Sweetening the Pans - Continuous Flow

[Big_Eddy]

The subject of Sweetening the Pans comes up regularly with new sugar makers looking to expand from a few taps and batching to more trees.

A continuous flow arch is usually filled at the beginning of the season, then left full through the season (except for long cold or dry spells and cleaning). The first boil is used to "sweeten" or build a gradient in the pans from raw sap at the inlet to finished syrup at the outlet.

For reference, I took some photos of my arch before and after sweetening that might help to illustrate the process. I usually run about 1" deep.


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Photo 1. Initial Fill
Arch initially filled with sap. Flue pan on right, syrup pan on left. Sap enters in the bottom right, moves to the front, crosses to the middle, back to the back, then to the far side and out the front of the flue pan. You can see the valve that allows me to isolate the syrup pan from the flue pan. Once it enters the syrup pan, it again follows a serpentine path before exiting at the front left.

Initial fill is 40litres or ~10 gals of sap.


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Photo 2 - Flue pan boiling
During boiling, there is violent mixing going in each section, so the concentration within each section is continually balanced. However, each section is being fed by the section before it, so as the sap moves through the evaporator, it gets more and more concentrated.


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Photo 3. Gradient established
I shut down after 300l or 80gals of sap and allowed it to cool to take some photos. You can see the gradient that has now been established. Bottom right is clear as that is where raw sap is entering. The front section is getting close to syrup. (took ~100 gals until the first draw)
The gradiant in the evaporator will stay like this through the season, unless I drain it to prevent freezing damage or for a long dry spell. At the end of every day, I close the valve between the flue and syrup pans to prevent mixing back, and open it up again when the pans start to boil.

At the end of the season when I am out of sap, I will close the isolation valve, drain the flue pan into a pail and add it to the syrup pan, then flood the flue pan with water to finish off the last batch.

[bigschuss]

Thanks Big Eddy. I am about to get started on my Mason 2x4 for the first time this year….just waiting for the sap to start flowing. I have been reading up on this. Can I ask a few follow up questions?

My 3 section pan does not have isolation valves between the sections. So, I understand the concept of sweetening the pan and going through roughly 100 gallons to get that done. At the end of my day, with no ability to keep the gradient in the 3 sections of my pan, my sap will all flow back to a uniform concentration. Any way to tackle this problem? Is there a way to re-establish the gradient the next morning? Or will the simple process of firing up the evaporator and introducing fresh sap do the trick?

Thanks in advance.

[TrentonMaple]

I have a question also - you mentioned freezing damage. I have about 10 gallons of sap in my evaporator now also. With the season being so crummy, I only get sap in small quantities in sputters - then it locks into deep freeze again. Am I ok leaving sweetened-but-not-yet-syrup in the pan for up to a week until I finally get more sap?

[Big_Eddy]

My 3 section pan does not have isolation valves between the sections. So, I understand the concept of sweetening the pan and going through roughly 100 gallons to get that done. At the end of my day, with no ability to keep the gradient in the 3 sections of my pan, my sap will all flow back to a uniform concentration.

At any given time, the sap is level in your pans. If you are running relatively shallow (1") in your pans and leave it at that depth when you shut down, there is nothing to make it "flow back". There will be some mixing at the margins, but unless you stir or mix it deliberately or pour in raw sap, your gradient should remain essentially in place. When you start back up, you want to get the boil going before you open the valves and start flooding in new sap. It may take a bit longer than usual to get your first draw as the gradient is reestablished, but should not be an issue.

If you do want to isolate the sections, something as simple as a chunk of maple plopped in front of the opening will prevent enough mixing to maintain the gradient.

[Big_Eddy]

I have a question also - you mentioned freezing damage. I have about 10 gallons of sap in my evaporator now also. With the season being so crummy, I only get sap in small quantities in sputters - then it locks into deep freeze again. Am I ok leaving sweetened-but-not-yet-syrup in the pan for up to a week until I finally get more sap?

Depends on a number of factors. The risk is that sap freezes and splits the pan. Flue pans are more at risk than flat pans. Some folks take a chance, others leave a light or heater in the arch or light a small fire every few days. I usually watch the forecast to see how hard I think it will freeze. If I think it will freeze solid, I siphon out into a stainless pail, then pour it back in before the next boil. Concentrated sap will slush before it freezes hard, so -5C or higher, I usually leave it in.

