TITLE.PM5

580 ENGINEERING THERMODYNAMICS

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Amount of heat supplied during process of reheat,
hreheat = (h 3 – h 2 ) kJ/kg ...(ii)
From Mollier diagram or h-s chart,
h 1 = 2920 kJ/kg, h 2 = 2660 kJ/kg
h 3 = 2960 kJ/kg, h 2 = 2335 kJ/kg
Now, by putting the values in eqns. (i) and (ii), we get
W = (2920 – 2660) + (2960 – 2335)
= 885 kJ/kg. (Ans.)
Hence work done per kg of steam = 885 kJ/kg. (Ans.)
Amount of heat supplied during reheat,
hreheat = (2960 – 2660) = 300 kJ/kg. (Ans.)
If the expansion would have been continuous without reheating i.e., 1 to 4′, the work output
is given by
W 1 = h 1 – h 4 ′
From Mollier diagram,
h 4 ′ = 2125 kJ/kg
∴ W 1 = 2920 – 2125 = 795 kJ/kg. (Ans.)
Example 12.19. A steam power plant operates on a theoretical reheat cycle. Steam at
boiler at 150 bar, 550°C expands through the high pressure turbine. It is reheated at a constant
pressure of 40 bar to 550°C and expands through the low pressure turbine to a condenser at
0.1 bar. Draw T-s and h-s diagrams. Find :
(i)Quality of steam at turbine exhaust ; (ii)Cycle efficiency ;
(iii)Steam rate in kg/kWh. (AMIE Summer, 1999)
Solution. Refer Figs. 12.27 and 12.28.

150 bar

40 bar

1

2

3

T

4

s

T-s diagram

0.1 bar

4

2

(^13) 550 Cº
40 bar
150 bar
h-s diagram
h
s
Fig. 12.27 Fig. 12.28
From Mollier diagram (h-s diagram) :
h 1 = 3450 kJ/kg ; h 2 = 3050 kJ/kg ; h 3 = 3560 kJ/kg ; h 4 = 2300 kJ/kg
hf 4 (from steam tables, at 0.1 bar) = 191.8 kJ/kg