Vertical fire-tube boiler

The more common form of vertical boiler,[2] which was very similar in external appearance, instead used a single flue and water-filled cross-tubes.

In any boiler, one of the most prone locations for tube and plate wastage is around the water level, where agitation and boiling is most active.

The upper boiler shell is extended upwards in an annular ring, so as to always maintain the whole length of the tubes submerged.

The firetubes were curved to 'cause eddies in the hot gases as they rise',[4] to allow for free expansion with heat and also to allow a perpendicular joint between tube and tubeplate.

The boiler's operating water level was always maintained within this belt space, keeping the tubes entirely submerged.

A similar approach may be seen as the upper bulge around the vertical boiler of the reconstructed GWR railmotor.