10-33
10-41 A steam power plant that operates on a reheat Rankine cycle is considered. The condenser pressure, the net power
output, and the thermal efficiency are to be determined.
Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible.
Analysis (a) From the steam tables (Tables A-4, A-5, and A-6),
P3  12.5 MPa  h3  3476.5 kJ/kg

 s 3  6.6317 kJ/kg  K
T3  550C
3
Turbine
Boiler
4
P4  2 MPa 
 h4 s  2948.1 kJ/kg
s 4 s  s3

T 
6
5
h3  h 4
h3  h4 s
Condenser
Pump
2
1
h4  h3  T h3  h4 s 
 3476.5  0.853476.5  2948.1
 3027.3 kJ/kg
T
3
P5  2 MPa  h5  3358.2 kJ/kg

T5  450C  s5  7.2815 kJ/kg  K
P6  ?

 h6 
x 6  0.95
P6  ? 
 h6 s 
s 6  s5 
5
12.5 MPa
4s 4
2
2s
(Eq. 1)
P=?
(Eq. 2)
1
6s 6
s
h h
T  5 6 
 h6  h5  T h5  h6 s   3358.2  0.853358.2  h6 s  (Eq. 3)
h5  h6 s
The pressure at state 6 may be determined by a trial-error approach from the steam tables or by using EES from the above
three equations:
P6 = 9.73 kPa, h6 = 2463.3 kJ/kg,
(b) Then,
h1  h f @ 9.73 kPa  189.57 kJ/kg
v1  v f @ 10 kPa  0.001010 m 3/kg
w p ,in  v1 P2  P1  /  p


 1 kJ

 / 0.90 
 0.00101 m 3/kg 12,500  9.73 kPa 
3
1 kPa  m 

 14.02 kJ/kg
h2  h1  w p ,in  189.57  14.02  203.59 kJ/kg
Cycle analysis:
qin  h3  h2   h5  h4   3476.5  203.59  3358.2  2463.3  3603.8 kJ/kg
q out  h6  h1  2463.3  189.57  2273.7 kJ/kg
W net  m ( qin  q out )  (7.7 kg/s)(3603.8 - 2273.7)kJ/kg  10,242 kW
(c) The thermal efficiency is
 th  1 
q out
2273.7 kJ/kg
 1
 0.369  36.9%
3603.8 kJ/kg
q in
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