Determine the minimum power input required if the

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Unformatted text preview: Px Heat FIGURE P7–140 7–141E A piston-cylinder device initially contains 15 ft3 of helium gas at 25 psia and 70°F. Helium is now compressed in a polytropic process (PV n constant) to 70 psia and 300°F. Determine (a) the entropy change of helium, (b) the entropy change of the surroundings, and (c) whether this process is reversible, irreversible, or impossible. Assume the surroundings are at 70°F. Answers: (a) 100 kPa 27°C (P 2 P2)1/3, Py 1 (P1P 2 )1/3 2 7–145 Steam at 7 MPa and 500°C enters a two-stage adiabatic turbine at a rate of 15 kg/s. Ten percent of the steam is extracted at the end of the first stage at a pressure of 1 MPa for other use. The remainder of the steam is further expanded in the second stage and leaves the turbine at 50 kPa. Determine the power output of the turbine, assuming (a) the process is reversible and (b) the turbine has an isentropic efficiency of 88 percent. Answers: (a) 14,930 kW, (b) 13,140 kW 7 MPa 500°C STEAM TURBINE (1st stage) (2nd stage) 1 MPa 0.016 Btu/R, (b) 0.019 Btu/R, (c) irreversible 7–142...
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This document was uploaded on 11/28/2012.

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