History

     In 1944, Gaugler /1/ patented a lightweight heat transfer device which was essentially the present heat pipe. However, the technology of that period presented no clear need for such a device and it lay dormant for two decades. The idea was resurrected in connection with the space program, first as a suggestion by Trefethen /2/ in 1962 and then form a patent application by Wyatt in 1963. It was not until Grover and his co-workers /3/ of the Los Alamos Scientific Laboratory rediscovered the concept in late 1963 and built prototypes that the impetus was provided to this technology. Grover also coined the name “heat pipe” and stated, “Within certain limitations on the manner of use, a heat pipe may be regarded as a synergistic engineering structure which is equivalent to a material having a thermal conductivity greatly exceeding that of any known metal”.
     The first heat pipe that Grover built used water as the working fluid and was followed shortly by a liquid sodium heat pipe for operation at 1100 °K. Both the high temperature and ambient temperature regimes were soon explored by many workers in the field. It was until 1966 that the first cryogenic heat pipe was developed by Haskin of the Air Force Flight Dynamic Laboratory at Wright – Patterson Air Force Base.
     The concept of Variable Conductance or Temperature Controlled Heat Pipe was first described by Hall of RCA in a patent application dated October 1964. However, although the effect of a noncondensing gas was shown in Grover’s original publication, its significance for achieving variable conductance was not immediately recognized. In subsequent years the theory and technology of Variable Conductance Heat Pipes was greatly advanced, notably by Bienert and Brennan at Dynatherm /4/ and Marcus at TRW /5/.
     On April 5, 1967, the first “zero g” demonstration of a heat pipe was conducted by a group of engineers of the Los Alamos Scientific Laboratory. This first successful flight experiment overcame the initial hesitation that many spacecraft designers had for using this new technology to solve the ever – present temperature control problems on spacecraft. Subsequently, more and more spacecrafts have relied on heat pipes either to control the temperature of individual components or of the entire structure. Some examples of this trend were the ARS – E, OAO, ATS F&G spacecrafts, and the Sky Lab.
     The development of terrestrial applications of heat pipes progressed at a much slower pace. In 1968, RCA developed a heat pipe heat sink for transistors used in aircraft transmitters. This probably represented the first commercial application of heat pipes.
     In the meantime, many other applications have firmly established that heat pipes can solve many critical problems in heat transfer and temperature control.
     More details about heat pipe history can be found out from the following link, which is part of a site belonging to Los Alamos National Laboratory, USA:

http://www.lanl.gov/orgs/esa/epe/Heat_Pipe_Site/ancient4.html#Cott_65

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Principles of Operation


      A heat pipe is a hermetically sealed evacuated tube normally containing a mesh or sintered powder wick and a working fluid in both the liquid and vapor phase.

 

     When one end of the tube is heated the liquid turns to vapor absorbing the latent heat of vaporization. The hot vapor flows to the colder end of the tube where it condenses and gives out the latent heat. The recondensed liquid then flows back through the wick to the hot end of the tube. 
      Since the latent heat of evaporation is usually very large, considerable quantities of heat can be transported with a very small temperature difference from one end to the other.
      The vapor pressure drop between the evaporator and the condenser is very small; and, therefore, the boiling – condensing cycle is essentially an isothermal process. Furthermore, the temperature losses between the heat source and the vapor and between the vapor and the heat sink can be made small by proper design. Therefore, one feature of the heat pipe is that it can be designed to transport heat between the heat source and the heat sink with very small temperature losses.
      The amount of heat that can be transported as latent heat of vaporization is usually several orders of magnitude larger than can be transported as sensible heat in a conventional convective system with an equivalent temperature difference. Therefore, a second feature of the heat pipe is that relatively large amounts of heat can be transported with small lightweight structures.
      The performance of a heat pipe is often expressed in terms of equivalent thermal conductivity. The huge effective thermal conductivity of the heat pipes can be illustrated by the following examples.
      A tubular heat pipe using water as the working fluid and operated at 150 ºC would have a thermal conductivity several hundred times that of a copper bar of the same dimensions.
      A heat pipe using lithium as the working fluid at a temperature of 1500 ºC will carry an axial heat flux of 10 - 20 kW/cm2.
      By suitable choice of working fluid and container materials it is possible to manufacture heat pipes for use at temperatures ranging from - 269 ºC to in excess of 2300 ºC. / 6 /

