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Enclosure cooling : blow fresh air or suck hot air ?

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jujun:
Hello,

I often see fan that suck heat from the inside of the enclosure to put it outide, but when air is heated, it's have a lower density, so It must be more efficient to blow fresh air inside an enclosure, and let the heated air flow outside by itself. Is it right ?

Do you know any good practice about enclosure cooling ?

Thank you
J

Conrad Hoffman:
Not sure about entire enclosures, but I do know it's vastly more effective to blow on a hot component to cool it, than to suck air by it. Internally, find out what's hot and arrange the ducting so air it forced towards it. It probably doesn't matter how that's achieved, fan pressurizing the case, or exhausting it.

jaromir:
Fan blowing cold air has higher life expectancy than fan sucking hot air.

SiliconWizard:
I would say the key point whatever you do is airflow. It would very much depend on the enclosure itself, where the openings are located and what's inside, which will all influence airflow.

Blowing outside ("cold") air in tends to accumulate more dust around the fan. A filter would of course be needed, but it will usually tend to clog faster in this setup, one of the reasons why it's rarely done IMO. Likewise it will tend to suck the most cold air in from the fan's opening, whereas blowing hot air out will tend to in turn have cold air sucked in from pretty much all openings in the enclosure. Those are general considerations, of course again it would require a proper airflow analysis to conclude.

Really it's all in the airflow. In a "perfect" airflow situation, I don't think it would make any difference as to efficiency. Whatever volume of air you're blowing out will be replaced by air sucked in, or conversely. Now airflow is never perfect in a real enclosure, there are many obstacles and turbulences, openings are often not balanced/symmetrical, etc. If you're forcing cold air inside the enclosure, I think the cold air will mix with the hot air before eventually getting out, whereas forcing hot air out will tend to make the hot air get out faster, so there should be less inertia in a typical "crowded" enclosure. (And again in an ideal setup where the enclosure is very large, contains only negligible obstacles and has symmetrical in and out openings, it would probably not make a difference.)


Nominal Animal:
For dust ingress, it is useful to have the pressure inside the enclosure very slightly higher than ambient.  The pressure differential creates a kind of a barrier for dust.

For best results, you'll want good airflow.  That is, you'll want the air to flow freely from the input vents to the output vents inside the enclosure, passing over heatsinks and components that need cooling.  You don't want laminar flow, where the airstream stays in a tight bunch, though; you'll want a little bit of turbulence to ensure there are no pockets of nonmoving air.

I've built silent and low-noise but high-performance PC enclosures for myself and some family members for fun, both normal-sized and mini-ITX ones.  I use both ingress and egress fans, in push-pull configuration, with plastic and foam baffles to direct the airflow.  Foam baffles also absorb some noise from e.g. fan motors, generating more easily tolerated wider spectrum noise instead.  (Whoosh instead of whine.)  When using input air filters, the fans were about equally powered; without input air filters, the intake fan a bit more powerful (not just in airflow, but also capable of generating a small pressure differential without dropping the airflow too much).

Use the largest fan you can, even if it is in a suboptimal position.  Small (Ø50mm or smaller) fans have to rotate too fast to move any air, and will whine.  Larger fans can rotate slower, and in the 80×80×25mm class (smaller 12V PC case fans) you have a large selection of fan types to choose from.  The 120×120×25mm 12V fan class has some excellent fans that can push quite a bit of air at 800 RPM, being basically silent; I like both Noctua and Scythe fans, but like I said, there is quite a lot of variety available today.

Using a much larger fan also means that laminar airflow is usually not a problem, as the flow is generated in a relatively large area.  This includes situations where there is very little room before/after the fan -- although in that case, I'd pick a fan with lower airflow but higher pressure differential capability.

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