a passage way for conducting a current of fresh, foul, or heated air from one place to another. A pipe or passage for conveying flame and hot gases through surrounding water in a boiler; distinguished from a tube which holds water and is surrounded by fire. Small flues are called fire tubes or simply tubes. Flue boiler. See boiler. Flue bridge, the separating low wall between the flues and the laboratory of a reverberatory furnace. Flue plate, the aggregate surface of flues exposed to flame or the hot gases.