As Groves said, if you can contain the fire, the better/faster you are going to evaporate. My first year took an entire weekend to boil down 10 gal. of sap using the grill. Second year used a 4qt and a 9qt dutch ovens over an open fire, I figured I was boiling off about 1/2 gal/hr. I boiled for almost a week and a half straight to make 5qts of syrup. Third year build the common foundatin block arch with 3 steam pans. The foundation block will crumble from the heat of boiling if it is not protected in some fashion from the heat of the fire; found that out the second year I boiled. Used sheet metal for a front cover/door and some 4in duct for a chimney. I had some old red brick that I used to line the fire box. I could boil about 3gal/hr with this setup.
Surface does make a difference on evaporation rate. The steam pans have more that a turkey fryer pot The turkey fryers are good setup though, burner and pot all together. The hard part is finding cheap method to contain the heat under whatever pan(s) you are boilng with.
Eric
06-07 3-8 taps, Dutch oven, open flame
08- 12-17 taps, Block arch w/3 steam table trays