Refrigeration & Air-Conditioning Practice Set-6

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1. In Electrolux refrigerator

Ammonia is absorbed in hydrogen

Ammonia is absorbed in water

Ammonia evaporates in hydrogen

Hydrogen evaporates in ammonia

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 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 m3 of dry air

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

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

Lack of cooling water

All of these

Dirty condenser surface

Water temperature being high

4. A good refrigerant should have

High specific volume and high latent heat of vaporisation

High latent heat of vaporisation and low freezing point

Low C.O.P. and low freezing point

High operating pressures and low freezing point

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

Ammonia

R-11

R-12

R-22

6. Pressure of water vapour is given by

μ/[1 - (1 - μ) (Ps/Pb)]

0.622 Pv/ (Pb - Pv)

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

None of these

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

Critical pressure

Suction pressure

Back pressure

Discharge pressure

8. The relative humidity lines on a psychrometric chart are

Vertical and uniformly spaced

Curved lines

Horizontal and non-uniformly spaced

Horizontal and uniformly spaced

9. The wet bulb temperature during sensible heating of air

Increases

Decreases

None of these

Remains constant

10. 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 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 to the heat absorbed by the evaporator

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

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

20 to 50°C

50 to 70°C

None of these

70 to 110°C

12. 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)

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

Two fluids

Three fluids

Single fluid

None of these

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

All of the above

It gives higher coefficient of performance

It has low operating pressures

It is miscible with oil over large range of temperatures

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

Very wet vapour

Dry vapour

Wet vapour

High pressure saturated liquid

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

Dry bulb, wet bulb and dew point temperature are same

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

17. The freezing point of R-12 is

-95.2°C

-107.7°C

-135.8°C

-86.6°C

18. 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

Operating the machine at lower speeds

Raising the higher temperature

Lowering the higher temperature

19. Nusselt number (N_N) is given by

N_N = hl/k

N_N = ρ V l /μ

N_N = V²/t.c_p

N_N = μ c_p/k

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

Aqua-ammonia

Freon

Ammonia

Water

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

Dry bulb depression

Dew point depression

Degree of saturation

Wet bulb depression

22. An evaporator is also known as

Chilling coil

All of these

Cooling coil

Freezing coil

23. 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

24. In chemical dehumidification process,

Wet bulb temperature decreases

Dew point temperature decreases

Dry bulb temperature increases

All of these

25. 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

26. 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

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

Less than

Greater than

Equal to

None of these

28. A vapour absorption refrigeration system

Cools below 0°C

Gives noisy operation

Requires little power consumption

Gives quiet operation

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

These react with oxygen and cause its depletion

These react with plants and cause greenhouse effect

These react with ozone layer

These react with water vapour and cause acid rain

30. 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

Low side float valve

Automatic expansion valve

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

T₁/(T₂ - T₁)

T₂/(T₂ - T₁)

(T₁ - T₂)/T₁

(T₂ - T₁)/T₁

32. 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

1.86 kW

3.72 kW

18.6 kW

7.44 kW

33. Bell Coleman cycle is a

Reversed Carnot cycle

Reversed Rankine cycle

Reversed Otto cycle

Reversed Joule cycle

34. 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

35. 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

Absolute humidity

Humidity ratio

Degree of saturation

36. 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

Greater than

Equal to

None of these

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

Required pressure drop cannot be achieved

Capacity control is not possible

Cost is too high

It is made of copper

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

Lower than

Same as

None of these

Higher than

39. The unit of thermal diffusivity is

m²/hK

m/hK

m/h

m²/h

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

21 kJ/min

210 kJ/min

420 kJ/min

620 kJ/min

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41. 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

42. 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

Dehumidification and pure sensible cooling

Cooling and dehumidification

Cooling and humidification

43. During dehumidification process, _________ remains constant.

Relative humidity

Dry bulb temperature

Wet bulb temperature

Specific humidity

44. 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

25 kJ/kg

125 kJ/kg

50 kJ/kg

45. 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 dehumidified

Cooled and humidified

Heated and dehumidified

Heated and humidified

46. 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

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

To the left of saturated liquid line

None of the above

Between the saturated liquid line and saturated vapour line

To the right of saturated liquid line

48. Air refrigeration cycle is used in

Domestic refrigerators

Air-conditioning

Commercial refrigerators

Gas liquefaction

49. 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

50. The bypass factor of a cooling coil decreases with

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

Increase in fin spacing and increase in number of rows

Refrigeration & Air-Conditioning Practice Set-6

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