What is "downcomer"

Downcomer \Down"com`er\, n. A pipe to conduct something downwards; specif.:

1. (Iron Manuf.) A pipe for leading the hot gases from the top of a blast furnace downward to the regenerators, boilers, etc.

2. (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.

n. 1 A pipe to conduct something downwards. 2 # A large external pipe in a water-tube boiler, carrying unheated water from the steam drum down to the water drum as part of the circulation path. 3 # A pipe for leading the hot gases from the top of a blast furnace downward to the regenerators, boilers, etc.