Steam iswater vapor, often mixed withair or anaerosol of liquidwater droplets. This may occur due toevaporation or due toboiling, where heat is applied until water reaches theenthalpy of vaporization.Superheated orsaturated steam is invisible; however,wet steam, a visiblemist or aerosol of water droplets, is often referred to as "steam".[1]: 6
When liquid water becomes steam, it increases involume by 1,700 times atstandard temperature and pressure; this change in volume can be converted intomechanical work bysteam engines such asreciprocating piston type engines andsteam turbines. Piston-type steam engines played a central role in theIndustrial Revolution[citation needed] and steam-based generation produces 80 percent of the world's electricity.[2] If liquid water comes in contact with a very hot surface or depressurizes quickly below itsvapor pressure, it can create asteam explosion.
Steam is traditionally created by heating a boiler by burning coal and other fuels, but it is also possible to create steam with solar energy.[3][4][5] Water vapour that includes water droplets is described as "wet steam". As wet steam is heated further, the droplets evaporate, and at a high enough temperature (which depends on the pressure) all of the water evaporates and the system is invapor–liquid equilibrium.[6] When steam has reached this equilibrium point, it is referred to as "saturated steam".
Superheated steam orlive steam is steam at a temperature higher than itsboiling point for the pressure, which only occurs when all liquid water has evaporated or has been removed from the system.[7]
Steam tables[8] contain thermodynamic data for water/saturated steam and are often used by engineers and scientists in design and operation of equipment where thermodynamic cycles involving steam are used. Additionally, thermodynamicphase diagrams for water/steam, such as a temperature-entropy diagram or aMollier diagram may be useful. Steam charts are also used for analysing thermodynamic cycles.
Inagriculture, steam is used forsoil sterilization to avoid the use of harmful chemical agents and increasesoil health.[9]
Steam's capacity to transfer heat is also used in the home: for cooking vegetables, steam cleaning of fabric, carpets and flooring, and for heating buildings. In each case, water is heated in a boiler, and the steam carries the energy to a target object. Steam is also used in ironing clothes to add enough humidity with the heat to take wrinkles out and put intentional creases into the clothing.
As of 2000 around 90% of all electricity was generated using steam as theworking fluid, nearly all by steam turbines.[10]
In electric generation, steam is typically condensed at the end of its expansion cycle, and returned to the boiler for re-use. However, inco-generation, steam is piped into buildings through adistrict heating system to provide heat energy after its use in the electric generation cycle. The world's biggest steam generation system is theNew York City steam system, which pumps steam into 100,000 buildings inManhattan from seven co-generation plants.[11]
In other industrial applications steam is used forenergy storage, which is introduced and extracted by heat transfer, usually through pipes. Steam is a capacious reservoir for thermal energy because of water's highheat of vaporization.
Fireless steam locomotives weresteam locomotives that operated from a supply of steam stored on board in a large tank resembling a conventional locomotive's boiler. This tank was filled byprocess steam, as is available in many sorts of large factory, such aspaper mills. The locomotive's propulsion used pistons and connecting rods, as for a typical steam locomotive. These locomotives were mostly used in places where there was a risk of fire from a boiler's firebox, but were also used in factories that simply had a plentiful supply of steam to spare.
Steam engines and steam turbines use the expansion of steam to drive apiston or turbine to performmechanical 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.Condensation of steam to water often occurs at the low-pressure end of a steam turbine, since this maximizes theenergy efficiency, but such wet-steam conditions must be limited to avoid excessive turbine blade erosion. Engineers use an idealisedthermodynamic cycle, theRankine cycle, to model the behaviour of steam engines. Steam turbines are often used in the production of electricity.
Anautoclave, which uses steam under pressure, is used in microbiology laboratories and similar environments forsterilization.
Steam, especially dry (highly superheated) steam, may be used for antimicrobial cleaning even to the levels of sterilization. Steam is a non-toxic antimicrobial agent.[12][13]
Steam is used in piping for utility lines. It is also used in jacketing and tracing of piping to maintain the uniform temperature in pipelines and vessels.
Steam is used across multiple industries for its ability to transfer heat to drive chemical reactions, sterilize or disinfect objects and to maintain constant temperatures. In the lumber industry, steam is used in the process ofwood bending, killing insects, and increasing plasticity. Steam is used to accentuate drying of concrete especially in prefabricates.Care should be taken since concrete produces heat during hydration and additional heat from the steam could be detrimental to hardening reaction processes of the concrete. In chemical andpetrochemical industries, steam is used in various chemical processes as a reactant.Steam cracking of long chain hydrocarbons produces lower molecular weight hydrocarbons for fuel or other chemical applications.Steam reforming producessyngas orhydrogen.
Used in cleaning of fibers and other materials, sometimes in preparation for painting. Steam is also useful in melting hardened grease and oil residues, so it is useful in cleaning kitchen floors and equipment and internal combustion engines and parts. Among the advantages of using steam versus a hot water spray are the facts that steam can operate at higher temperatures and it uses substantially less water per minute.[14]
