Thermosiphon 2700377 209522824 2008-05-01T19:16:50Z Oldengine 6365341 /* References */ [[Image:Solar heater dsc00632.jpg|thumb|Solar heating system featuring a thermosiphon]] [[Image:Thermosiphon2.png|thumb|Warm water supply system with thermosiphon (schematically):<br />1: water tab<br />2: isolated container<br />3: warm water inlet<br />4: solar thermal collector<br />5: fresh water supply<br />]] '''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. ==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 [[buoyancy|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 ([[heat pipe]]) thermosiphon== ===Heat Pipe=== {{main|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. ==="''[[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 (matter)|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]]. ===A thermosiphon is not a heat pipe=== The thermosiphon has been sometimes incorrectly described as a 'gravity return [[heat pipe]]' <sup>[http://www.btfsolar.com/specifications.htm]</sup>. A wick is usually a necessary feature of a heat pipe to allow the return of [[Condensation|condensate]] to the [[evaporator]] via [[capillary action]], whereas this function is not needed in a thermosiphon as gravity allows the movement of the liquids <sup>[http://cipco.apogee.net/ces/library/twhtherm.asp]</sup>. 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 <sup>[http://www.cheresources.com/htpipes.shtml]</sup>. (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. ==Solar energy== Thermosiphons are used in some liquid-based [[solar heating]] systems to heat a liquid such as [[water]]. The water is heated [[passive]]ly 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 [[CPU|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 [[condensation|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 (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. ==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== *[http://www.btfsolar.com/specifications.htm Solar company - definition of thermosiphon] *[http://cipco.apogee.net/ces/library/twhtherm.asp Definition of thermosiphon] *[http://www.hpl.hp.com/research/papers/2002/thermosyphon.pdf HP Labs report on thermosiphons for computer cooling (PDF)] *[http://www.hawaiiislandsolar.org/003.html Solar water heating in Hawaii] *[http://www.overclockers.com/articles1246/ Overclockers.com guide to a prototype thermosiphon] *[http://www.smokstak.com/forum/showthread.php?t=8737 Smokstak.com antique engine thermosyphon cooling] *[http://money.cnn.com/magazines/fortune/fortune_archive/2006/06/12/8379261/index.htm Fortune Magazine 2006 06 12 "Next Stop, Lhasa"] [[Category:Computer hardware cooling]] [[Category:HVAC]] [[Category:Convection]] [[de:Thermosiphonanlage]] [[es:Termosifón]] [[fr:Thermosiphon]]