# Reverse Flow Smoker... How to calculate build tutorial...

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#### daveomak

##### SMF Hall of Fame Pitmaster
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Reverse Flow Smoker... How to calculate build tutorial...
By DaveOmak

1. This is a tutorial and you will need a calculator and paper to note desired dimensions... This tutorial has been proven to make an excellent operating Reverse Flow smoker...

Here's to "Alien BBQ" and the folks that first put the calculator together and "1728 Software Systems" for this great "Circle Calculator" and all the folks at "SmokingMeatForums" , with a special shout out to Ribwizzard, that contributed their ideas and feedback while this thing was coming together..

The smoker and how it's supposed to work...... click on pics to enlarge.....

2. Edit: 2-16-2014 ; You will need a hand held calculator, smart telephone, or your computer.... calculator, to use this tutorial.. It does not crunch numbers for you.....

Calculations for a standard design, reverse flow smoker..

Volume of the Cook Chamber.... Use the Inside Diameter of the tank... All smoker calculations are based on the volume of the Cook Chamber....

Diameter X Diameter X 0.7854 X Length = Volume in cubic inches / 231 = Volume in gallons

Volume in cubic inches X 0.004 = FB/CC opening in square inches

Volume in cubic inches X 0.004 = Area under the RF plate in square inches

Volume in cubic inches X 0.004 = Area required at the end of the RF plate in square inches

Volume in cubic inches X 0.33 = minimum volume of the Fire Box

The above areas are necessary for great air/heat/smoke flow.... using less may cause an overheated FB.... Narrow / Long CC may require an increase in those numbers due to increased surface area friction to volume... Wide/Fat Short CC may use smaller numbers due to reduced surface area friction to volume... edit 8-16-16... small volume patio type smokers will need some increases in measurements also.. FB/CC, under RF plate, end of RF plate... ~20-30%..

Volume in cubic inches X 0.001 = FB air inlets in square inches...

Recommended upper and lower air inlets... Upper air inlet directly across from the FB/CC opening to facilitate moving heat from the FB to the CC, and insuring good air flow through the CC.. The lower air inlet should be situated at or below the fuel grate.... The two air inlets can share the designated square inches of opening.... approx. 20% upper and 80% lower...

Volume in cubic inches X 0.022 (0.017 Old value) = Exhaust Stack Volume in cubic inches, above the CC.... (ESV) .. (The increase in volume ~30% shows a dramatic improvement in equalized temperatures across the cooking surface... edit 6/19/15 )

Exhaust Calculation..

ESV in cubic inches_____________________________ ... = Stack Length in inches (36" +/-)
0.7854 X Stack Diameter X Stack Diameter

Adjust the diameter of the stack, until the proper length is achieved... be sure to measure the actual internal diameter of the pipe used..
this is for round stacks only..

3. This circle calculator is really cool... It does it all.... Just fill in the necessary blanks...
http://www.1728.org/circsect.htm

For calculating the FB/CC opening.... You are calculating for the GREEN area... that will be the cut-out area in the FB that mates to the CC....

The green area is a segment. It is called a "Segment Area"... That area is the same as the FB/CC opening....

4. When you open the "Circle Calculator" link, Click on the "bullet" to the left of Radius & Segment Height ED....

Circle Calculator

Click on the 2 variables you know

......Chord AB & Segment Height ED................Chord AB & Apothem OE
......Segment Height ED & Apothem OE...........Chord AB & Arc AB

You should have opened the circle calculator and a bunch of boxes will appear below..
and a box called [CALCULATE]..

Enter the radius in the box so marked.... If you tank is 24" OD, and has a 3/8" thick steel wall, the ID of the tank 23.25".... the radius is 11.625"..... these numbers need to be accurate if you want stuff to fit... ALSO, take into account the thickness of the FB steel when cutting out the tank...

Enter a guess for the Segment Height ED... I'd start with 6.0

Click on [CALCULATE]....

The other boxes will fill with numbers....

The [Segment Area] box is what you area going to compare with the FB/CC opening in square inches...

If the FB/CC opening number is smaller, change the number in the Segment Height ED box to 5.... continue changing that number until the [Segment Area] matches the FB/CC number.....

Now look at the colored circles above.... Segment height ED is how tall the green area is and corresponds to the area to be cut out.....

Also, [Chord AB] corresponds to the width of the RF plate... NOTE... for ease in fitment, the FB should be at least as wide as the RF plate..

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...

