One or more pipes that extended through the length of a boiler. The flue carried hot gases from the firebox to the breeching and aided in heating the water within the boiler. The "western river boiler" used on nearly all steamboats is designed on the return flue principle, that is, the flame is passed under the boilers from the flame bed and passed into the flues at the rear end, returning forward through the flues, and piped into the smokestacks immediately over top of the furnace doors. The flue could become red hot when exposed by low water in the boiler. This often then led to a collapse of the flue and an explosion of the boiler.
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