
large external pipes in many water-tube boilers, carrying unheated cold water from the steam drum down to the water drum as part of the circulation path.
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http://en.wikipedia.org/wiki/Glossary_of_boiler_terms

• (n.) A pipe to conduct something downwards; • (n.) A pipe for leading the hot gases from the top of a blast furnace downward to the regenerators, boilers, etc. • (n.) 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.D...
Found on
http://thinkexist.com/dictionary/meaning/downcomer/

a pipe through which flow is downward.
Found on
http://www.encyclo.co.uk/visitor-contributions.php
Down'com`er noun 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 w...
Found on
http://www.encyclo.co.uk/webster/D/116

a pipe, tube, or passage for conducting fluid materials downward.
Found on
https://www.infoplease.com/dictionary/downcomer
No exact match found.