What is "tube sheet"

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.]

1. An inclosed passage way for establishing and directing a current of air, gases, etc.; an air passage; esp.:

   1. A compartment or division of a chimney for conveying flame and smoke to the outer air.

   2. A passage way for conducting a current of fresh, foul, or heated air from one place to another.

   3. (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.

   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.