Refrigeration & Air-Conditioning Practice Set-6

Published July 15, 2021 · Updated July 15, 2021

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1. The relative humidity is defined as

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 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

2. 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

Cooling and humidification

Dehumidification

Cooling and dehumidification

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

None of the above

To the right of saturated liquid line

Between the saturated liquid line and saturated vapour line

To the left of saturated liquid line

4. The wet bulb temperature during sensible heating of air

Decreases

Remains constant

None of these

Increases

5. 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 absorbed by the evaporator to the heat supplied by the gas burner

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

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

Specific humidity

Moist air

Saturated air

Dry air

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

None of the above

Temperature of the evaporator

Pressure in the evaporator

Degree of superheat at exit from the evaporator

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

Low side float valve

Thermostatic expansion valve

High side float valve

Automatic expansion valve

9. 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

Operating the machine at higher speeds

Raising the higher temperature

10. 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

7.44 kW

1.86 kW

18.6 kW

3.72 kW

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11. 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

1.5

0.67

0.6

0.3

12. In chemical dehumidification process,

Dry bulb temperature increases

Wet bulb temperature decreases

Dew point temperature decreases

All of these

13. 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

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

None of these

50 to 70°C

20 to 50°C

70 to 110°C

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

It is made of copper

Cost is too high

Capacity control is not possible

Required pressure drop cannot be achieved

16. 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)

25 kJ/kg

50 kJ/kg

100 kJ/kg

125 kJ/kg

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

None of these

Greater than

Equal to

Less than

18. The bypass factor of a cooling coil decreases with

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

Increase in fin spacing and increase in number of rows

19. The relative humidity lines on a psychrometric chart are

Vertical and uniformly spaced

Horizontal and uniformly spaced

Curved lines

Horizontal and non-uniformly spaced

20. 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

Humidity ratio

Degree of saturation

Absolute humidity

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

Discharge pressure

Critical pressure

Back pressure

Suction pressure

22. Air refrigeration cycle is used in

Gas liquefaction

Domestic refrigerators

Commercial refrigerators

Air-conditioning

23. 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

22°C DBT and 60% RH

20°C DBT and 80% RH

24. In Electrolux refrigerator

Ammonia is absorbed in hydrogen

Ammonia is absorbed in water

Ammonia evaporates in hydrogen

Hydrogen evaporates in ammonia

25. Pressure of water vapour is given by

μ/[1 - (1 - μ) (Ps/Pb)]

None of these

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

0.622 Pv/ (Pb - Pv)

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

21 kJ/min

420 kJ/min

210 kJ/min

620 kJ/min

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

Dew point temperature is lower than wet bulb temperature

Dry bulb temperature is higher than dew point temperature

Dry bulb, wet bulb and dew point temperature are same

Dry bulb temperature is higher than wet bulb temperature

28. Bell Coleman cycle is a

Reversed Rankine cycle

Reversed Otto cycle

Reversed Carnot cycle

Reversed Joule cycle

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

(T₂ - T₁)/T₁

T₂/(T₂ - T₁)

T₁/(T₂ - T₁)

(T₁ - T₂)/T₁

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

These react with oxygen and cause its depletion

These react with water vapour and cause acid rain

These react with ozone layer

These react with plants and cause greenhouse effect

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31. 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

1/e₁ + 1/e₂

e₁e₂

(e₁ + e₂)/ e₁ + e₂ - e₁e₂

32. The freezing point of R-12 is

-135.8°C

-86.6°C

-95.2°C

-107.7°C

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

All of these

Dirty condenser surface

Water temperature being high

Lack of cooling water

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

All of the above

It has low operating pressures

It is miscible with oil over large range of temperatures

It gives higher coefficient of performance

35. The unit of thermal diffusivity is

m/h

m/hK

m²/hK

m²/h

36. 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)

(td₁ -td₃)/( td₂ -td₃)

(td₂ -td₃)/( td₁ -td₃)

(td₃ -td₁)/( td₂ -td₃)

(td₃ -td₂)/( td₁ -td₃)

37. A refrigeration system

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

None of the above

Rejects energy to a low temperature body

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

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

Equal to

Greater than

Less than

None of these

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

Three fluids

None of these

Two fluids

Single fluid

40. 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

Avogadro's hypothesis

Newton's law of gases

Kinetic theory of gases

Dalton's law of partial pressures

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41. An evaporator is also known as

Freezing coil

Cooling coil

Chilling coil

All of these

42. A vapour absorption refrigeration system

Cools below 0°C

Gives noisy operation

Requires little power consumption

Gives quiet operation

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

Wet vapour

High pressure saturated liquid

Dry vapour

Very wet vapour

44. 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 humidified

Cooled and humidified

Heated and dehumidified

Cooled and dehumidified

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

Higher than

Same as

None of these

Lower than

46. During dehumidification process, _________ remains constant.

Dry bulb temperature

Wet bulb temperature

Specific humidity

Relative humidity

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

Freon

Aqua-ammonia

Water

Ammonia

48. A good refrigerant should have

High specific volume and high latent heat of vaporisation

Low C.O.P. and low freezing point

High operating pressures and low freezing point

High latent heat of vaporisation and low freezing point

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

Dew point depression

Dry bulb depression

Degree of saturation

Wet bulb depression

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

Ammonia

R-11

R-12

R-22

Refrigeration & Air-Conditioning Practice Set-6

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