I'm late to this post, but I'd like to let you know about my rocket burner maple syrup boiler. I use 5 stainless cafeteria pans, the deep ones, side by side. Each is suspended over a rocket chimney, inside a well insulated concrete box. The chimneys are 8" ID and the fire boxes are 8" w x 10" h x 24" l. I can easily boil 12 gph if I use thoroughly dried wood and keep the fires well tended. The temperature at the top of the chimney is 1620 degrees F, no baffle under the pan. No caramelization ever, and the syrup is nice and light. I ladle the sap from pan to pan, collecting the concentrated sap in the bottom pan to finish inside. I designed the pans so that each one is 3/4" lower than the previous so that I could siphon from level to level but the boiling action seems to mess with the siphons. The rig burns virtually smoke free unless I over-fire it, at which point I can get a 2' tongue of fire coming out the smoke stack. I've used it two seasons now, and I'm only seeing minor cracking in the insulated castable refractory (ICF) chimneys, but this is because the ICF shrinks on curing, and cracks formed in the molds.
It hardly needs saying that rocket burners are THE way to go because they burn clean and efficient. My first boiler had the pans hanging over an insulated fire box. The fire could never burn hot and clean, and pans got coated with creosote, LOTS of smoke. This year I'd like to try some smoke stack heat recovery, although I'm mot sure how well that will work. The hot stack is necessary to get a good draft. I'd also like to try putting air- water heat exchangers over the pans to use the steam to preheat the sap. We'll see how that goes.
Here is a video if you are interested:
I hope this is helpful.
