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Steam is vaporized water. It is a transparent gas. 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 (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 mixes with the cooler air. This mist consists of tiny droplets of liquid water. Pure steam emerges at the base of the spout of a steaming kettle where there is no visible vapor.

Saturated steam

Saturated steam is steam at equilibrium with liquid water . It defines the boundary between wet steam and superheated steam on the temperature-enthalpy diagram.

Superheated steam

Main article: Superheated steam
Superheated steam is steam at a temperature higher than its boiling point at a given pressure. For superheating to take place one of two things must occur. Either all of the liquid water must have evaporated or, in the case of steam generators (boilers), the saturated steam must be conveyed out of the steam drum before superheating can occur, as steam can not be superheated in the presence of liquid water.

There are three stages of heating to convert liquid water to superheated steam. First the liquid water’s sensible (the heat that can be measured with a thermometer) heat is raised. Then latent heat (this heat does not raise the temperature of the fluid) is added. After all of the liquid is evaporated or the saturated steam is taken from the steam drum sensible heat is again added superheating the steam.

Industrial uses

Steam engines and steam turbines

A steam engine uses the expansion of steam in order to drive a piston or turbine to perform mechanical work. The ability to return condensed steam as water-liquid to the boiler at high pressure with relatively little expenditure of pumping power is important. Engineers use an idealised thermodynamic cycle, the Rankine cycle, to model the behavior of steam engines.

Steam turbines, are often used in the production of electricity.

Condensation of steam to water often occurs at the low-pressure end of a steam turbine, since this maximizes the energy efficiency, but such wet-steam conditions have to be limited to avoid excessive turbine blade erosion.

Energy storage

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 thermal energy because of water's high heat of vaporization.

Electricity generation

In the U.S., more than 86% of electricity is generated using steam as the working fluid, nearly all by steam turbines.

Cogeneration

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 through a district heating system to provide heat energy after its use in the electric generation cycle. The world's biggest steam generation system is the New York City steam system which pumps steam into 100,000 buildings in Manhattanmarker from seven cogeneration plants.

Sterilization

An autoclave, which uses steam under pressure, is used in microbiology laboratories and similar environments for sterilization.

Agricultural use

In agriculture steam is used for soil sterilization to avoid the use of harmful chemical agents and increase soil health.

Domestic uses

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.

Steam tables

Steam tables are tables of thermodynamic data for water/steam. They are often used by engineers and scientists in design and operation of equipment where thermodynamic cycles involving steam are used. Additionally, thermodynamic phase diagrams for water/steam, such as a temperature-entropy diagram or a Mollier diagram shown in this article, may be useful.

Steam explosion

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

See also



References

  1. Singh, R Paul. (2001). Introduction to Food Engineering. Academic Press. ISBN 978-0-12-646384-2.
  2. http://www.spiraxsarco.com/resources/steam-engineering-tutorials/steam-engineering-principles-and-heat-transfer/superheated-steam.asp
  3. Carl Bevelhymer, "Steam", Gotham Gazette, November 10, 2003


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