Help Tweak My Design Please

[EverydayDiesel]

Howdy,

This will be my first (and probably last) build so I really want to get this right. There are a ton of very smart guys on here and I am hoping you can give me some feedback on this design.

I already have a 250 gallon tank and I plan on using that to cook for the church, friends and close neighbors. I plan on using 3" of insulation around the firebox.

Do these numbers look ok?




Please excuse my drawing (I am 100% beginner at cad)
Here you can see how high the firebox sits on the cooking grate (30 inch CC and cooking grate will be in the middle at 15"). Should this be lower?



I am quite confused on where exactly to put the throat on both the CC and the firebox.
Should the opening be higher or lower in the FB?
Should the opening be higher or lower in the CC?
Should I move the FB up or down on the CC?


Thanks for any help you can give!

[hogaboomer]

3 inches of insulation seems like a bit of overkill. That firebox is gonna be spendy.

[EverydayDiesel]

Its a bit more then normal but the best I can tell this is an important area that shouldnt be skimped on. I would rather cry once/buy once and I only want to build this once. Luckly I have a few sheets of 1/4" 4x8 plate that I bought back when it was $125 each.

How does the placement of the throat look?

[hogaboomer]

I don't know, I never built one that big. Let Colson have a look at it, or some of the others.

[dacolson]

On a standard offset, the placement of the throat doesn’t matter as much as long as the top is no higher than halfway up. That’s what was causing Boomer’s big temp spikes at that end I believe. I put mine at halfway and have good temp control w the throat baffle I built. You create air restrictions at the stack so the throat can be a bit larger than recommended if you worry about airflow. I would put the throat as high up on your FB as you can. That keeps extra heat transfer from your FB to a minimum.

On insulation, IMO, I wouldn’t go more than 2”. I built a smaller pit with 2” and couldn’t get it to smoke at all when it heated up. Combustion was so clean I got very little smoke. I’ve never built one as big as a 250 gallon though so maybe it’s appropriate. I don’t think I’ll build a fully insulated FB again and I live in MN. I may test out a partially insulated design some time.


Sent from my iPhone using Tapatalk

[EverydayDiesel]

I thought no smoke or thin blue smoke was what well are after? I live in texas so it's pretty hot here. To be honest I don't really understand why we insulate other then to help build heat which causes the air to expand causing it to draw. In short, (I think) insulation raises temps which helps draw

[dacolson]

Thin blue, yes. No smoke, just heat = no smoke flavor. You gotta have a little smoke or it’s just an oven.


Sent from my iPad using Tapatalk

[hogaboomer]

You don't have to have an insulated firebox to get convection. People insulate to use less wood, or to eliminate risk of unwary kids getting burned.

[EverydayDiesel]

Thank you for helping me. You are a wealth of knowledge!

is it typical to insulate the back wall? (where the orange is)

[Brewmaster]

I'm in eastern Canada with a couple of smokers, one offset has an insulated firebox, and the other doesn't. I found the uninsulated benefited greatly by lining the bottom 1/3 of the firebox with thin firebrick, just seemed to make the whole thing run smoother/more steady temps. I build fires on firebox floor without fire-grates, also worthy of note they are both round fireboxes so it helps to cradle the coal bed similar to a v-bottom or v-style fire grate. I don't do a lot of cooks in shitty weather though, if the forecast is for snow sleet hail, and the like I'm not cooking, although I have been caught in it.

Regardless, a 250 gallon is a different beast, I don't think you want a Franklin-style baffle plate or anything similar involved. Most of the larger ones I've seen have the firebox/throat inlet at food grate height, or a couple of inches or so below that, and use no baffle plates of any sort.

Agree with dacolson, your throat is too small (calculations here show the smallest case scenario, bigger is better from everything I've learned - more free-flowing unrestricted gases = better convection) Also agree restriction of flow should come from air drafts/stack design/size and so on rather than the throat, so open that sucka up! haha

Also agree 3" insulation is overkill. 2" seems to be what most do and I don't think there could really be much if any benefit with 3". Great freakin score on the sheets of 1/4" you got before the world went to hell though!

