Thoughts on my plans for a 2x4 arch?

[sprucegrouse]

7780


Attached are plans (side view) for a 2x4 arch I plan to build this off season. Any thoughts on the dimensions? I wanted a fairly long firebox to use 18-20" wood I have on hand. I am sure it will work, but figured I would seek input from people who have already built similar. Does the angle/depth of the arch part seem ok assuming lined with 1" archboard or half brick? Is 6" under the back of the pan too much? Thanks for any input

[TerryEspo]

I was told that if you have 8 inch pipe, that you want the ramp to have 2.5" clearance, not more as your diagram shows 7", you need to close that up a bit, too much heat loss going up the stack.

You can build as planned, but lay bricks to take up some space.

Here is mine, my arch from inside, I use 6" pipe and left about 2" clearance just before it goes out the pipe.7781
Hope that helps.

Terry

[Zamkev]

I agree with Terry. You can use brick, as mentioned and then take up more space with sand if needed. "they say" that you want the flames to "lick" the bottom of your pan - even at the back. I think 7" is too much space and the flames will shoot up the stack without kissing your pan effectively. I'd aim for 2" and I don't think you'll be disappointed.

[fishman]

I built similar but left 6" in case i ever go to a drop flue . I just have a few extra fire bricks loose on top of the ramp and that seems to keep the flame up.

[Big_Eddy]

If you're building you're own - I would suggest 18"x60" or 24"x72".

2x4 is too square. The advantage of the arch is that the flames pass under the pans and transfer heat along their way to the smoke stack. By making a 2x4 arch with a deep firebox, you're going to lose a larger portion of your heat up the stack. A 2x4 and a 2x6 use the same amount of wood to keep the fire hot, but the 2x6 is going to evaporate at least 50% more sap in the same time with the same fuel.
An 18"x60 will use 25% less wood and still evaporate the same or more sap than the 2x4.

Just my 2 cents worth.

[sprucegrouse]

Thanks for the advice. I actually already have the 2x4 flat pan so I'm pretty well moving forward with that size for now. I've actually contemplated building the arch to accomodate a 2x6 even with drop flues in the future...if I were to do that, how far do they typically drop down? I have about a limitless supply of wood so I'm not too concerned about efficiency for now.

Is there some thought process to calculating the area under the pans...like maybe make it equivalent to the cross section of the stack? That seems like it would be consistant with the 2.5" figure if the total width is closer to 22" due to the insulation on the sides, my math shows about 50sq in for an 8" stack so 2 1/4" x 22" wide would be about right?

Is that 18" from the grates to the pan way out of line?

[Big_Eddy]

If it were me and I had a 2x4 flat pan to start with - this is what I would do.

I'd build a basic arch to fit the 2x4, but with provision to extend it another 2' longer in the future.
I'd make the fire box 20" front to back, to fit 16" wood. I'd have 6" of space UNDER the grates and 20"-24" of space above the grates for a pan bottom height of ~26-30".
I'd ramp on about a 30 degree angle from a point about 10" above the grates and I'd ramp it up to the point where I have 6 1/2" of space below the pans AFTER I have bricks installed.

Now I have an arch that fits my current pan and is ready to extend to 6' with a flue pan at the rear.

When I bricked the arch, I would fill the level section below the back of the pan to about 3" below with either a second layer of brick (no mortar), or sand.

Next year I would build a 2x2 syrup pan and a 2x4 flue pan, and cut the back of the arch to extend the extra 2'. Take the extra layer of bricks out and the flues will drop down to 1/2" above the brick.

[PerryW]

what material are the cut-up canoe rack grates made of?

[sprucegrouse]

what material are the cut-up canoe rack grates made of?

Ha, I forgot that was on there. That was a note to myself to scrounge some heavy wall 1" square stock I had a canoe laying on. I figured if I made that into my grate and drilled a bunch of 1/4" holes in it...air under fire? That might be a separate discussion on engineering that whole deal.

Big_Eddy, I think I'm starting to lean toward your concept on build the arch to accommodate a 2x6 with drop flues, and just block it up for the 2x4 for now. I think I will only have 100 taps or so next season so I think I'll be fine on the 2x4 but it seems like more taps each year is a theme around here. Also...I really wish I had seen your thread on building a flue pan before just now. Guys like you make it look almost too easy...or at least tempting for someone with limited skills to at least try.

Revisions attached

[Big_Eddy]

Thinking about it a bit more - I'd probably build the arch 6' long anyway and would build a 2'x2'x12" preheat pan to fit in the last 2'. The preheat tank would use the last 2' of your arch and would allow you to preheat 5-10 gallons of sap with the "free heat" that would be going up the chimney anyway. I'd build it out of mild steel (20 gauge 4'x4' sheet <$40) if I planned to replace it later.

100 taps on a 2x4 flat pan is still going to take some long boils. With a blower - you'll probably get 10 gals/hr consistently so plan on 30-50 hours boiling a week. The preheater and extra 2 feet would drop that to 20-35 hours a week.