Altair Fan - test-drb@ccmp.vtda.org

17 Jun 2005

Dwight:

	Interestingly, on the IMSAI, the slotted vents on the power supply
side are blocked to prevent the "path cheating" you alluded to. Also the
inside of the IMSAI case was virtually spotless with an inward-blowing fan
(and no filter) while the Altair, which I believe had an inward-blowing fan,
was filthy. 

Rich 

-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Dwight K. Elvey
Sent: Friday, June 17, 2005 1:08 PM
To: cctalk at classiccmp.org
Subject: Re: Altair Fan

Hi
 One should note that unless the fan is blowing directly onto
some particularly hot item, you'll get really poor air flow
distribution by blowing with a muffin fan. First the fast moving
air tended to stick to the first surface it finds ( Coanda effect ).
Also the air is rotation that tends to cause it to find corners
to travel along.
 When the air is pulls into the fan, it tends to have more
laminar flow from all directions.
 For better all around cooling, I recommend taping or masking
off most of the slots on the power supply side, near the fan
( for a IMSAI or Atair ). These keep the air from cheating by
taking the path of least resistance. More air flows across the
boards and along the power supply.
Dwight

...
 From: "Dave Dunfield" <dave04a at
dunfield.com>

At 21:31 16/06/2005 -0400, you wrote:
>All:
>
>	OK, this is a stupid question. I'm fixing up the 8800b that I got
>recently and I removed the fan to perform some work on the power supply.
>Unfortunately I didn't mark which direction the air blows. Can someone tell
...
  me if it blows
inward or outward?

	Thanks.

Rich 
Hi Rich,

Both of my 8800s have the fan blowing out.

The general rule is:

If the fan has a filter, then it should blow in.:

  By maintaining positive internal pressure, and
  filtering the air that comes in, you keep dust
  from entering the box.

If it does not have a filter, it should blow out:

  A fast moving stream of air will carry more dust
  than a slow one. If you blow the fan in, it will
  carry dust into the box which will drop out before
  it exits at slower speed (because of the much larger
  area of all the vents and other openings) - it also
  causes dust buildup to concentrate that certain
  points bacause of sudden changes in airflow (you can
  often see bands of thick dust in boxes with fans
  improperly installed). Going the other way reduces
  the entry speed (same reason) and thus the amount of
  dust that enters - what does get in tends to settle
  out in a more uniform distribution.

Regards,
Dave
-- 
dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
                http://www.parse.com/~ddunfield/museum/index.html