Refrigeration & Air-Conditioning Practice Set-6

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

Cost is too high

Required pressure drop cannot be achieved

Capacity control is not possible

It is made of copper

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

Degree of saturation

Dew point depression

Wet bulb depression

Dry bulb depression

3. 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 dehumidified

Heated and humidified

Cooled and dehumidified

4. 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 100% RH

25°C DBT and 40% RH

20°C DBT and 80% RH

5. 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

None of these

Less than

Equal to

6. The freezing point of R-12 is

-86.6°C

-135.8°C

-107.7°C

-95.2°C

7. 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

8. In Electrolux refrigerator

Ammonia evaporates in hydrogen

Hydrogen evaporates in ammonia

Ammonia is absorbed in water

Ammonia is absorbed in hydrogen

9. 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

7.44 kW

18.6 kW

1.86 kW

3.72 kW

10. A refrigeration system

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

Rejects energy to a low temperature body

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

None of the above

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11. 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

12. 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

13. Bell Coleman cycle is a

Reversed Otto cycle

Reversed Carnot cycle

Reversed Rankine cycle

Reversed Joule cycle

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

Three fluids

Single fluid

None of these

Two fluids

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

None of the above

Pressure in the evaporator

Degree of superheat at exit from the evaporator

Temperature of the evaporator

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

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

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

17. The relative humidity lines on a psychrometric chart are

Vertical and uniformly spaced

Horizontal and uniformly spaced

Curved lines

Horizontal and non-uniformly spaced

18. The bypass factor of a cooling coil decreases with

Decrease 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

Increase in fin spacing and increase in number of rows

19. A vapour absorption refrigeration system

Gives quiet operation

Requires little power consumption

Gives noisy operation

Cools below 0°C

20. 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

Operating the machine at higher speeds

Lowering the higher temperature

Raising the higher temperature

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21. 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

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

Back pressure

Discharge pressure

Critical pressure

Suction pressure

23. 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

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

21 kJ/min

620 kJ/min

420 kJ/min

210 kJ/min

25. In chemical dehumidification process,

Wet bulb temperature decreases

Dew point temperature decreases

Dry bulb temperature increases

All of these

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

Lower than

Higher than

None of these

Same as

27. The relative humidity is defined as

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

The mass of water vapour present in 1 kg 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 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

28. 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

Avogadro's hypothesis

Newton's law of gases

Kinetic theory of gases

Dalton's law of partial pressures

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

These react with oxygen and cause its depletion

These react with water vapour and cause acid rain

These react with plants and cause greenhouse effect

These react with ozone layer

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

It gives higher coefficient of performance

It has low operating pressures

All of the above

It is miscible with oil over large range of temperatures

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31. Pressure of water vapour is given by

μ/[1 - (1 - μ) (Ps/Pb)]

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

None of these

0.622 Pv/ (Pb - Pv)

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

Dry bulb temperature is higher than dew point temperature

Dry bulb temperature is higher than wet bulb temperature

Dry bulb, wet bulb and dew point temperature are same

Dew point temperature is lower than wet bulb temperature

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

Dirty condenser surface

Water temperature being high

Lack of cooling water

All of these

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

To the left of saturated liquid line

To the right of saturated liquid line

None of the above

Between the saturated liquid line and saturated vapour line

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

Very wet vapour

High pressure saturated liquid

Wet vapour

Dry vapour

36. Nusselt number (N_N) is given by

N_N = hl/k

N_N = μ c_p/k

N_N = ρ V l /μ

N_N = V²/t.c_p

37. The unit of thermal diffusivity is

m/h

m²/h

m²/hK

m/hK

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

Aqua-ammonia

Ammonia

Water

Freon

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

Automatic expansion valve

Thermostatic expansion valve

High side float valve

Low side float valve

40. 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

Dry air

Specific humidity

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41. A good refrigerant should have

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

Low C.O.P. and low freezing point

42. During dehumidification process, _________ remains constant.

Relative humidity

Wet bulb temperature

Dry bulb temperature

Specific humidity

43. 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 humidification

Dehumidification and pure sensible cooling

Cooling and dehumidification

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

Equal to

Greater than

Less than

None of these

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

Ammonia

R-11

R-22

R-12

46. 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)

(td₁ -td₃)/( td₂ -td₃)

(td₂ -td₃)/( td₁ -td₃)

(td₃ -td₂)/( td₁ -td₃)

(td₃ -td₁)/( td₂ -td₃)

47. An evaporator is also known as

Cooling coil

Freezing coil

All of these

Chilling coil

48. 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

1.5

0.67

0.3

49. The wet bulb temperature during sensible heating of air

Decreases

Increases

Remains constant

None of these

50. Air refrigeration cycle is used in

Air-conditioning

Domestic refrigerators

Gas liquefaction

Commercial refrigerators

Refrigeration & Air-Conditioning Practice Set-6

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