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Technical Question? Heat Pipe Cooler and phase change fluid

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June 2, 2007 12:22:10 PM

Looking at a review of a Thermalright SI-120, it went into detail on operation of the heat pipe system I had not previously understood.

http://www.systemcooling.com/thermalright_si-120-01.html

But basically it is a phase change process and depends on the condensed fluid flowing back to the heat source (CPU). In the above review, the test was done with the M/B horizontal.

It would seem to me, if I install this in tower case (all I use) where the M/B is vertical, it would appear to me that the condensed fluid would not go to the base but to the pipes near the base. It would appear the cooler would not be as efficient.

Am I missing something or is this an issue for tower users and heat pipe using coolers?
June 2, 2007 2:19:40 PM

This is more about learning than what I will use. As for the phase change, the Thermalright SI-120 is not listed as a phase change heat sink. However, the review referenced in my original post described the fact that the Heat Pipe uses water in a vacuum chamber (so phase change which really is the most effective heat removal process is claimed in this reveiw as how this unit works). I am assuming, based on the statements in this review, that heat pipes all use this process.

If a heat pipe is solid, than we are using conduction (that makes the most effective type copper), agreed, the above statement does not apply.

If some heat pipes use solid pipes and some use hollow, how do you tell? None of the specs I have read indicate one way or another.

My concern is, if a heat sink design depends on a phase change process to be effective, it could be important to know this because you want the heat to have as direct transfer process to the fluid as possible to achieve maximum effectiveness. (I had assumed the pipes to be solid unless the heat sink was claiming Phase change as the mechanism for its effectiveness).
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June 2, 2007 3:43:53 PM

In answer to your original question:
Quote:
It would seem to me, if I install this in tower case (all I use) where the M/B is vertical, it would appear to me that the condensed fluid would not go to the base but to the pipes near the base. It would appear the cooler would not be as efficient.

Am I missing something or is this an issue for tower users and heat pipe using coolers?

The article you linked to provides the answer (2nd page):
Quote:
The vaporized fluid then condenses back to liquid (cold-end) and the heat is dissipated into the air from the metal cooling fins. The working fluid returns to the hot end via capillary action thru an internal wicking structure (sintered metal coating, fine wire mesh, or grooves) so the heat pipe does not have to rely on gravity to recycle the working fluid. The key to a heat pipe’s high efficiency is the latent heat of vaporization.

Regarding whether it's phase change, I'm with you on this one. In purely scientific terms, the heatpipe employs a substance that changes phases between liquid and gas to enhance cooling performance.

The reason for the confusion is that there are other phase change cooling solutions that provide far superior cooling compared with standard solutions (heatsink + fan), and these have become known generically as 'phase change cooling' within the pc community.

I'm personally not aware of any heatpipe solutions that use solid copper pipes, and thought that they were all based on the same principal. I'm happy to be corrected on this if someone has better knowledge.
June 2, 2007 4:07:43 PM

Thanks for the response. Missed that one on the second page.

Even with the capillary action, I would be concerned some about the vapor displacing the liquid due to space limitation. However, I did find another review where the M/B was test in a vertical position and numbers were pretty good.

I had no idea about heat pipes in general being hollow (had always assumed them to be solid and using conductive heat transfer process to the fins).

Have seen references to this elsewhere recently (such as reports on hot running ASUS boards with he new P35 north bridge running very hot due to being hollow).

No comment on fill fluid, if any in the ASUS heat pipes on the motherboard. Are they purely conductive (my thought if they run so hot) or are they supposed to have a fluid in them for removing heat by latent heat of vaporization?

And the learning continues
June 2, 2007 4:19:41 PM

I too had some concerns regarding the potential loss of efficiency when mounting on a vertically positioned motherboard. I just sort of relied on the designers to have taken this into account considering most users of these high-end heatsinks would have tower PC cases.

Regarding the Asus motherboard heatpipes, I currently have 2 and always believed them to contain a liquid inside the heatpipes. I can confirm that they are hollow (I have removed both, and they are clearly not solid copper). I will be chopping them up soon as I need the mosfet heatsink (I have replaced the NB and SB heatsinks with aftermarket solutions) so I will let you know if they contain any liquid :) 
June 2, 2007 4:31:05 PM

That is cool. I am not on a very advanced system. No need for this info now. Will need to know more in near future to build my next computer. I build one about every 3 to 5 years.