[PerryW]

Great description of the continuous flow process and gradients which is a difficult concept to grasp!

I leave sap in my pans all season except an occasional draining of the front pan for cleaning. I let my pans freeze solid as it is impractical to drain everything and bring it up to the house and keep it warm, but I do take the precaution of bringing the levels as low as I dare when I shut down before an anticipated freeze.

Of course, this year I still haven't even bothered to finish tapping or set up the evaporator yet as it's too darn cold.

[eustis22]

pic of my pan as of this morning after approx 100 gals boiled...

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I'm perplexed as to root cause...fire not hot enough? not localized enough? I dunno

[feathercast]

I have a question also. It looks like you don't reverse your flow in your pan. Do you have to clean it very often of sugar sand?

[Big_Eddy]

I have a question also. It looks like you don't reverse your flow in your pan. Do you have to clean it very often of sugar sand?

I can rotate my front pan 180 but I never have. I remove it once or twice a season and give it a light cleaning. Do a thorough job at the end of the year.

[Big_Eddy]

pic of my pan as of this morning after approx 100 gals boiled...

I'm perplexed as to root cause...fire not hot enough? not localized enough? I dunno

Tell us more about your setup and how you are running it.

How deep is the sap? Was the level constant?
How are you adding sap - float valve, steady drip, a gallon at a time? Where?
What is your evaporation rate - about 6 gallons / hr?
Was the fire steady, or did you take a break for lunch, let the fire die down etc.?
Foaming - was there any, are you using defoamer?


To establish a gradient, especially with a small number of sections, you need a steady process. Every time you allow the sap level to change in the pan, you are moving sap between sections and mixing. Sap finds its own level. If you are running 1 1/2" deep in your 2x3 pan, you have about 6 gallons in the pan. Add 2 gallons of raw sap, and you just pushed 1/2 the second section into the third, and most of the first into the second, mixing the syrup and messing with your gradient. Likewise, if you boil hard for a while and the sap level drops to 1" and you refill back to 1 1/2", you've done the same thing. Foaming in the last section does the opposite. The foaming causes the level in the last section to rise, pushing almost syrup back into the middle section and mixing it with the partially evaporated sap there.

Establishing a visible gradient in a small pan is not easy. The higher the evaporation rate and the more sections in the pans, the easier it is to see the gradient develop and the more distinct the difference in concentration will be between sections. Regardless, if you keep adding sap at one end only, you will develop a gradient. If you can - test the concentration in each of your sections - I expect they will be different even if you can't see it visually.

[michiganphil]

At the end of my day, with no ability to keep the gradient in the 3 sections of my pan, my sap will all flow back to a uniform concentration. Any way to tackle this problem? Is there a way to re-establish the gradient the next morning?


I don't have a valve between my back and front pans either. Here's what I do to keep my gradient.
When I get to the end of a boiling session, I draw off some sweet (what is in the last channel by the draw valve). I take maybe a gallon or so, and set it aside. Then I let the fire go out. The next time I light a fire, I let it all get up to boiling, then start feeding the raw sap. Once this is all going good (takes me about a half hour), I slowly add the 'sweet' back to the last syrup channel right before the draw off valve. This gets the gradient back where I was before, and a short time later I can start regular draws of syrup.

[eustis22]

How deep is the sap? Was the level constant?
About 2 inches is where I'm comfortable right now

How are you adding sap - float valve, steady drip, a gallon at a time? Where?
Steady drip from my preheater at one end containing near boiling sap

What is your evaporation rate - about 6 gallons / hr?
Between that and 8, i think

Was the fire steady, or did you take a break for lunch, let the fire die down etc.?
No, I fire steadily keeping it high until I stop

Foaming - was there any, are you using defoamer?
There is a bit in the front half of the middle channel, which I defoam...sometimes there's some in the syrup channel, which I also defoam

> It looks like you don't reverse your flow in your pan. Do you have to clean it very often of sugar sand?

I've only boiled twice on it so far and I read elseforum that a pan this small doesn't really need to be rotated.

[eustis22]

adding a pic of the re-worked innards I'm curious as to how the draw might be affected with the reworking. I'll see tomorrow...I need more grate and have to trim the heat shield some so it's more flush...I'm worried about that wall. Only refractory cement is holding it up and the refractory cement, which is allegedly good to 2000 degrees, was gone from a lot of my upper bricking.

