Fans and Cooling (long)

[Ed Tanton]

>Date: Fri, 24 Jan 1997 22:10:39 -0500
>To: QRP-L Reflector <qrp-l@Lehigh.EDU>
>From: Ed Tanton <n4xy@avana.net>
>Subject: Fans and Cooling (long)
>
>Hi Everyone... Fans have been an interesting debate for years and years.
The major decision factor is NOT noise... it is dirt. Importing UNFILTERED
air into a precision electronic instrument is just plain stupid fellas (and
yet my Alpha 78 inducts air from the bottom, allows it to circulate around
the underside of the chassis, and exhausts it through the top via the metal
tube anodes and their chimneys. They saved money on filters at the risk of
the equipment lifespan-especially true around HV! Only saving grace there
is its extremely low usage here!) 
>
>You can easily buy filters and their holders made for-and sized for the
muffin fans we use. Under no circumstances should a fan pull air-and
therefore dirt & whatever else-into anything you value... without
filtration. And the major ploys to-sort of-avoid this: pulling it in from
underneath the unit or exhaust-only fans (where do you figure the air
you're exhausting comes from?)  are only methods to fool yourself and
postpone the maintenance. 
>
>Yes it does cool the unit-but the price is dirt/etc. all over the air
pathway inside your rig. Two words: FILTER IT!!! 
>
>And while you're at it, check whatever test equipment you own AND YOUR PCs
for clogged air holes. Even in an office where daily cleaning and vacuuming
is performed, I have opened PCs whose internal PS ventillation holes were
literally closed over completely after one or two years. (I now do PM on
all those machines to avoid nasty fires and failures that can result from
such conditions.)
>
>One simple-if-inelegant-solution to the OMNI VI problem (and others as
applicable) is to mount a fan EXTERNALLY that blows ACROSS the heat sink.
That alone ought to help significantly. 
>
>Another technique that will lower the internal temp of any metal cabinet
is to add an internal-only fan to move a small amount (10-40cfm) around the
cabinet. This allows the cabinet walls to become a gigantic heat sink
themnselves. In a 6  ft high 5 ft wide by 5 ft deep metal cabinet with
several thousand watts inside, I was able to reduce the interior
temperature by 5 - 7 decrees F using interior air circulation alone. This
is a significant number in those circumstances! (I cannot say what a small
unit like the OMNI VI where the electronics are crammed, and the cabinet
sides are not really exposed to circulation air would do-but it couldn't
hurt.) Even if totally without any exterior heatsink effect, the internal
fan will spread the heat around-probably enough to solve the problem.
>
>Your choices are: [1] leave it alone (my OMNI VI does not exhibit this
heat-bfo drift problem as far as I know;)  [2] add an external-only fan
blowing across the heatsink; and/or [3] add an internal-only fan
circulating the air without external air contamination; or [4] make certain
to use (and keep clean) a filter, and use an external fan importing clean,
filtered air for cooling.
>
>By the way, there are numerous fans available in various catalogs having
low enough volumes to perform this job well with imperceptable noise. Much
above 30 cfm and that is not so. 10 cfm ought to be plenty however. Also,
spring for the ball bearing fans-they last a LOT longer, and are usually
quieter for the same air rating than the cheaper sleeve bearing fans. (You
can always recognize the used ball bearing fans from the rest at
hamfests-they are the ones that spin so freely, and keep on spinning longer
than the others!) 
>
>Finally, use 12V fans for 12V if you can. They'll last longer and run
cooler. Using 12V on 24 works most of the time-and SOMETIMES there are fans
who are manufactured for ranges, but you'll be better off doing it right.
(And ALWAYS put a reverse diode across a DC fan. They make great generators
when winding down.)
72/73 

Ed Tanton N4XY    EMAIL: n4xy@avana.net   TEL: (770)579-3933 V/MBX/FAX
189 Pioneer Trail
Marietta, GA  30068-3466

QRP-ARCI#7663    G-QRP#6779    OK-QRP#172    QRP-L#758    AdvRC#140
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