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Making a couple 120 gallon offsets

antirice

Fire Starter
54
17
Joined Jul 3, 2019
Hi all -

Figured I'd start a thread to keep track of my progress on the smokers I'm designing/building. Nothing too extravagant - just a couple 120 gallon tank builds. I found a cheap supplier, so I bought three tanks. One for me, one for my dad, and one to either cut in half and use as fire boxes for our two smokers, or maybe build and sell a third one to help offset the costs associated with the project. We shall see. Not very many folks are looking for stick burners up where I'm at.



They're about perfect size for smaller backyard bbqs - in the end I should have enough space for about 4 briskets and a water pan with room to spare.

My initial idea was to build one that can be converted from RF to standard flow but, after reading about the success that folks seem to have with tuning plates, I think i'll just stick to the traditional flow. I want to have a hot and cold side if I'm smoking/cooking different things, but also want to be able to even things out in the event I'm actually cooking 4 briskets at once. I think tuning plates will be the way to go.

I do metal work (design and fab) for a living, so I will have this whole build modeled in Solidworks before I ever start cutting. If you're along for the ride, there will be plenty of screenshots and then, once the build starts, lots of pics too.

- Chris
 

JC in GB

Master of the Pit
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Joined Sep 28, 2018
Can't wait to see your build. Thanks for taking me along.

JC :emoji_cat:
 

antirice

Fire Starter
54
17
Joined Jul 3, 2019
First debate to settle: FB location and design.

If I'm cutting the third tank in half and using it, I can orient things a couple ways:

1)



2)



3)



4)

Any one of the above, but space the FB furter to the right and make a duct of sorts that'd connect the FB and CC.


In the interest of keeping costs and build time low, I'm leaning towards either 1 or 2 above. 3 would have a complicated door on the right hand side for access to the fire for adding logs and whatnot. But 1 would have a complex cut where the two meet and 2 would require two 24" discs welded to the ends instead of just one like if I went with 1 or 3.

Or I could just scrap the whole idea of using half of the third tank as a FB and instead make a square box.

Decisions...

- Chris
 

JC in GB

Master of the Pit
SMF Premier Member
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Joined Sep 28, 2018
I like #1 the curve looks nice and may even help with air flow. But what do I know?
 

bighoof

Newbie
15
0
Joined Jul 26, 2019
Number 1 seems the norm I can’t find another tank I may have to build a box around a 24” pipe or get a 30” pipe so more like #2
 

antirice

Fire Starter
54
17
Joined Jul 3, 2019
Made a little design progress at lunch today.



Still have a long ways to go. Next hurdle is deciding how tall I want the door opening to be. I have the bottom edge about 1" below centerline to accommodate the sliding grates I'll be putting in there and still have the cooking surface be right in the middle, but I'm never certain as to how much vertical clearance I'll need. I want to put an upper rack in there, so I'll need to cut the door fairly close to the top. But I don't want 100% of the heat escaping when I open the door either, so...

Right now the opening is about 75° of a circle (plus the 1" below centerline). This equates to roughly 11" of clearance above the grate. I think it'll work for now. I can still fine tune later.

- Chris
 

JC in GB

Master of the Pit
SMF Premier Member
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Joined Sep 28, 2018
That looks like a great start. Nice lines. Hope you get the height figured out.
 

antirice

Fire Starter
54
17
Joined Jul 3, 2019
made a little progress today, but spaced taking screen shots. Oh well, I can snag more in a day or two.

I've figured that I'll want the grate height around 38" (±2"). I build a rolling toolbox/workbench around that height and it was a really comfy height for me to work off of. I'll re-measure it just to make sure that I'm not remembering the dimensions wrong and adjust if need be. I'm not putting a shelf on the front of the smoker, so it should be pretty comparable to my workbench.

I've started drawing in the legs and lower rack. I also cut out a side door in the firebox as well as an upper vent located directly across from the opening to the CC. These will be fairly simply vents that have a rectangular cover that just slides out of the way to open up the vent - nothing too extravagant.

Speaking of the FB to CC opening - mine is a bit oversized as compared to what the calculator says I need. Even the RF calculator calls for 114sq in and I have 134. I have read about folks who use a baffle at the FB opening and tuning plates in the CC that have a hard time moving sufficient air to get the temps high enough for some cooks. I figure that if I'm looking at using the same setup, I might as well oversize the opening. It's easy enough to partially block it off if I am moving too much air.

- Chris
 

antirice

Fire Starter
54
17
Joined Jul 3, 2019
Finished the lower rack and added the stack today. I am toying with ways to smooth out the airflow transition from the horizontal plenum to the vertical stack. I know many people have things how I have it shown with quite a bit of success, but I'd like to see if I can improve things at all.


I know Aaron Franklin like a tall exhaust stack and, since he's more or less who I'm basing things off of, I figured I'd just throw the stack in there at a tall height. I can always cut some off after the fact, but it's kinda hard to stretch it an extra foot if it's too short.


- Chris
 

antirice

Fire Starter
54
17
Joined Jul 3, 2019
So I think I found a design I like that should slightly improve airflow without being too far out in left field. What do you guys think?



- Chris
 

Berettaclayshooter

Smoke Blower
141
54
Joined Jul 14, 2018
Your plenum looks real good. I would advise away from a top lid in the fire box and go with a door on the end. You'll loose a lot of heat every time you open the fire box with a top lid. Depending on where your from insulating the firebox isn't a bad idea.
 

antirice

Fire Starter
54
17
Joined Jul 3, 2019
Your plenum looks real good. I would advise away from a top lid in the fire box and go with a door on the end. You'll loose a lot of heat every time you open the fire box with a top lid. Depending on where your from insulating the firebox isn't a bad idea.
I'll have a door in the end of the FB for adding logs and whatnot. The top door was for access to the grate I'll have in there for searing. I figured it'd be a shame to waste a good fire, so I could use the FB for high direct heat cooking when needed. I will definitely need to make sure it's sealed really well, that's for sure!

As far as insulating goes, I live in WA State so it does get cold in the winter, but not crazy cold like the folks further north experience. I debated insulating the firebox, but I don't think I'm going to use it as much when it's that cold out. I spend most weekends in the winter out on the river fishing for steelhead and whatnot, so I don't do a lot of outdoor cooking then. I can always wrap with insulation later if it ends up being needed I suppose, or do you recommend making a metal "shell" for the FB and filling the void with insulation?

Thanks for the input!

- Chris
 

antirice

Fire Starter
54
17
Joined Jul 3, 2019
It's been busy at work, but I finally had time to work on the design more this morning.






I am still looking into exactly what I want to do with the baffle that I'd like to put at the FB to CC opening. I need to get my damper put on the exhaust as well. I'm going to make the grates out of 1x1x1/8 angle and expanded metal (unless anyone has a better idea - I'm all ears). Getting close to an end product I'm happy with though!

- Chris
 
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