I made my own triple wall. My stack is 12" heavy gauge galv from Leader. I then made 16" from 28 gauge sheet metal spaced out with little metal spacers i made and then 20" the same. Going up from the evap it is single wall until about 2 feet lelow the lower cord of the trusses (ie, ceiling height, if it had a ceiling), from there the 2 added layers are suspended from the trusses on chain and steel support rods, and this tripple wall goes tight to the steel roofing but onlu the single wall extends thru the roof. In my location we are required to get a permit and be inspected by the fire dept before we can burn wood, the fire dept really liked my design and the wood is protected. Think of it this way, for a single stack you must have 36" clearance to combustables, 1 layer of tin spaced at least 1 inch away cuts the distance to 18" needed, a nother layer cuts the distance to 9" and if you need more a nother layer cuts it to 4.5". I made my 16 by assembling 2 8" to make the pipe and the 20" was 2 @ 10" joined. If i redo it I will go with flat stock and screw it (or pop rivet) together all in 1 piece because it will be easier to keep it round. Mine is far from a neat round circle but it works good and look OK. For my spacers i made v shapes with feet on the v. I pop riveted the feet to the stack before assembling and had several around the circumfrance to hold the position. Doing it this way cost very little and the wood is protected from the heat. The first season i had this I put a thermometer on the hot side of the closest truss and the temp was under 120 degrees with the evap at full fire for a few hours. I do have mine at 2" spacing but 1" would still work to specs.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.