Natural circulation is the circulation of fluid within piping systems or
open pools that is due to the density changes caused by temperature
differences. Natural circulation does not require any mechanical
devices to maintain flow.
DEFINE natural circulation and forced circulation.
DEFINE thermal driving head.
DESCRIBE the conditions necessary for natural
circulation to exist.
EXPLAIN the relationship between flow rate and
temperature difference in natural circulation flow.
DESCRIBE how the operator can determine whether
natural circulation exists in the reactor coolant system
and other heat removal systems.
DESCRIBE how to enhance natural circulation flow.
Forced and Natural Circulation
In the previous chapters on fluid flow, it was explained that any time that fluid flows there is
some friction associated with the movement, which will cause head loss. It was pointed out that
this 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
Thermal Driving Head
Thermal driving head is the force that causes natural circulation to take place. It is caused by
the difference in density between two bodies or areas of fluid.