Case Airflow: Positive or Negative Pressure?

[FlamingAromr]

What about neutral pressure? By that I mean intake CFM = exhaust CFM. Something else to chew on.

Almost impossible to achieve perfect neutral pressure, though in concept, it would probably be a bad plan as it would result in dead zones. (Places where the air just stands still because the natural current avoids that path)

In my opinion positive pressure is the way to go, it provides more air into the case therefore more opportunity for heat to disperse. If you have negative pressure it creates a slight vacuum meaning air particles are farther apart providing less heat conductivity for the components inside.
that being said, you will need to clean inside the case more often with positive pressure. So if you are not one to clean often, to much positive pressure could result in over heating from "Dust-clogged" components.

 

[ScrewySqrl]

oh my god. this thread will never die

 

[Paperdoc]

Agree. I note also that, as far a definitive answers go, it's a total loss! Of the total posts, 51.5% actually voted one way or the other, so only half have firm opinions. Of those who did vote, the result is an even split. Regarding the material posted, my read is they contain a lot of opinions varying from theoretical through factual to entirely unfounded, and almost no experimental proof. So if anyone is looking for a definite answer, there is none. There are only personal preferences.

I can get away with saying this because I was an early poster in this thread.

 

[Kaitain Jones]

Some slightly strange claims being made here, along the lines of, "It's better to have more air coming out of the case than going in". Well that seems to be pretty unlikely unless the laws of physics are being violated.

Broadly, it's better to have cool air entering the case at the bottom and hotter air being expelled from the top. For reasons of personal comfort it's probably better to have the air coming out of the back/top/sides rather than the front as well.

Your overall plan should be to create a wind tunnel moving from bottom to top, and somewhat from front to back if possible. Intake fans at the bottom that are close to components expected to be hot will be A Good Thing. Intake fans that go through dust filters are also A Good Thing because then you can increase the ratio of intake air that is being filtered (vs not filtered). Biggest win overall is to have a large exhaust fan on top of the case and/or high up on the rear panel. Having positive or negative pressure is not terribly important relative to these things, but I would place slightly greater emphasis on having well-placed and powerful exhaust fans.

 

[Seoulja615]

i dont think intake airs blow that much air to cool down the components. I think you need to have some what intake to create air flow to move air out. So I say more Negative air flow

 

[vlc]

C'mon guys this isn't all that complicated. You want clean air-flow. Flow is what cools, not pressure or lack of it. Dust is a heat insulator so you want to exclude it.
1. Clean using air filters. Do not allow any unfiltered air into the system. Since the case is not sealed, you must use positive air-pressure, else unfiltered air will enter the case via any gap, port, etc.
2. Flow by having both an entry and an exit for air. Position the intake fan low and heat-exits high. Too many exits and you can't maintain positive pressure, so don't use an exit fan, unless you're sure the intake fan is able to move a much higher volume of air and your case is otherwise almost air-tight. But If you were to blow filtered air in at even high pressure, but blocked all exits, you'd not have any flow. Now, as most cases are not air-tight, just blow as much filtered air in at relatively high pressure as you can, and it will escape through the many holes in the case. If you think there is still heat accumulating at the top of the case, try tilting the case slightly (no much) forward to allow the hot air to flow more to the highest point where there are more exit holes. Try to allow the air-pressure to be released as much as possible at the top of the case, rather than at the bottom. In the unlikely event that you still think heat is building either drill a few small holes near the top, or add a small exhaust fan near the top.

 

[ScrewySqrl]

you forget how old this thread is, vlc.

 

[threepoint]

Well positive pressure = air pushed out of cracks in case in order to balance. So there is less dust, but low cool, hot air stays in the case for longer.
Negative pressure = air pulled into cracks in case in order to balance. So there is more dust, because the crack in case is not filtered. But you get more cool because there is more air pushed out of case.
Balanced = medium cooling, medium dust.
So do you want more cool, less dust, or balance? That will help you decide positive, negative or balanced

Sorry for bad English xD

 

[element5]

Based on comments in another thread, I now have a question about the ideal way to set up airflow through a case. It seems that there are two camps in this debate -- those that think the air pressure in the case should be positive, and those that think it should be negative. For the sake of clarification, I'll define what I think each of those terms mean:

Positive - there is more fresh air coming into the case than there is hot air exiting the case. Basically, the CFM from intake fans exceeds the CFM from exhaust fans.

