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 Post subject: Negative Pressure Cases...No Way!
PostPosted: Mon Jan 19, 2004 11:36 am 
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One man's experience, your's may differ....

I've read all the pros/cons on the subject. Most of the case projects I've done have been positive pressure (more air blowing in than out). But lately I built several negative pressure cases. I wanted all the incoming air to be filtered, so I sealed every crack, every hole, the little openings at the ports, the cards, the back of the optical drives, etc. Then I put filters over the openings so the output air was being sucked through the filters....

This worked fine in several builds with no problems....the cases were staying reasonably clean. Well lately I pulled the bezel off a DVD drive for painting. To my dismay I found a significant amount of dust that was being sucked through the drive...I guess around the ide cable. And this was an expensive piece of hardware.

That's it...it's positive or neutral pressure from now on. The replacement case I'm working on has one 120mm fan blowing out and another 120mm fan blowing in (filtered air/same fan motors). I control both fans manually with a controller that reads rpms. I set the intake fan about 200rpms higher than the output fan. Positive pressure.....no more dusty air being sucked in through the cracks.


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PostPosted: Mon Jan 19, 2004 11:58 am 
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what positive pressure cases are good with few to no mods? How about just an SLK3700 with a 7V intake and 5V exhaust?


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PostPosted: Mon Jan 19, 2004 12:12 pm 
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I guess you have to calculate the total airflow out/in. If your psu is also sucking out of the case, you have to add that flow to your case fan flow....for the total output. It's probably going to require some guess-work, unless you set both the case fans to blow in. Then you have to use two filters. Ugg....

I suppose you could duct the PSU to suck air from outside the case. Then you could try your case suggestion with different voltages...that would probably work ok. You still need to filter all the air blowing into the case, if you want to avoid dust.

You know it's easy to blow out the inside of the computer frequently, then you don't have to worry about filters. But that still leaves your negative pressure case sucking dusty air through the drives...can't avoid that with negative pressure, no matter how you do it. (apparently).


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PostPosted: Mon Jan 19, 2004 12:34 pm 
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Good point about calculating in the PSU exhaust.

And also the exhaust will blow more air than the intake per RPM because the intake is filtered and also not as direct as the exhaust...

I can definately see why you did some of the experiments that you did with the positive flow cases and their dual intakes.

Heh, one last thing to try with a negative pressure case: gasket seals on your drives.


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 Post subject:
PostPosted: Mon Jan 19, 2004 12:52 pm 
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I think the only fooproof way to keep dust out of your case is to hermetically seal the case. Then watercool everything with an external radiator.
Your system would be dust free as long as your cds are clean.


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 Post subject:
PostPosted: Mon Jan 19, 2004 12:59 pm 
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If you want a positve air pressure sytem then make a big filter box with a humongous fan in it and hook that to the back of the computer with a hose.

Ither that or undervolt a HEPA Shop Vac. Shop Vac hoses are about 80mm so you could stick one right to the back of your computer. You would give your cpu HSF a ton of air.


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 Post subject:
PostPosted: Mon Jan 19, 2004 1:41 pm 
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There is another solution to prevent dust in drives....get a case with a good door that is tightly sealed. Run the computer with the door closed.
The drive I took apart did have a good seal on the drive door. I guess the dust entered around the volume knob. I really don't think the dust problem can be solved with a negative pressure case.

If you have plenty of openings in the case, you could have a neutral pressure setup.....but then you're letting the noise out. I really don't see many problems with positive pressure. In my experiments the temps were the same either way.....what mattered with temperatures was total airflow, not positive or negative pressure.


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 Post subject:
PostPosted: Mon Jan 19, 2004 1:57 pm 
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I wouldn't mind a positive pressure case if the PSU weren't the only exhaust.

That's interesting about the total airflow being the most important variable.

Do you have a picture of the computer door? Couldn't you seal that off or remove the volume button or whatever? Like mpteach said, hermetically seal that baby!

Dag, well perhaps a door would be the answer :? . My Sonata has a door, but there are front connectors in the front that aren't behind it... The Silentmaxx website isn't working, but maybe the ST-11 would work.

What type of seal would you use on a door? It would need to be thin, but would still need to work as well as a refrigerator seal.

How about an SLK3700? Just block those front USBs. Really though, even if a little dust came through the Sonata or SLK front connectors, the drives should stay dust free.


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 Post subject:
PostPosted: Mon Jan 19, 2004 5:11 pm 
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Trip....the case doesn't have a door. The drive door has a little seal behind it when it closes. Didn't help keep out dust though. As far as sealing this case any further, well I did all I'm going to do in that respect. It was far easier to go over to positive pressure.

The Lian-Li 6070 looks to be the case with the best door. It's expensive though, and I won't buy one unless I see it in person. It wasn't available at the last two computer shows I went to, and nobody in town sells them. I think it could be turned into a fantastic case....mabe some time in the future for me.

For the present, I have moved my expensive burner to an external Firewire box....no fans, no noise, no dust. Heh...


