Consider two equal volumes of the same type of fluid. If the two volumes are not at the same
temperature, then the volume with the higher temperature will also have a lower density and,
therefore, less mass. Since the volume at the higher temperature will have a lower mass, it will
also have less force exerted on it by gravity. This difference in the force of gravity exerted on
the fluid will tend to cause the hotter fluid to rise and the colder fluid to sink.
This effect is seen in many places. One example of this is a hot air balloon. The force causing
a hot air balloon to rise is a result of a difference in density between the hot air inside the
balloon and the cooler air surrounding it.
Heat added to the air in the balloon adds energy to the molecules of air. The movement of the
air molecules increases and the air molecules take up more space. The air molecules inside the
balloon take up more space than the same amount of air molecules outside the balloon. This
means the hot air is less dense and lighter than the surrounding air. Since the air in the balloon
is less dense, gravity has less effect on it. The result is that the balloon weighs less than the
surrounding air. Gravity pulls cooler air down into the space occupied by the balloon. The
downward movement of the cooler air forces the balloon out of the space previously occupied,
and the balloon rises.
Conditions Required for Natural Circulation
Natural circulation will only occur if the correct conditions exist. Even after natural circulation
has begun, removal of any one of these conditions will cause the natural circulation to stop. The
conditions for natural circulation are as follows.
A temperature difference exists (heat source and heat sink exists).
The heat source is at a lower elevation than the heat sink.
The fluids must be in contact with each other.
There must be two bodies of fluid at different temperatures. This could also be one body of fluid
with areas of different temperatures. The difference in temperature is necessary to cause a
density difference in the fluid. The density difference is the driving force for natural circulation
The difference in temperature must be maintained for the natural circulation to continue.
Addition of heat by a heat source must exist at the high temperature area. Continuous removal
of heat by a heat sink must exist at the low temperature area. Otherwise the temperatures would
eventually equalize, and no further circulation would occur.
The heat source must be at a lower elevation than the heat sink. As shown by the example of
the balloon, a warmer fluid is less dense and will tend to rise, and a cooler fluid is more dense
and will tend to sink. To take advantage of the natural movement of warm and cool fluids, the
heat source and heat sink must be at the proper elevations.