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Creating positive air pressure

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October 8, 2012 11:20:01 AM

Hello,
I'm buying this case: http://www.newegg.com/Product/Product.aspx?Item=N82E168...

So if I get this right, as long as I will not add anymore fans (there are 3 optional as you can see), I will get negative air pressure, since I will have one 20cm intake fan (600-800 rpm), 20cm exhaust (600-800 rpm) and 12cm exhaust (1000 rpm).
As you can guess from the title, I want to create positive pressure, since from what I have read negative pressure causes the computer to become a dust magnet.

Here are my questions:
1) How much difference should be between the intake and the exhaust air flow? (f.e: +20cfm, +30cfm, etc...).
2) Assuming I will add intake fans, Should the extra fans have the same rpm and air flow as those I already have? Or can I get fans with more rpm and air flow?

* Please don't tell me what you think about the case, because although there are better options in USA, I'm not living there :D 

Thank you :) 
a b ) Power supply
October 8, 2012 11:56:10 AM

You could just move one of the exhaust fans to the side to have 2 intake and 1 exhaust. Your case has room for a 120mm side intake.
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October 8, 2012 1:25:03 PM

wanderer11 said:
You could just move one of the exhaust fans to the side to have 2 intake and 1 exhaust. Your case has room for a 120mm side intake.


Thanks for your answer, I didn't think about that. If I will put one exhaust fan as intake on the side, will I get good air circulation?
And can you answer my 2 original questions please (especially the first one)?
Thanks
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a b ) Power supply
October 8, 2012 1:33:25 PM

grizly said:
Thanks for your answer, I didn't think about that. If I will put one exhaust fan as intake on the side, will I get good air circulation?
And can you answer my 2 original questions please (especially the first one)?
Thanks


Well for your first question if you think about it you can't keep adding more air to your case than goes out. If you have more intake fans than exhaust the air will basically get pushed out rather than just having the exhaust fan suck it out. Make sense?

For the second question they don't have to be the same rpm. I have heard having 2 fans close together running at different rpm can be bad because one fan will have extra stress on it from the faster turning fan forcing more air through it. In your case it won't matter since they aren't next to each other.
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a c 134 ) Power supply
October 8, 2012 1:51:32 PM

grizly said:
As you can guess from the title, I want to create positive pressure, since from what I have read negative pressure causes the computer to become a dust magnet.


What you have read is greatly exaggerated. Simply put, the air going into the box must be equal to the air going out of the box of the box would simply explode / implode. In this simple example, let's say you have a 100 cfm fan blowing in and one 150 cfm fan blowing out, both with the same static pressure ....One would assume that such an installation would result in 50 cfm of air being sucked in through the case openings.....but that would be wrong.

As air flows thru a constriction, it created a "head loss" measured in static pressure. The overall head loss at each opening determines the flow thru that opening. What will actually happen in the above instance is that the 150 cfm will have its flow slightly reduced because it is trying to pull more air out than is being blown in .... lets say for arguments sake, this is reduced to 135 cfm. Because of the slight negative pressure created, inside the case, the intake fan will have its job made easier and will wind up pushing say 115 cfm, leaving 20 cfm to be pulled in thru the case openings.

Dust creation isn't so much a function of positive or negative air pressure as it is filtration. If ya fan intakes don't have filters, then you will have lotta dust. If ya intakes have filters, then you will have moderate levels of dust as the filters don't catch everything.

If ya have negative air pressure, then you'll want to add filters to ya other case openings.

http://www.xoxide.com/lascutfangri.html

If ya have positive air pressure and no filters on ya fan intakes, you are still gonna be loaded w/ dust.

In either instance, you will still be blowing dust outta ya machine.....I have 12 boxes here, all filtered, and blow them out at least quarterly.
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a b ) Power supply
October 8, 2012 2:09:33 PM

You need dust filters on all intake fans. No matter what kind of pressure you are aiming for. That's what helps the most against dust building up in the case. In theory, if you are going with negative air pressure, you would consequently have to add dust filters to every opening (since air/dust will be sucked in those openings). If that's practical is questionable.

Depending on your case, positive air pressure might have an edge in cooling for the reasons mentioned by JackNaylorPE.
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October 8, 2012 2:10:30 PM

I always preferred negative air pressure. More exhaust fans and you can remove the intake fans and let the exhaust fans do the work and use the intakes to guide airflow over the components you want cooled. It's not going to matter with dust, regardless. It's going to be sucked into the case, unless you have filters, regardless. Filters get clogged and then you have zero air-flow.
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a b ) Power supply
October 8, 2012 2:43:44 PM

egilbe said:
Filters get clogged and then you have zero air-flow.


