CC to FB calculations are beyond me.

[12ring]

I wasn't good at geometry and I'm obviously not good at following directions because I can't seem to figure out the calculations from the tutorial.

Right now I have a 40" long piece of pipe that is 18.5" inside diameter for my cook chamber. Then I have a 19" inside diameter pipe to use as my FB. Right now it is 23" long. I would prefer to cut it down some if that's ok. I was hoping for it to be 18"-20" long at most.

I probably sound like that kid in class that just wants someone to give him the answers but when I tried the calculations I only got so far. I came up with 1075 cu in for my cook chamber but after that I was lost.

Can someone tell me how big the opening between the CC and FB needs to be, how big my fire box needs to be and how big of diameter and lengths my smoke stack needs to be? I would really appreciate it.

 

[shyzabrau]

Are you using a reverse flow plate?

You are off by a factor of ten on your CC volume. It is 10,752 cubic inches.

You want a firebox that is 1/3 of the volume of your CC. That would be 3,584 cubic inches. That is a length of 12.64" (inside length).

For the opening between the two, Fendon calculates 28.7 sq. inches. Since you have two round pipes, will you cut out a curved hole on each, like the picture below?

Lens.gif

• shyzabrau
• May 24, 2017

What will you use for your chimney stack? The diameter determines the length.

 

[shyzabrau]

Solving the area in the overlapping circles is a non-trivial problem.

(See this write-up for the calculations!)

I didn't want to go through the exercise to solve for the distance between the centers, so I plugged the numbers into the equations and played with the distance until I got the opening that was required. I came up with 15", which gives an opening of 28.74 square inches.

That would look something like this:

Overlapping.jpg

• shyzabrau
• May 24, 2017

The distance of 15" refers to the distance between the two centers, as indicated by the line with the arrows at each end.

If you plan on reverse flow, the RF plate would need to be above the opening.

Edited once I realized that I used sin instead of arcsin...

 

[kam59]

This will work.

Link to BBQ Pit Calculator

Then use this to calculate your opening.

http://planetcalc.com/5155/

 

[shyzabrau]

 
Then use this to calculate your opening.

http://planetcalc.com/5155/

That was much faster than finding the equations and putting them into Excel...

And, the dam is an excellent and obvious aspect that I didn't consider...

 

[12ring]

Thank you both VERY much. I was beating myself up over why I couldn't figure it out. You guys laid it out there for me.
I appreciate that.

I should have mentioned from the beginning that I was doing a direct flow smoker. This is what I'm starting out with.
I will keep you guys updated.

IMG_2879.JPG

• 12ring
• May 24, 2017

 

[kam59]

 
That was much faster than finding the equations and putting them into Excel...

And, the dam is an excellent and obvious aspect that I didn't consider...

Thank you Shyzabrau. I have been putting grease bridges in my cookers for years. My opinion is that it is not a matter of if but when you will have a grease fire because the grease found its way to your firebox.