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 Post subject:
PostPosted: Mon Jan 19, 2004 7:41 pm 
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Airflow isn't very good in that Lian Li is it? I'll bet that door blocks airflow pretty well though.

What drive are you using that is sealed? Neither my optical, zip, or floppy drives have much of a seal.

I'll bet that volume buttom would let in dust. It seems like someone would design one with just a sealed eject button and no other gadgets.


Last edited by Trip on Mon Jan 19, 2004 7:48 pm, edited 1 time in total.

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 Post subject:
PostPosted: Mon Jan 19, 2004 7:46 pm 
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Far out. I recently upgraged a friends pc from a 900 to a 2.4. It was a piece of crap cheap case, cheap psu, stock cpu & gpu cooler. Not a fleck of dust was found to be worthy of note. It was quiet and cool to a point that if you didn't look there was no problem over a 2 year period. It's just as quiet now.

Sometimes, the more you play with things the worse they become.

=D

my 0.02c


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 Post subject:
PostPosted: Mon Jan 19, 2004 9:09 pm 
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dust buildup in/around the drives can be avoided if you modify your case so that the negative pressure causes air to be taken in via a much less restrictive opening. take the bqe for example, i've cut out the front grill and enlarged the bottom opening of the front bezel so that air goes in extremely easily now. also i taped up all the other holes on the front steel face thing.

and... no dust. anywhere. not even on the fan blades.

actually there is a fair amount of dust on the dust filter but that's what tha'ts there for, eh?


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 Post subject:
PostPosted: Mon Jan 19, 2004 9:40 pm 
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yea, lessening the negative pressure should definately reduce the amount of dust coming through the cracks. i think i saw i picture of your modded case. I may do something similar with my Sonata.


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 Post subject:
PostPosted: Mon Jan 19, 2004 11:20 pm 
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I was debating - should I get an internal DVD/RW or external?

I think the way to go is a cheapy optical in the case, and a quality one on USB2/FW - you won't bang it up going to LAN parties and it is out of the air and dust stream of your PC...


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 Post subject:
PostPosted: Tue Jan 20, 2004 4:00 am 
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Man I thought I had this dust thing solved, until I looked inside the drive. My problem is a really dusty house because of my other hobby....parrots. They fly around, produce feather dust, and then blow it everywhere with flapping wings. Positive pressure was an easy cure for me.


No....the parrots stay. :D


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 Post subject:
PostPosted: Tue Jan 20, 2004 6:20 am 
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chylld wrote:
dust buildup in/around the drives can be avoided if you modify your case so that the negative pressure causes air to be taken in via a much less restrictive opening. take the bqe for example, i've cut out the front grill and enlarged the bottom opening of the front bezel so that air goes in extremely easily now. also i taped up all the other holes on the front steel face thing.

and... no dust. anywhere. not even on the fan blades.

actually there is a fair amount of dust on the dust filter but that's what tha'ts there for, eh?


This is my exact approach too. I have very, very little problem with dust in the drives of any systems I set up this way. The dust filter gets pretty dusty and gets cleaned every month or so.

In the days before SLK3700's my standard cases were Antec SX10x0 and SX6xx cases that were set up to run positive pressure because it seemed logical that it should work better than negative pressure from a "dust intake" point of view.

The trouble was, no matter how I tried to configure them, I'd always get dust in the case, and to a lesser extent, the optical drives too. This was the time that I started taping up all the extraneous holes in my case. I was trying to increase the pressure inside the case and force more of the air out through the optical drives. This seemed to make absolutely no difference to the amount of dust that would collect inside the optical drives. I'd still also have a slight dust buildup inbetween the fins of the heatsink, and on tops of the vidcard and PCI cards, as well as dust laying in the bottom of the case. All this on positive pressured, air filter-equipped cases.

About 2 years ago I started getting interested in quiet computers and started thinking about how to reduce noise. The most obvious and easy thing to was to start getting rid of fans and undervolting the fans that were remaining. I started changing my configurations from positive pressure with front fans to negative/neutral pressure with very low flow (or sometimes no) front fans and one or 2 undervolted L1As as exhaust fans on the back of the case. This certainly helped reduce the noise levels quite a bit and after running in this sort of configuration for a while I noticed that there didn't seem to be any extra dust buildup in the case or the optical drives.

My current type of case setup is a very low flow exhaust fan, a very, very low flow (or no) front fan, taping up all extraneous holes in the case and a dust filter on the intake grill. This slight negative pressure is generally the coolest (temperature-wise) setup to run on most systems that I've played with and I see no particularly bad dust buildup either, certainly no worse dust buildup than on any of my older "positive-pressure" setups.

In my mind, the "positve pressure = less dust" theory is a myth BUSTED!! (as they say on TV).

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 Post subject:
PostPosted: Tue Jan 20, 2004 7:26 am 
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LOL I love myth busters!

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 Post subject:
PostPosted: Tue Jan 20, 2004 9:11 am 
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dammit Ralf, my world was so simple before that post. Can you think of any way dust entered your positive pressure setup? I need things to make sense... Do you think maybe your setup wasn't really positive pressure or maybe your filters weren't very good?

