Smoker Dimension Calculation

[Cheese Burger]

Hello Everyone

I've used Dave's calculation to determine the dimension of an offset smoker and I was wondering if you're able to quickly check the figures to make sure I've calculated them correctly. In particular the exhaust calculation, as it seems like there are two different formulas?

The dimension of the cooking chamber will be a pipe of 24inch in diameter and 51inch in length. I'm thinking of a 6inch diameter smoke stack.

Volume in cubic inches –
24 x 24 x 0.7854 x 51 = 23,072 cubic inch

FB/CC opening in square inches –
23,072 x 0.004 = 92.2 square inch

Minimum volume of firebox –
23,072 x 0.33 = 7,614 cubic inch

Area of Firebox air inlet –
23,072 x 0.001 = 23 square inch

Exhaust Stack Volume (ESV) –
23,072 x 0.022 = 508 cubic inch

Length of exhaust stack –
ESV divided by (0.7854 x stack diameter x stack diameter) ???
Therefore it'll be 508 divided by (0.7854 x 6 x 6) = 18 inch

Or is it ESV divided by ( 0.7854 x 36 ) = answer squared?

Thank you in advance.

 

[smokin peachey]

daveomak should be around soon he is probably getting his coffee brewed and will be around a little later. I look forward to seeing pics of your build.

 

[Cheese Burger]

I don't have much to contribute to the forum at the moment. But I'm planning to add photos of my build when I get started and hopefully have more to add to discussions down the track.

 

[daveomak]

I haven't got that too clear, do I.....
The calculation for the volume of a pipe is....
D" x D"x 0.7854 x L" = volume in cubic inches....
Sooooooooo,
Volume in cubic inches / D" x D" x0.7854" = Length"

508 / 4 x 4 x 0,7854 = ~35" is the number we are looking for....
This calculation above comes up ~40" ... Since a little bigger or taller is better than too small or too short, let's use a 4" diameter stack 35" tall above the CC.....

5" stack... 508 / 25 x 0.7854 = 25" tall.... a bit too much short.... A 35" tall stack might "suck" too much air...

 

[Cheese Burger]

Hello Dave

Thank you for clearing that up. Correct me if I interpreted it incorrectly, but all the figures are correct, except for the dimension of the stack?

So, you'd recommend a 4" diameter stack with a length of 35", from the TOP of the CC?

Thank you.

 

[Cheese Burger]

After going back though the post, I think I've figured out what Dave is trying to say in regards to the size and length of the smoke stack.

A 35" smoke stack (above cooking chamber) is the "ideal" height. Plus or minus. We then adjust the diameter of the stack to get near the Volume in cubic inches of the smoke stack that you've calculated.

So in my case, I worked out that the ideal volume of my smoke stack should be around 508 cubic inches. And a 4.3inch diameter stack will achieve that. But practically, it'll make sense to would go with a 4.5inch diameter stack at 35" above the cooking chamber.

Is this correct?

 

[Cheese Burger]

Should the opening from your cooking chamber to the plenum also be 4.5 inch in diameter? Or do I go bigger?

 

[daveomak]

Make the plenum wide and tall to collect gasses in the CC from a large area to exit the stack....

 

[Cheese Burger]

Would you advise placing the centre of the plenum at the midpoint of the cooking chamber, where the grate would be located? Or slightly higher?

 

[daveomak]

If you have 1 food grate, at the food grate.. If you have 2, between the 2 grates...

 

[Cheese Burger]

Thank you