A boiler is a closed vessel in which water or other fluid is heated under pressure. The steam or hot fluid is then circulated out of the boiler for use in various process or heating applications. A safety valve is required to prevent over pressurisation and possible explosion of a boiler.
Construction of boilers is mainly limited to steel and cast iron.
Sources of heat for the boiler can be the combustion of fuels such as wood, coal, oil or natural gas. Electric boilers use resistance or immersion type heating elements. Nuclear fission is also used as a heat source for generating steam. Waste-heat boilers use the heat rejected from other processes such as gas turbines.
Boilers can also be classified into fire-tube or water-tube boilers depending on whether the fire is inside or outside. For example, steam locomotives have fire-tube boilers, where the fire is inside the tube and the water on the outside. In water-tube boilers the water flows in tubes around the fire.
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