·add. ·noun A pipe to conduct something downwards;.
II. Downcomer ·add. ·noun A pipe for leading the hot gases from the top of a blast furnace downward to the regenerators, boilers, ·etc.
III. Downcomer ·add. ·noun 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.