Hydrodynamics and Thermal Boundary Layers and Types of Heat Exchangers

4. Regimes of boiling:

5. 1. Purely convective Boiling: The excess temperature is less than 5°C. The liquid in
contact with the hot surface gets superheated and slowly rises to the surface and
vapour formation generally occurs at the surface.
2. Nucleate Boiling: As the heating rate is increased bubbles of vapour will be seen to
form at favourable locations called nucleation sites on the surface. The bubbles will
grow and will detach themselves from the surface and will begin to rise to the
surface. For lower range of values of excess temperature, these bubbles will slowly
condense again and will get reduced in size as they rise. This will be the case when
the excess temperature is of the order of 10°C. Further increase in heat rate and
consequently the surface temperature (also the excess temperature) will cause
vapour bubbles to rise as a stream to the surface. Bubbles will also form at more
number of locations. The excess temperature will be about 20°C.
3. Film Boiling: A further increase in the heat flux causes a film of vapour to cover the
surface. This is due to the merging of favourable locations into an area. This reduces
the convection coefficient and excess temperature shoots up to a very high value. In
this situation the heat transfer is both by radiation and by conduction to the vapour.
If the material is not strong enough for withstanding this temperature, the
equipment will fail by damage to the material. This is known as burn out.

6. Condensation
• Film Wise Condensation
The liquid condensate wets the solid surface, spreads out and forms
a continuous film over the entire surface.
• Drop Wise Condensation
The vapour condenses into small liquid droplets of various sizes
which fall down the surface in a random fashion.

7. HEAT EXCHANGERS
In many industrial applications heat has to be transferred from one
flowing fluid to another through a solid barrier separating these fluids.