[Elecraft] K3 Cooling

Mon Apr 5 20:34:14 EDT 2010

Don,

I suspect it would make a big difference, especially with the SubRX 
installed - it would be worse with the air flow inward.
In order to provide effective cooling, the air flow needs to be laminar 
across the heat sink.  If there is turbulence, the cooling effectiveness 
will be reduced.  By blowing air onto the heat sink, first each fin 
would create its own bit of turbulence as the air strikes it, and then 
when the air flow reaches the blockage presented by the SubRX enclosure, 
additional turbulence is bound to occur, and that would create 
backpressure for the movement of the air stream.  More turbulence means 
less cooling.  With the exhaust fans, the air will come from wherever it 
can within the K3 enclosure and should flow in a more laminar fashion 
toward the fans, and then exit at the back.

Again, I believe this is counter-intuitive to our normal senses.  If we 
were to stick a finger into the air path, it would seem like the air is 
cooler if it is blowing on our finger.  The heat sink cooling is 
different than the sensation of our finger, cooling of a surface is 
highly dependent on a laminar flow where our 'finger test' would surmise 
that the turbulent flow should cool better.

Because of the effects of turbulence, there is also a point of 
diminishing returns when increasing the air flow with faster fans.  
Turbulence can also be created with exhaust fans, but not as readily 
(per volume of air moved) as with air pumped into the assembly.

I have not done any air flow studies of my K3 cooling, and I don't know 
to what extent Elecraft has done that during the development process, 
but it seems to be quite OK.  The highest temperatures I have seen 
reported is somewhere in the vicinity of 60 to 65 deg C, and if I recall 
correctly, Eric has stated that there is no problem below 80 deg C.  
(Someone correct me on that number if my memory is not correct).
15 deg C is a lot of margin.

73,
Don W3FPR

Don Rasmussen wrote:
> 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. ;-) 
>>   
>>
>>     
>
>
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