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Heat pipe
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== Structure, design and construction == {{Main|Vapor-compression refrigeration}} [[file:Heat Pipe Mechanism.svg|thumb|upright=1.4|alt=Longitudinal cross-section of a heat pipe. It is closed at both ends. The 'wick' coats the inside surface, while the inner cavity is filled with vapour. The diagram illustrates heat transfer: 1. (left end of the pipe) working fluid evaporates to vapour absorbing thermal energy; 2. vapour migrates along cavity to lower temperature end; 3. vapour condenses back to fluid and is absorbed by the wick, releasing thermal energy; 4. working fluid flows back to the lower temperature end.|Diagram showing components and mechanism for a heat pipe containing a wick]] [[file:Ekati Diamond Mine.jpg|thumb|upright=1.4|alt=A worker in high visibility clothing and a hard hat examines a long line of pipes about four times his height sticking out of rocky ground.|[[Heat pipes]] keep ground frozen and inhibit water transfer into the open pit during mining activities at [[Ekati Diamond Mine]]]] [[file:CFD IsoSkin Heat Pipe.gif|thumb|This 100 mm by 100 mm by 10 mm high thin flat heat pipe (heat spreader) animation was created using high resolution CFD analysis and shows temperature contoured flow trajectories, predicted using a [[Computational fluid dynamics|CFD]] analysis package.]] [[file:CFD Vapor Chamber Heat Sink Design v1.gif|thumb|This 120 mm diameter vapor chamber (heat spreader) heat sink design thermal animation was created using high-resolution CFD analysis and shows temperature contoured heat sink surface and fluid flow trajectories predicted using a [[Computational fluid dynamics|CFD]] analysis package.]] [[file:Laptop CPU Heat Pipe Cross Section.jpg|thumb|Cross section of a heat pipe for cooling the CPU of a laptop computer. Ruler scale is in millimetres.]] [[file:IsoSkin.png|thumb|Cut-away view of a 500 μm thick flat heat pipe with a thin planar capillary (aqua coloured)]] [[file:IsoSkin2.png|thumb|Thin flat heat pipe (heat spreader) with remote heat sink and fan]] A typical heat pipe consists of an envelope (sealed pipe), a wick, and a working fluid . The envelope is made of a material that is compatible with the working fluid such as [[copper]] for water heat pipes, or [[aluminium|aluminum]] for [[ammonia]] heat pipes. Typically, a [[vacuum pump]] removes the air from the pipe, which is partially filled with a working fluid and then sealed. The working fluid mass is chosen so that the heat pipe contains both vapor and liquid over the [[operating temperature]] range.<ref name="Faghri_2016">{{cite book |last=Faghri |first=A. |year=2016 |url=https://books.google.com/books?id=5n2kAQAACAAJ |title=Heat Pipe Science and Technology |edition=2nd |publisher=Global Digital Press|isbn=978-0-9842760-1-1 }}</ref> The operating temperature of a given heat pipe system is critically important. Below the operating temperature, the liquid is too cold and cannot vaporize into a gas. Above the operating temperature, all the liquid has turned to gas, and the environmental temperature is too high for the gas to condense. [[Thermal conduction]] is still possible through the walls, but at a greatly reduced rate of thermal transfer. In addition, for a given heat input, a minimum working fluid temperature must be attained; while at the other end, any additional increase (deviation) in the heat transfer coefficient from the initial design tends to inhibit the heat pipe action. This can be counterintuitive, in the sense that if a heat pipe system is aided by a fan, the heat pipe operation may potentially be severely reduced. The operating temperature and the maximum heat transport capacity—limited by its capillary or other structure used to return the fluid to the hot area (centrifugal force, gravity, etc.)—are closely related.<ref>{{Cite journal|last1=Praful|first1=S.|last2=Prajwal Rao|first2=V.|last3=Vijeth|first3=V.|last4=Bhagavath|first4=Skanda V.|last5=Seetharamu|first5=K. N.|last6=Narasimha Rao|first6=R.