Proper way of calculating air volume replace time for PC case?

Arika · July 13, 2022

It has been a while since i've been in school, so i'm probably missing something very obvious.

I'm thinking of changing my case to a smaller one and have been trying to figure out how often the full volume of air is replaced in my current case vs the predicted replacement of the new case.

I just took all the CFM values of my fans, added them together and then divided the cubic feet volume of my case into that total CFM number. I'm pretty sure I've done something wrong, because apparently the full volume would be replaced 127 times per minute (every 0.47 seconds) with all my fans at their lowest speed.

I know there will be some loss in efficiency, so the numbers wont be 100% accurate but surely this is wrong, right? even with a 50% discrepancy, that's still every second.

For Science! · July 13, 2022

and that is why "hot air rises" is a non-concept when taking fans into account....

I mean CFM numbers are ideal case-scenarios with zero obstruction, so expect the real numbers to be lower, but the reality is probably not too far off. Whether you also successfully achieve a "new air in, old air out" scenario is also largely dependent on the layout. But yes, I would absolutely not be surprised that if every few seconds a case's air is displaced.

Electronics Wizardy · July 13, 2022

35 minutes ago, Arika S said:

I just took all the CFM values of my fans, added them together and then divided the cubic feet volume of my case into that total CFM number. I'm pretty sure I've done something wrong, because apparently the full volume would be replaced 127 times per minute (every 0.47 seconds) with all my fans at their lowest speed.

Well a few issues.

Not all the fans will will be working together. Lots of fans will be moving air that anouther fan has already moved. So if you have an intake and an exhaust fan, you will have the cfm of one of the fans as the other will be moving the air the other fans already moved.

Next issue is the rated CFM is only at 0 pressure. Lots of nice fans have a graph of pressure vs airflow as the higher the pressure, the lower the flow.

Also not all the air in a case gets replaced equally. so lots of air in the case will likely almost never get moved(like under motherboard or in the corners), but some air will be replace much more often(like infront or behind a fan)

WoodenMarker · July 19, 2022

Listed CFM is at 100% rpm with no obstructions.

Assuming neutral airflow with equal intake and exhaust, a very rough calculation for effective CFM could be to account for just exhaust airflow, halve the listed CFM for obstructions like mesh / filters, and multiply that by fan speed percentage.