1. What is the formula for COP of an ideal vapour absorption refrigeration system?
Qg = the heat given to the refrigerant in the generator
Qe= the heat absorbed by the refrigerant in the evaporator
2. An ideal vapour absorption refrigeration system may be regarded as a combination of _________
a) Carnot engine and Carnot pump
b) Carnot refrigerant and Carnot pump
c) Carnot engine and Carnot refrigerant
d) Carnot engine alone
3. Find the COP of a vapour absorption refrigeration system using the following data.
Qg = 6600
Qe = 2100
a) 0.45
b) 0.65
c) 0.31
d) 0.35
4. Find the COP of a vapour absorption refrigeration system using the following data.
Te = 268
Tg = 393.2
Tc = 303
a) 1.53
b) 1.76
c) 1.66
d) 1.98
5. In case the heat is discharged at different temperatures in condenser and absorber then the formula for COP would be?
Ta = Temperature at which heat is discharged in the absorber
Te and Tc are the temperature limits for Carnot refrigerator
Tg and Tc are the temperature limits for Carnot engine
6. Find the COP of an ideal vapour absorption refrigeration system using the following data.
Qg = 1880
Qe = 1045
a) 0.47
b) 0.65
c) 0.69
d) 0.55
7. Which of the following is correct about VARS and VCRS?
a) VARS use mechanical energy, and VCRS use heat energy
b) VARS use heat energy, and VCRS use mechanical energy
c) Both use mechanical energy
d) Both use heat energy
Answer: b
Explanation: VCRS use mechanical energy i.e., VCRS uses compressor which withdraws energy from the evaporator and drawn into condenser after compression. VARS use heat energy i.e., heat exchanger or heat generators are used and by which desired effect is achieved.
8. The compressor from VCRS is replaced by which of the following in the VARS?
a) Absorber, Pump
b) Generator, Pressure reducing valve
c) Absorber, Pump, Generator, and Pressure reducing valve
d) Absorber, Rectifier, Generator, and Pressure reducing valve
Answer: c
Explanation: Compressor from the VCRS is replaced by an absorber, a pump, a generator, and a pressure reducing valve. These all components together do the same work as a compressor but by using the heat energy.
9. What is the purpose of using an absorber?
a) Heat absorption
b) Heat rejection
c) Pressure reduction
d) Work done
Answer: b
Explanation: Vapour refrigerant from the evaporator enters the absorber. Cooling water is circulated to transfer the heat, and refrigerant’s strong solution is discharged to the generator.
10. What is the purpose of using a pump?
a) Heat absorption
b) Heat rejection
c) Pressure reduction
d) Pressure increment
Answer: d
Explanation: The strong solution discharging from the absorber is pumped to the generator using a liquid pump. The liquid pump increases the pressure up to 10 bar.
11. What is the purpose of using a generator?
a) Heat supplied
b) Heat rejection
c) Pressure reduction
d) Pressure increment
Answer: a
Explanation: Generator generates heat. From the generator, heat is supplied to the refrigerant, and the strong refrigerant is heated further by using some external source like gas or steam.
12. What is the fundamental equation of the C.O.P. of Ideal VAR system?
a) Heat absorbed in the evaporator / Work done by a pump
b) Heat absorbed in the evaporator / Heat supplied in the generator
c) Heat absorbed in the evaporator / Work done by compressor + Heat supplied in the generator
d) Heat absorbed in the evaporator / Work done by pump + Heat supplied in the generator
Answer: d
Explanation: VAR uses the generator to do work as well as pump to pressurize the liquid to the generator. So, total work done is the summation of work done by a pump and generator. Refrigeration effect is the heat absorbed in the evaporator.
So, C.O.P. = Refrigeration effect / Total work done
= Heat absorbed in the evaporator / Work done by pump + Heat supplied in the generator.
13. What is the relation between QC, QG, and QE if they represent heat given to the refrigerant in the generator, heat discharging to atmosphere from condenser to absorber and heat absorbed by the refrigerant in the evaporator?
a) QC + QG = QE
b) QC = QG + QE
c) QC + QE = QG
d) QC = QG – QE
Answer: b
Explanation: According to the first law of thermodynamics, heat discharged to the atmosphere or cooling water from the condenser to absorber is equal to the heat given to the refrigerant in a generator, heat absorbed by the refrigerant in the evaporator and heat added to the refrigerant due to pump work. Heat added due to pump work is neglected due to minimal effect.
