Refrigeration & Air-Conditioning Practice Set-6

July 15, 2021

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1. 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

2. 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 mass of water vapour present in 1 kg 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

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

High side float valve

Thermostatic expansion valve

Automatic expansion valve

Low side float valve

4. An evaporator is also known as

Freezing coil

Chilling coil

All of these

Cooling coil

5. 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)

(td₂ -td₃)/( td₁ -td₃)

(td₁ -td₃)/( td₂ -td₃)

(td₃ -td₁)/( td₂ -td₃)

(td₃ -td₂)/( td₁ -td₃)

6. Bell Coleman cycle is a

Reversed Rankine cycle

Reversed Otto cycle

Reversed Joule cycle

Reversed Carnot cycle

7. The freezing point of R-12 is

-107.7°C

-135.8°C

-86.6°C

-95.2°C

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

T₁/(T₂ - T₁)

(T₁ - T₂)/T₁

T₂/(T₂ - T₁)

(T₂ - T₁)/T₁

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

210 kJ/min

21 kJ/min

620 kJ/min

420 kJ/min

10. A vapour absorption refrigeration system

Gives quiet operation

Requires little power consumption

Gives noisy operation

Cools below 0°C

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

Temperature of the evaporator

None of the above

Degree of superheat at exit from the evaporator

Pressure in the evaporator

12. A good refrigerant should have

Low C.O.P. and low freezing point

High latent heat of vaporisation and low freezing point

High specific volume and high latent heat of vaporisation

High operating pressures and low freezing point

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

Less than

Greater than

None of these

Equal to

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

Greater than

Equal to

Less than

None of these

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

Dirty condenser surface

All of these

Water temperature being high

Lack of cooling water

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

Same as

Lower than

Higher than

None of these

17. In Electrolux refrigerator

Ammonia is absorbed in water

Ammonia evaporates in hydrogen

Ammonia is absorbed in hydrogen

Hydrogen evaporates in ammonia

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

It has low operating pressures

All of the above

It gives higher coefficient of performance

It is miscible with oil over large range of temperatures

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

70 to 110°C

None of these

50 to 70°C

20 to 50°C

20. A refrigeration system

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

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

Rejects energy to a low temperature body

None of the above

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21. Air refrigeration cycle is used in

Commercial refrigerators

Gas liquefaction

Domestic refrigerators

Air-conditioning

22. 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 temperature is higher than wet bulb temperature

Dry bulb, wet bulb and dew point temperature are same

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

R-22

Ammonia

R-11

R-12

24. 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

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

High pressure saturated liquid

Wet vapour

Dry vapour

Very wet vapour

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

These react with plants and cause greenhouse effect

These react with ozone layer

These react with water vapour and cause acid rain

These react with oxygen and cause its depletion

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

Freon

Aqua-ammonia

Water

Ammonia

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

Single fluid

Two fluids

None of these

Three fluids

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

1/e₁ + 1/e₂

e₁e₂

(e₁ + e₂)/ e₁ + e₂ - e₁e₂

e₁ + e₂

30. In chemical dehumidification process,

All of these

Dry bulb temperature increases

Wet bulb temperature decreases

Dew point temperature decreases

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31. 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

Newton's law of gases

Kinetic theory of gases

Avogadro's hypothesis

32. Pressure of water vapour is given by

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

μ/[1 - (1 - μ) (Ps/Pb)]

None of these

0.622 Pv/ (Pb - Pv)

33. 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

0.67

0.6

1.5

34. During dehumidification process, _________ remains constant.

Relative humidity

Specific humidity

Wet bulb temperature

Dry bulb temperature

35. 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)

100 kJ/kg

125 kJ/kg

50 kJ/kg

25 kJ/kg

36. 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

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

Degree of saturation

Dry bulb depression

Dew point depression

Wet bulb depression

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

Cooled and humidified

Heated and dehumidified

Heated and humidified

Cooled and dehumidified

39. The unit of thermal diffusivity is

m/hK

m/h

m²/h

m²/hK

40. The relative humidity lines on a psychrometric chart are

Horizontal and non-uniformly spaced

Horizontal and uniformly spaced

Vertical and uniformly spaced

Curved lines

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41. The wet bulb temperature during sensible heating of air

Decreases

None of these

Remains constant

Increases

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

25°C DBT and 40% RH

25°C DBT and 100% RH

20°C DBT and 80% RH

22°C DBT and 60% RH

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

Dry air

Specific humidity

Saturated air

Moist air

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

Capacity control is not possible

Cost is too high

Required pressure drop cannot be achieved

It is made of copper

45. 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

Dehumidification and pure sensible cooling

Dehumidification

Cooling and dehumidification

Cooling and humidification

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

Raising the higher temperature

Operating the machine at higher speeds

Operating the machine at lower speeds

Lowering the higher temperature

47. Nusselt number (N_N) is given by

N_N = ρ V l /μ

N_N = hl/k

N_N = V²/t.c_p

N_N = μ c_p/k

48. The bypass factor of a cooling coil decreases with

Increase in fin spacing and increase in number of rows

Decrease in fin spacing and decrease in number of rows

Decrease in fin spacing and increase in number of rows

Increase in fin spacing and decrease in number of rows

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

Back pressure

Suction pressure

Discharge pressure

Critical pressure

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

To the right of saturated liquid line

To the left of saturated liquid line

None of the above

Between the saturated liquid line and saturated vapour line

Refrigeration & Air-Conditioning Practice Set-6

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