Refrigeration& Air Conditioning
1. A vapor compression refrigeration system is an improved type of air refrigeration system in which a suitable working substance, termed as refrigerant is used.
a) True
b) False
Answer: a
Explanation: A vapor compression refrigeration system is an improved type of air refrigeration system in which a suitable working substance, termed as refrigerant is used. Refrigerants such as ammonia, CO2 and Sulphur dioxide etc. are used as refrigerants in a vapor compression refrigeration system.
2. The vapor compression refrigeration system is similar to a ________
a) latent heat pump
b) latent heat engine
c) generator
d) evaporator
Answer: a
Explanation: The vapor compression refrigeration system is a latent heat pump, as it pumps its latent heat from the brine and delivers it to the cooler.
3. The first vapor compression refrigeration system was developed by ________
a) L. G. Hamilton
b) Jacob Perkins
c) Stephen Rossi
d) Lenard Sandwik
Answer: b
Explanation: The first vapor compression refrigeration system was developed in 1834 by Jacob Perkins using hand operations.
4. The COP of vapor compression refrigeration compared to simple air refrigeration system is ________
a) high
b) low
c) same
d) can’t say
Answer: a
Explanation: COP of vapor compression refrigeration compared to simple air refrigeration system is high due to the use of refrigerant and as it can be employed over large range of temperatures.
5. Which of the following is not an advantage of vapor compression refrigeration system over air refrigeration system?
a) Smaller size for a given capacity of refrigeration
b) It has less running cost
c) It can be employed over wide range of temperature
d) There is no problem of leakage in vapor compression refrigeration system
Answer: d
Explanation: In vapor compression refrigeration system the prevention of leakage of the refrigerant is a major problem. The advantages of it are smaller size for a given capacity of refrigeration, it has less running cost and it can be employed over wide range of temperature.
6. The low pressure and temperature vapor refrigerant enters the ______ of the vapor compression system.
a) compressor
b) condenser
c) receiver
d) evaporator
Answer: a
Explanation: In the compressor the low pressure and temperature vapor refrigerant from the evaporator is drawn through the suction valve, where it is compressed to high pressure and temperature.
7. The high pressure and temperature vapor refrigerant enters the ______ of the vapor compression system.
a) compressor
b) condenser
c) receiver
d) evaporator
Answer: b
Explanation: In the condenser the high pressure and temperature vapor refrigerant from the compressor is condensed, during which the refrigerant gives up its latent heat.
8. In any compression refrigeration system there are how many pressure conditions?
a) 1
b) 2
c) 3
d) 4
Answer: b
Explanation: In any compression refrigeration system there are 2 pressure conditions, namely high pressure and low pressure side.
9. Which of the following is not an advantage of vapor compression cycle over reversed Carnot cycle?
a) It’ COP and refrigeration effect can’t be increased
b) Use of expansion valve reduces the size and cost of plant
c) It is a practical cycle on which plant can be built
d) Wet compression of Carnot cycle is avoided
Answer: a
Explanation: It’ COP and refrigeration effect can be increased by use of superheated vapor at entry of compressor. The advantages are use of expansion valve reduces the size and cost of plant, it is a practical cycle on which plant can be built and wet compression of Carnot cycle is avoided.
10. The pipe line emanating from compressor up to the condenser is called ________
a) suction line
b) pipe line
c) liquid line
d) delivery line
Answer: d
Explanation: The pipe line emanating from compressor up to the condenser is called delivery line or discharge line or hot gas line. Whereas suction line is the low pressure vapor line from the exit end evaporator leading to the compressor suction side and liquid line carries the liquid from receiver to the control valve
11. In vapor compression cycle having dry saturated vapor at the end of compression the work done is given by?
H2 – Enthalpy of vapor after compression
H1 – Enthalpy of vapor before compression
a) H2 – H1
b) H2 + H1
c) H2 × H1
d) H2 ÷ H1
Answer: a
Explanation: Work is done on the compressor in the compression process (1 – 2), and the external work-done in the remainder process of the cycle is zero. Hence the net work-done equals to the compressor work.
12. In vapor compression cycle having dry saturated vapor at the end of compression the refrigeration effect is given by?
H1 – Enthalpy of vapor before compression
H4 – Enthalpy of vapor before entering evaporator
H2 – Enthalpy of vapor after compression
a) H1 – H4
b) H2 – H4
c) H1 × H4
d) H2 × H4
Answer: a
Explanation: Refrigeration effect is the heat absorbed by the vaporizing refrigerant in the evaporator during the process (4 – 1), therefore refrigeration effect = H1 – H4.
