Voltage vs CFM - is there a formula/ratio?

[Thrackan]

Hi all,

I'm currently in the process of dubbing which of the fans I have in my pile I will be using in my PC. I'm mainly looking at which fans will move decent air with decent noise, so some of my fans will need to be undervolted.

So I was going calculating when I realized I didn't know whether half the voltage means half the CFM. Say a fan blows 100CFM @ 12v, does it blow 50CFM @ 6v?
Or is there a fixed ratio or formula I can use>

If anyone can tell me, thanks a lot!

 

[W1zzard]

no, definitely not half. and half rpm is also not half cfm.

i'm not even sure if a fan with 100 cfm at 12v blows more air at 6v than a fan which has 90 cfm at 12v

try the fans, get the one with the most airflow as case fan. for cpu fans look for pressure. your hand and ears should be good enough for this testing

 

[Thrackan]

Thanks, now I know why it was difficult for me to find this info

To add to my own confusion, here is an example:

The Noiseblocker XL series consist of 2 fans with the exact same blade design.

The XL1 twirls around 1000 times a minute while producing an airflow around 33CFM
The XL2 twirls around 1400 times a minute while producing an airflow around 56CFM
1000/33 = approx. 30.3 RPM/CFM ratio
1400/56 = exactly 25 RPM/CFM ratio

But, those are specs, measurements on some reviews note the XL2 actually blows over 60CFM and the XL1 blows around 34CFM.
See a review here
1000/34 = 29.4
1400/67 = 20.9

That still doesn't make sense, and it's only 1 brand of fans

Also, I don't need them as CPU fans, I will have 2 blowing through a low restriction radiator and 2 outside the case.

 

[phanbuey]

1) no radiator is low restriction, not even the low restriction ones. High static pressure is a must for rads.

2) each fan has a different sound profile in a different environment... one fan can be dead silent, but as soon as you put something in front of it, it will become loud (like noctua fans), while another can be relatively unaffected.

3) the blade design will determine static pressure and RPM/CFM ratio, each blade design is different, and each fan is different - more RPM's means more static pressure as well, so the CFM's will not scale linearly. A rule of thumb is if you look straight at the fan, and there are virtually no gaps between the blades, then it has higher static pressure than a fan with similar characteristics and gaps between the blades.

4) Voltage/rpms depends on fan motor, each fan will be different, some will spin up at 5v some will not spin until 7v...

like W1zz said, the best way is to test - no matter how good a fan looks on paper, in reality they can do some really weird stuff like whistle at certain voltages, so test until you find what you are looking for.

 

[Thrackan]

1) no radiator is low restriction, not even the low restriction ones. High static pressure is a must for rads.

I know by experience that my San Ace fans on 5v or 12v make little difference in temps. I'm talking 1 or 2 degrees.

2) each fan has a different sound profile in a different environment... one fan can be dead silent, but as soon as you put something in front of it, it will become loud (like noctua fans), while another can be relatively unaffected.

3) the blade design will determine static pressure and RPM/CFM ratio, each blade design is different, and each fan is different - more RPM's means more static pressure as well, so the CFM's will not scale linearly. A rule of thumb is if you look straight at the fan, and there are virtually no gaps between the blades, then it has higher static pressure than a fan with similar characteristics and gaps between the blades.

I'm talking about fans I already have, so I know how noisy each one is.
I also know, by estimate (no specs on the San Aces) or spec, how much each fan blows at 12v, but I want to calculate the CFM at 5v.

4) Voltage/rpms depends on fan motor, each fan will be different, some will spin up at 5v some will not spin until 7v...

like W1zz said, the best way is to test - no matter how good a fan looks on paper, in reality they can do some really weird stuff like whistle at certain voltages, so test until you find what you are looking for.

You'd be 100% on the spot if I was shopping fans

So let me elaborate on my "problem" here.
I have basically 2 choices to put on the rad:
- The San Ace fans at 5v (otherwise the noise is unbearable)
- Noiseblocker XL2's at 12v or tuned down a bit, they are not loud imho

If I put the Noiseblockers on the rad, I have to put 2 Coolermaster fans on the exhaust spot, since the San Ace fans are too thick to fit there.
Also, I can't monitor the San Ace fans' RPM, since they draw more current than I feel safe to put on a motherboard header. It also seems splitting the RPM cable and putting that on a mobo header doesn't work either, I tried that too.

So, instead of screwing everything out of the case and trying how much they blow, I was trying to see if there was a formula that I could use to approximately see which one would "win".

That, and I was just interested if it is calculatable.

 

[EastCoasthandle]

Hi all,

I'm currently in the process of dubbing which of the fans I have in my pile I will be using in my PC. I'm mainly looking at which fans will move decent air with decent noise, so some of my fans will need to be undervolted.

So I was going calculating when I realized I didn't know whether half the voltage means half the CFM. Say a fan blows 100CFM @ 12v, does it blow 50CFM @ 6v?
Or is there a fixed ratio or formula I can use>

If anyone can tell me, thanks a lot!

As answered by others no, 6v does not mean 50CFM. However, on that train of thought you can buy a high CFM fan(s) and use a 25 Watt Rheostat or the 50 Watt Rheostat. This will allow you to tune the voltage and speed gradually to a level that you desire in both CFM and noise. That way you have more options then just 6V.

Side note:
At the time of this post the post above wasn't visible to me. And from reading it (now) you want to use existing fans. Having said that I would still suggest the Rheostat unless there is something.

 

[Thrackan]

As answered by others no, 6v does not mean 50CFM. However, on that train of thought you can buy a high CFM fan(s) and use a 25 Watt Rheostat or the 50 Watt Rheostat. This will allow you to tune the voltage and speed gradually to a level that you desire in both CFM and noise. That way you have more options then just 6V.

Side note:
At the time of this post the post above wasn't visible to me. And from reading it (now) you want to use existing fans. Having said that I would still suggest the Rheostat unless there is something.

I have a fan controller which I can use, even for the San Aces.

But as you all mention, there is no such thing as a formula apparently. Guess I'll have to test it.