Your throat design looks good to me, I like the straight horizontal top of the throat inlet, feel like it's even better the wider it is, to the point of being full chamber width. Allows for even heat/smoke inlet to begin with rather than a hot spot in middle on the top-rounded arch style. And having a dam in the bottom so no grease can run back in the firebox is solid.

[Brewmaster]

Thin blue, yes. No smoke, just heat = no smoke flavor. You gotta have a little smoke or it’s just an oven.


Sent from my iPad using Tapatalk

Agreed!

[Brewmaster]

is it typical to insulate the back wall? (where the orange is)

Most that I've seen insulate that end, but not inside. So if the uninsulated firebox were attached first, then any areas exposed would be insulated and jacketed. If that makes sense?

[EverydayDiesel]

All good information, thanks guys.

Is the calculator ID or OD for the exhaust stack?

[Pete Mazz]

ID

[Dirtytires]

I built my firebox fully insulated...bottom, back and even the door. To be honest, I'm not sure it is really needed but I wanted it for the safety factor in case somebody (most like me, after a beer or two) bumped into it. I used a 1 inch thick ceramic insulation and it has worked great. One thing to consider is the R-factor of the insulation. My 1 inch has better insulating properties than the popular 2 inch Roxul. It works great and would save a considerable amount of steel material to reduce both cost and weight.

Best part is my firebox gets hot but I'm not worried about 3rd degree burns if it gets bumped. I took internal wall/external wall temps and can post if anyone wants.

[EverydayDiesel]

What type insulation did you put in yours?

[Dirtytires]

Couldn't tell you the brand. I ordered it from Grainger (search their website for 'heat resistant insulation') a number of years ago. Comes in a bunch of different size rolls and r-factors. At the time, I looked up the r-value of the popular roxul and my goal was to go thinner/more efficient. The ceramic is also much easier to work with (cuts precisely and no fibers) and heat resistant to something like 2700 degrees.

Let me know if you need help, I'll dig into my notes.

[EverydayDiesel]

Should I plan on putting tuning plates in this offset? The original design doesnt really have them but now I am second guessing. What kind of modifications do I need to allow for in order to allow tuning plates as an option later?

[Big T]

You'll need to add a couple of rails for the TP's to sit on if you think that you may go that route. They'll need to be a little higher than the top of the throat and run most of the length of the CC.

[EverydayDiesel]

I am not an experienced cooker or smoker builder to say one way or another. Im not new to fabrication but this smoker stuff has me with my eyes crossed.

The best I can tell tuning plates allow you to make a more even temperature across the grate but I kind of like the idea of having different cooking 'zones' for cooking different things.

[Pete Mazz]

That's the upside of tuning plates. You can adjust the placement to create different zones.

[dacolson]

Just remember, if you want to plan for the option of tuning plates, keep the top of your throat low enough to put the rails above them and leave 3 or 4” of clearance below your cooking grates.

Here’s a thought. Build it like you’re going to use tuning plates. When it’s complete, mess around without the plates in place. See how you like it. Then mess around with them in and compare. If you don’t like either option, you can build in a baffle like I use or like Boomer. The baffle is the only option that would be very difficult to reverse. Lots of options.


Sent from my iPhone using Tapatalk

[EverydayDiesel]

I plan on putting the grate at 15" (middle of the 30" CC). I should put the top of the throat at 12"? So far I have found that higher on the throat means hotter/faster cook and lower on the throat means lower temps and slower cooking times.

[hogaboomer]

k.a.m., the smoker building guru on the Texas BBQ Forum, told me to put the tuning plates 6 inches below the grate. He said, if you put them too close, you tend to get more radiant heat cooking than smoking. It mostly depends on how tall your throat needs to be. I couldn't always get a full 6 inches. IIRC, one was 5 inches, and it still worked good.

[EverydayDiesel]

I appreciate the replies.

My crystal ball isnt working too good. Is it typical to put tuning places on a 250gallon? How do you know that tuning plates are needed before you build?