Refrigeration & Air-Conditioning Practice Set-6

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

(T₁ - T₂)/T₁

(T₂ - T₁)/T₁

T₁/(T₂ - T₁)

T₂/(T₂ - T₁)

2. 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

Cooled and dehumidified

Heated and dehumidified

3. The freezing point of R-12 is

-95.2°C

-135.8°C

-107.7°C

-86.6°C

4. 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

5. 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

6. The wet bulb temperature during sensible heating of air

None of these

Decreases

Increases

Remains constant

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)

100 kJ/kg

125 kJ/kg

50 kJ/kg

25 kJ/kg

8. 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

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

All of the above

It gives higher coefficient of performance

It has low operating pressures

It is miscible with oil over large range of temperatures

10. During dehumidification process, _________ remains constant.

Dry bulb temperature

Relative humidity

Specific humidity

Wet bulb temperature

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11. 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

Operating the machine at lower speeds

Raising the higher temperature

Lowering the higher temperature

12. In Electrolux refrigerator

Ammonia evaporates in hydrogen

Ammonia is absorbed in hydrogen

Hydrogen evaporates in ammonia

Ammonia is absorbed in water

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

R-11

R-22

R-12

Ammonia

14. 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

15. 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

1.86 kW

3.72 kW

18.6 kW

16. Air refrigeration cycle is used in

Commercial refrigerators

Air-conditioning

Domestic refrigerators

Gas liquefaction

17. 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

18. 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

19. 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

Relative humidity

Humidity ratio

Degree of saturation

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

Two fluids

Single fluid

Three fluids

None of these

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

Dew point temperature is lower than wet bulb temperature

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

22. 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

Cooling and humidification

Dehumidification

23. 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

24. A good refrigerant should have

Low C.O.P. and low freezing point

High latent heat of vaporisation and low freezing point

High specific volume and high latent heat of vaporisation

High operating pressures and low freezing point

25. The relative humidity lines on a psychrometric chart are

Horizontal and uniformly spaced

Horizontal and non-uniformly spaced

Vertical and uniformly spaced

Curved lines

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

These react with oxygen and cause its depletion

These react with plants and cause greenhouse effect

These react with ozone layer

These react with water vapour and cause acid rain

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

21 kJ/min

210 kJ/min

620 kJ/min

420 kJ/min

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

Water

Aqua-ammonia

Ammonia

Freon

29. 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

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

Suction pressure

Discharge pressure

Critical pressure

Back pressure

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31. An evaporator is also known as

All of these

Freezing coil

Cooling coil

Chilling coil

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

To the right of saturated liquid line

To the left of saturated liquid line

Between the saturated liquid line and saturated vapour line

None of the above

33. 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)

(td₁ -td₃)/( td₂ -td₃)

(td₃ -td₁)/( td₂ -td₃)

(td₃ -td₂)/( td₁ -td₃)

34. In chemical dehumidification process,

Dry bulb temperature increases

Dew point temperature decreases

All of these

Wet bulb temperature decreases

35. 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

Newton's law of gases

Dalton's law of partial pressures

Avogadro's hypothesis

Kinetic theory of gases

36. The bypass factor of a cooling coil decreases with

Decrease 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

Increase in fin spacing and increase in number of rows

37. The unit of thermal diffusivity is

m²/h

m/h

m/hK

m²/hK

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

Greater than

Less than

Equal to

None of these

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

Dry bulb depression

Degree of saturation

Dew point depression

Wet bulb depression

40. 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

0.3

0.67

0.6

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41. A vapour absorption refrigeration system

Requires little power consumption

Cools below 0°C

Gives noisy operation

Gives quiet operation

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

Thermostatic expansion valve

Low side float valve

Automatic expansion valve

High side float valve

43. The relative humidity is defined as

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

The mass of water vapour present in 1 m3 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

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

Same as

Higher than

Lower than

None of these

45. Bell Coleman cycle is a

Reversed Joule cycle

Reversed Otto cycle

Reversed Carnot cycle

Reversed Rankine cycle

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

Cost is too high

Capacity control is not possible

It is made of copper

Required pressure drop cannot be achieved

47. Pressure of water vapour is given by

μ/[1 - (1 - μ) (Ps/Pb)]

None of these

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

0.622 Pv/ (Pb - Pv)

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

Moist air

Specific humidity

Saturated air

Dry air

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

50. 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)/ e₁ + e₂ - e₁e₂

e₁ + e₂

1/e₁ + 1/e₂

Refrigeration & Air-Conditioning Practice Set-6

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