 
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Types

      Heat pipes are classified into two general types: “Conventional” and “Variable Conductance”.
      The conventional heat pipe is a completely passive device. It is not restricted to a fixed operating temperature but adjusts its temperature according to the heat load and the sink condition. Its thermal conductance is very high but, nevertheless, a nearly constant parameter.
      With minor modifications, the heat pipe can be made a device of variable thermal conductance. There are some means of achieving variable conductance but they are not discussed in this material. The theory of variable conductance heat pipes is outlined. However, because of the complexity of this theory, the reader is referred to special texts /5 / for detailed derivations.

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Operating Ranges

     In this material, the operating temperature ranges of heat pipes are referred to as “Cryogenic” (0 to 150 °K), “Low Temperature” (150 to 750 °K) and “High Temperature” (750 to 3000 °K).These ranges have been defined somewhat arbitrarily such that the currently known working fluids are generally the same type within each range, and each range is roughly four times as large as the preceding one.;
     Working fluids are usually elemental or simple organic gases in the cryogenic range, mainly polar molecules or halocarbons in the low temperature range, and liquid metals in the high temperature range.
      The approximate useful range of some working fluids is indicated in Figure 2. Also are indicated the limits of the three regimes as defined above. The limits of the ranges should only be considered as approximate since some of the fluids overlap into the next temperature range.

Figure 2. Approximate range of applicability of some working fluids in the various temperature regimes

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Applications of the Heat Pipe

     The heat pipe has been, and currently is being, studied for a wide variety of applications, covering almost the complete spectrum of temperatures encountered in heat transfer processes. The applications range from the use of liquid helium heat pipes to aid target cooling in particle accelerators, to cooling systems for state-of-the-art nuclear reactors and potential developments aimed at new measuring techniques for the temperature range 2000 – 3000 °C.

Broad Areas of Application


In general the applications come within a number of broad groups, each of which describes a property of the heat pipe. These groups are:

            • Separation of heat source and sink
            • Temperature flattening
            • Heat flux transformation
            • Temperature control
            • Thermal diodes and switches

The high effective thermal conductivity of a heat pipe enables heat to be transferred at high efficiency over considerable distances. In many applications where component cooling is required, it may be inconvenient or undesirable thermally to dissipate the heat via a heat sink or radiator located immediately adjacent to the component. For example, heat dissipation from a high power device within a module containing other temperature – sensitive components would be effected by using the heat pipe to connect the component to a remote heat sink located outside the module. Thermal insulation could minimize heat losses from intermediate sections of the heat pipe.

The second property listed above, temperature flattening, is closely related to source – sink separation. As a heat pipe, by its nature, tends towards operation at a uniform temperature, it may be used to reduce thermal gradients between unevenly heated areas of body. The body may be the outer skin of a satellite, part of which is facing the sun, the cooler section being in shadow. Alternatively, an array of electronic components mounted on a single pipe would tend to be subjected to feedback from the heat pipe, creating temperature equalization.

The third property listed above, heat flux transformation, has attractions in reactor technology. In thermionics, for example, the transformation of a comparatively low heat flux, as generated by radioactive isotopes, into sufficiently high heat fluxes capable of being utilized effectively in thermionic generators has been attempted /6/.

The fourth area of application, temperature control, is best carried out using the variable conductance heat pipe. This can be used to control accurately the temperature of devices mounted on the heat pipe evaporator section. While the variable conductance heat pipe found its first major applications in many more mundane applications, ranging from temperature control in electronics equipment to ovens and furnaces.

As with any other device, the heat pipe must fulfill a number of criteria before it becomes fully acceptable in applications in industry. For example, in the die-casting and injection molding the heat pipe has to be:

        • Reliable and safe
        • Satisfy a required performance
        • Cost – effective
        • Easy to install and remove.

Obviously, each application must be studied in its own right, and the criteria vary considerably. A feature of the molding processes, for example, is the presence of high frequency accelerations and decelerations. In these processes, therefore, heat pipes should be capable of operating when subjected to this motion, and this necessitates development work in close association with the potential users.

