Heat pipe 1

Heat pipe

A laptop heat pipe system

A heat pipe or heat pin is a heat-transfer device thatcombines the principles of both thermal conductivityand phase transition to efficiently manage the transferof heat between two solid interfaces. The idea of heatpipes was first suggested by R.S.Gaugler in 1942.However, it was not until 1962, when G.M.Groverinvented it, that its remarkable properties wereappreciated and serious development began.

At the hot interface within a heat pipe, which istypically at a very low pressure, a liquid in contact witha thermally conductive solid surface turns into a vaporby absorbing heat from that surface. The vapor thentravels along the heat pipe to the cold interface, condenses back into a liquid, releasing the latent heat. The liquidthen returns to the hot interface through either capillary action or gravity action where it evaporates once more andrepeats the cycle. In addition, the internal pressure of the heat pipe can be set or adjusted to facilitate the phasechange depending on the demands of the working conditions of the thermally managed system.

Structure, design and construction

Diagram showing components and mechanism for a heat pipecontaining a wick

A typical heat pipe consists of a sealed pipe or tubemade of a material with high thermal conductivity suchas copper (see: Copper in heat exchangers) oraluminium at both hot and cold ends. A vacuum pumpis used to remove all air from the empty heat pipe, andthen the pipe is filled with a fraction of a percent byvolume of working fluid (or coolant) chosen to matchthe operating temperature. Alternatively, the pipe isheated until the fluid boils, and sealed while hot.Examples of such fluids include water, ethanol,acetone, sodium, or mercury. Due to the partial vacuumthat is near or below the vapor pressure of the fluid,some of the fluid will be in the liquid phase and somewill be in the gas phase. The use of a vacuumeliminates the need for the working gas to diffuse through any other gas and so the bulk transfer of the vapor to thecold end of

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This 100mm by 100mm by 10mm high thin flat heat pipe (heat spreader) animation, wascreated using high resolution CFD analysis, and shows temperature contoured flow

trajectories, predicted using a CFD analysis package, courtesy of NCI [1].

Cross section of a heat pipe for cooling the CPU of a laptopcomputer. Ruler scale is in millimeters.

Cut-away view of a 500 µm thick flat heat pipe, with a thin planarcapillary (aqua colored)

the heat pipe is at the speed of themoving molecules. In this sense, theonly practical limit to the rate of heattransfer is the speed with which the gascan be condensed to a liquid at the coldend.[2]

Inside the pipe's walls, an optional wickstructure exerts a capillary pressure onthe liquid phase of the working fluid.This is typically a sintered metalpowder or a series of grooves parallel tothe pipe axis, but it may be any materialcapable of exerting capillary pressureon the condensed liquid to wick it backto the heated end. The heat pipe maynot need a wick structure if gravity orsome other source of acceleration issufficient to overcome surface tensionand cause the condensed liquid to flowback to the heated end.[citation needed]

A heat pipe is not a thermosiphon,because there is no siphon.Thermosiphons transfer heat bysingle-phase convection. (See also:Perkins tube, after Jacob Perkins.)

Heat pipes contain no mechanicalmoving parts and typically require nomaintenance, though non-condensinggases (that diffuse through the pipe'swalls, result from breakdown of theworking fluid, or exist as impurities inthe materials) may eventually reducethe pipe's effectiveness at transferringheat. This is significant when theworking fluid's vapour pressure islow.[citation needed]

The materials chosen depend on thetemperature conditions in which theheat pipe must operate, with coolantsranging from liquid helium forextremely low temperature applications(2–4 K) to mercury (523–923 K) &sodium (873–1473 K) and even indium

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Thin flat heat pipe (heat spreader) with remote heat sink and fan

(2000–3000 K) for extremely high temperatures. Thevast majority of heat pipes for low temperatureapplications use some combination of ammonia(213–373 K), alcohol (methanol (283–403 K) orethanol (273–403 K)) or water (303–473 K) as workingfluid. Water, for instance, at low pressure will boil atjust above 273 K (0 degrees Celsius) and so can start toeffectively transfer latent heat at this low temperature.

