fire box design.

[maplefool]

Okay, I've been reading this stuff for a couple years now, and I have whoat may be my first "dumb" question...n
I'm finishing up an arch I've been fabricating out of an old fuel oil barrel, but I'm not sure how to construct the portion from the back of the firebox to the chimney. I've seen both a gradual slope (30 degrees up and out) and a 90 degree straight up the back of the box, and flat back to the pipe under the pan.

I'm guessing there are advantages to both styles, but I would love some feedback.

I'm using one flat plan with a copper coil preheater and float valve.
Thanks for any info!

[Big_Eddy]

Many designs out there, and the manufacturers are likely experimenting to find what design is the very best for their arch and pan designs. It will vary depending on whether the evaporator is flat pan, drop flue or raised flue. It will also vary depending on arch length and width.

Considerations

Even heating through the pans. A steep wall and the heat must go up before it goes back. The front of the arch is hotter than the back. More ramp and the flames will follow the ramp and the front of the pan will be "cooler"
Extract as much heat as possible before the chimney. Too much ramp or too much gap and the flames will "race" towards the chimney and you are losing heat up the stack
Sufficient breathing room - you need enough space under the pan to allow for good air flow. Not enough space, not enough draft, not enough incoming oxygen for a hot fire. Minimum 1.5x to 2x the chimney area at the tightest spot.
Pan Length versus firebox depth. If you have a 20" deep firebox and a 6' long pan, you would want more ramp than if your pan is only 4' long.


For a 2x4 flat pan setup, not divided, my goal would be to extract as much heat as possible from my wood, and I would go with a steep back wall and a consistent gap under the rest of the pan (1.5x the area of the chimney). Slow the flames down, extract what you can at the front of the pan, and also along the pan length.
If you divide that 2x4 pan (across the width) and want a "syrup" section at the front, then I might ramp the top half of the wall at 30-45 to move the heat back on the pan and also have a slightly cooler syrup section at the front for a more controlled finishing process.
If your pan is 2x6 (divided or not), I would ramp the top half of the wall to get more of the heat under the back of the pan and I would increase to 2x the area of the chimney under to maintain the draft.
Certainly if you change to a flue pan, then there is much more opportunity to extract heat from the flame stream and I would ramp at 30-45 to get the heat to the flue pan AND leave me with a cooler front syrup pan. I would then leave almost no gap under the flues to drive the flames between them.

[maplefool]

Thanks for the info!
With a 2x4 flat pan, sounds like a steep back wall will work best.
I'm also excited to try some 1" arch board on the inside.