Steam

From Wikipedia, the free encyclopedia

You have new messages (last change).

For other uses, see Steam (disambiguation).

Castle Geyser in its steam phase

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). Pure steam (unmixed with air, but in equilibrium with water-liquid) has a temperature of around 100 degrees Celsius at standard atmospheric pressure, and occupies about 1,600 times the volume of liquid water (steam can of course be much hotter than the boiling point of water; such steam is usually called superheated steam). 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.

[edit] Uses

A steam engine uses the expansion of steam to drive a piston or turbine to perform mechanical work. In other industrial applications steam is used as a repository of energy, 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 90% of electric power is produced using steam as the working fluid, mainly 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 carefully controlled to avoid excessive 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.

The International Association for the Properties of Water and Steam (IAPWS), 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).