VAPOUR POWER CYCLES 573

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(ii)Thermal efficiency of the cycle, ηηηηηThermal :

ηThermal =

Work done

Heat supplied

=

806 93

2264 9

.

.

= 0.3563 or 35.63%. (Ans.)

(iii)Thermal efficiency of Rankine cycle, ηηηηηRankine :

ηRankine =

hh

hhf

04

(^04)
−
−

2905 2000
2905 137 8
−
−.
= 0.327 or 32.7%. (Ans.)
(iv)Theoretical gain due to regenerative feed heating
= 35. 32.
35.
63 7
63
− = 0.0822 or 8.22%. (Ans.)
(v)Steam consumption with regenerative feed heating

1 3600
Work done / kg
×

1 3600
806.93
×
= 4.46 kg/kWh. (Ans.)
Steam consumption without regenerative feed heating

1 3600
Work done / kg without regeneration
×

1 3600
04
×
hh−
= 1 3600
2905 2000
×
−
= 3.97 kg/kWh. (Ans.)
(vi) Quantity of steam passing through the last stage of a 50000 kW turbine with
regenerative feed-heating
= 4.46 (1 – m 1 – m 2 – m 3 ) × 50000
= 4.46 (1 – 0.088 – 0.0828 – 0.046) × 50000 = 174653.6 kg/h. (Ans.)
Same without regenerative arrangement
= 3.97 × 50000 = 198500 kg/h. (Ans.)
Example 12.17. A steam turbine plant developing 120 MW of electrical output is equipped
with reheating and regenerative feed heating arrangement consisting of two feed heaters—one
surface type on H.P. side and other direct contact type on L.P. side. The steam conditions before
the steam stop valve are 100 bar and 530°C. A pressure drop of 5 bar takes place due to throttling
in valves.
Steam exhausts from the H.P. turbine at 25 bar. A small quantity of steam is bled off at
25 bar for H.P. surface heater for feed heating and the remaining is reheated in a reheater to
550 °C and the steam enters at 22 bar in L.P. turbine for further expansion. Another small quan-
tity of steam is bled off at pressure 6 bar for the L.P. heater and the rest of steam expands up to
the back pressure of 0.05 bar. The drain from the H.P. heater is led to the L.P. heater and the
combined feed from the L.P. heater is pumped to the high-pressure feed heater and finally to the
boiler with the help of boiler feed pump.
The component efficiencies are : Turbine efficiency 85%, pump efficiency 90%, generator
efficiency 96%, boiler efficiency 90% and mechanical efficiency 95%. It may be assumed that the
feed-water is heated up to the saturation temperature at the prevailing pressure in feed heater.
Work out the following :
(i)Sketch the feed heating system and show the process on T-s and h-s diagrams.
(ii)Amounts of steam bled off.
(iii)Overall thermal efficiency of turbo-alternator considering pump work.
(iv)Specific steam consumption in kg/kWh. (AMIE Summer, 1998)