Downcomer

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. [ Webster 1913 Suppl. ]