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Understanding Wood Burning in a Firebox

post #1 of 10
Thread Starter 

This is closely related to my other recent post on "Firebox Vent Size Survey".

 

So let's say I have a good fire of nice dry oak burning in my firebox for the past six hours - nice coals, etc. Too keep my smokebox down to 220 degrees, I find that I have to really choke off my firebox.

 

When I open my firebox door, I see that if I haven't recently put new wood in, the fire is a mass of glowing wood coals. Sometimes, immediately upon opening the firebox door, there is almost an explosion of fire - the gasses escaping from the wood suddenly (almost explosively) catch fire and the entire firebox is a huge fireball and flames shoot out the door.

 

Clearly, during a long slow burn, like for a pork butt, there are lots of unburnt gasses from the wood that just pass from the firebox and pass through the smokebox and up the exhaust. When I open the firebox door, a large amount of oxygen enters the firebox and I have a near-explosive event of these gasses igniting. After the initial burst of flames, the fire settles down to glowing coals with some flames above - i.e., with the door open, now the fire has enough oxygen to burn the gasses that are coming off my wood as it burns.

 

Wow, it's almost like I'm answering my own questions as I'm writing this. My question is whether this is a good situation? I'm wondering if a fire that is that choked down is doing whatever it is that makes for optimal smoking in a smoker?

 

The more I think about it, the fellow in my other thread who recommended that I get my fire off the firebox floor and onto a grate might well really be on to something. I can now see that if I get a bit more air moving through the fire, I would be burning those gasses that I am now not burning. I'll bet that would indeed burn much hotter - and so I would only need a much smaller fire in size. AND maybe burn cleaner (as it is, IMHO, my firebox makes more white smoke than I like).

 

Any thoughts on this? Am I out in left field with my thinking and understanding of what is going on in my firebox? Am I seemingly on the right track with my thinking?

 

Thanks!!!!

 

Terry Farrell

Tampa Bay, Florida  

post #2 of 10
Quote:
Originally Posted by PianoV View Post

 

The more I think about it, the fellow in my other thread who recommended that I get my fire off the firebox floor and onto a grate might well really be on to something. I can now see that if I get a bit more air moving through the fire, I would be burning those gasses that I am now not burning. I'll bet that would indeed burn much hotter - and so I would only need a much smaller fire in size. AND maybe burn cleaner (as it is, IMHO, my firebox makes more white smoke than I like).

 

Ding ding ding, we have a winner. A smaller, hotter clean burning fire is what you want. It will give you a lighter but much better smoke flavor devoid of all the nasty, acrid byproducts of a larger snuffed down fire.

post #3 of 10
Thread Starter 

Yup, my head is clearing now. The sun is rising. Fresh air is flowing (not on my fire yet, but rather my brain!). I think I'm about to enter my new phase of improved smoking!

 

I've been smoking meat for 20 years with absolutely zero input from anyone (just don't know anyone else with a smoker!). I feel like it is 1964, I am a member of the Deep Sea Drilling/Coring Research Team, and I have just discovered plate tectonics!

 

This is all a total revelation to me. With ZERO doubt, I will obtain a grate today, start a fire this evening (and play with it a bit to learn how to regulate it - I can only assume it will behave a bit differently than my snuffed-out fire) and throw a nice pork butt in the smoke chamber maybe around 9 or 10 PM.

 

I'll drop a post here by 6 PM Sunday and let you know! Thank you, thank you all, this is sooooo terrific!!!!!!!   :yahoo:

post #4 of 10

Your thursty for knowledge,

 

Heres what you have:

 

Hearth Botom Grate

The drawing below shows the most common way of positioning wood when smoking. When wood is placed directly on the ground the whole air enters frome above. This is the way fireplaces work and though all air seems to bypass the wood, enough of it reacts with fire to maintain combustion. It is obvious from the drawing that a strong draft has very little influence on a combustion process as the air will only stream faster over the fire.

smokehouse firebox

Hearth bottom grate

 

 

The rate of combustion and temperature is controlled by the amount of wood placed in a fire pit and as the air is consumed, a new fresh air comes into its place. The rate of combustion is independent of the draft and the fire burns as if no chimney existed. To be able to control burning, the air opening has to be greatly decreased and a damper will start exercising control. Wood burns from front to back. When placing wood on the ground in a firebox with fresh air flowing in freely, the higher temperature will be in the back of a firebox or the barrel.

