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Old 08-19-2019, 08:11 PM
Midiamp Midiamp is offline
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Default How to calculate airflow properly?

Hi all, I'm thinking of switching from my current intake, 2x ML140 RGB to 3x LL120 RGB. I'm just wondering how to properly calculate the airflow from the new fan sets. Is it as simple as just multiplying the listed airflow from the tech specs?

For example, the ML140 is listed at 55.4 CFM while the LL120 is 43.25 CFM. So does that means at max speed, the total airflow from my current fan set is 110,8 CFM while the new fan set delivers 129.75 CFM? I'm just putting the numbers roughly off course, not considering blocked path/components or anything.

I'm considering to replace my case, Define C TG to Meshify C TG, I thought after all these time, why would I use an RGB fan on the intake without ever seeing it. Off course I bought the ML140 RGB not by choice, because there's nothing in the market that isn't an RGB anymore.
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Old 08-19-2019, 09:26 PM
c-attack c-attack is offline
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Theoretically. All specified values on the box are "free air" measurements -- no resistance. That condition does not exist in real life, but you can mostly ignore it. Where things get tricky is when you are mounting against a radiator or dense dust filter. Then the result are more elusive and not all fans are affected equally.

However, in the simple calculation (all you can really do), you can also factor in speed to make better comparisons. A 3000 rpm fan will have a very plump CFM value, but that only exists when running at 3000 rpm. Not something most people can't take. You can take a further step by multiplying in a fraction of desired speed/maximum speed x CFM at maximum. This can be useful when comparing fans with different maximum speeds or different sizes.

So perhaps you feel the fastest you are willing to run the ML140-RGB is 1000 rpm. You can multiply the 1000/1200 x 55.4 = ~46.2 cfm per fan (x2) = 92.4 total.

The 120mm fans you can stand up to 1400 rpm and thus 1400/1500 x 43.25 = 40.1 x 3 fans = ~120 cfm

Both those fans have relatively low maximums and the values are close to mark. This can be more useful for high speed fans you will never run anywhere close to their specified maximums. In reality, the P-Q curve of a fan is not perfectly linear. 140mm fans are usually close, but 120mm can be strange. Nevertheless, you are rarely given one to work with so the straight mathematical estimation is all you can do. All this said, 3x120 is going to beat 2x140 (at similar noise levels) every time.
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