How does an evaporator work??? I know...I know...

[dennisn418]

So, I'm new to this hobby and currently only have a half dozen trees tapped. Making syrup on the stove top in a stock pot. But as I get into it I'm having a hard time understanding what the evaporator does differently. Rather than try to explain what I know I'd rather just ask for a quick diagram or description of the system and all of its parts.....Thanks for the learning!

[wobbletop]

They don't do anything differently. It's all the same... boiling to remove water.

The bigger units just do it faster and more efficiently. The other difference is they can do it in a continuous system so that you are always drawing off finished syrup, as opposed to a "batch" method where you are stopping after boiling the sap to make syrup, and then starting again with a pot of sap.

[RileySugarbush]

So, I'm new to this hobby and currently only have a half dozen trees tapped. Making syrup on the stove top in a stock pot. But as I get into it I'm having a hard time understanding what the evaporator does differently. Rather than try to explain what I know I'd rather just ask for a quick diagram or description of the system and all of its parts.....Thanks for the learning!

It is just a big pot on a fire.

Embellishments include:


Make the pot flat and wide and keep the sap shallow so there is a big surface area
Split the pot into two parts and further increase the surface area of one a lot by adding slots in the bottom that fill with sap ( that is the flue pan)
Add some dividers to keep the sap flowing the way you want
Add a float valve to maintain the sap level automatically
Make the fire very hot

I'm sure more complete descriptions will follow, but that really is the gist of it.

[dennisn418]

What makes the syrup go from pan to pan? Or does it zig zag through each compartment?

[lords sugaring]

Here's a run down of how my evaporator works as do all double pan setups. Raw sap travels from the head tank into the first float box. The float box controls how much sap is in the flue pan. From the float box it travles into the flue pan via a stainless tube. The pan is divided and there is an opening in the rear to allow sap to be pushed into the other channel. From there it flows into the syrup side float box via another stainless tube. From there it goes into its first divider and moves to each divider via a opening in the divider. Once it's reaches the final divider in the front channel it moves into the draw off section where finished syrup is drawn from and the process continually happens over and over until all raw sap is processed for the day. Before running out of sap entirely I'll stopoading wood into the arch and let the pan finish off all sap so it sits where I started from and can pick back up where I left off on my next sap run. Hope this helps

[MISugarDaddy]

What makes the syrup go from pan to pan? Or does it zig zag through each compartment?
Evaporation and drawing off finished syrup from the syrup pan causes the movement of the unfinished syrup. In your original post, you said you were "making syrup on the stove top in a stock pot"...hopefully you are doing this outside and not in your significant other's kitchen, otherwise you are likely to have a very sticky mess when you are finished.
Gary

[DrTimPerkins]

Think of an evaporator as a long stainless steel gutter with continuous heat along the bottom. You pour a relatively constant stream of sap into one end. As the liquid progresses along the path in the gutter away from that end, more and more water evaporates so that the density of the solution increases. Eventually, at the other end from where the sap is entering, a much smaller amount of syrup runs out. The water was lost through evaporation, the sugar (with some water as well) keeps moving along.

[maple flats]

In essence an evaporator with multiple channels is just really one big long pan, the channels just keep the sap moving in one direction as evaporation takes some out and as new sap is added at the inlet end. It just goes back and forth because it's far more efficient to have a more compact pan to boil on. Lets for example take mine. The first pan has 2 channels and being a flue pan each of those channels has several sections surrounded by the sap and under the pan the flues are in the intense heat of the fire. In my case the flues go up and then down 10". This gives far more surface area to get better heat exchange. After the sap moves from the inlet thru the 2 channels (and 6 flues each) it flows thru a SS pipe which then feeds a second float box. From there the sap flows thru 4 channels with flat bottoms, each 9" wide and 3' long until it finally reaches syrup density thru the continuous evaporation as it moves along. At the end the syrup flows into a draw off box, where the syrup is drawn off. On my 3x8, the path in the evaporator was 10' in the flue pan, then 12' in the syrup pan, or 22' it has traveled. Additionally it traveled thru my pre-heater in the hood over the flue pan thru a series of 1/2" pipes in parallel about 54" one way, then up and back 54" the other way, taking heat from the steam. Then it flows to the inlet box to feed the flue pan, in a SS tube partially in the boiling sap in the flue pan, it goes in about 9", turns 90 degrees and then flows to the front of the flue pan. Having hit the inlet float box at 160-170F, then flowing thru the pipe partially submerged in boiling sap and the upper part of that tube in in the steam being created, it travels another 54" to the front of the first channel of the flue pan and is either at boiling temp or very near it. The flues add lots more surface area to exchange the heat. The boiling rate is related to the surface area, but that is not the surface area of the sap, but rather the surface area between the heat source and the sap.
On mine and likely all bigger evaporators we can reverse the sap flow. I do mine by simply closing one supply valve, move the float from that float box to the other float box and then open another valve. In that process the syrup pan float box became the draw off box and the previous draw off box became the inlet float box. We do this to help keep niter (sugarsand) in check. reversing the flow makes it so you can boil longer before a pan needs to be cleaned. On my current flue pan, sap always flows in the same direction, it is not reversible, my previous flue pan also was reversible simply by moving on plug in the inlet float box from on tube to another and then moving the float from one side to the other in the syrup pan, and closing one valve and opening another.
Using innovative channel configurations, some newer evaporators can be switched to reverse the flow without even moving a float. If you get a drop flue design, there is only 1 float box controlling the depth in the entire evaporator, with a raised flue design you have a separate float to control each, the flue pan and the syrup pan. Remember, the syrup pan has a flat bottom and the flue pan has flues that can be anywhere from 4-5" deep up to 16" deep, which add more heat exchange surface for a faster boil.