Very glad to hear filtered low negative pressure works well - you obviously have a very good (and suprisingly simple) setup.

Bluefront do you have any ideas?

Let's put Ralf's case to the parrot test :twisted: !


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 Post subject:
PostPosted: Tue Jan 20, 2004 9:41 am 
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Trip, don't go over to the dark side!

I used to work in pharmaceutical manufacturing, all their clean rooms where positive pressure, using hepa filters.

The key may be in realizing that you won't be able to filter all the dust out. So arrange high airflow over the parts that need it, and low flow over op drives and stuff that doesn't.

Easier said than done I know.


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 Post subject:
PostPosted: Tue Jan 20, 2004 10:11 am 
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Ralf Hutter wrote:
This slight negative pressure is generally the coolest (temperature-wise) setup to run on most systems that I've played with and I see no particularly bad dust buildup either, certainly no worse dust buildup than on any of my older "positive-pressure" setups.

But, then again, you are flowing less air.


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 Post subject:
PostPosted: Tue Jan 20, 2004 11:40 am 
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HammerSandwich wrote:
Ralf Hutter wrote:
This slight negative pressure is generally the coolest (temperature-wise) setup to run on most systems that I've played with and I see no particularly bad dust buildup either, certainly no worse dust buildup than on any of my older "positive-pressure" setups.

But, then again, you are flowing less air.


So? Why flow more air than you need? More airflow = more noise, and perhaps more dust as well!

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 Post subject:
PostPosted: Tue Jan 20, 2004 11:57 am 
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Ralf Hutter wrote:
More airflow = more noise, and perhaps more dust as well!

Exactly! You're now seeing about the same dust as with positive pressure and more airflow, so the negative pressure may not be keeping your box cleaner.


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 Post subject:
PostPosted: Tue Jan 20, 2004 12:46 pm 
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HammerSandwich wrote:
Ralf Hutter wrote:
More airflow = more noise, and perhaps more dust as well!

Exactly! You're now seeing about the same dust as with positive pressure and more airflow, so the negative pressure may not be keeping your box cleaner.


I didn't say that negative pressure keeps it cleaner, I said it doesn't get any dirtier with negative pressure.

In other words, the same amount of dust seems to end up inside my case whether it's setup with negative pressure or positive pressure.

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 Post subject:
PostPosted: Tue Jan 20, 2004 2:13 pm 
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Bluefront wrote:
No....the parrots stay. :D

I take it dressing the parrots in those Intel bunny suits is not an option either? :-)

Edit: Actually I'm surprised Bluefront, Master of the Duct, you didn't try an external box with huge filters to clean the air before feeding it to your case (the muffler thingie is what I have in mind, used for intake).


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 Post subject:
PostPosted: Tue Jan 20, 2004 2:57 pm 
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lenny i already recommended that


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 Post subject:
PostPosted: Tue Jan 20, 2004 3:09 pm 
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mpteach wrote:
lenny i already recommended that

You're right, I didn't read carefully enough :oops:


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 Post subject:
PostPosted: Tue Jan 20, 2004 3:58 pm 
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Ralf Hutter wrote:
I didn't say that negative pressure keeps it cleaner, I said it doesn't get any dirtier with negative pressure.

In other words, the same amount of dust seems to end up inside my case whether it's setup with negative pressure or positive pressure.

Please excuse my lack of clarity. I meant relatively cleaner. If you're seeing about the same amount of dust in both configurations, then the negative pressure is very possibly increasing the amount of dust relative to airflow while the lower flow compensates to keep the actual amount of dust comparable.

In other words, the same amount of dust seems to end up inside your case whether it's set up with low-flow negative pressure or higher-flow positive pressure.

Better phrased?


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PostPosted: Tue Jan 20, 2004 4:08 pm 
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this is a great thread, thanks guys!


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 Post subject:
PostPosted: Tue Jan 20, 2004 4:17 pm 
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I'm going to agree with Ralf on this one.

The entire positive/negative pressure and its effect on dust is vastly over-rated.

The air pressure gradient is simply too tiny at the CFM's we're talking about. I'd be surprised if you could even measure the change in air pressure inside a PC. (without thousands of dollars in lab equipment at least)

PC's are not setup like clean rooms or biohazard rooms. (biohazard rooms operate in negative pressure) We use axial fans sucking air out of perforated boxes. They use Screw or Root blowers forcing air into a hermetrically sealed chamber.


My advice:

Use positive, or negative, it doesn't really matter. If dust bothers you, put a filter on the intakes. (or reduce the dust in your environment, your lungs are negative pressure too :wink: ) Dust doesn't damage anything anyway.

But whatever you do, don't do both an intake and an exhaust fan. That's just doubling your noise production for the same amount of airflow. Blowing 30 CFM into your case, and sucking 30CFM out of your case does not give you 60CFM flowing through it... it gives you 30, only with twice the noise.


Last edited by Rusty075 on Tue Jan 20, 2004 4:19 pm, edited 1 time in total.

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PostPosted: Tue Jan 20, 2004 4:19 pm 
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anybody think that magnets on motors could have something to do with dust?

~EO


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