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bending aluminum - Page 5

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Posted by sam c on December 16, 2007, 8:12 pm
 
gary@builditsolar.com wrote:

hi gary
setting heat sink compound is available , because I have used it in my
current hw collector project , it is absurdly expensive , is it worth
the extra cost ? I cant tell you yet as the collector is not in service
,yet but I would doubt that the extra cost is worth it . would I use it
for this perpose again , no it is just to dear .
sam c.

Posted by RicodJour on December 16, 2007, 4:15 pm
 

brown/white

sheet.

It's that time of year.  ;)


That's one part of the calculation.  What's the energy hit when
comparing copper in direct contact with the aluminum as opposed to
having a .01 caulk barrier?


Why wouldn't you use thicker aluminum sheet in the first place?


Using an incorrectly sized jig or sketchy method to bend the aluminum
will guarantee you an unwanted air gap.  Aluminum springs back - use
that to your advantage.  Bend the aluminum more tightly using
something like one of the nice jigs others have posted and you
shouldn't have any appreciable air gap.


Black/white trim coil is readily available.  There are a lot of houses
- well, maybe not a _lot_ - with black trim.  How else would they wrap
trim and fascia?
http://www.acehardwareoutlet.com/ (iazh4u45eo0xkf45bdo3q155)/ProductDetails.aspx?SKU8022154

The emissivity was the reason I asked St. Nick (it's that time of
year) why he chose painted aluminum as opposed to mill finish.  I
really don't know the numbers and I was asking the difference.

R


Posted by Jeff on December 16, 2007, 5:18 pm
 RicodJour wrote:

brown/white

sheet.

   Well, go run the calcs. Nobody has measurements.

   In general the larger the contact area the less you need to worry
about thermal contact resistance. I'm running thermal compound at the
copper/aluminum boundary.

   Price versus availability. Aluminum and copper are very high these
past few years. Flashing is cheap and available. Of course it is a less
controlled alloy than buying commercial aluminum sheet or roll. Buying
what I wanted would have increased my collector cost about 2 1/2 times
even with a discount.

   Try getting it yourself...

Try this yourself. This is not as simple as you think. I have good tight
contact but would not think of mounting this naked, see below...

   On a side note, I had processor that had a very good metal to metal
contact with it's heat sink yet no heatsink compound. We are talking
virtual metal to metal contact. Yet adding heatsink compound lowered the
  CPU temp at least 15F. I used to repair amps and neither I nor the
manufacturer thought of mounting transistors without compound, now,
those transistors were tightly screwed down to the heatsink and both the
heatsink and transistor base were polished and well machined.

   When you are talking high heat density contact conductivity is
important. Go run the numbers, the formulas you know.

   Jeff

http://www.acehardwareoutlet.com/ (iazh4u45eo0xkf45bdo3q155)/ProductDetails.aspx?SKU8022154


Posted by Solar Mike on December 16, 2007, 11:24 pm
 Hi Jeff

I wouldnt put any thermal compound between your muiltiple sheets, too much
effort and cost.
if you pop rivet a flat thicker 6" piece of alumininum centered on the tube
from behind, then that will pull them together to give a good tight fit on
the copper tube, the extra metal will help conduct the heat in from the
edges. Any thermal compound used at the Cu-Al interface would be most
benificial. Silicon caulk would work here as it will be very thin, but watch
that it doesnt set on you before pop riveting together. I used a thin non
silicon heatsink compound available here in NZ in 500gm tins, brush it on
with a paint brush.

Best of luck
Mike (NZ)



Posted by Jeff on December 17, 2007, 2:24 am
 Solar Mike wrote:

   Hi Mike,

   Nice to hear from you.


   That's what I was thinking. But I wonder if a smear of something,
like silicone caulk or even moly grease might help. Something thin...


   I don't really have the thicker metal. The city I live in doesn't
have a good resource of salvage and metal shops, if anything they are
vanishing...

  I've cut and formed all the channels and made them so they stop just
short of the next tube. Then I have a flat cover sheet on top. That
gives me 3 thicknesses of .01" aluminum except for the edge that is only
two.

   I got most of one box riveted together before it started raining...
I'll look tomorrow to see how well the sheets meet. Rivets are in a
repeating dice 5 pattern. That's just over 200 rivets (about every 5")
for one 2' * 10' box. I've got a jig to drill and rivet it but it still
takes an hour or so!


  Any thermal compound used at the Cu-Al interface would be most

That sounds nice. I have some more typical thicker non-silicon compound
that I'm applying (at the Cu-Al junction) with a toothbrush! $5
US/pound. Incidental costs add up!

  >

   Thanks,

  Jeff


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