HEAT TRANSFER in Boiling Condition

Distillation column, Steam generation unit, evaporation system, petroleum refining industries in all of them an important step is involved that is heat transfer to boiling liquids. This is very much concerned in controlling the temperature of the chemical reactor. A heating surface, which is a source of heat supply to a liquid in above equipment, is used not only to heat up but also to change the phase of the liquid to vapour state by raising its temperature to the boiling point and provide necessary latent heat for its vaporization. This process of heating a liquid is an excellent physical phenomenon exhibiting various stages of accomplishing vaporization.

When a liquid flow over a heated tube or when a liquid is boiled in a vessel that has heating tube bundle in it, supplying constant heat. Bubbles are formed at the surface of the heat medium, these bubbles rise from the surface and collapse immediately before reaching the surface of the liquid because the temperature is not homogeneous in the liquid and when the liquid attain the equilibrium temperature to the corresponding boiling temperature at the pressure of the equipment. These bubbles rise to the surface and carried over the vapour space above the liquid surface. This is called as pool boiling of saturated liquid.
If the surface of the heat media has a temperature higher than the boiling temperature of the liquid, the liquid at the surface exceed then its saturation temperature and further heated up forming the layer of vapour film on the solid surface. This film acts as barrier reducing the heat transfer rate knows as film boiling.


For heat transfer in boiling, resistance is mainly offered by the film of vapour which forms immediately over the hot surface.
Q = h A (Tw – T1)
T1 is the saturated liquid temperature on one side of the film and
Tw is the surface temperature on the other side.
h is the boiling coefficient.

In condensation, the main resistance comes from the liquid film adjacent to the surface and the heat transfer rate is
Q = h A (Tv – Tw)
Tv = saturated vapour temperature and 'h' is the condensing coefficient.