Thermosiphon

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Solar heating system featuring a thermosiphon

Warm water supply system with thermosiphon (schematically):
1: water tab
2: isolated container
3: warm water inlet
4: solar thermal collector
5: fresh water supply

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.

1 Simple thermosiphon
2 Phase-change (heat pipe) thermosiphon
3 Solar energy
4 Computing
- 4.1 Uses
- 4.2 Drawbacks
5 Ground cooling
6 See also
7 References

[edit] 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.

[edit] Phase-change (heat pipe) thermosiphon

[edit] Heat Pipe

Main article: heat pipe

A heat pipe contains a phase change fluid, which transfers heat by evaporation and condensation. A heat pipe does not use a siphon nor does it rely on convection. This makes a heat pipe distinct from a thermosiphon. 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 not big enough to transfer all of the heat that is being applied to it using convection alone. To improve the performance in such a situation, either more fluid should be added (possibly in a larger thermosiphon), or all other fluids (including air) should be pumped out of the loop, in order to create a true heat pipe. In the case of the heat pipe, less fluid is needed.

[edit] "heat pipe thermosiphon"

(Note: This term should be used with care, since the terms "thermosiphon" and "heat pipe" refer to very different devices.)

In cases when the thermosiphon fluid incurs excessive resistance to flow, or excessive heat is applied, the liquid may be heated beyond its boiling point (assuming it is a liquid that boils), thus causing a phase change as the liquid evaporates to a gas (vapor) (such as steam). Since the gas is much less dense than the hot liquid, and thus much more buoyant, the convective pressure is increased considerably. This is may be referred to as a "heat pipe thermosiphon". In addition to thermosiphon convection, heat transfer is somewhat increased by the phase change of a fluid inside a closed system. It 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. 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.

A thermosiphon reboiler is also called a calandria.

[edit] A thermosiphon is not a heat pipe

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 transfer heat in the absence of gravity, which is useful for space applications. A thermosiphon is, in a sense, "simpler" than a heat pipe ^[3]. (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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] See also

[edit] References