Firebox dimensions. Starting a new build

[wango]

I'm helping my son build a small reverse flow offset with a smoker/warming cabinet over the firebox. We checked the pit calculator ratios and the tank we are planning to use for our firebox is about 40% bigger than recommended. Should we cut it down or plan to have smaller fires? Or should we add enough firebrick inside to get closer to the recommended size? Any info will be appreciated, this is our first offset build

[k.a.m.]

Welcome to the ring v, looking forward to seeing some pics of your cooks and following your cooker project.
40% over sized is pretty large in my opinion but doable. It does change the way the cooker will draft but once you learn it she should cook. What are the dimensions of the cooking chamber and proposed firebox?
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Always remember slow and steady wins the race.

Hybrid Cooker

[wango]

The cooking chamber is an old air compressor tank. 15" in diameter, 39" long overall, it has rounded ends. It doesn't have a tag stating how many gallons it holds, we estimated 35-40. Our firebox is going to be made from an old hydraulic tank that is rectangular 15x20x20. The warming cabinet/smoker on top of the firebox will 15x18 by 24" tall

[wango]

I just did the math on the cooking chamber, we may have estimated high, more like 30 gallons

[k.a.m.]

I would look for a larger diameter cooking chamber. The 15" diameter is too small in my opinion I would look for a tank with at least a 20" diameter.
_________________
Always remember slow and steady wins the race.

Hybrid Cooker

[SoEzzy]

Running the numbers, without the warming box, your firebox is 261% of the size needed for your cooking chamber, with the warming box, you are at 141% of the needed size, if you run your exhaust through the warming chamber.

But at the end of the day, the cooker will cook how it cooks, you can raise the fire grate, you can line the fire box but it's not really needed, as once you get it put together, and you learn how it likes to run, you'll build the fire small and hot, and keep it fed with enough wood or charcoal, and enough air to burn it hot and fast, and that's what will matter most.

The pit with the bigger firebox will take longer than a smaller sized pit to get fully up to temperature, when you are starting off on a cook, due to the extra mass of the firebox, sucking out more of the heat from the fire, than a smaller firebox will take, but that will likely be the only difference you will spot between the two options!

Good luck with your build.
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Here's a change Robert.

I still work here!

[wango]

Thanks for the replies, looks like I should look around for a slightly larger cook chamber or cut the firebox down some. He's building this on a budget, so we're trying to keep costs as low as possible. So far he has $0 invested because he's using scraps I had stashed away in my barn.

[k.a.m.]

I will be the first one to say re-purposing material is the way to go but in my opinion a 15" X 39" tank once built will be difficult to maintain temps and be very limited on cooking area.
I would rather see y'all build one that will perform and use the smaller one as a project cooker while y'all are cooking on the better one.
_________________
Always remember slow and steady wins the race.

Hybrid Cooker

[Wreckless]

+1 on kams last post. Not much finer reward than a fine cooker build on a low budget. Beware that the budget doesn't cripple you though. The cost of the pipe to move up to a 20" or 24" aside, I think you will find the increase in other materials to be small especially since you had already planned / budgeted for the larger FB. Your labor and time will for sure not increase enough to even notice. It takes the same amount of time and labor to cut a rail for a 24" as it will for a 15". The end result will be where you notice the most change, happily so I would think. Good luck!
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[wango]

Thanks for all the replies. We've taken your advice and found a 20" by 60" air compressor tank. It's really old and very thick, not sure how thick yet. Looking at the welds it's obviously heavier than what we had. Paid 50 on Craig's list for it. Our firebox is now 97% without calculating for the warming/smoking chamber. What calculations do you use to factor in the warming box? We want to add the warming box so now we're trying to decide if we should modify the firebox or build/find something else for a firebox.

[wango]

Our plan is to be able to use the warming box as a smoker or as a warming box. I.E. bbq in the cooking chamber and make jerky in the warming box at the same time. Is this realistic?

[k.a.m.]

Congrats on your cooking chamber find.
Do not figure the warming chamber as part of the equation. How do you plan on introducing your heat/smoke into the warming chamber?
_________________
Always remember slow and steady wins the race.

Hybrid Cooker

[wango]

We're planning on having an adjustable baffle between the chimney and the warming chamber, the warming chamber will have a separate chimney on top. Another member on here, Neths Redneck BBQ, posted pics of his new cooker from Shirley Fabrication. I tried to copy a link to his pic but it didn't work

[k.a.m.]

So you will have a sliding damper between the cooking chamber and the warmer like the one on my RF build here.
http://www.thesmokering.com/forum/viewtopic.php?t=32283&postdays=0&postorder=asc&start=100
If so only figure your cooking chamber and firebox. The warmer when used like this will act more like an exhaust. Just make sure your opening is large enough to allow the heat/smoke to flow smoothly.
I hope this helps.
_________________
Always remember slow and steady wins the race.

Hybrid Cooker

[wango]

[k.a.m.]

Thank You wango.
The warmer will run between 200° and 240° when the damper is open and cooker is warmed up and humming at around 250°.
_________________
Always remember slow and steady wins the race.

Hybrid Cooker

[wango]

Here is a pic of the compressor my son bought off craigslist. Its an old tank, the tag reads 1950, its 1/4" thick. 20" diameter, 52" long between the ends, .64" long overall It was a working compressor when we brought it home.

[wango]

I had this old rolling table/bench given to me many years ago. It had a plywood top, I used it for a couple projects and its been sitting in the corner of my shop cluttered up with crap ever since. We needed a frame and casters for the cooker and decided this would be a good time to re-purpose it. This pic was taken after we removed the plywood.

[wango]

I had never seen this tag until we removed the plywood. Apparently the frame was originally used by the Douglas Aircraft Co for some type of rocket storage. It says its rated for 850 lbs, I had 1000 lbs on a couple times and it held it fine. So the name of his grill is now "The Rocket" which is contradictory for a slow cooker, but hey with that tag, how can it be called anything else? The tag is going to stay on. Looking at the welds on the angle iron frame, its pretty clear that it was added later on.

[wango]

Removing the angle iron