Look up the thread by Warners point 2-27-2011 "2 x 4 drop tube flue pan" under the boiling forum. I have not seen one being used but thought it was an excellent idea to increase your boil rate. You would probably have to weld a skirt around the outside bottom of your pan, the drop tubes would sit on the burner and the skirt would keep the heat under the pan on the drop flues. You might have to make some heat exit slices in the skirt or something, but I guess that a little experimenting would tell you. I would think that even drop tubes 3"-5" would greatly increase your boil. Just a thought.
Joe
2004- 470 taps on gravity and buckets
2006- 590 taps on gravity and buckets 300 gph RO
2009- 845 taps on vacuum no buckets, 600 gph RO
2010- 925 taps on vacuum new 2 stage vacuum pump
2014- 3045 taps on vacuum, new 1200 gph RO
2015- 3104 taps on vacuum
2017- 3213 taps on vacuum
3' x 10' oil fired evaporator with steamaway