Random thoughts on airflow

[mackerel]

For practical purposes I've just put into action a kinda custom case. My original intent was to have 4 equal size holes, two intake on bottom, two exhaust on top, and only put fans on the exhaust. Now I'm wondering... should I have more passive intakes? Actually, the more I think about it, the better it seems as an idea. Due to the environment it will be used in, I don't want high speed on the intake, while retaining high volume flow. I might have answered my own question.

 

[EarthDog]

What can be more passive than no fans???

Not entirely sure what you are getting at. The thread feels like a thought bubble...

 

[mackerel]

Assuming a sealed box, if I have two exhaust fans moving air out, the same amount of air needs to go in whatever other holes there are. I was going to have equal size and number intakes, but now I'm thinking if I have more intakes, the flow through each would be slower. This may be advantageous for what I want to do in reducing the chance of picking up stuff from the outside.

It means I need to do another order though...

 

[Alaric]

Or filter the intakes-but that would reduce flow and require more intake area anyway...

 

[mackerel]

They will be filtered anyway. I'll start a proper thread on the "case" once I get the bits in, as it will be something rather different...

 

[Alaric]

I'll stay tuned for that. The best (ambient) cooling system in the world is useless if the air won't flow!

 

[trents]

All exhaust and no intake is a good formula for dust/pet hair intrusion because it will create negative pressure inside the case. Filters will help but . . .

 

[mackerel]

For practical purposes I'll have a sealed box (for other reasons), apart form intakes/exhausts. It doesn't matter what the pressure inside will be. The intakes will take in as much air as will be going out regardless.

 

[EarthDog]

All exhaust and no intake is a good formula for dust/pet hair intrusion because it will create negative pressure inside the case. Filters will help but . . .

He mentioned it is a sealed case. The only way dust gets in is through the intake which will be filtered.

That said, people will be surprised how little positive pressure vs negative makes a difference with dust in a case. Most cases with filters on them it wont really make a difference. The really isnt a 'pressure' difference in the first place. Air, like water, uses the path of least resistance to get from a to b and typically isnt through tiny cracks in the case.

 

[trents]

It just seems like common sense to me that if more air is being pushed in than is exhausted there will be at least a small pressure build up inside the case which will act like wall at the intake site, causing dust and hair to drop out before going very far.

 

[EarthDog]

If the case isn't sealed, like 99% are not, it matters very little (in my testing years ago with 2 different cases). I don't know if that is the law, but, thinking logically about it, there isn't a alot of dust that can go through the cracks and crevices in the first place.

 

[trents]

No, I agree that very little dust will enter through the cracks and crevices. I'm talking about entering through the intake fan(s) opening. Even though there is air moving through the case when intake exceeds exhaust it still meets some resistance at the interface like a speeding car running into the back of a car moving more slowly. This would cause some particulate matter to drop out of the air stream before it gets very far in the case. But if the filters are more effective then it is a moot point. It needs to also be mentioned that filters themselves need to be cleaned as well so I'm not sure it's worth it when you consider the expense of the filters themselves and the hassle of removing them to be cleaned. Personally, it just seems easier to me to open the case and blow everything out with an air compressor. Filters also cut down on ventilation and the cooling that affords.

 

[JrClocker]

What you are describing will present a high resistance to the exhaust fans. So you are going to need fans that can move air through that (i.e. a static pressure rating on fan, not just CFM).