# In need of a little help with the FB/CC Opening of my build

#### Horst92

##### Newbie
Hey guys,
my first post here... greetings from Germany,
just registered because I couldn´t exactly figure one main thing out in daveomak great tutorial for building a RF smoker.
First off, you guys did amazing work with that guide... Has really helped me so far. This is where I encountered a problem:

I´m building a reverse flow smoker with a round CC and a round FB. However, the CC and the FB have different diameters.
I couldn´t figure out how to calculate the Football opening between FB and CC.
These are my dimensions:
CC inner diameter: 19.52 inches
CC length: 39.37 inches
FB inner diameter: 15.59 inches
FB length: 23.62 inches

In an update to his post, Dave said something about different diameters:

"EDIT..2/13/17... A situation recently arose where the round FB was of different diameter than the CC.... In that event, it is imperative to use the radius of the CC to calculate the initial CC/FB opening.. THEN use the radius of the FB to calculate the FB/CC opening for the football dimensions... reason .. the football will NOT fit into the FB because of the larger diameter of the CC..."

This is the part I don´t understand.
Can anyone help me with that?
Thank you guys!

PS: I also attached a picture of the current state of our build:

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
Calculate the area needed for the FB/CC opening......
Using the radius of the CC, figure out the necessary opening in sq.in.... Using 1/2 those sq. in. and the radius of the FB, figure out ED..

When calculating for, and with a FB that is smaller radius, use the FB radius for the football opening...

If the total height of the football interferes with the placement of the RF plate, increase ED to get more sq. in. opening and remove a top couple inches of the football..

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
Or, cut off the top of the FB and elevate same to increase the opening....

The drawing below has overlapping thoughts/drawings.... try not to get confused...

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
Below is a design that Ahumidora sells, in Argentina....

#### Horst92

##### Newbie
Hey Dave,
thank you for the quick reply!
I just put in all the numbers and came up with the following:

FB/CC opening in square inches: 11,7819

If I put half of that into the calculator and try to guess ED, I end up with 1.09 inches.
Does that mean ED for the whole Football would be 2,18 inches?

Feels kind of small. However, you probably have much more experience with these numbers. Does that ED sound reasonable?

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
Diameter X Diameter X 0.7854 X Length = Volume in cubic inches
Volume in cubic inches X 0.004 = FB/CC opening in square inches

19.5x19.5x0.7854x39.4x.004 = 47sq. in. FB to CC opening

47/2= 23.5 sq. in. for 1/2 the football

19.5 / 2 = 9.75 radius of the CC

ED for 1/2 the football is 2 3/4".... returns 25.5 segment area...
When you add the 2 halves you get 5.5" "ED" which returns ~51 sq. in. for the FB/CC opening ...

.... bigger is better...

#### Horst92

##### Newbie
Thank you Dave,
that makes a ton of sense.. I adjusted everything and am now going forward with the build!