Abstract: The vapor pump constituted by a mass and heat exchanger comprises an outer enclosure, a set of exchanger tubes disposed inside the enclosure and extending lengthwise therein, opening out at one of the ends in a first radial plane and at their other ends in a second radial plane, which radial planes include means for positioning and supporting the tubes and for closing the empty spaces in the section of the enclosure situated outside the tubes, thereby defining between said radial planes both a first gaseous fluid flow circuit inside the tubes, and a distinct counterflow second fluid flow circuit inside the enclosure but outside the tubes. One of the first and second gaseous fluids constituted by moist hot combustion products from a thermal process while the other gaseous fluid is constituted by new combustion air. Humidifier means are provided for humidifying the air inserted into the enclosure.