A heat pipe is a heat transfer mechanism that can transport large quantities of heat with a very small difference in temperature between the hotter and colder interfaces. A heat sink (or heatsink) is an environment or object that absorbs and dissipates heat from another object using Thermal contact (either direct or radiant Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature
Inside a heat pipe, at the hot interface a fluid turns to vapour and the gas naturally flows and condenses on the cold interface. The liquid falls or is moved by capillary action back to the hot interface to evaporate again and repeat the cycle. Capillary action, capillarity, capillary motion, or wicking is the ability of a substance to draw another substance into it
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Structure, Design and Construction
A typical heat pipe consists of a sealed pipe or tube made of a material with high thermal conductivity such as copper or aluminium. In Physics, thermal conductivity, k is the property of a material that indicates its ability to conduct Heat. Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 WikipediaNaming A vacuum pump is used to exclude all fluids (both gases and liquids) from the empty heat pipe, and then the pipe is filled with a fraction of a percent by volume of working fluid, (or coolant), chosen to match the operating temperature. A coolant is a fluid which flows through a device in order to prevent its overheating transferring the heat produced by the device to other devices that utilize or dissipate it Some example fluids are water, ethanol, acetone, sodium, or mercury. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Acetone (also known as propanone, dimethyl ketone, 2-propanone, propan-2-one and β-ketopropane) is a colorless mobile flammable Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Due to the partial vacuum that is near or below the vapor pressure of the fluid, some of the fluid will be in the liquid phase and some will be in the gas phase.
The inside of the pipe's walls an optional wick structure exerts a capillary pressure on the liquid phase of the working fluid. In Fluid statics, capillary pressure is the difference in Pressure across the interface between two Immiscible fluids and thus defined as p_c=p_{\text{non-wetting This is typically a sintered metal powder or a series of grooves parallel to the pipe axis, but it may be any material capable of exerting capillary pressure on the condensed liquid to wick it back to the heated end. Sintering is a method for making objects from powder, by heating the material (below its Melting point - solid state sintering until its particles adhere The heat pipe may not need a wick structure if gravity or some other source of acceleration is sufficient to overcome surface tension and cause the condensed liquid to flow back to the heated end. For the work of fiction see Surface Tension (short story. Surface tension is a property of the surface of a Liquid that causes it to
A heat pipe is not a thermosiphon, because there is no siphon. Thermosiphon (alt thermosyphon) refers to a method of passive Heat exchange based on natural Convection which circulates liquid in a vertical closed-loop A siphon (also spelled syphon) is a continuous tube that allows liquid to drain from a reservoir through an intermediate point that is higher than the reservoir the flow being Thermosiphons transfer heat by single-phase convection. Convection in the most general terms refers to the movement of molecules within Fluids (i (See also: Perkins Tube, after Jacob Perkins. Jacob Perkins ( 9 July 1766 &ndash 30 July 1849) was an Anglo-American Inventor, Mechanical engineer and Physicist )
Heat pipes contain no mechanical moving parts and typically require no maintenance, though non-condensing gases (that diffuse through the pipe's walls, result from breakdown of the working fluid, or exist as impurities in the materials) may eventually reduce the pipe's effectiveness at transferring heat. This is significant when the working fluid's vapour pressure is low.
The materials chosen depend on the temperature conditions in which the heat pipe must operate, with coolants ranging from liquid helium for extremely low temperature applications (2–4 K) to mercury (523–923 K) & sodium (873–1473 K) and even indium (2000–3000 K) for extremely high temperatures. Helium exists in Liquid form only at very low Temperatures The Boiling point and critical point depend on the Isotope Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Indium (ˈɪndiəm is a Chemical element with chemical symbol In and Atomic number 49 The vast majority of heat pipes for low temperature applications use some combination of ammonia (213–373 K), alcohol (methanol (283–403 K) or ethanol (273–403 K)) or water (303–473 K) as working fluid. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor In Chemistry, an alcohol is any Organic compound in which a Hydroxyl group ( - O[[hydrogen H]]) is bound to a Carbon Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits, is a Chemical compound Water is a common Chemical substance that is essential for the survival of all known forms of Life.
