Moreira, Luis A wrote:
> I get what you guys mean and you are right. The only thing that the
> volume of the air in the enclosure makes a difference is on how long
> it will take from a state of no power dissipation, for the
> temperature in the enclosure to reach steady state temperature.

Theoretically yes, because at a fixed flow volume the enclosure volume
dictates the time between "air changes".  However, in a practical system
this will usually be much less than the time for the thing being cooled to
heat up.

I should point out something that Russell brought up and that I didn't
emphasize enough in my original post.  The black box view of the system
being cooled is valid and useful, and I believe my example was correct.
However, that says nothing about how that power is tranferred to the moving
air and what temperature the thing being cooled needs to be to tranfer that
power, and what temperature different sections of the internal airflow are.
In my example, the outflow is at 40degC, but some parts of the internal flow
could be much higher with others largely bypassing the hot thing and staying
near 20C.  The average outflow would still be 40C, or you could envision
turbulance so that the outflow air is well mixed and it's all at 40C.
Either way you still transfer the 683W, but there can be large differences
in what that means to the equipment being cooled.

Let's say for example you've got a common off the shelf heatsink for a TO-3
transistor that has a thermal resistance of 3degC/W (a plausible value for
such a thing).  If such a single heatsink had to dissipate 683W its
temperature would have to rise by 2050C.  Obviously that's no good.
However, if the 683W are spread evenly over 50 such heatsinks, each only
needs to rise 41C above the air around it.  Designing the air flow system
inside the cabinet so that each of these 50 heatsinks gets the proper flow
of cool air is no trivial feat.

The point is that just because 1 cubic foot per second and 20C rise works
from the black box point of view doesn't guarantee that it does what you
need inside, at least not without a lot of careful engineering.

Also picture the mass of 50 TO-3 heatsinks that is about plausible to
dissipate 680W to keep within semiconductor temperatures.  That's a lot of
heatsinks.  If I remember right, the OP wanted over a order or magnitude
more cooling.  Imagine the very large surface required to exchange that
amount of heat with air.  That's why Russell suggested water cooling.


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