So talking about firebox size?

March 15, 2015

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So talking about firebox size?

Original post by Forums Member

 QueTex » March 4th, 2015, 8:17 am

The small trailer smoker i built back in 2011 had a way oversize box, took a lot of time testing and tuning to get the openings right so it would cook evenly and get a clean burn. The smoker I currently use basically had the same dilemma, I did’t know about the feldmans calculator back in 2011 but now I do. Nonetheless I was sure I could tune it to make it run good without shrinking the box but after months of trial and error well I was wrong, in the end I still had to modify the box down to about 107% and now it burns nice and clean, still runs a little too hot but I can live with it, right at 300 to 325 will absolutely not cook under 300.
So what is it about a too large fb that will just not allow it to run right, what is the dynamics or physics behind that? Any way to explain it in common english? lol
QueTex
Just Got in trouble for buying tools


Re: So talking about firebox size?

Postby Rodcrafter » March 4th, 2015, 11:08 am

This should be good, you have just asked for a BTU explanation from Frank. :-B
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more……..
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Re: So talking about firebox size?

Postby QueTex » March 4th, 2015, 3:06 pm

My thoughts are that heat expands in a too big box and has trouble “flowing” to the chamber, have seen many cookers back flow out back even with stack wide open.
QueTex
Just Got in trouble for buying tools
 

Re: So talking about firebox size?

Postby forty_creek » March 13th, 2015, 12:11 pm

I’m on my first build but I have cooked on several pits of all sizes and makes. I think you are on track with the flow/ lack of draft with the heat being trapped in an oversized FB. I to had never found a pit calculator until this past early winter. From my experience and thinking… I am sure you have double checked your vent size, throat size and stack size… My first thought is something is a tad bit out of whack of the 3. You should be able to get the temp down below 300. Leans me to wonder about your exhaust stack…but with a damper on top you should be able to control the loss of heat which isn’t your problem. I am thinking through this… air comes in the vent goes through the throat down the BP back across the CC to the exhaust. So the heat source starts with the air vents to the FB… How’s the Q on the pit, smokey?

Now I am wondering what kind of pit do you have..RF, Cabinet, Offset???

Like I said I am not an expert…I just saw no one has responded since RC who has helped me tremendously with my build especially when my head goes to smoking, BTW. I find it an interesting question myself. I do understand the issue with an undersized FB. Have a buddy that has a 250 offset w/ an undersized FB. Of course I never knew it until I found the pit calc. But I have always thought his FB was undersized. Takes a while to get temps balanced and constantly adding wood and adjusting vents. He built a solid cooker but without any help. I am anxious to here a response on the dynamics and/or physics behind it.

Okay I have wasted however long it took you to read this… I was just thinking & typing before I get back to work on my build.

DRANKIN, SMOKIN and DREAMIN
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Re: So talking about firebox size?

Postby Rodcrafter » March 13th, 2015, 5:06 pm

Ok, so let’s see the pics of this cooker. Inside and out, with the vents and throat opening and BP opening if it has one. The stack also. List the dims and we will all see what is going on here.

Jm2cw

Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more……..

Re: So talking about firebox size?

Postby Frank_Cox » 1 minute ago

Sorry I’m just now getting to this post…. :oopssign:

Anyway… here goes…

Commencing the BTUing…..

The firebox is sized according to the volume of air needed to not only aid in complete combustion but also match the amount of air needed to evenly heat the cook chamber volume. this is done in several ways…

  • amount of fuel used in the firebox
  • controlling the volume and velocity of the air movement through the entire cooker
  • keeping the different parts of the cooker balanced.

Along with the firebox the Throat opening, Baffle Plate Gap, and the smokestack in an RF Smoker are very important in this balance. Even if you use the PitCalc you can still get these out of balance. I shoot for a number between 103% and 110% with 107% being the sweet spot for the firebox volume. if this is out of this range you will find the smoker will be difficult to control. Too Big and you will need more air flow for complete combustion which will lead to high temp swings and very uneven cook chamber temps. too small and you will lean to lower temp swings with very poor combustion, high temperature difference in the CC, and soot.
Now, the PitCalc (whoever’s you use…. I prefer ours tho) starts with the CC Volume and then moves to the firebox. this ratio sets the tune for everything else. If the firebox is oversized then everything else will be also. Even when I use a warming cabinet I do not oversize the FB. No Need to…
The throat opening is a good starting point for the Baffle Plate Gap. I use that as the Maximum BP Gap opening. I never go bigger than that. as a matter of fact I’m usually 50% or 75% of that size. The main function of the BP Gap is to choke down the volume of air but increase it’s velocity. :huh:
Yep that’s right…. Velocity and volume are 2 different things.
kinda like putting your thumb over the end of a water hose the amount of water coming out is less but it speeds what IS coming out way up…. Pause while your brain rests for a sec……
Pause…
Pause…
Pause…
In an RF smoker the Baffle plate gap controls this so we can slow the volume down under the BP and absorb more heat into it for radiant heat to help control CC Temps. In a standard Offset this is done using the smoke stack damper.
In an RF smoker we keep the damper at full open if the pit is balanced properly. If you need to move the damper to half closed then your BP gap is most likely too big or you have the pit out of balance somehow…

The smokestack size is also very important. too much volume and you will draw too much. too small and you will not draw enough.

Frank Cox

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