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

Horst92

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Hey guys,
my first post here... greetings from Germany,
just registered because I couldn´t exactly figure one main thing out in daveomak 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:
IMG_20200426_120859.jpg
 

daveomak

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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..
5f7 2.jpg 1 A Football opening 2.jpg

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

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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...

24 build flat top.jpg
 

daveomak

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Below is a design that Ahumidora sells, in Argentina....


ahumadora  flat top FB.jpg
 

Horst92

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Joined Apr 26, 2020
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

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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


2020-04-26.png


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

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Joined Apr 26, 2020
Thank you Dave,
that makes a ton of sense.. I adjusted everything and am now going forward with the build!
 

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