Spiral to vertical furnace tube transition

A furnace tube wall enclosure defining a furnace for a once-through steam generator comprising a lower furnace section of spaced inclined tubes which spiral upward to a transition zone, the inclined tubes being rigidly united by membranes disposed in the intertube spaces, an upper furnace section extending above the lower furnace section, the upper furnace section including load-carrying vertical tubes in fluid communication with the inclined tubes, the vertical tubes having lower portions disposed horizontally alongside of the upper portions of the inclined tubes, means for connecting said lower portions of the vertical tubes to said upper portions of the inclined tubes, said connecting means including vertical external support straps spaced peripherally about the lower furnace section and weldably connected to the membranes uniting the inclined tubes below the lower portion of the vertical tubes, and a plurality of securing plates secured to each of the straps and weldably connected to the vertical tubes alongside of the upper portion of the inclined tubes, wherein the support straps are mounted flush with the outside surface of the inclined tubes, the support straps including bars weldably interconnecting the membranes to the support straps, and wherein the vertical tubes have longitudinal centerlines approximately vertically in line with the centerlines of the support straps.