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Boilers

Boilers: a closed vessel in which a liquid is heated or vaporised by the direct application of heat to the outside of the vessel. The heated or vaporised fuild exits the boiler for use in various processes or heating applications.

Boilers have many applications. They can be used in stationary applications to provide heat, hot water, or steam for domestic use, or in generators and they can be used in mobile appliations to provide steam for locomotion in applications such as trains, ships, and boats. Using a boiler is a way to transfer stored energy from the fuel source to the water in the boiler, and then finally to the point of end use.
Construction of boilers in mainly in steel, stainless steel, and wrought iron. In live steam models, copper or brass is often used. Cast iron is used for domistic water heaters. Although these are usually termed "boilers", their purpose is to produce hot water, not steam, and so they run at low pressure and try to avoid actual boiling. The brittleness of cast iron makes it impractical for steam pressure vessels.

The source of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or natural gas. Electric steam boilers use resistance or immersion type heating elements. Nuclear fission is also used as a heat source for generating steam. heat recovery steam generators (HRSGs) use the heat rejected from other processes such as gas turbines.

Boilers can be classified into the following configurations:

Fire-tube Boilers: Here water partially fills a boiler barrel with a small volume left above to accommodate the steam (steam space). The heat source is inside a furnace or firebox that has to be kept permanently surrounded by the water in order to maintain the temperature of the heating surface just below boiling point. The furnace can be situated at one end of a fire-tube which lengthens the path of the hot gases, thus augmenting the heating surface which can be further increased by making the gases reverse direction through a second parallel tube or a bundle of multiple tubes (two pass or return flue boiler); alternatively the gases may be taken along the sides and then beneath the boiler through flues (3-pass boiler). In the case of a locomotive-type boiler, a boiler barrel extends from the firebox and the hot gases pass through a bundle of fire tubes inside the barrel which greatly increase the heating surface compared to a single tube and further improve heat transfer. Fire-tube boilers usually have a comparatively low rate of steam production, but high steam storage capacity. Fire-tube boilers mostly burn soild fuels, but are readily adaptable to those of the liquid or gas variety.

Water-tube Boiler: In this type, the water tubes are arranged inside a furnace in a number of possible configurations: often the water tubes connect large drums, the lower ones containing water and the upper ones, steam; in other cases, such as monotube boiler, water is circulated by a pump through a succession of coils. This type generally fives high steam production rates, but less storage capacity than the above. Water-tube Boilers can be designed to exploit any heat source including nuclear fission and are generally preferred in high pressure applications since the high pressure water/steam is contained within narrow pipes which can withstand the pressure with a thinner wall.

Flash Boiler: is a specialised type of water-tube boiler.

Fire-tube Boiler with Water-tube firebox: Sometimes the two above types have been combined in the following manner: the firebox contains an assembly of water tubes, called thermic syphons. The gases then pass through a concentional fire-tube boiler. Water-tube fireboxes were installed in many Hungarian locomotives.

Sectional Boiler: In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside cast iron sections. These sections are assembled on site to create the finished boiler.

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