The advantage of heat pipes is their great efficiency in transferring heat. They are a much better heat conductor than an equivalent cross-section of solid copper. A heat flux of more than 230 MW/m² has been recorded (nearly four times the heat flux at the surface of the sun). In the various subfields of Physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks [1]
Heating a volatile liquid at a fixed volume can be dangerous, since the pressure can exceed the strength of the container (see pressure vessel). A pressure vessel is a closed container designed to hold gases or liquids at a Pressure different from the ambient Pressure. The pipe must safely withstand the pressure that occurs when all of the fluid is in the vapor phase at high temperature. Most importantly, the maximum pressure in the heat pipe must be limited by carefully restricting the total mass of working fluid.
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 area available for vapor condensation. Thus a wider range of heat fluxes and temperature gradients can be accommodated with a single design.
A modified heat pipe with a reservoir having no capillary connection to the heat pipe wick at the evaporator end can also be used as a thermal diode. The term thermal Diode is sometimes used to describe a (possibly non-electrical device which causes heat to flow preferentially in one direction This heat pipe will transfer heat in one direction, acting as an insulator in the other.
Flat heat pipes
Thin planar heat pipes (heat spreaders) have the same primary components as tubular heat pipes. These components are 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 cross section for heat flow even with a very thin device. These thin planar heat pipes are finding their way into “height sensitive” applications, such as notebook computers, and surface mount circuit board cores. It is possible to produce flat heat pipes as thin as 0. 5 mm (thinner than a credit card). ISO 7813 defines standards for banking cards such as the thickness and the rounding for the corners
Heat transfer
Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant. Evaporative coolers (also called swamp, desert, or air coolers) are devices that cool air through the simple Evaporation of water Evaporation is the process by which Molecules in a Liquid state (e Condensation is the change of the physical state of aggregation (or simply state of matter from gaseous phase into liquid phase Heat pipes rely on a temperature difference between the ends of the pipe, and cannot lower temperatures at either end beyond the ambient temperature (hence they tend to equalise the temperature within the pipe).
When one end of the heat pipe is heated the working fluid inside the pipe at that end evaporates and increases the vapour pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporisation of the working fluid reduces the temperature at the hot end of the pipe. In Thermochemistry, latent heat is the amount of Energy in the form of Heat released or absorbed by a substance during a change of phase
The vapour pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapour pressure over condensing working fluid at the cooler end of the pipe, and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapour condenses, releases its latent heat, and warms the cool end of the pipe. Non-condensing gases (caused by contamination for instance) in the vapour impede the gas flow and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapour pressures are low. The velocity of molecules in a gas is approximately the speed of sound and in the absence of non condensing gases, this is the upper velocity with which they could travel in the heat pipe. In practice, the speed of the vapour through the heat pipe is dependent on the rate of condensation at the cold end.
The condensed working fluid then flows back to the hot end of the pipe. In the case of vertically-oriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action. Capillary action, capillarity, capillary motion, or wicking is the ability of a substance to draw another substance into it
When making heat pipes, there is no need to create a vacuum in the pipe. One simply boils the working fluid in the heat 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 suspected that a water charged heat pipe would only work when the hot end reached the boiling point (100 °C) and steam was transfered to the cold end. However, the boiling point of water is dependent on absolute pressure inside the pipe. In an evacuated pipe, water will boil just slightly above its melting point (0 °C). The melting point of a solid is the temperature range at which it changes state from solid to Liquid. The heat pipe will operate, therefore, when the hot end is just slightly warmer than the melting point of the working fluid. Similarly, a heat pipe with water as a working fluid can work well above the boiling point (100 °C), if the cold end is low enough in temperature to condense the fluid.
