The heat transfer of air-water-vapour mixtures in a desalination plant, using the aero-evapo-condensation process, has been studied theoretically and experimentally. The present unit consists of a falling-film evaporator and condenser, made of polypropylene. It was designed to work at low temperatures (of 60-90 degrees C) utilizing geothermal energy. A model for heat-and-mass transfers in the plant is developed, resulting in a set of classical equations. Two experimental pilot studies were used. The latter, installed in France and the south of Tunisia, were supplied by fuel and geothermal energy respectively. The influences of different thermal and hydrodynamic parameters on the unit performances were investigated. Experimental results were compared with the model predictions. From this comparison, it can be learnt that the model is well able to predict the trends of the heat-and-mass characteristics of the evaporator. The present process was compared with other desalination techniques. It was shown that in areas where geothermal water resources are available, the process is promising either for cooling or desalting geothermal water.