So, QC = QG + QE
14. What is the equation of VAR system in terms of entropy?
a) QC + QE / TC = QG / TG + QE / TE
b) QG + QC / TC = QG / TG + QE / TE
c) QG + QE / TC = QG / TG + QE / TE
d) QG + QE / TC = QC / TC + QE / TE
Answer: c
Explanation: As vapour absorption system is considered as a perfectly reversible system,
Hence the initial entropy of the system should be equal to the entropy of the system after some change in its conditions.
As entropy is heat absorbed divided by the temperature.
From the first law of thermodynamics, QC = QG + QE
So, the equation in terms of entropy QC / TC = QG / TG + QE / TE
Modified as, QG + QE / TC = QG / TG + QE / TE
15. What is the ratio of QG / QE in terms of temperature?
If TC, TG and TE if they represent temperature at which heat is given to the refrigerant in the generator, temperature at which heat is discharging to atmosphere from condenser to absorber and temperature at which heat is absorbed by refrigerant in the evaporator
a) [TC – TE / TE] [TG / TG – TC]
b) [TC – TE / TE] [TG / TG – TE]
c) [TC – TE / TE] [TC / TG – TC]
d) [TC – TE / TC] [TG / TG – TC]
Answer: a
Explanation: By using the phenomenon of perfectly reversible system,
QG + QE / TC = QG / TG + QE / TE
QG / TG – QG / TC = QE / TC – QE / TE
QG [TC – TG / TC x TG] = QE [TE – TC / TC x TE]
By rearranging, QG / QE = [TC – TE / TE] [TG / TG – TC]
16. What is the value of maximum C.O.P. in terms of heat?
a) QG / QE
b) QE / QG
c) QG / QC
d) QC / QE
Answer: b
Explanation: As VAR uses the generator to do work as well as pump to pressurize the liquid to the generator. So, total work done is the summation of work done by a pump and generator. Refrigeration effect is the heat absorbed in the evaporator.
So, C.O.P. = Refrigeration effect / Total work done
= Heat absorbed in the evaporator / Work done by pump + Heat supplied in the generator
For maximum C.O.P., due to the minimal effect of work done by a pump, QP is neglected.
QE = Heat absorbed in the evaporator and QG = Heat supplied in the generator
C.O.P. maximum = QE / QG
17. What is the value of maximum C.O.P. in terms of temperature?
a) [TC – TE / TE] [TG / TG – TC]
b) [TC – TE / TE] [TG / TG – TC]
c) [TC – TE / TE] [TG / TG – TC]
d) [TG – TC / TG] [TE / TC – TE]
Answer: d
Explanation: As VAR uses the generator to do work as well as pump to pressurize the liquid to the generator. So, total work done is the summation of work done by a pump and generator. Refrigeration effect is the heat absorbed in the evaporator.
So, C.O.P. = Refrigeration effect / Total work done
= Heat absorbed in the evaporator / Work done by pump + Heat supplied in the generator
For maximum C.O.P., due to the minimal effect of work done by a pump, QP is neglected.
QE = Heat absorbed in the evaporator and QG = Heat supplied in generator
C.O.P. maximum = QE / QG
By using the phenomenon of perfectly reversible system,
QG + QE / TC = QG / TG + QE / TE
QG / TG – QG / TC = QE / TC – QE / TE
QG [TC – TG / TC x TG] = QE [TE – TC / TC x TE]
By rearranging, QE / QG = [TG – TC / TG] [TE / TC – TE]
(C.O.P.) max = QE / QG = [TG– TC / TG] [TE / TC – TE]
18. Which of the following is true?
a) (C.O.P.)max = (C.O.P.)Carnot / ηCarnot
b) (C.O.P.)max = (C.O.P.)Carnot + ηCarnot
c) (C.O.P.)max = (C.O.P.)Carnot x ηCarnot
d) (C.O.P.)max = (C.O.P.)Carnot – ηCarnot
Answer: c
Explanation: As, (C.O.P.) max = QE / QG = [TG – TC / TG] [TE / TC – TE]
Where, it may be noted that, [TE / TC – TE] is the coefficient of performance of a Carnot refrigerator working between TE and TC temperatures.
And, [TG – TC / TG] is the efficiency of a Carnot engine working between TG and TC temperatures.
Hence, (C.O.P.)max = (C.O.P.)Carnot x ηCarnot
19. C.O.P. of the VAR system is lower than the C.O.P. of VCR system.
a) True
b) False
Answer: a
Explanation: Though the VAR system has many advantages over VCR, VAR gives lower C.O.P. as compared to VCR. Compressor used in the VCR system given higher pressure difference, which increases the temperature more and thus increasing the heat rejection. Overall impacting the C.O.P. to increase but in case of VAR system, heat supplied is limited so overall C.O.P. is not as high as in the VCR.