The main reason for the effectiveness of heat pipes is the evaporation and condensation of the working fluid. The heat of vaporization greatly exceeds the sensible heat capacity. The enthalpy of vaporization, (symbol \Delta{}_{v}H also known as the heat of vaporization or heat of evaporation, is the Energy required Specific heat capacity, also known simply as specific heat, is the measure of the heat energy required to increase the Temperature of a unit quantity Using water as an example, the energy needed to evaporate one gram of water is equivalent to the amount of energy needed to raise the temperature of that same gram of water by 540 °C (hypothetically, if the water was under extremely high pressure so it didn't vaporize or freeze over this temperature range). Almost all of that energy is rapidly transferred to the "cold" end when the fluid condenses there, making a very effective heat transfer system with no moving parts.
Origins and research in the United States
The general principle of heat pipes using gravity (commonly classified as two phase thermosiphons) dates back to the steam age. Thermosiphon (alt thermosyphon) refers to a method of passive Heat exchange based on natural Convection which circulates liquid in a vertical closed-loop The modern concept for a capillary driven heat pipe was first suggested by R. S. Gaugler of General Motors in 1942 who patented the idea. [2]The benefits of employing capillary action were independently developed and first demonstrated by George Grover at Los Alamos National Laboratory in 1963 and subsequently published in the Journal of Applied Physics in 1964. Los Alamos National Laboratory (LANL (previously known at various times as Site Y, Los Alamos Laboratory, and Los Alamos Scientific Laboratory) is a Journal of Applied Physics is a Scientific journal published by the American Institute of Physics (AIP [3] Grover noted in his notebook:[4]
"Heat transfer via capillary movement of fluids. The "pumping" action of surface tension forces may be sufficient to move liquids from a cold temperature zone to a high temperature zone (with subsequent return 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 no external pumps, may be of particular interest in space reactors in moving heat from the reactor core to a radiating system. In the absence of gravity, the forces must only be such as to overcome the capillary and 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 for commercial applications (though their work was mostly funded by the US government). During the late 1960s NASA played a large role in heat pipe development by funding a significant amount of research on their applications and reliability in space flight following from Grover's suggestion. NASA’s attraction to heat pipe cooling systems was understandable given their low weight, high heat flux, and zero power draw. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program Their primary interest however was based on the fact that the system wouldn’t be adversely affected by operating in a zero gravity environment. The first application of heat pipes in the space program was in thermal equilibration of satellite transponders. As satellites orbit one side is exposed to the direct radiation of the sun while the opposite side is completely dark and exposed to the deep cold of outer space. This article is about artificial satellites For natural satellites also known as moons see Natural satellite. Outer space, often simply called space, comprises the relatively empty regions of the Universe outside the escape velocities of Celestial bodies. 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 and demonstrated flawless operation with and without the influence of gravity. The developed cooling system was the first description and usage of variable conductance heat pipes to actively regulate heat flow or evaporator temperature.
Corporate R&D
Publications in 1967 and 1968 by Feldman, Eastman, & Katzoff first discussed applications of heat pipes to areas outside of government concern and that did not fall under the high temperature classification such as; air conditioning, 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 to turbine 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 commercial electronic products in place of both forced convection and passive finned heat sinks. Initially they were used in tuners & amplifiers, soon spreading to other high heat flux electronics applications. During the late 1990s increasingly hot microcomputer CPUs spurred a threefold increase in the number of U. S. heat pipe patent applications. As heat pipes transferred from a specialized industrial heat transfer component to a consumer commodity most development and production moved from the U. S. to Asia. Modern CPU heat pipes are typically made from copper and use water as the working fluid.
Applications
Grover and his colleagues were working on cooling systems for nuclear power cells for space craft, where extreme thermal conditions are found. The terms atomic battery, nuclear battery, tritium battery and radioisotope battery are used to describe a device which uses the emissions from a Radioactive A spacecraft is a Vehicle or machine designed for Spaceflight. Heat pipes have since been used extensively in spacecraft 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 in heat emission have resulted in greater demands on cooling systems. Heat pipes are typically used to move heat away from components such as CPUs and GPUs to heat sinks where thermal energy may be dissipated into the environment.
Solar Thermal
Heat pipes are also being widely used in solar thermal water heating applications in combination with evacuated tube solar collector arrays. In these applications, distilled water is commonly used as the heat transfer fluid inside a sealed length of copper tubing that is located within an evacuated glass tube and oriented towards the sun.
