Performance Model for a Horizontal Tube Falling Film Evaporator in a Desalination Unit

A model has been developed to predict the evaporative heat transfer coefficient in a horizontal tube falling evaporator and has been applied to evaluate overall performance of a desalination unit. Performance variation with different parameters like operation pressure, temperature, tube diameter, spatial arrangement of the tubes and the type of water distribution system has been studied. Also, variation of the mass flow rate of distilled water inside the exchanger is analyzed. The predicted transfer characteristics obtained from the simulations are compared with the experimental data. It has been observed that the model is able to predict the trends of heat transfer characteristics of the evaporator reasonably well. However, at low liquid film flow rate conditions, the model over predicts the heat transfer characteristics marginally. In order to improve the evaporative exchanger performance, it is observed that preheating of the liquid film before injection into the evaporator is desirable.