Source : Webster's Revised Unabridged Dictionary (1913)

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.