In solar thermal water heating applications, an evacuated tube collector can deliver up to 40% more efficiency compared to more traditional "flat plate" solar water heaters. Evacuated tube collectors eliminate the need for anti-freeze additives to be added as the vacuum helps prevent heat loss. These types of solar thermal water heaters are frost protected down to more than -3 °C and are being used in Antarctica to heat water.
Pipelines over permafrost
Heat pipes are used to dissipate heat on the Trans-Alaska Pipeline System. The Trans-Alaska Pipeline System ( TAPS) usually called the Alyeska Pipeline in Alaska or the Alaska Pipeline elsewhere is a major U Heat produced by friction and turbulence in the moving oil would conduct down the pipe's support legs and melt the permafrost which anchors them. Heat pipes with radiators at the top are used on each leg to keep them cold so they won't melt the permafrost and let the pipeline collapse.
Limitations
Heat pipes must be tuned to particular cooling conditions. The choice of pipe material, size and coolant all have an effect 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 condensation process will cease to occur; in such conditions, the heat pipe's thermal conductivity is effectively reduced to the heat conduction properties of its solid metal casing alone. In Physics, thermal conductivity, k is the property of a material that indicates its ability to conduct Heat. Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter from a region of higher Temperature to a region of lower As most heat pipes are constructed of copper (a metal with high heat conductivity), an overheated heatpipe will generally continue to conduct heat at around 1/80 of the original conductivity. Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29
In addition, below a certain temperature, the working fluid will not undergo phase change, and the thermal conductivity will be reduced to that of the solid metal casing. One of the key criteria for the selection of a working fluid is the desired operational temperature range of the application. The lower temperature limit typically occurs a few degrees above the freezing point of the working fluid.
References
- ^ Jim Danneskiold, Los Alamos-developed heat pipes ease space flight. Los Alamos News Release, April 26, 2000.
- ^ Gaugler, Richard (1944), Heat Transfer Devices, Dayton, Ohio: U. S. Patent Office, pp. 4, 2350348
- ^ Grover, G. M. , T. P. Cotter, and G. F. Erickson (1964). "Structures of Very High Thermal Conductance". Journal of Applied Physics 35 (6): 1990–1991. .
- ^ Heat Pipe research at LANL
Heat Pipe Science and Technology, Amir Faghri, Taylor and Francis 1995.
See also
- Thermosiphon: A similar mechanism in which thermal energy is transferred by fluid buoyancy rather than evaporation and condensation. Thermosiphon (alt thermosyphon) refers to a method of passive Heat exchange based on natural Convection which circulates liquid in a vertical closed-loop In Physics, buoyancy ( BrE IPA: /ˈbɔɪənsi/ is the upward Force on an object produced by the surrounding liquid or gas in which it is
- Vapor-compression refrigeration
- Evaporative cooling
- Heat sink
- CPU cooling
- Aircooling
- Watercooling
- Peltier or thermoelectric cooling
- Loop heat pipe
External links
Vapor-compression refrigeration is one of the many Refrigeration cycles available for use Evaporative coolers (also called swamp, desert, or air coolers) are devices that cool air through the simple Evaporation of water A heat sink (or heatsink) is an environment or object that absorbs and dissipates heat from another object using Thermal contact (either direct or radiant Computer cooling is the process of removing heat from computer components Air cooling is a method of dissipating Heat. It works by making the object to be cooled have a larger surface area or have an increased flow of air over its surface or both Water cooling is a method of Heat removal from components As opposed to Air cooling, Water is used as the heat transmitter The thermoelectric effect is the direct conversion of temperature differences to electric Voltage and vice versa Thermoelectric cooling uses the Peltier effect to create a Heat flux between the junction of two different types of materials A loop heat pipe (LHP is a two-phase heat transfer device that uses Capillary action to remove heat from a source and passively move it to a Condenser or RadiatorDapyx Software network: MP3 Explorer | Ebook Manager | Zenithic