Steam

(physics) Water vapor, or water in its gaseous state; the most widely used working fluid in external combustion engine cycles.

Water vapor, or water in its gaseous state. Steam is the most widely used working fluid in external combustion engine cycles, where it will utilize practically any source of heat, that is, coal, oil, gas, nuclear fuel (uranium and thorium), waste fuel, and waste heat. It is also extensively used as a thermal transport fluid in the process industries and in the comfort heating and cooling of space. The universality of its availability and its highly acceptable, well-defined physical and chemical properties also contribute to the usefulness of steam.

The temperature at which steam forms depends on the pressure in the boiler. The steam formed in the boiler (and conversely steam condensed in a condenser) is in temperature equilibrium with the water. Under these conditions, with steam and water in contact and at the same temperature, the steam is termed saturated. Steam can be entirely vapor when it is 100% dry, or it can carry entrained moisture and be wet. After the steam is removed from contact with the liquid phase, the steam can be further heated without changing its pressure. If initially wet, the additional heat will first dry it and then raise it above its saturation temperature. This is a sensible heat addition, and the steam is said to be superheated. Superheated steam at temperatures well above the boiling temperature for the existing steam pressure follows closely the laws of a perfect gas. Chiefly because of its availability, but also because of its nontoxicity, steam is widely used as the working medium in thermodynamic processes. It has a uniquely high latent heat of vaporization. Steam has a specific heat about twice that of air and comparable to that of ammonia. The specific heat of steam is relatively high so that it can carry more thermal energy at practical temperatures than can other usable gases. See also Boiler; Boyle's law; Charles' law; Dalton's law; Entropy; Steam engine; Steam-generating unit; Steam heating; Steam turbine; Water desalination.