Refrigeration & Air-Conditioning Practice Set-6

July 15, 2021

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1. The coefficient of performance of Electrolux refrigerator is the ratio of

Heat absorbed by the evaporator minus the heat supplied by the gas burner to the heat absorbed by the evaporator

Heat supplied by the gas burner minus the heat absorbed by the evaporator to the heat supplied by the gas burner

Heat supplied by the gas burner to the heat absorbed by the evaporator

Heat absorbed by the evaporator to the heat supplied by the gas burner

2. A good refrigerant should have

High specific volume and high latent heat of vaporisation

High latent heat of vaporisation and low freezing point

High operating pressures and low freezing point

Low C.O.P. and low freezing point

3. Air refrigeration cycle is used in

Commercial refrigerators

Air-conditioning

Domestic refrigerators

Gas liquefaction

4. The superheated vapour region, in a pressure enthalpy chart, is represented by the space

None of the above

To the left of saturated liquid line

Between the saturated liquid line and saturated vapour line

To the right of saturated liquid line

5. The ratio of the actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature and pressure, is called

Relative humidity

Degree of saturation

Absolute humidity

Humidity ratio

6. The relative humidity is defined as

The ratio of the actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature and pressure

The mass of water vapour present in 1 m3 of dry air

The ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure

The mass of water vapour present in 1 kg of dry air

7. Which of the following refrigerant has the lowest freezing point?

Ammonia

R-11

R-22

R-12

8. In a psychrometric process, the sensible heat added is 30 kJ/s and the latent heat added is 20 kJ/s. The sensible heat factor for the process will be

0.3

1.5

0.67

0.6

9. The C.O.P. of a heat pump working on a reversed Carnot cycle is

T₂/(T₂ - T₁)

(T₂ - T₁)/T₁

T₁/(T₂ - T₁)

(T₁ - T₂)/T₁

10. During dehumidification process, _________ remains constant.

Wet bulb temperature

Dry bulb temperature

Relative humidity

Specific humidity

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11. An infinite parallel planes with emissivities e₁ and e₂, the interchange factor for radiation from surface 1 to surface 2 is given by

(e₁ + e₂)/ e₁ + e₂ - e₁e₂

1/e₁ + 1/e₂

e₁ + e₂

e₁e₂

12. In a saturated air-water vapour mixture, the

Dry bulb temperature is higher than dew point temperature

Dew point temperature is lower than wet bulb temperature

Dry bulb, wet bulb and dew point temperature are same

Dry bulb temperature is higher than wet bulb temperature

13. It is desired to condition the outside air from 70% relative humidity and 45° C dry bulb temperature to 50% relative humidity and 25° C dry bulb temperature (room condition). The practical arrangement will be

Cooling and humidification

Dehumidification

Cooling and dehumidification

Dehumidification and pure sensible cooling

14. The difference between dry bulb temperature and wet bulb temperature, is called

Wet bulb depression

Degree of saturation

Dry bulb depression

Dew point depression

15. When the lower temperature of a refrigerating machine is fixed, then the coefficient of performance can be improved by

Lowering the higher temperature

Operating the machine at lower speeds

Raising the higher temperature

Operating the machine at higher speeds

16. The bypass factor, in case of sensible cooling of air, is given by (where td₁ = Dry bulb temperature of air entering the cooling coil, td₂ = Dry bulb temperature of air leaving the cooling coil, and td₃ = Dry bulb temperature of the cooling coil)

(td₃ -td₂)/( td₁ -td₃)

(td₃ -td₁)/( td₂ -td₃)

(td₂ -td₃)/( td₁ -td₃)

(td₁ -td₃)/( td₂ -td₃)

17. As relative humidity decreases, the dew point temperature will be ________ wet bulb temperature.

Higher than

Lower than

None of these

Same as

18. R-12 is generally preferred over R-22 in deep freezers since

It gives higher coefficient of performance

All of the above

It has low operating pressures

It is miscible with oil over large range of temperatures

19. Nusselt number (N_N) is given by

N_N = ρ V l /μ

N_N = hl/k

N_N = μ c_p/k

N_N = V²/t.c_p

20. Bell Coleman cycle is a

Reversed Carnot cycle

Reversed Otto cycle

Reversed Rankine cycle

Reversed Joule cycle

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21. A refrigeration system

Removes heat from a low temperature body and delivers it to a high temperature body

None of the above

Removes heat from a high temperature body and delivers it to a low temperature body

Rejects energy to a low temperature body

22. A vapour absorption refrigeration system

Gives quiet operation

Cools below 0°C

Gives noisy operation

Requires little power consumption

23. A pressure gauge on the discharge side of a refrigerant compressor reads too high. The reasons will be

Water temperature being high

Dirty condenser surface

All of these

Lack of cooling water

24. The total pressure exerted by the mixture of air and water vapour is equal to the sum of pressures which each constituent would exert, if it occupied the same space by itself. This statement is called

