Power Plant Engineering

Fig. 9.12

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278
POWER PLANT ENGINEERING
This equation will get reduced to,
η
t
= 1 – 
(
1) /
1
4
T
1
.
. ( )
T
(
)
k
k
p
ac
at
r
−












η η



 



...(1)
For ideal open cycle, 
η
ac
= 
η
at
= 1
η
t
= 1 – 
(
1) /
1
4
T
. ( )
T
k
k
p
r
−












...(2)
The regenerator should be designed properly to avoid any substantial.
Pressure loss in it, which might cancel out any gain in thermal efficiency. Because of some
pressure loss in the regenerator, the turbine output and the net output will be slightly less than for the
simple cycle.
9.4.2 REHEATING
In reheat cycle, the combustion gases are not expanded in one turbine only but in two turbines.
The exhaust of the high-pressure turbine is reheated in a reheater and then expanded in a low-pressure
turbine. By reheating, the power output of the turbine is increased but the cost of additional fuel may be
heavy unless a heat exchanger is also used. A reheat cycle is shown in Fig. 9.13. Considering the adi-
abatic expansions, the total work done in the two turbines will be equal to: (I
3
– I
4a
) + (I
5
– I
6a
).
S.M.
Alternator
L.P.T.
Exhaust
5
6
4
Reheater
Fuel
H.P.T.
3
Fuel
C.C.
2
A.C.
Air in
Starting motor
Fig. 9.13
2a
2
3
4
7a
4a
5
6
6a
H
φ
Re-heat cycle
0
0
5
10
15
10
20
30
40
20
T
h
e
rm
a
l E
ffi
ci
e
n
cy
%
Pressure Ratio
Reheat
Simple
Fig. 9.14
Fig. 9.15
If the combustion gases were expanded in one turbine only down to point 7a for the same pres-
sure ratio, then the work output would have been: (I
3
– I
7a
).
Now the constant pressure lines on the H-
Φ
chart diverge away from the origin and converge
towards the origin. Therefore the line 5–6a will be greater than 4a–7a. Hence reheating increases the

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GAS TURBINE POWER PLANT
279
power output. By reheating, the average temperature of heat addition is raised resulting in higher output
and efficiency of the cycle. If reheat cycle is to be adopted then the pressure ratio must be high as at low
pressure ratios, the thermal efficiency is lowered by reheating Fig. 9.14. Reheating reduces the airflow
through the cycle resulting in decreased input to the compressor. For ideal reheating; the working fluid
temperature after reheating is equal to the maximum permissible turbine inlet temperature. That is,
T
5
= T
3
The efficiency of the cycle will be given as,
η
t
= 
3
4
5
6
2
1
3
2
5
4
(T
T )
(T
T )
(T
T )
(T
T )
(T
T )
−
+
−
−
−
−
+
−