When I was in the woodstove and furnace business, 24 ga or heavier was fine, I'm not sure about lighter weights. Galvinized (or SS) is good, aluminum will not do it. The spacers need to be at least 1" long and non combustible. I've made them out of 24 ga galvanized sheet metal, 1" pieces of copper tubing or have used ceramic insulaters (electric fence type) but they are longer than 1" and each layer of 1" or more cuts the clearance needed by 50%. So no layers=36" 1=18", 2=9" and 3=4.5". You need to realize that for example, the 2=9" means 9" from the combustible surface not 9" from that layer of non combustible.
As mentioned above, an evaporator runs far hotter than a woodstove, but the clearance requirements are designed to protect the combustible surface in the event of a chimney fire, when the stove pipe is glowing hot.
When I put up my stack originally, I made a triple wall pipe, it was inspected by our fire dept which is required in my area. They liked it, I then needed to have our insurance agent get a picture of it, he also liked it. When I replaced that stack I then used 1" ceramic insulation wrapped 2x around the stack, then covered with a layer of light gauge SS. Again, both inspections were good.
I like the looks of the last method, it is much neater. At the bottom edge of the SS outer shell I cut slits every inch and bent them in at 90 degrees to give a finished appearance. The insulation and SS extend down 3' below the ceiling joists and it goes up to touch the underside of the roof steel. If you have wood close to the stack where it goes thru the roof, that is vulnerable too, it needs to be cut back and made safe. Also, you should have a spark arrestor on the stack unless you have a non combustible roof.
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.