Thermosiphon

Thermosiphon (alt. thermosyphon) refers to a method of passive heat exchange based on natural convection which circulates liquid in a vertical closed-loop circuit without requiring a conventional pump. 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.

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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 buoyant 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. In many cases the liquid flows easily because the thermosiphon is designed to have very little hydraulic resistance.

Phase change thermosiphon

In other cases when the loop has more resistance to flow, the liquid may be heated beyond its boiling point, causing a phase change as the liquid evaporates to a gas (such as steam). Since the gas is much more buoyant than the hot liquid, the convective pressure is increased considerably. This is known as a heat pipe thermosiphon. It allows the cooling and heating of objects by changing the phase of a liquid inside a closed system, and operates on the principles of buoyancy to move the fluid through the system.

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, then the system no longer operates on convection principles, so it is no longer a simple "thermosiphon". 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.

A thermosiphon reboiler is called a calandria.

The thermosiphon has been sometimes incorrectly described as a 'gravity return heat pipe' ^[1]. A wick is usually a necessary feature of a heat pipe to allow the return of condensate to the evaporator via capillary action, whereas this function is not needed in a thermosiphon as gravity allows the movement of the liquids ^[2]. The wick allows heat pipes to conduct heat exchange against gravity (so leading to heat pipe use in space applications); against this a thermosiphon can be seen as a 'simpler' heat pipe ^[3]. It follows that correct orientation is much more important for thermosiphons than for heatpipes.

Solar energy

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.

Computing

Thermosiphons are used in computing to describe a system for watercooling 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.

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 (158°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 thermosiphons' fan that cools the gas needs cool air to operate.

Ground cooling

Heat pipes are used at locations in higher latitudes like northern Alaska and Canada to prevent ice-rich permafrost 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.

See also

• Convection
• Heat pipe and Loop heat pipe
• Reboiler
• Vapor-compression refrigeration
• Siphon
• Solar heating
• Passive solar
• Watercooling

References

• Solar company - definition of thermosiphon
• Definition of thermosiphon
• HP Labs report on thermosiphons for computer cooling (PDF)
• Solar water heating in Hawaii
• Overclockers.com guide to a prototype thermosiphon
• Fortune Magazine 2006 06 12 "Next Stop, Lhasa"