Liquid oxygen of a low pressure column forms a bath 5 at the top of a heat exchanger 2 of the type having plates defining vertical passages 17, 18. This liquid is predistributed along every other passage 17 by a series of apertures 27 formed for example in the plates, and then is uniformly distributed in a fine manner in the same passages by a packing 24 so as to form a continuous running liquid film. The gaseous nitrogen of a medium pressure column is introduced into the remaining passages 18 and condensed by heat exchange with the oxygen which is vaporized.