Older pans were often equal length, such as 2' on each for 4' total, but that was before R.O.'s. My arch is 8' long and it originally had a 3x2 syrup pan and a x6 flue pan. I ordered a new set of pans 3x3 syrup and 3x5 flue. Running concentrate in anywhere from 8% to 13% sugar, that seems to do well, the syrup still has enough time to develop a great taste. On the new super R.O.'s getting high concentrations, like 30%+ they run a short flue pan and a long syrup pan to give the syrup time to develop the taste needed.
For you, if not going above 15-20% concentration, a pan set with 35-40% of the total being syrup pan and the rest being flue pan should be good.
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.