Steam

In physical chemistry and in engineering, steam refers to vaporized water. It is a pure, invisible gas (for mist see below), which at standard atmospheric pressure has a temperature of around 100 degrees celsius, and occupies about sixteen hundred times the volume of liquid water. Superheated steam is steam heated well beyond its boiling point at standard pressure. A steam engine uses the expansion of steam to drive a piston or turbine and so 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 the power is produced using steam as the working medium, 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.

A steam explosion (also called a littoral explosion) is a violent boiling or flashing of water into steam, typically occurring when water is superheated. The water changes from a liquid to a gas with extreme speed, increasing dramatically in volume (see above). A steam explosion sprays steam and boiling-hot water and the hot medium that heated it in all directions (if not otherwise confined, e.g. by the walls of a container), creating a danger of scalding and burning.

Steam explosions are most often encountered where hot lava meets sea water. A dangerous steam explosion can be created when liquid water encounters hot, molten metal. As the water explodes into steam, it splashes the burning hot liquid metal along with it, causing an extreme risk of severe burns to anyone located nearby and creating a fire hazard.

Events of this general type are also possible if, under extreme circumstances, the fuel of a liquid-cooled nuclear reactor becomes molten. (They are known as fuel-coolant interactions or fci). In these events the passage of the pressure wave through the predispersed material creates flow forces which further fragment the melt, resulting in rapid heat transfer, and thus sustaining the wave.

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. After gaseous steam has intermixed with air, it is no longer properly called steam and is instead referred to as water vapor.

cs:Pára de:Wasserdampf fr:Vapeur it:vapore ja:水蒸気 nl:Stoom pl:Para wodna zh:水汽