Harlon,
I wish I had a link for you. But a twin wall heat exchanger is not all that hard to build.
You'll need,
- , pieces of copper pipe 1/2", 3/4", & 1"
- , T fittings, on the straight through part of the T you'll need 3/4" on one end and 1" on the other. The third will depend on the plumbing coming to the heat exchanger from the solar collector.
- , 1/2" to 3/4" adapter
You'll probably need a piece of 1/2" and 3/4" steel bar stock. To drive through the fittings, so that the pipe's will slide all the way through the fittings. Using a round file will weaken the fitting's.
Assemble the 1" pipe with the 3/4 pipe inside of it, then put the T's on each end. You'll want at least 1" of the 3/4" pipe sticking out of the T's. Solder this together, it'll be your antifreeze jacket. Now insert the 1/2" pipe all the way through the 3/4" pipe and slide on the 1/2" to 3/4 adapter and solder. This will be the bottom of you heat exchanger. Again leave at least 1" of extra pipe sticking out each end to hook in to your water lines to the tank.
There will be a gap between the 1/2" and 3/4" pipes, that you will want to fill with a heat transfer medium from the link above if they have something pourable or use mineral oil. Mineral oil may not be the best for heat transfer but it's better than air and it's safe, one of it's uses is as a laxative.
If you want you can install a 1/2" to 3/4" adapter on the top, but do not solder it. That way if the 1/2" or 3/4" pipe were to spring a leak it will flow out the top, and you'll know about it.
I hope I made this easy to understand , And that you find it useful.
Gordy