Abstract: An improved multistage flash evaporator comprises a plurality of cylindrical shells parallelly and horizontally extending in spaced relation, tube sheets in pairs supported by opposite end portions of the cylindrical shells, each pair of the sheets holding a bundle of cooling tubes therebetween, a top plate connecting the upper portions of the shells to one another, internal bulkheads connecting the lower portions of the shells to a bottom plate, and demisters mounted between the shells, whereby the bulkheads are reduced in height and wall thickness to an economic advantage as compared with the conventional bulkheads, and the demisters are made adjustable in surface areas according to the evaporation loads to be encountered.