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From: FelineGrace@cup.portal.com (Dana B Bourgeois)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: The Fanning of the Amiga
Message-ID: <43540@cup.portal.com>
Date: 21 Jun 91 19:20:05 GMT
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Thad, I think your basic point is correct: A positive pressure cooling
system is 'better' for cleanliness than a negative one.  I just don't
see how commercial companies can implement it reliably at a reasonable
cost point.  I'd spend $10 or $20 bucks extra for positive cooling but
not $100 or $200.  I'd rather get out the vacuum cleaner at regular
intervals.  

As was mentioned several times, positive pressure systems are funny
creatures to design.  It's sorta like when you try to design a
waterfall.  Even very small variations in the lip can make big 
changes in the water flow and volume.  So inside the case, flow is
determined by the easiest path to the largest outlet.  But there are
many paths and small variations between paths (like a developing
air leak in the case, a new expansion card, or even the built up
crud on your filter) can result in large variations of air flow.
That variation can result in large temperature swings in the electronics.
So for cost and reliability reasons I can understand why home PCs are
either suck-cooled or naturally blown (by convection).  :)

Regarding your internal fan, I know you know that suspended inside the case
as you describe will spend most of its energy circulating its output
to its input without doing much 'work'.  So I have to ask, how does the
air get into your case?  (and out of it?)

Fan noise is caused by motor speed (actually blade tip speed), blade
edge shape, and by pressure drop.  A fan contained inside a case 
and working with a very small pressure drop across it would give off
minimal noise.  

Dana Bourgeois @ cup.portal.com