13. Entropy of dry-saturated vapor when Sf is 0.2513 and Sfg is 4.7878, is _______
Sf – Entropy of saturated liquid at pressure p
Sfg – Entropy change during vaporization
a) 6.987
b) 5.987
c) 5.0391
d) 4.776
Answer: c
Explanation: Entropy of dry-saturated vapor when Sf is 0.2513 and Sfg is 4.5819, is given by
Sg = Sf + Sfg = 0.2513+4.7878 = 5.0391.
14. One of the assumptions of the vapor compression cycle is that all of the processes are reversible.
a) True
b) False
Explanation: Vapor compression cycle with vapor being dry saturated after compression is a theoretical cycle having various assumptions. One of the assumptions is that all the processes are reversible.
15. In which case does the compression process remains in superheated state?
a) Wet state
b) Dry state
c) Semi-dry state
d) Always
Answer: b
Explanation: In case of dry state the compression process remains in superheated state i.e. for compression of dry-saturated vapor or superheated vapor.
16. Dry compression is preferred over wet compression.
a) True
b) False
Answer: a
Explanation: Dry compression is preferred over wet compression since it gives high volumetric efficiency and mechanical efficiency of the compressor with less chances of damage to it. However, in some cases wet compression is preferred when the compression is done using a screw or rotary compressor.
17. The ratio of COP of vapor compression cycle and COP of Carnot cycle is known as?
a) Relative COP
b) Ideal COP
c) Performance index
d) Theoretical COP
Answer: c
Explanation: The ratio of COP of vapor compression cycle and COP of Carnot cycle is known as the Refrigeration efficiency or performance index (P.I). Whereas relative COP is the ratio of Actual COP divided by Ideal COP.
18. Which case is when the refrigerant compression process is carried while it’s wet?
a) Wet state
b) Dry state
c) Semi-dry state
d) Always
Answer: a
Explanation: In case of wet state the vapor compression process remains in wet state i.e. for compression of refrigerant is carried out while it’s still wet and for dry state compression is done in dry state.
19. In case of reciprocating compressors, wet compression is avoided.
a) True
b) False
Answer: a
Explanation: In case of reciprocating compressors, wet compression is avoided as the liquid droplets in the refrigerant would enter the compressor and damage the valves and other moving parts.
20. Which one of the following is not an assumption in theoretical vapour compression cycle?
a) There are no pressure losses in the condenser, evaporator, compressor, valves and the connecting part lines
b) All processes are reversible
c) There are mechanical and fluid friction losses
d) There is no heat transfer between the system and surroundings except in the evaporator and condenser
Answer: c
Explanation: There are no mechanical or fluid friction losses, is one of the assumptions made in theoretical vapor compression cycle. The other assumptions are all processes are reversible, there is no heat transfer between the system and surroundings except in the evaporator and condenser and there are no pressure losses in the condenser, evaporator, compressor, valves and the connecting part lines.
21. Power consumption in dry compression is less than wet compression.
a) True
b) False
Explanation: It is found that the power consumption is about 8%-10% less with wet compression compared to dry-compression. Hence power consumption in dry compression is less than wet compression.
22. Due to what in wet-compression affects the heat transfer rates from the vapor refrigerant to the cooling medium?
a) Liquid droplets
b) Work consumed
c) Power required
d) In wet-compression the heat transfer remains unaffected
Answer: a
Explanation: In wet – compression the liquid droplets may carry the lubricating oil from the cylinder walls of the compressor. This oil would then pass into the condenser and adversely affect the heat transfer rates from the vapor refrigerant to the cooling medium.
23. The following p – h diagram indicates?
a) Vapor compression when vapor is dry at the end of the compression
b) Vapor compression when vapor is wet at the end of the compression
c) Vapor compression when vapor is dry – saturated at the end of the compression
d) Vapor compression when vapor is dry before of compression
Answer: b
Explanation: As from the diagram we can see that the compression line does not touch the saturated line it means that the vapor after compression is wet. For dry compression, it should’ve touched the saturation line.
24. Which of these is not an effect of superheat in suction vapour?
a) increase in refrigeration effect
b) increase in compression work
c) decrease the capacity of compressor per ton of refrigeration
d) condenser size decreases
Answer: d
Explanation: The effects of superheat in suction vapour are increase in refrigeration effect, increase in compression work, decreasing the capacity of compressor per ton of refrigeration and increasing the condenser size as the heat rejected in the condenser increases.