Negative - there is more air exiting the case via exhaust fans than fresh air entering the case via intake fans and vents. The CFM for exhaust fans exceeds the CFM from intake fans.

Now I know that very few of us have taken the time to run silicon caulking on all the seams of our computers to make them completely air tight, so there is more involved here than just the fan CFM -- positive pressure cases won't explode over a period of time and negative pressure cases won't implode. Also, the direction of airflow within a case has a lot to do with the effectiveness of the cooling, but let's ignore that for now.

The real question is this, I suppose. Is it better to pull air through the system or to push it? Or does it not really matter? Anyone have links to some good testing on this, or some empirical data to back up theories?

EDIT: I turned this into a poll with a slightly different flavor. Balancing a system's airflow exactly is probably impossible, so when designing your fan setup, should you make it more negative, or more positive?

If you take an aggregate of opinion on this topic it suggests that every person who has posted to this thread is correct. I did tons of research into the topic and there are strong arguments in both directions. One thing that seems to be widely agreed on happens to be the state that you probably can not achieve ... perfectly neutral case pressure.

Therefore I set about doing a bunch of testing. I found that for me slightly positive case pressure works best. When I say "works best" I mean considering temperature, dust accumulation, etc. The difference in positive/negative pressure delta T is just a couple of degrees (note that I did not test any crazy configurations ... ie; all exhaust). There are also too many variables to establish positive/negative solely based on fan capacity (it is though a good approximation absent an exhaustive evaluation ... pun intended). Thermodynamic complexities aside, in my case I have 7 140 mm fans set for slightly positive pressure with exhaust out the top. That said there are wild cards. For example, my PSU has a fan that spools up based on a temp profile, my graphics card has 2 fans that also operate on a profile, some of the fans are air flow and some static pressure (this is important ... fan design needs to be properly applied), my CPU radiator is exchanging heat. And the biggest wild card of all ... ambient room temperature. This makes more of a difference than everything else together. The computer case and the surrounding air will seek to achieve thermodynamic balance. So if you are exhausting hot air into an even hotter room eventually your case temp will go up not down.

So, with things so close I decided to look at it from the viewpoint of threat mitigation and the number one ranked "computer killer" over time (assuming no catastrophic failure) is dust. I think if you get the computer off the floor, set the case pressure to either slightly (emphasis on "slightly") negative or positive, keep the room temp as cool as possible and remove dust religiously you will have done everything possible.

Good luck.

 

[LeeBrown72]

i have always been a fan of negative pressure as i feel it will leave less standing air in the case

whether there is any scientific proof of this im not sure ive never read a test of it if anyone has a link to one id be grateful

youd have to bear in mind tho that for any pressure change to occur from outside pressure the whole case would have to be air tight as even gaps between cd drives would pull in air to balance out so unless you plan on spending ages sealingeverything your not going to see any difference

also as a side note things cool better witht the side off so you would be better off putting a mesh on your side panel

Yes, you are correct. A vacuum (negative pressure) is much stronger and better than pressure. That's why you have a vacuum brake booster on your car. A "pressure" assist would prove highly inadequate. Further, any object under even a slight vacuum dehydrates and as it dehydrates it cools (remember, all the air going through your case contains some amount of moisture. Positive pressure causes that moisture to collect on your components, and, over time cause corrosion). Create a vacuum in your case, allow for adequate intake air to be introduced (ventilation ports) on the far side (away from the exhausting fans) of any heat sources (CPU, RAM, GPU, etc.) and cool air will be drawn directly across them. Voila, a cooler case. If you had 5 exhaust fans and no pressure fans and adequate and strategically placed air vents (with filters installed) you would have the perfect air cooling scenario. All airflow would then be of clean dust free cooler outside air being drawn through your case and the vacuum would "vacuum" the component heated air out of your case. You would then be as near to ambient as you can get. I did this on my Thor case (which is full of vent holes) and dropped the board temps by an average of 7Cs. All I had to do on this Thor case was to reverse mount a few fans.
LeeBrown72

 

[Karadjgne]

