CFM matching between CPU and Exhaust Fans

john_c · Post by **john_c** » Mon Jan 08, 2007 10:40 am

I am planning a low-noise build using low speed fans. I have a Lian Li V2000 Plus II, and my conceptual design is to install a Kama Bay in the front and then replace the case fans making both the side panel and the CPU fan into exhausts. I am hoping that will give me good airflow through the case with dedicated exhaust at the hottest two components to keep warm air from accumulating inside.

My initial thinking was to make all of the case fans S-Flex 800 RPM models with a 1200 RPM model on the Ultra-120 CPU cooler. It has occurred to me though that by having greater CFM on the CPU fan than on the exhaust fan, it might not promote good flow of the warmed CPU air out the exhaust and might lead to some of the warmed air remaining in the case.

When CPU fans are directed at venting fans as with the Ultra-120, are there any rules of thumb for matching the fan RPMs and/or CFM? Should the fans be matched in speed? Or should the exhaust fan be higher than the CPU fan to provide additional vacuum? Or does the heatsink reduce enough airflow for it not to matter?

Thanks.

jaganath · Post by **jaganath** » Mon Jan 08, 2007 12:55 pm

Or does the heatsink reduce enough airflow for it not to matter?

bingo. The heatsink presents an impedance to airflow, so flow volume of the 1200rpm will be reduced.

pcy · Post by **pcy** » Mon Jan 08, 2007 1:28 pm

My tests suggest the airflow through a heatsink is typically about 1/3rd of the free air flow from teh same fan.

Peter