18 Nov
1999
18 Nov
'99
3:19 a.m.
> P.S.: When designing a new cooler
element/radiator, keep in mind that
> silver is a way better thermal conductor than Al or Cu.
Not quite accurate, Hans.
Silver and Copper are way better thermal conductors
than Alumin(i)um, but
there's not much to choose between the two of them.
Ok Ok, but it's better :)
>From Kaye and Laby, 15th edition: (all figures in
watts per metre per kelvin)
At -100 Celsius: Al, 241; Cu, 420, Ag, 432.
Interestingly enough, at these low
temperatures Beryllium is pretty good at 367.
At 0 Celsius: Al, 236; Cu, 403; Ag, 428. Be has
fallen to 218, behind even Al.
At 100 Celsius:Al, back up to 240; Cu, 395; Ag, 422.
According to my numbers: AL 210 Cu 380 Ag 408 - just I have no expliciet
temperature named (and I'm somewhat confused, since I belived that lambda
is not depending on any temperature, but rather a coefficient(?) to determinate
the conductance (?) - otherwise you'd have to integrate lambda over the
temperaturerange within the object (remember, its w/mK, where K is dT))
Conclusion: Cu and Ag are over 60% better than Al; at
low temperatures Ag is
better by 2%; at higher temperatures its lead increases to 6% but is still
nothing to write home about.
Well 6% are still 6%, especialy if the difference in price isn't that big.
FWIW Gold is the only other metal that makes it above
200, and is between Cu and
Al in all cases.
Anyway, it seems to me that the way to go is:
1. Peltier chip between CPU and heatsink. Heatsink is a large block of copper.
Adds more and more heat - an infinite loop, where you have to add a bigger
part to the 'interchange' on the hot side, hust for transporting the
Peltiers own heat. I found it better to optimise the transport within the
interchange element (as with using Cu or Ag, and mor efficient flow
structures) than just adding a Peltier. It's not about geting the
target temperature as low as possible, but rather transporting away
ad much (thermal) power as possible - that will inherently keep the
target from overheating.
Also, if we just remove the heat to keep the device (and all parts)
not below environment temperature we avoide all probems with
condensation (word?). We don't have to isolate all cooled parts
wathertight. Saving again a lot of recurces.
2. Use a refrigerant cycle similar to a domestic
freezer, but connect the
refrigerant circuit directly to holes bored in the heatsink block. No
intervening water circuit.
3. Of course, keep the refrigerant radiator well away
from the system, and
supply it with plenty of fans...
Or just use water and a real _big_ radiator to expell the heat.
After all, it's again about radiation a specific amount of
thermal power - and this can be done by either a high delta-T
or just a biger surface (phi = lamda * S * delta-T / delta).
4. Finally, try not to spill refrigerant if it's
one of those chlorinated
organics that the environmentalists are always going on about. It won't do any
good (although a discussion of whether it does harm is decidedly off-topic), and
will be well-nigh impossible to replace...
Or best of all, just use a matter with excelent properies and
no risk - like water (we are talking about a small amount, so
no risk of drowning) - water has one of the best termal capacity
numbers and good qualities in conductiviety (Just to avoide
questions why it isn't used in frigdes - Water can't be compressed -
so it is hard to reach temperatures below the environment).
Conclusion: I belive that a 'soft' aproach can give the same
result in most situation without spending endless resources
to push a single idea solution (brute force).
Gruss
H.
BTW: I considere this total on topic, since these are the same
problems since computers exist (I've been trained on cooled
CPU systems in the late 70s).
--
Der Kopf ist auch nur ein Auswuchs wie der kleine Zeh.
H.Achternbusch