Heat Transfer Performance of Falling Film Evaporators Used in Multi-effect Desalination (MED) Plants

In multi-effect desalination (MED) plants, the evaporators are usually of horizontal falling film type. For this kind of evaporators, high heat transfer coefficient can be achieved over low temperature difference and low spray density/liquid load. The current operating temperature range of MED plants is from 65 to 40 °C and researchers are trying to widen this range (85–5 °C), in order to enhance desalted water productivity. In this study, review of heat transfer coefficient correlations for falling films was carried out. The heat transfer coefficient prediction by different studies was compared with respect to film Reynolds number and working temperature. It was found that the empirical correlations by Fujita and Tsutsui, and Danilova et al. were better in evaluating heat transfer coefficient for MED application. It was concluded that increasing the top brine temperature (TBT) of MED evaporator is more beneficial than decreasing the lower temperature. The estimated heat transfer coefficient at 5 °C was 30–65% was less than that of heat transfer coefficient at 85 °C.