The advantage of heat pipes over many otherheat-dissipation mechanisms is their great efficiency intransferring heat. They are fundamentally better at heatconduction over a distance than an equivalentcross-section of solid copper (a heat sink alone, thoughsimpler in design and construction, does not takeadvantage of the principle of matter phase transition). Some heat pipes have demonstrated a heat flux of more than230 MW/m².[3]

Active control of heat flux can be effected by adding a variable volume liquid reservoir to the evaporator section.Variable conductance heat pipes employ a large reservoir of inert immiscible gas attached to the condensing section.Varying the gas reservoir pressure changes the volume of gas charged to the condenser which in turn limits the areaavailable for vapor condensation. Thus a wider range of heat fluxes and temperature gradients can be accommodatedwith a single design.

A modified heat pipe with a reservoir having no capillary connection to the heat pipe wick at the evaporator end canalso be used as a thermal diode. This heat pipe will transfer heat in one direction, acting as an insulator in theother.[citation needed]

Vapor Chamber or Flat heat pipesThin planar heat pipes (heat spreaders) have the same primary components as tubular heat pipes. These componentsare a hermetically sealed hollow vessel, a working fluid, and a closed-loop capillary recirculation system.Compared to a one-dimensional tubular heat pipe, the width of a two-dimensional heat pipe allows an adequate crosssection for heat flow even with a very thin device. These thin planar heat pipes are finding their way into “heightsensitive” applications, such as notebook computers, and surface mount circuit board cores. It is possible to produceflat heat pipes as thin as 0.5 mm (thinner than a credit card).[citation needed]

Loop heat pipeA loop heat pipe (LHP) is a two-phase heat transfer device that uses capillary action to remove heat from a sourceand passively move it to a condenser or radiator. LHPs are similar to heat pipes but have the advantage of being ableto provide reliable operation over long distance and the ability to operate against gravity. They can transport a largeheat load over a long distance with a small temperature difference.[][4] Different designs of LHPs ranging frompowerful, large size LHPs to miniature LHPs (micro loop heat pipe) have been developed and successfully employedin a wide sphere of applications both ground based as well as space applications.

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Heat transfer

A heat sink (aluminium) with heat pipe (copper)

Heat pipes employ evaporative cooling to transfer thermal energy fromone point to another by the evaporation and condensation of a workingfluid or coolant. Heat pipes rely on a temperature difference betweenthe ends of the pipe, and cannot lower temperatures at either endbeyond the ambient temperature (hence they tend to equalise thetemperature within the pipe).

When one end of the heat pipe is heated the working fluid inside thepipe at that end evaporates and increases the vapour pressure inside thecavity of the heat pipe. The latent heat of evaporation absorbed by thevaporisation of the working fluid reduces the temperature at the hotend of the pipe.

The vapour pressure over the hot liquid working fluid at the hot end ofthe pipe is higher than the equilibrium vapour pressure overcondensing working fluid at the cooler end of the pipe, and thispressure difference drives a rapid mass transfer to the condensing end where the excess vapour condenses, releasesits latent heat, and warms the cool end of the pipe. Non-condensing gases (caused by contamination for instance) inthe vapour impede the gas flow and reduce the effectiveness of the heat pipe, particularly at low temperatures, wherevapour pressures are low. The speed of molecules in a gas is approximately the speed of sound, and in the absence ofnoncondensing gases (i.e., if there is only a gas phase present) this is the upper limit to the velocity with which theycould travel in the heat pipe. In practice, the speed of the vapour through the heat pipe is limited by the rate ofcondensation at the cold end and far lower than the molecular speed.[citation needed]

