case fans

ultra99 · TweakNOOB Joined: 29 May 2005 Posts: 10

I was looking for a case at my local stores and found ones that have two 80mm fans slots at the back and a side fan, and others with one 120mm fan at the back and a side fan. Now i dont know which one of those would be more effective.

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Yoshida · UberTweaker Joined: 24 Dec 2003 Posts: 1219

the 120mm are for people who want a more quiet case, because those fans dont need to spin as fast to move the same amount of air. But i think the 2x80mm would move more air? but sure wouldnt be as quiet

Josh · Posted: Mon Jun 06, 2005 9:36 am Post subject:

fussnfeathers · Lord of the Tweak Joined: 14 Dec 2004 Posts: 2763

A well-designed 120mm fan will move as much air as two poorly designed 80mm fans. The bigger the blades, the more air they move. There's a whole debate on how much CFM you need (higher is generally better, though) compared to intake vs. output.

If you're looking at 120mm fans, with a side 80mm fan, the ideal situation would be to have the 120mm rear fan blowing out, the 80mm side fan blowing out, and two 80mm fans in the front blowing fresh air in........as long as the two fans in the front had less CFM than the two output fans.

At any rate, you're gonna have to put an intake fan or two in, I haven't seen too many case makers that include them.
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ToggleHead · TWEAKGURU Joined: 03 Mar 2004 Posts: 4360 Location: Jersey

Also, generally 120mm's tend to be quieter, as they dont need to spin as fast as an 80 to get similar air flow volume.
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Spiegel · Posted: Mon Jun 06, 2005 1:45 pm Post subject:

So it's supposed to be more out than in, right? I always get confused as to which it should be

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ToggleHead · TWEAKGURU Joined: 03 Mar 2004 Posts: 4360 Location: Jersey

Yes, its good practice to have more exhaust than intake.
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[KoG]^weaZel · Posted: Tue Jun 07, 2005 1:37 pm Post subject:

ToggleHead · TWEAKGURU Joined: 03 Mar 2004 Posts: 4360 Location: Jersey

hm....why more in..... i would imagine exhausting the hot air....and slightly tuggin on the good fresh air would be wiser.....explain
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2old2care · Posted: Tue Jun 07, 2005 1:55 pm Post subject:

Interesting...I've pondered this a time or two....

if intake > output then the case will be slightly positive in pressure.
if output > intake then the case will be slightly negative pressure.

But flow wouldn't be influenced much either way. If you have more than a slight pressure on the case, the flow still wouldn't be influenced much, but you would be wasting the energy of the fan trying to pressure the case.

I agree that it has to do with total cfm...and it's ability to get across the places it needs to cool.

Hmmm. **still ponders**
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[KoG]^weaZel · Posted: Tue Jun 07, 2005 2:09 pm Post subject:

Lets think about the psu for a minute.
Q: Where does the psu get its air from?
A: The inside of the case.

And lets say you are exhausting more than you are putting into the case. And if its a nice tight fitting case you are causing a lower pressure area inside the case. The psu fan now has to work harder to pull air across all those heat producing parts of the power supply. WIth less air available it will cause the psu to run hotter and you actually get less output from a hotter psu.

As Nathan has said many times in the past DO NOT take your psu airlfow into account when figuring case airflow. This is true but you cant forget about it all together. It has to have some air to flow in order to be able to cool.

I found this information out when I was testing the watercooling set-up in my current case. The case is an Antec P-160 aluminum case. It has room above the psu to mount the radiator to the inside of the top of the case. So I cut out the area so it has an opening to radiator and I mount the fans on the outside of the case. At first the fans were pushing air into the case, (the standard fan set-up is one intake and one exhaust both of the 120mm type). Now with the 3 intake and one exhaust everything was working fine no heat problems on any front. Then I tried switching the fans so that they would pull air out of the case. When I was using the system under little load I would notice the fan in the psu spinning up. And on the Turbo Cool 510 this can make quite a noise. Since that time I have moved the radiator to outside the case so that has no affect on the case airflow. So in the cases stock fan configuration the intake and exhaust should be balanced, and the psu fan doesnt spin up at all.

So, bottom line is you want to balance your airflow as much as you can. You dont want an excess either way.
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2old2care · Posted: Tue Jun 07, 2005 3:13 pm Post subject:

Agreed that as close to balance is probably best. And I think I will agree that slightly positive (or more intake) would be best if you are not counting the air for the PSU. Because if you were somewhat negative (or more exhaust) you might steal some flow from the PSU. Actually, slightly more intake would be okay even if you did count the PSU. Just don't let the PSU fan be considered as a "cooling" part of the solution.
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fussnfeathers · Lord of the Tweak Joined: 14 Dec 2004 Posts: 2763

It's debatable, and like Weaz says, it's really on a case by case basis (pun intended).

I've pretty much decided that a slight negative pressure is better. In my own case, I have two 80mm fans blowing, three 80mm fans blowing out, and the 120mm fan on my NeoPower. Given the fact that my case has vent holes along the top edges, and it's not......erm........well, ok, it's a $50 case.

This keeps my case temp at ambient, or just above. I found that when I turned the side fan around to blow IN, my case temps rose a couple of degrees. Nothing major, but worth noting. I figure that the negative pressure in the case was pulling more cool air in from the vent holes, while positive pressure was actually pushing air back out of the vents (and the drive bay covers, loose fitting side panel, blah blah) before it had any time to do any good.

I'm pretty much on the negative pressure side of things. Given the quality of the average computer case people buy (I mean, really.....why would somebody on a budget blow $200 on a Sliverstone case??) two in/three out seems to be an ideal situation. The negative pressure pulls in enough air from the vents and gaps to put the CFM rating within 10% of each other.

I'm also intrigued by the newer "media center" cases, which all use large fans at the rear and small fans at the front, which would indicate that the designers were going for a negative pressure design from the start.
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