Refrigeration & Air-Conditioning Practice Set-6

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1. A vapour absorption refrigeration system

Cools below 0°C

Requires little power consumption

Gives noisy operation

Gives quiet operation

2. 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)

(td₃ -td₂)/( td₁ -td₃)

(td₁ -td₃)/( td₂ -td₃)

(td₃ -td₁)/( td₂ -td₃)

3. 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

4. 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

Dalton's law of partial pressures

Kinetic theory of gases

Avogadro's hypothesis

Newton's law of gases

5. An evaporator is also known as

All of these

Chilling coil

Cooling coil

Freezing coil

6. 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

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

Critical pressure

Discharge pressure

Back pressure

Suction pressure

8. The unit of thermal diffusivity is

m²/h

m/hK

m/h

m²/hK

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

Cost is too high

It is made of copper

Required pressure drop cannot be achieved

Capacity control is not possible

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

1/e₁ + 1/e₂

e₁ + e₂

(e₁ + e₂)/ e₁ + e₂ - e₁e₂

e₁e₂

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11. 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

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

Thermostatic expansion valve

High side float valve

Low side float valve

Automatic expansion valve

13. 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

14. 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 ozone layer

These react with water vapour and cause acid rain

15. 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

16. 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

Dry air

Moist air

Specific humidity

17. 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

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

Lack of cooling water

Dirty condenser surface

Water temperature being high

All of these

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

It has low operating pressures

It gives higher coefficient of performance

It is miscible with oil over large range of temperatures

All of the above

20. The freezing point of R-12 is

-135.8°C

-86.6°C

-107.7°C

-95.2°C

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21. The wet bulb temperature during sensible heating of air

Decreases

Increases

None of these

Remains constant

22. In chemical dehumidification process,

Dry bulb temperature increases

Dew point temperature decreases

All of these

Wet bulb temperature decreases

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

None of the above

To the left of saturated liquid line

Between the saturated liquid line and saturated vapour line

To the right of saturated liquid line

24. Nusselt number (N_N) is given by

N_N = V²/t.c_p

N_N = ρ V l /μ

N_N = μ c_p/k

N_N = hl/k

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

Ammonia

Aqua-ammonia

Water

Freon

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

Wet bulb depression

Dew point depression

Dry bulb depression

Degree of saturation

27. The relative humidity lines on a psychrometric chart are

Vertical and uniformly spaced

Horizontal and uniformly spaced

Horizontal and non-uniformly spaced

Curved lines

28. 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 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

29. 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

0.3

1.5

0.67

0.6

30. The bypass factor of a cooling coil decreases with

Increase in fin spacing and decrease in number of rows

Increase in fin spacing and increase in number of rows

Decrease in fin spacing and decrease in number of rows

Decrease in fin spacing and increase in number of rows

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31. 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

20°C DBT and 80% RH

22°C DBT and 60% RH

32. Bell Coleman cycle is a

Reversed Rankine cycle

Reversed Otto cycle

Reversed Joule cycle

Reversed Carnot cycle

33. 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 kg of dry air

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

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

Dew point temperature is lower than wet bulb temperature

Dry bulb, wet bulb and dew point temperature are same

Dry bulb temperature is higher than dew point temperature

Dry bulb temperature is higher than wet bulb temperature

35. Air refrigeration cycle is used in

Gas liquefaction

Commercial refrigerators

Domestic refrigerators

Air-conditioning

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

620 kJ/min

21 kJ/min

420 kJ/min

210 kJ/min

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

Ammonia

R-12

R-11

R-22

38. A refrigeration system

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

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

39. 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

Cooling and dehumidification

Cooling and humidification

Dehumidification

Dehumidification and pure sensible cooling

40. Pressure of water vapour is given by

None of these

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

0.622 Pv/ (Pb - Pv)

μ/[1 - (1 - μ) (Ps/Pb)]

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

Single fluid

Two fluids

Three fluids

None of these

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

Lower than

Same as

Higher than

None of these

43. 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

125 kJ/kg

25 kJ/kg

100 kJ/kg

44. 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

None of these

Equal to

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

Equal to

None of these

Less than

Greater than

46. 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

18.6 kW

7.44 kW

3.72 kW

1.86 kW

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

70 to 110°C

None of these

20 to 50°C

50 to 70°C

48. In Electrolux refrigerator

Hydrogen evaporates in ammonia

Ammonia is absorbed in hydrogen

Ammonia evaporates in hydrogen

Ammonia is absorbed in water

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

None of the above

Pressure in the evaporator

Temperature of the evaporator

Degree of superheat at exit from the evaporator

50. During dehumidification process, _________ remains constant.

Wet bulb temperature

Relative humidity

Dry bulb temperature

Specific humidity

Refrigeration & Air-Conditioning Practice Set-6

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