|date=2020|title=On the operating temperature of heat pipes|journal=Journal of Physics: Conference Series|volume=1473|issue=1|page=012025|doi=10.1088/1742-6596/1473/1/012025|bibcode=2020JPhCS1473a2025P|issn=1742-6588|doi-access=free}}</ref> Working fluids are chosen according to the required operating temperatures, with examples ranging from [[liquid helium]] for extremely low temperature applications (2–4 [[Kelvin|K]]) to [[Mercury (element)|mercury]] (523–923 K), [[sodium]] (873–1473 K) and even [[indium]] (2000–3000 K) for extremely high temperatures. The vast majority of heat pipes for room temperature applications use [[ammonia]] (213–373 K), [[Alcohol (chemistry)|alcohol]] ([[methanol]] (283–403 K), [[ethanol]] (273–403 K)), or [[water]] (298–573 K). Copper/water heat pipes have a copper envelope, use water as the working fluid and typically operate from {{cvt|20|to(-)|150|C|K}}.<ref>{{cite web |date=May 6, 2013 |url=http://www.ecnmag.com/news/2013/05/improving-materials-convert-heat-electricity-and-vice-versa |title=Improving materials that convert heat to electricity and vice-versa |work=Electronic Component News |access-date=2013-05-07 |archive-url=https://web.archive.org/web/20130728092902/http://www.ecnmag.com/news/2013/05/improving-materials-convert-heat-electricity-and-vice-versa |archive-date=July 28, 2013 }}</ref><ref name="google1"/> Water heat pipes are sometimes partially filled with water, heated until the water boils and displaces the air, and then sealed while hot. The heat pipe must contain [[Vapor–liquid equilibrium|saturated]] liquid and its vapor (gas phase). The saturated liquid vaporizes and travels to the condenser, where it is cooled and condensed. The liquid returns to the evaporator via the wick, which exerts [[capillary action]] on the liquid. Wick structures include [[sintered]] [[Powder metallurgy|metal powder]], screen, and grooved wicks, which have a series of grooves parallel to the pipe axis. When the condenser is located above the evaporator in a gravitational field, gravity can return the liquid. In this case, the pipe is a [[thermosiphon]]. Rotating heat pipes use centrifugal forces to return liquid from the condenser to the evaporator.<ref name="Faghri_2016" /> Heat pipes contain no moving parts and typically require no maintenance, though non-condensable gases that diffuse through the pipe's walls, that result from breakdown of the working fluid, or that exist as original impurities in the material, may eventually reduce the pipe's effectiveness.<ref name="Faghri_2016" /> The heat pipe advantage over many other heat-dissipation mechanisms is their efficiency in transferring heat. A pipe one inch in diameter and two feet long can transfer {{cvt|3.7|kW|BTU/hour}} at {{convert|1800|F|C}} with only {{convert|18|F-change|0}} drop from end to end.<ref name="google1">{{cite magazine|url=https://books.google.com/books?id=iFzpLpXjYdkC&q=Heat+pipe&pg=PA102 |pages=102–03 |first=Ed |last=Edelson |title=Heat pipes – new ways to transfer energy |magazine=Popular Science |volume=204 |issue=6 |issn=0161-7370 |publisher=Bonnier |via=Google Books |date= June 1974|access-date=2013-05-07}}</ref> Some heat pipes have demonstrated a [[heat flux]] of more than 23 kW/cm<sup>2</sup>, about four times the that of the Sun's surface.<ref>{{cite press release |first=Jim |last=Danneskiold |url=http://www.lanl.gov/news/releases/archive/00-064.shtml |title=Los Alamos-developed heat pipes ease space flight |agency=[[Los Alamos National Laboratory]] |date=April 26, 2000 |archive-url=https://web.archive.org/web/20050320172533/http://www.lanl.gov/news/releases/archive/00-064.shtml |archive-date=2005-03-20}}</ref> Some envelope /working fluid pairs that appear to be compatible are not. For example, water in an aluminum envelope develops significant amounts of non-condensable gas within hours or days. This issue is primarily due to the oxidation and corrosion of aluminum in the presence of water, which releases non-condensable hydrogen gas.<ref>{{Cite journal |last=Zohuri |first=Bahman |date=January 2021 |title=Heat Pipe as a Passive Cooling System Driving New Generation of Nuclear Power Plants |url=https://www.researchgate.net/publication/348279721 |journal=Edelweiss Chemical Science Journal |volume=3 |issue=1 |pages=32 |via=ResearchGate}}</ref> In an endurance test, pipes are operated for long intervals and monitored for problems such as non-condensable gas generation, material transport, and corrosion.