With primary reference now to FIGS. 1 through 3, the elongated furnace tube 44 is of circular cross section, and is adapted to be positioned generally vertically relative to surface 31. Tube 34 is braced intermediately along its length by contact with preheat systems 78, 79, and is supported at its lower portion 87 by a coaxially positioned, rigid mounting ring 90 which is weldably secured to tube 44 by a plurality of radially spaced-apart members 92. The lower portion 87 of tube 44 penetrates a suitable orifice provided in base plate 51, and is secured to frusto-conical, tapered portion 46. Mounting ring 90 is positioned on the tops of a plurality of radially spaced apart tabs 96 which extend radially outwardly from a circular, rigid angle iron ring 98. More particularly, circumferentially aligned coaxially positioned iron ring 98 includes a top portion 100 and a side portion 101 secured to base plate 51. An air passageway 102 is defined between shell 42 and inner furnace tube 44. This passageway extends upwardly through the apparatus and is vented through a plurality of radially spaced apart holes 104 formed in plate 41, which forms a shoulder relative to shroud l6. Cooling air is inputted into the device by a conventional fan 70 communicating via duct 71, and an internal passageway 71A defined interiorly of the angle iron ring 98. A plurality of radially spaced apart venting orifices 110 are defined at spaced apart locations in section 101 of mounting ring 98 to facilitate admission of cooling air at uniform intervals about the periphery of furnace tube base 87. In the preferred form of the invention a circumferential insulation ring 114 is coaxially disposed between cooling region 102 and the outermost stainless steel shroudr42. Insulation layer 114 thus promotes thermal efficiency.