We just made up some feed water heater coils from 1"od 10gauge copper 12 foot long. The trick is to braze on a stopper one end, braze on a nut the other then anneal the pipe. Fill with 30 down kiln dried sand, from builders merchant. Tap it down then wind in a bolt to compress the sand. This will stop it collapsing when bent.
All the best Percy
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