how "positive" must airflow be for a tower case to keep dust out?

[Winey]

I'm upgrading a case (and the system inside it)? Actually two such
systems. I want to have positive air pressure in the case sufficient
to keep out dust. If you know this design, it has vents on the sides
of the case near the top, plus a row of vents in the back above the
adapter cards:

Both cases will have an ASUS A7M-w266 D,TBred 2000, no overclocking.

Power Supply is an Antec TruePower 550. (assume 0 cfm out at lowest
fan speed, 40 cfm at highest fan speed)

Cases are an Addtronics 7896A and a 6896A.

Assume roughly 140 cfm in. (2 120 mm Panaflow "L" model)

So, my question is:

How positive do I need to keep the case air pressure, to keep dust
out? That is, how much more should the cfm in be compared to the cfrm
out?

 

[Paul]

I'm upgrading a case (and the system inside it)? Actually two such
systems. I want to have positive air pressure in the case sufficient
to keep out dust. If you know this design, it has vents on the sides
of the case near the top, plus a row of vents in the back above the
adapter cards:

Both cases will have an ASUS A7M-w266 D,TBred 2000, no overclocking.

Power Supply is an Antec TruePower 550. (assume 0 cfm out at lowest
fan speed, 40 cfm at highest fan speed)

Cases are an Addtronics 7896A and a 6896A.

Assume roughly 140 cfm in. (2 120 mm Panaflow "L" model)

So, my question is:

How positive do I need to keep the case air pressure, to keep dust
out? That is, how much more should the cfm in be compared to the cfrm
out?

I just tried "computer dust positive pressure" on altavista.com
and one of the first hits was this.

http://www.dustfreepcinfo.com/pages/184710/index.htm

They use filters before their fans that pressurize the cabinet.
Thus, clean air is used, and blowing that clean air under positive
pressure, through any opening in the cabinet, prevents dust
from entering.

If your case blows dirty air in, I don't think it really matters
what differential there is between "in" and "out". As long as there
are dead spots in the case, where the air velocity is lower than it
was originally, the particulate will settle out. (My thinking here,
is that dust works on the same principle as silt in a river - as
soon as the river widens and the water velocity drops, the silt
falls out and collects on the bottom of the river.)

To implement a solution like those dustfreepc people, you will
need micron filters, a plenum, and your large 120mm fans. A plenum is
simply a length of plumbing leading from the filter to the fan, and
keeps the filter from getting too close to the fan blades. About
3" of plenum between the filter and the fan should be good. You
can try mounting the filter right on top of the fan, but you'll
probably hear more noise coming from the fan that way.

As soon as you use filters on a PC case, then you've got a maintenance
item. The filters have to be cleaned or changed every x months. You
have to remember to do it. If you forget, the computer might overheat.
The most endangered component is probably the disk drive, and
the computer will continue to run quite happily under conditions that
are trashing the disk.


Some other threads from my search:

http://forums.pcper.com/showthread.php?s=&postid=2029294

http://forums.overclockersclub.com/?showtopic=8412&st=10&

The interesting tidbit on the following site is

CFM = 3.16 x Watts / Delta_T_degrees_F

http://www.chassis-plans.com/cooling_and_noise.html

Using the AMD recommendation of a max case temp rise of 7C, which
is 12.6F, and assuming 200W for a high end PC, we get 50 CFM.

This one is a home brew solution, using an air cleaner filter
replacement as the filter for the air. Positive case pressure
is created by two identical fans, with one running 200RPM higher
than the other:

http://www.silentpcreview.com/Sections+index-req-printpage-artid-144.html

So, if the air is filtered, then a positive air flow seems to be
the solution offered by commercial designs. For unfiltered
situations, a balance between in and out, or a slight negative
pressure seem to be favored.

I can tell you that one computer I use, which has only exhaust
fans, is filthy inside. Yet, I've never had any trouble with
the floppy or the CD on that computer.