Smoker Calculator reasoning
The concept for the changes is below.... Alien and his associates, did an amazing job putting the original calculator together... 100% of my numbers are based on his original numbers... small changes were made for the improvements due to design changes in smokers.....
The original measurements were taken from SFB smokers with tuning plates I do believe.... The advent of the RF smoker did not make any changes to the calculator... Now the exhaust has to travel twice as far as before, and the original FB/CC opening restricted the air flow from the FB, causing the FB to over heat... Some would get up to 450 deg and higher while the CC wouldn't get over 250.... And then there is the increased friction from the steel.... When using natural draft, any restriction/friction is very detrimental to the flow....
So, on a whim, the FB/CC opening was enlarged, the area under the RF plate was increased and the area at the end of the RF plate was increased... all by 50%.... That solved ALL the problems.... fuel consumption was reduced.... folks were happy....
I personally experienced members improvements and fuel usage by the letters/PMs they wrote me on their improvements...... One additional modification was suggested.... members here, tested a second air inlet to the FB.... directly across from the FB/CC opening... That was a significant improvement of some/most smokers.... If the FB was too hot, it could be cooled by opening the upper air inlet.... Also, the fire had a separate air inlet to adjust it's heat output.... It should be installed at or below the wood fire grate....
All of this has been tested and proven by members building smoker on this forum.... The only place you will find using these numbers... or it used to be... I'm sure many builders are now using these ideas for their builds.... this process has been going for at least 2 years....

Thanks for all your help...... DaveOmak

================== edit 12/24/14

If you are planning a round FB and round CC.... Follow this.....

5. Volume in cubic inches X 0.004 = FB/CC opening in square inches.... the answer is divided by 2.... then that number is solved for in the circle calculator by manipulating the segment height.. when the radius and segment height box is checked... That will give you the lower and upper halves of the football opening... then double the new segment height number and run the calculator... (if originally it was 4", put in 8").... that's the width of the football, AND the new height of the RF plate... and if you look at the chord AB number, when the 8" segment height was crunched, that is the width of the RF plate....

Last edited:
If I apply the 150% recommendation I will need more than half of my 29 inch ID pipe. Can you please help me on this one in my threat?

Use the tutorial above and you will be OK.... Also read the whole thing.....

" Wide/Fat Short CC may use smaller numbers due to reduced surface area friction to volume.." That applies to your ~500 gallon tank also....
You can insert the FB into the CC to increase efficiency of air draw.. less friction....

Remy Bernabela
That applies to your ~500 gallon tank also....

Golden lead. I was working with the volume above the RF plate. I used the tank volume instead in Feldons' and it all makes sense now. Numbers are looking great!!! Thank you a lot.

Can someone please check my homework.....outside diameter is 30x90 long

tank is 1/4 thick

If the fire grate is raised to accommodate airflow and an ash pan does the recommended firebox volume include the entire firebox or just the area above the grate?

Thanks

Lance

The firebox dimensions are adequate to accommodate ash space etc... generally, folks have ~4" below the grate... some even install a sliding door for the ash to dump... a ~2" hole with a 4x4" plate to cover is adequate... guides for the 4x4" plate aren't required... just a loose plate works..

Don't use Feldon's... Use the tutorial we developed on this forum....

Thanks, Dave.

Hi guys,
I'm new here from Australia and in the process of building my first RF.
I've had a read through this post but don't really understand how it all works (sorry).
Any chance one of you top blokes could help me work out what size I need everything?
I already have my CC and it measures 1250mm x 750mm internal or 49.2" x 29.5" for you americans :)
What size should I be making my FB, Flue & FB/CC opening?
I'm looking at making the FB from 6mm or 1/4" plate and was hoping to make it no wider than 700mm or 27.5" so it sits well on the trailer.

Follow the tutorial... calculate 1 (one) step at a time... write the answer down and go to the next step....
You should have a calculator on your computer or phone that works well...
When you get to a step that you need help with, we are here...

Volume in cubic inches X 0.022 0.017 = Exhaust Stack Volume in cubic inches, above the CC.... (ESV) .. (The increase in volume ~30% shows a dramatic improvement in equalized temperatures across the cooking surface.

This part is confusing to me. Volume x.022 to volume x .017 ??? Is this giving a range? Other question is my CC is going to be a different size and shape that my FB. The CC is going to be a 100lb propane tank cut in half then add approximate 10" of plate in between the two halves to make a round rectangle type of shape. The FB is just going to be the rounded tank. Is this acceptable? Anything I should do different from these instructions? Or should I add the same amount of metal to the FB so its the same shape as the CC? Thanks for the guide!

If there is a lot of interest I can whip up a website to do the calculations... Shouldnt take but a night or two

If there is a lot of interest I can whip up a website to do the calculations... Shouldnt take but a night or two

Sounds interesting

This is what I have the site looking like so far...

That was the easy part... Now I have to get the circle calculator working

Last edited:
tobycat
Well... Better yet... Should I keep working on it?

Well... Better yet... Should I keep working on it?

Since you have it started you should keep it going.

You might want to start a new thread with it though so you aren’t stepping on anyone’s toes. Some folks are easier offended than others.

If you start another thread let me know do I can keep an eye on the progress.

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