Cross-flow and Reversing

[twigbender]

Ok, I'll admit it: I'm just a simple, flat pan sort of guy. And I like it that way, thank you. But I do have a question for the more experienced of you out there, to wit: I understand with a cross-flow that the fresh sap comes into the pan at one corner, flows back and forth across the pan along the baffles and ends up at the opposite corner as concentrated sap or syrup.

Why doesn't this stuff mix with the new sap? Does the increasing density while it is flowing and boiling off water hold back the less concentrated sap? How do you get that process started so it isn't all pure sap in the pan at once?

What reason would you reverse the flow in the pan?

I know a whole bunch of you can answer those questions in your sleep, so have at it, please. And thank you for the good info!

[RileySugarbush]

I'll try,

Somebody let me know if I get it right or wrong!

What you are talking about is the gradient in concentration and therefore density. The Sap is dilute where it enters and concentrated to syrup on the far end. To get a good mental picture of what is going on, imagine your pan as stretched out in a long straight line from left to right, with even heat along the bottom. Starting with all raw sap on the left, it evaporates evenly along the length, but as you add sap to the inlet end to keep that level constant, there is a gradual and continuous flow towards the right. If you think about a little section of the sap in the middle, it wants to flow right since sap is being evaporated over there and the level is dropping. So it does, pushed by the more dilute sap to it's left. Since the sap way on the right is far from the dilute sap on the left, it continues to evaporate and concentrate, and is replenished with slightly less concentrated sap to it's left. It is a continuous gradient along the length as the water vapor escapes. If you had a perfect steady syrup draw off, you could have a continuous system with raw sap on one end and syrup coming out the other at one 40th (+/-) the rate, and water vapor escaping the whole length. There is little back mixing because the channel is shallow and narrow and there is nothing to force the flow back.

The baffled pans are effectively just a folded up version of this long trough so it fits on the arch.

Reversing flow allows the raw sap to re-dissolve the built up sugar sand from the previous concentrated end.

[twigbender]

Thanks John. That explains it well. Do you tap around Eden Prairie? When do you anticipate your season will begin? I'm about 5 hours north of you and we don't start until after St. Pat's Day at the earliest. I am considering going to Fillmore County (south of Rochester) to get a head start, but I'll probably have to get that set up this coming fall for next season. I understand that they are often done by the middle of March. Any experience down that way?

[RileySugarbush]

I tap on 5 acres around my house in Eden Prairie. It really varies around here. The trees are probably running today, it's 39° already! But there is another cold snap is on the way with -14 tomorrow night and no thaw in sight.

I don't have my records with me but late Feb/to early March is probably the best guess for a tap date for me in the Twin Cites, and often still going into April.

[Brent]

John

When I've tried to clean pans, ( flat pan on Leader Half Pint ) I never noticed that the sand re-dissolved, and when I reversed the flow of by feeding into difference corners, I also never saw the sand level change. Does it really dissolve and I was not observant enough.

I could see that more would dissipate out as it got more and more concentrated towards the draw off point, and that reversing the flow would change the corner of the pan that this happened, possibly extending the time between shutdowns and cooling.

I have a new 2x6 on order and the maker says in a small pan I will never see the
difference if I get reverse or cross flow. He says we'll get almost the same precipitation rate everywhere and that reversing in a small rig is not worth the time or expense.

[RileySugarbush]

Brent,

I'm only a year out of my steam table pans so I'm definitely not the right guy to comment on that. Just regurgitating what I have heard on the flow reversing. On my 2x6 I am using a half pint pan as my syrup pan, and with our relatively small production I doubt I would even need to think about rotating it.

[Father & Son]

Brent,
John was right about the build up not showing up in a small operation very fast but it will happen. I boiled my first three years on a 2 x 6 reverse flow Leader. I would not reverse the flow (which only involved moving a plug in the flue pan and turning the float pan around) every boil. The best way to tell if you're getting build up is by running your scoup/skimmer along the bottom of the channels of the syrup pan. If you check the channels from feed side to drawoff side you know when it is time to reverse just by the "feel" of the channel closest to the draw off. It will feel bumpy or rough. Reversing the flow will put a weaker concentration of sap through that channel and the buildup will dissolve. I never reversed flow in the middle of a boil, I just waited till the next day. I've been told the bigger operations will.

Jim

[WESTVIRGINIAMAPLER]

In my opinion, reversing never dissolves buildup or at least completely, but it may help loosen some of it and get it going in the opposite direction. What happens is sugar sand continues to build up in the last section or drawoff section the worst as the continual flow of sap pushes it over then and after a while, you have the bottom of the section completely covered and this can result in the section possibly scorching, smoking and/or giving the syrup some burnt taste, etc, you get the picture.

[super sappy]

Last year I did not switch drawoff side for several boils. I had all I could do to get everything up and running with a new broken leg. Well just like Brandon said I got a little scorching in the syrup pan and did not realize it until after I had drawn it off into the flat filter. It tainted the whole batch for the day with a burnt flavor.That mistake hit me in my scottish wallet. Lesson learned alwayse change drawoff side at least every day- Just my 2 cents-ss

[Sugarmaker]

Changing sides helps loosen and move the sugar sand to avoid scorching. the aggressive boiling and movement of the fresh sap seems to agitate and loosen the sugar sand. Also remember that sugarsand amounts can vary year to year and run to run. So if you dont have any your lucky some guys have a lot and must watch it more often.

Chris