Die casting and Injection Molding

Die casting and injection molding processes, in which metal alloys or plastics are introduced in molten form into a die or mould and rapidly cooled to produce a component, often of considerable size and complexity, have enabled mass production on a considerable scale to be undertaken. The production rate of very small plastic components may be measured in cycles per second, while alloy castings such as covers for car gearboxes may be produced at upwards of one per minute. Aluminum zinc and brass are the most common metals used in the die-cast components, but stainless steel components may now be made using this technique.

The removal of heat during the solidification process is the most obvious requirement, and nearly all dies are water-cooled. However, difficulties are sometimes experienced in taking water-cooling channels to inaccessible parts of the die. A common solution is to use the inserts made of more highly conducting material such as molybdenum, which conducts the heat away to more remote water-cooling channels. Furthermore, it is often inconvenient to take water-cooling to movable or removable nozzles, sprue pins, and cores.

Possibly a more important aspect of die cooling is the need to minimize thermal shock, thus ensuring a reasonable life for the components. With quite large temperature differences between the molten material and the cooling water, which must be tolerated by the intervening die, the life of the die can be shortened. What these parts clearly require is a means of rapidly abstracting heat from their working surfaces at a temperature more nearly approaching that of the molten metal.

Two more thermal problems may be mentioned. In some processes it may be necessary or desirable to heat parts of the die to ensure continuous flow of the molten material to the more inaccessible regions remote to the injection point. To obtain the subsequent rapid solidification, a change from heating to cooling is required in a minimum amount of time to keep cycle times as short as possible.

The heat pipe in its simple tubular form has properties that make it attractive in two areas of application in dies and moulds. Firstly, the heat pipe may be used to even out temperature gradients in the die by inserting it into the main body of the die, without connecting it to the water-cooling circuits.

Probably the most important application is in assisting heat transfer between the die face and the water-cooling path in areas where hot spots occur.

Cooling of Electronic Components

At present the largest application of heat pipes in terms of quantity used is the cooling of electronic components such as transistors, other semiconductor devices, and integrated circuit packages.

There are two possible ways of using heat pipes:

        1. mount the component directly onto the heat pipe, and
        2. mount the component onto a plate into which heat pipes are inserted.

Spacecraft

Heat pipes, certainly at vapour temperatures up to 200 °C, have probably gained more from developments associated with spacecraft applications than from any other area. The variable conductance heat pipe is a prime example of this “technological fall-out”. In the literature can be found details about the following types of application:

        • Spacecraft temperature equalization
        • Component cooling, temperature control and radiator design
        • Space nuclear power sources
          – Moderator cooling
          – Removal of the heat from the reactor at emitter temperature. (Each fuel rod
          would consist of a heat pipe with externally attached fuel).
          – Elimination of troublesome thermal gradients along the emitter and
          collector.

Energy Conservation

The heat pipe, because of its effectiveness in heat transfer, is a prime candidate for applications involving the conservation of energy, and has been used to advantage in heat recovery systems, and energy conversion devices.

Energy conservation is becoming increasingly important as the cost of fuel rises and the reserves diminish, and the heat pipe is proving a particularly effective tool in a large number of applications associated with conservation.

There are a large number of techniques for recovering heat from exhaust air or gas streams or from hot water streams. Details and explanations about heat pipe heat exchangers can be found in this material. Also, a lot of details can be found visiting the Web pages belonging to heat pipe manufacturers presented in this chapter.

Features of heat pipe heat exchangers that are attractive in industrial heat recovery applications are:

    • No moving parts and no external power requirements, implying high reliability.
    • Cross-contamination is totally eliminated because of a solid wall between the hot and cold fluid streams.
    • Easy to clean.
    • A wide variety of sizes are available, and the unit is in general compact and suitable for all.
    • The heat pipe heat exchanger is fully reversible – i.e. heat can be transferred in either direction.
    • Collection of condensate in the exhaust gases can be arranged, and the flexibility accruing to the use of a number of different fin spacing can permit easy cleaning if required.

The application of heat pipe heat exchangers fall into three main categories:

1. Recovery of waste heat from processes for reuse in the same process or in another, e.g. preheating of combustion air. This area of application is the most diverse and can involve a wide range of temperatures and duties.