[dennisn418]

Ok. Starting to make sense now.

How do you control the flow rate through the pans? As the density changes the flow would change....is this just accomplished by angles and experience?

Similarly, how does the sap go from the Flue Pan to the next pans? Are you manually opening up a valve?

The staging of all of this is where I get hung up.

[lords sugaring]

The float boxes controls the flow sort of. They both can be adjusted on how much sap enters the pans and will automatically shut the plunger type valve off once at the proper level. Think of it like the back of your toilet, there's a float and a valve inside to automatically shut the water off once it reaches the proper level, same principals in the float box. Each float box does have ball valves so you can block off all sap entering and exiting into the flue pan. I close both after cool down to stop more sap from entering from head tank and stop sap entering my sweetened front pan. There's really no way to control how fast it flows, every unit is different and evaporates more or less depending on size.

[RileySugarbush]

The whole gradient and flow question is tough to understand. Here is a simplified way of thinking of it. Kind of simplified......

First of all, we will talk about a 2x6 drop flue system, since it only has one float box and one fluid level.

The thing to keep in mind is despite the "gradient" term, the pans are all level with the sap/syrup essentially the same depth throughout. Not counting the deep slots that for some reason we call flues.

Here we go....

For our explanation, lets take the zig zags out and straighten out the dividers into one big long and skinny pan. The flue pan may have one or two dividers. If it is four feet long and has one divider, the sap travels 8 feet and we will mentally straighten that out to a 1x8 foot pan, with raw sap coming in through a float box at one end and exiting through a pipe into the syrup pan on the other.

The syrup pan has a flat bottom with no flues, since the thick syrup could easily burn in a flue. That pan is 2' x 2'. Lets give it three dividers and straighten out the four channels into a 6"x 8' channel.

So now we have a really long, 16' flat pan. The first 8 feet have slots full of sap in the bottom for good heat transfer and is 12 inches wide. The last 8' is only half a foot wide and has a flat bottom.

Open up the fill line and set the float valve to 1". If you have no heat, the fill valve will stop the flow when the whole thing gets to the setting, 1" deep ( except the deep flues). Nothing happens. Just a big long pan of sap.

Ah, but now the fire comes in! Light a big fire under the whole thing. ( this isn't quite right since the fire is on one end and the hot exhaust gasses heat up the rest, but good for this discussion). Eventually the whole thing starts to boil and the level starts to slowly go down. The sap on the flat pan end goes down, the sap in middle goes down, and most importantly, the sap at the float valve goes down. The float valve does what it is made to do and opens a bit and lets in more sap so the level there goes up. If the level goes up at the inlet, it has to go up everywhere, because thats what fluids do. So some of the new sap slides forward, pushing the older sap in front of it until the level goes up. The flow is only one way and doesn't slop around and mix much because the depth is shallow and the channels are fairly narrow. Repeat that mental exercise a few times and you can see that the area around the float valve has 1" of fresh raw sap around it, and the sap near the other end has been boiling a while and is getting more dense, but is about 1" deep too.

Keep it up, longer than you you think it should take, and the end far from the inlet is almost syrup. The pans are now "sweetened", and there is a density and sugar content gradient from one end to the other.

If you are paying attention, you let a little of the syrup out. That drops the level there a little bit, so that drops the level all over because it's a fluid, remember? So more sap comes in and the level returns, pushing everything forward in the process. If you draw off syrup smoothly and slowly, and the fire is even, you can trickle the syrup out at a steady rate and let sap in at about 30 times that rate. In between, the difference between the slow rate out and the fast rate in, is going up into the air as water vapor, all along the pans.

But if you take out big gulps of syrup, and let the fire die down and then throw in a big pile of pine to "catch up", that smooth continuous flow becomes an sloshing, oscillating mess and bad things happen.

So smooth operation. Even firing. Steady, steady. All is good. You got it! Until the neighbor stops by and starts asking questions, you forget to draw off, and you burn the syrup in the font pan. We have all done it, or will soon!


Back to our pans. Fold them back up into a channelled 2x2 and 2x4. Connect with a pipe. Set on an arch with hot fire under the front pan channel the hot gasses under the flue pan.

And there you go!

[Run Forest Run!]

What a great explanation. I could almost smell the syrup and steam. Unfortunately it makes me want to upgrade from steam pans now. :cool:

[brocken]

Thank you so much for the description, it was kind of what I was thinking but cleared up a little of the haze.

[RileySugarbush]

I remember trying to figure out what the flue pans looked like, back when I was using steam pans. The pans in the pictures were so reflective I couldn't figure out what was going on! I finally went out the Minnesota Landscape Arboretum. Nobody was around but the sugarhouse was open and and pans cleaned and upside down on the arch. What a revelation! Before that I was so confused by term "flue pan"

[Haynes Forest Products]

Hey some day we can have an in depth discussion about the different meaning of GRADIENT and how it applies to world peace.

[Mikemartin274]

Just like the pan in the stove that you boil your fruit loops in.

[Big_Eddy]

Here's the picture I usually show to explain the process.17840

[DrTimPerkins]

Or...from the North American Maple Syrup Producers Manual...

17841

[DrTimPerkins]

Hey some day we can have an in depth discussion about the different meaning of GRADIENT and how it applies to world peace.

Uh oh....everybody just keep quiet and slowly back away. :o