Richard,
Opinions are free but "rigorous data"? That you'll have to pay dearly for.
;-) Actually, I could probably write several paragraphs on thermal
conductivity and a comparison of aluminum versus copper and even throw in
some data too, the bottom line of which is it doesn't make a nickel's worth
of difference in a radiator. Before I say why this is so, I might point out
that the purity of a metal has a very big influence on its thermal (and
electrical) conductivity. Stainless steel, for example, is a very poor
thermal conductor compared to pure iron (or pure nickel). I could explain
this in terms of phonons and quantum mechanics, but you can get a feel for
this classically by thinking about colliding billiard balls. When the
billiard balls are the same size, they transfer momentum most efficiently.
If you have a pure metal, all the nuclei (billiard balls) are the same size
and it will conduct heat pretty well. If you alloy two very different size
nuclei together, it really screws up the momentum exchange and the thermal
conductivity goes to hell. The effect at very low temperatures can be
several orders of magnitude; in fact, this is a pretty good way to measure
purity. For completeness, I should also point out that in a typical metal,
approximately half the heat is conducted by the conduction electrons, the
same ones that carry electrical current. That's why metals are generally
better heat conductors than insulators, although there are some very high
thermal conductivity insulators like diamond and beryllium oxide, for
example. (I'll resist the temptation to explain this phenomenon. ;-)
The reason the purity discussion is relevant to comparing copper to
aluminum is that industrial copper is relatively pure, whereas most
aluminum structural parts are highly alloyed for strength, which makes the
thermal conductivity of typical aluminum alloys a lot less than copper,
radiators included. (BTW, an interesting engineering fact about structural
aluminum I learned a while back is its failure limit is zero; i.e., it will
fail eventually, no matter how small the applied stress is. Makes you
think twice about flying in an old airplane, huh?)
From the previous argument, it would seem that copper is the best material
to build a radiator out of. However, the thermal conductivity of the
radiator is scarcely a factor in its efficiency. You could even make one
out of stainless steel and it would work just about as well as copper or
aluminum. The biggest impediment to heat transfer in a radiator is the
air/surface interface, and to a lesser extent, the coolant/surface
interface. There is scarcely any temperature drop across the thickness of
the cooling tubes. Other factors, like weight, are very important though. I
had occasion to pick up an empty copper radiator for a Tiger, one of the
really good ones which, incidentally, worked extremely well. It weighed a
ton! I couldn't believe how heavy it was. I didn't actually weigh it (see,
you have to pay extra for real numbers), but it must have easily weighed
twice or three times as much as my aluminum radiator. If you don't mind an
extra twenty pounds or so on the front of your Tiger, then copper has the
practical advantage of being pretty easy to repair compared to aluminum.
I understand by private communication that there is a better aluminum
radiator undergoing testing on a Tiger with very impressive results. It is
not as thick as the Ron Davis radiator. The source is Fluidyne, who
supplies radiators to NASCAR, etc. You can check them out at:
http://www.fluidyne.com/
Right now, a radiator for a Tiger would be special order, but if there is
enough interest, maybe one of the clubs like CAT will have them make a
limited production run. I'm sure it won't be available for this summer though.
Hope this amuses if not instructs.
Well, TTFN
Bob
At 05:34 PM 3/18/00 +0000, Richard F Flynn wrote:
>I hesitate to ask, but does anyone have and rigorous data on aluminum or
>copper
>is better at dissipating heat? I have been told by a HVAC type that copper is
>usually the metal of choice for cooling systems in buildings because it is
>more
>efficient than aluminum, yet people on the list seem to have had improved
>their
>cooling situations with aluminum radiators.
>
>Any hard data out there?
>
>thanks in advance,
>
>rick
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