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Some background info on what an evaporator is: It is a system of metal pans set over a heat source. Sap constantly enters the first pan controlled by a float valve to keep a constant depth. The pans have channels which are open to each other at alternating ends, so that the sap flows back and forth in a serpentine pattern. At the end of the pan system there is a manual valve where you periodically draw off finished syrup. From the beginning of the pan system to the end there is a ever-increasing concentration of sugar.

My question is, how does the sap get more concentrated towards the end of the pan? Or conversely, how is it that the sap doesn't maintain a homogenous sugar concentration throughout? (If you let the fire go out, once the sap cools it will redistribute itself until it is homogenous.)

I think somebody will probably tell me it has to do with the distance the sap has flowed through the pan by the time it gets to the other end (it would be a linear distance of about 24 feet in my system) but I don't think that answers the question because when you first start a batch, you flood the whole system with cold, fresh sap, all of uniform sugar content, and then you start the fire. It doesn't take long before there is a visible color difference between each channel of the final pan, with the darkest one being next to the draw-off valve.

And no, it's not flow caused by the pans being angled downwards either (a BS answer posted on yahoo!answers to a similar question). The pans are perfectly level. And as far as I can tell the heat intensity from the fire below is uniform at least for the final four channels, which are where you really begin to see visible color graduation as it becomes syrup.

here is a quick drawing of the setup that may help with visualization

There is one detail that I didn't describe at first because I couldn't figure out how to without a drawing, so now that I've added the drawing I'll elaborate on that. The first pan, where the sap goes in, has flues, which are deep, narrow vertical pockets in the bottom of the pan, which reach all the way down into the stove itself. The flames actually pass between the flues on their way to the stovepipe. This (obviously) greatly increases the surface area being heated. I don't know if this changes anything but I thought I should add it.

evaporator (cold)

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Could you break this into more readable paragraphs, please? Also, if you can find some photos of the set up, that would be helpful in visualizing the apparatus. –  Mark Eichenlaub Apr 8 '11 at 6:35
Welcome to physics.SE! I see your linebreaks in the source, you have to leave one line empty to start a new paragraph. Also, to improve readability you can emphasize the core of your question by using the quote symbol > for that lines –  Tobias Kienzler Apr 8 '11 at 7:42
Here is a picture link from a previous question on foam at maply syrup boiling. sites.google.com/site/lindsayssugarbush/_/rsrc/1240515239201/… –  Georg Apr 8 '11 at 9:32
really nice picture! –  lurscher Jun 14 '11 at 20:51

4 Answers 4

What's going on is that there are two flows here, a gas flow (produced by boiling) and a liquid flow. The liquid flow is from the entrance to the exit while the gas flow is to the air. The gas flow is controlled by allowing only one exit for the gas, near the place where you add syrup.

The longer the fluid spends in the contraption the more water is boiled out of it and the more sugar that is left. So by the time the liquid gets to the output spigot it's spent a lot of time boiling and has lost a lot of water.

The process is similar to distillation in that the steam preferentially carries off water rather than sugar. This suggests that a more efficient method of processing maple syrup is distillation, and that designing an evaporator so that it mimics the distillation process as much as possible is a good idea. The US Forest Service agrees:

Processing Maple Syrup with a Vapor Compression Distiller: An Economic Analysis Lawrence D. Garrett, Forest Service Research Paper NE-385, 1977
Forest Service, U.S. Department of Agriculture,
Northeastern Forest Experiment Station
6816 Market Street, Upper Darby, PA 19082

A test of vapor compression distillers for processing maple syrup revelaed that (1) vapor compression equipment tested evaporated 1 pound of water with 0.047 pounds of steam equivalent (electrical energy); open-pan evaporators of similar capacity required 1.5 pounds of steam equivalent (oil energy) to produce 1 pound of water; (2) vapor compression evaporators produced a syrup of equal quality to that from a conventional open-pan evaporation plant; and (3) a central plant producing 8,000 gallons of syrup per year should yield a return of 16 percent on investment. Increasing annual product output should increase the return on investment.

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""The second reason for running the apparatus this way is that the fewer of the volatiles will be sent out with the gas."" This is nonsense. ""The pans are organized in a manner similar to a distillation apparatus."" I'd like to see such "distillation apparatus. In general: rechurning -1 –  Georg Apr 11 '11 at 10:08
@Georg; I know a lot more about distillation than you think. I was the VP of engineering at Liquafaction corporation. My paper on black holes: arxiv.org/abs/0907.0660 lists my address as 720 Road N NE. This is, in fact, an ethanol distillation plant. See page 12 of: newprojectnews.com/jan08.pdf for the announcement that the same address was permitted for ethanol distillation production. –  Carl Brannen Jun 12 '11 at 4:48
Here's a link to the processing of maple syrup referred to as distillation: "The exact origins of the discovery of maple sap and its distillation, or processing, into syrup is impossible to determine with any firm historical accuracy." associatedcontent.com/article/2718987/… Another reference: massmaple.org/history.php –  Carl Brannen Jun 13 '11 at 4:02
An ethanol distillery typically has two products, ethanol and distiller's grains. The equivalent products for a maple syrup distillation is water and maple syrup. The portion of a distillation tower who's action is equivalent to a maple syrup processor is the "stripper". Instead of "bottoms" coming out of the bottom, you have maple syrup. But in both cases, the gas coming out of the top is largely water. –  Carl Brannen Jun 13 '11 at 19:00
The gas escapes along the entire surface area. Most large scale sugaring operations use reverse osmosis on the front end of the process - then they feed the concentrated sap into the evaporator. –  Tim Jun 29 '11 at 21:08

