O-type boiler

It is named, like the D-type and A-type boilers, from the approximate shape of its tubes.

They are characterised by single steam and water drums vertically above each other, with curved vertical water tubes to the sides forming an overall cylindrical volume.

There is no grate at the base of this furnace space, so they are fired by liquid burners, oil or gas, rather than a solid fuel furnace producing ash.

[1] The large radiant heating area available allows a combustion rate, for a given furnace volume, of around twice that for a contemporary boiler, such as the Yarrow.

O-type package boilers appeared post-World War II, with the general shift away from coal and to more automated boilers needing fewer human operators.