How do you calculate the volume of air a fan pushes?

[RetroUO]

I saw a post saying that you want a slightly positive case pressure and I'm preparing to begin making, and modding) my first computer in the next few months and figured this is something I should know for when I setup my fans. How can volume be calculated? I was thinking size*speed(maybe the volts going to it for speed). Does that sound correct? If not how is it done?

[myv65]

1) Read the fan specs.
2) Realize that no fan delivers its claimed flow
3) Know that positive case pressure only matters if you'll be using intake filters and cleaning them religiously

The whole point is to keep dust intrusion at a minimum. The only way to do this is positive pressure using filtered air. If you don't keep your filters clean, they will gradually decrease the intake air (read: you'll go from positive to negative pressure). There's nothing inherently wrong with zero exhaust fans provided you have enough intake fans AND a good exhaust plan (exhaust openings near the top/back of the case). That'll guarantee positive case pressure even if you don't clean the filters, but you'd lose overall airflow and have higher temperatures if you don't do the routine maintenance.

[BaldHeadedDork]

Since I was one of the guys advising positive case flow, here's how I calculate airflow:

Say I'm going to use four 80mm case fans on a new rig I'm building. I'm going to buy two fans for the intake that are rated one volume level higher than the exhaust fans. For example, I'll use a pair of 32CFM fans for the intake and a pair of 26CFM fans for the exhaust.

The difference between the two will create a positive case pressure if the miscellaneous gaps can't pass more than 12CFM. You can check this by wetting your finger and running it around the bezels or the edge of your side panel. You should feel a little air coming out of the case. If you don't the differential volume isn't sufficient and you either need to run more powerful fans on the intake, or slower exhaust fans, to raise the differential volume.

Myv65 is right, claimed flow rates can be pure fiction. But if you stick to one brand of fans for your project you should be able to get a differential in flow rates that you can work with.

And he is spot on that filtering the intake and keeping those filters clean is essential. I've picked up the habit of cleaning them monthly, but if you don't smoke and have a low-dust environment you can stretch that out.



BHD

[BaldHeadedDork]

Oh, and any internal fans (like for your HSF and GPU) do not get counted into calculating the airflow. You're only concerned with the fans that push air into and out of the case.


BHD

[RetroUO]

Thx, I think that answered my questions. So pick one brand and just go ahead and use thier volumes to 'calculate' it but just know that if I ever mix brands I need to know they probly aren't actually that, only good for in-brand comparing.
And we have 2 dogs and we don't exactly dust our house THAT often so I'd probly clean my filters once a month, it's not that hard is it?