Refrigeration & Air-Conditioning Practice Set-3

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1. Most thermostatic expansion valves are set for a superheat of

20°C

5°C

15°C

10°C

2. The minimum temperature to which water can be cooled in a cooling tower is

Ambient air temperature

Dew point temperature of air

Dry bulb temperature of air

Wet bulb temperature of air

3. Rating of a domestic refrigerator is of the order of

40 tons

10 tons

0.1 ton

5 tons

4. Mass flow ratio of NH₃ in comparison to Freon-12 for same refrigeration load and same temperature limits is of the order of

1: 1

9: 1

1: 3

1: 9

5. A reversible engine has ideal thermal efficiency of 30%. When it is used as a refrigerating machine with all other conditions unchanged, the coefficient of performance will be

3.33

1.33

4.33

2.33

6. A certain refrigerating system has a normal operating suction pressure of 10 kg/cm gauge and condensing pressure of about 67 kg/cm. The refrigerant used is

Ammonia

Freon

Brine

Carbon dioxide

7. A human body feels comfortable when the heat produced by the metabolism of human body is equal to the

(C) Sum of (A) and (B)

(D) Difference of (A) and (B)

(B) Heat stored in the human body

(A) Heat dissipated to the surroundings

8. If a heat pump cycle operates between the condenser temperature of +27°C and evaporator temperature of -23°C, then the Carnot COP will be

0.2

1.2

6

5

9. The condition of refrigerant after passing through the condenser in a vapour compression system is

Superheated vapour

Dry saturated vapour

Saturated liquid

Wet vapour

10. In a flooded evaporator refrigerator, an accumulator at suction of compressor is used to

Collect liquid refrigerant and prevent it from going to compressor

Collect vapours

Superheat the vapour

Detect liquid in vapour

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11. For obtaining high COP, the pressure range of compressor should be

High

Any value

Optimum

Low

12. Which of the following cycles uses air as the refrigerant?

Carnot

Stirling

Ericson

Bell Coleman

13. Highest temperature encountered in refrigeration cycle should be

At critical Temperature

Above critical temperature

Near critical temperature of refrigerant

Much below critical temperature

14. The domestic refrigerator uses following type of compressor

Piston type reciprocating

Axial

Centrifugal

Miniature sealed unit

15. In actual air-conditioning applications for R-12 and R-22, and operating at a condenser temperature of 40° C and an evaporator temperature of 5° C, the heat rejection factor is about

5.12

1.25

1

2.15

16. Which of the following is not a desirable property of a refrigerant?

Good electrical conductor

Large latent heat

Low boiling point

High risibility with oil

17. The boiling point of ammonia is

-10.5°C

-33.3°C

-30°C

-77.7°C

18. The C.O.P. of a refrigerator working on a reversed Carnot cycle is (where T₁ = Lowest absolute temperature, and T₂ = Highest absolute temperature)

(T₂ - T₁)/T₁

T₁ / (T₂ - T₁)

(T₁ - T₂)/T₁

T₂/ (T₂ - T₁)

19. In a refrigeration system, the expansion device is connected between the

Receiver and evaporator

Compressor and condenser

Evaporator and compressor

Condenser and receiver

20. In a refrigeration system, heat absorbed in comparison to heat rejected is

Less

More

More for small capacity and less for high capacity

Same

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21. The general rule for rating refrigeration systems (excepting for CO₂ system) is to approximate following h.p. per ton of refrigeration

0.5 to 0.8 h.p. per ton of refrigeration

0.1 to 0.5 h.p. per ton of refrigeration

1 to 2 h.p. per ton of refrigeration

2 to 5 h.p. per ton of refrigeration

22. Freon group of refrigerants are

Nontoxic and non-inflammable

Non-inflammable and toxic

Toxic

Inflammable

23. Critical temperature is the temperature above which

A gas will immediately liquefy

Water will evaporate

A gas will never liquefy

Water will never evaporate

24. The value of C.O.P in vapour compression cycle is usually

Any one of the above

Always less than unity

Always more than unity

Equal to unity

25. The bank of tubes at the back of domestic refrigerator is

Refrigerant cooling tubes

Evaporator tubes

Condenser tubes

Capillary tubes

26. The refrigerant after condensation process is cooled below the saturation temperature before throttling. Such a process is called

None of these

Super-cooling

Normal cooling

Sub-cooling or under-cooling

27. The operating pressure for refrigerating units using R-12 as a refrigerant is

8 bar

15 bar

2 bar

30 bar

28. On the pressure-enthalpy diagram, condensation and desuperheating is represented by a horizontal line because the process

Involves no change in volume

Takes place at constant entropy

Takes place at constant pressure

Takes place at constant temperature

29. The freezing point of sulphur dioxide is

-135.8°C

-56.6°C

-75.2°C

-77.7°C

30. The atmospheric air at dry bulb temperature of 15°C enters a heating coil maintained at 40°C. The air leaves the heating coil at 25°C. The bypass factor of the heating coil is

0.4

0.376

0.67

0.6

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31. The capillary tube, as an expansion device, is used in

All of these

Domestic refrigerators

Room air conditioners

Water coolers

32. A bootstrap air cooling system has

Four heat exchangers

Two heat exchangers

One heat exchanger

Three heat exchangers

33. When the temperature of the surrounding is higher than the temperature of the body, then the heat loss by convection from the body to the surrounding will be

Positive

None of these

Zero

Negative

34. Which of the following statement is correct?

(A) The human body can lose heat even if its temperature is less than the atmospheric temperature.

(B) The increase in air movement increases the evaporation from the human body.

(C) The warm air increases the rate of radiation of heat from the human body.

(D) Both (A) and (B)

35. Cooling water is required for following equipment in ammonia absorption plant

Evaporator

Condenser, absorber and separator (rectifier)

Absorber

Condenser

36. The change in evaporator temperature in a refrigeration cycle, as compared to change in condenser temperature, influences the value of C.O.P.

Unpredictable

Equally

More

Less

37. In a lithium bromide absorption refrigeration system

Ammonia is used as a refrigerant and lithium bromide as an absorbent

Lithium bromide is used as a refrigerant and water as an absorbent

None of these

Water is used as a refrigerant and lithium bromide as an absorbent

38. A thermostatic expansion valve in a refrigeration system

None of the above

Ensures the evaporator completely filled with refrigerant of the load

Maintains different temperatures in evaporator in proportion to load

Is suitable only for constant load systems

39. The curved lines on a psychrometric chart indicates

Dry bulb temperature

Relative humidity

Wet bulb temperature

Dew point temperature

40. The vapour compression refrigerator employs the following cycle

Carnot

Rankine

Reversed Carnot

Reversed Rankine

Refrigeration & Air-Conditioning Practice Set-3

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