Steam

Steam, water in the vapour 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 largely depends on the generation of steam, whether the heat is produced by burning coal or gas or by the nuclear fission of uranium (see Steam Engine; Turbine). Steam is also still much in use for space heating purposes, and it propels most of the world's naval vessels and commercial ships.

The boiling point of water at the sea-level atmospheric pressure of 101.3 kilopascal (kPa), or 14.7 lb/sq in, is about 100° C (212° F). At this critical temperature, the addition of 226 joules of heat will convert 1 kg (2.2 lb) of water to steam at the same temperature. For water under pressure, the boiling point rises with the increase of pressure up to a pressure of 222.1 kPa (32.2 lb/sq in) according to Boyle's law. At this pressure, water boils at a temperature of 374.15° C (705.47° F), its critical point. Beyond the critical pressure and temperature there is no distinction between liquid water and steam.

Pure steam is an invisible gas. In many cases, however, when water is boiling, small droplets of water are taken up with the steam, and the resulting whitish mixture is visible. 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 clouds seen when a kettle boils on a stove. Such steam is said to be wet.

Steam that is at 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 it is throttled by being passed through a valve from a high-pressure vessel to a low-pressure vessel. Throttling causes the temperature of the steam to drop somewhat, but its temperature is nevertheless higher than that of saturated steam at the corresponding pressure. Steam in its superheated state is generally used in modern power generation systems.