Dalton's law of partial pressures

Avogadro's hypothesis

Kinetic theory of gases

Newton's law of gases

25. The coefficient of performance of Heat Pump is always _________ one.

None of these

Less than

Equal to

Greater than

26. The pressure at the outlet of a refrigerant compressor is called

Critical pressure

Discharge pressure

Suction pressure

Back pressure

27. The bypass factor of a cooling coil decreases with

Decrease in fin spacing and increase in number of rows

Increase in fin spacing and decrease in number of rows

Increase in fin spacing and increase in number of rows

Decrease in fin spacing and decrease in number of rows

28. A refrigerating system operating on reversed Brayton refrigeration cycle is used for maintaining 250 K. If the temperature at the end of constant pressure cooling is 300 K and rise in the temperature of air in the refrigerator is 50 K, then the net work of compression will be (assume air as working substance with C_p = 1 kJ/kg)

50 kJ/kg

100 kJ/kg

25 kJ/kg

125 kJ/kg

29. The temperature of ammonia after compression in a vapour compression system is

20 to 50°C

70 to 110°C

None of these

50 to 70°C

30. The relative humidity lines on a psychrometric chart are

Vertical and uniformly spaced

Horizontal and non-uniformly spaced

Curved lines

Horizontal and uniformly spaced

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31. The unit of thermal diffusivity is

m²/h

m²/hK

m/h

m/hK

32. The freezing point of R-12 is

-95.2°C

-135.8°C

-107.7°C

-86.6°C

33. An evaporator is also known as

All of these

Freezing coil

Cooling coil

Chilling coil

34. A mixture of dry air and water vapour, when the air has diffused the maximum amount of water vapour into it, is called

Moist air

Saturated air

Dry air

Specific humidity

35. For evaporators and condensers, for the given conditions, the logarithmic mean temperature difference (t_m) for parallel flow is ________ that for counter flow.

Equal to

None of these

Less than

Greater than

36. Environmental protection agencies advice against the use of chlorofluorocarbon refrigerants since

These react with ozone layer

These react with water vapour and cause acid rain

These react with plants and cause greenhouse effect

These react with oxygen and cause its depletion

37. Pressure of water vapour is given by

0.622 Pv/ (Pb - Pv)

None of these

μ/[1 - (1 - μ) (Ps/Pb)]

[Pv (Pb - Pd)]/ [Pd (Pb - Pv)]

38. A valve which maintains a constant degree of superheat at the end of the evaporator coil, is called

Low side float valve

High side float valve

Thermostatic expansion valve

Automatic expansion valve

39. A vapour absorption refrigerator uses _________ as a refrigerant.

Aqua-ammonia

Water

Freon

Ammonia

40. In Electrolux refrigerator

Ammonia is absorbed in hydrogen

Hydrogen evaporates in ammonia

Ammonia is absorbed in water

Ammonia evaporates in hydrogen

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41. In a vapour compression system, the condition of refrigerant before entering the expansion or throttle valve is

Dry vapour

High pressure saturated liquid

Wet vapour

Very wet vapour

42. The capillary tube is not used in large capacity refrigeration systems because

It is made of copper

Capacity control is not possible

Cost is too high

Required pressure drop cannot be achieved

43. The wet bulb temperature during sensible heating of air

Decreases

Remains constant

Increases

None of these

44. One tonne of refrigeration (1TR) means that the heat removing capacity is

210 kJ/min

420 kJ/min

620 kJ/min

21 kJ/min

45. In chemical dehumidification process,

Wet bulb temperature decreases

Dry bulb temperature increases

Dew point temperature decreases

All of these

46. In a spray washing system, if the temperature of water is higher than the dry bulb temperature of entering air, then the air is

Heated and dehumidified

Cooled and humidified

Cooled and dehumidified

Heated and humidified

47. The operating temperature of a cold storage is 2°C. The heat leakage from the surrounding is 30 kW for the ambient temperature of 40°C. The actual C.O.P. of refrigeration plant used is one fourth that of ideal plant working between the same temperatures. The power required to drive the plant is

18.6 kW

3.72 kW

7.44 kW

1.86 kW

48. An Electrolux refrigerator is called a _________ absorption system.

None of these

Two fluids

Single fluid

Three fluids

49. The thermostatic expansion valve operates on the changes in the

Pressure in the evaporator

Temperature of the evaporator

Degree of superheat at exit from the evaporator

None of the above

50. The comfort conditions in air conditioning are at (where DBT = Dry bulb temperature, and RH = Relative humidity)

25°C DBT and 100% RH

25°C DBT and 40% RH

20°C DBT and 80% RH

22°C DBT and 60% RH

Refrigeration & Air-Conditioning Practice Set-6

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