Steam

steam

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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steam

1. the gas or vapour into which water is changed when boiled

2. the mist formed when such gas or vapour condenses in the atmosphere

3. get up steam (of a ship, etc.) to work up a sufficient head of steam in a boiler to drive an engine

4. Austral slang cheap wine

5. driven, operated, heated, powered, etc., by steam

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steam [stēm]

(physics)

Water vapor, or water in its gaseous state; the most widely used working fluid in external combustion engine cycles.

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Steam

Water vapor, or water in its gaseous state. Steam is the most widely used working fluid in external combustion engine cycles, where it will utilize practically any source of heat, that is, coal, oil, gas, nuclear fuel (uranium and thorium), waste fuel, and waste heat. It is also extensively used as a thermal transport fluid in the process industries and in the comfort heating and cooling of space. The universality of its availability and its highly acceptable, well-defined physical and chemical properties also contribute to the usefulness of steam.

The temperature at which steam forms depends on the pressure in the boiler. The steam formed in the boiler (and conversely steam condensed in a condenser) is in temperature equilibrium with the water. Under these conditions, with steam and water in contact and at the same temperature, the steam is termed saturated. Steam can be entirely vapor when it is 100% dry, or it can carry entrained moisture and be wet. After the steam is removed from contact with the liquid phase, the steam can be further heated without changing its pressure. If initially wet, the additional heat will first dry it and then raise it above its saturation temperature. This is a sensible heat addition, and the steam is said to be superheated. Superheated steam at temperatures well above the boiling temperature for the existing steam pressure follows closely the laws of a perfect gas. Chiefly because of its availability, but also because of its nontoxicity, steam is widely used as the working medium in thermodynamic processes. It has a uniquely high latent heat of vaporization. Steam has a specific heat about twice that of air and comparable to that of ammonia. The specific heat of steam is relatively high so that it can carry more thermal energy at practical temperatures than can other usable gases. See Boiler, Steam engine, Steam-generating unit, Steam heating, Steam turbine, Thermodynamic cycle, Thermodynamic principles

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Steam 

the gaseous state of water. Water is heated to the point of vaporization in various typs of heat exchangers, for example, steam boilers and evaporators. It is the working medium in steam power plants and serves as the heat carrier in ventilation systems as well as in heating and water-supply systems. Steam is also used for industrial purposes. When water is heated to 100 °C at a pressure of 101.325 kilonewtons per square meter (kN/m²), or 760 mm Hg, it begins to boil, and steam forms. The temperature of the steam is also 100°C, but the steam occupies a much greater volume than the water. As long as any water remains in the liquid state, the temperature of the system is constant despite the continued addition of heat. When water and steam are in equilibrium, the system has reached the state of saturation, which can be precisely characterized by a specific saturation pressure and saturation temperature. The temperature can begin to rise again only after all the water is converted into steam; the volume of steam at 100°C is 1,673 times greater than the volume of water at 4°C. Upon further heating above the saturation temperature, steam passes from the saturated state into the superheated state. If vaporization is performed at various pressures, the temperature of vaporization changes as a function of pressure (see Table 1).

Table 1. Temperature and density dependence of saturated water and steam on pressure of saturated steam
Steam pressure MN/m2 (kgf/cm2)	Temperature °C	Density kg/m3
		Water	Steam
0.101
(1) ................	99.1	959	0.58
1.01
(10) ...............	179	887.9	5.05
10.1
(100)...............	309.5	691.9	54.2
22.3
(220)...............	372.1	420	229

The heat that is required to raise 1 kg of water from 0°C to the saturation temperature is called the enthalpy of water, while the heat that is expended to convert 1 kg of water at the saturation temperature into a dry, saturated vapor is called the heat of vaporization. At the critical pressure, the heat of vaporization equals 0, but if heating is performed at higher pressures, the supply of heat causes a continuous change of temperature. This change is accompanied by a continuous increase in volume without a concurrent separation of material into liquid and gaseous phases. Water is at the critical point when the pressure is 22.1 meganewtons per square meter (MN/m²), or 225.65 kilograms-force per square centimeter (kgf/cm²); when the temperature is 374.15°C; and when the density is 303 kg/m³. Steam is heated above the critical point in steam boilers. As a rule, steam engines and turbines use superheated instead of saturated steam, since machines that are powered by superheated steam are more efficient than those powered by saturated steam; superheated steam is often called live steam. In the USSR and abroad, the strongest steam power plants use steam at a pressure of 25 MN/m² (225 kgf/cm²) and at a temperature of 545°C. For heating purposes, for example, in space heaters, the use of saturated steam is economically feasible because the heat-transfer coefficient for condensating saturated steam is substantially higher than for superheated steam.

The properties of water vapor were first studied in the 16th and 17th centuries. At the beginning of the 17th century, the Italian scientist G. della Porta investigated the specific volume of water vapor. At the same time, the French scientist S. de Caus researched aspects of steam condensation. Various properties of water vapor were investigated at the end of the 18th century: the relationship between vaporization temperature and pressure were studied by D. Papin; heat of vaporization was investigated by J. Black and J. Watt; and Watt also conducted research on the specific volume of steam at the pressure of 0.1 MN/m². The study of the properties of steam as a working medium in steam engines was undertaken in the 1840’s by the French scientist A V. Regnault. In 1904 the German scientist R. Mollier proposed an enthalpy-entropy diagram, called a Mollier diagram, for water vapor.

Studies on the properties of water vapor were carried out in Russia during the 19th century by several scientists, including L. G. Bogaevskii, B. B. Golitsin, and A. I. Nadezhdin. The Soviet scientist I. I. Novikov derived a theoretical equation of state for superheated steam, which he treated as a nonideal gas. Far-reaching experimental studies of the thermodynamic and physical properties of water vapor were performed by several noted scientists, among them Professor M. P. Vukalovich, Professor N. B. Vargaftig, Academician V. A. Kirillin, and Professor D. L. Timrot. Based on the studies of Soviet scientists, tables and diagrams were compiled in the USSR concerning the thermodynamic properties of water and water vapor at pressures of up to 100 MN/m² and temperatures of up to 1000°C. Skeleton tables that contain data on the properties of steam were adopted in New York City in 1963 by the Fourth Conference of the International Association on the Properties of Steam.

REFERENCES

Vukalovich, M. P., and I. I. Novikov. Tekhnicheskaia termodinamika, 4th ed. Moscow, 1968.
Kirillin, V. A., V. V. Sychev, and A. E. Sheindlin. Tekhnicheskaia termodinamika. Moscow, 1968.
Vukalovich, M. P. Tablitsy termodinamicheskikh svoistv vody i vodianogo para, 7th ed. Moscow-Leningrad, 1963.
Vukalovich, M. P., S. L. Rivkin, and A. A. Aleksandrov, Tablitsy teplofizicheskikh svoistv vody i vodianogo para. Moscow, 1969.