Refrigeration & Air-Conditioning Practice Set-6

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

Dry bulb temperature is higher than dew point temperature

Dry bulb, wet bulb and dew point temperature are same

Dry bulb temperature is higher than wet bulb temperature

Dew point temperature is lower than wet bulb temperature

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

Between the saturated liquid line and saturated vapour line

To the left of saturated liquid line

None of the above

To the right of saturated liquid line

3. 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 -td₁)/( td₂ -td₃)

(td₂ -td₃)/( td₁ -td₃)

(td₃ -td₂)/( td₁ -td₃)

(td₁ -td₃)/( td₂ -td₃)

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

Ammonia

Water

Freon

Aqua-ammonia

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

High side float valve

Low side float valve

Automatic expansion valve

Thermostatic expansion valve

6. 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

7. 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

8. In chemical dehumidification process,

All of these

Dew point temperature decreases

Wet bulb temperature decreases

Dry bulb temperature increases

9. The relative humidity lines on a psychrometric chart are

Curved lines

Horizontal and non-uniformly spaced

Horizontal and uniformly spaced

Vertical and uniformly spaced

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

Lowering the higher temperature

Operating the machine at higher speeds

Operating the machine at lower speeds

Raising the higher temperature

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11. Nusselt number (N_N) is given by

N_N = hl/k

N_N = μ c_p/k

N_N = V²/t.c_p

N_N = ρ V l /μ

12. 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

Equal to

None of these

Greater than

13. 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

Avogadro's hypothesis

Newton's law of gases

14. The freezing point of R-12 is

-107.7°C

-95.2°C

-86.6°C

-135.8°C

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

Cooled and humidified

Heated and humidified

Cooled and dehumidified

Heated and dehumidified

16. Bell Coleman cycle is a

Reversed Rankine cycle

Reversed Otto cycle

Reversed Carnot cycle

Reversed Joule cycle

17. A good refrigerant should have

High latent heat of vaporisation and low freezing point

High operating pressures and low freezing point

High specific volume and high latent heat of vaporisation

Low C.O.P. and low freezing point

18. 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

Moist air

Saturated air

Specific humidity

19. An evaporator is also known as

Freezing coil

Cooling coil

Chilling coil

All of these

20. Pressure of water vapour is given by

0.622 Pv/ (Pb - Pv)

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

None of these

μ/[1 - (1 - μ) (Ps/Pb)]

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21. 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

100 kJ/kg

125 kJ/kg

25 kJ/kg

22. 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

23. The relative humidity is defined as

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 mass of water vapour present in 1 m3 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

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

Less than

Greater than

None of these

Equal to

25. The wet bulb temperature during sensible heating of air

Decreases

Increases

None of these

Remains constant

26. A vapour absorption refrigeration system

Cools below 0°C

Gives quiet operation

Requires little power consumption

Gives noisy operation

27. In Electrolux refrigerator

Ammonia is absorbed in hydrogen

Ammonia evaporates in hydrogen

Ammonia is absorbed in water

Hydrogen evaporates in ammonia

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

It is miscible with oil over large range of temperatures

It gives higher coefficient of performance

It has low operating pressures

All of the above

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

R-11

R-12

Ammonia

R-22

30. 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

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31. During dehumidification process, _________ remains constant.

Specific humidity

Dry bulb temperature

Relative humidity

Wet bulb temperature

32. The bypass factor of a cooling coil decreases with

Increase 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

Decrease in fin spacing and increase in number of rows

33. 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

Dehumidification

Dehumidification and pure sensible cooling

Cooling and humidification

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

Degree of saturation

Dew point depression

Dry bulb depression

Wet bulb depression

35. A refrigeration system

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

None of the above

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

None of the above

Temperature of the evaporator

Pressure in the evaporator

Degree of superheat at exit from the evaporator

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

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

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

Back pressure

Discharge pressure

Critical pressure

Suction pressure

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

All of these

Lack of cooling water

Water temperature being high

Dirty condenser surface

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

Three fluids

Two fluids

Single fluid

None of these

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41. 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

Absolute humidity

Degree of saturation

Humidity ratio

Relative humidity

42. Air refrigeration cycle is used in

Commercial refrigerators

Air-conditioning

Domestic refrigerators

Gas liquefaction

43. 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

18.6 kW

7.44 kW

1.86 kW

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

70 to 110°C

50 to 70°C

20 to 50°C

None of these

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

Wet vapour

High pressure saturated liquid

Very wet vapour

Dry vapour

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

Higher than

Same as

Lower than

None of these

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

21 kJ/min

210 kJ/min

620 kJ/min

420 kJ/min

48. 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

49. The unit of thermal diffusivity is

m/h

m²/h

m²/hK

m/hK

50. 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)

Refrigeration & Air-Conditioning Practice Set-6

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