Any time that fluid flows there is some friction associated with the movement, which will cause head loss. Head loss is commonly compensated for in piping systems by pumps that do work on the fluid, compensating for the head loss due to friction. Circulation of fluid in systems by pumps is referred to as forced circulation.
It is possible to design some fluid systems in a manner that does not require the presence of pumps to provide circulation. The head required to compensate for the head losses is created by density gradients and elevation changes. Flow that occurs under these circumstances is called natural circulation.
Natural circulation refers to the ability of a fluid in a system to circulate continuously, with gravity and possible changes in heat energy. The difference of density being the only driving force. If the differences of density are caused by heat, this force is called as "thermal head" or "thermal driving head."
A fluid system designed for natural circulation will have a heat source and a heat sink. Each of these is in contact with some of the fluid in the system, but not all of it. The heat source is positioned lower than the heat sink.
Most materials that are fluid at common temperatures expand when they are heated, becoming less dense. Correspondingly, they become denser when they are cooled. At the heat source of a system of natural circulation, the heated fluid becomes lighter than the fluid surrounding it, and thus rises. At the heat sink, the nearby fluid becomes denser as it cools, and is drawn downward by gravity. Together, these effects create a flow of fluid from the heat source to the heat sink and back again.