Repeat Prob. 11–56 for a flash chamber pressure of 0.6 MPa.
(a)
The fraction of the refrigerant that evaporates as it is throttled to the flash chamber.
Answer to Problem 57P
The fraction of the refrigerant that evaporates as it is throttled to the flash chamber is
Explanation of Solution
Show the T-s diagram for compression refrigeration cycle as in Figure (1).
From Figure (1), write the specific enthalpy at state 5 is equal to state 6 due to throttling process.
Here, specific enthalpy at state 5 and 6 is
From Figure (1), write the specific enthalpy at state 7 is equal to state 8 due to throttling process.
Here, specific enthalpy at state 7 and 8 is
Express the fraction of the refrigerant that evaporates as it is throttled to the flash chamber
Here, specific enthalpy at saturated vapor is
Conclusion:
Perform unit conversion of pressure at state 1 from
Refer Table A-12, “saturated refrigerant-134a-pressure table”, and write the properties corresponding to pressure at state 1
Here, specific entropy and enthalpy at state 1 is
Refer Table A-13, “superheated refrigerant 134a”, and write the specific enthalpy at state 2 corresponding to pressure at state 2 of
Write the formula of interpolation method of two variables.
Here, the variables denote by x and y is specific entropy at state 2 and specific enthalpy at state 2 respectively.
Show the specific enthalpy at state 2 corresponding to specific entropy as in Table (1).
Specific entropy at state 2 |
Specific enthalpy at state 2 |
0.9500 | 270.83 |
0.9519 | |
0.9817 | 280.60 |
Substitute
Thus, the specific enthalpy at state 2 is,
Perform unit conversion of pressure at state 3 from
Refer Table A-12, “saturated refrigerant-134a-pressure table”, and write the property corresponding to pressure at state 3
Perform unit conversion of pressure at state 5 from
Refer Table A-12, “saturated refrigerant-134a-pressure table”, and write the property corresponding to pressure at state 5
Here, specific enthalpy at saturated liquid is
Substitute
Refer Table A-12, “saturated refrigerant-134a-pressure table”, and write the property corresponding to pressure at state 8
Substitute
Refer Table A-12, “saturated refrigerant-134a-pressure table”, and write the specific enthalpy at evaporation and pressure of
Substitute
Hence, the fraction of the refrigerant that evaporates as it is throttled to the flash chamber is
(b)
The rate of heat removed from the refrigerated space.
Answer to Problem 57P
The rate of heat removed from the refrigerated space is
Explanation of Solution
Express the enthalpy at state 9 by using an energy balance on the mixing chamber.
Here, the rate of total energy entering the system is
Express the mass flow rate through the flash chamber.
Here, mass flow rate through condenser is
Express The rate of heat removed from the refrigerated space.
Conclusion:
Substitute
Substitute
Substitute
Hence, the rate of heat removed from the refrigerated space is
(c)
The coefficient of performance.
Answer to Problem 57P
The coefficient of performance is
Explanation of Solution
Express compressor work input per unit mass.
Express the coefficient of performance.
Express entropy at state 4.
Here, specific entropy at state 3 is
Conclusion:
Refer Table A-12, “saturated refrigerant-134a-pressure table”, and write the property corresponding to pressure at state 3
Here, specific entropy at saturated vapor is
Substitute
Refer Table A-13, “superheated refrigerant 134a”, and write the specific enthalpy at state 4 corresponding to pressure at state 4 of
Show the specific enthalpy at state 4 corresponding to specific entropy as in Table (2).
Specific entropy at state 4 |
Specific enthalpy at state 4 |
0.9389 | 285.47 |
0.9444 | |
0.9733 | 297.10 |
Use excels and substitute value from Table (2) in Equation (IV) to get,
Substitute
Substitute
Hence, the coefficient of performance is
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Chapter 11 Solutions
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