Modern Engineering Thermodynamics

478 CHAPTER 13: Vapor and Gas Power Cycles
Q B + Q R
Primary
boiler
Reheater
1
6
3
2
Stage
1
Stage
2
W T
T
2s
1
3
Boiler
feed
pump
W P
Condenser
5
Q C
(a) Equipment schematic
4
5,6s 4s
s
(b) Thermodynamic state diagram
FIGURE 13.27
A Rankine cycle power plant with reheat.
The thermal efficiency of the Rankine cycle power plant with reheat shown in Figure 13.27 can be computed
from the general thermal efficiency definition as
ðη T Þ Rankine
cyc1e with
one reheat unit
W
= _ pm − j _W p j ð
= h 1 − h 2 Þ+ ðh 3 − h 4 Þ− ðh 6 − h 5 Þ
_Q B + _Q R
ðh 1 − h 6 Þ+ ðh 3 − h 2 Þ
and, if the liquid condensate is considered to be incompressible, then Eq. (13.7b) can be used to give
ðη T Þ Rankine
cyc1e with
one reheat unit
ð
= h 1 − h 2 Þ+ ðh 3 − h 4 Þ− v 5 ðp 6 − p 5
ðh 1 − h 6 Þ+ ðh 3 − h 2 Þ
where (η s ) p is the isentropic efficiency of the boiler feed pump. Using Eq. (13.4a), we can introduce the isentropic
efficiencies of the two turbine stages (η s ) pm1 and (η s ) pm2 as
Þ/ ðη s
Þ p
and
ðη s Þ pm1
= h 1 − h 2
(13.13)
h 1 − h 2s
Finally, the thermal efficiency of the cycle can be written as
ðη s Þ pm2 = h 3 − h 4
(13.14)
h 3 − h 4s
ðη T Þ Rankine
cycle with
one reheat unit
ð
= h 1 − h 2s
Þðη s Þ pm1
+ ðh 3 − h 4s Þðη s Þ pm2
− v 5 ðp 6 − p 5
ðh 1 − h 6 Þ+ ðh 3 − h 2 Þ
Þ/ ðη s Þ p
(13.15)
where the values of h 2 and h 6 in the denominator of this equation are calculated from Eqs. (13.13) and
(13.7b) as
and
h 2 = h 1 − ðh 1 − h 2s Þðη s Þ pm1
(13.16)
h 6 = h 5 + v 5 ðp 6 − p 5 Þ/ ðη s Þ p
(13.17)