Refrigeration & Air-Conditioning Practice Set-6

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1. Air refrigeration cycle is used in

Commercial refrigerators

Domestic refrigerators

Air-conditioning

Gas liquefaction

2. 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

125 kJ/kg

100 kJ/kg

50 kJ/kg

3. A vapour absorption refrigeration system

Cools below 0°C

Requires little power consumption

Gives quiet operation

Gives noisy operation

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

None of the above

Between the saturated liquid line and saturated vapour line

To the right of saturated liquid line

To the left of saturated liquid line

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

Moist air

Dry air

Specific humidity

Saturated air

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

Less than

Greater than

None of these

Equal to

7. 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)

(td₁ -td₃)/( td₂ -td₃)

(td₂ -td₃)/( td₁ -td₃)

(td₃ -td₁)/( td₂ -td₃)

8. An evaporator is also known as

All of these

Cooling coil

Chilling coil

Freezing coil

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

Wet bulb depression

Degree of saturation

Dew point depression

Dry bulb depression

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

R-22

Ammonia

R-12

R-11

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

Freon

Aqua-ammonia

Water

Ammonia

12. 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

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

Raising the higher temperature

Lowering the higher temperature

Operating the machine at lower speeds

Operating the machine at higher speeds

14. A refrigeration system

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

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

Rejects energy to a low temperature body

None of the above

15. 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

16. A good refrigerant should have

High operating pressures and low freezing point

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

17. 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

18. The relative humidity lines on a psychrometric chart are

Horizontal and uniformly spaced

Vertical and uniformly spaced

Curved lines

Horizontal and non-uniformly spaced

19. 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

Less than

None of these

Equal to

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

Dry bulb temperature is higher than wet bulb temperature

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

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21. 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)

22. In Electrolux refrigerator

Ammonia is absorbed in water

Ammonia is absorbed in hydrogen

Hydrogen evaporates in ammonia

Ammonia evaporates in hydrogen

23. The unit of thermal diffusivity is

m²/h

m/h

m²/hK

m/hK

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

High pressure saturated liquid

Dry vapour

Wet vapour

Very wet vapour

25. 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 kg of dry air

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

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

Low side float valve

High side float valve

Thermostatic expansion valve

Automatic expansion valve

27. The bypass factor of a cooling coil decreases with

Increase in fin spacing and decrease in number of rows

Decrease in fin spacing and decrease in number of rows

Increase in fin spacing and increase in number of rows

Decrease in fin spacing and increase in number of rows

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

20 to 50°C

None of these

50 to 70°C

70 to 110°C

29. Pressure of water vapour is given by

0.622 Pv/ (Pb - Pv)

μ/[1 - (1 - μ) (Ps/Pb)]

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

None of these

30. The wet bulb temperature during sensible heating of air

None of these

Increases

Remains constant

Decreases

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31. Nusselt number (N_N) is given by

N_N = ρ V l /μ

N_N = μ c_p/k

N_N = hl/k

N_N = V²/t.c_p

32. 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

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

Lower than

Higher than

None of these

Same as

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

620 kJ/min

420 kJ/min

210 kJ/min

21 kJ/min

35. During dehumidification process, _________ remains constant.

Specific humidity

Wet bulb temperature

Relative humidity

Dry bulb temperature

36. 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

Newton's law of gases

Dalton's law of partial pressures

Avogadro's hypothesis

37. 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

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

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

Back pressure

Suction pressure

Critical pressure

Discharge pressure

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

Degree of superheat at exit from the evaporator

None of the above

Temperature of the evaporator

Pressure in the evaporator

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41. 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

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

Cost is too high

Capacity control is not possible

Required pressure drop cannot be achieved

It is made of copper

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

These react with water vapour and cause acid rain

These react with ozone layer

These react with plants and cause greenhouse effect

These react with oxygen and cause its depletion

44. 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

45. Bell Coleman cycle is a

Reversed Otto cycle

Reversed Joule cycle

Reversed Rankine cycle

Reversed Carnot cycle

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

Single fluid

Two fluids

None of these

Three fluids

47. In chemical dehumidification process,

Dew point temperature decreases

All of these

Wet bulb temperature decreases

Dry bulb temperature increases

48. 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

49. 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

Humidity ratio

Relative humidity

Absolute humidity

Degree of saturation

50. The freezing point of R-12 is

-86.6°C

-107.7°C

-135.8°C

-95.2°C

Refrigeration & Air-Conditioning Practice Set-6

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