In distillation columns, Steam generation units, evaporation systems, and petroleum refining industries in all of them, an important step involved is heat transfer to boiling liquids. This is very much concerned with controlling the temperature of the chemical reactor.
A heating surface, which is a source of heat supply to a liquid in the above equipment, is used not only to heat up but also to change the phase of the liquid to a vapour state by raising its temperature to the boiling point and providing 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 a 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 attains the equilibrium temperature to the corresponding boiling temperature at the pressure of the equipment. These bubbles rise to the surface and are carried over the vapour space above the liquid surface. This is called a 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 exceeds then its saturation temperature and is further heated up forming a layer of vapour film on the solid surface. This film acts as a barrier reducing the heat transfer rate known 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.
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