Arch Design

[NW Ohio]

I have been thinking about building a masonry arch for a while. Thinking about a small one to accommodate a 2x4 hybrid pan. A couple of questions: 1) I see an arch that a guy posted on here last year (http://mapletrader.com/maplegallery/evaporator/p841-build.html) and wonder why I couldn't leave the "fire box" open all the way to the back but use a piece of steel plate (or something) to force the heat forward, then up and through the drop flues. I picture an arch similar to his but with the capability of burning longer wood all the way back under the flues, with the heat still coming up under the front of the pan then traveling back. Any thoughts? 2) If this idea makes since... how much space should there be between the bottom of the flues and the piece of steel? 3) Will block like this (or even clay brick) holdup if the arch is lined with fire brick, should there be anything between the fire brick and the other masonry?

Thanks for any input!

[lpakiz]

Ohio,
If you get a good hot fire going, a sheet of steel even a 1/2 inch thick will warp terribly. Good concept, tho. It works in my wood furnace just fine, but the heat is much less.

[NW Ohio]

Are you saying it can't be done or are you saying that it would need to 1" thick steel?

[lpakiz]

Well, perhaps if you could shield it or protect it from heat, it would survive. Like ceramic blanket or refractory-covered.
I know right where you are coming from. My first home-made was 2 feet wide and 3 feet long. I had an arch built into the back of the firebox, and had to cut wood about a foot long.
Good idea, but uncommon.

[bowhunter]

Mild steel starts to yield (melt) at about 800 degrees F. Steel is an excellent conductor of heat so if it's exposed to heat on one side and a cooler environment on the other side the steel can usually transfer enough heat to keep it below 800. That's why it works in applications such as a wood stove firebox. When you put steel in the firebox where it has nowhere to transfer heat it will get up to the same temperature as the firebox pretty quickly and start to melt. Man has been melting iron/steel in wood fires for centuries to make iron tools. Even if you insulate the steel it still has to have a cool side to transfer heat away. If it doesn't it will still get up to the firebox temperature. A natural gas flame has a temperature of about 3,400 degrees F. I can't remember what a typical wood flame would be but I know it exceeds 2,500 degrees F. Steel grates survive because the air coming into the arch is cooling the steel. So bottom line you can make the plate as thick as you want it, but if its only exposed to the environment inside the arch on all sides it will yield(buckle).

[morningstarfarm]

Ok, perhaps I am misunderstanding this...but why would yu want to burn longer wood Witt a flu pan? It seems to me that by burning smaller wood you are getting much more heat under your pans...I can't count how many times I had a jet engine coming out of my stack back in the day...and moaning about all the lost heat...when I went Ito a flu pan with shorter wood I actually cut my wood use by roughly half...just a thought...

[morningstarfarm]

Another quick thought...why go with stone? It would seem to me to be very difficult to expand later...and as we all now....expansion is a concern to all of us...I have never met a sugar maker that hasn't expanded...even when they say they have no interest in doing so...

[NW Ohio]

I am thinking longer just for ease. On the hybrid 2 x 4 pans there are only 18" from the front of the flues to the front of the pan (so a little less than that on the evaporator). I figure that if I can keep the wood lower in the firebox, I don't have to worry about banging up the flues with a big stack of wood. I am not a "combustion engineer" so maybe it doesn't work like this but my thought is that if I burn a fire stacked lower in the box (but still send the heat forward and make it come back through the flues) I can get the same results. "Why stone?" I evaporate outdoors (no shack). I can't see having a steel evaporator and leaving it sit outside (rusting away). Expansion might be in the form of a summer kitchen with a lean-to over the evaporator (maybe). This is just a hobby for me, no expansion into commercial size production (I know what you all are thinking!).