2. Recovery of waste heat from a process to preheat air for space heating.

3. Heat recovery in air – conditioning systems, normally involving comparatively low temperatures and duties.

Preservation of Permafrost

One of the largest contracts for heat pipes was placed with McDonnell Douglas Corporation by Alyeska Pipeline Service Company for nearly 100,000 heat pipes for the Trans – Alaska pipeline.

The function of these units is to prevent thawing of the permafrost around the pipe supports for elevated sections of the pipeline. Diameters of the heat pipes used are 5 and 7.5 cm, and lengths vary between 8 and 18 m.

The system developed by McDonnell Douglas /8/ uses ammonia as the working fluid, heat from the ground being transmitted upwards to a radiator located above ground level.

Details and photographs of Trans–Alaska Pipeline can be found at this link:

http://wwwndo.ak.blm.gov/dalton/pipe1.htm

Snow Melting and Deicing

An area of application, and one in which work in Japan has been particularly intense, has been the use of heat pipes to melt snow and prevent icing.

The operating principle of the heat pipe snow melting (or deicing) system is based upon the use of heat stored in the ground as the heat input to the evaporators of the heat pipes.

Heat Pipe Inserts for Thermometer Calibration

Heat pipe inserts have been developed at IKE, Stuttgart, for a variety of duties, including thermocouple calibration. The heat pipes are normally operated inside a conventional tubular furnace. The built-in enclosures provide isothermal conditions, a necessary pre-requisite for temperature sensor calibration. The isothermal working spaces can also be used for temperature sensitive processes, such as fixed-point cell heating, crystal growing and annealing.

High Temperature Heat Pipe Furnace

Under contract from the European Space Agency, IKE developed a high temperature heat pipe surface, for materials processing in a micro gravity environment in the temperature range 900 to 1500 °C /9/

Miscellaneous Heat Pipe Applications

To assist the reader in lateral thinking, a number of other applications of heat pipes are listed below.

        • Heat pipe roll-bond panels for warming bathroom floors (Japan)
        • Heat pipe-cooled dipstick for cooling motor bike engine oil (Japan)
        • Passive cooling of remote weather station equipment (Canada)
        • Cooling of drills (Russia)
        • Thermal control of thermoelectric generators (USA)
        • Cooling of gas turbine blades (Czech Republic)
        • Thermal control of electric storage heaters (Byelorussia, UK)
        • Cooling of semi-automatic welding equipment (Russia)
        • Deicing fish farms and ornamental ponds (Romania)
        • Heating heavy oil in large tanks (Romania)
        • Cooling of soldering iron bit (UK)
        • Cooling of bearings for emergency feed water pumps (UK)
        • Cooling of targets in particle accelerators (UK)
        • Isothermalisation of bioreactors (China)
        • Cooling of snubber pins in the synthetic fiber industry (UK)
        • Thermal control in electric batteries Dehumidifiers (USA)
        • Car passenger compartment heating
        • Domestic warm air heaters (USA)

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Companies involved in heat pipe field

            1. 1973, Stuttgart, Germany.
            2. 1976, Bologna, Italy.
            3. 1978, Palo Alto, USA.
            4. 1981, London, UK.
            5. 1984, Tokyo, Japan.
            6. 1987, Grenoble, France.
            7. 1990, Minsk, Russia.
            8. 1992, Beijing, China.
            9. 1995, Los Alamos, USA.
            10. 1997, Stuttgart, Germany.
            11. 1999, Tokyo, Japan.
            12. 2002, Moscow, Russia.


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Heat Pipe Technology into the Third Millennium – the
most recent International Heat Pipe Symposium

The sixth International Heat Pipe Symposium – 2000 (6IHPS-2000) is a sequel to a series of Heat Pipe Symposia that were originally held in Japan and organized by the Japan Association for Heat Pipe (JAHP). The International Heat Pipe Symposium in Chiang Mai, Thailand offers an excellent means of sharing heat pipe technology and its applications to all the countries.  It will also provide a splendid opportunity to present and share with other countries around the world the latest advances in heat pipe science and technology.
Details on the Website: http://ns.eng.cmu.ac.th/6ihps

