BOILING AND CONDENSATION

 Boiling and condensation


At constant pressure if the temperature of the liquid is raised to saturation temperature then the liquid boils.  Similarly condensation occurs when the temperature of vapour is lowered to saturation temperature at constant pressure condition, the vapour changes to liquid. Such heat transfer occurring during boiling and condensation is categorized under convection mode of heat transfer. As purely heat is transferred by fluid motion.

Condensation and boiling depends mainly of the three factors. These are surface tension, latent heat of vaporization and liquid-gas interface. Large amount of latent heat is absorbed and released during boiling and condensation. At equilibrium condition the temperature remains constant. The rate of heat transfer during condensation and boiling is very high as compared to other modes of convection heat transfer.

BOILING

Many of us gets confused when we hear the two words i.e. boiling and evaporation. Most of us think that both these words have same meaning. To some extent it is correct because in both boiling and evaporation phase change takes place from liquid to vapour. But friends, at grass root level there is large difference between these two words.

At certain temperature when the vapour pressure is less than the saturation pressure evaporation occurs. The best example for it is evaporation of sweat from our body.

Boiling can be understood as when the vapour pressure crosses the saturation pressure at certain temperature. The example can be heating of water. Saturation pressure is nothing but the pressure exerted by the atmosphere or the weight of atmosphere. At any instant when the vapour pressure of liquid exceeds the atmospheric pressure, the liquid starts boiling.

This can be explained at microscopic level.When we heat the liquid, it molecules absorbs energy and gains kinetic energy. At certain temperature the kinetic energy crosses threshold limit and overcomes the saturation pressure or atmospheric pressure and it starts boiling. The process is started by formation of bubble at the interface of liquid and vapour and slowly it starts rising. The bubbles are formed at super critical temperature. The bubbles are formed due to surface tension at the interface region and as the temperature increases the surface tension decreases, as a results bubbles are lost and vapour rises.

Boiling is of two types depending on the motion of bulk fluid:-

1) Pool boiling
2) Flow boiling

When there is no bulk fluid motion is is referred as pool boiling. The fluid is stationary and any motion is by natural convection current. On the other hand flow boiling is associated with bulk motion of fluid. Flow is provided by applying external devices like pump.

BOILING REGIMES


1) Natural convection boiling
2) Nucleate boiling
3) Transition boiling
4) Film boiling

When the liquid is heated two- three degrees above the saturation temperature then the fluid motion is by natural convection current.

Nucleate boiling
 can be sub divided into two parts. In the first part very few bubbles are formed at the interface and they are dissipated as soon as they leave the surface. In the second part the bubble formation increases. Colum of vapour is generated. The column of vapour rises up to the interface and then rises up in atmosphere. The bubbles at the bottom forms the cluster and it decreases then heat transfer rate.

In transition boiling as large number of bubbles covers the heater surface heat transfer rate decreases. This will act as insulator. This region lies in between nucleate boiling and film boiling.

Film boiling is important as because if the temperature of the heater surface is further increased radiation mode of heat transfer becomes predominant. Under film boiling the heat flux is minimum. This is because the heater surface is completely covered by vapour and bubble. There is no possibility of transfer of heat. But if the temperature of heater is further increased heat transfer is increased and radiation heat transfer is observed.

Previous
Next Post »