This is one of those questions, where if you ask twelve
people, you'll get a dozen different answers

HTH,
Paul

 

[Wizard Prang]

It's probably as good idea to get one of these...

http://www.directron.com/110alert.html

I have one near my HD.

Prang

 

[BigBadger]

Unless you have filters on the air inlets the dust is gonna get into the
case anyway.... and if you do have filters then the performance of the fans
will be much reduced.
If you have filters and want to ensure positive pressure then the only sure
fire way is don't have any exhaust fans.

 

[Michael S.]

How would that keep out the dust?? It would seem if any fans are blowing in,
they would bring in dust with the inbound air and any positive air pressure
inside the case would limit the amount of incoming air for ventillation.
Don't get me wrong, I would like to see a solution beside airflow limiting
filters since I have cleaned out dustbunnies of record size and number from
a few hundred enterprise computers over the years and I would like very much
to keep the dust out of my new build.

MkeSp

 

[borolad]

I'm upgrading a case (and the system inside it)? Actually two such
systems. I want to have positive air pressure in the case sufficient
to keep out dust. If you know this design, it has vents on the sides
of the case near the top, plus a row of vents in the back above the
adapter cards:

Both cases will have an ASUS A7M-w266 D,TBred 2000, no overclocking.

Power Supply is an Antec TruePower 550. (assume 0 cfm out at lowest
fan speed, 40 cfm at highest fan speed)

Cases are an Addtronics 7896A and a 6896A.

Assume roughly 140 cfm in. (2 120 mm Panaflow "L" model)

So, my question is:

How positive do I need to keep the case air pressure, to keep dust
out? That is, how much more should the cfm in be compared to the cfrm
out?

I do remember reading an excellent site years ago that gave
scientifically measured answers to your question - but I don't have
the URL anymore, if I come across it I'll post here.

I do however want to make the following observations :

- at 1+1/2 to one going through filters and case perforations you will
need probably to go for 2+ 1/2 to one to achieve the required CFM.
Those filters need to be external, internal ones hardly ever get
changed often enough, people just can't be bothered opening the case.

I'm sure you won't want 5.5k deltas so bigger sizes the better running
at 5 or 7 volts will keep the noise down and the flow up. I've been
fitting 120's for about 7 years now but almost without exception this
requires case mods.

- all cases need a gentle laminar airflow input, a simple ' hot air
rises ' is often the cause of 'dead spots', all cases have thousands
of holes and air will take the path of least resistance. Baffles can
be used to eliminate re-circulation.

- chassis 12 volt cooling fans draw very little power; less than two
watts for a 60mm fan, less than four watts for a 120 mm fan. Even if
you have six 120 mm fans and four 60 mm ..fans, the total power draw
from the 12 volt line will be less than three amperes.

- removing the chassis perforations from both input & exhaust fans
will boost the throughput substantially, this would particularly be
the case for the exhaust / PSU fans. It's a contradiction in terms
because to maximize airflow, any resistance should be minimized but
filtered positive pressure does exactly the opposite.

input fans should be sealed against the panel, if they are not air
leaks at a rate of knots, reducing input by a considerable margin.
Pairs of fans should both suck - or - blow otherwise the tend to
cancel each other out.

- you don't give the VID, but modern AGP cards generate lot's of heat,
if you can, leave the slot next to the AGP 'open' to allow airflow',
you should do this anyway to avoid sharing an IRQ address with AGP
slot.

- if you can the rule is mount your MOBO low and your Drives high, all
drives produce a lot of heat particularly 7-10k RPM, I find the two
platter 3 year warrant ' spinpoint ' line from samsung are the coolest
drives I've ever used and certainly they are very very quiet.

