Refrigeration & Air-Conditioning Practice Set-6

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

25°C DBT and 100% RH

22°C DBT and 60% RH

25°C DBT and 40% RH

20°C DBT and 80% RH

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

R-12

Ammonia

R-22

R-11

3. 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

Kinetic theory of gases

Newton's law of gases

4. 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)/ e₁ + e₂ - e₁e₂

e₁e₂

1/e₁ + 1/e₂

e₁ + e₂

5. An evaporator is also known as

All of these

Chilling coil

Cooling coil

Freezing coil

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

To the right of saturated liquid line

Between the saturated liquid line and saturated vapour line

To the left of saturated liquid line

None of the above

7. In Electrolux refrigerator

Hydrogen evaporates in ammonia

Ammonia is absorbed in hydrogen

Ammonia is absorbed in water

Ammonia evaporates in hydrogen

8. 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

Greater than

Equal to

9. In chemical dehumidification process,

Dry bulb temperature increases

Wet bulb temperature decreases

Dew point temperature decreases

All of these

10. 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)

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11. The unit of thermal diffusivity is

m/hK

m/h

m²/h

m²/hK

12. Bell Coleman cycle is a

Reversed Otto cycle

Reversed Joule cycle

Reversed Carnot cycle

Reversed Rankine cycle

13. 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

14. A vapour absorption refrigeration system

Gives quiet operation

Requires little power consumption

Gives noisy operation

Cools below 0°C

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

All of the above

It is miscible with oil over large range of temperatures

It has low operating pressures

It gives higher coefficient of performance

16. A refrigeration system

Rejects energy to a low temperature body

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

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

None of the above

17. The relative humidity lines on a psychrometric chart are

Horizontal and uniformly spaced

Vertical and uniformly spaced

Curved lines

Horizontal and non-uniformly spaced

18. The freezing point of R-12 is

-95.2°C

-135.8°C

-107.7°C

-86.6°C

19. 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 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 absorbed by the evaporator minus the heat supplied by the gas burner to the heat absorbed by the evaporator

20. Nusselt number (N_N) is given by

N_N = V²/t.c_p

N_N = hl/k

N_N = μ c_p/k

N_N = ρ V l /μ

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21. 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 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 mass of water vapour present in 1 kg of dry air

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

22. 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

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

Suction pressure

Critical pressure

Discharge pressure

Back pressure

24. Pressure of water vapour is given by

0.622 Pv/ (Pb - Pv)

None of these

μ/[1 - (1 - μ) (Ps/Pb)]

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

25. The wet bulb temperature during sensible heating of air

Decreases

None of these

Remains constant

Increases

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

21 kJ/min

620 kJ/min

210 kJ/min

420 kJ/min

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

Wet vapour

High pressure saturated liquid

Dry vapour

Very wet vapour

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

Thermostatic expansion valve

High side float valve

Low side float valve

Automatic expansion valve

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

None of these

Two fluids

Single fluid

Three fluids

30. 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

1.86 kW

3.72 kW

7.44 kW

18.6 kW

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31. 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

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

Lower than

Same as

Higher than

None of these

33. 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

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

Lack of cooling water

Water temperature being high

Dirty condenser surface

All of these

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

Greater than

Equal to

None of these

Less than

36. 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

0.3

0.67

37. 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

38. 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

Cooling and dehumidification

Dehumidification and pure sensible cooling

39. 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

Degree of saturation

Humidity ratio

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

Capacity control is not possible

Required pressure drop cannot be achieved

It is made of copper

Cost is too high

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41. During dehumidification process, _________ remains constant.

Wet bulb temperature

Dry bulb temperature

Specific humidity

Relative humidity

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

Degree of superheat at exit from the evaporator

Temperature of the evaporator

Pressure in the evaporator

None of the above

43. Air refrigeration cycle is used in

Commercial refrigerators

Domestic refrigerators

Gas liquefaction

Air-conditioning

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

Degree of saturation

Dew point depression

Wet bulb depression

Dry bulb depression

45. 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

Heated and dehumidified

Cooled and dehumidified

Cooled and humidified

46. 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

100 kJ/kg

25 kJ/kg

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

Aqua-ammonia

Freon

Ammonia

Water

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

70 to 110°C

20 to 50°C

50 to 70°C

None of these

49. 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

Specific humidity

Moist air

Dry air

50. A good refrigerant should have

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

Low C.O.P. and low freezing point

Refrigeration & Air-Conditioning Practice Set-6

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