20. In a VAR system, heating, cooling, and refrigeration take place at temperatures 200°C, 30°C, and -10°C respectively. What is the value of maximum C.O.P. of the system?
a) 2.202
b) 2.808
c) 3.404
d) 2.404
Answer: d
Explanation: Given: TG = 200°C = 200 + 273 = 473 K
TC = 30°C = 273 + 30 = 303 K
TE = – 10°C = 273 – 10 = 263 K
As, (C.O.P.)max = QE / QG = [TG – TC / TG] [TE / TC – TE]
= [263 / 303 – 263] [473 – 303 / 473]
= [263/ 40] [173 / 473]
= [45499 / 18920]
= 2.404.
21. What is the value of maximum C.O.P. if the VAR system has a capacity of 12 TR and heat given to the refrigerant in the generator is 40 kW?
a) 0.9523
b) 0.4878
c) 1.05
d) 2.05
Answer: c
Explanation: Given: QE = 12 TR = 12 x 3.5 = 42 kW
QG = 40 kW
As, C.O.P. maximum = QE / QG
= 42 / 40
= 1.05.
22. The device which divides the high pressure side and the low pressure side of a refrigerating system is known as _____________
a) condenser device
b) evaporator device
c) receiver device
d) expansion device
Answer: d
Explanation: The expansion device is an important device that divides the high pressure side and the low pressure side of a refrigerating system. It is also known as metering device or throttling device.
23. Which of the following function is not performed by the expansion device?
a) It reduces the high pressure liquid refrigerant to low pressure liquid refrigerant
b) It maintains the desired pressure difference between the high and low pressure sides of the system
c) It controls the flow of refrigerant according to the load on the evaporator
d) It compresses the vapor
Answer: d
Explanation: The expansion device performs the following functions, it reduces the high pressure liquid refrigerant to low pressure liquid refrigerant, it maintains the desired pressure difference between the high and low pressure sides of the system and It controls the flow of refrigerant according to the load on the evaporator.
24. The expansion device used with flooded evaporators is known as expansion valves.
a) True
b) False
Answer: b
Explanation: The expansion device used with flooded evaporators is known as float valves and the expansion device used with dry expansion evaporators are called expansion valves.
25. The expansion device is placed between which two components?
a) Condenser and evaporator
b) Compressor and condenser
c) Evaporator and compressor
d) Receiver and evaporator
Explanation: The expansion device is placed between receiver (containing liquid refrigerant at high pressure) and evaporator (containing liquid refrigerant at low pressure).
26. Which of the following is not an advantage of capillary tube?
a) The cost of capillary tube is less
b) A high starting motor is not required
c) No receiver is needed
d) A capillary tube designed for a specific condition will also work efficiently for other conditions
Answer: d
Explanation: A capillary tube designed for a specific condition will not work efficiently for other conditions as the length is directly and inner diameter is indirectly proportional to the frictional resistance. The longer the tube and smaller the diameter, greater is the pressure drop created in the refrigerant flow.
27. Capillary tube, as an expansion device is used in?
a) Water coolers
b) Domestic refrigerators
c) Room air conditioners
d) All of the mentioned
Answer: d
Explanation: The capillary tube as an expansion device is used in small capacity hermetic sealed refrigeration units such as in water coolers, domestic refrigerators and room air conditioners, etc.
28. Which one of the following is also known as a constant superheat valve?
a) Capillary tube
b) Hand-operated expansion valve
c) Thermostatic Expansion valve
d) Low side float valve
Answer: d
Explanation: Thermostatic expansion valve is also called a constant superheat valve because it maintains a constant superheat of the vapor refrigerant at the end of the evaporator coil, by controlling the flow of liquid refrigerant through the evaporator.
29. Thermostatic expansion valves are usually set for a superheat of?
a) 10°C
b) 5°C
c) 8°C
d) 15°C
Answer: b
Explanation: Thermostatic expansion valves are usually set for a superheat of 5°C for better efficiency of the refrigeration cycle.
30. The low-side float valve is located between the condenser and evaporator.
a) True
b) False
Answer: b
Explanation: The low-side float valve is located in the low pressure side i.e. between the evaporator and the compressor suction line. Whereas the high side float valve is located in the high pressure side i.e. between the condenser and evaporator.
31. The thermostatic expansion valve operates on the changes in the ___________
a) degree of superheat at exit from the evaporator
b) temperature of the evaporator
c) pressure in the condenser
d) pressure in the evaporator
Answer: a
Explanation: The thermostatic expansion valve operates on the changes in the degree of superheat at exit from the evaporator. Thermostatic expansion valves are usually set for a superheat of 5° C for better efficiency of the refrigeration cycle.
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