Refrigeration & Air-Conditioning Practice Set-6

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1. The relative humidity is defined as

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

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

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

620 kJ/min

21 kJ/min

210 kJ/min

420 kJ/min

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

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 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

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

Operating the machine at higher speeds

Raising the higher temperature

Lowering the higher temperature

Operating the machine at lower speeds

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

To the left of saturated liquid line

Between the saturated liquid line and saturated vapour line

None of the above

To the right of saturated liquid line

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

22°C DBT and 60% RH

25°C DBT and 40% RH

25°C DBT and 100% RH

20°C DBT and 80% RH

7. Pressure of water vapour is given by

μ/[1 - (1 - μ) (Ps/Pb)]

None of these

0.622 Pv/ (Pb - Pv)

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

8. 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 - e₁e₂

e₁ + e₂

1/e₁ + 1/e₂

9. 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

0.6

0.67

1.5

0.3

10. A vapour absorption refrigeration system

Gives noisy operation

Requires little power consumption

Cools below 0°C

Gives quiet operation

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11. An evaporator is also known as

All of these

Freezing coil

Chilling coil

Cooling coil

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

Ammonia

R-22

R-11

R-12

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

T₁/(T₂ - T₁)

(T₂ - T₁)/T₁

T₂/(T₂ - T₁)

(T₁ - T₂)/T₁

14. 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

Kinetic theory of gases

Dalton's law of partial pressures

Newton's law of gases

Avogadro's hypothesis

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

Equal to

Greater than

None of these

Less than

16. Bell Coleman cycle is a

Reversed Rankine cycle

Reversed Otto cycle

Reversed Joule cycle

Reversed Carnot cycle

17. A good refrigerant should have

High operating pressures and low freezing point

High latent heat of vaporisation and low freezing point

Low C.O.P. and low freezing point

High specific volume and high latent heat of vaporisation

18. 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

Cooling and dehumidification

Dehumidification and pure sensible cooling

Cooling and humidification

19. The unit of thermal diffusivity is

m²/h

m²/hK

m/hK

m/h

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

It gives higher coefficient of performance

It is miscible with oil over large range of temperatures

It has low operating pressures

All of the above

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21. In Electrolux refrigerator

Hydrogen evaporates in ammonia

Ammonia is absorbed in hydrogen

Ammonia evaporates in hydrogen

Ammonia is absorbed in water

22. 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 humidified

Cooled and dehumidified

Heated and dehumidified

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

Same as

Higher than

None of these

Lower than

24. The relative humidity lines on a psychrometric chart are

Curved lines

Vertical and uniformly spaced

Horizontal and non-uniformly spaced

Horizontal and uniformly spaced

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

Wet bulb depression

Dry bulb depression

Dew point depression

Degree of saturation

26. The wet bulb temperature during sensible heating of air

Increases

None of these

Remains constant

Decreases

27. 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

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

Less than

Equal to

Greater than

None of these

29. During dehumidification process, _________ remains constant.

Specific humidity

Dry bulb temperature

Relative humidity

Wet bulb temperature

30. 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

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31. A refrigeration system

Rejects energy to a low temperature body

None of the above

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

32. Air refrigeration cycle is used in

Gas liquefaction

Air-conditioning

Commercial refrigerators

Domestic refrigerators

33. The freezing point of R-12 is

-135.8°C

-86.6°C

-95.2°C

-107.7°C

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

Ammonia

Freon

Water

Aqua-ammonia

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

None of these

20 to 50°C

50 to 70°C

70 to 110°C

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

Back pressure

Critical pressure

Discharge pressure

Suction pressure

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

None of the above

Degree of superheat at exit from the evaporator

Pressure in the evaporator

Temperature of the evaporator

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

Water temperature being high

All of these

Lack of cooling water

Dirty condenser surface

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

Dry bulb, wet bulb and dew point temperature are same

Dew point temperature is lower than wet bulb temperature

Dry bulb temperature is higher than wet bulb temperature

Dry bulb temperature is higher than dew point temperature

40. 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

1.86 kW

18.6 kW

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41. 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

Moist air

Specific humidity

Saturated air

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

Two fluids

Three fluids

None of these

Single fluid

43. The bypass factor of a cooling coil decreases with

Increase in fin spacing and increase in number of rows

Decrease in fin spacing and increase in number of rows

Decrease in fin spacing and decrease in number of rows

Increase in fin spacing and decrease in number of rows

44. In chemical dehumidification process,

All of these

Wet bulb temperature decreases

Dew point temperature decreases

Dry bulb temperature increases

45. 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

Absolute humidity

Degree of saturation

Humidity ratio

Relative humidity

46. Nusselt number (N_N) is given by

N_N = V²/t.c_p

N_N = hl/k

N_N = ρ V l /μ

N_N = μ c_p/k

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

Automatic expansion valve

High side float valve

Thermostatic expansion valve

Low side float valve

48. 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 oxygen and cause its depletion

These react with water vapour and cause acid rain

49. 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)

50. 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

Refrigeration & Air-Conditioning Practice Set-6

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