In a Rankine cycle with reheat steam leaves the boiler at 17.5 MPa and 500 C, an

Question

In a Rankine cycle with reheat steam leaves the boiler at 17.5 MPa and 500 C, and then it is isentropically expanded through a high-pressure turbine to 3 MPa. It is reheated at constant pressure to 500 C and isentropically expanded through a low-pressure turbine to 70 kPa. The water is then condensed to saturated liquid at 70 kPa and isentropically compressed to 17.5 MPa to repeat the cycle. (a) Draw the cycle on a T-s diagram and calculate (b) the specific heat added (kJ/kg) , (c) the specific heat rejected (kJ/kg), and (d) the thermal efficiency of the cycle.

Explanation / Answer

P1 = 17.5 MPa
T1 = 500 C
h1 = 3275.64
s1 =6.2416

P2 = 3 Mpa
s2 =s1 isentropic
h2 = 2831.77

P3 = 3MPa
T3 = 500
h3 = 3456.39
s3 = 7.2345

P4 = 70 kPa
s4 =s3 isentropic
h4 = 2570.7

P5 = 70 kPa
x5 = 0
h5 = 376.77
s5 = 1.1921

P6 = 17.5 Mpa
s6 = s5 isentropic
h6 = 394.78

All h in kJ/kg, s in kJ/kgK
Q supplied= (3275.64 - 394.78) + (3456.39 - 2831.77) = 3505.48 kJ/kg
Q rejected = 2570.7 - 376.77 = 2193.93 kJ/kg
Net work = (3275.64 - 2831.77) + (3456.39 - 2570.7) - (394.78 - 376.77) = 1311.55 kJ/kg
Eff= 1311.55/3505.48 = 37.4142 %

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