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I've made a new system, it has a 92mm exhaust fan at the upper rear. I have two fans here, one is 92mm, the other is 120mm. This will go on the lower front and will be the air intake.

Which would be best?

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closed as not a real question by Breakthrough, random Apr 19 '12 at 16:12

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Larger fans can move the same volume of air at a lower blade speed, which equates to less noise for the same cooling potential. AFAIK, 120mm fans are also more common, and often cheaper than a 92mm equivalent (simply due to a more saturated 120mm fan market). –  Breakthrough Apr 19 '12 at 10:46
    
As for which fan you should use, it is irrelevant as to the fan size. The only thing that truly matters is how much air it can push (usually given in cubic feet per minute, or cubic meters per hour), which is a function of the blade size and speed. To balance these, you'll usually focus on how much noise a fan emits, so larger is usually better. –  Breakthrough Apr 19 '12 at 10:50

4 Answers 4

As somewhat of a physicist, I'd say: use the larger fan as exhaust. This will get you the best flow in real situations, and also: the exhaust fan in a case is of more use than the intake fan since the exhaust fan is close to the CPU and GPU which generate the most heat. Thus use the best (=biggest, rpm and other variables considered equal) fan there.

Is it a large enough difference to be noticeable? Perhaps.

Can it rely on a lot more factors in this case (e.g. exact placement of cables/drives, dimensions of CPU/GPU cooling)? Yes. But my above recommendation should hold in more cases than not, and you'd need to analyze the airflow very closely to find the real-life flow. The indicators you have are the temperature sensors in the box: if you try it one way and they are higher than the other: switch.

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I would also say use the larger as exhaust. The larger fan moves more air, but air intake happens not only via the intake fan but also via all the little holes in the case, underside and everywhere. Getting the hot air out of the case is the primary concern, not getting the fresh one in (that's an afterthought, by comparison). –  Carighan Maconar Apr 19 '12 at 6:55

Best is to measure mounting bracket.... probably quietest 12cm ventilator pumps as much air as the loudest 9cm one....

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They just spin slower moving same amount of air (which would give same chilling effect) –  ZaB Apr 19 '12 at 7:49

If you have a way to control your fan speeds (eg SpeedFan, if you are using Windows), I would go with the larger one. A large fan at lower speed will move as much air as a smaller fan running faster. I assume you are looking at the same line of fans from a single manufacturer.

The noise curve does not necessarily increase linearly with rotation speed, so while the two fans may operate in the same dB range, I find that the larger ones can achieve the same effect while running a little quieter. Remember that even 1dB reduction in sound level is a significant improvement.

Also, since your case is not sealed, you generally don't have to pull as much air into the case as you push out. There will be an optimum balance of airflow where the load on your fans from both pulling and pushing air will yield the best airflow-to-noise ratio. Experiment.

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The poster has two fans and wants to use them both - the question is which one should go in the back and which one in the front. –  Daniel Andersson Apr 19 '12 at 6:36
    
Oh I didn't pick up on that. In that case I would simply experiment. One argument is that the exhaust will 'push' its noise out of the case more readily, whereas the other argument says put the quietest fan at the front, because that's where the direct sound path to your ears is. In the time it takes to pick through responses to this question, both configurations could be tried. –  paddy Apr 19 '12 at 22:50

Depends on which size the case accepts (ie. has mounting holes for). If it can take both, then choose whichever better matches your requirements in terms of airflow and noise level. Usually larger fans can move more air with less noise than smaller ones, but without knowing exactly which fans you have, it's impossible to give a definitive recommendation.

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They're Zalman ZM-SF2 and Zalman ZM-SF3. Case takes them both at the front. 92mm > at the rear. Which will be quieter? –  Dee Apr 18 '12 at 19:08
    
According to the official specs (ZM-SF2, ZM-SF3) they both have the same noise level - 18-23 dBA. Unfortunately I can't find specs for airflow, but it seems like they're pretty much equal, so use whichever one you want, it doesn't look like there'll be much of a difference. –  Indrek Apr 18 '12 at 19:12
    
The poster has two fans and wants to use them both - the question is which one should go in the back and which one in the front. –  Daniel Andersson Apr 19 '12 at 6:36
    
If both fans are running, wouldn't the noise level be the same? It wouldn't be a bad idea to check if its THAT noise sensitive, but it shouldn't make a huge difference –  Journeyman Geek Apr 19 '12 at 7:30

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