Refrigeration & Air-Conditioning Practice Set-6

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1. 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

Newton's law of gases

Kinetic theory of gases

Avogadro's hypothesis

Dalton's law of partial pressures

2. In chemical dehumidification process,

Wet bulb temperature decreases

Dew point temperature decreases

All of these

Dry bulb temperature increases

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

Cost is too high

Required pressure drop cannot be achieved

It is made of copper

Capacity control is not possible

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

T₂/(T₂ - T₁)

(T₁ - T₂)/T₁

T₁/(T₂ - T₁)

(T₂ - T₁)/T₁

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)

(td₃ -td₁)/( td₂ -td₃)

(td₁ -td₃)/( td₂ -td₃)

(td₃ -td₂)/( td₁ -td₃)

(td₂ -td₃)/( td₁ -td₃)

6. Air refrigeration cycle is used in

Gas liquefaction

Domestic refrigerators

Commercial refrigerators

Air-conditioning

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

e₁e₂

e₁ + e₂

(e₁ + e₂)/ e₁ + e₂ - e₁e₂

1/e₁ + 1/e₂

8. Pressure of water vapour is given by

0.622 Pv/ (Pb - Pv)

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

μ/[1 - (1 - μ) (Ps/Pb)]

None of these

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

Automatic expansion valve

Thermostatic expansion valve

Low side float valve

High side float valve

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

These react with plants and cause greenhouse effect

These react with oxygen and cause its depletion

These react with water vapour and cause acid rain

These react with ozone layer

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11. During dehumidification process, _________ remains constant.

Wet bulb temperature

Relative humidity

Specific humidity

Dry bulb temperature

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

Water

Aqua-ammonia

Ammonia

Freon

13. An evaporator is also known as

Chilling coil

Cooling coil

Freezing coil

All of these

14. A refrigeration system

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

None of the above

Rejects energy to a low temperature body

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

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

All of the above

It gives higher coefficient of performance

It is miscible with oil over large range of temperatures

It has low operating pressures

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

Critical pressure

Suction pressure

Discharge pressure

Back pressure

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

Dry bulb temperature is higher than wet bulb temperature

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

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

Between the saturated liquid line and saturated vapour line

To the left of saturated liquid line

To the right of saturated liquid line

None of the above

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

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

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

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

20 to 50°C

70 to 110°C

50 to 70°C

None of these

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

Pressure in the evaporator

Degree of superheat at exit from the evaporator

None of the above

Temperature of the evaporator

22. Nusselt number (N_N) is given by

N_N = V²/t.c_p

N_N = μ c_p/k

N_N = ρ V l /μ

N_N = hl/k

23. A good refrigerant should have

Low C.O.P. and low freezing point

High specific volume and high latent heat of vaporisation

High operating pressures and low freezing point

High latent heat of vaporisation and low freezing point

24. The relative humidity lines on a psychrometric chart are

Vertical and uniformly spaced

Horizontal and uniformly spaced

Horizontal and non-uniformly spaced

Curved lines

25. The unit of thermal diffusivity is

m²/h

m/h

m²/hK

m/hK

26. The freezing point of R-12 is

-95.2°C

-86.6°C

-135.8°C

-107.7°C

27. 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

Cooled and dehumidified

Heated and dehumidified

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

Saturated air

Moist air

Specific humidity

Dry air

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

Dirty condenser surface

Lack of cooling water

Water temperature being high

All of these

30. Bell Coleman cycle is a

Reversed Carnot cycle

Reversed Joule cycle

Reversed Otto cycle

Reversed Rankine cycle

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

Higher than

None of these

Same as

Lower than

32. The wet bulb temperature during sensible heating of air

Increases

None of these

Decreases

Remains constant

33. 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

3.72 kW

7.44 kW

18.6 kW

1.86 kW

34. The relative humidity is defined as

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

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

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 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

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

Operating the machine at lower speeds

Lowering the higher temperature

Raising the higher temperature

Operating the machine at higher speeds

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

420 kJ/min

21 kJ/min

210 kJ/min

620 kJ/min

37. In Electrolux refrigerator

Ammonia evaporates in hydrogen

Ammonia is absorbed in water

Ammonia is absorbed in hydrogen

Hydrogen evaporates in ammonia

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

Very wet vapour

Wet vapour

High pressure saturated liquid

Dry vapour

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

25°C DBT and 100% RH

20°C DBT and 80% RH

25°C DBT and 40% RH

22°C DBT and 60% RH

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

Three fluids

Two fluids

None of these

Single fluid

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41. 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

42. 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

Equal to

Greater than

Less than

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

Degree of saturation

Dew point depression

Wet bulb depression

Dry bulb depression

44. 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

Degree of saturation

Relative humidity

Humidity ratio

Absolute humidity

45. 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

0.3

0.6

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

Equal to

None of these

Greater than

Less than

47. The bypass factor of a cooling coil decreases with

Decrease in fin spacing and increase in number of rows

Increase in fin spacing and increase in number of rows

Increase in fin spacing and decrease in number of rows

Decrease in fin spacing and decrease in number of rows

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

R-22

R-12

R-11

Ammonia

49. A vapour absorption refrigeration system

Requires little power consumption

Cools below 0°C

Gives noisy operation

Gives quiet operation

50. 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 dehumidification

Cooling and humidification

Dehumidification

Refrigeration & Air-Conditioning Practice Set-6

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