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From: rjg@sialis.mn.org (Robert J. Granvin)
Newsgroups: comp.sys.att
Subject: Re: Fan Recommendation
Message-ID: <1436@sialis.mn.org>
Date: 6 May 89 20:12:09 GMT
References: <1567@jhunix.HCF.JHU.EDU> <485@limbic.UUCP> <664@devildog.UUCP> <1611@neoucom.UUCP> <3053@hound.UUCP>
Reply-To: rjg@sialis.mn.org (Robert J. Granvin)
Organization: Dr. Ho Laboratory and Day Care Center
Lines: 48

>Tell you what I did with my second fan.   I reversed the direction of air
>flow by turning the fan around.  This creates the effect of an air tunnel;
>one fan sucks in cool air, the second blows out the warmed air.  I used to
>have thermal problems until I did this.  By the way, if you are having
>thermal problems severe enough to cause your plastic casing to deform, you
>have a serious fan problem.  You should probably replace the fan and make
>sure that it is always blowing directly on the power supply side.  [...]

Ummm... Again, I'm sure someone will correct me if I'm wrong, but I
recall a lot of discussion in the past that described this as a Very
Bad Thing To Do.

While blowing air directly onto the power supply may seem like a good
thing to do, and certainly isn't a bad thing to do, it completely
defeats the air-flow design of the 3b1 (7300).

The 3b1 has grills along the front, side and top right which sucks
outside air into the machine, and blows it out through the right side
fan.  The effect is that all major components of the machine:
motherboard, RAM, drive, floppy _and_ power supply are cooled
adequately.  As a matter of fact, covering the top right vents has, in
numerous occasions, resulted in warped casing from overheat of the
power supply.  That much alteration in the air flow is Very Bad, and
if you're seeing that kind of problem, check your vents.  (Some people
have been surprised to find that there are indeed vents on the top of
the machine, even though it looks like there should be.  NEVER cover
these!)

Reversing the fan to blow _into_ the machine may keep the power supply
happy, but the rest of the airflow won't operate the same.  Air will
stagnate and escape through a different path, and the air, being
forced to escape through a wider area, will cause the airflow to
actually diminish.  The other components of the machine will not only
see less airflow, but will have to contend with the air heating from
the air that blows across the power supply.  This of course is for the
one fan method of 3b1 cooling...

How this actually interacts with two fans, one blowing in, one blowing
out, is a different matter, but I would suspect that the airflow,
especially at the front of the machine would be at least as bad, since
there would already be a high amount of air in the machine, not
allowing air to enter from the front.  Has anyone actually determined
what would happen in this case?

-- 
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