In a natural circulation boiler the circulation is achieved by the difference in

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

when the water in the

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centra ...

is heated. In natural circulation steam boilers the circulation of water is by

convection currents Convection is single or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convect ...

, which are set up during the heating of water. In most of the boilers there is a natural circulation of water the fundamental principle of which is based on the principle of

Thermosiphon A thermosiphon (or thermosyphon) is a device that employs a method of passive heat transfer, heat exchange based on natural convection, which circulates a fluid without the necessity of a mechanical pump. Thermosiphoning is used for circulation ...

See also