Superheated steam is steam at a temperature higher than its vaporization (boiling) point at the absolute pressure where the temperature is measured.
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 saturated steam (a mixture of both gas and saturated vapor) is heated at constant pressure, its temperature will also remain constant as the vapor quality (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 occurs 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. Superheated steam is also not useful for heating. Saturated steam has a much higher useful heat content. Slightly superheated steam may be used for antimicrobial disinfection of biofilms 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 incompressible. 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. Superheating and pressure reduction through expansion, ensures that the steam flow throughout its passage through a turbine or an engine, always remains as a compressible gas, which will not damage the internal moving parts of the turbine or engine through which the steam passes.