downcomer

Down"com`er, n. A pipe to conduct something downwards; specif.: (a) (Iron Manuf.) A pipe for leading the hot gases from the top of a blast furnace downward to the regenerators, boilers, etc. (b) (Steam Engin.) In some water-tube boilers, a tube larger in diameter than the water tubes to conduct the water from each top drum to a bottom drum, thus completing the circulation.

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