[Elecraft] Thermal control for Q1 / Q2 in KPA100?

[Darwin, Keith]

Very key point.  If you want to take heat out of a box, you're far
better to point the fans out and let them draw in cool air from the
other side than to point the fan in and "blow cool air in".  Here's why.

When you blow a fan into a box (K2, PC, or your house in the summer)
you're blowing into a confined space and the pressure goes up.  That
increase in pressure fights against the fan and reduces the amount of
air that flows.  Mechanical efficiency and air transfer go down.  Turn
the fan around and blow it out of the box into the "universe".  The
outside world cannot be pressurized by the fan so the fan moves air more
effectively.

And I believe the air being blown into the box helps to set up dead
zones where hot air can pool up.  The flow through the box is better in
the exhaust case.

I've tested the theory both with my computer and with using fans to cool
my house in the summer.  In both cases, better cooling is achieved with
fans blowing out than in.

Makes me wonder about reversing the fan in the K2.  It just might make
it more effective and allow it to say on low longer.  I have to open it
up to tighten the HW and insert one more shield spring so I'll get
another chance to give it a good lookin' over.

- Keith N1AS -
- K2 5411.ssb.100 -


-----Original Message-----
From: Don Wilhelm

The fact that an exhaust fan flow cools objects better than blowing air
onto a surface is a bit counter-intuitive because standing in the
downstream side of a fan feels cooler to the skin - but that is only
because of evaporation.
Careful measurement will show that the cooling effectiveness for an
object like a heat sink is more effective by moving the heated air away
from the object.  Getting the heat away is the objective, and blowing on
it is less effective than exhausting the heated air.