Abstract: Heat exchanger with streaming liquid to vaporize a liquid by heat exchange with a second fluid, of the type comprising a parallelepipedal body formed form an assembly of parallel vertical plates (4) defining between them a multitude of flat passages (18, 19) distributed in an assembly of vaporization passages (18), and in an assembly of heating passages (19). Each passage contains an undulant-spacer (20) with vertical generatrices. Structure for distribution of the liquid is provided at the upper end of the exchanger (2) to distribute the liquid over all the length of the vaporization passages (18). Structure (9) is provided to direct the second fluid into the heating passages (19). The distribution structure is disposed in compartments (23) closed at their upper end and situated each above a heating passage (19), from which the compartment is separated by a horizontal strip (22).