My current computer is really getting long in the tooth like me and needs to be replaced (AMD 939 X64 3000, not me hehe). That is why I am updating my obsolete knowledge. My cooling system on this case is no sides on case with a decent cpu cooler and my case temp is typically about 23C and CPU temp is 29 to 32C when playing Oblivion.

Not too sure what my Diamond X1950Pro is running, hear it runs hot but I feel little heat with my hand by it (easy access for hand temp monitoring, lol)

PS: Already have case (Cooler Master 830 stacker and they are not stackable :D  )

Had to correct CPU, list my other computer cpu
June 2, 2007 4:34:35 PM

Curiosity got the better of me....

I decided to break open one of those Asus heatpipes, and I can confirm that there is no liquid inside.

The interior walls of the pipes seems to be covered with a crystalline copper-coloured solid, but no liquid.

It's possible there was a pressurised gas in there, but nothing that I could notice when opening.
June 2, 2007 4:40:29 PM

Ahh, must be a cat by nature, hehe

OK, that answered

You say a crystalline substance as in a coating of the inside of the pipe? may be for more surface area inside pipe for heat transfer surface, but that would infer a medium to transfer the heat to.

Sounds strange to me, wonder who their engineers are or more to the point, what type?

I may teach heat transfer and fluid flow, and I am not an engineer, but something just ain't right here with their heat pipes.
June 2, 2007 4:57:01 PM

Hmm...wouldn't we prefer more surface area on the outside of the pipe, seeing as that's where we want the heat to dissipate from?

I was wondering whether it may have been part of the 'capilliary action' solution, but with no liquid, I'm not so sure.

All in all, I'm a bit confused with my findings :?


PS. Good choice on the case - shame about it not being stackable :D . Make sure you get one of the cross-flow fans as I find they make a big difference to the cooling.
June 2, 2007 5:03:45 PM

Quote:
It's possible there was a pressurised gas in there


I don't know that much about heat pipes but isn't it possible the it's a liquid under pressure and when you released the pressure it turned to gas therefore no liquid.

Quote:
wouldn't we prefer more surface area on the outside of the pipe


Heat pipes are designed to move heat not dissipate it.
June 2, 2007 5:46:39 PM

Quote:
Quote:

I don't know that much about heat pipes but isn't it possible the it's a liquid under pressure and when you released the pressure it turned to gas therefore no liquid.


Very good point. Very possibly a refrigerant gas (maybe R134a) that under pressure is a liquid. If it flashed as "aoe" cut it opened and due to the small size, it is very possible that she would have missed it, I would have.

That would explain the rough surface on the inside to aid in cooling the refrigerant to return it to a liquid state.

"aoe" commented that wouldn't we want more surface area outside rather than inside. Actually, we want enough inside and outside for the amount of heat (btu) we want to move from one place to another. This applies to the inside and outside. As she identified, the inside was granular. The inside of a pipe, you are limited in options to increase the effective heat transfer surface. One very effective way is to create a very granular surface. This is especially effective in low flow rate situations.

Outside of the pipe, other options exist. Typical heat pipes have fins attached that effectively increase the surface area. Heat is moved to the fins by conduction and the fins lose heat by radiant and convective transfer.
June 2, 2007 6:56:40 PM

Quote:
It's possible there was a pressurised gas in there


I don't know that much about heat pipes but isn't it possible the it's a liquid under pressure and when you released the pressure it turned to gas therefore no liquid.
Yeah, that's sort of what I meant - I should have worded that better :p 

Quote:
wouldn't we prefer more surface area on the outside of the pipe

Heat pipes are designed to move heat not dissipate it.
Sorry - was being a bit cynical there :) 

I was originally waiting to cut it open because I didn't have the right cutting tools available to do so. However, my curiosity got the better of me so I used the old fashioned method - stressing the metal by bending :D 

Obviously, with such a primitive method, It's possible that I may have missed a small amount of gas escaping.
June 2, 2007 7:05:15 PM

aoe, personally, I thank you for your curiosity. It helped lead the way down this road and I gained a better understanding.

I do think ASUS has a heat pipe design problem though.
June 5, 2007 6:33:01 PM

I just cut open my other Asus heatpipe and was pleasantly surprised to find some liquid inside :) . This was from the P5W-DH Deluxe (the other was an A8N32-SLi).

No capilliary though, but apparently that is not unusual for heatpipes on motherboards. Same copper-coloured crystalline solid coating the inside walls, but not so much on this one.
!