I can't vouch for the answer here, but it purports to be the reason.

The evaporator is perfectly level. If that's the case, visitors often wonder how it is that sap moves through the pan. Forgotten is the fact that rising steam is the equivalent of dipping or draining liquid from the pan. A level bottom means that sap moves under the pull of gravity as water leaves the pan as steam. (That, of course, is the whole objective of the process!) The sugar stays behind forming a syrup of increasing sweetness to make the finished product. At that point, the sugar concentration is at 60% and the boiling point is elevated about 7 degrees. The difference in viscosity coupled with the baffles in the pan allows finished syrup to accumulate in the last section without our ever having to mechanically separate the final product. At the end of the season or if, for some reason, we wish to shut down the process and empty the pans, it is possible to draw off virtually all the syrup. One could not, of course, simply drain the pan. The fire would cause melt-down in a matter of seconds. Instead, soft water from the farm pond takes the place of the entering raw sap and supplies the necessary cooling liquid. Boiling continues, and because of their different viscosities, the syrup and pond water do not mix significantly. This same situation occurs when oils of different densities flow sequentially in a transcontinental pipeline. Very little mixing occurs at the interface between the two products. Virtually all the syrup can be drawn off before the pond water arrives at the discharge valve.

I suppose the big question is why doesn't the liquid become homogeneous? That's probably due to the baffles limiting the interface significantly - reducing the the ability of the most concentrated fluid to mix with the most diluted.

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""The evaporator is perfectly level"" Maybe, but the surface of the liquid isn't! When You watch the Missisipy down in Loisiana it looks quite level, but nevertheless there is some inclination of the surface towards the mouth! With the syrup gaining density by evaporation things are more complicated –  Georg Jun 30 '11 at 9:53

In answer to how does your evaporator work: in the pan there is a frontal boundary of sugar concentrations, the long narrow compartments minimize this frontal boundary. The effectiveness of the evaporator is determined by the travel speed through the compartments versus the migration of sugar as it attempts to dissolve into the solution, thus segregating the fresh incoming sap from that boiled for a period of time allowing us to draw syrup continuously as sap is added continuously. Keeping steady feed, steady heat, minimal foam, and steady draw minimizes the intermixing that might occur. The density or the viscosity of the product has absolutely no bearing on this process.

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I have been boiling sap all weekend and have been wondering the exact same thing. When you are not drawing off syrup, and only adding sap to one end of the pan, why is the concentration of sugar more at the end (where the syrup tap is) and the lowest where you drip the sap in?

Okay - visualize the zig zag pattern of the pan as a long trough that goes left to right. You dump sap in the left side only. You are not drawing out of the right side yet. Equivalent boiling is occurring over the entire surface. Although there is no liquid coming out of the pan, the net flow of sap is from left to right because you are adding it to the right side, and it is being lost to the air at an even rate over the whole thing. The speed of the water flow is the fastest on the left side, and the slowest on the right side. As the water moves to the right, it also goes up, into the air as steam. As the sugar in the water moves to the right is does not go up, it only stays in the pan. The water/sugar on the left is moving faster, so it "leaves less sugar behind" and the water on the right moves slower, so "more sugar is left behind" leaving the sugar concentration on the right (or the end of the pan) greater than the concentration on the left (the beginning of the pan).

I will illustrate an analogy: Imagine a 1000 meter running race. No imagine that the runners (the sap) have pockets full of sand with a holes in their pockets. As soon as they start running, the sand starts running out of the holes in their pockets. They are running really fast at first and the amount of sand on the ground represents the sugar content in the liquid in the evaporator pan. They are getting tired half way through the race, thus leaving more sand per square inch on the ground. At the end, they are pooped, and moving really slowly. The sand is still spilling out onto the ground at the same rate (equal boiling rate over the whole pan). The speed of each runner approaches zero as the near the end of the race. The race track is covered with sand at the end. More runners keep starting the race at you dribble sap in and they all run the same way - fast in the beginning, slow at the end. Sand spills out at the same rate throughout the race. Sprinkles of sand in the beginning, piles at the end.

I'm pretty sure this is whats going on here. Osmosis is what describes why it becomes a uniform concentration when you stop dripping sap in and let it boil without taking any syrup off.

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