Question 12.20: A turbine is supplied with steam at a pressure of 32 bar and...
A turbine is supplied with steam at a pressure of 32 bar and a temperature of 410°C. The steam then expands isentropically to a pressure of 0.08 bar. Find the dryness fraction at the end of expansion and thermal efficiency of the cycle.
If the steam is reheated at 5.5 bar to a temperature of 395°C and then expanded isentropically to a pressure of 0.08 bar, what will be the dryness fraction and thermal efficiency of the cycle ?
First case. Refer Fig. 12.29.
From Mollier chart :
h_1=3250 kJ/kg
h_2=2170 kJ/kg
Heat drop (or work done) =h_1-h_2
= 3250 – 2170 = 1080 kJ/kg
Heat supplied =h_1-h_{f_2}
= 3250 – 173.9 \left[h_{f_2}=\right. kJ/kg at 0.08 bar]
= 3076.1 kJ/kg
Thermal efficiency =\frac{\text { Work done }}{\text { Heat supplied }}=\frac{1080}{3076.1}= 0 . 8 5 1 or 35.1%.
Exhaust steam condition, x _2
= 0.83 (From Mollier chart).
Second case. Refer Fig. 12.30 (b).
From Mollier chart :
h_1=3250 kJ/kg ;
h_2=2807 kJ/kg ;
h_3=3263 kJ/kg ;
h_4=2426 kJ/kg.
Work done Work done =\left(h_1-h_2\right)+\left(h_3-h_4\right)=(3250-2807)+(3263-2426)=1280 kJ/kg
Heat supplied =\left(h_1-h_{f_4}\right)+\left(h_3-h_2\right)
= (3250 – 173.9) + (3263 – 2807) = 3532 kJ/kg
Thermal efficiency =\frac{\text { Work done }}{\text { Heat supplied }}=\frac{1280}{3532}=0.862 \text { or } 86.29%.
Condition of steam at the exhaust,
x _4=0.985 [From Mollier chart].