25. The COP of the cycle may increase or decrease or remain unchanged depending upon the range of pressure and the degree of superheating.
a) True
b) False
Answer: a
Explanation: The COP of the cycle may increase or decrease or remain unchanged depending upon the range of pressure and the degree of superheating as it can affect the refrigeration effect, compression work, heat rejected in condenser etc. and thus the COP.
26. In which case will the refrigeration effect will not increase and the COP of cycle with superheated vapour in suction phase will be less than the COP of ideal cycle?
a) Superheating the vapour from state 1 to 1′
b) Superheating the vapour from state 1 to 2
c) Superheating the vapour from state 1′ to 2′
d) COP always increases when superheating vapour in suction vapour
Answer: a
Explanation: As we can see from the below picture of a cycle with superheated vapour in suction vapour, the refrigeration effect is not increased and thus the COP will be less than that of ideal COP.
27. Which one of the following is not effect of liquid sub cooling?
a) increase the refrigeration effect
b) reduce the piston displacement
c) increase the COP
d) increase the compressor power per ton of refrigeration
Answer: d
Explanation: Sub cooling the liquid reduces the compressor power per ton of refrigeration due to reduced mass flow rate of refrigerant. It also increases the refrigeration effect, reduce the piston displacement and increase the COP.
28. Which one of the following is not effect of change in suction pressure?
a) reduces the net refrigeration effect
b) reduces the mass flow rate of refrigerant per ton of refrigeration
c) reduces the COP
d) increases the compressor power per ton of refrigeration
Answer: b
Explanation: Change in suction pressure reduces the net refrigeration effect thus increasing the mass flow rate of refrigerant per ton of refrigeration.
29. Effect of high side pressure or discharge pressure or condenser pressure is to reduce the COP of the cycle.
a) True
b) False
Answer: a
Explanation: Effect of high side pressure or discharge pressure or condenser pressure is to reduce the COP of the cycle as the refrigeration effect is reduced and the compression work increases as well as the compressor power increases per ton of refrigeration due to increased mass flow and additional energy supplied in compression.
30. What is the effect of using a flash chamber in VCR system?
a) C.O.P. increases
b) C.O.P. decreases
c) C.O.P. remains the same
d) Mass of refrigerant flowing through evaporator increases
Answer: c
Explanation: As C.O.P. of the VCR system containing flash chamber is given by,
C.O.P. = h1 − hf3 / h2 − h1
Which is same as the simple VCR cycle. The purest forms of refrigerant enter the respective system but effectively do not give enormous impact on C.O.P., So C.O.P. remains the same.
The mass of refrigerant required to flow through the evaporator decreases as even the smaller amount can produce the same refrigeration effect from its pure liquid form, resulting in a reduction in the size of evaporator.
31. From the following line diagram and p-h chart, what is the refrigeration effect in terms of the mass of mixture i.e., m2?
a) m2 (h1 − hf3)
b) m2 (h2 − h1)
c) m2 (h1 − h2)
d) m2 (h4 − h1)
Answer: a
Explanation: Refrigerating effect is considered in the evaporator i.e. heat absorbed by the evaporator is called as refrigerating effect.
R.E. = m1 (h1 − hf4’)
And also, m1 / m2 = h1 − h4 / h1 − hf4’
Hence, R.E. = m2 (h1 − hf3)
32. What is the value of C.O.P. in the VCR with flash chamber?
a) h1 – h2 / h1 – h4
b) h1 – hf3 / h2 – h1
c) h2 – h1 / h4 – h1
d) h1 – hf4’ / h1 – h4
Answer: b
Explanation: C.O.P. = Refrigerating effect / Work done
Refrigerating effect in terms of m2 = m2 (h1 – hf3)
Work = m2 (h2 – h1)
C.O.P. = m2 (h1 – hf3) / m2 (h2 – h1)
C.O.P. = (h1 – hf3) / (h2 – h1)
33. What is the effect of using accumulator in the VCR system?
a) C.O.P. increases
b) C.O.P. decreases
c) Total dry compression of refrigerant occurs
d) Sub-cooling happens
Answer: c
Explanation: As C.O.P. of the VCR system containing accumulator is given by,
C.O.P. = h1 – hf3 / h2 – h1
Which is same as the simple VCR cycle. The purest forms of refrigerant enter the respective system but effectively do not give enormous impact on C.O.P. So C.O.P. remains the same.
The C.O.P., R.E., and power required are the same, but it protects the liquid refrigerant from entering the compressor. Dry compression is always ensured by using accumulator or pre-cooler.
