Heat Pipe Selection Guide
Heat Pipe Introduction
Heat pipes are used to transport heat over a distance with very
low thermal resistance. This is very helpful when small or distant
heat sources need to be dissipated over a larger area or moved to
a remote heat exchanger. Heat pipes are a Fluid Phase Change
application, often referred to as “re-circulating,” because they use
a closed loop to transfer heat quickly through evaporation and
condensation within the heat pipe.
Heat pipes do not actually dissipate the heat to the environment, but
serve to move heat efficiently within a thermal system. A heat pipe is
a copper tube with an internal wick structure that is sealed on both
ends with a small amount of water inside. As heat is applied to the
pipe, the water will boil and turn to a gas, which then travels to the
colder section of the heat pipe where it condenses back to a liquid.
It is the evaporating and condensing of the water that form a
pumping action to move the water (and thus the heat) from end to
end of the pipe.
There are many types of wick structure that can be used within the heat pipe
and they are generally classified into grooved, mesh, powder and hybrid. A
grooved heat pipe is a copper tube with a series of shallow grooves around the
internal perimeter of the heat pipe. While the water is a liquid, it travels in the
grooves and while it is a vapor it travels in the open space of the pipe. Grooved
pipes can be used in horizontal orientations, but are very limited in
performance if used above 15° out of horizontal. A mesh heat pipe is a smooth
wall copper tube with a woven copper mesh installed along the interior of the
pipe. The mesh is designed to remain in contact with the walls of the pipe in
areas where the pipe may be bent or flattened. Mesh pipes can be used in
horizontal and orientations up to 30° out of horizontal. A powder wick heat
pipe can also be known as a sintered heat pipe.
During the manufacturing process a mandrel is
installed in the center of the pipe and copper
powder is poured into the pipe around the mandrel.
After the powder is sufficiently packed, the parts are
placed into a sintering oven. Once at temperature,
the copper powder will stick to the pipe and to
itself, forming numerous internal pockets like a
sponge. Because of the small pocket sizes, sintered
pipes can efficiently move the water and can be
used horizontally, vertically and all points in
between including upside down. Wakefield-Vette
primarily sells sintered, or powder, style heat pipes
due to their higher performance and the best heat
pipe for your application.
Grooved Heat Pipe
Mesh Heat Pipe
Powder Wick Heat Pipe
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Heat Pipe Selection Guide
Why Use Heat Pipes?
Heat pipes are used in many harsh environments such as:
Key Features
•
Material: Copper
•
Wick Structure: Powder Sintered Copper
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Light Weight
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Versatile with high thermal performance
•
•
•
•
Telecommunications
Aerospace
Transportation
Computers and Datacenters
Heat pipes have proven to be robust and reliable over many
years in these types of applications. The next section will give
more technical detail on the performance of heat pipes
depending on diameter, length, and angle of use.
Many thermal systems benefit from the addition of heat pipes, especially when heat sources are dense
and/or remote to the final heat exchanger. Computer applications, such as processors, graphics cards and
other chip-sets, have high thermally dissipated power in a small area. Fan heat sink combinations used in
these applications can offer high-performance dissipation to the ambient, but much of the battle is to bring
the heat to the heat exchanger with as little temperature change as possible. Heat pipes excel at this and
can transport large heat loads from small areas with very little temperature difference.
How Heat Pipes Operate
Heat Source
1.
2.
3.
4.
5.
6.
Working fluid absorbs heat while evaporating to vapor
Vapor transfers along the cavity to the lower temperature area
Vapor condenses back to fluid, discharging heat
Fluid is absorbed by the sintered/powdered wick structure
Fluid returns to high temperature end via capillary force in the wick structure
Natural or forced convection air flow dissipates excess heat to ambient
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Heat Pipe Selection Guide
Heat Pipe Basics
Heat Pipe Basics
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•
•
•
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Picking the correct pipe
Transport
General parameters
Bending
Flattening
When selecting the diameter and length of heat pipe it is
important to consider the orientation with respect to gravity
and overall heat load for the thermal system. The transport
of vapor within the heat pipe is responsible for the thermal
conduction from one end to the other. A larger diameter
heat pipe can transport more vapor, translating into a larger
heat carrying capacity. Also, the orientation of the pipe with
respect to gravity plays a role in the thermal capacity of a
heat pipe.
When selecting the diameter and length of heat pipe it is important to consider the orientation with respect to The
thermal capacity is increased when the heat source is lower than the condenser (or ambient heat exchanger) because
gravity assists the return of condensed water back to the heat source. The opposite is also true as the thermal
capacity is reduced when the condensed water must move by capillary forces back to the heat source against gravity.
This effect is exaggerated with longer heat pipes and testing has shown that the gravity effect can nearly the double
the thermal capacity in the advantageous direction and cut the capacity in half in the deleterious direction from the
heat pipe in the horizontal orientation. In the short heat pipe extreme (3”-4” length), this effect is nearly zero, so
please consult with Wakefield-Vette engineers to find the right solution for your application.
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Heat Pipe Selection Guide
Flattening Heat Pipes
Flattening is another aspect of heat pipes that effect their performance.
Often it is necessary to flatten a heat pipe to fit into a desired shape or
gap or to increase the contact area of the pipe with the heat. Since
flattening reduces the effective cross-sectional area of the round pipe,
the thermal capacity is reduced, just as if a smaller diameter pipe was
being used. The larger diameter of the starting heat pipe, the larger
reduction of thermal capacity is seen when flattening. Also, the larger
diameter pipes cannot be flattened to the same ultimate dimension as
the smaller pipes without disrupting heat flow altogether. This is also
true for bending of pipes. The radius of bending is usually 3-5x the
diameter of the heat pipe depending on the pipe diameter and the
process of bending the pipe. The potential danger is to collapse the
pipe, effectively cutting off vapor and thermal transport.
Bending radius for heat
pipes of different
diameters depending on
the method of bending.
Bending
•
By Hand:
•
4mm: 4 x diameter
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6mm: 4 x diameter
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8mm: 5 x diameter
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Tooling:
•
4mm: 3 x diameter
•
6mm: 3 x diameter
•
8mm: 4 x diameter
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Heat Pipe Selection Guide
Heat Pipe Assemblies
Interfacing heat pipes with plates and heat exchangers is
predominately about maximizing contact area while adhering to the
flattening and bending guidelines mentioned above. In most cases,
the heat pipes are slotted into channels/grooves in the plate to
maximize contact. The heat pipe can be secured into the groove
using solder or thermal epoxy, which also augments the contact area
of the heat pipe. The heat pipe can also be clamped between two
plates with matching channels/grooves which are fastened together.
In the clamped configuration, thermal grease can be used to increase
the contact of the heat pipe to the plates to reduce the thermal
resistance of the contact interface, just as the thermal epoxy and
solder did in the prior example.
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