Let me explain this picture a bit.
It's fairly hard to shove one tube inside another tube. It's especially hard as the lengths required get long and longer. When making a heat exchanger using 3/16 and 3/8" tube, it becomes an exercise in insanity.
Both tubes must be nearly perfectly straight.
That means, even the tinyiest imperfections become a problem. As you begin to push one through, friction becomes a huge problem. Even a degree or two at a tiny spot pushes parts of the tube against the inside of the other tube.
This isn't a problem with performance, simply that it becomes impossible to push one tube in any farther. You must then withdraw it, figure out what's stopping you, touch it a tiny little bit, and try again.
My method involves 5 ft long steel beams and clamps.
Clamp the tubes straight as possible.
And try.
When your looking down a 20ft long barrel though, things get interesting.
This took 3 days.
