Superheated steam
Encyclopedia
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. 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
. 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.
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, 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.
s. 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.
Superheated steam is not usually used in a heat exchanger due to low heat transfer co-efficient. In refining and hydrocarbon industries superheated steam is mainly used for stripping and cleaning purposes.
Steam is induced into the soil which causes almost all organic material to deteriorate. Soil steaming is an effective alternative to chemicals in agriculture.
Steam
Steam is the technical term for 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 a temperature higher than water's boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
. If saturated steam
Saturated steam
In thermodynamics, the state of saturation of a fluid indicates that the fluid is at its boiling point temperature. This term can be applied in several ways:*Saturated liquid: fully in the liquid state but is about to vaporize....
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. 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
Thermodynamic equilibrium
In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. The word equilibrium means a state of balance...
, 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
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
is passed through a separate heating device (a 'super heater') which transfers additional heat to the steam by contact or by radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
.
Superheated steam is not suitable for sterilization
Sterilization (microbiology)
Sterilization is a term referring to any process that eliminates or kills all forms of microbial life, including transmissible agents present on a surface, contained in a fluid, in medication, or in a compound such as biological culture media...
. 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
Fourier number
In physics and engineering, the Fourier number or Fourier modulus, named after Joseph Fourier, is a dimensionless number that characterizes heat conduction. Conceptually, it is the ratio of the heat conduction rate to the rate of thermal energy storage. Together with the Biot number, it...
kill value. Superheated steam is also not useful for heating. Saturated steam has a much higher useful heat content.
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 pointBoiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
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
Steam turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....
blades, 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.
Steam engine
Superheated steam was widely used in main line steam locomotiveSteam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
s. Saturated steam has three main disadvantages in a steam engine
Steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
: 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.
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. In refining and hydrocarbon industries superheated steam is mainly used for stripping and cleaning purposes.
Pest control
Superheated steam is used for soil steaming.Soil steam sterilization
Soil steam sterilization is a farming technique that sterilizes soil with steam in open fields or greenhouses. Pests of plant cultures such as weeds, bacteria, fungi and viruses are killed through induced hot steam which causes their cell structure to physically degenerate. Biologically, the...
Steam is induced into the soil which causes almost all organic material to deteriorate. Soil steaming is an effective alternative to chemicals in agriculture.