Apparatus for converting a liquid to vapour. A boiler consists of a furnace in which fuel is burned, surfaces to transmit heat from the combustion products to the water (or other liquid), and a space where steam (or vapour) can form and collect. A conventional boiler burns a fossil fuel or waste fuel; a nuclear reactor may instead supply the heat. There are two types of conventional steam boiler. In a fire-tube boiler, the water surrounds the steel tubes through which hot gases from the furnace flow; easy to install and operate, fire-tube boilers are widely used to heat buildings and to provide power for factory processes, as well as in steam locomotives. In a water-tube boiler, the water is inside tubes, with the hot furnace gases circulating outside the tubes; water-tube boilers, which produce more and hotter steam, are used in ships and factories. The largest are found in the central-station power plants of public utilities; other large units are used in steel mills, paper mills, oil refineries, and chemical plants. Seealso steam engine.

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Machine that uses steam power to perform mechanical work through the agency of heat (hence a prime mover). In a steam engine, hot steam, usually supplied by a boiler, expands under pressure, and part of the heat energy is converted into work. The rest of the heat may be allowed to escape, or, for maximum engine efficiency, the steam may be condensed in a separate apparatus, a condenser, at comparatively low temperature and pressure. For high efficiency, the steam must decrease substantially in temperature as it expands within the engine. The most efficient performance (i.e., the greatest output of work in relation to the heat supplied) is obtained by using a low condenser temperature and a high boiler pressure. Seealso Thomas Newcomen, James Watt.

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Invisible gas consisting of vaporized water. When mixed with minute droplets of water, it has a white, cloudy appearance. In nature, steam is produced by the heating of underground water by volcanic processes and is emitted from hot springs, geysers, fumaroles, and some volcanoes. Steam also can be generated on a large scale by technological systems, such as those using fossil-fuel-burning boilers and nuclear reactors. Modern industrial society relies on steam power; water is heated to steam in power plants, and the pressurized steam drives turbines that produce electric current: thermal energy is converted to mechanical energy, which is converted into electricity.

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In physical chemistry, and in engineering, steam refers to vaporized water. It is a pure, completely invisible gas (for mist see below). At standard temperature and pressure, pure steam (unmixed with air, but in equilibrium with liquid water) occupies about 1,600 times the volume of an equal mass of liquid water. In the atmosphere, the partial pressure of water is much lower than 1 atm, therefore gaseous water can exist at temperatures much lower than 100 C (see water vapor and humidity).

In common speech, steam most often refers to the white mist that condenses above boiling water as the hot vapor ("steam" in the first sense) mixes with the cooler air. This mist is made of tiny droplets of liquid water, not gaseous water, so it is no longer technically steam. In the spout of a steaming kettle, the spot where there is no condensed water vapor, where there appears to be nothing there, is steam.


A steam engine uses the expansion of steam in order to drive a piston or turbine to perform mechanical work. In other industrial applications steam is used for energy storage, which is introduced and extracted by heat transfer, usually through pipes. Steam is a capacious reservoir for energy because of water's high heat of vaporization. The ability to return condensed steam as water-liquid to the boiler at high pressure with relatively little expenditure of pumping power is also important. Engineers use an idealised thermodynamic cycle, the Rankine cycle, to model the behaviour of steam engines.

In the U.S., more than 86% of electric power is produced using steam as the working fluid, nearly all by steam turbines. Condensation of steam to water often occurs at the low-pressure end of a steam turbine, since this maximises the energy efficiency, but such wet-steam conditions have to be limited to avoid excessive turbine blade erosion.

When liquid water comes in contact with a very hot substance (such as lava, or molten metal) it can flash into steam very quickly; this is called a steam explosion. Such an explosion was probably responsible for much of the damage in the Chernobyl accident and for many so-called 'foundry accidents'.

Steam's capacity to transfer heat is also used in the home: for cooking vegetables, steam cleaning of fabric and carpets, and heating buildings. In each case, water is heated in a boiler, and the steam carries the energy to a target object. "Steam showers" are actually low-temperature mist-generators, and do not actually use steam.

In electric generation, steam is typically condensed at the end of its expansion cycle, and returned to the boiler for re-use. However in cogeneration, steam is piped into buildings to provide heat energy after its use in the electric generation cycle. The world's biggest steam generation system is Con Edison in New York City which pumps steam into 100,000 buildings in Manhattan from seven cogeneration plants.

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