VAPOUR POWER CYCLES 579
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\M-therm\Th12-3.pm5
Advantages of ‘Reheating’ :
- There is an increased output of the turbine.
- Erosion and corrosion problems in the steam turbine are eliminated/avoided.
- There is an improvement in the thermal efficiency of the turbines.
- Final dryness fraction of steam is improved.
- There is an increase in the nozzle and blade efficiencies.
Disadvantages :
- Reheating requires more maintenance.
- The increase in thermal efficiency is not appreciable in comparison to the expenditure
incurred in reheating.
+Example 12.18. Steam at a pressure of 15 bar and 250°C is expanded through a turbine
at first to a pressure of 4 bar. It is then reheated at constant pressure to the initial temperature of
250 °C and is finally expanded to 0.1 bar. Using Mollier chart, estimate the work done per kg of
steam flowing through the turbine and amount of heat supplied during the process of reheat.
Compare the work output when the expansion is direct from 15 bar to 0.1 bar without any reheat.
Assume all expansion processes to be isentropic.
Solution. Refer Fig. 12.26.
2125
2335
2660
2920
2960
h(kJ/kg)
s (kJ/kg K)
0.1 bar
4
4 ¢
Saturation line
Superheat line
15 bar 4 bar 250 Cº
Reheating
2
1
3
Fig. 12.26
Pressure, p 1 = 15 bar ;
p 2 = 4 bar ;
p 4 = 0.1 bar.
Work done per kg of steam,
W = Total heat drop
= [(h 1 – h 2 ) + (h 3 – h 4 )] kJ/kg ...(i)
