In this study, heat and mass transfer in a horizontal-tube falling-film evaporator, destined to be used in a desalination plant functioning by aero-evapo-condensation process, is studied theoretically. The present exchanger is made of polypropylene and it was designed to work at low temperatures (60 degrees C-90 degrees C) utilising geothermal energy. The one-dimensional model developed, uses basic aerodynamic, hydrodynamic and heat/mass transfer information to predict the performance of the exchanger. The predicted transfer characteristics obtained from the simulations are compared with experimental data. From this comparison it can be learnt that the model is well able to predict the trends of heat and mass characteristics of the evaporator. However, in low liquid film now rate conditions, the model was found to overpredict the transfer characteristics slightly and the reasons for the differences are discussed. The influence of the different thermal, hydrodynamic and geometric parameters on the evaporator performances was investigated. The variations of the distilled water amount inside the exchanger are analysed. The results are used to determine the operational conditions corresponding to a maximum evaporative performances. The model was also used to optimise the different components of the exchanger.