Superlen, Thank you! Glad you enjoyed it!
I think double wall is going to hurt your heat transfer.
There is zero worry with the single walled pipe. 1/4" is some pretty tough stuff, and copper has been used to carry potable water for a very long time with no problems. It would be a pain to get anything bigger into the small size element hole.
Please let us know how your build goes!
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שְׁמַע יִשְׂרָאֵל ה' אֱלֹהֵינוּ ה' אֶחָד
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