The Collaborative International Dictionary
Flue \Flue\ (fl[=u]), n. [Cf. OF. flue a flowing, fr. fluer to flow, fr. L. fluere (cf. Fluent); a perh. a corruption of E. flute.]
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An inclosed passage way for establishing and directing a current of air, gases, etc.; an air passage; esp.:
A compartment or division of a chimney for conveying flame and smoke to the outer air.
A passage way for conducting a current of fresh, foul, or heated air from one place to another.
(Steam Boiler) 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.
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In an organ flue pipe, the opening between the lower lip and the languet.
Flue boiler. See under Boiler.
Flue bridge, the separating low wall between the flues and the laboratory of a reverberatory furnace.
Flue plate (Steam Boiler), a plate to which the ends of the flues are fastened; -- called also flue sheet, tube sheet, and tube plate.
Flue surface (Steam Boiler), the aggregate surface of flues exposed to flame or the hot gases.
Usage examples of "fire tubes".
They looked like highly intricate cannons with fire tubes, solenoids, and coolant piping wrapped around a long transparent shaft.
All this posed a continued threat, for clever men were spread across the world, and spurred by such stories they might begin to recreate the fire tubes of Heruta.
Unfortunately, EB11/R doesn't explain how superheating was carried out, and EA contents itself with a cryptic, 'this mechanism returned the steam through the fire tubes of the boiler for reheating.
That's one thing that made us decide to cast and use those fire tubes.