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.
[Webster 1913 Suppl.]