The condensed working fluid then flows back to the hot end of the pipe. In the case of vertically-oriented heat pipesthe fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned bycapillary action.When making heat pipes, there is no need to create a vacuum in the pipe. One simply boils the working fluid in theheat pipe until the resulting vapour has purged the non condensing gases from the pipe and then seals the end.An interesting property of heat pipes is the temperature over which they are effective. Initially, it might be suspectedthat a water charged heat pipe would only work when the hot end reached the boiling point (100 °C) and steam wastransferred to the cold end. However, the boiling point of water is dependent on absolute pressure inside the pipe. Inan evacuated pipe, water will boil just slightly above its melting point (0 °C). Thus the heat pipe can operate athot-end temperatures as low as just slightly warmer than the melting point of the working fluid. Similarly, a heatpipe with water as a working fluid can work well above the boiling point (100 °C), if the cold end is low enough intemperature to condense the fluid.[citation needed]

The main reason for the effectiveness of heat pipes is the evaporation and condensation of the working fluid. Theheat of vaporization greatly exceeds the sensible heat capacity. Using water as an example, the energy needed toevaporate one gram of water is 540 times the amount of energy needed to raise the temperature of that same onegram of water by 1 °C. Almost all of that energy is rapidly transferred to the "cold" end when the fluid condensesthere, making a very effective heat transfer system with no moving parts.[citation needed]

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Origins and research in the United StatesThe general principle of heat pipes using gravity (commonly classified as two phase thermosiphons) dates back tothe steam age. The modern concept for a capillary driven heat pipe was first suggested by R.S. Gaugler of GeneralMotors in 1942 who patented the idea.[5] The benefits of employing capillary action were independently developedand first demonstrated by George Grover at Los Alamos National Laboratory in 1963 and subsequently published inthe Journal of Applied Physics in 1964.[6] Grover noted in his notebook:[7]

"Heat transfer via capillary movement of fluids. The "pumping" action of surface tension forces may besufficient to move liquids from a cold temperature zone to a high temperature zone (with subsequentreturn in vapor form using as the driving force, the difference in vapor pressure at the two temperatures)to be of interest in transferring heat from the hot to the cold zone. Such a closed system, requiring noexternal pumps, may be of particular interest in space reactors in moving heat from the reactor core to aradiating system. In the absence of gravity, the forces must only be such as to overcome the capillaryand the drag of the returning vapor through its channels."

Between 1964 and 1966, RCA was the first corporation to undertake research and development of heat pipes forcommercial applications (though their work was mostly funded by the US government). During the late 1960sNASA played a large role in heat pipe development by funding a significant amount of research on their applicationsand reliability in space flight following from Grover's suggestion. NASA’s attraction to heat pipe cooling systemswas understandable given their low weight, high heat flux, and zero power draw. Their primary interest however wasbased on the fact that the system wouldn’t be adversely affected by operating in a zero gravity environment. The firstapplication of heat pipes in the space program was in thermal equilibration of satellite transponders. As satellitesorbit, one side is exposed to the direct radiation of the sun while the opposite side is completely dark and exposed tothe deep cold of outer space. This causes severe discrepancies in the temperature (and thus reliability and accuracy)of the transponders. The heat pipe cooling system designed for this purpose managed the high heat fluxes anddemonstrated flawless operation with and without the influence of gravity. The developed cooling system was thefirst description and usage of variable conductance heat pipes to actively regulate heat flow or evaporatortemperature.

Corporate R&DPublications in 1967 and 1968 by Feldman, Eastman, & Katzoff first discussed applications of heat pipes to areasoutside of government concern and that did not fall under the high temperature classification such as: airconditioning, engine cooling, and electronics cooling. These papers also made the first mentions of flexible, arterial,and flat plate heat pipes. 1969 publications introduced the concepts of the rotational heat pipe with its applications toturbine blade cooling and the first discussions of heat pipe applications to cryogenic processes.Starting in the 1980s Sony began incorporating heat pipes into the cooling schemes for some of its commercialelectronic products in place of both forced convection and passive finned heat sinks. Initially they were used intuners & amplifiers, soon spreading to other high heat flux electronics applications. During the late 1990sincreasingly hot microcomputer CPUs spurred a threefold increase in the number of U.S. heat pipe patentapplications. As heat pipes transferred from a specialized industrial heat transfer component to a consumercommodity most development and production moved from the U.S. to Asia. Modern CPU heat pipes are typicallymade from copper and use water as the working fluid.[8]

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Applications

Alaska pipeline support legs cooled by heat pipes tokeep permafrost frozen.