Just audible is 10 dBA
Soft whisper at 15 feet is 30 dBA
A quiet room is about 28dBA
A noisy drive measures about 36-38 dBA
A quiet office is about 40 dBA
Air conditioner, normal speech, 60 dBA
Noisy restaurant, freeway traffic, noisy office, 70 dBA
Hearing protection recommended at 80 dBA
Lawn mower on grass is 85 dBA
Heavy truck in traffic measures 90 dBA
Rock concert is 110 dBA
Auto horn at 3 ft, maximal vocal effort results in 120 dBA
Thunderclap is 130 dBA
Jet air ops on a US Navy carrier deck is 140 dBA

- look a series of numbers keeps going through my head, I've no idea
how factual / reliable they are but I seem to remember reading
somewhere that ....... " 35 CFM of airflow is required for a system
that dissipates 200W "

All the above ramble is experience, guesstimates and my personal
twopenny worth.

BoroLad

 

[Winey]

Well, I would think as long as the pressure inside the case is the same or
more than out by any margin, dust wouldn't be sucked in. There's just one
problem. Where are you going to get the air to blow into the case? I'm not
sure you'll find filters that will allow enough air through to keep a
positive flow into the case and still filter out minute dust particles.
Personally, I just clean mine twice a year or more.

As the OP, agree 100%. I guess I should have stated my question a bit
differently:

How can I keep * a lot off the dust* from settling in my case.

If there is positive air pressure, some of the dust might get
exhausted, and some will simply have to be cleaned out every X months.

Both my Addtronics cases have a cleanable mesh filter on the
removeable front of the case. It does trap some dust, and the total
filter area is big enough so that a small amount of dust obstruction
probably doesn't affect fan performances.

These cases aren't 100% dust-free in use, but they are a lot cleaner
than systems without any dust filters. And that is probably good
enough.

 

[Amir Facade]

I think the exhaust hose on a medium size shop vac would do the trick
nicely.

Amir

 

[Ben Pope]

I think the exhaust hose on a medium size shop vac would do the trick
nicely.

You included over 100 lines of previous post, what ARE you talking about?

Ben

 

[Amir Facade]

I'm upgrading a case (and the system inside it)? Actually two such

 

[Ben Pope]

Bloody hell. learn how to post.

The exhaust system from a vac would be warm air and not ideal. Might be a
bit noisy too.

Ben

 

[NEM]

Hi Michael S.,

How would that keep out the dust??

By making the room the computer sits in have less dust to begin with.
I'm NOT talking about a million dollar Clean Room, but there are things
that can be done to minimize the amount of dust that could get inside.

Carpet, as one example, is bad. As the carpet gets older, it breaks off
and becomes airborne and gets sucked into a computer. Pets that think
the computer case is a companion, since you spend more time with the
computer then with them. <G> Hey, they get feed, what more do they want.
Heh. Consider what makes up dust, human byproduct. Stop shedding. <G>

The CPU fan in this computer was the main culprit. The fan just plasters
dust into the heat sink and no longer allows the air to flow through the
fins.

Or, stop using fans and use water cooling. Or pantyhose. I read an
article that suggested that the material used to make pantyhose is
perfect for allowing air flow and keeping a fair amount of dust out of
the case. A simular material is used in recording studios in front of
microphones.

One way or another, it is a good idea to go into the case and clean it
out. There is no reason to spend less time in there cleaning, then you
would your own living space two or three times a year. <G>

 

[notritenoteri]

ever thought of vacuming out the guts once in a while?

 

[Ben Pope]

ever thought of vacuming out the guts once in a while?

Yeah, but the static build up worries me.

Of course compressed air (or LPG or..) would be more suitable

Ben

 

[Ed Light]

By making the room the computer sits in have less dust to begin with.

Air cleaner with filter!
Ion generator to stick dust to the walls!


--
Ed Light

Smiley :-/
MS Smiley :-\

Send spam to the FTC at
(e-mail address removed)
Thanks, robots.

 

[Roger Hamlett]

Yeah, but the static build up worries me.

Of course compressed air (or LPG or..) would be more suitable

Brush the dust out with a soft artists brush. Have a vacuum cleaner
sucking beside the case, to stop you inhaling the dust, and stop it from
just settling back where it'll go through the computer again.
Dust was the reason the original IBM design, had the rear fans blowing
into the case. The idea was that the intake here was higher, and further
from the carpet/user, and the outflow of air at the front would keep the
floppy disk from filling with fluff. It was one of those ideas that was
'forgotten' with the passage of time.

