Heat Transfer, Refrigeration and Air Conditioning – Section 2

11. For evaporators and condensers, for the given conditions, the logarithmic mean temperature difference ™ for parallel flow is __________ that for counter flow.

A. equal to
B. less than
C. greater than

Correct Answer: A. equal to

12.  A vapour absorption refrigerator uses __________ as a refrigerant

A. water
B. ammonia
C. freon
D. aqua-ammonia

Correct Answer: D. aqua-ammonia

13. Thermal conductivity of air increases with rise in temperature.

A. Correct
B. Incorrect

Correct Answer: A. Correct

14. The material of pipelines for a system using ammonia as a refrigerant should be copper.

A. Yes
B. No

Correct Answer: B. No

15. The wave length corresponding to the maximum energy is inversely proportional to the absolute temperature. This statement is called Planck’s law.

A. Yes
B. No

Correct Answer: B. No

16. Which of the following refrigerant has the highest freezing point.

A. Ammonia
B. Carbon dioxide
C. Sulphur dioxide
D. R-12

Correct Answer: B. Carbon dioxide

17. Air conditioning means

A. cooling
B. heating
C. dehumidifying
D. all of these

Correct Answer: D. all of these

18. The emissivity for a black body is

A. 0
B. 0.5
C. 0.75
D. 1

Correct Answer: D. 1

19. Which of the following would lead to a reduction in thermal resistance?

A. In conduction, reduction in the thickness of the material and an increase in thermal conductivity.
B. In convection, stirring of the fluid and cleaning the heating surface.
C. In radiation, increasing the temperature and reducing the emissivity.
D. all of the above

Correct Answer: D. all of the above

20. The ratio of the thickness of the thermal boundary layer to the thickness of hydrodynamic boundary layer is equal to (Prandtl number)n, where n is equal to

A.-1/3
B.-2/3
C.1
D.-1

Correct Answer: A.-1/3

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