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[bigschuss]

I don't have a valve between my back and front pans either. Here's what I do to keep my gradient.
When I get to the end of a boiling session, I draw off some sweet (what is in the last channel by the draw valve). I take maybe a gallon or so, and set it aside. Then I let the fire go out. The next time I light a fire, I let it all get up to boiling, then start feeding the raw sap. Once this is all going good (takes me about a half hour), I slowly add the 'sweet' back to the last syrup channel right before the draw off valve. This gets the gradient back where I was before, and a short time later I can start regular draws of syrup.

Thanks. Great tip.

[bigschuss]

At any given time, the sap is level in your pans. If you are running relatively shallow (1") in your pans and leave it at that depth when you shut down, there is nothing to make it "flow back". There will be some mixing at the margins, but unless you stir or mix it deliberately or pour in raw sap, your gradient should remain essentially in place. When you start back up, you want to get the boil going before you open the valves and start flooding in new sap. It may take a bit longer than usual to get your first draw as the gradient is reestablished, but should not be an issue.

If you do want to isolate the sections, something as simple as a chunk of maple plopped in front of the opening will prevent enough mixing to maintain the gradient.

Thanks Big Eddy.

[eustis22]

I'm happy to say the draw was fine, the back of the pan boiled, and I think I was getting approx 10 gph. Wall hasn't collapsed, so I'm happy. Still looking for the alleged gradient....will go check color in the channels after the rain later.

[superiorsyrup]

Big Eddy,

Thanks for the right up. Why is it that some people reverse the flow?

We are new to the "high tech" world we run 100 taps and boil around 800 gallons of sap per year. This year we made a new arch from a wood furnace. Our arch measures 22" x 93"1/2. I am making a two pan divided set up. Syrup pan is 22" x 24" and sap pan is 22" X 69.5". I am wondering what would be best for placement (direction) and amount of dividers in each pan. Also if reversing flow is a big deal I though that I can have two taps on pre heater so that I can draw from either of them along with having connections and valves on either side connecting the sap and syrup pans.

Thanks in advance
Brad

[Big_Eddy]

Big Eddy,

Why is it that some people reverse the flow?


Reversing the flow is done to reduce nitre build up on the pans. Most of the nitre precipitates out of the syrup as the last few Brix are reached. Therefore the last section of the syrup pan sees the most build up. By reversing the flow, the build up is spread over more sections (and there are some views that nitre will dissolve off the pans when exposed to lower density boiling sap) and there is less chance of scorching syrup and longer periods between cleaning. Larger evaporators have a larger problem, as there is correspondingly more syrup and therefore more nitre being produced in the same time period.

With my evaporator, I see almost no nitre build up in the flue pan over the season, but do get some in the last syrup section. After about 500 gallons through, I'm currently due to remove the syrup pan and give it a quick scrub. I'll give it a thorough cleaning at the end of the year. The last time I boiled I had 4x as much nitre as the earlier boils. Not sure why that is, but there are always some sap runs that do.


We are new to the "high tech" world we run 100 taps and boil around 800 gallons of sap per year. This year we made a new arch from a wood furnace. Our arch measures 22" x 93"1/2. I am making a two pan divided set up. Syrup pan is 22" x 24" and sap pan is 22" X 69.5". I am wondering what would be best for placement (direction) and amount of dividers in each pan.
Thanks in advance
Brad

Divider placement in a flue pan is always front to back to align with the flues. A flat pan (sap or syrup) can be front to back or side to side.

Syrup pan alignment varies by vendor and sugar-maker. More sections allows for smaller more frequent draws, but it also slows the flow in the entire system and can cause a "lag" in backfilling when boiling hard or drawing off. Side to Side placement can result in the middle (or back) section of the syrup pan boiling harder than the front and syrup being made in the middle section before the front. This makes for difficulties managing the draws. Front to back eliminates this with a 2 section pan, but can add it back with a 3 section pan if the arch is hotter in the middle than the sides. Larger Evaps often use multiple syrup pans instead of dividers, so in those cases, side to side is the only option.

