a)
The average temperature of the room after 30 min.
a)
Answer to Problem 206RP
The average temperature of the room after 30 min is
Explanation of Solution
Write the expression for the energy balance equation for closed system without air in the room.
Here, energy transfer into the control volume is
Write the expression to calculate the final vapor quality.
Here, final vapor quality is
Write the expression to calculate the final internal energy of the system.
Here, final internal energy of the system is
Write the expression to calculate the final entropy of the system.
Here, final entropy of the system is
Write the expression to calculate the mass of the steam.
Here, mass of the steam is m, volume of the steam is
Write the expression to calculate the ideal gas equation, to find the mass of the air.
Here, mass of the air is
Write the expression to calculate the total work done by the fan.
Here, rate of work done by fan is
Conclusion:
Substitute 0 for
Here, change in internal energy of system is
From the Table A-6, “Superheated water table”, obtain the following properties of steam at temperature of
From the Table A-6, “Saturated water-Pressure table”, obtain the following properties of steam at pressure of
Here, Saturated liquid specific volume is
Substitute
From the Table A-6, “Saturated water-Pressure table”, obtain the following properties of steam at pressure of
Here, Saturated liquid internal energy is
Substitute
From the Table A-6, “Saturated water-Pressure table”, obtain the following properties of steam at pressure of
Here, Saturated liquid entropy is
Substitute
Substitute
Substitute 0.01388 kg for m,
Substitute
Here, heat transfer in is
From the Table A-1, “the molar mass, gas constant and critical point properties table”, select the gas constant of air as
Substitute
Substitute
From the Table A-2, “the ideal–gas equation specific heats of various common gases table”, select the specific heat at constant pressure for air as
Substitute 12.6 kJ for
Thus, the average temperature of the room after 30 min is
b)
The entropy change of the steam.
b)
Answer to Problem 206RP
The entropy change of the steam is
Explanation of Solution
Write the expression to calculate the change of entropy of the steam.
Here, mass of the steam is m, final entropy is
Conclusion:
Substitute 0.01388 kg for m,
Thus, the entropy change of the steam is
c)
The entropy change of the air.
c)
Answer to Problem 206RP
The entropy change of the air is
Explanation of Solution
Write the expression to calculate the entropy change of air.
Since,
Conclusion:
Substitute 98.5 kg for m,
Thus, the entropy change of the air is
d)
The entropy generation in the turbine.
d)
Answer to Problem 206RP
The entropy generation in the turbine is
Explanation of Solution
Write the expression for the entropy balance equation of the system.
Here, rate of net entropy in is
Conclusion:
Substitute 0 for
Here, entropy change of air is
Substitute
Thus, the total entropy generated during the process is
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Chapter 7 Solutions
Thermodynamics: An Engineering Approach
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