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Solar DHW: Copper or PEX - Page 3

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Posted by Morris Dovey on October 26, 2007, 5:48 pm
 
daestrom wrote:

| ISTR that even copper systems can have a problem in direct sun with
| no flow. If the water/glycol gets too hot it can melt the solder
| used in the connections.  (or maybe that is only concentrating
| collectors...?)

I think solder melts somewhere around 450F - a problem for some
concentrating collectors, but shouldn't be a problem for
reasonably-designed non-concentrating collectors.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/



Posted by daestrom on October 28, 2007, 10:30 pm
 


I looked and found there are several versions of 'solder'.  The traditional
tin-lead stuff melts around 361 F.  But a lead-free version with bismuth
fails as low as 194C

So I guess the moral is to pay attention to what kind of solder you use :-)

http://en.wikipedia.org/wiki/Solder

daestrom


Posted by Jeff on October 28, 2007, 11:54 pm
 daestrom wrote:

   I was advised to go with a silver bearing solder for my collectors.
Not because it has a higher temperature but because it has a much higher
shear strength. Of course it also has the higher temp, it's just so darn
expensive!

   Jeff


Posted by Solar Flare on October 29, 2007, 12:49 am
 PEX is not good for UV exposure. Sounds like more to the story than
meets the eye. I find it hard to believe you can get glycol too hot
for PEX tubing unless the pressure relief valave didn't work.

"pressure relief"? what pressure relief?




Posted by Steve Shantz on October 29, 2007, 2:01 am
 Engineeringtoxbox.com says a 50% solution of Propylene glycol in water
will boil at 222F at atmospheric pressure.  They do not list the
boiling temperatures at elevated pressures, but even with a 15 psi
relief valve, the boiling point could reasonably be expected to be 235
or higher.  I think I've seen some of the better grades of PEX Al PEX
with max temperature rating of 200F at 50 or 60 psi.  I suspect that
temperatures don't have to go much higher than this before the PEX
portion of the sandwich rapidly looses strength.

If all of the above is true, I wouldn't want PEX on the return line.
It might make it through a stagnation / restart event, but it would
not be a sure thing.... not the kind of robustness I would want in a
system design.

As for the solder, if the max temp of a stagnated boiling pressure
relieving system is about 230 - 250F, then I would think a standard
95/5 Sn/Sb solder should work fine, as it melts around 450 - 460F.

Steve


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