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Concept
Superheated steam
Résumé
Superheated steam is steam at a temperature higher than its vaporization point at the absolute pressure where the temperature is measured.
Superheated 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 contains 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. 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; while it has more energy and can do more work than saturated steam, its heat content is much less useful. This is because superheated steam has the same heat transfer coefficient of air, making it an insulator - a poor conductor of heat. Saturated steam has a much higher wall heat transfer coefficient.
Slightly superheated steam may be used for antimicrobial disinfection of biofilms on hard surfaces.
Source officielle
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