PHASE CHANGE HEATTRANSFER AND HEAT
1.What is meant by pool
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.
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
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
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
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
12. How are heat exchangers classified
based on flow arrangement?
Parallel flow, counter flow, compact and cross flow