----- Original Message -----
From: "TC Dailey" <[email protected]>
To: <[email protected]>
Sent: Tuesday, August 23, 2011 11:38 AM
Subject: [Drakelist] Cooling Fans
In the "for what it's worth" department - I used to hold a
muffin fan ATOP my SB-220 and SB-200, with a pair of
toothpicks. It made NO permanent holes, held the fan
where it did some good, AND I had it "sucking out", as
opposed to "blowing in". Any physics or fluid-mechanics
guy will tell you that when you compress a fluid (like
air), it heats up (albiet, in this case - very slightly),
a negative pressure [sic] tends to lower the temp -
whatever works for you, but my fans PULL - they don't
PUSH.
Tom - W0EAJ
Having a sucking fan on top takes advantage of the
normal flow of the heated air due to convection. It also
does not blow heated air over the rest of the chassis. Since
the amount of air to be moved is not great a low speed fan
works fine. Mounting on the back of the final cage works
fine despite the theoretical advantage of top mounting (I've
tried it).
The size of the fan and design of the blades and its
speed determine the volume of air it moves and also the
amount of back pressure it can work against. Both are not
critical at all here. However, the _noise_ a fan makes also
depends on size, speed, and blade shape. For the same volume
of air a small fan must run faster. Some miniature fans are
nearly sirens, they are necessary for some applications but
not for cooling fairly large vacuum tube gear so a much
quieter fan can be used.
I suppose one could devise a physics lab problem of
determining both compressional heating and the heating due
to frictional forces on the flowing air. That would require
measuring a _lot_ of parameters. While this is silly here
there are engineering applications where such things _do_
need to be calculated.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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