Where can I find a 3/4" Copper Coil

I've seen a similar setup with the coil suspended in boiling water, rather than directly exposed to the flames. For that matter, I've used a similar apparatus for heating water.Having tried both methods, my experience is that the boiling water bath is MUCH safer, and still heats the water quite satisfactorily.
With the coil directly exposed to flame, you'd be making steam.
Properly designed, that could work, but steam is not a tame lion.

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III

if you need to make a coil from copper tubing fill it with sand it will keep it from creasing. then bend around something the size you want' a large fence post would work good.

Alum tube with water in it won't get close to melting.
Could you use a conduit bender and keep moving it along?