The Collaborative International Dictionary of English v.0.48:
Flue \Flue\ (fl[=u]), n. [Cf. OF. flue a flowing, fr. fluer to
flow, fr. L. fluere (cf. Fluent); a perh. a corruption of
1. An inclosed passage way for establishing and directing a
current of air, gases, etc.; an air passage; esp.:
(a) A compartment or division of a chimney for conveying
flame and smoke to the outer air.
(b) A passage way for conducting a current of fresh, foul,
or heated air from one place to another.
(c) (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.
2. In an organ flue pipe, the opening between the lower lip
and the languet.
[Webster 1913 Suppl.]
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.