Is "heat pipe" technology any better than solid copper?

[rgwalt]

Also note that this "process" is basically what goes on in EVERY air conditioning system. Freon (or some nicer fluid) changes from a liquid to a vapor (or vice versa) to absorb/dissipate heat in places with different temperatures. Your radiator in your car also works on a similar principle.

Not quite... In an AC or refridgeration unit there is a compression step and an expansion step. These steps are what allows heat to be pumped from one area to another. I don't mean to nitpick here, but making these sorts of comparisons can give people the wrong idea of how a heat pipe actually works.

Ryan

 

[HokieESM]

True.... I should have pointed that out. I was just speaking of the general evaporation-condensation process. Good point.

Its is important to note that the water MUST be under vacuum (relative to ambient temperature)... otherwise, you won't get a vapor.

Leo, also, since you seem to get my point earlier... what do you think the chances of the water actually changing phase are? It would seem that you would have to have the water within a pretty narrow operating range.... because if it reaches a state where it DOESN'T change phase.... things could get very ugly, very quickly.

 

[Evadman]

Is it actually water? I assumed is was a type of refigerant.

 

[rgwalt]

The websites I've seen indicate that the heatpipes contain water with additives under a vaccum. However, to get pure water to boil at 30C, you need to bring it down to 0.04 atm.

Ryan

 

[CTho9305]

Originally posted by: Evadman
Is it actually water? I assumed is was a type of refigerant.

I built a big heatpipe using rubbing alcohol at low pressure and it still wasn't a low enough vaporization point. it is not water that they use.

 

[HokieESM]

linky

So, it supposedly is water + additives.

rgwalt, I also pulled out my "steam tables" (its been a long time since I took thermo) looking up the evaporation temperature vs. pressure for water. As you stated, at 30 degrees C, the saturation pressure is about 0.04 atm. The ONLY problem I see is that if the pressure is varied slightly (say manufacturing tolerances), the saturation temperature could be 40 degrees C... and it might never reach that temperature... thus remaining water, and the heat transfer would drop seriously. Same if the pressure was too low... it might permanently be vapor.

Also, if the room temperature is too high (or the fan doesn't bring across enough air), then the water may never condense, resulting in other problems.

I'm sure the engineers at Shuttle (or whoever developed this particular set) would have considered this. Its obvious that heat pipes must be made specific for the application.... because if the heat transfer/temperature is out of spec, they are GREATLY reduced in efficiency. I've just worked a little with fluids under vacuum.... exact control over the pressure can be achieved, but only with considerable effort. Of course, these things don't come cheap. 🙂

Still an interesting/very plausible concept.

 

[rgwalt]

Well, one would think that they would have some serious quality control in order to make sure the fluid will boil at the correct temperature. However, this is a closed system, so as the liquid boils or condenses, the pressure increases or decreases until a new equilibrium is reached, so there will be some tolerance in the operating temperature. I bet the designers pull a hard vaccum and fill the pipe with enough liquid so that there is a sufficient supply over a range of operating temperatures. Doing so will ensure that the system will always be saturated assuming the two ends are at the same temperature. As long as the heat sink end (cold end) is at a lower temperature than the heat source end (hot/cpu end), it should function just fine as the system should reach a equilibrium temperature and pressure distribution.

Breaking out the steam tables... I like it. What did you use? I busted out Perry's Handbook.

Ryan

 

[rgwalt]

Originally posted by: CTho9305

Originally posted by: Evadman
Is it actually water? I assumed is was a type of refigerant.

I built a big heatpipe using rubbing alcohol at low pressure and it still wasn't a low enough vaporization point. it is not water that they use.

How low of a vaccum did you pull? The article states water with additives. You need to pull a pretty hard vaccum in order to get the equilibrium behavior you want.

Ryan

 

[CTho9305]

Originally posted by: rgwalt

Originally posted by: CTho9305

Originally posted by: Evadman
Is it actually water? I assumed is was a type of refigerant.

I built a big heatpipe using rubbing alcohol at low pressure and it still wasn't a low enough vaporization point. it is not water that they use.

How low of a vaccum did you pull? The article states water with additives. You need to pull a pretty hard vaccum in order to get the equilibrium behavior you want.

Ryan

No quality control - I just used a blowtorch to boil the alcohol at the bottom, then after I had reached some arbitray amount (I figured it should have pushed most of the air out and lowered the pressure) I sealed the pipe.

 

[HokieESM]

Since I'm still in school (working on my PhD--but in SOLID mechanics 🙂 ), my textbooks are still handy--I just grabbed my thermo book.

I suppose they just need a relatively good vacuum.... I was just wondering, over varying conditions of the processor (idle to full load), wouldn't there be some pretty drastic differences in the operational effectiveness? I mean, if the processor end is at 40 deg C (plausible for a P4) and the other end is 25 deg C (a bit above ambient)... sure. But if BOTH are at 25 deg C or BOTH are at 40... I could see that making a big difference.

In any event... the system is relatively foolproof if they can get the vacuum down to the 0.04 to 0.05 atm level (I don't want people to doubt the process because of my conjectures!). A friend of mine (who does fluids processing research) confirmed that in a factory setting, they're probably just pulling as hard a vacuum as possible--0.04 atm is about as low as you can go with standard pumps with any reasonable speed.

 

[MadRat]

The heat pipe allows you to insulate the heatsink away from the surface that is ultimately being cooled. There can be a layer of insulation in between the contact point and the final destination of the heat in order to keep heat energy from traveling through the system in the wrong direction.