Firebox questions

[Arcwelder]

Hi all, presently, I am building an offset smoker that is: 24”x48”x1/2”. I hear from many that the firebox should be 1/3 of the CC, however as I am using the same diameter pipe for my firebox as I am for my CC it seems it will be too small. I’ve heard Aaron Franklin say he likes a long firebox so that he has room to pull the wood back from the smoke/heat opening giving him better control.
Also, if one looks at the offsets he is now manufacturing they appear to have fireboxes of the same diameter as the as the CC and seem to be close to 1/2 the length of the CC.
He also claims to have the firebox to CC opening just under the cooking grates. I have cut a 1/2 moon shape for mine that at its highest point is 10”.
Can you all respond with your critisms, thoughts and ideas. I truly find the firebox and it’s opening to the CC one of the most important but the least explained with the least amount of pictures of the internal FB opening to the CC.
I think if someone would make YouTube videos to explain and show what to do and how to shape the opening they will have lots and lots of viewers.

Thanks Everyone!
Arcwelder

[hogaboomer]

Franklin's fireboxes are double walled, that's probably why it looks long, the ID is smaller than just using a single wall. Are you using one of the pit calculators? There are several online, including one right under the banner at the top of this page.

[Arcwelder]

Thanks I’ll check it out

[hogaboomer]

Here's another more detailed one. The guy that made that is an administrator here. I do pipe on pipe, so this is the one I use.

https://planetcalc.com/4580/

[dacolson]

The size of the throat in a standard offset is not as important as long as it’s at least as big as the calc calls for. Half moon, rectangle, oval, etc probably matters a bit but area matters more. Franklin talks a lot about air flow and not impeding it but his restaurant pits are massive and act very differently from the small pits he sells.
Placement of the top of the throat at grate level is common and helps get heat up in the CC faster. I like to put a throat baffle right in front of the throat opening in the CC directing heat up to the top of the CC ASAP. It also redirects radiant heat to keep from scorching meat at that end. Frank Cox here suggests it and Jambo makes them this way as well. It helps to improve convection.
I built a 120 gallon for a buddy like this and the grate temp heat stays super even left to right.

[Dirtytires]

We hear it all the time...”Franklin said.....”

Keep in mind he has a dozen pits running 24/7 and has dedicated pit masters that build up hundreds of hours of experience each month. He builds them for simplicity and uniformly. With that said, I don’t recommend building yours just like his for home use. I smoke a lot but I’ll never build up that experience in my entire lifetime.

Follow the calculator dimensions for all sizes and specs and you will have a pit that will function well and be easy to maintain temps. Build it like Franklin and expect to spend a few hundred hours figuring it out.

Just my opinion.....it’s your pit.

[Chriz]

The size of the throat in a standard offset is not as important as long as it’s at least as big as the calc calls for. Half moon, rectangle, oval, etc probably matters a bit but area matters more. Franklin talks a lot about air flow and not impeding it but his restaurant pits are massive and act very differently from the small pits he sells.
Placement of the top of the throat at grate level is common and helps get heat up in the CC faster. I like to put a throat baffle right in front of the throat opening in the CC directing heat up to the top of the CC ASAP. It also redirects radiant heat to keep from scorching meat at that end. Frank Cox here suggests it and Jambo makes them this way as well. It helps to improve convection.
I built a 120 gallon for a buddy like this and the grate temp heat stays super even left to right.

Do you have a picture of the throat, grate and baffle?
Curious to see how it looks

[dacolson]

Yup. Here you go.
The first two show how I built mine. The throat baffle is on the back of the firebox, inside the cook chamber. The third pick shows the throat baffle in front of the cook rack.






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[Chriz]

Holy! That’s one impressive baffle.
I love it!!

Thanks for the pics

[D . Jur]

Whats about that kind of baffle? Is it work ?

[Big T]

Whats about that kind of baffle? Is it work ?

That design seems counter productive to me. Heat naturally rises so putting a baffle that angles down would make it a little more difficult for the heat to leave the FB. Also once the heat clears the baffle it will rise so it looks like you would just move the hot spot down a few inches. I feel that a baffle similar to Dave's or tuning plates are the 2 best options for an offset smoker but I could be wrong and it wouldn't be the first time.

[D . Jur]

Whats about that kind of baffle? Is it work ?

That design seems counter productive to me. Heat naturally rises so putting a baffle that angles down would make it a little more difficult for the heat to leave the FB. Also once the heat clears the baffle it will rise so it looks like you would just move the hot spot down a few inches. I feel that a baffle similar to Dave's or tuning plates are the 2 best options for an offset smoker but I could be wrong and it wouldn't be the first time.

I thought that a plate of this kind was just right, so that the upper shelf closest to the firebox would not be made useless due to the very high temperature.

[D . Jur]

By the way, Big T, the calculator only counts a square СС, can this formula be used for an octagon СС ?

[dacolson]

For an octagon FB, you need to figure out the volume of your octagon and then work backwards into the the cylinder size. I use this calc:
https://www.vcalc.com/wiki/KurtHeckman/Octagon+Volume
Then just plug in values that are close to actual in the pipe volume calculator until you get your calculated volume from above.

