Definition - What does Natural Circulation mean?
Natural circulation is the ability of a fluid in a system to circulate continuously, with the difference in density being the only driving force.
In another way, natural circulation is caused by convection currents which result from the uneven heating of the water in a boiler. It may be either free or accelerated, and is primarily used in boilers and evaporators.
Natural circulation evaporators are based on the natural circulation of the product caused by the density differences that arise from heating. Natural circulation evaporators are necessary in effluent treatment plants, and in both the chemical and pharmaceutical industry.
Corrosionpedia explains Natural Circulation
In natural circulation boilers, the circulation of water depends on the difference between the density of an ascending mixture of hot water and steam, and a descending body of relatively cool and steam-free water. The difference in density occurs because the water expands as it is heated, and thus, becomes less dense. In most boilers there is a natural circulation of water, the fundamental principle of which is based on the principle of thermosiphon.
In a boiler with free natural circulation, the generating tubes are installed almost horizontally, with only a slight incline toward the vertical. When the generating tubes are installed at a much greater angle of inclination, the rate of water circulation is greatly increased. Therefore, boilers in which the tubes slope quite steeply from steam drum to water drum are said to have natural circulation of the accelerated type.
Natural circulation evaporators are very simple and are normally used where the effluent has a high viscosity, higher levels of hardening agents, un-dissolved solids, for products which are affected by their own high temperatures and longer residual times. The operation can be continuous, batch or semi-batch and does not require pumps for recirculation or intermediate product transfer.
The overall circulation rate (flow) in natural circulation systems mostly depends on the height, operating pressure and heat input rate of the boiler.