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View Full Version : Air gap design to raised flue in 2x5 evaporator



tgormley358
03-26-2019, 08:24 AM
I’m having a good season in MA, Probably finishing up this week, with 125 taps, Hopefully 25+ gallons. Here’s an evaporator question. My Lapierre Jr 2x5 has run great, no complaints (with the exception of a
minor rust issue posted previously here, I’m getting good support from Lapierre). But I get a good hard boil Only in the middle section of the flue pan, and I’m wondering if relates to the arch and bricking design, because I’ve read others here saying different things about the gap between the top of the arch rails and the bottom of the raised flues (around .5” seems to be recommended). My arch has that gap of about .5”, but only for about 1’ of length, right in the middle where I get the hardest boil. From the fire box back, the fire box is around 24” long til the ramp starts which goes about 1’, then it flattens for about 1’ where the gap is .5”, but then the last 12-18” drops down 4-5” before it reaches the stack. It’s that 4-5” drop I’m wondering about, do others have that, and is it the cause of a slower boil that last 18” in the flue pan? Should the whole Pan boil hard like the middle? Would you add sand in that last 12-18” before the stack to make the gap a consistent .5” the whole way? I plan to call lapierre about it also since it’s their design and they obviously know what they’re doing.

n8hutch
03-26-2019, 09:00 AM
You are correct, you only need 3-4" at the very back of your flue pan to have that drop, technically you only need the equivalent of your stack volume so assuming that you have a 6" stack that would be roughly 29sqaure inches so a 3" opening would be enough, I'd probably do 4 to be sure you have good flow.

I started out with almost a foot of transition area at the back of my flue pan , but quickly closed it down to 6" after the first season, I have a 12" stack.

I have my arch insulation level with my rails and just rely on my 1" pan gasket to give me the desired gap.

tgormley358
03-26-2019, 10:31 AM
Ok good to know I’m on the right track. I’ll experiment with it the way you did to reduce that length. My stack is 8” (or 10? I have to double check but I think 8), so I’m wondering what that calculation is Nate, to get 29 sq” from a 6” stack? And how to go from 29 sqin to 3-4” before the stack? My stack also has the tapered starting portion about 12-18” wide at the bottom tapering to the 8” stack. Does that factor in?

Is the first calc pie-r-sq? For an 8” stack then roughly 50”. My arch is 24” wide. Thanks!

Tom

n8hutch
03-26-2019, 11:25 AM
Ok good to know I’m on the right track. I’ll experiment with it the way you did to reduce that length. My stack is 8” (or 10? I have to double check but I think 8), so I’m wondering what that calculation is Nate, to get 29 sq” from a 6” stack? And how to go from 29 sqin to 3-4” before the stack? My stack also has the tapered starting portion about 12-18” wide at the bottom tapering to the 8” stack. Does that factor in?

Is the first calc pie-r-sq? For an 8” stack then roughly 50”. My arch is 24” wide. Thanks!

Tom , yes 3.14x radius squared, so 8" would be about 50 so you would still be fine with 4"

maple flats
03-27-2019, 08:21 AM
i'm not sure. On my raised flue I do like Nate for clearance under the flues, but at the back end of my arch it drops down about 8" under the stack and makes the drop at about 3" before the end of the flues pan. Mine boils hard all the way back. On my 3x8 the stack is 12" and 17.5' tall counting the length added with the flip open top cap.

mellondome
03-27-2019, 12:55 PM
3 to 4 inches should be the sweet spot
For the 3x8... it is the same... as you are 12" wider. You may want to lessen the space under the flues. Air passing through that space is not helping with any boil. You want all that air passing through the flues. After having reconfigured my 2x6 many times, the best boil rate is with the bottom of the flues just touching the blanket underneath them.