This is honestly a trick question. My daughters Sony vaio has 2 fans, 1x cpu and 1x exhaust. Because of case design, its impossible to mount an intake fan. So the whole thing runs negative, and I'm cleaning it every 3-4 weeks. My 690 II Adv has 2x 120mm exhaust and 2x 140mm. It's running positive as I keep fan speeds approximately equal. Clean it every 6 months. Case temps on the Sony run 42-45°. Case temps on the 690 run 30-32°. Just as many gaps, slots, leaks etc, a case is a case. I much prefer the positive approach. With as many gaps as there are in any given case, true positive or negative is impossible, all you really get is a hair above or below ambient pressure. The difference becomes that very tiny hair difference preventing or helping dust accumulation. Won't make a difference on temps or moisture or airflow.

 

[PeTeY735]

Negative or Positive, it's really about where the air flows. Getting a good amount of air through and hitting all the high heat areas!!! I see a lot of talk about more air in or out... Once a pressure is achieved, negative or positive, air in = air out. That is a fact. So really, get air directed over your important heat loads, get air mixing good, and get a good exchange rate. The effects of negative or positive pressure (distance between molecules, dehydration and cooling, and all the other random theory I've see in here) WON'T HAVE AN EFFECT!!! How much pressure are we talking here, barely measureable and insignificant! Unless you got some overstock fans from a NASA vacuum chamber, I think we can ignore that!

What I've found is taking air from lower is good, exhaust higher, get the air to circulate inside to avoid "dead zones", make sure it is going over VRMs, GPU, CPU, and RAM areas (depending on their cooling, air over CPU is not as important if you have liquid cooling...)

Personally, I like positive to keep the dust from coming in openings, filters and radiators drop flow (pressure drop across them). So I have filters on intake with fans just slightly faster than exhaust fans. 140mm 1800rpm on intake, 140mm 1400rpm on exhaust. 3 in, 3 out, all aimed to mix the air good and hit the important parts. Cables out of the way to let the air flow.

 

[Karadjgne]

Pc case fans are incapable of creating either positive or negative pressure in a case. (1) too many avenues of escape through fan vents especially, (2) what goes in, must come out and vice versa.

The only thing positive or negative is airflow. Positive airflow being enough intake to maximise exhaust and any leftover escaping out holes in the case. Negative airflow is not having enough intake to match exhaust, so air is drawn in through the case sides. Sounds like pressure, often mistaken for such, but it's not. It's like voltage and current.

Get enough air movement out, you cut off dust entrance at gaps. Get enough air movement in, you get dust. Either way, intake cfm will equal exhaust cfm according to airflow availability. The further you choke airflow in either direction, the less effective the fans become. Choke fully any airflow and a 2000 rpm fan will move 0 cfm. The blades just spin and nothing happens. This is why exhaust is more important than intake. Case designers know this, which is why there is always an exhaust fan, whether or not there is an intake fan. You cannot fully choke airflow in a case, even the old At cases had front vents and enough gaps to allow airflow.

Best scenario with best airflow is slightly more intake potential than exhaust. Minimizes interior dust, maximizes airflow.

There, done. Can we close this ancient thread now?

 

[Namdhda]

I'm slightly on the side of positive pressure. My tower has a 230mm intake in front, a 230mm side intake, a 120mm intake on the bottom, a 140mm exhaust in back, a 230mm exhaust on top, and a 120mm exhaust on top, plus a 120mm behind the hard drive cage pointed toward the back to promote airflow through the tower, and it's behind a shroud that directs the air right across the gpu. The cpu cooler (212 evo of course) has air pointed at the 140. My tower stays really close to room temperature.

 

[Outlander_04]

I think its time people stopped using the labels positive and negative pressure . Neither can be correct in a computer case , and neither is desirable .

For there to be either there would have to be flow AND restriction .
Cases have air flow. Yes that is caused by a transient pressure variation , but the only way to raise the pressure would be to pump air in and not let it out . The inverse is obviously true for negative pressure .

Air flow over hot components is what is required .
A very good general rule is : In at the front and bottom , out at the top and back . This works best because the hotter air wants to naturally rise .
Its also true that inlet fans tend to be quieter .
And that fans on the side of a case often do more harm to flow through the case than good .

 

[ScrewySqrl]

this thread keep getting necro'ed

 

[Karadjgne]

So close it.