My mentor's bro works for PVI - Pressure vessels INC. They do heat exchangers, and scrubbers. He welded us up what we call the sputnik over three years ago - it is for waste burning oil. Drip oil into it and my advice is to have a day tank like on a fishing boat. It might finally be hooked up with a flue this week after three years.
We tested it out in the driveway years ago, finally bringing it inside after years of saying "tis the season to burn waste oil." We have two 55 gallon barrels almost full, nothing better to do with it of course. I will report how it works, no pics at this point but it has a cover and a bunch of tubes to extract heat, kinda cool. Looks like a rocket motor.
Oh yeah, it has a 40x60 building around it and we'll prolly leave it off while gone, or who knows leave a bit of oil dripping after we leave, but it will be burning something.
Other than that we use a propane top of the tank radiant heater.