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I think I understand how heat pipes work, and thus the following one has me flummoxed:-

Chinese heat pipe

It's from a typical bulk order Chinese web site, but I've seem many similarly contorted arrangements. They are very common in the over clocking /modding community. There is never discussion as to which way the heat sink must be oriented for efficient operation.

My example seems ridiculous. With an U bend in the middle, I don't see how heat sinks like this can evaporate at the hot end and condense at the cold end. Surely the condensate will just pool in the U bend? Even with condensate wicking, it must be easier to wick downwards with gravity than upwards fighting gravity.

Are these types of heat sinks just a con? Overclocking /modding heat sinks are never spec'd with a deg. C /W rating or recommended orientation. This would not do in the engineering world. Can it work equally effectively in any orientation?

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  • $\begingroup$ Is it for water cooling? I'm amazed electronics would run at a high enough temperature to produce steam? I must be missing something $\endgroup$
    – Jayy
    Commented Aug 30, 2016 at 2:03
  • $\begingroup$ @Jodes I do believe that heat pipes often use water as working fluid because of its high specific heat and evaporation heat. Electronics don't have to get 100°C to evaporate water, namely by lowing the pressure inside the pipes the boiling point decreases. I also believe that this boiling point can shift because as more water turns into steam due to higher heat input the pressure will increase. $\endgroup$
    – fibonatic
    Commented Aug 30, 2016 at 3:39
  • $\begingroup$ I do believe that the majority of the fluid flow is forced by capillary action. However gravity might still have a significant influence. If so, then one could test this. $\endgroup$
    – fibonatic
    Commented Aug 30, 2016 at 3:43
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    $\begingroup$ Those on the Alyeska pipeline use anhydrous ammonia . Many liquids could be used. $\endgroup$
    – blacksmith37
    Commented Jan 29, 2021 at 0:56

3 Answers 3

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Orientation does often matter. As Carl's answer mentions, the liquid can get from the condenser to the hot interface via capillary action, but most common heat pipes are designed assuming gravity will do the job.

Capillary action is much more effective in space where there is no gravity, but produces very little flow when it has to work against gravity. Therefore, even heat pipes designed to transport the liquid via capillary action in space need to be oriented correctly on earth.

Make sure the radiator is above the hot interface, especially where there is no datasheet available.

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Orientation is in fact irrelevant. Heat pipes are used in space, among other places. Quoting from guess-what-reference-page,

At the hot interface of a heat pipe a liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid – releasing the latent heat. The liquid then returns to the hot interface through either capillary action, centrifugal force, or gravity, and the cycle repeats.

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    $\begingroup$ Wouldn't that mean orientation is very relevant? What if gravity + orientation means that the liquid naturally pools at the "cold interface"? Then the hot interface gets so hot that the cold interface also gets hot, and the liquid boils but now the cold interface is hot, so how does the vapor condense? "The liquid then returns to the hot interface through <mechanisms>" but again, centrifugal force or gravity, the heat pipe has to be oriented correctly for those forces to return the liquid to the correct end. $\endgroup$
    – Chuck
    Commented Aug 30, 2016 at 12:25
  • $\begingroup$ @Chuck Please read what I quoted. IN some systems, gravity helps; in others, the design is purely for capillary action $\endgroup$
    – Carl Witthoft
    Commented Aug 30, 2016 at 13:16
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    $\begingroup$ You say "capillary action" later in the answer, but you headline with the first sentence, "Orientation is in fact irrelevant." This is not true. $\endgroup$
    – Chuck
    Commented Aug 30, 2016 at 13:34
  • $\begingroup$ @CarlWitthoft Your answer is self contradictory. How can capillary action act independently of gravity (on Earth)? If this were true, the glass of water in front of me would empty itself as the meniscus pulls it up and over the sides of the glass! $\endgroup$
    – Paul Uszak
    Commented Aug 30, 2016 at 21:06
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    $\begingroup$ @PaulUszak, you are not taking into account that the ordinary heat pipes are constructed with long fiber-like material along internal walls, which has tremendous surface. This surface provides the return of condensed agent back to evaporation area via capillary action, which has overwhelmingly stronger suction and vastly dominates gravity. That's why heat pipes work in any orientation. If you put this wick fabric into your glass and give it enough area to evaporate, it will empty your glass in no time. $\endgroup$
    – Ale..chenski
    Commented Sep 29, 2016 at 4:02
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Orientation effects the Heatpipe's Qmax, the maximum watt that particular pipe can carry.

So, a well design Heatsink + Heatpipe system will stay below that. If you are below that in a well designed system, orientation will not affect it.

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