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 Steam phase eruption of Castle Geyser in Yellowstone Park |
 A temperature-versus-entropy diagram for steam |
 A Mollier enthalpy-versus-entropy diagram for steam |
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 Template:Convert (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. Pure steam is present at the base of the spout of a steaming kettle where there is no visible vapor.
[edit] Saturated steam
Saturated steam is steam at equilibrium with liquid water [1]. It defines the boundary between wet steam and superheated steam on the temperature-enthalpy diagram.
[edit] 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.[2]
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.
[edit] Steam engine
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 behaviour of steam engines.
[edit] 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 energy because of water's high heat of vaporization.
[edit] Electricity generation
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.
[edit] 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 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.[3]
[edit] Sterilization
An autoclave, which uses steam under pressure, is used in laboratories for sterilization.
[edit] 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.
[edit] 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.
[edit] Steam explosion
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'.
[edit] See also
- Electrification
- Food steamer or steam cooker
- Geyser—geothermally-generated steam
- IAPWS—an association that maintains international-standard correlations for the thermodynamic properties of steam, including IAPWS-IF97 (for use in industrial simulation and modelling) and IAPWS-95 (a general purpose and scientific correlation).
- Industrial Revolution
- Live steam
- Mass production
- Nuclear power—and power plants use steam to generate electricity
- Psychrometrics—moist air/vapour mixtures, humidity and air conditioning
- Steam locomotive
- Oxyhydrogen
[edit] References
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