Can anyone explain a drop flue pan to a newbie?

[Andy VT]

I'm pretty new to all this and will probably be evaporating in steam pans or at best flat pans for a while yet, but there's something I don't get after googling about raised and dropped flue pans. In a drop flue, why doesn't the sap just sit in the flue and eventually turn to hard sugar? What makes sap flow into the flue and back out, since it appears (this might be my problem) that the bottom of the drop flues are the lowest point in the system? Is it just the bubbling of the boil that makes some sap jump out of the flue and other sap fall back into it, and thus keep it "rotated"? This question will probably get some chuckles out of pretty much everyone who understands exactly how these work :-D
I know, I should just walk into someplace that sells them and ask, but its bugging me right now and they're all closed
Andy

[DrTimPerkins]

Is it just the bubbling of the boil that makes some sap jump out of the flue and other sap fall back into it, and thus keep it "rotated"?

As you guessed, you've already answered your own question.

Note that you typically will run a drop flue pan with an inch or so of liquid ABOVE the top of the flues. So there is a continuous liquid connection between the liquid in the entire pan.

If you take a glass of water, and add more water to it...the water doesn't stay separated. It mixes. Toss in a lot of agitation from heating, and the liquid stays very well mixed.

[Andy VT]

Thanks, that was simpler than I thought and makes more sense now than I thought it would now that you've explained it that way!
Andy