Will this work ? Chimney box idea

[TerryEspo]

Hi All:

As I am building my new 2 x 4 arch, I have a crazy idea.

In the pics the cardboard box represents the steel box, the coffee can represents my 7" chimney. The block of wood represents a pan of sap that is heated from the top of the "box"

I am thinking of a raised "box" at the rear of my arch. The box will start after the back of the pan. Instead of a flat top arch, I want this "box" so a (pan sitting on top of "box") to be above my 7" pan, I can then gravity feed from my preheater into the main pan.

I am thinking that once the heat leaves the pan area, I may as well use it. I will also insulate the sides of the "box" as to have max heat on top.

Will this affect my chimney performance?
Am I doing something I should not be doing?

Please feel free to give your opinion.

Thanks
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[maple flats]

There is a reason why arches have a wide base stack, that allows the heat to be across the entire width. If you draw from just a 7" pipe, even though the box is whole width, you will lose potential heat transfer as the flow of heat goes to the stack. The corners will get very little. Make the base stack a near full width transition from arch to stack. You can likely do it in 3 or 4 feet height, then use the 7" up from there as planned.If you are welding the arch and can't easily make an oval base to round, try just welding with squared corners and tapered sides, then just a flat piece on top not much bigger than the 7" stack and cut a hole to go up with thw 7" from there. If you make it out of 18-22 gal sheet metal and have the ability to shape it, make the transition to 7" from the wide base start. If necessary, a tin shop can make it for you.

[bushmoose]

I agree with Maple flats suggestion, but something else you might try is to incorporate some sort of baffle inside the box that will distribute the heat heading towards the stack? Perhaps a 2 or 3 inch flat bar welded on the underside of the top surface, all the way across, just at the edge of the stack opening?

[Bernie/MA]

I'm experimenting with about the same thing except I'm putting the warming pan against the bottom of the stack on a shelf over the pan. I'm hoping to get heat from the stack as well heat from the steam fom the pan. My grandson has a rig I made with an 8" stack and I'm working on a shelf to clamp onto the stack. That might work for you if it works and your stack is close (or could be) to the pan.

[Waynehere]

Yeah, I did something similar based on another members arch I saw. I had a preheater pan made, the width of the arch, but then put drop tubes in it to collect the heat better after it left the flu pan. Works like a charm. You can take a gander at my link below to get a better idea. I would also make the base stack as wide as possible to keep the heat dispersed all the way across the bottom of the pans.

[TonyL]

I did something similar, although I opened up the top of the box to allow better heat transfer to the warming pan. I've boiled twice so far this year, and it works great. I keep 3-4" of sap in the warming pan, and while I've never seen it boil, the sap temperature going into the main pan runs 160-170 degrees most of the time.

The main pan (2 x 3) will get a full roiling boil in about 15 minutes.
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[TerryEspo]

TonyL, that looks great, great use of lost heat.
Exactly what I hope to achieve. As I am work, I am thinking what my best way to do it is, season is right around the corner. I wish I had longer to get a custom made preheater pan. Maybe I can use mild steel this year and set it into the arch to catch maximum heat, maybe just a stainless pan sitting on top, dripping into main pan.

Anyway, some great ideas from everyone, thank-you for all suggestions.