Sheet metal heat exchange stack or fire tube for gas fired hot air furnaces

Claim 1

A GENERALLY FLAT AND RECTANGULAR HEAT EXCHANGE FIRE TUBE OF THE CHARACTER DESCRIBED AND ADAPTED TO BE POSITIONED VERTICALLY ABOVE A COMBUSTION CHAMBER FOR PASSAGE OF THE PRODUCTS OF COMBUSTION ISSUING FROM SAID CHAMBER UPWARDLY THROUGH THE FIRE TUBE, SAID FIRE TUBE COMPRISING TWO SIMILAR COUNTERPART SHEET METAL SECTIONS WELDED TOGETHER ALONG MARGINAL SEAM LINE TO PROVIDE A HOLLOW TUBULAR STRUCTURE WHICH IS OPEN AT ITS TOP AND BOTTOM AND HAVING OPPOSED CLOSELY SPACED SIDE WALLS OF BOARD RECTANGULAR EXPANSE, AND WIDELY SPACED NARROW VERTICALLY EXTENDING FRONT AND REAR WALLS, SAID WALLS DEFINING AN INTERNAL PASSAGE FOR COMBUSTION GASES, AT LEAST ONE OF SAID SIDE WALLS BEING FORMED WITH AN INWARDLY PROJECTING OFFSET PROTUBERANCE IN THE MEDIAL REGIONS THEREOF EXTENDING PARTIALLY ACROSS SAID INTERNAL PASSAGE AND TERMINATING IN CLOSE PROXIMITY OF THE OTHER SIDE WALL, SAID PROTUBERANCE AND THE DIRECTLY OPPOSED REGION OF SAID OTHER SIDE WALL BEING DESIGNED FOR ENGAGEMENT WITH EACH OTHER WHEN EITHER SIDE WALL EXCEEDS A PREDETERMINED DEGREE OF INWARD BULGING TOLERANCE.