Using T- diagrams , explain why superheating and reheating is necessary in a vapor power cycle from the functional perspective . Also show how it helps improve the efficiency of the cycle .
The main practical advantage of reheat (and of superheating) is the decrease in moisture content in the turbine because most of the heat addition in the cycle occurs in the vaporization part of the heat addition process.
Super Heating:- It is the process of increase the steam temperature above the steam temperature.
Super heated vapour or superheated steam is a vapor at a temperature higher than its boiling point at the absolute pressure where the temperature is measured.
The process of superheating is the only way to increase the peak temperature of the Rankine cycle (and to increase efficiency) without increasing the boiler pressure. This requires the addition of another type of heat exchanger called a superheater, which produces the superheated steam.
The T-S diagram with super heating is shown in below figure.
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In the superheater, further heating at fixed pressure results in increases in both temperature and specific volume. The process of superheating of water vapor in the T-s diagram is provided in the figure between state E and saturation vapor curve. As can be seen also wet steam turbines (e.g. used in nuclear power plants) use superheated steam especially at the inlet of low-pressure stages. Typically most of nuclear power plants operates multi-stage condensing wet steam turbines (the high pressure stage runs on saturated steam)
With the increase of temperature and specific volume due to superheating of the steam the better expansion can be done in turbine blades. So the efficiency of the power plant also increases. We can clearly observe the change where enthalpy values of the conditions of the turbine inlet and outlets when super heating is done.
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