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Static Pressure vs. CFM

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post #1 of 35 (permalink) Old 05-20-2010, 04:46 PM - Thread Starter
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This thread is meant to be a conversational (friendly debate) piece.

I have been doing a lot of research on Static Pressure (SP) and Cubic Feet per Minute (CFM) in our case fans. And I have a few things that I think could be debatable about the whole importants of Static Pressure for Radiator (Rad) or Heat Sink (HS) cooling.

Static Pressure:
When talking about cooling a Rad or HS most people will tell you to look at the SP of a fan as opposed to the CFM. A good SP rated fan will be able to overcome the flow restrictions that the Rad or HS fins create (by blocking air flow). A fan with a good CFM and a low SP rating will not be able to push the air through the fins as efficiently as a fan with a good SP rating.

CFM:
Fans with a good CFM will usually have a lower SP rating. Meaning they move a ton of air as long as there is nothing in their way.

Two Jobs Two Fans:
We all see people getting say 6 of the same fan for their case. 2 for the Rad push/pull, 2 for exhaust and 2 for intake. However, to me the Rad and/or HS fans are and should be different fans than your case fans.

IMO, case fans should have as much CFM as you can stand to hear. Look at it this way, CFM is for moving air in and out. So for me a fan with good SP is not as good as a fan with a lot of CFM since I want to get the cool air in and then get it out as fast as possible. A SP fan is not going to move as much air and when we are talking about the transfer of air I want CFM.

Now for Rads and HS you can get better cooling from a fan with good SP but herein is my friendly debate. I want a fan with good SP yes, but I also want a fan that will move a lot of air.

So I opt to get a fan with decent SP but a better CFM rating. My thinking is I need to move as much cool air as possible over the fins of my cooling device.

Example:
As an example here are two fans to look at. (Disclaimer: the Magma has 200rpm more but I don’t think it does much)
  • Noctua NF-P12 = 1.68mm SP & 54.3 CFM
  • Enermax Magma UCMA12 = 1.4mm SP & 69.15 CFM
Now in looking at these two fans the NF-P12 has a 0.28mm-H20 gain on the Magma but the Magma has a 14.85CFM advantage over the NF-P12.

So for me, I will give up the 0.28 of SP to get the almost 15CFM more with the thinking that I will move more air over the cooling device which at the end of the day is what we all want.

More to the point I think by strictly looking at SP you could be doing yourself a disservice. There needs to be a happy medium.

Just my $0.02 worth…

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post #2 of 35 (permalink) Old 05-20-2010, 05:19 PM
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Quote:
Originally Posted by Michael Cox View Post
Fans with a good CFM will usually have a lower SP rating.
how many fans have you looked at have specs that support that statement? i ask because one class of fans, the delta focused flows, not only have extremely high cfm ratings, but extremely high pressure as well (with extreme power draw and noise to match!). if you are just looking at specs, it seems to be that lower rpm fans will generally have lower cfm and pressure, and that smaller fans seem to have higher pressure (but not always higher cfm) than larger fans, all else being equal. simple example from me, my 60mm delta black label has way more pressure than my 180mm case fan, but probably moves less air overall.

also, i don't think you can compare specs from one manufacturer to another, as they may not conduct tests the same way. not only that, but manufacturer specs should be taken with a grain of salt, so really they are only good for comparing fans from one specific manufacturer.

finally, without testing to prove anything, i would hardly make any decision based on specs when the difference is .28mm or 15 cfm. those differences seem so small as to be insignificant. honestly, i'd make a choice based on looks and price if they are that close to eachother, unless several reviews said yes, this fan is better (in whatever way that matters to me) than that fan.

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post #3 of 35 (permalink) Old 05-20-2010, 05:22 PM
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The fan game seems to be such a complicated one. Ultra Kazes on a fan controller this, Gentle Typhoons that, Yate loons this.

Since I'm cheap, more air at a low price seems to do the job. I got a 2500RPM 120mm fan out of a gateway case and running full blast on my heatsink on one end and the stock Heatsink's fan on the back pulling air seems to keep things nice and cool. If I ever have the money to nitpick about static pressure, I will.

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post #4 of 35 (permalink) Old 05-20-2010, 05:29 PM - Thread Starter
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Fair play Cyco-Dude. I am only looking at 120mm fans since that is what is used mostly for case and rad/hs fans. And as I said this is just my opinion and you know what they say about opinions!

However, I would think there has to be a point when SP and CFM would cross on a graph and become a situation when one would over power the other for a bit but then they would trade places. Hope that made sense it did in my head. lol

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post #5 of 35 (permalink) Old 05-20-2010, 05:31 PM
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From what I understand is you would want fans with higher static pressure for your heatsink fan(s) and higher cfm for your intake/exhaust.


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post #6 of 35 (permalink) Old 05-20-2010, 05:35 PM
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San ace has both?

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post #7 of 35 (permalink) Old 05-20-2010, 05:40 PM
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Quote:
Originally Posted by NomNomNom View Post
San ace has both?
San aces have pretty good static pressure and high CFM xD. If your willing to dish out the cash.

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post #8 of 35 (permalink) Old 05-20-2010, 05:42 PM
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Quote:
Originally Posted by BNT View Post
From what I understand is you would want fans with higher static pressure for your heatsink fan(s) and higher cfm for your intake/exhaust.
Pretty much. So like slipstreams or r4's for case fans and panaflos, san aces, or gentletyphoons for heatsinks and rads.

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post #9 of 35 (permalink) Old 05-20-2010, 05:45 PM
 
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My fans have 78CFM and 3.90mmH20. 28dB
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post #10 of 35 (permalink) Old 05-20-2010, 05:47 PM
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Originally Posted by NomNomNom View Post
San ace has both?
You can not compare San Ace with Noctua, Enermax .... Those are more like case fans not heatsink or radiator fans
So , yes San Ace has both 100 CFM & 7.2 mm SP

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