The dividers in my syrup pan are side to side and I have 3 sections to allow for rotation (which I never do.) My arch boils slightly harder in the middle section than the front, but not enough to make syrup there. I like it that way as it makes the draws a little easier to manage.
I also have 3 sections in my flue pan. I'm not sure the extra section helps with gradient, but this way the coldest sap is entering at the back (coolest point in the flue pan) and not killing the boil, and after completing the serpentine, the piping to the syrup pan is short. If I had decided on 2 sections, I would have had to feed and exit at the front, or run a long pipe from the back of the flue pan to the syrup pan. It was easier to toss in an extra divider and it certainly doesn't hurt the gradient.

[Sugarmaker]

Gradient will not stay in position if you don't block off each partition. All the sap will mix in the pans when not boiling, like over night. When you start up it again will be like having high concentrate sap in the entire evaporator. Just means your first draws may be larger due to more extra sweet sap in the pans. Just watch it closer because it will make syrup faster than the raw sap.
Nice job on the start up thread Big Eddy.
Regards,
Chris

[eustis22]

will a slower boil affect the forming of the gradient?

[Big_Eddy]

As you boil, there is always some mixing that is happening within the pans. The slower the boil, the more mixing takes place between sections in the pan, so it will affect the gradient to some degree. Same with sap depth - deeper, more mixing. But what has much more effect is allowing the levels in the pans to fluctuate up and down. As the levels move up and down, sap / syrup from each section has to move into the next section or the previous section to maintain the same level throughout the pans. That movement back and forth causes a lot more mixing than happens during a stable process. So slower boiling will make a difference, but will not cause a problem.

In my evaporator, I used 3/4" connections between pans and to the float box. I can evaporate faster than my float box can keep up so when I am firing hard, my overall level will drop. If I take a break and go in for lunch, the float box will catch up, and push everything forward, diluting my syrup pan. There is still a gradient, each section is just a lot less concentrated than it was before the break. It always takes me an hour to an hour and a half after a break to re-establish the gradient to the point that I can start drawing regularly again.

At the end of the day, when there is still heat in the arch, my sap pan with much more surface area will continue to evaporate off longer than my syrup pan. If I don't close the valve between, the syrup pan will backflow into the sap pan until the levels are even again.

The reality is that it doesn't really matter. If sap is going in and steam is coming off, then eventually you will get syrup out. The "steeper" the gradient you can develop in the evaporator, the smaller and more frequent the draws will be. That's really all the difference it makes.

[Fort Wisers]

Sorry to revive such an old thread....
Just wanted to say thanks to Big Eddy for the initial write-up and to everyone else who contributed.
We just recently (and sort of unexpectedly) moved from batch boiling to continuous flow.
I'm sure you're all wondering why we're on continuous flow with only 6 taps, the story of where we started an how we got here is to long and sorted to post and clutter up this thread.

I can honestly say we knew there'd be a change in process but we were unprepared in how much of a change and what to expect.
Its funny, no matter how much you read and try to prepare, you never really know until you actually do it first hand......

This write-up has certainly helped to guide us, calm some nerves and give us a bit of re-assurance that we're on the right path (or at least half way on the path anyhow LOL).
Thanks folks!

[alexa.holleran]

I second Fort Wisers' comment; thanks to Big Eddy and others who've taken the time (years ago, now) to explain the sweetening calculations! They're super helpful to those of us new to continuous flow. I boiled about 160 gallons yesterday and was baffled (pun intended!) not to have syrup to draw off yet by nighttime...but with these explanations and GOwin's calculator form (http://mapletrader.com/community/sho...efore-draw-off), I can see it will take me about 250 gallons to sweeten, so I'm not crazy. It's a remarkably long time/large amount, though! Trying to stay patient...

[anchorhd]

I have the same unit. Bricked 1.25" from top of wall to the pan. Sometimes when I load a little to rough a I can knock a brick loose on top. I should use more cement. I just place it back until final cleaning.
I think you are running it too deep to see the gradiant. I run mine by looking in the windows by the draw off. I think that opening is 2". I run 1/2 the window. This year I am leaving the sweet in the pan when I am done. I works so much faster the next boil. I used to waste approximately 50 gallons to sweeten the pan. Now it starts sweet. I rotate the warming tank feed pipe every boil. If I clean it I mix 1/2 water ,1/2 white vinegar. A pan brush from bacoms.18074

[eustis22]

I notice that theres a LOT of sludge in my filter after drawing @218/219. Enough so that a good pint is still in the paper filter after setting for a day. Is it possible I am overcooking my sap? It's always been very dark (golden syrup is a myth to me) but it has been somewhat translucent in earlier years.