Reheat Rankine cycle

Operating pressure limits of a steam powerplant are 10 kPa and 10 MPa with net-
work output as 80 MW. Steam enters the high-pressure turbine and reaches 500C
and then it is reheated to its initial condition in low pressure turbine at 1 MPa.
Isentropic efficiency of turbines is 80% and that of pump is 95%.
a. Determine steam quality or temperature at turbine exits
b. Mass flow rate of steam
c. Thermal efficiency of cycle

Sol:-

State 1
10 MPa, 5000C
h1 = 3375.1 kJ/kg ---- from superheated steam table
s1 = 6.5995 kJ/kg-K ---- from superheated steam table

State 2s
1 MPa, s2s = s1 = 6.5995 kJ/kg-K
h2s = 2783.86 kJ/kg ---- from superheated steam table

State 2
ηis = h1 - h2 / h1 - h2s
0.8 = 3375.1 - h.1 - 2783.86
h2 = 2902.1 kJ/kg

T2 = 3450C ---- from superheated steam table

State 3
1 MPa, 5000C
h3 = 3479.1 kJ/kg ---- from superheated steam table
s3 = 7.7642 kJ/kg-K ---- from superheated steam table

State 4s
10 kPa, s4s = s3 = 7.7642 kJ/kg-K

sf = 0.6492 kJ/kg-K , sg = 8.1488 kJ/kg-K ---- from saturated pressure table
s4s = sf + x4s (sg - sf)
7.7642 = 0.6492 + x4s (8.1488 - 0.6492)
x4s = 0.9487

hf = 191.81 kJ/kg, hg = 2583.9 kJ/kg ---- from saturated pressure table
h4s = hf + x4s (hg - hf)
h4s = 191.81 + 0.9487 (2583.9 - 191.81)
h4s = 2461.1857 kJ/kg

State 4
ηis = h3 - h4 / h3 - h4s
0.8 = 3479.1 - h.1 - 2461.1857
h4 = 2664.7685 kJ/kg
T4 = 800C ---- from superheated steam table