TITLE.PM5

VAPOUR POWER CYCLES 593

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\M-therm\Th12-4.pm5

(ii) Steam generated per hour :

Work developed per kg of steam in the turbine

= 1(h 1 – h 2 ′) + (1 – m) (h 2 ′ – h 3 ′)

= (3300 – 2697) + (1 – 0.157) (2697 – 2234) = 993.3 kJ/kg

Actual work developed by the turbine

=

×

=

×

22500 22500

ηηalt. mech. 094 097.. = 24676.5 kW

∴ Steam generated per hour = 24676 5

993 3

3600

1000

.

.

× tonnes/h = 89.43 tonnes/h. (Ans.)

(iii) The overall efficiency of the plant, ηoverall :

Net power available deducting pump power

= 22500 (1 – 0.09) = 20475 kW

Heat supplied in the boiler =

89 43 1000×−

086

.() 15

.

hhf

kJ/h

=

×−

×

89 43 1000 3300 561.4)

0 86 3600

.(

.

kW = 79106.3 kW

∴ ηoverall = Net power available
Heat supplied by the boiler
=^20475
79106.3
= 0.2588 or 25.88%. (Ans.)
Example 12.24. A steam power plant of 110 MW capacity is equipped with regenerative as
well as reheat arrangement. The steam is supplied at 80 bar and 55°C of superheat. The steam is
extracted at 7 bar for feed heating and remaining steam is reheated to 350°C, and then expanded to
0.4 bar in the L.P. stage. Assume indirect type of feed heaters. Determine :
(i)The ratio of steam bled to steam generated,
(ii)The boiler generating capacity in tonnes of steam/hour, and
(iii)Thermal efficiency of the cycle.
Assume no losses and ideal processes of expansion.
Solution. The schematic arrangement of the plant is shown in Fig. 12.39 (a) and the proc-
esses are represented on h-s chart in Fig. 12.39 (b).
Given : Capacity of plant = 110 MW ;
t 1 = 350°C i.e., ts at 80 bar ~− 295°C + 55°C = 350°C)
p 2 = p 3 = 7 bar ; t 3 = 350°C ; p 4 = 0.4 bar
l Locate point 1 corresponding to the condition p 1 = 80 bar and t 1 = 350°C, on the h-s chart.
l Locate point 2 by drawing vertical line through point 1 till it cuts the 7 bar pressure line.
l Locate point 3 as the cross point of 7 bar and 350°C temperature line.
l Locate point 4 by drawing vertical line through the point 3 till it cuts the 0.4 bar pressure
line.
From h-s chart, we find :
h 1 = 2985 kJ/kg ; h 2 = 2520 kJ/kg ;
h 3 = 3170 kJ/kg ; h 4 = 2555 kJ/kg.
Also, from steam tables, we have :
hf 2 (at 7 bar) = 697.1 kJ/kg ; hf 4 (at 0.4 bar) = 317.7 kJ/kg.