Grover and his colleagues were working on cooling systems fornuclear power cells for space craft, where extreme thermalconditions are found. Heat pipes have since been used extensively inspacecraft as a means for managing internal temperature conditions.

Heat pipes are extensively used in many modern computer systems,where increased power requirements and subsequent increases inheat emission have resulted in greater demands on cooling systems.Heat pipes are typically used to move heat away from componentssuch as CPUs and GPUs to heat sinks where thermal energy may bedissipated into the environment.[citation needed]

Solar Thermal

Heat pipes are also being widely used in solar thermal water heatingapplications in combination with evacuated tube solar collectorarrays. In these applications, distilled water is commonly used as theheat transfer fluid inside a sealed length of copper tubing that islocated within an evacuated glass tube and oriented towards thesun.[citation needed]

In solar thermal water heating applications, an individual absorber tube of an evacuated tube collector can deliver upto 40% more efficiency when compared to more traditional "flat plate" solar water collectors. This is largely due tothe vacuum that exists within the tube which slows down convective and conductive heat loss. Relative efficienciesof the evacuated tube system are reduced however, when compared to "flat plate" collectors because "flat plate"collectors have a larger aperture size and can absorb more solar energy per unit area. This means that while anindividual evacuated tube has better insulation (lower conductive and convective losses) due to the vacuum createdinside the tube, an array of tubes found in a completed solar assembly absorbs less energy per unit area due to therebeing less absorber surface area pointed toward the sun because of the rounded design of an evacuated tube collector.Therefore, real world efficiencies of both designs are about the same.Evacuated tube collectors reduce the need for anti-freeze additives to be added as the vacuum helps slow heat loss.However, under prolonged exposure to freezing temperatures the heat transfer fluid can still freeze and precautionsmust be taken in the design to ensure that the freezing liquid does not damage the evacuated tube. Properly designedsolar thermal water heaters can be frost protected down to more than -3 °C with special additives and are being usedin Antarctica to heat water.[citation needed]

Permafrost coolingBuilding on permafrost is difficult because heat from the structure can thaw the permafrost. To avoid the risk ofdestabilization, heat pipes are used in some cases. For example, on the Trans-Alaska Pipeline System residualground heat remaining in the oil, as well as that produced by friction and turbulence in the moving oil could conductdown the pipe's support legs and melt the permafrost on which the supports are anchored. This would cause thepipeline to sink and possibly sustain damage. To prevent this each vertical support member has been mounted with 4vertical heat pipes.[9]

Heat pipes are also used to dissipate heat alongside parts of the Qinghai–Tibet Railway. The embankment and trackabsorb the Sun's heat. Vertical heat pipes either side of the formation prevent that heat spreading any further into thesurrounding ground.

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CookingThe first commercial heat pipe product was the "Thermal Magic Cooking Pin", developed by Energy ConversionSystems, Inc., and first sold in 1966. [10] The cooking pins used water as the working fluid. The envelope wasstainless steel, with an inner copper layer for compatibility. During operation, the heat pipe is poked through theroast. One end of the pipe extends into the oven where it draws heat to the middle of the roast. The high effectiveconductivity of the heat pipe cut the cooking time for large pieces of meat to one-half of the usual period. [11]

The principle has also been applied to camping stoves, transferring a large volume of heat at low temperature toallow baking of goods in camping-type situations, as well as cooking other dishes. An example of this is theBakepacker system.[12]

