Arch inside design

[Grabouss]

Not considering flue location, what is the main design pros and cons to each of these designs? We see some that have a wall/baffle in the back part of the arch to direct heat up towards the pan and then goes back to 9-10” deep (green on drawing). The wall/baffle usually goes up to 2-3” from the bottom of the pan. We also see some that the whole back part of the pan is shallow at 2-5” from the bottom of the pan (red on drawing). Considering that I run a flat bottom 2x3 pan, what are the pros and cons to both designs?

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Thanks in advance!

[ecolbeck]

From what I have seen (you probably have noticed this too), there is a direct connection between flue location and ramp design. Rear flue exit designs (e.g. WF Mason and Leader 1/2 Pint) utilize the wall/baffle, and top flue exit designs use the shallow ramp. Shallow ramps seem to be used on higher end hobby units (and obviously all professional rigs). The prevalence of the top exit and shallow ramp design suggests it may work better. My guess is that it keeps heat closer to the pan for a longer period of time.

[wmick]

Id agree with ecolbeck...
The hot gasses need to make contact with the pan, in order to transfer heat. I'm no expert, by any means, but this is my theory..

Seems to me that the ramp design is maybe trying to accomplish the following... At the beginning, lots of space so the velocity of the gasses can move slowly, tumble around, giving lots of time to hang around and transfer heat... Ramping it up on the way out, speeds up the velocity, but also "forces" all the gases to come in close contact with the pan before exiting... (trying to transfer as much heat as possible before it hit the chimney...

Re: The wall/baffle design.... Seems to me that it allows too much space for the gasses to make their way to the chimney without touching the pan.

[tcross]

i have a new 2.5x8 arch from Thor and mine has two ramps/baffles that go up to about 1" from the flues after the firebox ends. i asked them if i should fill in between them and they said not to. one roughly 6-8" where the fire box ends and another about 1' away from the chimney... (aprox locations). they are slopped on the fire box side. the arch i had before that (which i made) was a 4'wide by a 4' long. that, i built up there entire back section to about 2" under the flat pan (only had flat pans), and that rig boiled very well also! ......... for what it's worth.

[maple flats]

A little more info please. Can you show some pictures or at least a description of the set up as you have it now. If the fire is under the entire flat pan I would think a wall near the back of the pan, up to about 2-2.5" below the pan then back down to exit the exhaust. If the fire is all near the front, then run a ramp, starting maybe 10-12" below the pan, immediately behind the firebox area, then ramp up to near the pan bottom at the back. The space near the back should be slightly more than the area inside the stack pipe. Area is Pi r2, (Pi x r x r) or 3.14x diameter/2 squared.
For example, if the stack is 8", 1/2 is 4, x4= 16 x 3.14=50.24 sq in., if your arch is 24' wide, leave the space at 2.25 to 2.5" max.
The ramp keeps forcing the heat up to the pan bottom.
If you do like the green sketch and the space behind the wall is more than a couple of inches you will get a very good boil directly over the wall, but little if any behind the wall. How deep is the firebox , front to back and does your stack exit out the top or the back?

[wobbletop]

Not much to add, but the green version could be modified to approximate the red version by adding a bunch of insulation behind the wall and under the pans.

[Sugarmaker]

I tend to agree with maple flats. You need some heat and fire under that rear pan. Keep the area under the pan similar in size to your stack should work. I think the area under my flue pan and between the flues is slightly more than the round stack size of 12 inches
Regards,
Chris

[Grabouss]

Thanks folks. My initial and current plan is the red sketch. Both designs are definitely to direct more heat to the bottom of the pan but I just fail to see a benefit of the baffle other than allowing a bit more room for the flue. I can't think of any performance advantage. As maple flats said, I think there would be a hot spot above the baffle and reduced boil behind, just my theory.

Maple flats, my current setup is a propane burner I built last year to fit the pan. This year I’m building a new arch out of an oil tank. On the front, it will have my 2x3 pan, behind that will be a “step up” about 1” taller than the pan going back another 12” or so. This “step up” will be closed up on top where I can set a pre heater, kettle etc. The flue will come out of the back. I might not get the best results with the flue in the back vs the top but I want to have that closed flat spot for a kettle (bribe the kids with hot chocolate on demand, something along those lines) and pre heater. It will be insulated with 1” of rock wool rigid board and bricked in the firebox and everything behind the firebox will have 2” rock wool insulation. I will have an ash box on the bottom. The ash door will double as a draft or blower port. In general that’s the plan, skipping a bunch of technical details. I’m just trying confirm before I start building it, that what I have in mind will be as good as possible and not cause any issues that would have to be tore up and rebuilt.

Thanks again!