New Firebox Concept

[Kcd2016]

So I was sitting around the fire pit the other day getting hypnotized by the flames as one does and I had an idea. Has anyone applied the "secondary burn" concept to a smoker's firebox?

To clarify what I'm talking about specifically, the firepit I was sitting around was a solo stove. It's one of these smokeless firepits that draws in cold air from the bottom, heats the air within the walls of the pit, and finally the heated air ignites as it leaves the top of the pit reducing dirty smoke.



What if we tried to apply the same concept to a firebox? Could we get away with less fuel or smaller splits? Or smaller/lighter fireboxes? Would it be worth while to reduce white smoke? Here's a concept I drew up, it's a double wall firebox with the outer walls removed for clarity.



So the idea is the hot air would come out of the holes below the exit of the fb and ignite an clean up dirty smoke before entering the cc.

[tinspark]

I have been seeing more of these on other forums too. Mostly as back yard firepits that guys are interested in building. So your saying that the air that circulates between the inner and outer skin will ignite. I am not sure where it gets it's fuel. away from the wood thats on the other side of the metal wall. 3 things are needed to support combustion. 1) heat 2) fuel and 3)oxygen. Take any of those away any the fire is gone. add any of the 3 elements of fire with the support of the other 2 and fire gets bigger. (welding school/safety 101- how to put fires out) Not doubting it because it works somehow, but Still a little confused how the thing actually ignites without solid fuel unless it is somehow igniting gasses (byproduct of unburnt soot/ ignitable gasses??) released from the flame maybe?.. .. Can it be proved out where the fire between the skins actually getting it's fuel source? or will there be a flame shooting out those ports? <maybe Pete /mechanical engineer could explain concept. I have never understood it, But it would be cool!!
Also, would too clean of a fire in a smoker diminish the taste, which is what a smoker is all about ... maybe get rid of all of teh smoke including the blue smoke... IDK?? I think you should build one and test it out !! HAahahah

[dacolson]

I’ve seen - and tried - it said that you should have something like 20% of your air intake above the fire to aid in getting a cleaner burn. Did that on my first build. It’s also how all modern wood burning stoves are designed to burn cleaner and use less fuel. Honestly, I’m not sure I see the point in a FB. It’s not that difficult with properly sized vents to get clean smoke as it is. Too clean and - like Tinspark says - you get no smoke on your meat.
Now, that said, I do like me a science and engineering project! If built is good, isn’t over-built better?

[Kcd2016]

I have been seeing more of these on other forums too. Mostly as back yard firepits that guys are interested in building. So your saying that the air that circulates between the inner and outer skin will ignite. I am not sure where it gets it's fuel. away from the wood thats on the other side of the metal wall. 3 things are needed to support combustion. 1) heat 2) fuel and 3)oxygen. Take any of those away any the fire is gone. add any of the 3 elements of fire with the support of the other 2 and fire gets bigger. (welding school/safety 101- how to put fires out) Not doubting it because it works somehow, but Still a little confused how the thing actually ignites without solid fuel unless it is somehow igniting gasses (byproduct of unburnt soot/ ignitable gasses??) released from the flame maybe?.. .. Can it be proved out where the fire between the skins actually getting it's fuel source? or will there be a flame shooting out those ports? <maybe Pete /mechanical engineer could explain concept. I have never understood it, But it would be cool!!
Also, would too clean of a fire in a smoker diminish the taste, which is what a smoker is all about ... maybe get rid of all of teh smoke including the blue smoke... IDK?? I think you should build one and test it out !! HAahahah

it's the same concept as a rocket stove or the like. Basically the air preheats and when it exits the ports and makes contact with uncombusted solids and fumes within the initial smoke it ignites the smoke itself... at least that's my basic understanding. You can see it pretty clearly in the second photo above, all the air ports around the top edge of the fire pit are blowing flames! It's a pretty neat thing to see for the first time.

[Kcd2016]

I’ve seen - and tried - it said that you should have something like 20% of your air intake above the fire to aid in getting a cleaner burn. Did that on my first build. It’s also how all modern wood burning stoves are designed to burn cleaner and use less fuel. Honestly, I’m not sure I see the point in a FB. It’s not that difficult with properly sized vents to get clean smoke as it is. Too clean and - like Tinspark says - you get no smoke on your meat.
Now, that said, I do like me a science and engineering project! If built is good, isn’t over-built better?

yeah, my first feeling is it's probably not worth the trouble. but for the sake of science!

I was also kind of wondering on how I could build it in a less permanent and more of a quick and easy prototype kind of way.

[tinspark]

I’ve seen - and tried - it said that you should have something like 20% of your air intake above the fire to aid in getting a cleaner burn. Did that on my first build. It’s also how all modern wood burning stoves are designed to burn cleaner and use less fuel. Honestly, I’m not sure I see the point in a FB. It’s not that difficult with properly sized vents to get clean smoke as it is. Too clean and - like Tinspark says - you get no smoke on your meat.
Now, that said, I do like me a science and engineering project! If built is good, isn’t over-built better?

Maybe that's why some guys crack their doors. Gets some mixture on top of the fire too? The consensus here is to keep the FB air intakes at or below the bottom of teh coal bed with a raised fire to facilitate combustion.
I was watching a video tour posted on YouTube by a comp. pitmaster, Harry Soo . He took a video of the Franklin kitchen and one of Franklins pitmasters says he keeps the doors cracked on his 5 large offset smokers for a cleaner fire. I dont recollect any fresh air inlets in his FB or even a raised fire. Maybe same concept done differently. But the proof is in the pudding. They had people lined up at sun-up in lawn chairs for 4 hours. Doin' something right!!