Ventilation heat recoveryIn heating, ventilation and air-conditioning systems, HVAC, heat pipes are positioned within the supply and exhaustair streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the heatenergy.The device consists of a battery of multi-row finned heat pipe tubes located within both the supply and exhaust airstreams. Within the exhaust air side of the heat pipe, the refrigerant evaporates, taking its heat from the extract air.The refrigerant vapour moves towards the cooler end of the tube, within the supply air side of the device, where itcondenses and gives up its heat. The condensed refrigerant returns by a combination of gravity and capillary actionin the wick. Thus heat is transferred from the exhaust air stream through the tube wall to the refrigerant, and thenfrom the refrigerant through the tube wall to the supply air stream.Because of the characteristics of the device, better efficiencies are obtained when the unit is positioned upright withthe supply air side mounted over the exhaust air side, this allows the liquid refrigerant to flow quickly under gravityback to the evaporator. Generally, gross heat transfer efficiencies of up to 75% are claimed by manufacturers.[citation

needed]

Nuclear Reactor CoolingSince the early 1990s, numerous nuclear reactor power systems have been proposed using heat pipes for transportingheat between the reactor core and power conversion system.[13] The first nuclear reactor to produce electricity usingheat pipes, Demonstration Using Flattop Fission was first operated September 13, 2012. [14]

LimitationsHeat pipes must be tuned to particular cooling conditions. The choice of pipe material, size and coolant all have aneffect on the optimal temperatures in which heat pipes work.When heated above a certain temperature, all of the working fluid in the heat pipe will vaporize and the condensationprocess will cease to occur; in such conditions, the heat pipe's thermal conductivity is effectively reduced to the heatconduction properties of its solid metal casing alone. As most heat pipes are constructed of copper (a metal with highheat conductivity), an overheated heatpipe will generally continue to conduct heat at around 1/80 of the originalconductivity.In addition, below a certain temperature, the working fluid will not undergo phase change, and the thermalconductivity will be reduced to that of the solid metal casing. One of the key criteria for the selection of a workingfluid is the desired operational temperature range of the application. The lower temperature limit typically occurs afew degrees above the freezing point of the working fluid.Most manufacturers cannot make a traditional heat pipe smaller than 3mm in diameter due to material limitations (though 1.6mm thin sheets can be fabricated). Experiments have been conducted with micro heat pipes, which use piping with sharp edges, such as triangular or rhombus-like tubing. In these cases, the sharp edges transfer the fluid

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through capillary action, and no wick is necessary.[citation needed]

References[1] http:/ / www. novelconceptsinc. com/[3] Jim Danneskiold, Los Alamos-developed heat pipes ease space flight (http:/ / www. lanl. gov/ news/ releases/ archive/ 00-064. shtml). Los

Alamos News Release, April 26, 2000.[7] Heat Pipe research at LANL (http:/ / www. lanl. gov/ orgs/ esa/ epe/ Heat_Pipe_Site/ ancient. shtml)[9] http:/ / www. alyeska-pipe. com/ InTheNews/ MonthlyNews/ 2004/ December/ dec2004_featurestory. asp[10] Midwest Research Institute, Heat Pipes (http:/ / ntrs. nasa. gov/ archive/ nasa/ casi. ntrs. nasa. gov/ 19750007248_1975007248. pdf), NASA

Report NASA CR-2508, pg. 19, Jan 1, 1975.

