"Joe Gwinn" wrote in message
...
....
Have liquid propane in the sealed copper boiler with a tube going
uphill to a copper condenser cooled by dry ice in alcohol.
....
The copper stuff can be ordinary plumbing tubing and fixtures. For a
one-off, plumbers solder is good enough. For a true seal, braze with
phosphorus-copper brazing filler, as used for HVAC systems.
....
Joe Gwinn

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The pipe fitting that enables making a condenser or similar concentric tube
structure is a pipe reducer with the smaller end bored through on a lathe.
Brass is easier than copper to chuck and turn. A 6-jaw chuck helps hold the
reducer without (much) distortion, or you can jam a fitted wood plug into
the large end.

When I learned industrial refrigeration in the 1970's this is what we brazed
joints with:
https://www.amazon.com/Lucas-Milhaup.../dp/B06Y1N5517