Think of a continuous flat pan like this:
Assume a 2x4 flat pan with (4) 6" channels. Rearrange them so they are four 6" by 4' channels end to end, with openings in the touching ends so there is free flow between them. That is what the opening in the channels are. Now you have a 16' long pan 6" wide.
Start trickling sap into one end and it will slowly flow along the length and fill to say one inch. If you have a float to regulate it, it stops and you are done, with a long skinny pan with one inch of sap.
Now add heat. Even fire along the length. All along the pan, water starts evaporating and as the level drops, the float lets a bit more in on that end and the new sap pushes the old along the pan. As this continues, the density gradient develops. Not really a depth gradient. The fresh sap doesn't mix along the full length because the pan is shallow and narrow. There is nothing stirring and mixing them together along that long length. The sap that is farther from the float has been in there longer and gets denser. Keep cooking and eventually the far end, which hasn't seen hardly any of the new sap, gets to syrup, so you draw a bit off that end. This does about the same thing as the new sap, it drops the level a bit, so it's neighbors have to move towards it to keep everything level. That drops the level at the inlet so the float lets in a bit more sap.
If the heat is even and constant, and you set the valve perfectly to draw syrup at a slow and constant rate, your float will replace it with sap at the same rate, a continuous process. Perfection!
That never happens for me, any variation in the fire, even opening the door to load wood, disturbs the delicate balance and the process surges and get out of whack. So you have to watch and regulate the flow all the time. If the heat is very unevenly distributed, you can get syrup somewhere other than the draw off valve. Small pans can be more difficult than bigger ones.
If it was easy, everyone would do it. As it is, it's just us dopes.
Sorry to duplicate the description. I got distracted while typing this and the New Yorker beat me to it!
Last edited by RileySugarbush; 02-24-2017 at 09:43 AM.
Reason: Just saw Urban's Explanation
John
2x8 Smokylake drop flue with AOF/ AUF
180 taps on sacks
75 on 3/16 tubing with shurflo
Eden Prairie, Minnesota