how "positive" must airflow be for a tower case to keep du..

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?

In article <0m89g09l8i0g8fcq6e5eqhcq9ir5atusg8.DeleteThis@4ax.com>,
NOSPAMME.DeleteThis@no-one-here.com wrote:

 > 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.

<a style='text-decoration: underline;' href="http://www.dustfreepcinfo.com/pages/184710/index.htm" target="_blank">http://www.dustfreepcinfo.com/pages/184710/index.htm</a>

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:

<a style='text-decoration: underline;' href="http://forums.pcper.com/showthread.php?s=&postid=2029294" target="_blank">http://forums.pcper.com/showthread.php?s=&postid=2029294</a>

<a style='text-decoration: underline;' href="http://forums.overclockersclub.com/?showtopic=8412&st=10&" target="_blank">http://forums.overclockersclub.com/?showtopic=8412&st=10&</a>

The interesting tidbit on the following site is

CFM = 3.16 x Watts / Delta_T_degrees_F

<a style='text-decoration: underline;' href="http://www.chassis-plans.com/cooling_and_noise.html" target="_blank">http://www.chassis-plans.com/cooling_and_noise.html</a>

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:

<a style='text-decoration: underline;' href="http://www.silentpcreview.com/Sections+index-req-printpage-artid-144.html" target="_blank">http://www.silentpcreview.com/Sections+index-req-printpage-artid-144.html</a>

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<!-- ~MESSAGE_AFTER~ -->