Refrigeration & Air-Conditioning Practice Set-6

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

Dirty condenser surface

All of these

Lack of cooling water

Water temperature being high

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

Dry bulb, wet bulb and dew point temperature are same

Dry bulb temperature is higher than wet bulb temperature

Dry bulb temperature is higher than dew point temperature

Dew point temperature is lower than wet bulb temperature

3. 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 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 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

4. A good refrigerant should have

High latent heat of vaporisation and low freezing point

Low C.O.P. and low freezing point

High operating pressures and low freezing point

High specific volume and high latent heat of vaporisation

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

Automatic expansion valve

Low side float valve

Thermostatic expansion valve

High side float valve

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

Degree of superheat at exit from the evaporator

Temperature of the evaporator

None of the above

Pressure in the evaporator

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

R-12

R-11

Ammonia

R-22

8. A vapour absorption refrigeration system

Gives noisy operation

Requires little power consumption

Cools below 0°C

Gives quiet operation

9. 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

(e₁ + e₂)/ e₁ + e₂ - e₁e₂

e₁e₂

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

Specific humidity

Saturated air

Moist air

Dry air

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11. 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

1.86 kW

7.44 kW

18.6 kW

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)

(td₁ -td₃)/( td₂ -td₃)

(td₃ -td₁)/( td₂ -td₃)

(td₃ -td₂)/( td₁ -td₃)

(td₂ -td₃)/( td₁ -td₃)

13. The unit of thermal diffusivity is

m²/h

m/h

m²/hK

m/hK

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

Freon

Aqua-ammonia

Ammonia

Water

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

Back pressure

Critical pressure

Discharge pressure

Suction pressure

16. In chemical dehumidification process,

Dew point temperature decreases

Wet bulb temperature decreases

All of these

Dry bulb temperature increases

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

620 kJ/min

21 kJ/min

210 kJ/min

420 kJ/min

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

Three fluids

Single fluid

Two fluids

None of these

19. An evaporator is also known as

All of these

Cooling coil

Freezing coil

Chilling coil

20. During dehumidification process, _________ remains constant.

Dry bulb temperature

Relative humidity

Specific humidity

Wet bulb temperature

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21. The relative humidity lines on a psychrometric chart are

Horizontal and uniformly spaced

Curved lines

Horizontal and non-uniformly spaced

Vertical and uniformly spaced

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

22°C DBT and 60% RH

20°C DBT and 80% RH

25°C DBT and 40% RH

25°C DBT and 100% RH

23. The wet bulb temperature during sensible heating of air

Remains constant

Increases

Decreases

None of these

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

It is miscible with oil over large range of temperatures

It has low operating pressures

It gives higher coefficient of performance

All of the above

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

Very wet vapour

Wet vapour

Dry vapour

High pressure saturated liquid

26. Nusselt number (N_N) is given by

N_N = V²/t.c_p

N_N = μ c_p/k

N_N = ρ V l /μ

N_N = hl/k

27. 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

125 kJ/kg

50 kJ/kg

25 kJ/kg

28. The freezing point of R-12 is

-107.7°C

-86.6°C

-95.2°C

-135.8°C

29. 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

Humidity ratio

Absolute humidity

Degree of saturation

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 increase in number of rows

Decrease in fin spacing and decrease in number of rows

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

Required pressure drop cannot be achieved

Capacity control is not possible

It is made of copper

Cost is too high

32. 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

33. 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

34. 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 dehumidified

Heated and humidified

Cooled and dehumidified

Cooled and humidified

35. A refrigeration system

None of the above

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

Rejects energy to a low temperature body

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

36. Bell Coleman cycle is a

Reversed Otto cycle

Reversed Carnot cycle

Reversed Rankine cycle

Reversed Joule cycle

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

None of these

20 to 50°C

70 to 110°C

50 to 70°C

38. 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

Dalton's law of partial pressures

Newton's law of gases

Kinetic theory of gases

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

Dehumidification

Dehumidification and pure sensible cooling

Cooling and dehumidification

Cooling and humidification

40. 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

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

Lower than

Higher than

Same as

None of these

42. Air refrigeration cycle is used in

Gas liquefaction

Commercial refrigerators

Air-conditioning

Domestic refrigerators

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

Dew point depression

Dry bulb depression

Degree of saturation

Wet bulb depression

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

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

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

45. 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

Equal to

None of these

46. 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

47. Pressure of water vapour is given by

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

0.622 Pv/ (Pb - Pv)

μ/[1 - (1 - μ) (Ps/Pb)]

None of these

48. In Electrolux refrigerator

Ammonia is absorbed in hydrogen

Hydrogen evaporates in ammonia

Ammonia is absorbed in water

Ammonia evaporates in hydrogen

49. 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

0.6

0.67

0.3

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

Equal to

Less than

Greater than

None of these

Refrigeration & Air-Conditioning Practice Set-6

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