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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 by some amount, resulting in a lowering of its temperature without changing state from a gas, to a mixture of saturated vapor and liquid. If unsaturated steam is heated at constant pressure, its temperature will also remain constant as the vapor quality increases towards 100%, and becomes dry 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 situations. To produce superheated steam in a power plant or for processes the saturated steam drawn from a boiler is passed through a separate heating device 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, but it has more energy and can do more work than saturated steam, but the 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.