34. What is the value of C.O.P. in the VCR with a flash chamber?
a) h1 – h2 / h1 – h4
b) h1 – hf3 / h2 – h1
c) h2 – h1 / h4 – h1
d) h1 – hf4’ / h1 – h4
Answer: b
Explanation: C.O.P. = Refrigerating effect / Work done
Refrigerating effect in terms of m2 = m2 (h1 – hf3)
Work = m2 (h2 – h1)
C.O.P. = m2 (h1 – hf3) / m2 (h2 – h1)
C.O.P. = (h1 – hf3) / (h2 – h1)
35. Which of the following cooling towers possess maximum heat transfer from air to water?
a) Natural Draft
b) Mechanical Draft
c) Natural and Mechanical Draft
d) Atmospheric Draft
Answer: b
Explanation: Natural Draft is where air circulates naturally without any external force, and the atmospheric draft is a type of natural draft. So, the mechanical draft is the type of cooling tower which gives the maximum heat transfer due to usage of fans for the circulation. More the force more heat can be transferred.
36. How is the performance of the cooling tower indicated?
a) Wet-bulb temperature
b) Dry bulb temperature
c) Approach
d) Range
Answer: c
Explanation: Approach can be defined as the temperature difference between water leaving the cooling tower and ambient wet-bulb temperature. If the approach is low, then the performance is good and vice-versa. This approach value denotes how close the cooling tower gets the water to the wet-bulb temperature of the ambient or surrounding air.
37. What is the difference between the temperature of entering and leaving water in the cooling tower?
a) Wet-bulb temperature
b) Dry bulb temperature
c) Approach
d) Range
Answer: d
Explanation: Range can be defined as the difference between entering water temperature and leaving water temperature. This is also an indicator of the performance of cooling tower but for the effective performance, a better indicator, i.e., the approach is used.
38. What is the formula for the effectiveness of cooling tower from the following?
a) Range / (Range + Approach)
b) Approach / (Range + Approach)
c) Range / Approach
d) Approach / Range
Answer: a
Explanation: Approach can be defined as the temperature difference between water leaving the cooling tower and ambient wet-bulb temperature. The range can be defined as the difference between entering water temperature and leaving water temperature.
Hence, the effectiveness can be given as,
Effectiveness = Range / (Range + Approach).
39. A cooling tower brings water temperature to _________
a) WBT
b) DBT
c) DPT
d) Ambient WBT
Answer: d
Explanation: Approach can be defined as the temperature difference between water leaving the cooling tower and ambient wet-bulb temperature. If the approach is low, then the performance is good and vice-versa. This approach value denotes how close the cooling tower gets the water to the wet-bulb temperature of the ambient or surrounding air. So, cooling tower brings the water temperature to ambient wet bulb temperature for the operation.
40. What is the correct representation of range from the following?
a) Range = Cooling tower water outlet temperature – Wet bulb temperature
b) Range = Cooling tower water inlet temperature – Wet bulb temperature
c) Range = Cooling tower water outlet temperature – Dry bulb temperature
d) Range = Heat load in kcal per hour / Water circulation in liters per hour
Answer: d
Explanation: Range can be defined as the difference between entering water temperature and leaving water temperature. It is also defined as the heat load per water circulation. As in the given options, the first three are incorrect as one of them represents approach so correct representation is,
Range = Heat load in kcal per hour / Water circulation in liters per hour
Range = Cooling tower water inlet temperature – Cooling tower water outlet temperature.
41. What is L / G in terms of the cooling tower?
a) Length / Girth
b) Length / Gradient
c) Air mass flow rate / Water flow rate
d) Water flow rate / Air mass flow rate
Answer: d
Explanation: The L / G is called a liquid-gas ratio. It is defined as the ratio of water flow rate to the air mass flow rate in the cooling tower. This L / G ratio affects the effectiveness of the cooling tower.
42. Mechanical draft cooling tower size is ________ the size of the natural draft cooling tower.
a) smaller than
b) larger than
c) equal to the
d) very much larger than
Answer: a
Explanation: The mechanical draft cooling towers are smaller in size than that of the natural draft cooling towers with the same capacity due to the fact that a large volume of forced air increases the cooling capacity.
43. The cooling capacity of mechanical draft cooling towers cannot be varied.
a) False
b) True
Answer: a
Explanation: The main advantage of mechanical draft cooling towers is cooling capacity can be controlled. This can be achieved by controlling the amount of forced air entering the cooling tower.
