Superheated steam
Superheated steam is steam at a temperature higher than water's boiling point. If saturated steam is heated at constant pressure, its temperature will also remain constant as the steam quality (think dryness) increases towards 100% dry saturated steam. Continued heat input will then generate superheated steam. This will occur if saturated steam contacts a surface with a higher temperature.[1] The steam is then described as superheated by the number of degrees it has been heated above saturation temperature.
Superheated steam and liquid water cannot coexist under thermodynamic equilibrium, as any additional heat simply evaporates more water and the steam will become saturated steam. However this restriction may be violated temporarily in dynamic (non-equilibrium) situations. To produce superheated steam in a power plant or for processes (such as drying paper) the saturated steam drawn from a boiler is passed through a separate heating device (a 'super heater') which transfers additional heat to the steam by contact or by radiation.
Superheated steam is not suitable for sterilization.[2] This is because the superheated steam is dry. Dry steam must reach much higher temperatures and the materials exposed for a longer time period to have the same effectiveness; or equal Fo kill value. Superheated steam is also not useful for heating. Saturated steam has a much higher useful heat content[citation needed].
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[edit] Saturated steam
Saturated steam is, in contrast to superheated steam, steam that is in equilibrium with heated water at the same pressure, i.e. it has not been heated past the boiling point for that pressure.
If saturated steam is reduced in temperature (whilst retaining its pressure) it will condense to produce water droplets, even if it is still considerably above the boiling point of 100°C at standard pressure. These condensation droplets are a cause of damage to steam turbine blades,[3] the reason why such turbines rely on a supply of dry, superheated steam.
Dry steam is saturated steam that has been very slightly superheated. This is not sufficient to change its energy appreciably, but is a sufficient rise in temperature to avoid condensation problems, given the average loss in temperature across the steam supply circuit. Towards the end of the 19th century, when superheating was still a less than certain technology, such steam-drying gave the condensation-avoiding benefits of superheating without requiring the sophisticated boiler or lubrication techniques of full superheating.[4]
[edit] Uses
[edit] Steam engine
Superheated steam was widely used in main line steam locomotives. Saturated steam has three main disadvantages in a steam engine: it contains small droplets of water which have to be periodically drained from the cylinders; being precisely at the boiling point of water for the boiler pressure in use, it inevitably condenses to some extent in the steam pipes and cylinders outside the boiler, causing a disproportionate loss of steam volume as it does so; and it places a heavy demand on the boiler because a large amount of water has to be evaporated per unit volume of steam.
Superheating the steam dries it effectively, raises its temperature to a point where condensation is much less likely and increases its volume significantly. Added together, these factors increase the power and economy of the locomotive. The main disadvantages are the added complexity and cost of the superheater tubing and the adverse effect that the "dry" steam has on lubrication of moving components such as the steam valves. Shunting locomotives did not generally use superheating.
The normal arrangement involved taking steam after the regulator valve and passing it through long superheater tubes inside specially large firetubes of the boiler. The superheater tubes had a reverse ("torpedo") bend at the firebox end so that the steam had to pass the length of the boiler at least twice, picking up superheat as it did so.
[edit] Processing
Other potential uses of superheated steam include: drying, cleaning, layering, reaction engineering, epoxy drying and film use where saturated to highly superheated steam is required at one atmospheric pressure or at high pressure. Ideal for steam drying, steam oxidation and chemical processing. Uses are in surface technologies, cleaning technologies, steam drying, catalysis, chemical reaction processing, surface drying technologies, curing technologies, energy systems and nanotechnologies. Superheated steam is not usually used in a heat exchanger due to low heat transfer co-efficient.[1] In refining and hydrocarbon industries superheated steam is mainly used for stripping and cleaning purposes.
[edit] Pest control
Superheated steam is used for soil steaming.[dubious ] Steam is induced into the soil which causes almost all organic material to deteriorate. Soil steaming is an effective alternative to chemicals in agriculture.
[edit] See also
[edit] References
- ^ a b Superheated Steam : International site for Spirax Sarco. Spiraxsarco.com. Retrieved on 2012-01-25.
- ^ William D. Wise, "Succeed at steam sterilization, "Chemical processing" 27 November 2005. Retrieved 2010-10-10.
- ^ Roy, G.J. (1975). Steam Turbines and Gearing. Kandy Marine Engineering Series. Stanford Maritime. pp. 36–37. ISBN 0-540-07338-5.
- ^ Hills, Richard L. (1989). Power From Steam. Cambridge University Press. p. 203. ISBN 0-521-45834-X.
