Thermosiphon
Encyclopedia
Thermosiphon refers to a method of passive heat exchange
based on natural convection
which circulates liquid without the necessity of a mechanical pump. This circulation can either be open-loop, as when liquid in a holding tank is passed in one direction via a heated transfer tube mounted at the bottom of the tank to a distribution point - even one mounted above the originating tank - or it can be a vertical closed-loop circuit with return to the original vessel. Its intended purpose is to simplify the pumping of liquid and/or heat transfer, by avoiding the cost and complexity of a conventional liquid pump.
than the cooler water in the bottom of the loop. Convection moves heated liquid upwards in the system as it is simultaneously replaced by cooler liquid returning by gravity. Ideally, the liquid flows easily because a good thermosiphon should have very little hydraulic resistance.
Heat can still be transferred in this system by the evaporation
and condensation
of vapor; however, the system is properly classified as a heat pipe
. If the system also contains other fluids, such as air, then the heat flux density will be less than in a real heat pipe, which only contains a single fluid.
The thermosiphon has been sometimes incorrectly described as a 'gravity return heat pipe
' http://www.btfsolar.com/specifications.htm. Heat pipes usually have a wick to return the condensate
to the evaporator
via capillary action
. A wick is not needed in a thermosiphon because gravity moves the liquid http://cipco.apogee.net/ces/library/twhtherm.asp. The wick allows heat pipes to transfer heat when there is no gravity, which is useful in space. A thermosiphon is "simpler" than a heat pipe http://www.cheresources.com/htpipes.shtml.
(Single-phase) thermosiphons can only transfer heat "upward", or away from the acceleration vector. Thus, orientation is much more important for thermosiphons than for heatpipes. Also, thermosiphons can fail because of a bubble in the loop, and require a circulating loop of pipes.
than the liquid, the convective pressure is greater. This is a well known invention called a reboiler
. A group of reboilers attached to a pair of plena
is called a calandria
.
The term "phase change thermosiphon" is a misnomer and should be avoided.
When phase change occurs in a thermosiphon, it means that the system either does not have enough fluid, or it is too small to transfer all of the heat by convection alone. To improve the performance, either more fluid is needed (possibly in a larger thermosiphon), or all other fluids (including air) should be pumped out of the loop.
Thermosiphons are used in some liquid-based solar heating systems to heat a liquid such as water
. The water is heated passively by solar energy and relies on heat energy being transferred from the sun to a solar collector
. The heat from the collector can be transferred to water in two ways: directly where water circulates through the collector, or indirectly where an anti-freeze solution carries the heat from the collector and transfers it to water in the tank via a heat exchanger
. Convection allows for the movement of the heated liquid out of the solar collector
to be replaced by colder liquid which is in turn heated. Due to this principle, it is necessary for the water to be stored in a tank above the collector.
the internal computer components, most commonly referring to the processor. While any suitable liquid can be used, water is the easiest liquid to use in thermosiphon systems. Unlike traditional watercooling
systems, thermosiphon systems do not rely on a water pump (or a pump for other liquids) but rely on convection for the movement of heated water (which may become vapour) from the components upwards to a heat exchanger. There the water is cooled and is ready to be recirculated. The most commonly used heat exchanger is a radiator
where air is blown actively through a fan system to condense
the vapour to a liquid. The liquid is recirculated through the system, thus repeating the process. No pump is required - the vaporization and condensation cycle is self sustaining.
and radiator
. As engine power increased, increased flow was required and so engine-driven pumps were added to assist circulation. More compact engines then used smaller radiators and required more convoluted flow patterns, so the circulation became entirely dependent on the pump and might even be reversed against the natural circulation. An engine cooled only by thermosiphon is also very sensitive to low coolant level i.e. missing only a small amount of coolant stops the circulation, a pump driven system is much more robust and can handle low coolant level.
from melting below buildings and other infrastructure such as schools, air hangars, community water tanks, and even some stretches of highway. Heat pipes are also a common feature along the length of the Trans-Alaska Pipeline System
. In these applications the solution in the pipes is often carbon dioxide
or ammonia
. At the bottom of the heat pipe, heat from the ground warms the liquid and converts it to a vapor. Cooling from the heat sink fins above ground releases this heat to the atmosphere and causes the vapor to condense on the outer pipe wall, which then drains back into the liquid pool at the bottom of the heat pump.
