A steam power plant operating on a simple ideal Rankine cycle maintains the boiler at 6000 kPa, the turbine inlet at 600 °C, and the condenser at 50 kPa. Compare the efficiency of this cycle when it is operated so that the liquid enters the pump as a saturated liquid against that when the liquid enters the pump 11.3 °C cooler than a saturated liquid at the condenser pressure.

ηa=0.349

ηb=0.345

Explanation:

The enthalpy and entropy at state 3 are determined from the given pressure and temperature with data from table:

The quality at state 4 is determined from the condition   and the entropies of the components at the condenser pressure taken from table:

The enthalpy at state 4 then is:  

Part A

In the case when the water is in a saturated liquid state at the entrance of the pump the enthalpy and specific volume are determined from A-5 for the given pressure:  

The enthalpy at state 2 is determined from an energy balance on the pump:

    =346.67 kJ/kg

The thermal efficiency is then determined from the heat input and output in the cycle:  

Part B  

In the case when the water is at a lower temperature than the saturation temperature at the condenser pressure we look into table and see the water is in a compressed liquid state. Then we take the enthalpy and specific volume for that temperature with data from  and the saturated liquid values:  

The enthalpy at state 2 is then determined from an energy balance on the pump:  

    =299.79 kJ/kg  

The thermal efficiency in this case then is: