# Standard Reverse Flow Smoker Calculator... by DaveOmak and others... Ready to use.. rev5.. 6/19/15.

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
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..

Gallons X 231 = volume in cubic inches.... Sometimes the tag, in gallons, reflects a 20% air space for expansion....

Volume of the Cook Chamber.... Use the Inside Diameter of the tank...

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

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...

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

Recommended 20% upper and 80% 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....

Volume in cubic inches X 0.022 = 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

ESV in c.i. = D2 ... Diameter of the stack squared...
0.7854 x 36
10/3/2019 edit

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...
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

......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...

When calculating for, and with a FB that is smaller radius, use the FB radius for the football opening...

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..

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....

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#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
Evening folks...... Please try this calculator out to help find errors..... This thing was a PITA to put together.... cut and paste... and other stuff I don't know about...
DO NOT COPY until we have all the errors straightened away..... then have at it...

HAVE AT IT !!!!! any problems... I'm here.....

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#### lendecatural

##### Smoking Fanatic
Looks Great Dave!

I just ran the numbers both ways based on tank volume in gallons and by dimensions against Feldon's @ 100% firebox with the 1.5 factor and the differences were in the noise. It's ready for prime time viewing and use.

edit:
ESV in cubic inches / 0.7854 X (Diameter of the stack)2
You may want to write this one like this so some may not get confused and multiply by two instead of using the square.

ESV

0.7854 X Stack Diameter X Stack Diameter

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#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
Good idea.... I'm trying to do this on WordPad and it's a PITA.....

DONE..... Actually a great idea...... Thank you....

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
Looks Great Dave!

I just ran the numbers both ways based on tank volume in gallons and by dimensions against Feldon's @ 100% firebox with the 1.5 factor and the differences were in the noise. It's ready for prime time viewing and use.

When I was converting Feldon's to the Cooking Chamber volume base, I took a few liberties to EASIFY the calculator... Rounding etc... I didn't have too many concerns within +/- 5%...

#### gary s

##### SMF Hall of Fame Pitmaster
OTBS Member
Hey Dave, Nice,  I have been working on one similar just a fill in the blank ant it will calculate  everything for you, been working on it off and on for about six months, I sent an early version to a couple of members to see what they thought  and work out the rest of the bugs.

Let me pull up what I have and maybe we can fine tune them together,

Gary

#### gary s

##### SMF Hall of Fame Pitmaster
OTBS Member
Site keeps telling me "this type file not allowed" when I try to download it    I use Excel Spreadsheet ????

Gary   .........

#### smokingunny

##### Smoking Fanatic
I have a math type program that I have to use for my math class. It has all of the symbols and formulas and crap. If you want to send me the formulas I can put them in the program and post.

#### smokingunny

##### Smoking Fanatic
And thank you for this post, a lot of my questions have been answered.

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
I have a math type program that I have to use for my math class. It has all of the symbols and formulas and crap. If you want to send me the formulas I can put them in the program and post.

All the formulas are in the original post.... volume in cubic inches X 0.004 = etc....

Volume of the Cook Chamber... dia x dia x 0.7854 x length = volume in cubic inches.. etc.....

And the circle calculator has a link as to the original calculations... be sure to acknowledge 1728 Software Systems in your calculator and Alien BBQ...

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
OK FOLKS...... Is it easy to understand now.... what changes need to be made.... I can do this stuff in my sleep but I'm weird... I want everyone to be able to do the math......

#### gary s

##### SMF Hall of Fame Pitmaster
OTBS Member
Below is the link to the calculator I've been working on for several months

The reason I started this project is two fold. I enjoy helping people, and seeing Dave continually answer the same questions

Dave was the inspiration behind this. When I first found this site I had already Fabricated my first RF, I ended up doing some

modifications which improved the efficiency of my smoker, Thanks to Dave.

I think this is pretty close, but probably needs some tweaking.  Hoping Dave will tell me what I need to do to make this thing

the best it can be.

Remember  a work in progress feel free to comment

Thanks

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#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
Gary.... evening..... have at it... I can't open the file... My computer is bare bones....

#### lendecatural

##### Smoking Fanatic
I  have to pay the Bill Gates tax too, I have the older copy of Office, it opens .xls but not xlsx. Anything to keep us updating...... I think you can save it back down to an earlier file type and more people can give it a look.

#### gary s

##### SMF Hall of Fame Pitmaster
OTBS Member
I'll see if I can change it to an earlier version,  I know it still needs work, If you do get it open maybe you can help me finish it up

Gary

#### lendecatural

##### Smoking Fanatic
I'll be more than happy to help!

OTBS Member
Another format

#### daveomak

##### SMF Hall of Fame Pitmaster
OTBS Member
SMF Premier Member
Gary.... that comes up with lines of programming.... Do you have it in a "form" where you "fill in the blanks"....

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#### wolfman1955

##### Master of the Pit
Below is the link to the calculator I've been working on for several months

The reason I started this project is two fold. I enjoy helping people, and seeing Dave continually answer the same questions

Dave was the inspiration behind this. When I first found this site I had already Fabricated my first RF, I ended up doing some

modifications which improved the efficiency of my smoker, Thanks to Dave.

I think this is pretty close, but probably needs some tweaking.  Hoping Dave will tell me what I need to do to make this thing

the best it can be.

Remember  a work in progress feel free to comment

Thanks
This file opens fine in the newer Excell You need to enable edit to enter numbers in the cells.

#### wolfman1955

##### Master of the Pit
Another format
This file opens in the older Excell only it is in protected mode and requires a password to enable editing to put numbers in the cells.