I built a small natural gas evaporator using stainless u burners and then rebuilt it a couple years ago to be bigger and added straight stainless pipe burners down the middle of each U.
There are air/gas mixers for natural gas on each burner. They make similar ones for LP.
I have a small partition under the front 2 burners that seperates the air intake from the air intake for the back burners which are set lower for a drop flue back pan.
It comes up between the front 2 and the back 2 about 1 inch below the bottom of the front pan, right where it meets the back pan. That is where the exhaust gasses and heat go back to the flue pan. The separate air intake for the back burners and controlling the front exhaust so it's far above the flame base of the back burners was key. Also have a very small blower to assist in getting the draft and exhaust gasses out at the right rate.
This works extremely well. After the season I will try to get some better pictures of the partition I described. Pictures I have when building it don't show it.
Some pictures and videos:
https://www.roseummaple.com/2019/03/...vaporator.html
https://youtu.be/vmpdcf81HhY
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