Steam
In physical chemistry, and in engineering, steam refers to
vaporized water. It is a pure, completely invisible gas . Pure steam has a temperature of around 100 degrees
Celsius at standard atmospheric pressure, and occupies about 1,600 times the volume 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 100 C .
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.
Encyclopedia
In physical chemistry, and in engineering,
steam refers to
vaporized water. It is a pure, completely invisible gas . Pure steam has a temperature of around 100 degrees
Celsius at standard atmospheric pressure, and occupies about 1,600 times the volume 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 100 C .
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 is made of tiny droplets of liquid water, not gaseous water, so it is no longer technically steam.
Uses
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 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 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 , maintains international-standard correlations for the
thermodynamic properties of steam, including IAPWS-IF97 and IAPWS-95 .
See also
