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

[Mike S]

At 10:08 AM 12/6/2006, Darwin, Keith wrote...
>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.

It's not that simple. How does the fan know which side is confined? 
It's not a matter of "pressure fights" the fan in only one case - it's 
equally valid to say that "vacuum fights" in the other. A fan is a 
pump, which creates a pressure differential. At the extreme, one can 
only achieve a suction equal to atmospheric pressure, while the amount 
of pressure can be virtually unlimited. So, in the extreme case, more 
flow can be achieved by blowing than by sucking.

For the application at hand, it doesn't make much difference to total 
airflow whether the fan is sucking or blowing - the fan is limited by 
the pressure differential it can create. Whether that differential is 
between atmospheric and a (very) slight vacuum, or atmospheric and a 
(very) slight pressure simply doesn't make any practical difference.

For electronics, airflow direction may be determined by the available 
fan location(s), and a desire to have the most heat sensitive 
components receive the coolest airflow. Another consideration is that 
by having the fan blow into the device, all airflow through the device 
travels through a well defined area - so it can be easily filtered.

"Pressurizing the enclosure is the preferred method, since incoming air 
can be readily filtered. In addition, a pressurized enclosure will 
prevent dust entering through cracks or crevices. The fan is also 
handling cooler, denser air, and it will therefore have a slightly 
higher pressure capability (this may be a very slight advantage for low 
heat dissipating systems). An important feature of a pressurized system 
is that the fan life and reliability are increased due to the fan 
ambient temperature being lower. The disadvantage of pressurization is 
that heat generated by the fan is dissipated into the enclosure." - 
Mike Turner, Comar Rotron (a major fan manufacturer) - 
http://www.electronics-cooling.com/Resources/EC_Articles/MAY96/may96_01.htm