Fire box to cook chamber opening

[The Czar]

You'll have to forgive OB1 .....he has been under an incredible time delay ....he will be around soon to answer ....or just call him...

[lostbuckaroo]

No worries found this on another site posted by a gentleman named DaveOmak - this was very helpfull

FB to CC opening
What is the diameter of our tank ??? Use the circle calculator below for your specific diameter....
Click on( O Radius and Segment Height) adjust segment height ( D ) until the "segment area" = 64" sq. inches...
That will give a number based on your specific tank design.....
The reason I recommend this route.... I do not know where they get the "half moon" diameter numbers and how they could know the diameter of your tank....
Dave

examples: 16"D ++ 8 r and 5.7 seg. height = 64.2 sq in.
20"D ++ 10 r and 5.2 seg. height = 64.9 sq. in.

http://www.1728.org/circsect.htm

The RF Plate
Once you have that number, I recommend the area under the RF plate to be 1.5X the area of the FB to CC opening.... Use the same calculator to figure that number.... The increased area is designed to reduce friction loss in air/smoke flow under the plate... and the opening at the end of the RF plate should be the same or larger area (96 sq. in.) because the air/smoke has to take a 90 degree turn at that point.... Those considerations will make your smoker the best it can be.... IMHO... Dave

Example: 20"D tank... 10 r and 6.9 seg. height = 96 sq. in.
Set the RF plate 6.9 in. above the bottom of the tank.....