Planning arch build

[Mrk59rsk]

I am in the planning stages for an arch build for next season. My flat pan ( no dividers) will be 18x34x6.
I am using the pyromatic plans found on this site, a few questions:
Since the arch is small, can I just have the rear firebox wall vertical as in the plan or is an angled ramp better?
Stack opening (6"dia.)out the back end ok or thru the top better?
Any and all tips, suggestions to keep my screw ups to a minimum are appreciated by my wallet and my better half!
The forum members are very generous with their time and knowledge, thanks in advance!

[mol1jb]

Ramp vs no ramp - With as small as you are building you should be fine. But to be the most efficient, I would build a ramp after your grates ended.

Stack out back or top - Either will work fine. With out the back and an elbow up you may need a bit higher stack.

If you are not planning a blower for this build currently I would make it so if you wanted to add one in the future it would be easy to do.

I looked up the pyromatic plans and if you follow those you should be pleased with the result.

[NhShaun]

Going through the back or top should both be fine. I would suggest a transition from a rectangle the width of your pans to your 6 " stove pipe. That way the flames will not be forced to the center where your stove pipe is, but allow the heat to travel evenly across the bottom of the pans. It might not really matter on a smaller arch like yours, but if you're building it from scratch, why not include that?

[Mrk59rsk]

Thanks for the input Mol and Shaun!

[NhShaun]

No problem. What are you planning for insulation or bricks in the arch ? I would recommend at least 1 inch of ceramic insulation lining the entire arch. Fire bricks in the Firebox are pretty much a given. Though not totally necessary to line the entire ramp or sides, but it does help hold the heat and makes things easier to clean. If you end up just bricking the Firebox and not the whole ramp, I'd suggest at least putting bricks upright at the very end of the ramp where the flames will go 90degrees into your stack. That corner tends to get blasted with heat.

[Mrk59rsk]

Shaun, early plans are to use 2x2x3/16 angle iron for framing the arch, wanted the 2" to accommodate splits ( 1.25" ) plus Roxul/Rockwool batts (1.5") behind the splits in the firebox. This may compress the batts a bit, but not to bad. Leaning toward a straight 90 degrees at the rear of the firebox up to 3" from the pan bottoms. The rest of the arch would be insulated with the Rockwool batts up to 3" from the pan, then parged over to protect the insulation and help with total arch weight. Still not sure of stack location, thru back or top, or how to design stack area after the pans. Thanks for the interest.

[maple flats]

At the back of the firebox area on those smaller arches it is more common to build a wall out of firebrick. Have the top of the wall close to the pan bottom and level from side to side. To calculate the space figure the cross sectional area of the stack in sq. inches, then divide that by the width. The result is what space you want above the wall. Behind the wall there is then open space to let the smoke drop down to exit the stack. Some fill that space with anything to just fill the void, from fire bricks to vermiculite to sand. The issue with sand is that it tries to push the wall forward. For a 34-36" long arch the firebox might be 20-22", then 4.5" wide firebricks, leaving 7.5- 9.5" behind the wall if you go out the back. If you go out the top with the curve of the original tank top as a base stack you will have more space to fill, but it will be a more efficient design.

[Mrk59rsk]

Thanks for the input Dave. The arch will be angle iron, nice and square hopefully. I do plan on the rear firebox wall being bricks up to approx. 3" from pan bottoms then maintain the 3" distance from pan bottoms the length of the pans, approx. 46". I don't know how to finish to the stack opening, ramp down to the bottom of opening if going thru back or maintain the 3" if coming up thru a top plate? (6" stack pipe)

[Super Sapper]

I would come out the top with the stack and keep 2 inches under your pan instead of 3. A 6 inch stack would only need about 1.6 inches to equal area. If you are thinking of a preheat pan just increase the length of the arch to allow you to set it behind you other pan instead of on top. You can always use a heavy sheet of steel in that spot if you are not going to have on the first year or so.

[Mrk59rsk]

SuperSapper, thanks for the input.I am leaning toward going thru the top as you suggest. As it so happens I will have one pan 18x34x6 and a full size steam pan I used this year on a block arch for the new build, as you suggest I will use the steam pan for boiling instead of preheating, seems more efficient use of heating surface.