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/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”.
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 oK.
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.
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/.
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.
the meantime, many other applications have firmly established that heat
pipes can solve many critical problems in heat transfer and temperature
More details about heat pipe history can be found out from this link: http://en.wikipedia.org/wiki/Heat_pipe
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|The 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.|
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.
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.
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.
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
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.
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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pipes are classified into two general types: “Conventional” and
In the recent years appeared a new type generally named Loop Heat Pipe.
In the recent years appeared a new type generally named Loop Heat Pipe.
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.
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
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this material, the operating temperature ranges of heat pipes are referred
to as “Cryogenic” (0 to 150 oK), “Low Temperature” (150
to 750 oK) and “High Temperature” (750 to 3000 oK).
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.
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
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 oC.
Areas of Application
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
Heat flux transformation
Thermal diodes and switches
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.
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
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/.
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.
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
Cost – effective
Easy to install and
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.
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.
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.
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
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
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
the most important application is in assisting heat transfer between the
die face and the water-cooling path in areas where hot spots occur.
of Electronic Components
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.
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.
pipes, certainly at vapour temperatures up to 200 oC, 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:
temperature control and radiator design
Space nuclear power
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.
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
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
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
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.
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
The heat pipe
heat exchanger is fully reversible – i.e. heat can be transferred in
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
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.
Recovery of waste heat
from a process to preheat air for space heating.
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:
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.
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
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
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 oC
Miscellaneous Heat Pipe Applications
assist the reader in lateral thinking, a number of other applications of
heat pipes are listed below.
Noren Products, Inc. :
Thermacore, Inc. : http://www.thermacore.com/
Heat Pipe Technology, Inc. : http://www.heatpipe.com/
Los Alamos National
Isoterix Ltd. :
Two – Phase Heat
Transfer Laboratory Texas A&M University, USA:
Research and Diffusion Center for Heat Pipe, China:
Aerospace AG: http://www.dasa.com/dasa/index_e.htm?/dasa/e/dornier.htm
Energy Saving Products, Inc.: http://www.espnw.com/index.html
Enerton, Inc. :
S&P Coil Products:
Fujikura America, Inc. :
A comprehensive list of links to companies and institutions working in the heat pipe field can be found out at this site:
|1st IHPC||Stuttgart, Germany||1973|
|2nd IHPC||Bologna, Italy||1976|
|3rd IHPC||Palo Alto, CA, USA||1978|
|4th IHPC||London, UK||1981|
|5th IHPC||Tsukuba, Japan||1984|
|6th IHPC||Grenoble, France||1987|
|7th IHPC||Minsk, Belarus||1990|
|8th IHPC||Beijing, China||1992|
|9th IHPC||Albuquerque, NM, USA||1995|
|10th IHPC||Stuttgart, Germany||1997|
|11th IHPC||Tokyo, Japan||1999|
|12th IHPC||Moscow, Russia||2002|
|13th IHPC||Shangai, China||2004|
|14th IHPC||Florianopolis, Brazil||2007|
|15th IHPC||Clemson, SC, USA||2010|
|16th IHPC||Lyon, France||2012|
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:
1. P. D. Dunn, D. A.
Reay, “Heat Pipes” Pergamon Press
D. Fetcu, V. Ungureanu, “Tuburi Termice” (In Romanian)
Gaugler, R. S., “Heat Transfer Device”,
U. S. Patent 2,350,348. | Back |
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.
/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 |
B. I., “Nuclear thermionic energy converter”, Proceedings of 20 th
Annual Power Sources Conf., May 1966, pp 172 – 175.
/7/. P.D. Dunn & D.A. Reay,
"Heat Pipes" Fourth Edition, Pergamon.
/8/. Waters, E. D., “Arctic tundra kept frozen by heat
pipes”, The Oil and Gas Journal
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.
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