A refrigerator operating on the vapor-compression refrigeration cycle using refrigerant-134a as the refrigerant is considered. The temperatures of the cooled space and the ambient air are at 10°F and 80°F, respectively. R-134a enters the compressor at 20 psia as a saturated vapor and leaves at 140 psia and 160°F. The refrigerant leaves the condenser as a saturated liquid. The rate of cooling provided by the system is 45,000 Btu/h. Determine (a) the mass flow rate of R-134a and the COP, (b) the exergy destruction in each component of the cycle and the second-law efficiency of the compressor, and (c) the second-law efficiency of the cycle and the total exergy destruction in the cycle.
(a)
The mass flow rate of R-134a and the COP.
Answer to Problem 33P
The mass flow rate of R-134a and the COP is
Explanation of Solution
Show the T-s diagram for vapor-compression refrigeration cycle as in Figure (1).
From Figure (1), write the specific enthalpy at state 3 is equal to state 4 due to throttling process.
Here, specific enthalpy at state 3 and 4 is
Express the work input.
Here, specific enthalpy at state 2 and 1 is
Express heat supplied to the cooled space.
Express the heat removed from the cooled space.
Express quality at state 4.
Here, specific enthalpy at saturated liquid and evaporation and
Express specific entropy at state 4.
Here, specific entropy at saturated liquid and evaporation and
Express mass flow rate of R-134a.
Here, rate of heat lost is
Express the COP of the cycle.
Conclusion:
Refer Table A-12E, “saturated refrigerant-134a-pressure table”, and write the properties corresponding to initial pressure
Here, specific entropy at state 1 is
Refer Table A-13E, “superheated refrigerant-134a”, and write the properties corresponding to pressure at state 2
Here, specific entropy at state 2 is
Refer Table A-12E, “saturated refrigerant-134a-pressure table”, and write the properties corresponding to pressure at state 3
Here, specific entropy at state 3 is
As specific enthalpy at state 3 is equal to specific enthalpy at state 4,
Refer Table A-12E, “saturated refrigerant-134a-pressure table”, and write the properties corresponding to pressure at state 4
Substitute
Substitute
Substitute
Substitute
Substitute
Substitute
Substitute
Hence, the mass flow rate of R-134a and the COP is
(b)
The exergy destruction in each component of the cycle and the second-law efficiency of the compressor.
Answer to Problem 33P
The exergy destruction in compressor is
Explanation of Solution
For compressor:
Express the exergy destruction in compressor.
Here, surrounding temperature is
For condenser:
Express the exergy destruction in condenser.
Here, entropy generation during process 2-3 is
For expansion valve:
For evaporator:
Express the exergy destruction in evaporator.
Here, entropy generation during process 4-1 is
Express the power input of the compressor.
Express second law efficiency of the compressor.
Conclusion:
Perform unit conversion of surrounding temperature from
Perform unit conversion of high temperature medium from
Perform unit conversion of low temperature medium from
Substitute
Hence, the exergy destruction in compressor is
Substitute
Hence, the exergy destruction in condenser is
Substitute
Hence, the exergy destruction in expansion valve is
Substitute
Hence, the exergy destruction in evaporator is
Substitute
Substitute
Hence, the second-law efficiency of the compressor is
(c)
The second-law efficiency of the cycle and the total exergy destruction in the cycle.
Answer to Problem 33P
The second-law efficiency of the cycle is
Explanation of Solution
Express the exergy of the heat transferred from the low temperature medium.
Determine the second law efficiency of the cycle.
Express the total exergy destruction in the cycle.
Conclusion:
Substitute
Substitute
Hence, the second-law efficiency of the cycle is
Substitute
Hence, the total exergy destruction in the cycle is
Want to see more full solutions like this?
