- Superheated Steam Definition: Superheated steam is steam that has been heated above its saturation temperature.
- Generating Superheated Steam: To make superheated steam, pass saturated steam through a secondary heat exchanger.
- Applications in Power Plants: Superheated steam is crucial in steam turbines for efficiency and to prevent condensation.
- Steam Phase Diagram: This diagram shows the relationship between enthalpy, temperature, and pressure, helping to understand steam properties.
- Critical Point: The critical point on the steam phase diagram indicates where liquid water turns directly into steam.
Superheated Steam
A steam boiler can pass dry saturated steam through superheater surfaces, where additional heat raises its temperature above saturation at the same pressure.
Steam is superheated when T exceeds Tsat at its pressure. The degree of superheat is T – Tsat, so a temperature without its pressure cannot establish the superheated state.
At a fixed subcritical pressure, heating a wet steam-water mixture first evaporates the remaining liquid at the saturation temperature. Further heat then raises the dry vapor into the superheated region. A boiler superheater, reheater or separate heat exchanger can supply this heat, subject to material-temperature limits.
Superheated steam is used in power plants to generate electrical power. In a steam turbines cycle, the steam expands from high inlet pressure and enthalpy to a lower exhaust pressure. Turbine work is related to the enthalpy drop, while real output also reflects isentropic efficiency and mechanical losses.
Superheat at the turbine inlet raises the temperature of heat addition and helps control moisture during expansion. It does not guarantee a dry exhaust under every pressure ratio or turbine efficiency.
A multistage turbine extracts energy as steam passes towards the condenser. Depending on the inlet state and exhaust pressure, expansion can cross the saturated-vapor boundary and enter the two-phase region. Cycle designers may use reheat or moisture separation as well as inlet superheat.
Liquid droplets in later stages can reduce efficiency and erode blades and shrouds. This wet-vapor erosion is different from Water Hammer in piping. Turbine suppliers specify acceptable exhaust quality and use stage design, drainage, reheat or moisture separation to control damage while super heated steam conditions remain within material limits.
For a given cycle design, raising steam temperature can improve the efficiency of a steam turbine plant by increasing the average temperature of heat addition. The result depends on pressure, reheat, regeneration, component efficiency, condenser conditions and heat losses.
Heat-engine efficiency can be evaluated with an ideal upper bound or a specified cycle model:
Carnot Cycle efficiency: For reservoirs at fixed absolute temperatures, η = 1 – Tc/Th. Both temperatures must be in kelvins, and this is an upper bound rather than a steam-plant calculation.
Rankine cycle efficiency: Ideal Rankine efficiency is net work divided by boiler heat input. Net work equals turbine work minus pump work, with each heat and work term obtained from state enthalpies.
2. The following data illustrate a Carnot Cycle comparison and a Rankine-cycle calculation.
Example states:
A turbine receives superheated steam at 96 bar and 490oC. The exhaust is at 0.09 bar with 12% moisture, corresponding to quality x = 0.88.
The saturation temperature at 0.09 bar is about 43.7oC.
Determine and compare the Carnot upper bound and the ideal Rankine cycle efficiency using one consistent steam-property source.
For the Carnot expression, convert the hot and cold temperatures to kelvins.
For Rankine efficiency, determine turbine work, pump work and boiler heat input from the selected state enthalpies.
The calculation must state whether the turbine outlet is a measured state or the result of an isentropic-efficiency model.
The saturated-liquid enthalpy at 0.09 bar is about 183.3 kJ/kg in the property source used by the example; retain its source and rounding.
3. 
The Steam-Phase diagram is a schematic view of water and steam property data. A complete diagram must identify its axes and pressure path. At a fixed subcritical pressure, saturated-liquid specific enthalpy hf is the value at the bubble point. The line A-B represents sensible heating only if the plotted path and pressure are held as shown. The enthalpy reference convention, such as a stated value at 0oC, must also be identified.
Enthalpy of Vaporization (hfg): At fixed saturation pressure, hfg is the specific enthalpy difference between saturated vapor and saturated liquid. The saturated-vapor enthalpy is hg = hf + hfg, and B-C represents phase change only for the stated constant-pressure path.
Dryness Fraction (x): In an equilibrium two-phase mixture, x is the vapor mass divided by total mixture mass. Its enthalpy is h = hf + xhfg at the stated pressure. If B-C is plotted linearly against enthalpy, its midpoint corresponds to x = 0.5, while point C corresponds to dry saturated vapor with x = 1.
Line C-D represents further constant-pressure heating from saturated vapor into the superheated region.
Liquid region → States on the compressed-liquid side of the saturated-liquid boundary.
Superheated-vapor region → States on the vapor side of the saturated-vapor boundary below the critical pressure.
Two-phase region → Equilibrium mixtures inside the saturation dome, where quality ranges from 0 to 1.
Critical point → The endpoint where saturated-liquid and saturated-vapor states become identical and hfg falls to zero. Above the critical temperature, there is no liquid-vapor phase boundary and no boiling transition.
Supercritical fluid does not have a separate liquid or vapor phase across this region.
Critical temperature → 373.946oC
Critical pressure → 22.064 MPa, equivalent to 220.64 bar.
Supercritical steam cycles can raise the average temperature of heat addition, but efficiency depends on the complete cycle. Higher pressure and temperature also impose materials, chemistry, control, pump-power and cost constraints.
