VAPOUR POWER CYCLES 581

dharm

\M-therm\Th12-3.pm5

(i)Quality of steam at turbine exhaust, x 4 :

x 4 = 0.88 (From Mollier diagram)

(ii)Cycle efficiency, ηcycle :

ηcycle =

()( )

()()

hh hh

hh hhf

12 34

(^1324)
−+−
−+−

()()
(.)( )
3450 3050 3560 2300
3450 191 8 3560 3050
−+−
−+− =
1660
3768 2. = 0.4405 or^ 44.05%. (Ans.)
(iii)Steam rate in kg/kWh :
Steam rate =
3600
()()hh hh12 34−+−

3600
()()3450 3050−+−3560 2300

3600
1660
= 2.17 kg/kWh. (Ans.)
Example 12.20. A turbine is supplied with steam at a pressure of 32 bar and a tempera-
ture of 410°C. The steam then expands isentropically to a pressure of 0.08 bar. Find the dryness
fraction at the end of expansion and thermal efficiency of the cycle.
If the steam is reheated at 5.5 bar to a temperature of 395°C and then expanded isentropically
to a pressure of 0.08 bar, what will be the dryness fraction and thermal efficiency of the cycle?
Solution. First case. Refer Fig. 12.29.
h (kJ/kg)
h = 3250 1
h = 2170 2
2
0.08 bar
32 bar410 Cº
x = 0.83 2
Saturation line
s
1
Fig. 12.29
From Mollier chart :
h 1 = 3250 kJ/kg
h 2 = 2170 kJ/kg
Heat drop (or work done) = h 1 – h 2
= 3250 – 2170 = 1080 kJ/kg