cas cooling: airflow

[VJ]

Hello,

I'm trying to optimize the airflow/cooling in my pc case.

As airflow related stuff, it has a Noctua NF-B9 intake fan, 2 Noctua NF-R8 outtake fans and an Enemermax Modu82+ PSU.
The Noctua fans come supplied with adapters to slow them down, for the NF-B9, this yields possible airflows of 64.3 m³/h, 52,6 m³/h and 41,1 m³/h. For the NF-R8, it yields possible airflows of 53 m³/h, 39 m³/h and 26 m³/h.

Now, the question is: should I optimize the speeds so that there is more airflow for the intake fans (positive pressure in the case) or more airflow for the outtake fans (negative pressure in the case). Ideally, they should be balanced, but as this is virtually impossible, the setup will lean to either one of the choices.

As I gather, negative pressure in the case is better for cooling, but worse for dust-buildup.

How do you guys feel about it?

[Fat Tone]

My gut feeling is that you'll get a higher cooling rate to the gas that has higher density.
I suspect its all very marginal though and you'll end do doing whatever is quietest and keeps your system stable.

[VJ]

As I found on various sites, it is claimed that:

positive pressure helps keeps dust out (air only enters via intake fans, which should have filters), but has a higher risk of causing air buildups (where air just sits still). If there are no such build-ups, the fact that there is more air is an argument for better cooling.

negative pressure may pull more dust in, but due to less turbulence, airflow is better and thus also cooling. Also, as the main pull comes from the rear fans, it can be more silent... The fact is also that most cases have more outtake then intake fans... so are most case designs wrong?

So there are arguments for either...
I know it won't matter much, but was just curious.

[WxDude]

I remember seeing a video on the net not long ago from one of the case manufacturers.
They wanted to show the benefit of positive pressure. One of the biggest benefits is that
you can SIGNIFICANTLY reduce the amount of dust buildup in the case (and thus improve
cooling) with positive pressure as you can filter the air at the fans sucking air into the case
and no air will leak in (with dust) through the cracks in the case.

Here is a link to an article from Silverstone about the benefits of positive pressure:

http://www.silverstonetek.com.tw/tech/wh_positive.php

And here is a link to the Youtube Smoke video:

http://www.youtube.com/watch?v=Qe-2Z...layer_embedded

- Mike

[Nowhere]

More outtake imho - ATX standard itself seems to be optimised for it, with "out" fans simply strenghtening convective airflow. Positive pressure might work slightly against it...

(that said, I have one intake fan, spinning extremely slowly (mostly for silence), with airflow directed just below HDD and towards expansion cards, horizontal placement of which can inhibit "natural" ATX airflow after all; however most of the job is done by PSU fan ("bottom-fanned", 12cm) and backplate fan)

As for the dust...I don't think you can really prevent it; you have to keep house/PC clean anyway (reminds me, recently I had to do something inside my PC, looks like vacuum cleaner (one year)) A bit of compressed air from time to time, in a "safe" place, doesn't seem really that harder to me than cleaning filters/having stack of new ones.

[VJ]

My gut feeling is that you'll get a higher cooling rate to the gas that has higher density.
I suspect its all very marginal though and you'll end do doing whatever is quietest and keeps your system stable.

The origin of my main question is simple: should I slow down my rear outtake fans...?

Intake capacity is the same, ignoring the PSU fan, I currently have 1x 64 m^3/h in and 2x 53 m^3/h out. I can slow them down to 2x 36 (still negative!) or even to 2x 29 (slightly positive, depending on the airflow of the PSU cooler).

I should add that all the fans are slowed down (together) by the mainboard when temperature permits. But given the fact that they all are slowed down similarly, the difference remains.

[Mehen]

Dusty room? Air filters? What do you need to cool?

IMO if you aren't overclocking, and nothing is overheating - go the less dust route.

You could always go with a negative pressure case and cover up and dust-leaks with bits of used drier sheets.

[VJ]

Yes...
For me, I'll just test some configurations to find out which is the most silent; I know it won't matter much (no overclocking or so).

I just find it odd that such an - in essence - simple question turns out to be so difficult to answer. It triggered my curiosity...


Jörg

[TransformX]

negative pressure may pull more dust in, but due to less turbulence, airflow is better and thus also cooling. Also, as the main pull comes from the rear fans, it can be more silent... The fact is also that most cases have more outtake then intake fans... so are most case designs wrong?

You should see my DVD drive if I don't clean my computer in a while. Dust and cat hair block the intakes and then air comes in from other small cracks, such as the CD drive etc...

[Marshmallowman]

More intake fans with filters is the way to go. It reduces dust but also adds a bit extra air flow to dual slot video cards and psu's.

If you have extra output fans you reduce the effectiveness of some video card coolers and psu's.

Also it keeps you DVD drives clear of dust

[Evildead666]

Yup, Marshmallowman, on the head.
Graphics cards sometimes have outward cooling fans, and you have to have a positive pressure to ensure enough airflow.

On my parents case, which is stock, I have set a fan below the PSU to blow IN, thus creating positive pressure, since there are no other case fans.
The PSU above is plenty powerful, and the cool air blows out slightly from the front bottom intake fans (off). very quiet, and the cpu gets cool air. the air the psu blows out is warm and goes up
Its and Athlon x2 2200Mhz with an SI-120 on it, and a 120mm fan. have tried prime95 for a few hours no probs.

[VJ]

Ah, but my videocard does not blow out the back (single slot card)... The case has 1 92mm fan, 2 80mm fans blowing out and the PSU; no other devices pull air in our out. There is one vent on the rear above the card brackets. Slowing down the rear fans triggers fan speed alerts, so I don't have that much room to play with.

[Evildead666]

If it was me, i'd try and have the two 80mm fans blowing IN and the 92mm on the front switched off, just to see what the thermals are like....
but thats me

[VJ]

Interesting thought about reversing the airflow... but the rear vents are unfiltered and the case makes mounting fans quite difficult (noise isolation stuff), I gather much dust could be sucked in (granted with the open vent in the rear and a negative pressure, the same holds). It could also create a loop with the PSU fan. This is a rear view of the case:

I may try it though, bit it would also require me to reposition the CPU cooler, it is not the one in the photo, but the airflow is similar:


I tried slowing down the rear fans , but it triggers fan alerts when the mainboard slows the fans down further. Ignoring those, the system runs hotter (full load test) with these fans at low speeds than with them at high speeds. I assume the higher fan speed causes more airflow which seems to wheigh up against the upside of positive pressure in the case.

[Evildead666]

The PSU fan will not be able to suck ALL the cool air coming in from the back, and yes, you will have to modify the CPU fan if its mounted like that.
it would have to go on the other side...

The Fans at the rear will pull less dust than a low front fan, which picks up a lot of dust, especially if on the floor, since your feet will generate small amounts of dust...