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Books on Heat Pipes

All the titles below (except 1 & 2) are links for details. If this does not work all the books listed below can be found at the following link:
http://www.amazon.com/exec/obidos/search-handle-form/102-9105007-2240957  

1.
 P. D. Dunn, D. A. Reay, “Heat Pipes” Pergamon Press
2.
 D. Fetcu, V. Ungureanu, “Tuburi Termice” (In Romanian)
3.
 Advances in Heat Pipe Technology : Proceedings. Fourth Conference Held Sept 7-10, 1981
by England)/ Reay, D. A. International Heat Pipe Conference 1981 London. Hardcover (December 1981)
4.
 Heat Pipe Science and Technology by Amir Faghri. Hardcover (March 1995)
5.
 Heat Pipe Technology : Fundamentals and Experimental Studies by L.L. Vasiliev. Hardcover (June 1993)
6.
 Heat Pipe Technology : Materials and Applications by L.L. Vasiliev. Hardcover (June 1993)
7.
 Heat Pipe Technology: Theory, Applications and Prospects by Vic.)/ Akbarzadeh, International Heat Pipe Symposium 1996 Melbourne. Hardcover
8.
 Heat Transfer in Gas-Cooled Annular Channels (Experimental and Applied Heat Transfer Guide Books) by J. Vilemas. Hardcover (February 1987)
9.
 Radiation and Combined Heat Transfer in Channels (Experimental and Applied Heat Transfer Guide Books) by M. Tamonis, A. Zukauskas (Editor). Hardcover (February 1987)
10.
 Topics in Heat Transfer : Microgravity Heat Transfer and Flow/Heat Transfer in Space Energy Systems/Nonconventional Heat Pipe Systems/Heat Transfer I by R.S. Downing, et al. Paperback (July 1992)
11.
 Advances in Heat Pipe Technology
12.
 Fluid-structure interaction, transient thermal-hydraulics, and structural mechanics, 1993 : presented at the 1993 Pressure Vessels and Piping Conference, Denver, Colorado, July 25-29, 1993
13.
 The heat pipe by D. Chisholm.
14.
 Heat pipe theory and practice : a sourcebook by S. W. Chi.

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Bibliography

/1/. Gaugler, R. S., “Heat Transfer Device”, U. S. Patent 2,350,348. | Back |

/2/. Trefethen, L., “On the Surface Tension Pumping of Liquids or a Possible Role of the  
Candlewick in Space Exploration”, G. E. Tech. Info., Ser. No. 615 D114, Feb. 1962 | Back |

/3/. Grover, G. M., Cotter, T. P. and Erikson, G. F., “Structures of Very High Thermal
Conductivity”, J. Appl. Phys., 35, 1990 (1964)  | Back |

/4/ Bienert, W. B., Brennan, P. J., “Transient Performance of Electrical Feedback Controlled   
Variable – Conductance Heat Pipes”, ASME Paper 71 – Av – 27, SAE/ASME/AIAA Life Support and Environmental Control Conference, San Francisco, California, July 12 – 14, 1971. | Back |

/5/. Marcus, B. D., “Theory and Design of Variable Conductance Heat Pipes”, Reports no. 1and 2, TRW 13111 – 6027 – RO – 00, Contract NAS 2 – 5503, April 1971. | Back 1 | | Back 2 |

/6/. Leefer, B. I., “Nuclear thermionic energy converter”, Proceedings of 20 th Annual Power Sources Conf., May 1966, pp 172 – 175. | Back |

/7/. P.D. Dunn & D.A. Reay, "Heat Pipes" Fourth Edition, Pergamon. | Back |

/8/.  Waters, E. D., “Arctic tundra kept frozen by heat pipes”, The Oil and Gas Journal (US), pp 122-125, August 26, 1974. | Back |

/9/. Brost, O. et al., “High temperature lithium heat pipe furnace for space applications: Investigation of temperature stability and reproducibility”, Preprints, 7th Int. Heat Pipe Conf., Minsk, 1990. | Back |

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History Principles of Operation Types Operating Ranges Applications of the Heat Pipes Companies involved in the Heat Pipe Field International Heat Pipe Conferences Heat Pipe Technology into the Third Milennium Books on Heat Pipes Bibliography