Liquid water at 200 kPa and 15°C is heated in a chamber by mixing it with superheated steam at 200 kPa and 150°C. Liquid water enters the mixing chamber at a rate of 4.3 kg/s, and the chamber is estimated to lose heat to the surrounding air at 20°C at a rate of 1200 kJ/min. If the mixture leaves the mixing chamber at 200 kPa and 80°C, determine (a) the mass flow rate of the superheated steam and (b) the rate of entropy generation during this mixing process.
FIGURE P7–154
a)

The mass flow rate of the superheated steam.
Answer to Problem 156P
The mass flow rate of the superheated steam 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
Write the expression to calculate the mass balance of the system.
Here, inlet mass flow rate is
Conclusion:
Substitute 0 for
Here, mass flow rate at entry 1 is
Substitute
Rewrite the Equation (IV) to calculate the mass flow rate at entry 2.
From Table A-4, “the saturated water table”, select the initial enthalpy at entry 1
From Table A-6, “Superheated water”, select the initial enthalpy at entry 2
From Table A-4, “the saturated water table”, select the enthalpy at exit
Substitute
Thus, the mass flow rate of the superheated steam is
b)

The rate of heat entropy generation during the process.
Answer to Problem 156P
The rate of heat entropy generation during the process is
Explanation of Solution
Write the expression for the entropy balance during the process.
Here, rate of net input entropy is
Conclusion:
Substitute
Here, entropy at entry 1 is
Substitute
Substitute
Substitute
Thus, the rate of heat entropy generation during the process is
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Chapter 7 Solutions
EBK THERMODYNAMICS: AN ENGINEERING APPR
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