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Old 02-19-16, 11:01 AM   #23
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I can't believe I missed this thread! Sorry to revive something long dead, but I didn't do it first...

Ben, how did this experiment turn out? Did it work? If so, did it do well? Did you retire the rig? I'm interested if you can't tell.

I know from experience that this type of supplemental heating can work really well. Not just for solar, but for other types of heat sources as well. From a design standpoint, maximum efficiency of the heat exchanger is not paramount. With a low-grade heat source, there are only a certain number of raw btu available to transfer each pass through, so even with a superconducting exchanger, the gains are not enormous like with steam or a flame. As long as the heat doesn't leak out of the loop, it will come back through on the next pass through.

With this "shotgun" heat exchanger, the best way to improve the efficiency is easy. The unit can be built just like Ben made his, then a length of refrigeration-grade tubing (or a quadruple helix or what have you) can be coiled around the straight length of hard pipe, in between the tees. The heat transfer fluid line can be teed off at the supply and return ends, so that the fluid flows both through the center of the water pipe as well as around the outside. This greatly increases the surface area of both loops, and allows for a higher flow rate through the heating loop. Both factors increase hx effectiveness. If you feel the need to maximize contact area on the outside wall, and don't want to solder the tubing to the pipe, toothpaste or zinc oxide ointment are nearly as good at conducting heat.
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