Vapor generator

In a forced circulation fluid heating unit, upright walls forming a furnace chamber, at least one of the walls including upper and lower panels, each of said panels being comprised of a multiplicity of upwardly extending laterally spaced tubes rigidly united throughout most of their length, mixing header means receiving fluid from the lower panels and discharging the mixed fluid to the upper panels, some of said upper and lower panels being bent out of the plane of the wall at first and second levels, respectively, for connection to the header means and interlaced with and laterally spaced from each other in the plane of the wall between the first and second levels, the first level being subjacent the second level, the remaining upper and lower panels being bent out of the plane of the wall at about the first and second levels, respectively, for connection to the header means, and means for supporting said one wall including metallic webs weld uniting the interlaced upper and lower panels and transmitting the load of the lower panels to the upper panels, and means for top supporting said upper panels.