[dacolson]

Those big pits will take almost as much air as you can give them. I’m thinking that’s why they just use the doors.

[tinspark]

I have been seeing more of these on other forums too. Mostly as back yard firepits that guys are interested in building. So your saying that the air that circulates between the inner and outer skin will ignite. I am not sure where it gets it's fuel. away from the wood thats on the other side of the metal wall. 3 things are needed to support combustion. 1) heat 2) fuel and 3)oxygen. Take any of those away any the fire is gone. add any of the 3 elements of fire with the support of the other 2 and fire gets bigger. (welding school/safety 101- how to put fires out) Not doubting it because it works somehow, but Still a little confused how the thing actually ignites without solid fuel unless it is somehow igniting gasses (byproduct of unburnt soot/ ignitable gasses??) released from the flame maybe?.. .. Can it be proved out where the fire between the skins actually getting it's fuel source? or will there be a flame shooting out those ports? <maybe Pete /mechanical engineer could explain concept. I have never understood it, But it would be cool!!
Also, would too clean of a fire in a smoker diminish the taste, which is what a smoker is all about ... maybe get rid of all of teh smoke including the blue smoke... IDK?? I think you should build one and test it out !! HAahahah

You can see it pretty clearly in the second photo above, all the air ports around the top edge of the fire pit are blowing flames! It's a pretty neat thing to see for the first time.

Yeah, thats teh part I dont understand. It almost looks like it runs like a gas burner, but I am not sure where it is getting the fuel from that inner wall.
I have built rocket stoves,, and have several cnc files for different types of them that I burn on my cnc machine, and they are pretty straight forward, easy to build, and cheap, but really dont have much practical application other than they are cool and make a hot fire!! The fuel is at the fire with plenty of oxygen. But this thing is a little different set-up IMO
My vote is that U biuild one and prove it out for us

[Kcd2016]

The smoke itself is the fuel... i think. The ports aren't providing fuel but rather heated oxygen.

[tinspark]

Found this video
looks like a similar concept in a wood stove
a few interesting comments below the video..
one fella wasn't getting it to work til he choked down his primary combustion air intake on his wood stove.
https://www.youtube.com/watch?v=9rLQSx-RQMk

[dacolson]

The smoke itself is the fuel... i think. The ports aren't providing fuel but rather heated oxygen.

You are correct. White or grey smoke contains unburned fuel. At high temp with an injection fo additional air/O2, it recombusts and leaves you with CO2 and water vapor ... or much closer to it.

[dacolson]

The secondary burn are those lazy tongues of fire curling around at the top of your stove and out of the holes on the sides of the solo stove.

[dacolson]

Kdc2016 - In your design above, are the ribs in a spiral pattern so the airflow from the intake would circle the firebox all the way up? I'm assuming that the holes below the throat would be isolated from the actual throat opening so that heated fresh air would vent into the FB to combust there.

[Kcd2016]

Yep, they spiral the air around the fb a few times before channeling it all to the holes beneath the throat. And yes the bottom of the cc would be between the holes and throat so the air would have to exit the small holes into the fb before getting to the throat.

I'm no firebox engineer so I have no idea what I'm doing. Just spitballing about an idea stuck in my head.

[dacolson]

I bet you could vent those holes all the way around the top of the FB as well. Like the Solo. Or part way at least. There would be a big ole hot spot at the grates at that end I bet with the secondary burn right there.

Again, I don't think the end result would be optimal. You'd lose most of your smoke flavor.

[Kcd2016]

That's an interesting point as I've always aimed for little to no smoke coming out of my stack. I've always thought of the term "smoker" as something of a misnomer. I know the calculators are already designed to help us get full combustion as it is. So you're thinking a fb supercharger like this might just totally remove the smoke flavor.

[dacolson]

Yup. common complaint with thickly insulated fireboxes. With proper vents, they are so efficient you don't get hardly any of the delicious parts of the smoke that flavor the meat. It all burns up. I think it's part of the reason you see a lot of partially insulated pits. More efficient but not over the top.

[Dirtytires]

My 24x60 pit uses so little fuel now I'm not sure that decreasing fuel is a priority. Interesting concept tho....

[Kcd2016]

So let's scratch this idea.

What about drawing in at the top of the outside of the firebox and channeling it down before entering the inside of the fb at the bottom? This would avoid wind/weather issues and preheat the air a bit. As long as the channel is large enough would this still present any air flow issues?

[dacolson]

So let's scratch this idea.

What about drawing in at the top of the outside of the firebox and channeling it down before entering the inside of the fb at the bottom? This would avoid wind/weather issues and preheat the air a bit. As long as the channel is large enough would this still present any air flow issues?

As long as there’s enough airflow. Should be fine. Wind will always be an issue though. Wind across an opening creates pressure regardless if it’s offset or not. Probably good to have them on either side of the fb still.


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[Kcd2016]

Like this




Need to think on constructability...

[Big T]

That will probably work fine but if not, it looks like a lot of work to correct the issue. Not to mention all of the work to build it the first time.

[Kcd2016]

Agreed! I've since come to the conclusion that it would be best to go ahead and put the intake in the door... Easy enough to build a new door.

[Kcd2016]

If you made an insulated firebox, and each wall had a few ribs (thinking like a torsion box), could you get away with 1/8 for the inside wall?

[dacolson]

I’d worry about warping.


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