Question
Consider a steam power plant operating on an ideal reheat Rankine cycle. The work input to the pump is 20 kJ/kg. The work output from the high pressure turbine is 750 kJ/kg. The work output from the low pressure turbine is 1500 kJ/kg. The thermal efficiency of the cycle is 50 %. The enthalpy of saturated liquid and saturated vapour at condenser pressure are 200 kJ/kg and 2600 kJ/kg, respectively. The quality of steam at the exit of the low pressure turbine is _______________ % (round off to the nearest integer).
Answer :
Correct answer is : 92.91
Pump work, Wp = 20 kJ/kg, Thermal efficiency (η) = 0.5
High pressure Turbine work (WT) = 750 kJ/kg, Low pressure Turbine work (WT’) = 1500 kJ/kg
Now,
At condenser pressure,
Enthalpy of saturated liquid, hf = 200 kJ/kg
Enthalpy of saturated vapour, hg = 2600 kJ/kg
Wnet = WT + WT’ - Wp
Wnet = 700 + 1500 – 20
∴ Wnet = 2230 kJ/kg
Now,
h4 – h5 = 2230 kJ/kg
h5 = hf = 200 kJ/kg
∴ h4 = 2430 kJ/kg
Now,
Quality of steam at lower turbine outlet,
h4 = hf + x (hg - hf)
2430 = 200 + x(2600 - 200)
∴ x = 93 %
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