Natural circulation in an evaporator is caused by the thermal difference between the heating medium and the liquid. This thermal change is called "thermo-siphon action". Sufficient heat must be available to provide the necessary thermal difference to cause circulation.The operation of natural circulation is extremely simple. The body and tubes are partially filled with liquid and steam is admitted to the steam heat. Dilute liquid at the proper temperature is fed to the chamber below the lower tube sheet.When the boiling point is reached, bubbles of vapor are formed in the column of liquid, as they expand and increase in volume they push the liquid ahead with increasing velocity. As the upper part of the tubes is reached, the liquid is either in the form of drops or a film in contact with the wall of the tubes.The mixture of vapor and liquid  merges from the tubes and enters the upper chamber from which it passes through a short connection into the vapor separator. Concentrated liquid is removed from the bottom of the vapor separator as a finished product or to serve as feed for another effect.Continuous operation is the ideal method of operating an evaporator, but there are processes where batch operation is necessary. The separator can be designed for any required holding capacity.


The Advantage of Natural Circulation Evaporator:
1.Batch or continuous operation.
2.Relatively low retention time.
3.Minimum floor space but large amountof heating surface
4.Minimum maintenance requirements.
5.Suitable for recirculation.

Natural Circulation Applications:
1.Heat-sensitive materials
2.Foamy liquids
3.Materials requiring sanitary operation
4.Non-viscous materials
5.Non-crystal forming materials