Steam, water in vapor state, used in the generation of power and on a large scale in many industrial processes. The techniques of generating and using steam, therefore, are important components of engineering technology. The generation of electricity is largely accomplished by first generating steam, whether the heat is produced by burning coal or gas or by the nuclear fission of uranium (see Nuclear Energy; Steam Engine; Turbine). Steam also is still much in use for space heating purposes (see Heating, Ventilating, and Air Conditioning), and it propels most of the world's naval vessels and commercial ships (see Ships and Shipbuilding).
The boiling point of water at sea-level atmospheric pressure (760 torr or 14.7 lb/sq in) is about 100 C (212 F). At this critical temperature, the addition of 970.3 Btu of heat will convert 0.454 kg (1 lb) of water to 0.454 kg of steam at the same temperature. For water under pressure, the boiling point rises with the increase of pressure up to a pressure of 218 atmospheres of pressure (165,000 torr or 3200 lb/sq in). At this pressure, water boils at a temperature of 374 C (705 F), its critical point. Beyond critical pressure and temperature there is no distinction between liquid water and steam.
Pure steam is a dry and invisible vapor. In many cases, however, when water is boiling, a quantity of small droplets, or particles, of water are taken up with the steam, and the resulting mixture is visible as a white vapor. A similar effect occurs when dry steam is exhausted into the comparatively cool atmosphere. Some of the steam cools and condenses, forming the familiar white vapor seen when a kettle boils on a stove. Such steam is said to be wet.
Steam that is heated to the exact boiling point corresponding to the existing pressure is called saturated steam. Heating steam beyond this temperature produces so-called superheated steam. Superheating also occurs if saturated steam is compressed or if saturated steam is throttled by passing the steam through a valve from a high-pressure vessel to a low-pressure vessel. Throttling causes the temperature of the steam to drop somewhat, but the temperature of the throttled steam is still higher than that of saturated steam at the corresponding pressure. Steam in its superheated state is generally used in modern power generation systems.