The locomotive type of boiler has a square fire box at one end, but the cylindrical type of marine-shell boiler is fitted with one or more corrugated furnaces such as is shown in Fig. 221. These furnaces are divided along the center by the grate bars, and they are corrugated to enable them to resist the collapsing pressure of the water which surrounds them in the boiler. Each furnace is usually about 40 inches in diameter and 7 feet long.

These furnaces are made in the United States only by The Continental Iron Works of Brooklyn, New York. Briefly, the process of making a furnace is as follows: A mild steel plate from the rolling mill is bent into a cylinder in the bending rolls. The two edges are lap-welded by passing the lap, at a welding heat, between two disc rollers pressed against the seam by hydraulic pressure. After welding, the cylinder is heated to a bright red heat in a furnace. It is then lifted by the crane and carried to the corrugating machine, the vertical rolls of which are suitably shaped to press the corrugations gradually on the cylinder as it revolves repeatedly between the rolls. After the corrugations are pressed, the end is heated and flanged to the shape required, and the whole furnace is then annealed.

These furnaces are always made to order.