Drop Flue Vs. Raised Flue

[EriobNY]

I'm Purchasing a 2X6 evaporator for 2024 season and am looking for some clarification/reasons to select a drop or raised flue pan. I'm thinking that an engineering or operation reason exist - some measurement of efficiency?. After some time I've come up with the following explanations;

Raised pan thoughts:
1. For larger evaporators the raised pan creates a pressure difference between the pans and allows for a faster/more smooth flow to the front pan when drawing off.
1a. If this is true, what would be the cut off on evaporator size that makes the raised flue worth the extra cost?

2. The back pan can be run at a lower sap level, increasing boil the efficiency, while the front pan can be run higher level for safety allowing for more frequent smaller draws which reduces large temperature changes in the front pan.

3. Much less chance of damaging the flues that don't extend below the base while cleaning.

What research has been done on this at UVM and Cornell?

The difference in price is about +$600 -$700 for the raised flue pan.

Thoughts?

[Father & Son]

I have boiled on both and you are correct, the both have advantages. Here we go with the Ford/Chevy debate. But with that being said after boiling on both, I personally prefer the raised flue. Evaporator size or cut off isn’t really and issue. The added cost is the second float box. The second float box lets the rig respond quicker to the draw off and keeping the pans at the proper depth. A good friend boils on a 3 x 10 drop flue and from his one float on the rear pan there is 30 ft of channel from where the sap enters the flue pan to the draw off on the syrup pan. Big draws cause concern in keeping levels deep enough for him. Now let’s hear from a drop flue guy.

[Ed R]

I've only boiled on drop flue evaporator. I'm sure I would love boiling on a raised flue if I had a chance. For me using wood, you really have to be careful about your wood length in relation to the flues. It really comes into play when you are using pallet scrap type wood that is longer than standard chainsaw cut wood.

[Brian]

I run a 4x12 drop flue and run an inch deep at around 20% concentrate and have a 4ft front pan with an 8ft drop flue pan made by Thor and an arch that I designed and built. I make about 40 gallons the first hour and by the sec hour I am up to around 60 gallons an hour. The only thing I don't like about the raised flue is how much liquid it takes to fill the flue pan and how much sits between boils. I believe the faster you process the sap the lighter it is. My flue pan and front pan probably hold a 100 gallons of liquid. The whole thing is in a full boil in 3-5 minutes and for that reason I went with a drop flue style pans. Ps my wife is the co pilot!!

[johnallin]

I have a Leader 2x6 with Patriot raise flue pan. Purchased in ‘09 after running a Half Pint for 2 years.
I can only say that with the 2 float boxes, it’s easy to control levels is both pans. As mentioned, when a big draw comes on, it’s nice to have the head pressure to feed the syrup pan. At the end of the boil I do make sure to close the butterfly valve separating the flue and syrup pans…forgot more than once early on and came back to a flooded syrup pan!
Also firing is without concern of damaging flues. My vote would be raised flue, I think you’ll be better off.
Good luck with your search.

[darkmachine]

I'm looking at purchasing a new set of pans for my 2x6 as well, thinking of converting to raised flue. This is the first season i had what i'll call "flow back". It's caused all my syrup to be darker this year. I have an older model smoky lake hybrid(modified with 2 baffles in the flue portion), mating to a 2x2 flat syrup pan. This effectively gives me a 2x3 syrup pan and a 2x3 flue pan. I think because of the addition of the baffles and the inlet in the float box only being 3/4" the flow just can't be enough to keep up during a vigorous boil, and I end up pulling almost syrup back into the flue pan. It can happen at any time during the boil, but most often when the head pressure is low toward to end of the boil, so i have had to adapt and ease back the fire when my head tank starts to get low. Still great tasting syrup, but it's a little darker than i would like. Also I can't leave just anyone at the shack to feed the evaporator. honestly i don't think it was an issue until i started to boil concentrate instead of straight sap. and after typing all that, I've pretty well talked myself into a raised flue...lol. than again if i didn't add the baffles, and another 2 foot of syrup pan, maybe the 3/4" inlet would work just fine?

.http://mapletrader.com/community/attachment.php?attachmentid=22974&stc=1

[maple flats]

I also have boiled on both, I far preferred the raised flue, better control of sap level.

[Pdiamond]

I am with the raised flue crowd. I like the fact of having the two float boxes and sight glasses for each. I run my flue pan 1 1/4" over the flues and the syrup pan at 2". I like the trickle effect when syrup begins to come off the pan usually in a long continuous manner. With the auto draw all you have to do is watch that the temperature doesn't climb to high. Then you may have to open the valve a little more. The nice thing since I have had my raised flue pan is I have only had 1 year that I had to clean the flue pan before we were finished for the year.

[maple flats]

I ran my level at 1/2" over the flues and 1-1.25" in the syrup pan

[BAP]

I’ve boiled on both. I definitely prefer raised flue over drop flue. 2 float boxes, ability to keep the sap level in flue pan lower so it boils faster, and easier to drain and clean are the features I like.

[Pdiamond]

Dave, I am still just not comfortable going down that far in the pans. To me that's just to close to a danger zone.