3.1.3 Practical cycle using superheat
The first development of the Rankine cycle into a more practical steam cycle involves raising the pressure and temperature of the steam entering the turbine. The superheated steam cycle is shown schematically in Fig 1.24, and on the T-S diagram in Fig 1.25.
In the superheat cycle, the saturated dry steam leaving the boiler drum is further heated before entering the turbine. For the same quality of steam entering the condenser (i.e., same point D), the increase in work done is shown on Fig 1.25. The quantity of heat rejected to the condenser is the same as for the Rankine cycle. Hence, there is an improvement in the cycle efficiency.
The superheat efficiency is greater than the Rankine efficiency as Area (P + Q)/(P + Q + R) is greater than Q/(Q + R).
This superheat cycle was chosen to have the same turbine exhaust conditions as the previous Rankine cycle. However, a major advantage of superheating steam is that for increasing cycle temperature and pressure, the exhaust wetness in the turbine can be maintained within the physical limits mentioned earlier.