# Refrigeration and Air Conditioning Multiple Choice Questions

## Refrigeration and Air Conditioning

Question 1.

Nusselt number (NN) is given by

 A. NN = hl/k B. NN = μ cp/k C. NN = ρ V l /μ D. NN = V²/t.cp
Explanation :

Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid.

A Nusselt number of value one represents heat transfer by pure conduction.

Nusselt number (NN) is given by NN = hl/k

Question 2.

In case of sensible heating of air, the coil efficiency is given by (where B.P.F. = Bypass factor)

 A. B.P.F. - 1 B. 1 - B. P.F. C. 1/ B.P.F. D. 1 + B.P.F.
Explanation :

In case of sensible heating of air, the coil efficiency is given by 1 - B. P.F.

Question 3.

The undesirable property of a refrigerant is

 A. Non-toxic B. Non-flammable C. Non-explosive D. High boiling point
Explanation :

It is very important that the refrigerant has a low boiling point, that is it should have the ability to turn into gas easily when it absorbs heat.
If it has a high boiling point, the compressor will have to create too much of a vacuum in order to lower the pressure to make vaporization happen.

Question 4.

The desirable property of a refrigerant is

 A. Low boiling point B. High critical temperature C. High latent heat of vaporisation D. All of these
Explanation :

Physical Properties of Refrigerants

Low Condensing Pressure.
High Evaporator Pressure.
High Critical Pressure.
High Vapor Density.
High Dielectric strength.
High Latent Heat of Vaporization.
High Heat Transfer Coefficient.

Question 5.

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

 A. 2 bar B. 8 bar C. 15 bar D. 30 bar
Explanation :

The operating pressure for refrigerating units using R-12 as a refrigerant is 8 bar