<ref>[http://www.1-act.com/advanced-technologies/heat-pipes/heat-pipe-life-tests/ Life Tests] {{webarchive|url=https://web.archive.org/web/20141103195037/http://www.1-act.com/advanced-technologies/heat-pipes/heat-pipe-life-tests/ |date=2014-11-03 }}. Advanced Cooling Technologies.</ref><ref>{{cite web|url=http://www.1-act.com/examples-of-incompatible-fluidenvelope-pairs/|title=Incompatible Heat Pipe Fluid/Envelope Pairs|website=www.1-act.com|access-date=2014-11-03|archive-date=2018-07-08|archive-url=https://web.archive.org/web/20180708103837/https://www.1-act.com/examples-of-incompatible-fluidenvelope-pairs/}}. Advanced Cooling Technologies.</ref> The most commonly used envelope/wick/fluid combinations include:<ref>{{cite web |url=http://www.1-act.com/heat-pipe-materials-working-fluids-and-compatibility/ |title=Heat Pipe Materials, Working Fluids, and Compatibility |publisher=Advanced Cooling Technologies |access-date=2014-11-03 |archive-date=2016-04-22 |archive-url=https://web.archive.org/web/20160422072523/http://www.1-act.com/heat-pipe-materials-working-fluids-and-compatibility/ |url-status=dead}}</ref> * Copper envelope/water fluid for [[Thermal management of electronic devices and systems|electronics cooling]]. This is by far the most common type. * Copper or steel envelope with refrigerant [[1,1,1,2-Tetrafluoroethane|R134a]] fluid in [[HVAC]] systems. * Aluminum envelope with ammonia fluid for [[spacecraft thermal control]]. * [[Superalloy]] envelope with [[alkali metal]] ([[cesium]], [[potassium]], [[sodium]]) fluid for high temperature applications, most commonly for calibrating primary temperature measurement devices. Other combinations include stainless steel envelopes with nitrogen, oxygen, neon, hydrogen, or helium working fluids at temperatures below 100 K, copper/methanol for electronics cooling when the heat pipe must operate below the water range, aluminum/[[ethane]] heat pipes for spacecraft thermal control in environments when ammonia can freeze, and [[Refractory metals|refractory metal]] envelope/lithium fluid for high temperature (above {{convert|1050|C|K F}}) applications.<ref>{{cite web|url=http://www.1-act.com/compatible-fluids-and-materials/|title=Compatible Heat Pipe Fluids and Materials – Heat Pipe Technology|publisher=Advanced Cooling Technologies|access-date=2014-11-03|archive-date=2019-03-28|archive-url=https://web.archive.org/web/20190328192200/https://www.1-act.com/compatible-fluids-and-materials/}}</ref> Heat pipes must be tuned to particular cooling conditions. The choice of pipe material, size, and coolant all affect the optimal temperature. Outside of its design heat range, [[thermal conductivity]] is reduced to the [[heat conduction]] properties of its envelope. For [[copper]], that is around 1/80 of the design flux. This is because below the range, the working fluid never vaporizes, and above the range it never condenses. Few manufacturers can make a traditional heat pipe smaller than 3 mm in diameter due to material limitations.<ref>{{Cite web|url=https://www.enertron-inc.com/things-to-consider-when-bending-or-flattening-a-heat-pipe/|title=Things to Consider When Bending or Flattening A Heat Pipe |publisher=Enertron|language=en-US|access-date=2019-04-22|archive-date=2019-04-22|archive-url=https://web.archive.org/web/20190422203306/https://www.enertron-inc.com/things-to-consider-when-bending-or-flattening-a-heat-pipe/|url-status=dead}}</ref> Heat pipes containing graphene have been demonstrated can improve cooling performance in electronics.<ref>{{cite journal | url=https://onlinelibrary.wiley.com/doi/10.1002/nano.202000195 | doi=10.1002/nano.202000195 | title=A lightweight and high thermal performance graphene heat pipe | date=2021 | last1=Liu | first1=Ya | last2=Chen | first2=Shujing | last3=Fu | first3=Yifeng | last4=Wang | first4=Nan | last5=Mencarelli | first5=Davide | last6=Pierantoni | first6=Luca | last7=Lu | first7=Hongbin | last8=Liu | first8=Johan | journal=Nano Select | volume=2 | issue=2 | pages=364–372 | arxiv=2002.11336 }}</ref>
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