Water at 20 psia and 50°F enters a mixing chamber at a rate of 300 lbm/min where it is mixed steadily with steam entering at 20 psia and 240°F. The mixture leaves the chamber at 20 psia and 130°F, and heat is lost to the surrounding air at 70°F at a rate of 180 Btu/min. Neglecting the changes in kinetic and potential energies, determine the rate of entropy generation during this process.
FIGURE P7–140E
The rate of entropy generation during the process.
Answer to Problem 140P
The rate of entropy generation during the process is
Explanation of Solution
Write the expression for the energy balance equation for closed system.
Here, rate of net energy transfer in to the control volume is
The rate of change in internal energy of the system is zero at steady state,
Write the expression for the mass balance of the system.
Here, inlet mass flow rate is
Write the expression for the entropy balance during the process.
Here, rate of net input entropy is
Conclusion:
Substitute 0 for
Here, mass flow rate at inlet 1 is
Substitute
Substitute
Refer to Table A-4E, “Saturated water—Temperature table”, obtain the below properties at the pressure of
Here, fluid entropy is
Refer to Table A-4E, “Saturated water—Temperature table”, obtain the below properties at the pressure of
Here, entropy at inlet 2 is
Refer to Table A-4E, “Saturated water—Temperature table”, obtain the below properties at the pressure of
Here, entropy at exit 3 is
Substitute
Substitute
Substitute
Substitute
Thus, the rate of entropy generation during the process is
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Chapter 7 Solutions
THERMODYNAMICS: AN ENGINEERING APPROACH
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