[Tfug] Cooling

[Bexley Hall]

Hi,

I suspect this one is a bit harder to get a
*definitive* answer -- in the general case.   :<
*But*, who knows, maybe somebody out there
actually *does* this sort of thing for a living!

I had to replace the fan in one of my tape
drives (noisey).  Of course, fans exhibit
*loads* of symmetry (i.e., there are 8 ways
you can mount one -- though, ralistically, the
choices boil down to exactly TWO!)  So, since
I didn't make a deliberate note of the original
fan's orientation, I had to think this one
through...

Fortunately, the fan had a filter on it -- which
makes the choice easy (you don't need to filter
EXHAUST air)!  So, the fan should be oriented to
blow air *into* the enclosure.

This makes sense for a tape device since it helps
keep crud from getting into the mechanism.  Same
principle as maintaining positive pressure in sterile
environments, etc.

But, beyond this, any "rules" for air flow seem to
be pretty vague... "ad hoc", almost.

In the past, ME's et al. have made these decisions
in product designs in which I've been involved.
And, I've never been able to infer any *real* rules
as to "which is better" (and in which circumstances).

A common approach is to exhaust air from things like
power supplies.  This makes sense if there are other
fans in the device that can move the heated air
generated by other components in the assembly.  Note
that exhausting air pulled directly from a CPU's
heatsink fits this same class -- the heat source is
concentrated and, as such, you can readily pull air
directly from it.

But, what about devices that don't have these nice
identifiable hotspots?  Are there any rules as to
why or when it is best to pull vs. push air?  I
know the turbulence created by obstructions greatly
affects the cooling ability of the air flow.  But,
does the turbulence differ if the is air being
*pushed*
into that region vs. *pulled* though it?

(yeah, it's one of those questions that I can -- and
probably *will* -- die without knowing but it *is*
puzzling!)

Thx,
--don



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