is 34 feet long and is supported by 25 ribs 13 which are equally spaced at 16 inch intervals the assembled inner vessel 11 and surrounding ribs 13 are enclosed by an external enclosure formed of welded sheet steel skin plates 14 each stiffener plate or rib 13 is preferably fabricated from four symmetrical and mutually interlocking quadrants 15 which are sequentially welded together to surround and support the vessel 11 as the chamber assembly progresses each quadrant 15 is provided with dovetail like joining means 16 whereby each rib sub assembly comprising four interlocking elements is substantially self supporting making the final welding process significantly easier and more accurate each individual quadrant 15 is also provided with outward facing lugs 17 which after each group of four elements is welded together to form an individual rib 13 are enabled to engage a corresponding slot in its corresponding skin plate 14 in addition certain of the ribs 13 are provided with upstanding external lifting lugs 18 by which the chamber may be lifted and positioned for transport or use each of the ribs 13 of the present embodiment is made of one inch steel plate certain of the ribs 13 are also provided with one inch thick sheet steel external feet 19 positioned at spaced intervals on each side of the chamber along its length.