A vapor generator comprising a plurality of tubes connected together and arranged to form the boundary walls of a gas-tight enclosure, said tubes extending at an acute angle with respect to a horizontal plane for at least a portion of the height of said enclosure, at least one division wall associated with said enclosure, said division wall comprising a plurality of interconnected tubes and having a first portion extending within said enclosure and a second portion extending integrally with said first portion and penetrating one of said boundary walls, portions of the tubes forming said one boundary wall being bent outwardly from the plane of said boundary wall to form the area of penetration, seal means cooperating with said one boundary wall and said division wall portion for sealing the area of penetration of said one boundary wall by said second division wall portion to maintain said gas-tightness, said seal means comprising a corrugated plate extending over said area of penetration, a first portion of the corrugated plate extending along the plane of said one boundary wall, a second portion of the corrugated plate extending at an angle to the plane of said boundary wall to define a space, refractory material positioned in the space, heating means associated with said enclosure and means for passing fluid through said tubes to apply heat to said fluid.