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Thermal contact

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Posted by Biff on August 19, 2006, 1:03 pm
 
Making a solar thermal panel with copper tubing on alluminium flat
plate.  What's a good practical material to improve thermal contact
between tube and plate?


Posted by Jeff on August 19, 2006, 3:49 pm
 
Biff wrote:

I'd suggest searching the groups archives for plans and looking through
<URL: http://builditsolar.com  />

   My experience is limited, but here is a summary of what you will find.

    1) You'll get an electrolytic reaction between the the two disimilar
materials, you'll want to prime it. Black etch primer has been recom mended.

   2) You'll want a good physical bond between tubing and plate. Look at
the jigs made of wood that you just pound or press a rod into the plate.

   3) Although it appears to be possible to make this out of soft copper
tubing, you'll want to use hard copper. It comes straight, not rolled up.

   4) A thin smear of heat sink compound is enough. You can look at
<URL: http://didgikey.com  /> if you need to buy in quantities larger
than your electronic jobber carries.

   5) Other suggestion have included silicone caulk (which adds a
mechanical bond), and roofing compound. You'll get the greatest heat
transfer if the amount of filler material is as thin as possible.

   Jeff

Posted by Biff on August 20, 2006, 8:26 am
 
Thanks Jeff.  I've already sorted points 1,2 and 3., and rejected 4 on
cost gorounds - I have 50 metres of copper tube!  Yes, builders
silicone was my first choice but wondered if there were better
alternatives.


Posted by KDV on August 20, 2006, 5:23 pm
 


For avoiding corrosion and not for better heat transfer I use heat sink
compund from Dow Corning, between the copper tubes and the alu-plate.
The probleme is that this compound is very thick and you can't bring it on
very thin.
To avoid all this I have maded the next collectors in copper plates. I can't
see a difference in operation between the alu- or copper
plate collector.
So I wonder if I am going to use copper again as collector plate. Copper is
also mutch expensive and
is very heavy too.

Jeff,

A question for the clever guys:
What is the best in heat transfer? If I use alu-plates of 0.6 mm thickness
or I
use copper plates of 0.3 thickness? Knowing that the heat transfer of copper
is about twice as from alu.
Why this question?
For a better heat transfer I want to use copper plates. But 0.6 mm copper is
too thick for the metal press so 0.3 mm would be better to stamp and is
cheaper too. Now we have copper plates of 0.3 mm and a heat transfer of 390
W/(m*K) or we have aluminium of 0.6 mm thickness and a heat transfer of 237
W/(m*). So the heat transfer is the about the same for both?

--
Kris De Voecht
Homepage: www.devoecht.com



Posted by Jeff on August 21, 2006, 3:02 am
 KDV wrote:

Don't cover completely, it will spread when you clamp it. (At one time I
fixed a lot of amplifiers!) You can test this... but I don't think you
need to worry about a perfect connection. Try lowering the bond
conductance in the link posted below and you'll see what I mean.


Well, that's not me!


   Gary found this a while ago:

<URL: http://sel.me.wisc.edu/codepro/new_codepro.html  />

   Also, search back through the archives for some posts from Mike that
list suggested thickness, materials and spacing. Also, a wealth of
construction details.

<quote Mike in NZ>

Practical guidelines:
Copper fins 0.25mm thickness = 125mm width, 0.45mm = 140mm, 0.7mm = 160mm
Aluminium 0.5mm = 125mm, 0.75mm = 140mm, 1mm = 150mm
Steel 0.5mm = 75mm, 1mm = 90mm, 1.5mm = 100mm

</quote>

Basically, you'll balance the distance between tubes against the
thickness of the plate. But it's not exactly directly related to
thickness and heat transfer as so many things are going on.

   On a side note. You might consider not bending the plates at all.
Instead bend a strap of thinner material and bond that to the plate with
rivits and a smear of heat sink compound. You may wish for wiser minds
than mine to weigh in on that!

   Jeff


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