Abstract: A vapor generator including an upright furnace section the boundary walls of which are formed by a plurality of tubes, a portion of which extend at an angle with respect to a horizontal plane. A plurality of divisional walls are provided which extend from an area externally of the boundary walls and which penetrate a boundary wall and terminate within the furnace section. The latter boundary wall is provided with a seal assembly extending over the area of penetration of the boundary wall. Fluid is passed sequentially through the tubes forming the boundary walls and the division walls to heat the fluid and convert it to vapor.