Analysis of preheating process on the thermal performance of multi-effect evaporation for seawater desalination

In this work, three conventional feeding arrangements of multi-effect evaporation systems (forward-feed, parallel-feed, backward-feed) without preheater are studied. The results show that the feedwater temperature of the first effect is too low for the forward-feed and parallel-feed processes to affect the system performance. So it is considered that adding preheating process in the conventional forward-feed and parallel-feed. Compared with the previous work, the influence of preheating position and preheater number on the system performance is studied. It showed that compared with the system without preheating, the Gained output ratio (GOR) increased and the specific heat transfer $(A_{\mathrm{s}})$ decreased. After the number of preheaters is determined, preheating positions have important effects on thermal performance. As the preheating position moved backward, the GOR of the forward-feed system increased first and then decreased, and the $A_{\mathrm{s}}$ showed a linear increase trend. Both the GOR and $A_{\mathrm{s}}$ of the parallel-feed system increase first and then decrease. Last, the performance of five multi-effect evaporation systems, including forward-feed (F-MEE), forward-feed with preheating (PF-MEE), parallel feed (P-MEE), parallel feed with preheating (PP-MEE), and backward-feed (B-MEE) are compared which provides theoretical guidance for the selection of high efficiency and energy saving systems.