[Elecraft] K3 Cooling

[Guy Olinger K2AV]

I believe that if you examine the intake slots above the PA heat sink,
you will see that cool intake will come in equally across the fins,
with the fins farthest from the fans getting cool air directly from
above, and the portion of fins closest, though pulling warm air from
parts of the fins away from the fans, pulls the air with increasing
velocity as the fans are approached, cooling the heat sink near
equally.  That does NOT happen if the flow is reversed as the fins
farthest away get the LEAST velocity with the hottest air.  Flow
should definitely PULL over the fins and exhaust out the back.

In a minor disagreement with Don, I think that the unequal cooling is
the killer issue not the fan raising the air temp blowing in.

I'm not going to do it on mine (I like it working), but if someone
actually reverses that and goes, I'd guess +10 to +20C in the temp and
kicking in the overtemp protection.

As to how good the cooling is, just to see what happened, I let the K3
transmit at 100 watts for a half hour into a dead band.  It never hit
60C. I set there watching it the whole time waiting for a temp spike
that never happened. It got up to the high fan speed and blew fairly
warm air out the back, but stayed steady.  K3 is brick on key.
Something must be correctly engineered.

After that was over it occurred to me that I could have fried my
Astron RS35A, but that held up as well.  : >)

73, Guy.

On Mon, Apr 5, 2010 at 7:08 PM, Don Rasmussen <wb8yqj at yahoo.com> 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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