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 ?

Question

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 ?

Accepted Answer

First case. Refer Fig. 12.29.

From Mollier chart :

[latex]h_1=3250[/latex] kJ/kg

[latex]h_2=2170[/latex] kJ/kg

Heat drop (or work done) [latex]=h_1-h_2[/latex]

= 3250 – 2170 = 1080 kJ/kg

Heat supplied [latex]=h_1-h_{f_2}[/latex]

= 3250 – 173.9 [latex]\left[h_{f_2}= ight.[/latex] kJ/kg at 0.08 bar]

= 3076.1 kJ/kg

Thermal efficiency [latex]=\frac{ ext { Work done }}{ ext { Heat supplied }}=\frac{1080}{3076.1}= 0 . 8 5 1[/latex] or 35.1%.

Exhaust steam condition, [latex]x _2[/latex]

= 0.83 (From Mollier chart).

Second case. Refer Fig. 12.30 (b).

From Mollier chart :

[latex]h_1=3250[/latex] kJ/kg ;

[latex]h_2=2807[/latex] kJ/kg ;

[latex]h_3=3263[/latex] kJ/kg ;

[latex]h_4=2426[/latex] kJ/kg.

Work done Work done [latex]=\left(h_1-h_2 ight)+\left(h_3-h_4 ight)=(3250-2807)+(3263-2426)=1280[/latex] kJ/kg

Heat supplied [latex]=\left(h_1-h_{f_4} ight)+\left(h_3-h_2 ight)[/latex]

= (3250 – 173.9) + (3263 – 2807) = 3532 kJ/kg

Thermal efficiency [latex]=\frac{ ext { Work done }}{ ext { Heat supplied }}=\frac{1280}{3532}=0.862 ext { or } 86.29[/latex]%.

Condition of steam at the exhaust,

[latex]x _4=0.985[/latex] [From Mollier chart].

Question 12.20: A turbine is supplied with steam at a pressure of 32 bar and...

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