Heatpipes are sealed pipes that carry heat from one end to the other via the effect
of vaporisation and condensation. As an example of how a heatpipe works, let’s imagine
that such a heatpipe is placed vertically. The heatpipe contains a small amount of
water (or other fluid) sealed at a specific internal pressure. If we now heat the
bottom of the heatpipe, the fluid inside will vaporize (temperature of vaporization
will depend on the fluid and internal pressure) and the vapour will move up to the
top of the pipe. At the top, we link the pipe to a heatsink or other means of extracting
heat. So as the vapour reaches the top of the unit, it condenses as it gives off
its heat to the heatsink. The liquid now drips back to the bottom of the heatpipe
and the cycle start all over again. Because the liquid drips back by gravity, the
design of a heatpipe can be influenced by its orientation. Heatpipes are often designed
with wicks (grooves or metal mesh material) in order to get capillary action assisting
the movement of the liquid.
Heatpipes can be used to carry the heat from a restricted area (where we cannot place
a proper heatsink) to an area where there is greater space to extract the heat. As
an example, laptops often use heatpipes to carry the heat from the CPU to an area
near the edge of the laptop enclosure where a small fan can be used (in conjunction
with a heatsink) to remove the heat.
Heatpipes are sometimes also used as heat spreaders for heatsink applications. Heatpipes
can be embedded in the heatsink base resulting in greater effective thermal conductivity
of the base. In some cases, as shown in figure 1, the heatpipes are used to carry
the heat from the base of the heatsink directly to the fins, thus minimizing fin
efficiency issues where the fins have very high aspect ratio.
Vapour chambers can also be used as heat spreaders. Working on a similar principle
as heatpipes, vapour chambers will act as a very high conductivity block . The block
can be the base of a heatsink or simply a high conductivity block used to spread
the heat out to a larger surface area.
Figure 1: Heatpipe used for CPU cooling application
If you have any questions regarding the above or for a specific design question which
you may have, please contact us and we will be pleased to help you.