44. Mechanical Draft cooling towers do not depend on atmospheric air.
a) True
b) False
Answer: a
Explanation: One of the advantages of mechanical draft cooling towers is that they do not depend upon the atmospheric air. Fans do the operation. Due to the fact of independence, this type of cooling towers can be even installed inside the building.
45. Among the following refrigerants, which is having the lowest freezing point?
a) R – 21
b) R – 11
c) R – 12
d) R – 22
Answer: d
Explanation: Freezing temperature is a crucial property to carry out refrigeration. Among the given refrigerants, R – 22 is having the lowest freezing point, valued at -160°C. R – 11, R – 12 and R – 21 having freezing points as -111°C, -157.5°C and -135°C respectively.
46. What is the boiling point of Ammonia at atmospheric pressure?
a) -10.5°C
b) -30°C
c) -33.3°C
d) -77.7°C
Answer: c
Explanation: Ammonia has a boiling point at -33.3°C. This low boiling point makes Ammonia possible to get refrigeration below 0°C even at atmospheric pressure.
47. R – 717 is the refrigerant number of which of the following refrigerant?
a) Ammonia
b) Water
c) Carbon dioxide
d) Sulphur dioxide
Answer: a
Explanation R – 717 is the refrigerant number of Ammonia. Whereas, refrigerant number of Water, Carbon dioxide, and Sulphur dioxide is R – 118, R – 744 and R – 764 respectively.
48. What is the boiling point of Carbon dioxide at atmospheric pressure?
a) -20.5°C
b) -73.6°C
c) -50°C
d) -68°C
Answer: b
Explanation: Carbon dioxide has a boiling point at -73.6°C at given conditions. Carbon dioxide is having the lowest boiling point among all the refrigerants which are commonly used due to high capacity o refrigeration.
49. What is the freezing point of R – 12 at atmospheric pressure?
a) -86.6°C
b) -73.6°C
c) -157.5°C
d) -160°C
Answer: c
Explanation: R – 12 is having a boiling point at -157.5°C at given conditions. R – 12 is having the second-lowest freezing point after R – 22, which is valued at -160°C.
50. What is the refrigerant number of Water?
a) R – 717
b) R – 744
c) R – 118
d) R – 100
Answer: c
Explanation: Water is having refrigerant number as R – 118. Whereas, R – 717, 744, 100 corresponds to Ammonia, Carbon dioxide, and ethyl chloride.
51. Among the following refrigerants, which is having the lowest boiling point?
a) Ammonia
b) R – 12
c) Carbon dioxide
d) Sulphur dioxide
Answer: c
Explanation: Boiling point temperature is a crucial property to carry out refrigeration. Among the given refrigerants, Carbon dioxide is having the lowest boiling point, valued at –73.6°C. Ammonia, R -12 and Sulphur dioxide are having boiling points as -33.3°C, -29°C and -10°C respectively.
52. Among the following refrigerants, which is having the lowest C.O.P for refrigeration system working under the temperature limits of -15°C and 30°C as evaporator and condenser temperature respectively?
a) Ammonia
b) R – 12
c) R – 30
d) Carbon dioxide
Answer: c
Explanation: Coefficient of performance is the ratio of refrigeration produced and work done to get that refrigeration effect. C.O.P. is a crucial entity. The more the C.O.P., more the refrigeration effect is produced, and lower temperatures can be achieved. Among the given options, Carbon dioxide is having least C.O.P. for given temperatures conditions, which is valued at 2.56. Whereas, Ammonia, R – 12 and R – 30 is having C.O.P. as 4.76, 4.70 and 4.90 respectively.
53. R – 11 is having the highest C.O.P for refrigeration system working under the temperature limits of -15°C and 30°C as evaporator and condenser temperature respectively.
a) False
b) True
Answer: b
Explanation: Coefficient of performance is the ratio of refrigeration produced and work done to get that refrigeration effect. C.O.P. is a crucial entity. The more the C.O.P., more the refrigeration effect is produced, and lower temperatures can be achieved. For a Carnot cycle, C.O.P. under given conditions is 5.74 and R – 11 posses 5.09 C.O.P. very close to the ideal cycle.
54. A refrigerant should have a low latent heat of vaporization at the evaporator temperature.
a) True
b) False
Answer: b
Explanation: A refrigerant should have a high latent heat of vaporization at the evaporator temperature. More the latent heat of vaporization more is the capacity of refrigerant to absorb heat in the evaporator. So, Higher the value of latent heat of vaporization, higher is the refrigeration effect produced.
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