[dacolson]

Whats about that kind of baffle? Is it work ?

That design seems counter productive to me. Heat naturally rises so putting a baffle that angles down would make it a little more difficult for the heat to leave the FB. Also once the heat clears the baffle it will rise so it looks like you would just move the hot spot down a few inches. I feel that a baffle similar to Dave's or tuning plates are the 2 best options for an offset smoker but I could be wrong and it wouldn't be the first time.

I thought that a plate of this kind was just right, so that the upper shelf closest to the firebox would not be made useless due to the very high temperature.

The top rack is always hotter. Especially in a standard offset. Even with a downward deflector. The idea is to kickstart convection as fast as you can. Works for me. Lots of folks seem to like the other way too so ... up to you.

[Kcd2016]

Yup. Here you go.
The first two show how I built mine. The throat baffle is on the back of the firebox, inside the cook chamber. The third pick shows the throat baffle in front of the cook rack.






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I'm just over here minding my own business and here you go getting me thinking about building a new smoker... I've spent the last 30 minutes reading up on jambo's design too (also didn't realize franklin was building pits now). Interesting concept vs the reverse flow. So basically as I understand it instead of using the reverse flow plate to distribute and radiate heat from the bottom up, this design evenly radiates heat from the top down?

I'm getting rid of my old reverse flow and this throat baffle design might be the ticket for my next build!

Are there any other key engineering/design constraints to keep in mind when designing the throat baffle? For example I'm guessing the calculated throat area needs to be maintained at the baffle opening area too. And I'm guessing this works best with the stack at grate level otherwise heat just runs out the top?

[dacolson]

You are correct on the stack level. Couple thoughts on the engineering. And I use that term loosely. I tried to make the baffle so there were no constrictions. If the widest section of the throat was 6 inches for example I made sure there was no spot in the baffle that was under 6 inches. Also, I would extend the back lip of the baffle up above grade level by a few inches. Just to further redirect hot air up. I added an extra 6 inches. That seemed to help reduce the hotspot at that end of the pit right where the grate meets up with the baffle. My buddy bought this pit and lives in Tennessee now but I talk to him a couple of times a month. He cooks on it regularly. It was 15 to 25° difference from end to end when I sent it down. Except the last foot of the great by the fire box where it jumped up another 25°. The extra lip brought that down to just a few degrees higher from radiant heat off the metal.

[Chriz]

What is the effect of the smoke? Does it not go up and over the first part of the grate with those 6 inches?

[Kcd2016]

Yeah, looking at the jambo's the baffle appears to sit about 6" above the grate.

What are the dimensions of the cc you're referring to?

[Kcd2016]

Upon further research it looks like jambo keeps their baffle at grate level now. But this shows a plate like what D. Jur asked about above
https://youtu.be/i7cZkNoIwbw?t=76

The throat also isn't at the top edge of the fb but a few inches down...?

[Chriz]

Interesting plate, to direct the smoke down I guess?
Thoughts on this?


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[dacolson]

This pit is 120 gallons. I have the same setup on a smaller - 80 gallon pit as well

[dacolson]

Upon further research it looks like jambo keeps their baffle at grate level now. But this shows a plate like what D. Jur asked about above
https://youtu.be/i7cZkNoIwbw?t=76

The throat also isn't at the top edge of the fb but a few inches down...?

I'm starting another standard offset - 150 gallon - in a few weeks. I'll be experimenting with this design. Interested by confused by the additional angled baffle above the scoop. It looks like his grates right there are fried. Grease is all cooked off. Guessing that space is pretty unusable.

[Chriz]

I’ll leave it off gor now, let us know[emoji56]

[Big T]

Upon further research it looks like jambo keeps their baffle at grate level now. But this shows a plate like what D. Jur asked about above
https://youtu.be/i7cZkNoIwbw?t=76

The throat also isn't at the top edge of the fb but a few inches down...?

Jamie insulates all of his fire boxes on the trailer model so I think that the throat is still at the top of the FB but it gives the appearance of being lower. I haven't saw one with the baffle over the top of the throat before so that may be something new but I agree with Dave that it looks like it's burning up the grate right there. Also something that is often overlooked on the jambo is the way that the exhaust stack is mounted, if you look at the pit from the outside you can see that the bottom of the exhaust starts at the top of the bottom grate and goes up. However if you look at it from the inside you can see that he has a baffle similar to the one at the throat but not as large, it is turned up towards the upper rack. I've never cooked on a jambo but I've heard that the air moves in a swirling pattern through the CC and that's supposedly the reason that they cook so well. I remember one of the guys that was at the gathering where Frank debuted Lou Ann saying that the air flowed in such a way that the ends of the CC were one temperature and the middle of the CC was a different temperature. I think Pete was there so he may have a more detailed description of how it worked. I also saw a video that Frank released the other day of a 1000 gal pit that they had just finished and all 3 thermometers were sitting on 250 so I'm guessing that it had a different setup than Lou Ann. I tried to find some pictures of the jambo stack exit and the few that I found all were just a square opening so he must have changed the design at some point. I haven't saw one in person in several years and I've read that Jamie doesn't like pictures of his inner workings so I guess he done away with the stack baffle.