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Solved: Applying the Exergy Balance: Closed SystemsAs
Chapter 7, Problem 29P(choose chapter or problem)
As shown in Fig. P7.29, 1.11 kg of Refrigerant 134a is contained in a rigid, insulated vessel. The refrigerant is initially saturated vapor at \(-28^{\circ} \mathrm{C}\). The vessel is fitted with a paddle wheel from which a mass is suspended. As the mass descends a certain distance, the refrigerant is stirred until it attains a final equilibrium state at a pressure of 1.4 bar. The only significant changes in state are experienced by the refrigerant and the suspended mass. Determine, in kJ,
(a) the change in exergy of the refrigerant.
(b) the change in exergy of the suspended mass.
(c) the change in exergy of an isolated system of the vessel and pulley-mass assembly.
(d) the destruction of exergy within the isolated system.
Let \(T_0\) = \(293 \mathrm{~K}\left(20^{\circ} \mathrm{C}\right)\), \(p_0\) = 1 bar.
Questions & Answers
QUESTION:
As shown in Fig. P7.29, 1.11 kg of Refrigerant 134a is contained in a rigid, insulated vessel. The refrigerant is initially saturated vapor at \(-28^{\circ} \mathrm{C}\). The vessel is fitted with a paddle wheel from which a mass is suspended. As the mass descends a certain distance, the refrigerant is stirred until it attains a final equilibrium state at a pressure of 1.4 bar. The only significant changes in state are experienced by the refrigerant and the suspended mass. Determine, in kJ,
(a) the change in exergy of the refrigerant.
(b) the change in exergy of the suspended mass.
(c) the change in exergy of an isolated system of the vessel and pulley-mass assembly.
(d) the destruction of exergy within the isolated system.
Let \(T_0\) = \(293 \mathrm{~K}\left(20^{\circ} \mathrm{C}\right)\), \(p_0\) = 1 bar.
ANSWER:
a.)
Step 1 of 5
We have to determine the change in exergy of the refrigerant.
The change in the exergy of the refrigerant can be determined by finding the initial and final exergy values. The initial and final exergy of the refrigerant can be evaluated using the expression,
With the assumption that the only significant changes in state are experienced by the refrigerant and the suspended mass and for the refrigerant, there is no change in kinetic and potential energy .
Thus, the exergy at the initial state is
From Table A-10 oC
= 211.29 kJ/kg ,
-28 oC
= 0.2052 m3/kg ,
oC
= 0.9411 kJ/kg
K
From Table A-12 at 1 bar, 20 oC, 264.67 kJ/kg,
0.23349 m3/kg and
1.0829 kJ/kg
K
Hence,