Heat exchange
Heat exchange may refer to:*Heat transfer, an area of engineering concerned with the transfer of thermal energy *Heat exchanger, a device built for heat transfer from one medium to another...
based on natural convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
which circulates liquid without the necessity of a mechanical pump. This circulation can either be open-loop, as when liquid in a holding tank is passed in one direction via a heated transfer tube mounted at the bottom of the tank to a distribution point - even one mounted above the originating tank - or it can be a vertical closed-loop circuit with return to the original vessel. Its intended purpose is to simplify the pumping of liquid and/or heat transfer, by avoiding the cost and complexity of a conventional liquid pump.
Simple thermosiphon
Convective movement of the liquid starts when liquid in the loop is heated, causing it to expand and become less dense, and thus more buoyantBuoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
than the cooler water in the bottom of the loop. Convection moves heated liquid upwards in the system as it is simultaneously replaced by cooler liquid returning by gravity. Ideally, the liquid flows easily because a good thermosiphon should have very little hydraulic resistance.
Heat Pipes
In some situations the flow of liquid may be reduced further, or stopped, perhaps because the loop is not entirely full of liquid. In this case, the system no longer convects, so it is not a "thermosiphon."Heat can still be transferred in this system by the evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
and condensation
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
of vapor; however, the system is properly classified as a heat pipe
Heat pipe
A heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces....
. If the system also contains other fluids, such as air, then the heat flux density will be less than in a real heat pipe, which only contains a single fluid.
The thermosiphon has been sometimes incorrectly described as a 'gravity return heat pipe
Heat pipe
A heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces....
' http://www.btfsolar.com/specifications.htm. Heat pipes usually have a wick to return the condensate
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
to the evaporator
Evaporator
An evaporator is a device used to turn the liquid form of some chemical into its gaseous form. For example, an evaporator is used in an air conditioning system to allow the compressed cooling chemical to evaporate from liquid to gas, absorbing heat in the process.-Uses:As stated above, an...
via capillary action
Capillary action
Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...
. A wick is not needed in a thermosiphon because gravity moves the liquid http://cipco.apogee.net/ces/library/twhtherm.asp. The wick allows heat pipes to transfer heat when there is no gravity, which is useful in space. A thermosiphon is "simpler" than a heat pipe http://www.cheresources.com/htpipes.shtml.
(Single-phase) thermosiphons can only transfer heat "upward", or away from the acceleration vector. Thus, orientation is much more important for thermosiphons than for heatpipes. Also, thermosiphons can fail because of a bubble in the loop, and require a circulating loop of pipes.
Reboilers and Calandria
If the piping of a thermosiphon resists flow, or excessive heat is applied, the liquid may boil. Since the gas is more buoyantBuoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
than the liquid, the convective pressure is greater. This is a well known invention called a reboiler
Reboiler
Reboilers are heat exchangers typically used to provide heat to the bottom of industrial distillation columns. They boil the liquid from the bottom of a distillation column to generate vapors which are returned to the column to drive the distillation separation....
. A group of reboilers attached to a pair of plena
Plenum chamber
A plenum chamber is a pressurised housing containing a gas or fluid at positive pressure . One function of the plenum can be to equalise pressure for more even distribution, because of irregular supply or demand...
is called a calandria
Calandria
A calandria is the reactor core of the CANDU reactor. The calandria contains heavy water, a moderator used to moderate neutrons to achieve nuclear fission....
.
The term "phase change thermosiphon" is a misnomer and should be avoided.
When phase change occurs in a thermosiphon, it means that the system either does not have enough fluid, or it is too small to transfer all of the heat by convection alone. To improve the performance, either more fluid is needed (possibly in a larger thermosiphon), or all other fluids (including air) should be pumped out of the loop.
Solar energy
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. The water is heated passively by solar energy and relies on heat energy being transferred from the sun to a solar collector
Solar collector
-See also:*Solar thermal collector*Solar water heating*Solar air heating*Photovoltaic module*Renewable heat*Concentrating solar power...
. The heat from the collector can be transferred to water in two ways: directly where water circulates through the collector, or indirectly where an anti-freeze solution carries the heat from the collector and transfers it to water in the tank via a heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
. Convection allows for the movement of the heated liquid out of the solar collector
Solar collector
-See also:*Solar thermal collector*Solar water heating*Solar air heating*Photovoltaic module*Renewable heat*Concentrating solar power...
to be replaced by colder liquid which is in turn heated. Due to this principle, it is necessary for the water to be stored in a tank above the collector.
