New Smoker Build

[sankie13]

I'm in the process of designing and building a new smoker. The one I have now is cheap and leaks smoke and air, so like many before me I'm going to take the plunge. The main reason for this is mainly I enjoy building things but also I have most of the materials free in the form of old scrap metal around the farm. I do have a pipe for the main chamber and firebox and this is where my question comes in. The pipe I have is unused oil pipeline that is about 7' by 20" and 3/8 thick. The design in my head is a 30" main with a 12" firebox. My question is if a 3/8 thick main chamber is too much. I know it will require additional fuel to get it up to temp and then it should hold pretty well from there. I think a 3/8 firebox will be pretty nice but the main chamber is my concern. Does the extra 1/8 of an inch over a 1/4 main make that much of a difference? Does it add a large amount of time to the warm-up process. The bigger question is if a 1/4 is that much better that it's worth potentially having to pay for a pipe or barrel. Does anyone have experience with the two different sizes. With a main chamber that's only 30" I'm not sure that it would require just a massive amount of fuel to get it started each time or a lot of time but I could be wrong.
TIA

 

[kruizer]

You will certainly use more fuel to heat it up but the advantage is your temps will be very stable.

 

[kmmamm]

Yes, it will take a bit more energy to get it heat soaked. However, the additional mass will definitely do a better job of keeping the internal cc temps stabilized. Personally, I find fabricating with 3/8” plate and pipe is faster and more forgiving than its lighter cousin.

 

[sankie13]

My plans right now have an oversized firebox is that going to be detrimental?

 

[kmmamm]

My plans right now have an oversized firebox is that going to be detrimental?

Depends on how oversized you intend to go and which calculator you use. I try to aim for at least 15% over on the firebox volume ...a number that always seems to work pretty well for me. I figure the extra volume helps make up for volume lost to internal things like fire grating, ash accumulation, etc.
Be sure your total available inlet volume is also increased in about the same proportion to the added firebox volume.

 

[sankie13]

Ok right now I'm at 160% to get a firebox I can put full length logs in, that's pretty oversized but I didn't know if that's going to cause issues with airflow or something

 

[kmmamm]

60% over is pretty generous...I would be a little nervous about going that far over. Unless your cc and associated airflow can support a large fire, I wouldn’t recommend “full” logs....too much mass, they will cause large temp swings, a lot of white smoke (creosote) and it will be difficult to get enough air across them to get an even burn.

 

[sankie13]

Ok so if a 12 inch in length firebox with a 20" diameter is big enough that puts me at a firebox 40% the size of my chamber which is the same 20" diameter by 30 inches. That was my original plan but was thinking about going 16" to fit the logs which like you say may not be beneficial anyways.

 

[sankie13]

What effect does an oversized firebox have? Does it affect airflow?

 

[kmmamm]

Which build calculator are you working from Dave’s, Feldon’s, smoker builder, etc? I want to be sure we are apples to apples.
in general, what I have found is 20% oversized helps flow combustion air around the entirety of the fuel. I have also found keeping the size of the sticks to about 80% of the firebox‘s overall length is also helpful in maintaining a uniform burn with minimal temp swings. However, it isn’t simply the firebox size...inlets need to be siz3d and placed properly, the opening between fb and cc has to be sized properly, the area under and around the Rf plate has to be at least as large as the fb to cc opening, and the exhaust opening and length are also critical towards achieving proper air flow.

 

[sankie13]

I'm using Feldon's calculator. I have no problem with a 12 inch firebox which would be just a hair oversized supposedly at 40% the chamber size but I'm afraid it might not be wide enough for the logs I usually get from HEB. I can always chop them in half I suppose, I cut the chamber today at 30", which is plenty big for me, at least 2 brisket on that size chamber. Just concerned about going bigger on the firebox and any drawbacks to that. I hear it can cause issues and I hear it really doesn't matter. Not sure which is true

 

[sankie13]

Ok, got a new question for you. So with firebox that's 20" diameter 14 inches long and a cook chamber 20" diameter 30 inches long I needed a 4 inch smokestack that was 17.5 inches. Instead of putting it in the top of the smoker I put it coming out of the middle of the far side. Now being that it starts 8 inches below the top of the cooker do I need to add 8 inches to the smokestack or does the same 17.5 inch calculation still apply?

 

[kmmamm]

No need to add additional length. If anything I would go with a slightly shorter 6” stack equipped with an adjustable plate baffle on top to choke it down if necessary. (You probably will never need it for cooking but a baffle is also always good to have for storage). I am a fan of larger diameter exhaust tubes as they seem to flow better and are less restrictive. I also highly recommend installing an exhaust plenum to optimize airflow and temps through the cc. Plenums take a little extra time to design, build and install, but the results are always worth the effort.