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 (see water vapor and humidity).
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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 (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.
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.
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.
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.
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.
