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CFM, RPM and Noise

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Robbiem01

Member
Joined
Feb 9, 2002
If I know the stock rpm and cfm of a fan then I turn it down to say 1/2 rmp with a rheostat will the cfm also be half of the stock cfm or is there some other formula to use? I ask because I'm trying to find a good speed to run my Delta 50 db fan so its about 30-40 db but I also dont want to sacrifice more then 5 or 6*C. Motherboard monitor tells me the RPM and I would like to know the cfm of the fan. I want to see if a 80 cmf fan tuned down to 30 cfm is quieter then a fan that is ment to be run at 30 cfm in the first place. Any help would be appreciated! Thanks in advance.
 

Diggrr

Underwater Senior Member
Joined
Nov 29, 2001
Well, if your using a rheostat, just tune for the best balance between noise and temp.
I don't think cfm and rpm follow the same curves (half rpm might not equal half cfm) I don't think even the fan makers have worked this out, and two different brands would be different from each other. The only way to find the real cfm is with a machine meant to measure it.

I will say this though, the higher cfm fan is likely to have more air pressure running at a slower speed than the small fan that runs that cfm naturally. So it should have the better performance.
 

chuckerants

Member
Joined
Mar 8, 2002
Location
Phoenix
FWIW, I have a AX-7 with the 80mm Delta 68CFM fan that I just installed last weekend. OMG was that thing LOUD! I connected the rheostat the next day and now I have it turned down to the lowest RPM.

As for figuring out CFM, I'm willing to bet you'll probably need some alphabet soup formula to figure out. ;-)
 

RnPgrosz

Member
Joined
Dec 14, 2001
Location
NJ
chuckerants after you changed it to the lowest rpm how much did temps go up?
 

Kosmic

Member
Joined
Mar 25, 2002
Location
Washington State
Robbie,

Your Delta will still cool better with less noise at whatever CFM you dial it into than another fan designed for that RPM, especially if it has the secondary fixed blades to correct the air flow. Measuring the CFM is hard to do, so go with Digrr's suggestion of dialing the fan into a good balance between cooling and noise.

Ya know, delta should really make their fans come standard with rheostats...

K1
 

chuckerants

Member
Joined
Mar 8, 2002
Location
Phoenix
At the lowest RPM, the Delta 68 CFM seem to run at 29C when I wake up in the morning. After I've been working on it for a while, it will get upto 35C.

This is with a telnet session and [email protected] CLI running 24/7.

I really haven't run it at full RPM since the 24 hours of running the PC w/o a rheostat.

I'll run it at full RPM tonite when I get home and let you know.
 
OP
R

Robbiem01

Member
Joined
Feb 9, 2002
I turned my delta down so it is just as loud as my other fans and now I dont have to blast my speakers when I listen to music. I used to watch tv at vol 20 but now I can hear just as well at 12! I only lost 6*C which I can fix by opening the window every few hours to cool off the room (hmm dont know what i'll do about that in the summer ... I live in michigan.) Anyways thanks for the help. I can hear my self think ... well I dont think but I bet if I did think I would be able to hear it!