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  1. #1
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    Positive or Negative air pressure in case?

    Title says it all. I've heard/read different opinions and I tend to believe slight positive pressure is best since it helps keep out dust. Is this correct?

    EDIT - Which would be better for maximum airflow/cooling?
    Last edited by One Lazy Guy; 08-09-2003 at 08:00 AM.
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  2. #2
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    to me yes that is correct

  3. #3
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    If you are a filter fanatic, yes positive will keep it cleaner. If you don't have filters on all intake fans, it doesn't make any difference. The dust comes in through the fan as easily as the case vents.

    I think most cases are better designed for negative pressure and will have a more even distribution that way. Unless you add a side panel intake fan, I'd look carefully at air flow around the PCI slots and the optical drives.

  4. #4
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    this may sound stupid but can someone here explain to me what is negative and positive air pressure in a case?

    positive = more outtakes than intakes?

    negative = more intakes than outtakes?


    damn

  5. #5
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    other way around

    Positive = more intakes than exhausts
    negative = more exausts than intakes

  6. #6
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    lol thanks

  7. #7
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    I have to say that negative pressure (more air blowing out-more exhaust fans) in a case will have less dirt/dust buildup. I have two cases in front of me, one with negative pressure and the other with positive. On the np case, I've got a 120mm blowing in from the bottom with a 120mm on the side as an exhaust, two 80mm fans on the back as exhaust plus the two exhaust fans on the psu. Dust was never a real issue with the box but became even less of one when I put the 120mm exhaust fan on the side panel. This box has very little dust...period. The pp box just has a 120mm fan on the side panel blowing in with no exhaust fans (just two 80mm holes in the rear of the case where they should be cause I haven't put 'em back in...I'm lazy) and the dust is ridiculous. I'd consider dust bunnies a good thing, instead of these dust bears. There's no way that you can tell me that once I put the rear exhaust fans back in, it'll fix my dust problem.

  8. #8
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    Since positive pressure would increase the density of air inside the case, wouldn't that mean that heat is transferred more easily via convection? Also, would it increase the static pressure the fans exert?

  9. #9
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    I agree for the heat increase but I think the pressure would only increase on the intake fans.

  10. #10
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    if you are asking this q to find out the best way to cool your case, imho, niether is correct..............

    if you want optimum cooling, you really need to balance the incoming air with the outgoing air, so that you get a continous, solid flow of air from bottom front to top rear............... of course if youve got a fan controller, you could always put some fans in the side to give a little extra air to the gpu and cpu, and as long as they are kept at a lower speed (ie pulling in a minimal amount of fresh air at a low velocity), they should affect the airflow too much, and would benefit the cpu

    just my 2 penneth worth
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  11. #11
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    OK, two issues raised above. The pressure difference between positive and negative is not going to be enough to change the density. Air velocity is much more improtant.

    The last thing you want is exact balanced flow. You will note your case has a lot of other openings besides the fans. Air passing through these is the main way you get some cooling to things like CD drives, PCI cards, etc. If you had an exact balance, the flow would tend to "streamline" directly from the intake fan to the exhaust fan. If you look at the AMD website, they recommend having equal number of intake and exhaust fans with the extra capacity of the PSU fan providing the negative pressure.

  12. #12
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    Originally posted by nodark
    I have to say that negative pressure (more air blowing out-more exhaust fans) in a case will have less dirt/dust buildup. I have two cases in front of me, one with negative pressure and the other with positive. On the np case, I've got a 120mm blowing in from the bottom with a 120mm on the side as an exhaust, two 80mm fans on the back as exhaust plus the two exhaust fans on the psu. Dust was never a real issue with the box but became even less of one when I put the 120mm exhaust fan on the side panel. This box has very little dust...period. The pp box just has a 120mm fan on the side panel blowing in with no exhaust fans (just two 80mm holes in the rear of the case where they should be cause I haven't put 'em back in...I'm lazy) and the dust is ridiculous. I'd consider dust bunnies a good thing, instead of these dust bears. There's no way that you can tell me that once I put the rear exhaust fans back in, it'll fix my dust problem.
    Do you use filters on the intakes?
    I have tried numerous setups and personally I prefer positive pressure with filters on the intakes.
    My cases are generally near dust free with the exception of the one that is negative pressure with no filters (soon to change)
    Having little dust inside plugging up heat sinks is a plus for me.

    I can see how the negative pressure would keep less dust inside if no filters were present. The big 120mm would suck most of it out before it settles in.
    Last edited by DHutson; 08-09-2003 at 11:26 PM.
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  13. #13
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    I go with Keng's theory

  14. #14
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    Originally posted by KenG
    OK, two issues raised above. The pressure difference between positive and negative is not going to be enough to change the density. Air velocity is much more improtant.

    The last thing you want is exact balanced flow. You will note your case has a lot of other openings besides the fans. Air passing through these is the main way you get some cooling to things like CD drives, PCI cards, etc. If you had an exact balance, the flow would tend to "streamline" directly from the intake fan to the exhaust fan. If you look at the AMD website, they recommend having equal number of intake and exhaust fans with the extra capacity of the PSU fan providing the negative pressure.
    I have a PC-60. Would you know why when I up my positive, my cpu temp goes up but my case temp goes down?

  15. #15
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    I have a PC-60. Would you know why when I up my positive, my cpu temp goes up but my case temp goes down?
    No way to be sure w/o hearing/seeing more, but it is likely the flow pattern. With positive case pressure air is flowing from intakes thoughout the case to every available exit path and recirculation zones can develop where these paths split and near other stagnation points. If your CPU cooler exhausts into one of these areas the chances of recirculation from HSF exhaust into HSF inlet increases, raising the inlet temp and CPU temp along with it. A little judicious ducting to isolate CPU HSF inlet from its exhaust can often cure this. If you want to maintain positive case pressure to lower dust accumulation, see to it that the negative pressure inlet zone of the CPU cooler couples well to a high pressure zone fed by case intakes w/o crossing HSF or case exhaust regions. The brute force solution is a side intake close enough to the CPU HSF inlet to provide external air to the cooler and help suppress development of lateral recirculation cells. The trick is to avoid competing with cooler exhaust a/o direct couplin to case rear a/o upper exhausts.
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