Superheated steam is

steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...

at a

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

higher than its

vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomen ...

point at the

absolute pressure Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressur ...

where the temperature is measured. Superheated steam can therefore cool (lose

internal energy The internal energy of a thermodynamic system is the total energy contained within it. It is the energy necessary to create or prepare the system in its given internal state, and includes the contributions of potential energy and internal kinet ...

) by some amount, resulting in a lowering of its temperature without changing state (i.e., condensing) from a

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

, to a mixture of

saturated Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds ** Saturated and unsaturated compounds **Degree of unsaturation ** Saturated fat or fatty ac ...

vapor and

liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...

. If unsaturated steam (a mixture which contains both water vapor and liquid water droplets) is heated at constant

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

, its temperature will also remain constant as the

vapor quality In physics, a vapor (American English) or vapour (British English and Canadian English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R. H. Petrucci, W. S. Harwood, and F. G. Her ...

(think dryness, or percent saturated vapor) increases towards 100%, and becomes dry (i.e., no saturated liquid) saturated steam. Continued heat input will then "super" heat the dry saturated steam. This will occur if saturated steam contacts a surface with a higher temperature. Superheated steam and liquid water cannot coexist under

thermodynamic equilibrium Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In the ...

, 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 A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many pow ...

or for processes (such as drying paper) the saturated steam drawn from a

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central h ...

is passed through a separate heating device (a

superheater A superheater is a device used to convert saturated steam or wet steam into superheated steam or dry steam. Superheated steam is used in steam turbines for electricity generation, steam engines, and in processes such as steam reforming. There ar ...

) which transfers additional heat to the steam by

contact Contact may refer to: Interaction Physical interaction * Contact (geology), a common geological feature * Contact lens or contact, a lens placed on the eye * Contact sport, a sport in which players make contact with other players or objects * ...

or by

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

. 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 F0 kill value. Superheated steam is also not useful for heating, but it has more energy and can do more

work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking ** Working animal, an animal t ...

than saturated steam, but the heat content is much less useful. This is because superheated steam has the same

heat transfer coefficient In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the proportionality constant between the heat flux and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ). ...

of air, making it an insulator - a poor conductor of heat.

Saturated steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...

has a much higher wall heat transfer coefficient. Slightly superheated steam may be used for antimicrobial disinfection of

biofilms A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular po ...

on hard surfaces. Superheated steam's greatest value lies in its tremendous internal energy that can be used for kinetic reaction through mechanical expansion against turbine blades and reciprocating pistons, that produces rotary motion of a shaft. The value of superheated steam in these applications is its ability to release tremendous quantities of internal energy yet remain above the condensation temperature of water vapor; at the pressures at which reaction turbines and reciprocating piston engines operate. Of prime importance in these applications is the fact that water vapor containing entrained liquid droplets is generally

incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow ( isochoric flow) refers to a flow in which the material density is constant within a fluid parcel—an infinitesimal volume that moves with the flow velocity. An eq ...

at those pressures. In a reciprocating engine or turbine, if steam doing work cools to a temperature at which liquid droplets form, then the water droplets entrained in the fluid flow will strike the mechanical parts with enough force to bend, crack or fracture them. Superheating and pressure reduction through expansion ensures that the steam flow remains as a compressible gas throughout its passage through a turbine or an engine, preventing damage of the internal moving parts.

Saturated steam

Saturated steam is steam that is in equilibrium with heated water at the same pressure, i.e., it has not been heated above the

boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envir ...

for its pressure. This is in contrast to superheated steam, in which the steam (vapor) has been separated from the water droplets then additional heat has been added. These condensation droplets are a cause of damage to

steam turbine A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam turbin ...

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. By contrast, water vapor that includes water droplets is described as wet steam. If wet steam is heated further, the droplets evaporate, and at a high enough temperature (which depends on the pressure) all of the water evaporates, the system is in

vapor–liquid equilibrium In thermodynamics and chemical engineering, the vapor–liquid equilibrium (VLE) describes the distribution of a chemical species between the vapor phase and a liquid phase. The concentration of a vapor in contact with its liquid, especially a ...

, and it becomes ''saturated steam''. Saturated steam is advantageous in heat transfer due to the high latent heat of vaporization. It is a very efficient mode of heat transfer. In layman's terms, saturated steam is at its dew point at the corresponding temperature and pressure. The typical latent heat of vaporization (or condensation) is 970 Btu/lb (2256.5 kJ/kg) for saturated steam at atmospheric pressure.

Uses

Steam engine

Superheated steam was widely used in main line

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the locomot ...

has three main disadvantages in a

steam engine A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be trans ...

: 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. 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 heat 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. The application of superheated steam for sanitation of dry food processing plant environment has been reported. Hyeon Woo Park, Jie Xu, V. M. Balasubramaniam, and Abigail B. Snyder. 2021. The effect of water activity and temperature on the inactivation of Enterococcus faecium in peanut butter during superheated steam sanitation treatment. Food Control. Volume 125, Article 107942 Superheated steam is not usually used in a heat exchanger due to low heat transfer co-efficient.Superheated Steam : International site for Spirax Sarco
Spiraxsarco.com. Retrieved on 2012-01-25. In refining and hydrocarbon industries superheated steam is mainly used for stripping and cleaning purposes.

Pest control

Steam has been used for soil steaming since the 1890s. Steam is induced into the soil which causes almost all organic material to deteriorate (the term "sterilization" is used, but it is not strictly correct since all micro-organism are not necessarily killed). Soil steaming is an effective alternative to many chemicals in agriculture, and is used widely by greenhouse growers. Wet steam is primarily used in this process, but if soil temperatures above the boiling point of water are required, superheated steam must be used.

References