Concept explainers
A piston–cylinder device initially contains 1.4 kg of refrigerant-134a at 100 kPa and 20°C. Heat is now transferred to the refrigerant from a source at 150°C, and the piston, which is resting on a set of stops, starts moving when the pressure inside reaches 120 kPa. Heat transfer continues until the temperature reaches 80°C. Assuming the surroundings to be at 25°C and 100 kPa, determine (a) the work done, (b) the heat transfer, (c) the exergy destroyed, and (d) the second-law efficiency of this process.
FIGURE P8–48
(a)
The work done.
Answer to Problem 48P
The work done is
Explanation of Solution
Express the boundary work done.
Here, mass is
Conclusion:
Perform the unit conversion of initial pressure and final pressure from
Refer Table A-13, “superheated refrigerant-134a”, and write the properties corresponding to initial pressure of
Here, initial specific volume, internal energy and entropy is
Refer Table A-13, “superheated refrigerant-134a”, and write the properties corresponding to final pressure of
Write the formula of interpolation method of two variables.
Here, the variables denote by x and y is final pressure and final specific volume respectively.
Show the final specific volume at
Final pressure |
Final specific volume |
0.10 | 0.28465 |
0.12 | |
0.14 | 0.20242 |
Substitute
Show the final specific internal energy at
Final pressure |
Final specific internal energy |
0.10 | 297.10 |
0.12 | |
0.14 | 296.77 |
Substitute
Show the final specific entropy at
Final pressure |
Final specific entropy |
0.10 | 1.2573 |
0.12 | |
0.14 | 1.2289 |
Substitute
Thus, write the values obtained from interpolation method:
Substitute
Hence, the work done is
(b)
The heat transfer.
Answer to Problem 48P
The heat transfer is
Explanation of Solution
Express heat transfer.
Conclusion:
Substitute for ,
(c)
The exergy destroyed.
Answer to Problem 48P
The exergy destroyed is
Explanation of Solution
Express the exergy destruction.
Here, entropy generation is
Express the entropy generation by taking entropy balance on an extended system.
Here, net entropy transfer by heat and mass is
Conclusion:
Substitute
Substitute
Hence, the exergy destroyed is
(d)
The second law efficiency of the process.
Answer to Problem 48P
The second law efficiency of the process is
Explanation of Solution
Express exergy expended.
Here, efficiency for reversible cycle is
Express the second law efficiency.
Conclusion:
Substitute
Substitute
Hence, the second law efficiency of the process is
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Chapter 8 Solutions
Thermodynamics: An Engineering Approach
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