Natty Bumppo
5 year old buck +
My pans are 10 inches deep and i always keep at a minimum 6 in them to keep from scorching the sides.
Sap depth and evaporation rates are inversely related....the lower the depth in the pan, the higher the evaporation rate. One, if you're depth is 4, 5, 6" or so, much more of the energy in the wood (or propane) is being consumed just to keep that huge volume of sap at boiling temps. You can boil a lower depth with less wood and less time. Also, the surface area to volume ratio plays a role. With a huge volume of sap in a pan, the water molecules have to move that much farther to evaporate. With 1.5 to 2" you get very quick evaporation rates with less wood.
Boiling at a low depth though has risk. I know a few guys who will push their evap. rates by boiling at 3/4". You've got to be very careful though. Any lapse in concentration and you'll get a burned pan. The high sides of a sap pan are simply to keep all of the boiling sap inside, not to fill it up.
I boil on a little 2x4' hobby evaporator and run anywhere from 1" to 1 and 1/2" inches.