Computing
Thermosiphons are used in computing to describe a system for watercoolingWatercooling
Water cooling is a method of heat removal from components and industrial equipment. As opposed to air cooling, water is used as the heat conductor. Water cooling is commonly used for cooling automobile internal combustion engines and large industrial facilities such as steam electric power plants,...
the internal computer components, most commonly referring to the processor. While any suitable liquid can be used, water is the easiest liquid to use in thermosiphon systems. Unlike traditional watercooling
Watercooling
Water cooling is a method of heat removal from components and industrial equipment. As opposed to air cooling, water is used as the heat conductor. Water cooling is commonly used for cooling automobile internal combustion engines and large industrial facilities such as steam electric power plants,...
systems, thermosiphon systems do not rely on a water pump (or a pump for other liquids) but rely on convection for the movement of heated water (which may become vapour) from the components upwards to a heat exchanger. There the water is cooled and is ready to be recirculated. The most commonly used heat exchanger is a radiator
Radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in automobiles, buildings, and electronics...
where air is blown actively through a fan system to condense
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
the vapour to a liquid. The liquid is recirculated through the system, thus repeating the process. No pump is required - the vaporization and condensation cycle is self sustaining.
Uses
Modern processors get relatively hot. Even with a common heat sink and fan cooling the processor, operating temperatures may still reach up to 70 °C (160 °F). A thermosiphon can handle heat output at a much wider temperature range than any heat sink and fan, and can maintain the processor 10–20 °C cooler. In some cases a thermosiphon may also be less bulky than a normal heat sink and fan.Drawbacks
Thermosiphons must be mounted such that vapor rises up and liquid flows down to the boiler with no bends in the tubing for liquid to pool. Also, the thermosiphon’s fan that cools the gas needs cool air to operate.Engine cooling
Early cars and motor vehicles used thermosyphon circulation to move cooling water between their cylinder blockCylinder block
A cylinder block is an integrated structure comprising the cylinder of a reciprocating engine and often some or all of their associated surrounding structures...
and radiator
Radiator (engine cooling)
Radiators are used for cooling internal combustion engines, mainly in automobiles but also in piston-engined aircraft, railway locomotives, motorcycles, stationary generating plant or any similar use of such an engine....
. As engine power increased, increased flow was required and so engine-driven pumps were added to assist circulation. More compact engines then used smaller radiators and required more convoluted flow patterns, so the circulation became entirely dependent on the pump and might even be reversed against the natural circulation. An engine cooled only by thermosiphon is also very sensitive to low coolant level i.e. missing only a small amount of coolant stops the circulation, a pump driven system is much more robust and can handle low coolant level.
Ground cooling
Heat pipes are used at locations in higher latitudes like northern Alaska and Canada to prevent ice-rich permafrostPermafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
from melting below buildings and other infrastructure such as schools, air hangars, community water tanks, and even some stretches of highway. Heat pipes are also a common feature along the length of the Trans-Alaska Pipeline System
Trans-Alaska Pipeline System
The Trans Alaska Pipeline System , includes the Trans Alaska Pipeline, 11 pump stations, several hundred miles of feeder pipelines, and the Valdez Marine Terminal. TAPS is one of the world's largest pipeline systems...
. In these applications the solution in the pipes is often carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
or ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
. At the bottom of the heat pipe, heat from the ground warms the liquid and converts it to a vapor. Cooling from the heat sink fins above ground releases this heat to the atmosphere and causes the vapor to condense on the outer pipe wall, which then drains back into the liquid pool at the bottom of the heat pump.
See also
- Thermic siphon
- ConvectionConvectionConvection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
- Geothermal heat pump
- Heat pipeHeat pipeA heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces....
and Loop heat pipeLoop heat pipeA loop heat pipe is a two-phase heat transfer device that uses capillary action to remove heat from a source and passively move it to a condenser or radiator. LHPs are similar to heat pipes but have the advantage of being able to provide reliable operation over long distance and the ability to... - Passive solar
- ReboilerReboilerReboilers are heat exchangers typically used to provide heat to the bottom of industrial distillation columns. They boil the liquid from the bottom of a distillation column to generate vapors which are returned to the column to drive the distillation separation....
- SiphonSiphonThe word siphon is sometimes used to refer to a wide variety of devices that involve the flow of liquids through tubes. But in the English language today, the word siphon usually refers to a tube in an inverted U shape which causes a liquid to flow uphill, above the surface of the reservoir,...
- Solar heating
- Vapor-compression refrigerationVapor-compression refrigerationVapor-compression refrigeration is one of the many refrigeration cycles available for use. It has been and is the most widely used method for air-conditioning of large public buildings, offices, private residences, hotels, hospitals, theaters, restaurants and automobiles...
- WatercoolingWatercoolingWater cooling is a method of heat removal from components and industrial equipment. As opposed to air cooling, water is used as the heat conductor. Water cooling is commonly used for cooling automobile internal combustion engines and large industrial facilities such as steam electric power plants,...