- 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
When saturated steam from a steam boiler passes through heat transfer surfaces, its temperature increases beyond its saturation point.
Steam is described as super heated, if its temperature is more than that of its saturation temperature. Degree of super-heat is directly related with the temperature of the steam heated above the saturation temperature.
Superheat can only be added to dry, saturated steam, not steam with moisture. To achieve superheat, saturated steam must pass through a secondary heat exchanger. Hot flue gas from the boiler is an effective way to heat the saturated steam.
Superheated steam is used in power plants to generate electrical power. In steam turbines, superheated steam enters at one end and exits into a condenser at the other end. The energy difference between the inlet and outlet steam makes the turbine rotor turn, with steam energy gradually decreasing as it passes through the turbine.
It is essential to have sufficient superheat at the turbine inlet to avoid condensation of wet steam in the later stages of the turbine rotor.
Basically steam turbine rotor has got number of stages and the steam has to pass through each stage before reaching the condenser. So if the enough superheat is not provided in the steam at the turbine inlet, then the steam may get saturated while reaching the later stages of the rotor and subsequently get wetter while passing through the each successive stage.
Wet steam at the tail end of the rotor is very dangerous as it may lead to Water Hammer and severe erosion at the last stages of the turbine blades. In order to over-come this problem it is advisable to design the inlet steam parameters of steam turbine inlet in such way that super heated steam allow to enter at the turbine inlet and the turbine exhaust are designed to match the steam parameters close to saturated conditions.
A major reason for using superheated steam in steam turbine is the significant improvement in thermal efficiency.
Heat engine efficiency can be found by using either:
Carnot Cycle efficiency: Ratio of temperature difference between inlet and outlet to inlet temperature.
Rankine cycle efficiency: Ratio of heat energy at the turbine inlet and outlet to the total heat energy taken from the steam.
2. Example of calculating the Carnot Cycle and Rankine Cycle Efficiency.
Explained by Example:
A turbine is supplied with superheated steam at 96 bar at 490oC. The exhaust is at 0.09 bar and at 12 % wetness.
Temperature of saturated steam is : 43.7oC
Determine and Compare the Carnot Cycle and Rankine cycle.
Procedure to determine the Carnot cycle efficiency :
Procedure to determine the Rankine cycle efficiency :
Where,
Sensible heat in condensate corresponding to exhaust pressure of 0.09 bar in KJ/Kg = 183.3
3. 
Steam-Phase diagram is a graphical representation of data provided in the steam table. Steam-Phase diagram provides the relationship between enthalpy, temperature corresponding to various pressures. Liquid Enthalpy hf. This is represented by line A-B on the phase-diagram. When the water starts receiving heat from 0o C, then it receives all its liquid entahlpy along the saturated water line A-B on the phase diagram
Enthalpy of Saturated Steam (hfg): Any further heat addition results in change in phase to saturated steam and is represented by (hfg) on phase diagram i.e B-C.
Dryness Fraction (x): When heat is applied then the liquid start changing its phase from liquid to vapour and then the dryness fraction of the mixture starts increasing i.e moving towards unity. In the phase diagram dryness fraction of the mixture is 0.5 at exactly mid of the line BC. Similarly at point c on the phase diagram dryness fraction value is 1.
Line C-D Point c is in the saturated vapour line, any further heat addition results in increasing the steam temperature i.e beginning of steam superheating represented by line C-D.
Liquid Zone → Region towards left side of the saturated liquid line
Super heat zone → Region towards right side of the saturated vapour line
Two phase Zone → Area between the saturated liquid and saturated vapour line is mixture liquid and vapour. Mixture with varied dryness fractions.
Critical Point → It is the Apex point where saturated liquid and saturated vapour lines meet. Enthalpy of evaporation diminishes to zero at critical point, it means that water changes directly to steam at critical point and thereafter.
Maximum temperature which liquid can attain or exist is equivalent to critical point.
Critical point Parameters → Temperature 374.15oC
Pressure → 221.2 bar
Values above this are super-critical values and are useful in increasing the efficiency of the rankine cycle.
