Vessels or containers of every conceivable shape have been used as boilers. Many of the older types are now obsolete, but the following are these most commonly used:—
VERTICAL BOILERS.
[61]. Ordinary centre flue boiler. Sometimes the centre flue is surrounded with tubes, as [No. 65].
[62]. Vertical multitubular.
[63]. Vertical boiler, with diagonal tubes and smoke boxes.
[64]. Vertical return-flue.
[65]. “Pot” boiler.
[66]. “Field” boiler; with suspended tubes and internal circulating tubes.
[67]. Vertical egg-end boiler; with spiral flue. Large vertical boilers sometimes have cross flues, or large tubes.