Refrigeration and Air Conditioning

Question: An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m2). The critical thickness of insulation will be
[A].

25 mm

[B].

40 mm

[C].

160 mm

[D].

800 mm

Answer: Option A

Explanation:

No answer description available for this question.

Question: Fourier’s law of heat conduction gives the heat flow for
[A].

irregular surfaces

[B].

non-uniform temperature surfaces

[C].

one dimensional cases only

[D].

two dimensional cases only

Answer: Option C

Explanation:

No answer description available for this question.

Question: A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h1, h2 and h3 respectively. The average heat transfer coefficient for the cube is
[A].

h1 + h2 + h3

[B].

(h1h2h3)1/3

[C].

[D].

none of these

Answer: Option D

Explanation:

No answer description available for this question.

Question: Film coefficient is defined as the ratio of
[A].

thermal conductivity to the equivalent thickness of the film of fluid

[B].

temperature drop through the films of fluids to the thickness of film of fluids

[C].

thickness of film of fluid to the thermal conductivity

[D].

thickness of film of fluid to the temperature drop through the films of fluids

Answer: Option A

Explanation:

No answer description available for this question.

Question: A furnace is made of a red brick wall of thickness 0.5 m and conductivity 0.7 W/mK. For the same heat loss and temperature drop, this can be replaced by a layer of diatomite earth of conductivity 0.14 W/mK and thickness
[A].

0.5 m

[B].

0.1 m

[C].

0.2 m

[D].

0.5 m

Answer: Option B

Explanation:

No answer description available for this question.

Question: A composite slab has two layers of different materials with thermal conductivities k1 and k2. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be
[A].

k1k2

[B].

k1 + k2

[C].

[D].

Answer: Option D

Explanation:

No answer description available for this question.

Question: 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

Answer: Option D

Explanation:

No answer description available for this question.

Question: A steam pipe is to be lined with two layers of insulating materials of different thermal conductivities. For the minimum heat transfer,
[A].

the better insulation must be put inside

[B].

the better insulation must be put outside

[C].

one could place either insulation on either side

[D].

one should take into account the steam temperature before deciding as to which insulation is put where

Answer: Option A

Explanation:

No answer description available for this question.

Question: The average value of thermal conductivity for water at 20° C saturate is about 0.51.
[A].

True

[B].

False

Answer: Option A

Explanation:

No answer description available for this question.

Question: According to Newton’s law of cooling, the heat transfer from a hot body to a cold body is
[A].

directly proportional to the surface area

[B].

directly proportional to the difference of temperatures between the two bodies

[C].

either (a) or (b)

[D].

both (a) and (b)

Answer: Option D

Explanation:

No answer description available for this question.