Refrigerant-134a at 1 MPa and 90°C is to be cooled to 1 MPa and 30°C in a condenser by air. The air enters at 100 kPa and 27°C with a volume flow rate of 600 m3/min and leaves at 95 kPa and 60°C. Determine the mass flow rate of the refrigerant.
FIGURE P5–81
The mass flow rate of the refrigerant.
Answer to Problem 81P
The mass flow rate of the refrigerant is
Explanation of Solution
Consider the system is in steady state. Hence, the inlet and exit mass flow rates are equal.
The mass flow rate of air
The mass flow rate of refrigerant
Write the energy rate balance equation for one inlet and one outlet system.
Here, the rate of heat transfer is
The system is at steady state. Hence, the rate of change in net energy of the system becomes zero.
Neglect the work transfer, heat transfer to the surrounding, potential and kinetic energies.
The Equations (I) reduced as follows for air.
The Equations (I) reduced as follows for refrigerant.
Combining Equation (II) and (III).
Substitute
Write the formula for change in enthalpy
Substitute
For refrigerant:
At inlet:
The refrigerant is at the state of superheated condition.
Refer Table A-13, “Superheated refrigerant-134a”.
Obtain the inlet enthalpy
At exit:
The refrigerant is at the state of saturated liquid.
Refer Table A-11, “Saturated refrigerant-134a-Temperature table”.
Obtain the exit enthalpy
For air:
Refer Table A-1, “Molar mass, gas constant, and critical-point properties”.
The gas constant of air
Refer Table A-2, “Ideal2gas specific heats of various common gases”.
The specific heat at constant pressure
Write the formula for mass flow rate of air
Here, the volumetric flow rate of air is
Conclusion:
Substitute
Substitute
Thus, the mass flow rate of the refrigerant is
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Chapter 5 Solutions
Thermodynamics: An Engineering Approach
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