Modelling of heat and mass transfer in a horizontal-tube fallingfilm condenser for brackish water desalination in remote areas

Horizontal-tube falling film condensers are used in many fields: chemical industries, refrigeration, desalination, cooling, etc. However, their performances cannot be predicted precisely due to the complexity of the phenomena. In this paper, heat and mass transfer in a horizontal-tube falling-film condenser used in an innovative desalination plant was studied theoretically. The polypropylene exchanger was designed to work at relatively high temperatures (25– 35°C). In fact, the desalination unit was designed to function in remote areas where the ambient temperature frequently exceeds 30°C [1]. The 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 were compared to experimental data [2]. From this comparison, it was seen that the model is well able to predict the trends of heat and mass characteristics of the condenser. The influence of the different thermal, hydrodynamic and geometric parameters on the condenser performances was investigated. The variations of the distilled water amount inside the exchanger were analysed. The results were used to determine the optimum operational conditions. Finally, the model was also used to optimise the different exchanger components.

Rein.g An analytical study to predict the heat and mass transfer characteristics of the plastic horizontal-tube falling-film condenser was developed. The variations in fluid properties during their progression in the exchanger due to temperature evolution were considered. The elaborated model is based on the resolution of heat and mass transfer equations in each cell of the exchanger. This resolution was effected by using fluid properties calculated at the average temperature in the cell. The influence of inlet air velocity and cooling liquid inlet temperature on heat and mass transfer in the exchanger was investigated. Numerical results show an increase in condensing exchanger performance when the inlet cooling water decreases and air velocity increases. For the conditions corresponding to the real environment of southern Tunisia, the model predicted that the amount of condensed water cannot exceed 2 m3/d.