, which simulates 2 phase (vapour liquid) flow and which was evaluated with data resulting from experimental work. Heat exchangers with air water mixtures were studied on the TRIO Code was validated to include vapour liquid mixtures. A 2-phase heat exchanger was also developed for applications in refrigeration. This will be installed for testing in a 250 kW refrigerator, with R22 as the working fluid. In this experimental set up sensors which measure the mass flow of the vapour liquid mixture will be used. Measurements have been made of maldistribution in shell and tube heat exchangers using a 2-phase probe developed during the project. Data show that improvements could be made to the design of these heat exchangers, leading to better performance (eg increased evaporation duty and reduced pressure drop).