Chapter 11 Solutions
Thermodynamics: An Engineering Approach
- The operating condition for the single compressor in a household refrigerator is the lowest box temperature, which is typically A. 0F B. -20F C. 20F D. 40Farrow_forwardA refrigerated room is kept at −18◦C by a vapor-compression cycle with R-134a as the refrigerant. Heat is rejected to cooling water that enters the condenser at 14◦C at a rate of 0.35 kg/s and leaves at 22◦C. The refrigerant enters the condenser at 1.2MPa and 50◦C and leaves at the same pressure subcooled by 5◦C. If the compressor consumes 5.5 kW of power, determine (a) the mass flow rate of the refrigerant, (b) the refrigeration load and the COP, (c) the second-law efficiency of the refrigerator and the total exergy destruction in the cycle, and (d) the exergy destruction in the condenser. Take specific heat of water to be 4.18 kJ/kg·◦C.arrow_forwardHow to express the coefficient of performance of a standard refrigerator in terms of the enthalpies of the fluid ?arrow_forward
- A commercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at 10°C by rejecting waste heat to cooling water that enters the condenser at 18°C at a rate of 0.25 kg/s and leaves at 26°C. The refrigerant enters the condenser at 1.2 MPa and 50°C and leaves at the same pressure subcooled by 5°C. If the compressor consumes 2.3 kW of power, determine the COP?arrow_forwardConsider a refrigerator that operates on the vapor compression refrigeration cycle with R-134a as the working fluid. The refrigerant enters the compressor as a saturated vapor at 140 kPa, and exits at 800 kPa and 60 degrees Celsius, and leaves the condenser as a saturated liquid at 800 kPa. The COP of this refrigerator is:arrow_forwardA commercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at –35°C by rejecting waste heat to cooling water that enters the condenser at 18°C at a rate of 0.25 kg/s and leaves at 26°C. The refrigerant enters the condenser at 1.2 MPa and 50°C and leaves at the same pressure subcooled by 5°C. If the compressor consumes 3.3 kW of power, determine the mass flow rate of the refrigerant,arrow_forward
- A commercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at –35°C by rejecting waste heat to cooling water that enters the condenser at 18°C at a rate of 0.25 kg/s and leaves at 26°C. The refrigerant enters the condenser at 1.2 MPa and 50°C and leaves at the same pressure subcooled by 5°C. If the compressor consumes 3.3 kW of power, determine the minimum power input to the compressor for the same refrigeration load.arrow_forwardConsider a refrigeration system that operates on an actual vapor-compression refrigeration cycle with refrigerant 134a as the working fluid with an isentropic efficiency of a compressor of 75.1%. The refrigerant enters the compressor as saturated vapor at 140 kPa and is compressed to 800 kPa. Determine the value of h2 in kj/hg, answer in 4 decimal places with unit analysis. Subject: Thermodynamics 2arrow_forwardA commercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at –30°C by rejecting its waste heat to cooling water that enters the condenser at 18°C at a rate of 0.25 kg/s and leaves at 26°C. The refrigerant enters the condenser at 1.2 MPa and 65°C and leaves at 42°C. The inlet state of the compressor is 60 kPa and –34°C and the compressor is estimated to gain a net heat of 450 W from the surroundings. Determine the theoretical maximum refrigeration load for the same power input to the compressor. Water 26°C ↑ 18°C 1.2 MPа 42°C 65°C Condenser Expansion Win valve Evaporator Compressor 60 kPa -34°Carrow_forward
- Why is the reversed Carnot cycle executed within the saturation dome not a realistic model for refrigeration cycles?arrow_forwardConsider a two-stage cascade refrigeration system operating between the pressure limits of 2 MPa and 0.05 MPa. Each stage operates on an ideal vapor-compression refrigeration cycle with R-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counterflow heat exchanger where both streams enter at 0.5 MPa. If the mass flow rate of the refrigerant through the upper cycle is 0.25 kg/s, What would the COP be if the heat exchanger pressure were 0.7 MPa. Group of answer choices 2.50 1.50 1.48 1.51arrow_forwardAn ideal vapor-compression refrigeration cycle uses R-134a as the refrigerant. The refrigerant enters the evaporator at 160 kpa with a quality of 25% and leaves the compressor at 65 °C. If the compressor consumes 800W of power, determine (a) the mass flow rate of the refrigerant, (b) the condenser pressure, and (c) the COP of the refrigeratorarrow_forward
- Refrigeration and Air Conditioning Technology (Mi...Mechanical EngineeringISBN:9781305578296Author:John Tomczyk, Eugene Silberstein, Bill Whitman, Bill JohnsonPublisher:Cengage Learning