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

[d.cutter at ntlworld.com]

I'm always a little wary about changing a design.  I ask myself: what was in the designer's mind when he did this?  What characteristic was he trying to achieve and what information did he have that I do not?

Consult the fan design and see how its rating varies with pressure on one side or the other.  The air flow rate drops dramatically from its headline advertised value as soon as it encounters any back pressure in front of it and with restriction on the inlet side.  The manufacturer should know which is best for his product.

There are lots of fan types about and some are better at sucking than blowing.  

David
G3UNA
> 
> From: "Darwin, Keith" <Keith.Darwin at goodrich.com>
> Date: 2006/12/06 Wed PM 03:08:22 GMT
> To: <elecraft at mailman.qth.net>
> Subject: RE: [Elecraft] Thermal control for Q1 / Q2 in KPA100?
> 
> 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.
> 
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