It's easier to clean filters once a week than your whole case, isn't it? Especially since good cases have easily removable filters which makes the whole process a matter of just a few minutes. :sarcastic: 

Not cleaning and/or no dust filters means you have all the dust on your components/heatsinks, which would reduce cooling, too.
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October 8, 2012 2:47:56 PM

Thank you all for your answers :) 

So it seems you recommend negative pressure, so I will go with negative.
So now my question is whether or not to add more fans? (If I won't, I will still get negative pressure) If yes, How much fans? Where? And what kind (intake or exhaust)?

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a b ) Power supply
October 8, 2012 3:54:17 PM

OK, Quess I'll chimn in.

I think Ideally you want close to ballanced. Both Positive and negative have an adverse effect on CPU/GPU cooling, for opposite reasons but with same results., Negative pressure reduces the air molecules hitting the HSF cooling fins (molecules furter apart), and positive, while moelcules are closer the air flow is reduced.

LOLs jack "box would simply explode / implode" seriously dought case fans could ever increase/decrease air pressure that much. have never seen a case that air tight.

If Negative, air is pulled in from every opening in addition to thru the air filters. In some cases air will even be sucked into the DVD drive door (not all DVDs are "air" tight) creating dust build-up.
Just look for dust arround door, and other openings to verify
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a b ) Power supply
October 8, 2012 4:07:03 PM

Not all fan blades are equally efficient against a pressure head so the simple math of keeping intake CFM greater than exit CFM won't answer your question completely although it's a good rule of thumb. Blade inefficiency shows itself as turbulence replacing smooth flow so the CFM is reduced. The CFM ratings you see for fans are in free air.
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a b ) Power supply
October 8, 2012 4:36:25 PM

whatsthatnoise said:
It's easier to clean filters once a week than your whole case, isn't it? Especially since good cases have easily removable filters which makes the whole process a matter of just a few minutes.

Very much so.

I take my old PCs apart about once every other year to clean dust and the process easily takes two hours since I even open the PSU to vacuum-clean it and there sometimes is a surprising amount of it.

In my newest PC which I configured for positive pressure to take advantage of the case's front filters, I haven't noticed significant dust buildup beyond the HDDs after five years. Large dust particles are caught by the front grille and filter, smaller particles settle while passing between HDDs over the first 2-3", the rest appears practically dust-free.
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October 8, 2012 6:30:27 PM

RetiredChief said:
I think Ideally you want close to ballanced.


So if I want it close to balanced but slightly negative, is the current fan setting is alright in your opinion? Or should I add more fans?
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a b ) Power supply
October 8, 2012 6:45:38 PM

The only essential exhaust case fan is the one drawing the CPU's HSF hot air out the case. The other exhausts (not counting the PSU/GPU which are dedicated to a specific component) are unnecessary.

You would need to go outrageously imbalanced to need to worry about exhaust. Any "excess intake" will create a slight pressure increase and a corresponding passive exhaust through every opening in the case, which is perfectly fine... I would even say this is exactly what positive pressure build is about.
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a b ) Power supply
October 8, 2012 9:05:13 PM

You currently will have 2 Out ( a 120 and 200 mm) then depending on how you mount your PSU (bottom Mount) could have 3 OUT. Most newer cases allow for two ways to mount the PSU, A) with the intake fan on the bottom, draing ambient air in and then out back of PSU (probably best, but should have vent holes in bottom just below fan intake), or intake is from inside case and out back. In las configuration then it adds to the exhaust creating even a greater imbalance (neg). I'd probably add an intake at the Bottom, provided it is not directly below the PSU. anlternate is side intake.

Mine. Rear exhaust, top exhaust, two front intake. I then Blocked the vent holes on the side and the vent holes on top (where a 2nd Fan would mount). I think I replaced My rear with a 140 x 140 x 30 or (35) mm - been a while.

Added: Even thoe you have a top fan (larger than rear) I think you will find that most air HSF work best with air flow toward the back exhaust fan. Have two systems one each way. Think it has to do with the designed physics of heat pipes.
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October 8, 2012 10:16:55 PM

Thanks for everyone! I have no more questions for now :) 
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October 8, 2012 10:17:37 PM

Best answer selected by grizly.
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March 3, 2013 10:21:34 PM

Don't forget when figuring your positive or negative pressure of your case, that total airflow matters greatly.

Example A: 1- fan intake at 40 cfm matched with 1- fan exhaust at 50 cfm would result, in theory, in a 10 cfm neg. pressure.

Example B: 3- fan intakes at 50 cfm each matched with 3 - fan exhausts at 50 cfm each (again with all head loss being equal)
would result in a balanced pressure.

Of the above examples, example B would cool much better. So always try to move as much air thru your case as possible,
given mounting room and noise levels. And design good airflow patterns thru out the case.

Good luck with your build!!!!
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