> 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))
For really accurate studies you do indeed have to do just that. Thermal
conductivity varies with temperature in much the same way as electrical
conductivity - and is (AFAIK) similarly infinite in superconductors, which no
doubt helps in cooling them...
>> 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
Yes, but not much more. I have never seen a figure for efficiency of peltier
chips, but I have always assumed around 200%, i.e. removes about twice as much
heat as it generates. (YMMV - Efficiency varies with temperature difference)
> 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
Possibly true in a lot of applications ...
> target temperature as low as possible, but rather transporting away
> ad much (thermal) power as possible - that will inherently keep the
> target from overheating.
I have not much experience with cooling CPUs, but with other semiconductors
(mainly power transistors) I have found that the biggest thermal resistance in
the circuit is almost invariably semiconductor to package exterior. The amount
of heat you can remove depends therefore mainly on maximum allowable temperature
of semiconductor and temperature of package surface. Getting the package
surface down another few degrees will in a lot of cases more than make up for
the extra heat the peltier chip produces. It may even do so without extra
cooling on the water (etc.) side - a few degrees lower on the IC balanced by a
few degrees higher on the water may transport all the extra heet you need.
Especially if your cooling is decent (low thermal resistance) between water and
ambient.
> 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.
Condensation could be a problem, although it didn't seem to be in project
EUNUCH. Some sort of drying arrangement for the ventilation should be
sufficient though - this too can be done with cooling...
>> 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).
Still keep the radiator well away from the system and supply it with plenty of
fans!
See above for my reasons for keeping the temperature down...
> 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).
Perhaps. But your solution is limited by the ambient temperature, the maximum
chip temperature and the thermal resistance within the IC. For any better heat
transfer than that, you _have_ to cool below ambient. I was simply exploring
ways of doing this.
Philip.
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Received on Wed Nov 24 1999 - 12:19:21 GMT
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