Power Plant Engineering

SOLVED EXAMPLES
Example 1. Steam at a pressure of 15 kg/cm
2
 (abs) and temperature of 250°C. is expanded
through a turbine to a pressure of 5 kg/cm
2
 (abs.). It is then reheated at constant pressure to a
temperature of 200°C after which it completes its expansion through the turbine to an exhaust
pressure of 0.1 kg/cm
2
(abs). Calculate theoretical efficiency.
(a) Taking reheating into account
(b) If the steam was expanded direct to exhaust pressure without reheating.
Solution. From Mollier diagram
H
1
= Total heat of steam at 15 kg/cm
2
and 250°C = 698 Kcal/kg
H
2
= Total heat of steam at 5 kg/cm
2
= 646 Kcal/kg
Now steam is reheated to 200°C at constant pressure
H
3
= Heat in this stage = 682 Kcal/kg
This steam is expanded to 0.1 kg/cm
2
H
4
= Heat in this stage = 553 Kcal/kg
H
w4
= Total Heat of water at 0.1 kg/cm
2
= 45.4 Kcal/kg
Theoretical efficiency = 
1
2
3
4
1
3
2
4
{(H
H )
(H
H )}
{H
(H
H )
H
}
−
+
−
+
−
−
w

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30
POWER PLANT ENGINEERING
= 
{(698
646)
(682
533)}
{698
(642
646)
45.4}
−
+
−
+
−
−
= 0.293 or 29.3% Ans.
Example 2. Determine the thermal efficiency of the basic cycle of a steam power plant (Rankine
Cycle), the specific and hourly steam consumption for a 50 mW steam turbine operating at inlet condi-
tions: pressure 90 bar and temperature 500°C. The condenser pressure is 0.40 bar.