Something or someone that comes down; specifically, a pipe or channel through which a fluid descends in industrial or building systems.
From 'downcome' (a descent) plus the agent suffix '-er'; the technical meaning developed from industrial and plumbing contexts where fluids travel downward through pipes.
In a chemical plant or refinery, the 'downcomer' is a critical pipe where hot liquid falls downward to cool—it's named for the simple fact that stuff comes down through it! Industrial vocabulary often has these practical, descriptive compound words.
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