Concept explainers
Which is a more valuable resource for work production in a closed system −15 ft3 of air at 100 psia and 250°F or 20 ft3 of helium at 60 psia and 200°F? Take T0 = 77°F and P0 = 14.7 psia.
The more valuable resource for work production in a closed system; air or helium.
Answer to Problem 29P
The air has the more valuable resource for work production in a closed system.
Explanation of Solution
Express the mass in the system.
Here, mass is
Express the entropy change between the given state and dead state.
Here, entropy change between the given state and dead state is
Express the specific volume at the given state.
Express the specific volume at dead state.
Express the specific closed system exergy.
Here, specific internal energy is
Express the total exergy available for the production of work.
Here, mass is
Conclusion:
Solve for AIR:
Refer Table A-1E, “molar mass, gas constant and critical point properties”, and write the gas constant of air.
Refer Table A-2E, “ideal gas specific heats of various common gases”, and write the properties of air.
Substitute
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Solve for HELIUM:
Refer Table A-1E, “molar mass, gas constant and critical point properties”, and write the gas constant of helium.
Refer Table A-2E, “ideal gas specific heats of various common gases”, and write the properties of helium.
Substitute
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On comparing the total exergy available for the potential of work from Equations (VII) and (VIII), it is obtained that air has more potential.
Hence, the air has the more valuable resource for work production in a closed system.
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