External links• Frontiers in Heat Pipes (FHP) - An International Journal (http:/ / www. thermalfluidscentral. org/ e-journals/

index. php/ Heat_Pipes)• House_N Research (mit.edu) (http:/ / architecture. mit. edu/ house_n/ web/ resources/ tutorials/ House_N Tutorial

Heat Pipes. htm)• What is a Heat Pipe? (http:/ / www. cheresources. com/ htpipes. shtml)• Heat pipe selection guide (pdf) (http:/ / www. enertron-inc. com/ enertron-resources/ PDF/

How-to-select-a-heat-pipe. pdf)

Article Sources and Contributors 9

Article Sources and ContributorsHeat pipe  Source: http://en.wikipedia.org/w/index.php?oldid=543086472  Contributors: 7severn7, Alphapage, Andrea.gf, Apoc2400, Arielco, Arnero, Audriusa, Bdentremont, Beefman,BenFrantzDale, BillAnderson71, Biscuittin, Bobblewik, Bragit, Buster2058, Cacycle, CanOfWorms, Cassnat, Charles Gaudette, Charles Matthews, ChrisJMoor, Conicer, Dan100,DeathMetalParrot, Delldot, Dr. Morbius, Drpixie, Duff-95SHO, Editore99, Enviromet, Everyking, Evil genius, Falcon8765, Firsfron, Foober, Fromeout11, Fuhghettaboutit, Fuzzform, Gameyer4,Gerriegijsen, Glenn, GliderMaven, Goldom, Gracefool, Grafen, Guy Macon, Hankwang, Hds619, Heatlord, Husond, Hustvedt, Iridescent, Isofilm, Jaganath, Jdpipe, Jesse Viviano, Jesuitson,Jlascar, JonHarder, Josh Parris, KDS4444, Kbrose, Ken g6, Khalid hassani, Kristoferb, Lepreshaun, Lexor, Lightmouse, LordZaylor, LorenzoB, Luce nordica, Maniago, Matt Smith4, Mikiemike,Mindmatrix, Mitch Ames, Mmeijeri, Moncrief, Newuy, NickFr, Old Guard, Omegatron, Other-thing, Pabouk, Pahazzard, Pce3, Pihmpdaddi, Psychofox, Quadell, RexNL, Rholton, RichFarmbrough, Rjstott, Rjwilmsi, Roidroid, Roo72, Saga City, Sam Hocevar, Shaddack, Sharpinnovations, Sloggerbum, Snori, Sourpearpirate, Spinningspark, Srleffler, Ssd, Stephan Leeds,Sumone10154, Supak barua, Swaaye, TERdON, Theodore.cackowski, Tsemii, Twilight, Vanished user 5zariu3jisj0j4irj, Vertium, Whaa?, Wheels998, Wolfkeeper, Xaosflux, Yyash.vijay,ZeroOne, Zootalures, Zzedar, 166 anonymous edits

Image Sources, Licenses and ContributorsFile:Laptop Heat Pipe.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Laptop_Heat_Pipe.JPG  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Kristoferb(talk). Original uploader was Kristoferb at en.wikipediaFile:Heat Pipe Mechanism.png  Source: http://en.wikipedia.org/w/index.php?title=File:Heat_Pipe_Mechanism.png  License: Creative Commons Attribution-Sharealike 2.5  Contributors:Original uploader was Zootalures at en.wikipediaImage:CFD IsoSkin Heat Pipe.gif  Source: http://en.wikipedia.org/w/index.php?title=File:CFD_IsoSkin_Heat_Pipe.gif  License: GNU Free Documentation License  Contributors: HeatlordFile:Laptop CPU Heat Pipe Cross Section.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Laptop_CPU_Heat_Pipe_Cross_Section.jpg  License: Creative Commons Zero Contributors: User:EpbernardFile:IsoSkin.png  Source: http://en.wikipedia.org/w/index.php?title=File:IsoSkin.png  License: Public Domain  Contributors: User:IsofilmFile:IsoSkin2.png  Source: http://en.wikipedia.org/w/index.php?title=File:IsoSkin2.png  License: Public Domain  Contributors: User:IsofilmFile:Heatsink with heat pipes.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Heatsink_with_heat_pipes.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:HustvedtFile:Alaska Pipeline Closeup Underneath.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Alaska_Pipeline_Closeup_Underneath.jpg  License: GNU Free Documentation License Contributors: Photo by and (c)2005 Derek Ramsey (Ram-Man)

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