Natural circulation
Encyclopedia
Natural circulation refers to the ability of a fluid in a system to circulate continuously, with gravity and changes in heat energy (temperature) being the only driving force. This force is known 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.
Systems of natural circulation include tornado
es and other weather systems
, ocean current
s, and household ventilation
. Some solar water heaters use natural circulation.
In a nuclear reactor
, natural circulation can be a design criterion. It is achieved by reducing turbulence and friction in the fluid flow (that is, minimizing head loss), and by providing a way to remove any inoperative pumps from the fluid path. Also, the reactor (as the heat source) must be physically lower than the steam generators or turbines (the heat sink). In this way, natural circulation will ensure that the fluid will continue to flow as long as the reactor is hotter than the heat sink, even when power cannot be supplied to the pumps.
Notable examples are the S5G
and S8G
United States Naval reactor
s, which were designed to operate at a significant fraction of full power under natural circulation, quieting those propulsion plants. The S6G reactor
cannot operate at power under natural circulation, but can use it to maintain emergency cooling while shut down.
By the nature of natural circulation, fluids do not typically move very fast, but this is not necessarily bad, as high flow rates are not essential to safe and effective reactor operation. In modern design nuclear reactors, flow reversal is almost impossible. All nuclear reactors, even ones designed to primarily use natural circulation as the main method of fluid circulation, have pumps that can circulate the fluid in the case that natural circulation is not sufficient.
A fluid system designed for natural circulation will have a heat source and a heat sink
Heat sink
A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator in a car...
. 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
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
. 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.
Systems of natural circulation include tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es and other weather systems
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
, ocean current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
s, and household ventilation
Ventilation (architecture)
Ventilating is the process of "changing" or replacing air in any space to provide high indoor air quality...
. Some solar water heaters use natural circulation.
In a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
, natural circulation can be a design criterion. It is achieved by reducing turbulence and friction in the fluid flow (that is, minimizing head loss), and by providing a way to remove any inoperative pumps from the fluid path. Also, the reactor (as the heat source) must be physically lower than the steam generators or turbines (the heat sink). In this way, natural circulation will ensure that the fluid will continue to flow as long as the reactor is hotter than the heat sink, even when power cannot be supplied to the pumps.
Notable examples are the S5G
S5G reactor
The S5G reactor was a prototype naval reactor designed for the United States Navy to provide electricity generation and propulsion on submarines...
and S8G
S8G reactor
The S8G reactor is a naval reactor used by the United States Navy to provide electricity generation and propulsion on warships. The S8G designation stands for:* S = Submarine platform* 8 = Eighth generation core designed by the contractor...
United States Naval reactor
United States Naval reactor
United States Naval reactor refers to nuclear reactors used by the United States Navy aboard certain ships to produce power for propulsion, electric power, catapulting airplanes in aircraft carriers, and a few more minor uses. Such Naval nuclear reactors have a complete power plant associated with...
s, which were designed to operate at a significant fraction of full power under natural circulation, quieting those propulsion plants. The S6G reactor
S6G reactor
The S6G reactor is a naval reactor used by the United States Navy to provide electricity generation and propulsion on warships. The S6G designation stands for:* S = Submarine platform* 6 = Sixth generation core designed by the contractor...
cannot operate at power under natural circulation, but can use it to maintain emergency cooling while shut down.
By the nature of natural circulation, fluids do not typically move very fast, but this is not necessarily bad, as high flow rates are not essential to safe and effective reactor operation. In modern design nuclear reactors, flow reversal is almost impossible. All nuclear reactors, even ones designed to primarily use natural circulation as the main method of fluid circulation, have pumps that can circulate the fluid in the case that natural circulation is not sufficient.