Abstract: A paired stage flash evaporator unit is provided with an elongated shell having respective higher temperature and lower temperature evaporation chambers extending along a bottom portion of the shell space and separated from each other by an interstage elongated partition wall. A condenser tube bundle extends longitudinally between opposite shell end walls in an upper portion of the shell space. A transverse interstage partition separates the tube bundle into separate condenser chambers. Each condenser chamber is paired with an evaporation chamber by means of shroud and other isolation structure thereby forming two paired stages in the unit.In each stage, a liquid separator mesh extends coextensively with the evaporation chamber and both condenser chambers to direct vapor from the evaporation chamber into a flow path above the mesh. A distillate collection tray is supported beneath the tube bundle and it is divided into stage-separated tray portions by the transverse interstage partition.