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 fire. Fuel is burned inside the furnace, creating hot gas which boils water in the steam-generat ...

. It is named, like the D-type and A-type boilers, from the approximate shape of its tubes. They are characterised by single

steam Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...

and

water drum Water drums are a category of membranophone characterized by the filling of the drum chamber with some amount of water to create a unique resonant sound. Water drums are used all over the world, but are found most prominently in a ceremonial as w ...

s 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 An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) and lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturate ...

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. Growing to tall, it is characterized by small whitish flowers, a tall stem of fernlike leaves, and a pungent odor. The plan ...

. Small examples of the O-type are used as some package boilers. Most package boilers are

fire-tube boiler A fire-tube boiler is a type of boiler invented in 1828 by Marc Seguin, in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tube ...

s, often used for heating or

process steam A process is a series or set of Action (philosophy), 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 p ...

, and generally work at lower pressures than propulsion boilers. If high pressure steam is needed, such as for

steam turbine A steam turbine or steam turbine engine is a machine or heat engine that extracts thermal energy from pressurized steam and uses it to do mechanical work utilising a rotating output shaft. Its modern manifestation was invented by Sir Charles Par ...

s, 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 (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

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

References

{{Boilers Water-tube boilers Steam boiler types