Rear exhaust fan cfm question

[Bigmac80]

Should my rear exhaust fan have a higher cfm rating then my fans on my cpu heatsink? I have 2 140mm 80cfm fans on my prolimatech armageddon blowing air out the rear but the exhaust fan is only about 60cfm. Shouldn't my rear exhaust be atleast 80cfm or is it fine the way its already set up?

 

[jim45682]

I would think you would want your exhaust = to or greater than your cpu cooler, think of it this way, the air comming out of your cpu cooler is hot having just drawn heat from your cpu, you want that hot air out of the case as fast as possible before it has a chance to transfer that heat back into your components

 

[Bigmac80]

yeah i just got the cooler master excalibur for my exhaust...they are kinda loud but oh well...it blows a ton of air

 

[jb6684]

If you have two (2) rear exhaust 140mm fans, you don't need to go too crazy on RPMs (which while high RPMs produce more CFMs if you don't need 'em is just extra noise...)

These are very quiet, only 1000 RPM : (variety of cools available...)

http://www.newegg.com/Product/Product.aspx?Item=N82E16817139011


Although the spec on the Cooler Master are Very similar....

http://www.coolermaster.com/product.php?product_id=123


Unfortunately I've found I had to try several fans be for I found the ones that were "just right" for my cases.... Spec don't seem to mean a heck of a lot.

Also, try connecting to fan controllers on your motherboard and set BIOS on "silent" for fan controllers. That will slow and quiet 'em down a bit too.

 

[jim45682]

If you have two (2) rear exhaust 140mm fans, you don't need to go too crazy on RPMs (which while high RPMs produce more CFMs if you don't need 'em is just extra noise...)

These are very quiet, only 1000 RPM : (variety of cools available...)

http://www.newegg.com/Product/Product.aspx?Item=N82E16817139011


Although the spec on the Cooler Master are Very similar....

http://www.coolermaster.com/product.php?product_id=123


Unfortunately I've found I had to try several fans be for I found the ones that were "just right" for my cases.... Spec don't seem to mean a heck of a lot.

Also, try connecting to fan controllers on your motherboard and set BIOS on "silent" for fan controllers. That will slow and quiet 'em down a bit too.

his 2 140mm fans are on his cpu cooler in a push pull aimed with airflow front to rear, meaning his single rear exhaust cant keep up removing the hot air.

 

[rubix_1011]

Push/pull doesn't mean you can add the airflow of both fans together. You'll only realistically break even if the fans are the same, but typically you can take the average airflow/2 and even assume it will be less than that due to restriction over the cooler...and if the fans are even running at the speed rated for the CFM.

 

[jim45682]

I assumed when he stated 2 140mm 80cfm fans he was indicating 80cfm each, with only a single 60cfm exhaust
hence my reccomendation of upping his exhaust, prehaps I was in error, my bad

 

[Unolocogringo]

Most fans are rated at no static pressure. So in the open they blow or move 80cfm. Put that same fan on a thin fin design heatsint with a lot of static back pressure and cfm will drop significantly on some fans and by half on others.
Two fans in push pull do not add the ratings together.
Meaning 80+80 does not equal 160.
More like 80-back pressure=50(for a good fan)+20% for 2nd pull fan = total of 60cfm flow through heatsink.

 

[nukemaster]

Most fans are rated at no static pressure. So in the open they blow or move 80cfm. Put that same fan on a thin fin design heatsint with a lot of static back pressure and cfm will drop significantly on some fans and by half on others.
Two fans in push pull do not add the ratings together.
Meaning 80+80 does not equal 160.
More like 80-back pressure=50(for a good fan)+20% for 2nd pull fan = total of 60cfm flow through heatsink.

+1 to this.

Take a fan in the open and place your hand in front of it(feel the air flow), then feel the back of the heatsink. there is allot of restriction on heat sinks. It would surprise me if you need more exhaust.

Same thing goes for a fan with a filter, wow does it cut airflow down.

 

[rubix_1011]

^yes, that's the point I was making. 2 fans @ 80cfm doesn't = 160cfm. Fan rating is without restriction, and even then it's questionable if you actually get those results.

2 fans with 80cfm back to back with nothing between them 'MIGHT' net you 80cfm....but they'd actually work against each other with pressure differences at minor RPM differences...so even then (best case scenario) you can't achieve what your fans are rated.

 

[chris189]

It is also preferred to have more air volume being sucked out than actual cfm. Volume in quantity is better than speed of small air volume. What i mean by this is 4 120mm fans @ 110CFM will yield about as much air volume as a single 200mm 140cfm fan. Try to create a pressurized zone where the exhaust is helping the intake fans blow which will yield better performance.

 

[rubix_1011]

You are still referring to CFM if that is the case. Yes, you'd rather have volume than speed. However, this refers to different things with different size fans and speeds they run at. CFM is the same regardless of RPM; it is simply the product of RPM*fan opening, which are 2 different variables for different sized fans.