[Elecraft] K3 Cooling

[Don Rasmussen]

Don (FPR type),

Do you really think it matters in K3 - with micro fan motors and such a simple cage?

If a K3 is setup to run TX RTTY in a controlled environment,
and then repeat the test with the fan direction reversed, how much
of a temperature delta would you expect?

[Elecraft] K3 Cooling
Don Wilhelm w3fpr at embarqmail.com 
Mon Apr 5 15:20:41 EDT 2010 

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Don,

In my fluid mechanics labs it became apparent that exhaust cooling is 
much more effective than forced cooling for situations that are similar 
to the K3 PA 'cage'.
First is that the exhaust fan does not add to the temperature of the 
air, and second, the flow can be more easily controlled.  The exhaust 
fans remove heated air from the hottest spots - who cares where the 
cooler makeup air comes from, it can be supplied from any vent holes in 
the cabinet.

As humans, this is counter-intuitive because we feel cooler with air 
blowing onto us, but that is mainly because of evaporation cooling, not 
air flow.  We do not usually feel cooler standing on the other side of a 
fan.  Electronics do not have that evaporative cooling effect (unless 
you have poured a liquid on the electronics!), so in most cases, exhaust 
cooling near the heated areas does a better job.

If you question the fact that a fan adds heat to the air-stream, measure 
the temperature on the upstream side of a fan and the downstream side.  
I can assure you that the downstream air is warmer - the fan provides 
work to move the air, so it must heat the air because of that work-force 
(it is not the heat of the fan motor that I am referring to, but the 
motor heat may be present too.

One can design to contain the flow on the downstream side of the fan, 
but I see no air channel (pipe) in the K3 to accomplish that.  With old 
vacuum tube designs, we often used fans to pressurize the area below the 
chassis and direct the flow up around the tubes with chimneys around the 
tubes.

73,
Don W3FPR

Don Rasmussen wrote:
> I've seen this thread on every ham list I've read - goes on forever. 
>
> I always wonder - how does the K3 know which are the intake holes in the cabinet and which are exhaust? The cool air comes in replacing the hot air, right? ;-) 
>
> On tube transceivers, the finals may be oriented at the rear of the chassis near the fan, so a pulling fan will bring the cool air to the finals first, then that warmed air goes to the rest of the smaller tubes on the chassis. Reverse the air flow, the finals dont get the room temp air first, the smaller tubes on the chassis do. This -may- be important on some rigs, guess it depends. I have the Drakes setup for exhaust.
>
> K3 has these fins. I am going to take a poll with my fins and find out which ones care about the direction of the air travelling across them. ;-) 
>   
>