An O-type boiler is a form of

water-tube boiler A high pressure watertube boiler (also spelled water-tube and water tube) is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which boils water in the steam-gen ...

. It is named, like the D-type and

A-type boiler A type or type A may refer to: * A-type asteroid, a type of relatively uncommon inner-belt asteroids * A type blood, a type in the ABO blood group system * A-type inclusion, a type of cell inclusion * A-type potassium channel, a type of voltage-gat ...

s, 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. O-type boilers first appeared as marine boilers, such as the Johnson boiler, once the switch from coal firing to oil firing began. 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 ''Achillea millefolium'', commonly known as yarrow () or common yarrow, is a flowering plant in the family Asteraceae. Other common names include old man's pepper, devil's nettle, sanguinary, milfoil, soldier's woundwort, and thousand seal. The ...

. Small examples of the O-type are used as some package boilers. Most package boilers are fire-tube boilers, often used for heating or

process steam A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process, activities that produce a specific se ...

, and generally work at lower pressures than propulsion boilers. If high pressure steam is needed, such as for steam turbines, then a water-tube boiler may be preferred and these are mostly O-type. 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. O-type boilers are available with or without end water-walls. They may also use tube and

refractory In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...

cement infill, or else a steel membrane wall between the tubes.

References

{{Boilers Water-tube boilers Steam boiler types