A multi-stage flash evaporator is constructed so that the evaporator elements are dimpled plates mounted so that they extend horizontally. One, two, or more sets of plates may be mounted in each evaporator stage, the sets stacked upon one another in a tier arrangement. Each stage may be formed by a circular cross-section vessel, with I-beams--or like supports--extending horizontally at about the horizontal diameter of the vessel, and supporting the dimpled plates on them. A condensate collection trough is mounted below the dimpled plates and at the I-beams, the I-beams having openings in them through which condensate collected by the trough passes from one end of the vessel to the other. The hot liquid --such as sea water to be desalinated--to be evaporated is passed into a bottom chamber of each stage vessel, with vapor flowing upwardly through demistors and channels, into contact with the dimpled plates, and the condensate falling as a thin film down the plates and collecting in the condensate trough. The brine is preheated by passing it into the passageways within the dimpled plates counter to the direction of flow of the brine into and through the bottom chambers. The plates are preferably titanium or aluminum, but may be stainless steel if brackish water is distilled. A sacrificial anode may be mounted adjacent the plates.