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near-vacuum pressure induced cooling - Page 3

post #21 of 27
At first, positive pressure would be better for cooling. More air => more heat transfer. It's just that with more air flowing in than out, there's gonna be a LOT of dust getting in the case, and not a lot coming out. So in the long run negative pressure is better, because there would be less dust settling on the radiator and fans.

EDIT: lol, sorry. is it better?
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post #22 of 27
Quote:
Originally Posted by thiru View Post
Actually I think positive pressure would be better ...negative pressure is better.
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post #23 of 27
It's really a game of competing mechanics. Negative pressure would decrease the temperatures SLIGHTLY as well as decrease the thermal conductivity of the air as it's less dense. I feel that both processes are insignificant with the amounts of air we're talking about moving. Conversely positive pressure would increase temps and thermal conductivity. However, creating positive pressure is easier than creating negative in the sense that a pump can push fluid further pushing than it can pulling. (No matter how strong a water pump is, it can only pull a column of water so high while pulling that's determined by atmospheric pressure. But pushing it can go much much higher as it relies totally on the strength of the pump itself.)

Then it comes down to the fact that the only time there is 'significant' compression and decompression of gas is as it squeezes through TIGHT openings because of pressure.

With negative pressure you're theoretically only removing air with fans and now the incoming air is compressing as it goes through small cracks in your case, compressing/heating the outside and then expands/cools as it enters the tight openings. Cool air in, good.

On the other hand, positive pressure theoretically pushes ONLY air in but instead heats the air inside the case as it compresses the air while it finds its way out through small cracks, then expands and cools on the OUTSIDE of the case. Bad mojo.

But what you're REALLY looking for is more along the lines of the greatest air flow, as the expansion/contraction bit is VERY VERY SMALL AT THESE delta-P's. You'd rather just concentrate on moving the most air THROUGH your case because the OVERWHELMING heat input is the CPU warming the air inside. Getting that out as efficiently as you can would amount to much much more benefit as (getting a 1/3C decrease in temps from negative pressure is nothing compared to moving more 20C air from outside the case across the heatsink rather than the 30C heated air already inside.)

Your fans operate the most efficiently when they have the lowest load upon them. This basically means REDUCING ALL PRESSURE GRADIENTS IN YOUR CASE TO AS CLOSE TO ZERO AS POSSIBLE. Meaning you want a delta-P of almost zero.... meaning, you want the same amount of air moving INTO the case as moving OUT of the case. This eliminates loads on the fans stemming from working against the air trying to move in the opposite direction it's pushing in and instead only works against the native inertia of the air.

This is analogous to saying you don't want a headwind against your car if you want it to go faster. You'd like the wind to be moving IN CONCERT with your car (tailwind) which partially happens with two fans working in concert (push-pull).

Needless to say, this DOES NOT TAKE INTO ACCOUNT THE SPECIFIC DYNAMICS OF YOUR CASE. If you've got a choice for a fan to equalize pressure as an exhaust or create positive pressure as an intake... but it'd also directly feed your HSF a stream of cooler outside air (like a door fan) you'll likely see better temps with the positive pressure get up than the neutral.
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post #24 of 27
Quote:
Originally Posted by thiru View Post
It's just that with more air flowing in than out, there's gonna be a LOT of dust getting in the case, and not a lot coming out.

EDIT: lol, sorry. is it better?
It's actually the other way around, positive pressure caused by fans(assuming they have filters) will actually lessen the amount of dust in your case since it will prevent air and dust entering thru unfiltered cracks and seams in your case.
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post #25 of 27
I don't get it...

Why do people keep coming up with this insane theories that a vacumm = lower temps, and all this negative pressure stuff.

Just apply a little common sense, and it would be obvious that no air = no removal of heat = heat stays on your cpu and it burns up.

Not that you could create a vacumm with pc fans anyways....

There is a reason you don't see pics of vacummed PCs at overclock.net, because it doesn't work. If it did you would surely see them.
post #26 of 27
Some basic things to think about when developing a new cooling method.


Three methods of heat transfer:
Conduction: heat transfer through an object without the object moving
Convection: heat transfer through the motion of mass
Radiation: heat transfer through electromagnetic waves

Heatsinks by conducting heat into a large metal surface area. Then it transfers the heat into the air around it. This air then transfer thermal energy to cooler air (Convection)

In watercooling, phase change, DICE, and LN2, heat is conducted through a metal block into one of the above. Then through convection the fluids/gas moves the heat away. Eventually this heat is transferred to the air. Then exhaust source is cooled through convection

Radiation does not rely on matter to move energy. However, it is a very weak and low power way of heat transfer. It pretty much does not matter in computers.

The heat has to ALWAYS go somewhere....
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post #27 of 27
Thread Starter 
alright i actually understand now sorry guys
    
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