Important Questions and Answers - Heat and Mass Transfer - Radiation

PHASE CHANGE HEATTRANSFER AND HEAT EXCHANGERS

1.What is meant by pool boiling?

If heat is added to a liquid from a submerged solid surface, the boiling process is referred to as pool boiling. In the case the liquid above the hot surface is essentially stagnant and its motion near the surface is due to free convection and mixing induced by bubble growth and detachment.

2. State the difference between the drop wise condensation and film wise condensation.

Filmwise condensation:

The liquid condensate wets the solid surface, spreads out and forms a continuous film over the entire surface is known as film wise condensation.

Drop wise condensation:

In drop wise condensation, the vapor condenses into small liquid droplets of various sizes which fall down the surface in a random fashion.

3. What is meant by LMTD  ?

We know that the temperatures difference between the hot and cold fluids in the heat exchanger varies from point to point. In addition various modes of heat transfer are involved. Therefore based on concept of appropriate mean temperature difference, also called logarithmic mean temperature difference, the total heat transfer rate in the heat exchanger is expressed as

Q= UA(  T)m,

Where  U –Over all heat coefficient W/    K

A –Area, m

T-  Temperature difference.

4. What is meant by Foulingfactor?

We know the surface of a heat exchanger do not remain clean after it has been in use for some time. The surfaces become fouled with scaling or deposits. The effect of these deposits affecting the value of overall heat transfer coefficient. This effect is taken care of by introducing an additional thermal resistance called the fouling factor.

5. What is meant by condensation?

The change of phase from vapor to liquid state is known as condensation.

6. What is compact heat exchanger?

There are many special purpose heat exchangers called compact heat exchangers .They are generally employed when convective heat transfer coefficient associated with one of the fluids is much smaller than the associated with the other fluid.

7. Define Effectiveness.

The heat exchanger effectiveness is defined as the ratio of actual heat transfer to the maximum possible heat transfer.

Effectiveness        =  Actual heat transfer/ maximum possible heat transfer.

8. What is meant by parallel flow heat exchangers and counter flow heat exchanger?

Parallel flow heat exchanger:

In this type of heat exchanger, hot and cold fluids move in the same direction. Counter flow heat exchanger:

In this type of heat exchanger,hot and cold fluids move in parallel but in opposite directions.

9. What is heat exchanger?

A heat exchanger is defined as anequipment which transfers the heat from a hot fluid to a cold fluid.

10. Give the expression for NTU.

Number of Transfer Units (NTU) = UA / Cmin.

11. List the various promoters used for maintaining drop wise condensation.

Oleic acid, benzyl, certain fats and waxes are effective promoters used for maintaining drop wise

condensation.

12. How are heat exchangers classified based on flow arrangement?

Parallel flow, counter flow, compact and cross flow heat exchanger.