Steam Information

Steam is the technical term for the invisible water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air. At lower pressures, such as in the upper atmosphere or at the top of high mountains water boils at a lower temperature than the nominal 100 °C (212 °F) at standard temperature and pressure. If heated further it becomes Superheated steam.

The Enthalpy of vaporization is the energy required to turn water into the gaseous form when it increases in volume by 1,600 times at standard temperature and pressure; this change in volume can converted into mechanical work by steam engines and more recently steam turbines. Steam engines played a central role to the Industrial Revolution and modern steam turbines are frequently used to generate electricity. If liquid water comes in contact with a very hot substance (such as lava, or molten metal) it can create a steam explosion which were responsible for many foundry accidents and possibly also for much of the damage in the Chernobyl accident.

Types of steam and conversion

A gas can only contain a certain amount of steam (the quantity varies with temperature and pressure). When a gas has absorbed its maximum amount it is said to be in vapor-liquid equilibrium^[1] and if more water is added it is described as 'wet steam'.

Superheated steam is steam at a temperature higher than its boiling point for the pressure which only occurs where all the water has evaporated or, in the case of steam generators (boilers), the saturated steam has be conveyed out of the steam drum.^[2]

Steam tables contain thermodynamic data for water/steam and are often used by engineers and scientists in design and operation of equipment where thermodynamic cycles involving steam are used. Additionally, thermodynamic phase diagrams for water/steam, such as a temperature-entropy diagram or a Mollier diagram shown in this article, may be useful. Steam charts are also used for analysing thermodynamic cycles.

Uses

Agricultural

In agriculture, steam is used for soil sterilization to avoid the use of harmful chemical agents and increase soil health.

Domestic

Steam's capacity to transfer heat is also used in the home: for cooking vegetables, steam cleaning of fabric and carpets, and heating buildings. In each case, water is heated in a boiler, and the steam carries the energy to a target object. "Steam showers" are actually low-temperature mist-generators, and do not actually use steam.

Electricity generation (and cogeneration)

In the U.S., more than 86% of electricity is generated using steam as the working fluid, nearly all by steam turbines.

In electric generation, steam is typically condensed at the end of its expansion cycle, and returned to the boiler for re-use. However in cogeneration, steam is piped into buildings through a district heating system to provide heat energy after its use in the electric generation cycle. The world's biggest steam generation system is the New York City steam system which pumps steam into 100,000 buildings in Manhattan from seven cogeneration plants.^[3]

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 thermal energy because of water's high heat of vaporization.

Lifting gas

Owing to its low molecular mass, steam is an effective lifting gas, providing approximately 60% as much lift as helium and twice as much as hot air. It is not flammable, unlike hydrogen, and is cheap and abundant, unlike helium. The required heat, however, leads to condensation problems and requires an insulated envelope. These factors have limited its use thus far to mostly demonstration projects.^[4]

Mechanical effort

A steam engine and steam turbines use the expansion of steam 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. Condensation of steam to water often occurs at the low-pressure end of a steam turbine, since this maximizes the energy efficiency, but such wet-steam conditions have to be limited to avoid excessive turbine blade erosion. Engineers use an idealised thermodynamic cycle, the Rankine cycle, to model the behavior of steam engines. Steam turbines are often used in the production of electricity.

Sterilization

An autoclave, which uses steam under pressure, is used in microbiology laboratories and similar environments for sterilization.

Steam in piping

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.

See also

• Electrification
• Food steamer or steam cooker
• Geyser—geothermally-generated steam
• IAPWS—an association that maintains international-standard correlations for the thermodynamic properties of steam, including IAPWS-IF97 (for use in industrial simulation and modelling) and IAPWS-95 (a general purpose and scientific correlation).
• Industrial Revolution
• Live steam
• Mass production
• Nuclear power—and power plants use steam to generate electricity
• Oxyhydrogen
• Psychrometrics—moist air/vapor mixtures, humidity and air conditioning
• Steam locomotive

References

1. ^ Singh, R Paul (2001). Introduction to Food Engineering. Academic Press. ISBN ISBN 978-0-12-646384-2.
2. ^ "Superheated Steam : International site for Spirax Sarco". http://www.spiraxsarco.com/resources/steam-engineering-tutorials/steam-engineering-principles-and-heat-transfer/superheated-steam.asp.
3. ^ Carl Bevelhymer, "Steam", Gotham Gazette, November 10, 2003
4. ^ Steam Balloon JBFA Article

External links

• Steam Tables & Charts by National Institute of Standards and Technology, NIST
• What Is Steam? (general article about the properties of water/steam)
• Steam Tracing
• Online steam properties calculator (Spirax Sarco)
• Free Steam Tables Online calculator based on IAPWS-IF97 (MegaWatSoft)
• Online Steam Tables Calculator based on IFC-67

Categories: Forms of water | Water in gas | Steam power

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