CFM parameters, is 2 better than one?

[Wull]

Hello,

If I have two 120mm fans both with CFM stated at 80 each.

Does installing the two 120s side by side on the case side total a CFM of 160 or is it not that simple?

I'm wondering whether to go for a large side fan such as a 200 or 250mm or multiple 120s.



Also, balance of air being pulled in and air being pulled out. Should this match? The case I'm looking at is an old thing I picked up for £5. Currently has accomodates 80mm on front and on 80mm on back. Would like to change these two to 120mms then put something large on the side as my AMD dual 3Ghz really heats up every board it sits in. Should a a 4th fan be added to compensate for what the 3rd fan is doing?

[nevermind1534]

I'd imagine that it would be either 80, or 160.

~~~NVM, i misread, I would say 160

[crenn]

Depending on the fluid dynamics, you could get an average of 160CFM. As for balance for air being pulled in/pushed out, not really needed. A larger fan is normally used to push a fair bit of air while being more quiet, however, it's a larger power draw (depending).

[Luke122]

Larger fans can also be quieter, as they can spin more slowly, and still more more air than smaller fans. If noise is a concern (and 2 - 120mm fans is too loud), then a single 250 would be quieter, if running at lower rpms, and would still more more air.

As always, it depends on the fans themselves, so check flow (CFM), RPM, and DB (noise) ratings on any fans before purchase to avoid having to return them.

[FuzzyPlushroom]

If you have strong negative pressure (more being exhausted than sucked in) it'll get dustier than it would otherwise; beyond that, it doesn't really matter. Two 120s at 80 CFM each will indeed push 160 combined, probably slightly less than that since they're next to each other, but not by much. It'll be close enough for government work.

[Wull]

What happens with one fan piggy backing on another? Combine the CFM or will they both lose a little due to the proximity of the other?

[crenn]

Same CFM, different air pressure.

[Wull]

Thanks all for the advice.

Still not 100% clear on the piggy backed fans. I'm guessing same would mean the 2 fans both with CFM of 100 would still only produce 100CFM when piggybacked.

What if the values were different for the fans, say one was a 100 and the other 80?

When you say different air pressure, how does that air pressure affect the temp, for good or no difference? (this only comes up because I saw a mod where someone put a fan with LEDs on the back of a heavier fan that had a prop attached and wanted to know its effect).

Also, if I mount one on the back of the other will that count as 2 towards the whole negative and positive pressure?

All these questions, everytime I get an answer it seems to bring up another 2 questions for me.

[nevermind1534]

The difference with the piggybacked fans would not be great. I think the biggest difference that those would make would be the noise level.

[silverdemon]

if you look at a fan as just a simple hole, there can flow air through at a speed of 80 CFM (cubic feet / minute)
putting two fans beside each other means you have two holes: so the airflow doubles to 160 CFM.
putting two fans behind each other does not do anything with the airflow, because the hole will remain the same size, thus two fans behind each other will result in 80 CFM airflow.

pressure is a different story, I'm not really into that. But since pressure is measured in force/area i'd guess that:
putting two fans besides each other will give you the same pressure (double the airflow, but also double the area)
putting two fans behind each other will give you very little change... normally you would think that the pressure doubles (two fans -> two times the airflow but the same area)
but we just said that the airflow won't change when putting two fans behind each other. So my guess is that putting two fans behind each other is not doing anything due to the fact that they are not nearly as efficient as they are when placed on their own.

I hope this help

[Wull]

Very helpfull, thanks.

The mod that made me ask about this can be seen at,
http://www.youtube.com/watch?v=Zo9U33IIy_c&feature=related

Looks pretty good what he's done with the fan on the inside for lighting effect, but if it doesn't actually do anything airflow wise not sure if I would want the extra noise in there.

[Spawn-Inc]

it would be 80cfm, just double the amount of air being moved, not double the cfm.

larger fans can push the same air but at a slower speed.