add. ·noun a pipe to conduct something downwards;.
2. downcomer ·add. ·noun a pipe for leading the hot gases from the top of a blast furnace downward to the regenerators, boilers, ·etc.
3. 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.