Octagon RF Build

[DDave]

I have been asked by the local high school shop teacher to to the calculations and draw plans for a reverse flow smoker for one of his night school students. I have only done that for two so far. The one that they built for me (which I am pretty happy with) and another one for the metal shop which is still in the works.

But this student wants to build one with an octagon cook chamber and (thankfully) a square firebox. So I found an octagon volume calculator and put some numbers in the Pit Calc to come up with the appropriate numbers.

Below is a rough drawing of the end view. The resolution on the dimensions are a bit high but the shop does have a CNC plasma cutter.



Not having a lot of experience building pits, I have several questions and concerns.

Does the FB to CC opening look to small? The area according to the Pit Calc should be 29.79 square inches and the area in the drawing is 30.34 square inches so it calculates fine but it just looks small. Should it maybe be not as wide and taller? An 8" x 3.75" opening would equal 30 square inches.

I'm not sure where he wants to put the bottom rack. Obviously on an octagon the widest point can be below the center but if it is too low, assuming the RF plate is 3" below that, I'm concerned that will really restrict airflow. What should the minimum distance be from the bottom of the CC to the baffle plate?

Also is the baffle plate generally even with the top of the firebox? I seem to remember being told that when I designed mine and that's the way it was built and seems to work fine but I just want to confirm.

Thanks for your help.

Dave

[Big T]

The shape does look tight but the calculator is based off of volume so it shouldn't be an issue. I would work out where I wanted the racks and then tweak it from there to get the opening as tall as possible within reason. The BP really only has to be as high as the top of the throat but most guys raise them an inch or so and put a shadow plate below the BP for the first 12'' or so of the CC length. That is not a must have but it helps avoid a hot spot at the throat. The top of the FB can be the same height as the BP but the main concern is that the throat needs to be at the highest point of the FB so that the heat can flow out as smooth as possible.

[Dirtytires]

It’s only a 20 inch chamber. I would count on only 2 racks but would want them as deep aspossible.. My bottom rack is about 3-4 inches above the baffle plate.

[DDave]

I would work out where I wanted the racks and then tweak it from there to get the opening as tall as possible within reason.

Would something like this be a better option?



I guess my question would be is taller better or is wider better?

[Big T]

It should work either way but I would personally go with the taller opening rather than the narrow wider option. Normally I like a wider throat but 2.6'' just seems a little tight to me. Just be sure that there's not a lip on the top of the throat where it exits the FB, it needs to be a smooth transition.

[DDave]

Just be sure that there's not a lip on the top of the throat where it exits the FB, it needs to be a smooth transition.

So something along these lines?

[mp4]

yup - that looks good

[Big T]