# firebox to cooking chamber opening

Discussion in 'Reverse Flow' started by ribrustler67, Jul 1, 2013.

1. ### ribrustler67Fire Starter

hi all,been awhile since last posting,but my brother and I finally got a start on our reverse flow smoker,i have one problem maybe someone has a simple way to calculate,what is the best or easiest way to measure  the firebox opening to cooking chamber,i know its suppose to be 16 inches according to the calculator,but whats the easiest way to measure,im at 100% on my firebox size,i cant afford a mistake here,any help would be appreciated,its gonna be a half moon shape,26" pipe for both the cooking and firebox,right now I know its too big of an opening,would a paper templet work?thanks

2. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

ribrustler, evening....     Using the circle calculator in my signature line.........   Circle Calc

Click on the Radius and Segment Height ED portion of the calculator.......

Enter the radius of the cook chamber where called for...........

Adjust the Segment Height, in the box, until the (green area) segment area matches what the calculator says for the FB/CC opening...

That gives the largest and best opening for the build.....

Then adjust the Segment Height until the segment area is 1.5 times the FB/CC opening.......  that is the area under the RF plate needed for adequate air/heat flow for a great smoker.....

When you have that number, the Chord AB is the proper width for the RF plate to meet that area....

Using that calculator will give you the best air flow/heat flow available for building a great smoker.....

Dave

If you need help, I am here, or somewhere close...

Last edited: Jul 1, 2013
3. ### ribrustler67Fire Starter

thanks dave,i appreciate the help,if I have everything correct,26"x72" round cc and 26"x25" fb, for a half moon opening,it looks like it needs to be about a 9" opening,with a segment area of 175.5 and the 24" width rf plate is really close,does this sound correct to you? thanks again

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4. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

CC volume  =   38207 cu. in. actual

FB volume  =    12736 cu. in. recommended

Chimney volume  =  .05 x 12736  =  636 cu. in.  ( 5" ID stack X 32" tall above the CC )

FB to CC recommended opening  =    .008  X  12736  =  102 Sq. in.  ( 13" r x 6.5" height )

RF plate area     102  X  1.5  =    153 sq. in.  ( 13" r x 8.5" height )

Circle Calc.      radius 13"    Seg. Hgt. of 6.5"   =    Seg. Area 103.8 Sq. in.

RF Hgt. calc.    radius 13"   Seg. Hgt. of   8.5"  =  Seg. Area  150.85 Sq. in.

Chord AB is 24.39"  that is the width of the RF plate..  check the inside diameter of the tank.... re run the numbers... especially before cutting the RF plate.... it is based on a 26" inside diameter CC..

I use the recommended sizes, calculated from the CC size, for all calculations except the FB.....  The FB should not affect other dimensions.... Still the same amount of heat flow regardless of how big the FB is....

That's what I came up with....

Dave

5. ### ribrustler67Fire Starter

that's pretty close dave,i allowed 1.5 times the recommended,which I thought was a lot,if I backed the numbers down alittle bit,its close to what you have,thanks a lot for your help danny