Best Wishes

 

[David Maynard]

you'd think that computer case engineers (designers?) would at least be
keeping an eye on things like that...

Well, they do. But the other half of the story that's untold there is that,
after making the design, they also learned that people hate having their
computers 'blow' on them.

 

[Ben Pope]

you'd think that computer case engineers (designers?) would at least be
keeping an eye on things like that...

BTX design should improve a whole load of things, apparently.

Ben

 

[Bx.Cornwell]

Brush the dust out with a soft artists brush. Have a vacuum cleaner
sucking beside the case, to stop you inhaling the dust, and stop it from
just settling back where it'll go through the computer again.
Dust was the reason the original IBM design, had the rear fans blowing
into the case. The idea was that the intake here was higher, and further
from the carpet/user, and the outflow of air at the front would keep the
floppy disk from filling with fluff. It was one of those ideas that was
'forgotten' with the passage of time.

Best Wishes

you'd think that computer case engineers (designers?) would at least be
keeping an eye on things like that...

 

[Jay T. Blocksom]

I'm upgrading a case (and the system inside it)? Actually two such
systems. I want to have positive air pressure in the case sufficient
to keep out dust.

[snip]

Good idea.

If you know this design, it has vents on the sides
of the case near the top, plus a row of vents in the back above the
adapter cards:

[snip]

Sounds typical. But the specific details generally aren't really relevant to
your core approach -- *all* cases have sufficient vents and gaps of various
sorts to allow all manner of air ingress, if a (partial) vacuum exists inside
the case.

Both cases will have an ASUS A7M-w266 D,TBred 2000, no overclocking.

Power Supply is an Antec TruePower 550.

[snip]

Offhand, that seems like serious overkill, unless there's something else
you're not telling us. But that's another issue.

(assume 0 cfm out at lowest
fan speed, 40 cfm at highest fan speed)

[snip]

Both assumptions are probably too extreme. AFAIK, the PSU fan(s) will never
shut off completely (at least under the PSU's own control), so no "0 CFM"; and
if you believe any manufacturer's "maximum CFM" ratings, I've got a toll
bridge to sell you in New York.

Cases are an Addtronics 7896A and a 6896A.

Assume roughly 140 cfm in. (2 120 mm Panaflow "L" model)

[snip]

Not familiar with that particular model; but *if* both of those fans are
drawing through proper filters, that should be more than adequate unless
you're doing something silly like "serious" overclocking.

So, my question is:

How positive do I need to keep the case air pressure, to keep dust
out? That is, how much more should the cfm in be compared to the cfrm
out?

It doesn't work quite like that.

First, "flow" and "pressure" are two different things. Further, they tend to
oppose each other, in any given situation.

Second, as implied above, manufacturer "CFM" ratings are mostly a crock, in
part because *any* "raw" CFM rating presented without all the context details
(i.e., what sort of test jig; how much pressure differential is the fan
maintaining, etc.) is effectively near-meaningless. And even then, the
typical test jig doesn't come close to representing the real-world conditions
under which an *installed* fan operates. (For that matter, I strongly suspect
that at least most of the fans sold for hobbyist computer applications are
*never* actually tested on a proper flow bench.) At best, these ratings can
be a *very* rough guide to "this fan is more powerful than that fan", within a
single manufacturer's line.

Besides, the degree of pressure differential is not really important, as long
as it is even slightly positive (in fact, as long as it is even non-negative).
Even a zero pressure differential would still be sufficient to keep from
drawing dirty air into the case via all those assorted vents, gaps, etc.; and
that's all you're really trying to do WRT to managing the internal case
pressure. Beyond that, it mostly becomes a question of trading off total air
flow (i.e., cooling) vs. noise. All other things being more-or-less equal,
more fan speed (and/or bigger fans) == more flow == better cooling (presuming
decent overall system layout, so that you don't get serious internal
"stagnation" areas).

--

Jay T. Blocksom
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