 

 

Here is what most people do;

Stool Grate

The drawing below is an example of a closed fire box used in a heating stove. The entering air below the wood has a greater density than the hot air above the wood, this creates a difference of pressure (draft) and the only possible way is to go up through the wood. The burning coals will deprive the streaming air of its oxygen and that sets the temperature as no more fresh air is available.

smokehouse fire pit

Stool grate

Placing wood on a grate permits the introduction of primary air below the burning wood and better combustion. Here the coal bed physically separates fresh air below from the warm air above, a natural draft is created and all air has to flow through burning coals or wood. This is the most effective solution permitting a great deal of control.

 

 

Here is what I recomend:

Raised Stool Grate

The drawing below depicts a smaller amount of air flowing through the burning wood and the larger portion blows over the wood increasing the rate of burning. In this case the rate of combustion can be easily controlled by adjusting the air supply under the fire.

smokehouse-fire pit

post #5 of 10

Most smokers you see will be just the traditional stool grate. A design that gives you excellent control over your heat.  The problem is by the time the air travels through the coals , most of the oxygen has been depleted, and the top of the fire is starving. This decreases the efficiency of your fire and also creates the  'dirty" smoke due to the un spent particles. By adding the secondary vent above the fire grate, you can introduce oxygen to the upper part of the fire and get better ( I did not say complete)  combustion, increasing efficiency and allowing cleaner smoke.  But the more cool air you allow through that top vent can drop your temperature, so you have to find the right ratio.

 

If you only knew how many hours over a lifetime of playing around with these things , that Ive spent peeking through the vent openings, watching the fire and learning what the shape and length of the flame meant.........

 

Anyway, shoot for about a 60/40 or 65/35 ratio. Greener, dirtier wood needing more upper oxygen, and well seasoned clean wood needing less.

 

I could take you further and start talking about making it even more efficient with more ways to get closer to complete combustion, but remember, its just a smoker and what information I just shared along with the other members suggestions should be all that's necessary to get a proper burning fire.

post #6 of 10

Wiz-

 

very interesting!!

 

thanks!!

 

I may add an upper vent to my firebox for some added control

post #7 of 10
Thread Starter 

That raised stool grate drawing is what I am shooting for. I agree that it will likely be a good idea to install an air path for the upper part of the firebox. I will likely have to go in through the side of the firebox as the door on the firebox completely takes up the entire upper 80% of the firebox front. If I can remember how I built the darn thing, I think the sides of the firebox are constructed of red bricks on the outside, then concrete blocks filled with sand and an old (clean) broken-up porcelain potty (hey, we remodeled and I was trying to figure out how to dispose of it - well, actually two of them...) and then the layer of firebrick on the inside - that's a good 16" of wall to drill through! Hmmmmm, maybe I can just angle in from the front.

 

"...shoot for about a 60/40 or 65/35 ratio."  60% air from above and 40% air directed below the fire?  So I'll want my top-air inlet to be at least as large a diameter as where my bottom vent is adjusted with a mature fire.  Sounds like a plan!

post #8 of 10

No, 60/65 from under, 40/35 from above. at least that is what works in my pits,

post #9 of 10
Thread Starter 

Gotcha. Thanks. That'll just help me size my upper inlet a bit. It's just easier to install a smaller vent - so I may even go with something like 1" ID or so (with a damper control of course). Thanks again.

post #10 of 10

One thing I have not seen discussed much.

 

When you choke off oxygen to the fire box, you will generate CO, (carbon monoxide).

 

CO is what meat packagers use to keep meat pink.

 

Electric smokers generate not much CO, if at all.

 

 

dcarch

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