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Fins 4 panels, not Aluminium?

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Posted by Anti-Spam on March 4, 2009, 11:09 pm
Fins 4 panels, not Aluminium?  Just about to finish some home made
panels, I have made, but read somewhere that fitting Aluminium fins to
my copper pipe risers, may not be a good idea because of the chemical
reaction between these two materials, any suggestions on what I should
use instead pleae?

Posted by Anti-Spam on March 4, 2009, 11:14 pm

Sorry forgot to add, that the aluminium fins I was proposing to use,
would have been painted powder coat black, does that make a difference
in the right direction?

Posted by amdx on March 5, 2009, 12:51 pm

Only if the powder coat is between the alum/copper interface.

  I'm in the same position, just starting to build aluminum over copper
solar collectors. I have the same concern about a galvanic corrosion of the
 I think there needs to be some type of barrier between the two, but it
can't impead
heat flow.
 I picked this up with a quick google search.
"If paints or coatings are used for isolation, they must be compatible with
both metals. Bituminous or zinc chromate primers can be used between copper
and aluminum."
 I wonder about the thermal characteristics of zinc chromate. I look into
that today.

Posted by Anti-Spam on March 6, 2009, 7:49 pm

After re-reading what I said, I think powder coat black is fine but
not on the contact areas between Fin and Riser, that was a bad idea. I
am now considering maybe tin, or very thin zinc plated steel, and
powder coating the exposed areas, but masking the contact areas, what
do people think, of those ideas?

Posted by barefoot on March 7, 2009, 10:15 am
 There shouldn't be corrosion if the joins dont get and stay wet (or
suffer from stray electrical currents which will promote
electrolysis). Refrigeration evaporator coils are made by sliding
aluminium fins down a bank of thin wall (0.4mm ?) copper tubes and
then "hydraulicing" the tubes (with hydraulic oil) to expand them into
the fins - a bit hard to carry out at home. The aluminium fins are pre-
punched (flared) to make a shoulder for each pipe to expand against
which increases the surface contact area. Its hard to see given the
differing coefficients of expansion of the two metals and constantly
varying temperatures that the press fit would remain tight at all
times. As far as I'm aware any corrosion treatment is carried out
after assembly and is usually labelled passivation, thin film epoxy or
varnish. At refrigeration temperatures even though the coils may be
always wet there isn't much corrosion except where the air is
circulated over corrosive product because the low temperatures inhibit
it. The exact same coil used for air conditioning heating (with high
temperature refrigerant or water inside) doesn't seem to corrode - the
air may be humid at times but its rarely got free water and heated air
is usually very low RH. Condensers are essentially the same coil
installed full time out in the weather and they don't usually suffer
from corrosion unless the air has contaminants (although the above
coatings are usually available and a commonly used). In general once
aluminium forms an oxide its very durable unless continuously washed
in something that can eat it. Depending upon fin spacing (closer
spacing gives a coil better heat transfer but it tends to ice up more)
the copper pipes may or may not be completely covered by the aluminium
shoulder. Heatcraft Refrigeration which is a refrigeration wholesaler
and also evaporator manufacturer here and in the US used to sell coil
varnish in a spraycan that could be applied to a new coil to be
installed in corrosive atmospheres.
I've created the same effect as manufactured fins with shoulders by
drilling or punching holes a bit smaller in the aluminium plate then
creating the shoulder with an expanding type Rigid/Rothenberger pipe
swager but you could do a similar thing with a mechanical punch the
right size or just by driving the fins with smaller holes down the
pipe (the aluminium might need annealing once or twice).


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