Abstract: A plate evaporator comprises several heat exchange plates arranged substantially vertically in spaced face-to-face relationship to define between the plates evaporation passages alternating with heating medium passages. An increasing cross-sectional area and a decreasing perimeter of the evaporation passages are achieved by so arranging the two heat transfer surfaces of adjacent plates defining an evaporation passage that they diverge from each other in the flow direction of the medium to be evaporated and by decreasing the width of said heat transfer surfaces in said direction.

Abstract: A plate evaporator of the rising film type and/or the falling film type comprises several heat exchange plates essentially vertically arranged in spaced face-to-face relationship to define evaporation passages alternating with heating medium passages. Evaporation passages with increasing cross-sectioned area and decreasing perimeter in the flow direction of the feed liquid are formed by providing adjacent plates encompassing one evaporation passage with bulges (for example, ridges) which bulges, compared with imaginary planar plates, entail reduced volume of the evaporation passages. The size of said bulges (that is, the volume-reducing effect of said bulges) is gradually reduced in the flow direction of the medium to be evaporated. Thus, the cross-sectional area of the evaporation passages is increased while their perimeter is reduced in said direction.