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

AllTuckerdOut

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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.

I can start a new post - I didn't to take any traffic away from daveomak daveomak post since I am using his calculations. He did all the hard work!

But if the correct thing is to start a new post, I can do that. I didn't mean to step on anyone's toes
 

daveomak

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2019-05-03.png

Chord AB is the width of the RF plate... Segment Area should be close to Area under the RF plate etc, for good uniform flow... Segment height and area are the green area...
 

jlc191

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After running the circle calculations i am a bit stumped. How do the numbers get affected with having two different sizes of FB/CC? My 500 gal tank CC is 37” ID and 250 gal FB is 29.5”. How do i adjust for the different sizes? I would think that this would cause my RF plate to need to be wider/higher. Any help is greatly appreciated.
 

daveomak

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You can't make a FB/CC opening bigger than the 29.5" FB will allow...
Is your CC being made from a full 500 gallon tank ?? Or, are you cutting it down to a smaller CC ??
Do you have a build thread going ??
 

jlc191

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You can't make a FB/CC opening bigger than the 29.5" FB will allow...
Is your CC being made from a full 500 gallon tank ?? Or, are you cutting it down to a smaller CC ??
Do you have a build thread going ??
No thread going yet but plan on it. It is a full 500 gal tank with a 250 gal cut in half. This may be the decision to turn this into a standard offset if i can’t get the numbers to work for an RF.
 

daveomak

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Using a 500 gal. tank, you will need to build a square/rectangle FB to get a large enough FB/CC opening... Let me know the thread you start... PM me with that info so I can help out...
 

smokin peachey

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jlc191

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Sounds like a good build. I have done the same. Have you watched franklin’s video?
Here is a link to my build.
https://www.smokingmeatforums.com/threads/500-gallon-tank-build.250565/
daveomak daveomak i will let you know the thread and reach out with any questions. This ship has sailed so i May just finish with enthusiasm!!

smokin peachey smokin peachey i have read your post multiple times and it has been helpful. My theory up to today was based on franklins 1000g CC with half a 250g FB works for him so surely a 500gal tank would work just as well. Just thinking of changing my strategy from RF if it absolutely won’t cook.
 

Sharahso

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Should i follow the instructions above since it has been edited or add another 50% on the calculations here? Should i also do exactly the measurements of the fb/cc openings or can i add a little more and just put adjustments on the opening of fb?:) thank you!

>
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.....


    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 = 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..





    This circle calculator is really cool... It does it all.... Just fill in the necessary blanks...
    ......click on this link......
    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....

    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

    ......Radius and Central Angle...........................Radius & Chord AB
    [X] Radius & Segment Height ED.....................Radius & Apothem OE
    ......................................................Radius & Arc AB
    ......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.....

    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....
 

daveomak

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The build tutorial is up to date... Follow it......
 
Last edited:

AllTuckerdOut

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daveomak daveomak would these calculations work for an offset as well? I want to see about modifying my Oklahoma Joe to get some more airflow. If I use your equations, my exhaust pipe is about 20" too short haha
 

daveomak

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daveomak daveomak would these calculations work for an offset as well? I want to see about modifying my Oklahoma Joe to get some more airflow. If I use your equations, my exhaust pipe is about 20" too short haha
Yes... It works for an offset.... Too short a stack reduces the draw through the smoker... May cause an overheated fire box and uneven temps across the smoker..
 

Ajacobs52

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I have a reverse flow pit that we built several years ago. It runs pretty evenly but I can’t run the temp over about 225 and the wood consumption is pretty high. I am working on it right now to insulate the FB and to change up the intake vents. The FB size is about 10% smaller than what the calculations recommend but I hope that insulation will help to overcome that and put more BTU’s in the CC where they need to be. The main question I have is in regards to intake and exhaust. Right now, we have adjustable intake vents (all on the bottom, gonna put some up high) and a damper on the exhaust. I know a lot of folks are using wide open exhausts these days. Are the calculations in this tutorial based on using an open exhaust and controlling temp through the intake only or do these builds control temp through exhaust as well? I appreciate all of the info on this forum and look forward to any comments or suggestions that you folks may have. Thanks!

Alan
 
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daveomak

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Ajacobs52, morning...
First put in an upper air inlet... Use it to move the heat from the FB to the CC....
Your Fire Box probably runs pretty hot and you are wasting BTU's and firewood....
You RB should probably run about 50F above your CC temp when running correctly....
Skip the exhaust damper... It might be impeding the exhaust flow and screwing up your CC temperatures... I would remove it....

1579616155307.jpeg
 

Trus

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Joined Feb 11, 2020
Hi, Initially I was going with a square box design but now i just picked up a large old propane tank for a good deal. It measured 88" x 32" I would like to know if im starting this calculation off correctly. Please correct me if im wrong.,

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 Gallons = (32x32x0.7854x88) / 231 = 70773.9648cu in / 231 = 306.3808 gal
FB/CC opening in square inches = (70773.9648 x 0.004) = 283.0958 sq in

Please let me know if its correct before i move on to the rest of the calculations.

Thank you.
 

Trus

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Volume in Gallons = (32x32x0.7854x88) / 231 = 70773.9648cu in / 231 = 306.3808 gal
FB/CC opening in square inches = (70773.9648 x 0.004) = 283.0958 sq in
Area under the RF plate in square inches = (70773.9648 x 0.004) = 283.0958 sq in
Area required at the end of the RF plate in square inches = (70773.9648 x 0.004) = 283.0958 sq in
minimum volume of the Fire Box = (70773.9648 x 0.33) = 23355.408 cu in

FB air inlets in square inches = (70773.9648 x 0.001) = 70.77 sq in
ESV = (70773.9648 x 0.022) = 1557.0272 cu in
Stack Length = {1557.0272 / (0.7854 x 8 x 8)} = 30.976 in

Okay so heres the rest of the the calculations. Bare with me since im not too familiar with cu in and sq in and such.
With the fire box calculations says 23355.408 cu in minimal. Would that be 23355.408 / 3 = 7785.14 in so would this be my WxLxH for my fire box? im confuse with that caclulation and how to convert the cu in into WxLxH that i need to build my fire box.
 

daveomak

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WxLxH = cubic whatever.... inches, feet, miles.... etc..
Make the FB wider than the CC and cut it into the tank so it's past the weld.
Makes fit up easier...
Weld seam seen the the inside of the tank....
Propane Tank weld joint INSIDE.jpg
..
 

Trus

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WxLxH = cubic whatever.... inches, feet, miles.... etc..
Make the FB wider than the CC and cut it into the tank so it's past the weld.
Makes fit up easier...
Weld seam seen the the inside of the tank....
View attachment 432156 ..
Dave, FB 23355.408 cu in, If I made a FB 26"x26"x36" (26x26x36 =24336) would this be suffient?
FB/CC Opening 283.0958 sq in , would 14"x20" (280sq in) be suffient?
 

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