• Welcome to Overclockers Forums! Join us to reply in threads, receive reduced ads, and to customize your site experience!

Mutiplying CFM

Overclockers is supported by our readers. When you click a link to make a purchase, we may earn a commission. Learn More.

DumpALump

Member
Joined
Jun 24, 2002
Location
California, US
I think if they were one after another, the cfm would raise only slightly, but the pressure would double. This would be the same if the fans were on a HC.
 

TheWesson

Member
Joined
Dec 7, 2003
Location
Portland, OR usa
Yeah the basic story is that if you put two fans inline -> A -> B -> then CFM is only a little improved.

The more complicated story is that doing that helps with a pressure differential. If there's a pressure drop right behind B, then B would be moving a much reduced quantity of air. However with -> A -> B ->, then A helps take care of the pressure drop between A and B.

In other words, if you have 'stuff' in the airflow between "in" and "out" then having a fan on "in" AND a fan on "out" helps get CFM back to where it should be. Of course this is a very common factor in computer cases!

Just to substantiate this, I've tried putting fans inline (almost right after each other) and it helps very little. (Also of course if A and B are within a few cm, then you may get a lot of wind noise.)

But having both an intake and an exhaust fan does help - some. Not like doubling your airflow.

the wesson
 
OP
O

OutS|der

Member
Joined
Feb 3, 2004
Location
Crackatoea
k so it wouldnt be doubled? i got one silen-x 120mm fan followed by a jr-120 rad followed by another silen-x 120mm fan and i was trying to figure out how much CFM that would be pushing out so i can figure out what i need to have positive pressure in my case........its set up as fan A>rad>fan B>
 

DumpALump

Member
Joined
Jun 24, 2002
Location
California, US
The cfm is equal to the cfm that one outputs. Just you have better pressure. For HC/Rad more pressure is better to get through the fins. Since axial fans don't have much pressure to begin with, a dual fan setup works out better.
 

TheWesson

Member
Joined
Dec 7, 2003
Location
Portland, OR usa
yeah you should count that as probably a little less than the rated CFM for one silen-x 120 fan.

radiators would have a lot of impedance.

anyhow if you want to check case pressure just hold some tissue paper or a sheet of toilet paper next to some hole in your case. if it tries to "stick" to the case when you hold it close, then you have negative pressure ...

the wesson