Steam enters a turbine at 100 bars and 400degreeC. At the exit the pressure is 1

Question

Steam enters a turbine at 100 bars and 400degreeC. At the exit the pressure is 1.0 bar and the entropy has increased by 0.600 kJ/kg . K under adiabatic conditions. The changes in kinetic and potential energies may be neglected, (a) Determine the work, in kJ/kg. For the same initial conditions and final pressure the turbine is now run without insulation. The entropy production due to internal irreversibilities is assumed to be the same as in pan (a). However, the environment at 300 K is found to have an entropy change of +0.0800 kJ/(kg steam) . K. Also, the entropy change of the steam due to heat exchange with the environment has an absolute value of 0.0460 kJ/kg . K. (b) Determine the quality of the steam at the turbine outlet, and compare to the answer from pan (a). Also, (c) determine the heat transfer to the environment, in kJ/kg steam, and (d) find the amount of work output of the turbine, in kJ/kg. and compare to pan (a).

Explanation / Answer

state 1 : 100 bar, 400 centigrade

h1= 3099.9 kj/kg, s1 = 6.218 kj/kg K

sate 2: 1 bar

s1 - s2 = 0.6 kj/kg K

s2 = 5.618 kj/ kg K

from h-s chart, h2 = 2080 kj/kg

(a) work = h1 - h2 = 3099.9 - 2080 = 1019.9 kj /kg

5.618 = 1.303+ x 6.057

x = 0.663

(b)   s2 = 0.6+0.08+0.0460 = 5.744 kj/kg K

s2 = 5.744 kj/ kg K = 1.303 + x 6.057

x = 0.733

dryness fraction in this case is more than case a

heat transfer = 5.744 * 300 = 1723.2 kj/kg

work = 3099.9 - 2140 -1723.2 = - 763.3 kj/kg

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