steam engine, machine for converting heat energy into mechanical energy using steam as a medium, or working fluid. When water is converted into steam it expands, its volume increasing about 1,600 times. The force produced by the conversion is the basis of all steam engines. Steam engines operate by having superheated steam force a piston to reciprocate, or move back and forth, in a cylinder. The piston is attached by a connecting rod to a crankshaft that converts the back-and-forth motion of the piston to rotary motion for driving machinery. A flywheel attached to the crankshaft makes the rotary motion smooth and steady. The typical steam engine has an inlet valve at each end of the cylinder. Steam is admitted through one inlet valve, forcing the piston to move to the other end of the cylinder. This steam then exits through an exhaust valve. Steam from the other inlet valve then pushes the piston back to its original position, and the cycle starts again. In a single-cylinder steam engine the exhaust steam is usually expelled directly into the atmosphere. A compounded steam engine has several cylinders, which the steam passes through successively until, leaving the last cylinder, it is condensed into water and returned to the boiler. From the Greek inventor Heron of Alexandria to the Englishmen Thomas Newcomen and John Cawley, many persons contributed to the work of harnessing steam. However, James Watt's steam engine, patented in 1769, provided the first practical solution. Earlier engines depended on atmospheric pressure to push the piston into the cylinder, where a vacuum was created by sudden cooling of its steam content. Watt's use of a separate condenser resulted in a 75% saving in fuel. It also made possible the use of steam pressure to move the piston in both directions. Watt's continuing efforts produced a governor, a mercury steam gauge, and a crank-flywheel mechanism, all of which prepared the steam engine for a major role in the Industrial Revolution. Sailing vessels gave way to steamboats, and stagecoaches yielded to railroad trains as the steam engine was perfected. Transmitted by belts, ropes, shafts, pulleys, and gears, the energy from steam engines drove machines in factories and mills. Now, however, steam engines have been replaced in most applications by more economical and efficient devices, e.g., the steam turbine, the electric motor, and the internal-combustion engine, including the diesel engine. They are still sufficiently economical to be used in industries where steam is necessary for some purpose in addition to that of driving an engine.

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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