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For a heatpipe that is connected to a 50 Celsius object on one side,and a heatsink at 23 Celsius air at other side, what fluid filling will give better heat conductivity? Plain water, or some kind of refrigerant like Perflenapent/perfluoropentane ( c5f12 ) that boils at 28 degrees Celsius?

I saw on Linus youtube channel that he showed some prototype pc from a new company that uses refrigerant to have something like pumpless cooling - really just a flexible heatpipe filled with some unknown refrigerant connected to conventional watercooling style cpu block and large passive heatsink.

I read that water is the best fluid for heatsinks that operate at room temperature. What is truth, is water best or could some kind of refrigerant,possibly one with boiling point above room temperature but bellow cpu block temperature cool better than water?

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closed as off-topic by JMac, Kyle Kanos, Jon Custer, Chris, Michael Seifert Mar 7 '18 at 14:15

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    $\begingroup$ I'm pretty sure it would completely depend on how you engineer the system. It makes sense to me that a passive phase change heat pipe would do better than a passive water based system. Comparing it to active cooling solutions would be an unfair comparison unless you're only concerned about raw performance; and have no concerns regarding power consumption or noise. $\endgroup$ – JMac Mar 6 '18 at 15:18
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If a fluid is used to transfer heat between two locations, heat transfer equals the product of specific heat capacity and mass flow rate. A refrigerant is not required to transfer the fluid. A term 'heat capacity rate' has been developed to quantify the heat carrying capacity of a fluid (ref):

$$C = c_p{{dm}\over{dt}}$$ where $C$ is the heat capacity rate for a particular fluid, $c_p$ is the specific heat capacity of that fluid, and $dm/dt$ is the mass transfer rate for that fluid in a particular situation.

You can achieve a certain heat transfer rate by pumping or blowing the fluid to transfer it fast enough, then it doesn't really matter what fluid you use as the cooling effect can be be the same. You might use more energy to pump depending on the fluid and circuit design.

If you use a refrigerant within a refrigeration cycle, you can improve heat carrying capacity, but you need additional energy to run the refrigerant compressor, which requires more energy than a simple pump for the same mass transfer because it has to work against the restriction valve in the refrigeration cycle.

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