Thermosiphon (or thermosyphon) is a method of passive heat exchange, based on natural

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...

, which circulates a

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...

without the necessity of a mechanical pump. Thermosiphoning is used for circulation of liquids and volatile gases in heating and cooling applications such as heat pumps, water heaters, boilers and furnaces. Thermosiphoning also occurs across air temperature gradients such as those utilized in a wood fire chimney or solar chimney. This circulation can either be open-loop, as when the substance 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 container. Its purpose is to simplify the transfer of liquid or gas while avoiding the cost and complexity of a conventional pump.

Simple thermosiphon

Natural convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

of the liquid starts when

heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy ( heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conducti ...

to the liquid gives rise to a temperature difference from one side of the loop to the other. The phenomenon of

thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kin ...

means that a temperature difference will have a corresponding difference in density across the loop. The warmer fluid on one side of the loop is less dense and thus more buoyant than the cooler fluid on the other side. The warmer fluid will "float" above the cooler fluid, and the cooler fluid will "sink" below the warmer fluid. This phenomenon of natural convection is known by the saying "heat rises". Convection moves the heated liquid upwards in the system as it is simultaneously replaced by cooler liquid returning by gravity. A good thermosiphon has very little

hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counte ...

resistance so that liquid can flow easily under the relatively low pressure produced by natural convection.

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 usual "thermosiphon". Heat can still be transferred in this system by the

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when h ...

and

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapo ...

of vapor; however, the system is properly classified as a

heat pipe A heat pipe is a heat-transfer device that employs phase transition to transfer heat between two solid interfaces. At the hot interface of a heat pipe, a volatile liquid in contact with a thermally conductive solid surface turns into a vapor b ...

thermosyphon. 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 contains only a single substance. The thermosiphon has been sometimes incorrectly described as a 'gravity return

'. Heat pipes usually have a wick to return the condensate to the

evaporator An evaporator is a device used to turn the liquid form of a chemical substance, such as water, into a vapor. Uses Air conditioning and refrigeration Some air conditioners and refrigerators use a compressed liquid with a low boiling point, su ...

via

capillary action Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...

. A wick is not needed in a thermosiphon because gravity moves the liquid. 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. (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 buoyant than the liquid, the convective pressure is greater. This is a well known invention called a

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 distillatio ...

. A group of reboilers attached to a pair of

plena Plena is a genre of music and dance native to Puerto Rico. Origins The plena genre originated in Barrio San Antón, Ponce, Puerto Rico, around 1900. It was influenced by the bomba style of music. Originally, sung texts were not associated wit ...

is called a calandria. In some circumstances, for example the cooling system for an older (pre 1950s) car, the boiling of the fluid will cause the system to stop working, as the volume of steam created displaces too much of the water and circulation stops. 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

Thermosiphons are used in some liquid-based

solar heating A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating, but may refer to large power generating installations such as solar parabolic troughs and sola ...

systems to heat a liquid such as

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

. The water is heated passively by

solar energy Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture. It is an essen ...

and relies on

heat energy In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...

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 A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct conta ...

. 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

Architecture

In locations historically dominated by permafrost conditions, thermosiphons may be used to counter adverse geologic forces on the foundations of buildings, pipelines and other structures caused by the thawing of the permafrost. A study published in 2006 by oil giant

ConocoPhillips ConocoPhillips Company is an American multinational corporation engaged in hydrocarbon exploration and production. It is based in the Energy Corridor district of Houston, Texas. The company has operations in 15 countries and has production in ...

reports that Alaska's permafrost, upon which much of the state's infrastructure is built, has degraded since 1982 amid record warm temperatures. According to the Alaska Climate Research Center at the

University of Alaska Fairbanks The University of Alaska Fairbanks (UAF or Alaska) is a public land-grant research university in College, Alaska, a suburb of Fairbanks. It is the flagship campus of the University of Alaska system. UAF was established in 1917 and opened for c ...

, between 1949 and 2018 the average annual temperature in Alaska rose 4.0 degrees Fahrenheit, with an increase of 7.2 degrees Fahrenheit over the winter.

Computing

Thermosiphons are used for watercooling internal computer components, most commonly the

processor Processor may refer to: Computing Hardware * Processor (computing) **Central processing unit (CPU), the hardware within a computer that executes a program *** Microprocessor, a central processing unit contained on a single integrated circuit (I ...

. 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 pump but 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 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 cars, buildings, and electronics. A radiator is always ...

, 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 cycle of evaporation and condensation is driven by the difference in temperature.

Uses

Without proper cooling, a modern processor chip can rapidly reach temperatures that cause it to malfunction. Even with a common heat sink and fan attached, typical processor operating temperatures may still reach up to 70 °C (160 °F). A thermosiphon can efficiently transfer heat over a much wider temperature range and can typically maintain the processor temperature 10–20 °C cooler than a traditional heat sink and fan. In some cases, it is also possible that a thermosiphon may cover multiple heat sources and, design-wise, be more compact than an appropriately sized conventional 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. The system has to be completely airtight; if not, the process of thermosiphon will not take effect and cause the water to only evaporate over a small period of time.

Engine cooling

Thermo-syphon cooling circulation (Manual of Driving and Maintenance)

Early cars, motor vehicles and engine-powered farm and industrial equipment used thermosiphon circulation to move cooling water between their

cylinder block In an internal combustion engine, the engine block is the structure which contains the cylinders and other components. In an early automotive engine, the engine block consisted of just the cylinder block, to which a separate crankcase was attac ...

and

. They depended on forward movement of the car and fans to move enough air through the radiator to provide the temperature differential that caused the thermosiphon circulation. 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 susceptible to overheating during prolonged periods of idling or very slow travel when airflow through the radiator is limited, unless one or more fans are able to move enough air to provide adequate cooling. They are also very sensitive to low coolant level, i.e., losing only a small amount of coolant stops the circulation; a pump driven system is much more robust and can typically handle a lower coolant level.

References

External links

HP Labs report on thermosiphons for computer cooling (PDF)
{{HVAC Computer hardware cooling Heating, ventilation, and air conditioning Convection