what is superheated steam?why is it invisible? can it ignite a match stickwhen placed in its path?
Answers
The steam can therefore cool (lose internal energy) by some amount, resulting in a lowering of its temperature without changing state (i.e., condensing) from a gas, to a mixture of saturated vapor and liquid. If unsaturated steam (a mixture which contain both water vapor and liquid water droplets) is heated at constant pressure, its temperature will also remain constant as the vapor quality (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, 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 superheater) which transfers additional heat to the steam by contact or by radiation.
Superheated steam is not suitable for sterilization.[1] 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. Saturated steam has a much higher wall heat transfer coefficient.[2]
Slightly superheated steam may be used for antimicrobial disinfection of biofilms on hard surfaces.[3]
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 at those pressures. If steam doing work in a reciprocating engine or turbine, cools to a temperature at which liquid droplets form; the water droplets entrained in the fluid flow will strike the mechanical parts of engines or turbines, with enough force to bend, crack or fracture them.[4] 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.
Contents [hide] 1Saturated steam2Uses2.1Steam engine2.2Processing2.3Pest control3See also4ReferencesSaturated steam[edit]Saturated steam is 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. This is in contrast to superheated steam, in which the steam (water vapor) has been separated from the water and mist then additional heat has been added.
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,[5] 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.[6]
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 for saturated steam at atmospheric pressure.[7]