Abstract: A heat exchanger for the condensation of vapors, comprises a coil pipe having an upper inlet end and a lower outlet end and a plurality of lengths of pipe arranged in zigzag fashion between the upper and lower ends. Liquid is sprayed from above onto the coil pipe, and air is moved upwardly along the coil pipe. The lengths of pipe are inclined downwardly at acute angles to the horizontal from the inlet end to the outlet end. These acute angles progressively increase downwardly in such a way that the angles between pairs of adjacent lengths of pipe progressively increase downwardly for a plurality of those pairs. The inclination to the horizontal of the lowermost length of pipe is not more than 30.degree. and that of the uppermost not less than 3-5.degree.. The increment of angle increase, from pipe length to pipe length, is likewise 3-5.degree.. Vertical strips of plate are disposed between the lengths of pipe, to increase the evaporation surface.