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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 copper, steel and cast iron. In Live steam toys, brass is often used.
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, or HRSG's 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. The goal in both cases is to maximise the area of contact between the water and the hot gases heating it. For example, steam locomotives have fire-tube boilers, where the fire is inside the tube and the water on the outside. These usually take the form of a set of straight tubes passing through the boiler through which hot gas flows.
In water-tube boilers the water flows through a large number of narrow tubes around the fire. The tubes frequently have a large number of curves and sometimes fins to maximise the surface area. This type of boiler is generally preferred in high pressure applications since the high pressure water/steam is contained within narrow pipes which makes fabrication easier.
There are other types of boiler, largely of historical interest. For example, the Cornish boiler developed around 1812 by Richard Trevithick for generating steam for steam engines. This was both stronger and more efficient than the simple boilers which preceded it. It was a cylindrical water tank around 27 feet long and 7 feet in diameter, and had a coal furnace placed in a single cylindrical tube about three feet wide which passed centrally along the long axis of the tank. The fire was tended from one end and the hot gases from it travelled along the tube and out of the other end, to be circulated back along flues running along the outside of the boiler before being expelled via the chimney. This was later improved upon in the Lancashire boiler which had a pair of furnaces in separate tubes side-by-side. This was an important improvement since each furnace could be stoked at different times, allowing one to be cleaned whilst the other was operating. These designs are really primitive fire tube boilers, and led on to the Scotch boiler which was a popular fire tube design.
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