Refrigeration & Air-Conditioning Practice Set-6

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1. 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

Cooled and dehumidified

Cooled and humidified

Heated and humidified

2. The bypass factor of a cooling coil decreases with

Increase in fin spacing and increase in number of rows

Decrease in fin spacing and decrease in number of rows

Increase in fin spacing and decrease in number of rows

Decrease in fin spacing and increase in number of rows

3. 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 minus the 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

4. 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

3.72 kW

1.86 kW

7.44 kW

5. An evaporator is also known as

Cooling coil

All of these

Freezing coil

Chilling coil

6. In Electrolux refrigerator

Ammonia is absorbed in water

Ammonia evaporates in hydrogen

Ammonia is absorbed in hydrogen

Hydrogen evaporates in ammonia

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

Temperature of the evaporator

Pressure in the evaporator

None of the above

Degree of superheat at exit from the evaporator

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

Lower than

Same as

None of these

Higher than

9. Bell Coleman cycle is a

Reversed Otto cycle

Reversed Rankine cycle

Reversed Joule cycle

Reversed Carnot cycle

10. The C.O.P. of a heat pump working on a reversed Carnot cycle is

(T₂ - T₁)/T₁

T₁/(T₂ - T₁)

T₂/(T₂ - T₁)

(T₁ - T₂)/T₁

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11. 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 dehumidification

Dehumidification

Cooling and humidification

12. During dehumidification process, _________ remains constant.

Dry bulb temperature

Wet bulb temperature

Relative humidity

Specific humidity

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

20°C DBT and 80% RH

25°C DBT and 40% RH

22°C DBT and 60% RH

25°C DBT and 100% RH

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

Discharge pressure

Suction pressure

Back pressure

Critical pressure

15. A good refrigerant should have

Low C.O.P. and low freezing point

High operating pressures and low freezing point

High specific volume and high latent heat of vaporisation

High latent heat of vaporisation and low freezing point

16. 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

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

Aqua-ammonia

Ammonia

Freon

Water

18. The unit of thermal diffusivity is

m²/hK

m/hK

m/h

m²/h

19. Nusselt number (N_N) is given by

N_N = μ c_p/k

N_N = V²/t.c_p

N_N = hl/k

N_N = ρ V l /μ

20. For evaporators and condensers, for the given conditions, the logarithmic mean temperature difference (t_m) for parallel flow is ________ that for counter flow.

None of these

Less than

Equal to

Greater than

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21. 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

Dew point temperature is lower than wet bulb temperature

Dry bulb temperature is higher than wet bulb temperature

22. A vapour absorption refrigeration system

Cools below 0°C

Gives noisy operation

Requires little power consumption

Gives quiet operation

23. Pressure of water vapour is given by

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

None of these

0.622 Pv/ (Pb - Pv)

μ/[1 - (1 - μ) (Ps/Pb)]

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

Lowering the higher temperature

Raising the higher temperature

Operating the machine at higher speeds

Operating the machine at lower speeds

25. 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

None of the above

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

26. 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 right of saturated liquid line

To the left of saturated liquid line

None of the above

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

Three fluids

Single fluid

Two fluids

None of these

28. 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

29. 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)

(td₃ -td₂)/( td₁ -td₃)

(td₁ -td₃)/( td₂ -td₃)

(td₂ -td₃)/( td₁ -td₃)

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

50 to 70°C

20 to 50°C

None of these

70 to 110°C

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31. In chemical dehumidification process,

Wet bulb temperature decreases

Dew point temperature decreases

Dry bulb temperature increases

All of these

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

Less than

None of these

Greater than

Equal to

33. 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

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

R-11

Ammonia

R-12

R-22

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

Dry vapour

Wet vapour

High pressure saturated liquid

Very wet vapour

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

Wet bulb depression

Degree of saturation

Dry bulb depression

Dew point depression

37. 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 m3 of dry air

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

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

Kinetic theory of gases

Avogadro's hypothesis

Dalton's law of partial pressures

Newton's law of gases

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

Capacity control is not possible

It is made of copper

Cost is too high

Required pressure drop cannot be achieved

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

It has low operating pressures

All of the above

It is miscible with oil over large range of temperatures

It gives higher coefficient of performance

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41. 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)

125 kJ/kg

100 kJ/kg

25 kJ/kg

50 kJ/kg

42. The wet bulb temperature during sensible heating of air

Remains constant

Increases

None of these

Decreases

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

21 kJ/min

420 kJ/min

620 kJ/min

210 kJ/min

44. 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

0.6

0.67

1.5

45. The relative humidity lines on a psychrometric chart are

Horizontal and uniformly spaced

Curved lines

Horizontal and non-uniformly spaced

Vertical and uniformly spaced

46. Air refrigeration cycle is used in

Commercial refrigerators

Gas liquefaction

Domestic refrigerators

Air-conditioning

47. The freezing point of R-12 is

-95.2°C

-135.8°C

-107.7°C

-86.6°C

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

These react with water vapour and cause acid rain

These react with plants and cause greenhouse effect

These react with oxygen and cause its depletion

These react with ozone layer

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

e₁e₂

e₁ + e₂

1/e₁ + 1/e₂

(e₁ + e₂)/ e₁ + e₂ - e₁e₂

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

Automatic expansion valve

Thermostatic expansion valve

Low side float valve

High side float valve

Refrigeration & Air-Conditioning Practice Set-6

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