# Building offset, question about smoke stack size

#### jdoyle112

##### Newbie
Hello everyone. I'm about to begin building a custom offset and I'm just trying to determine what the length of my smoke stack should be. Feltons calculator says about 22" but that seems really small.. just wanted to get some second opinions. People seem to think you can go a bit longer than Feltons suggested length so I was thinking 32". Let me know what ya'll would recommend. I was thinking 4" diameter for the stack by the way.

Cook chamber - standard cylinder 20" (D) x 42" (L)
Firebox - square 16" (W) x 16" (H) x 20" (D)

Let me know if I can provide any additional information. Thanks!

#### Brokenhandle

##### Master of the Pit
I'm sure someone more knowledgeable than me will pipe in, but in my opinion it would be easier to shorten your stack if it's too long than it would be to lengthen it if it's too short.

Ryan

#### Brokenhandle

##### Master of the Pit
Welcome from Iowa! Good luck on your build, and remember to take pics so we can see your progress!

Ryan

#### JC in GB

##### Master of the Pit
SMF Premier Member
Hello everyone. I'm about to begin building a custom offset and I'm just trying to determine what the length of my smoke stack should be. Feltons calculator says about 22" but that seems really small.. just wanted to get some second opinions. People seem to think you can go a bit longer than Feltons suggested length so I was thinking 32". Let me know what ya'll would recommend. I was thinking 4" diameter for the stack by the way.

Cook chamber - standard cylinder 20" (D) x 42" (L)
Firebox - square 16" (W) x 16" (H) x 20" (D)

Let me know if I can provide any additional information. Thanks!
Here are the calculations that I used for my design. Haven't started the build as COVID has delayed my ability to get into the workshop.

I got these from daveomak

JC

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....

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.....

FB-CC relationship round to round.jpg

daveomak

Dec 24, 2014

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: Apr 26, 2020​

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#### jdoyle112

##### Newbie
Here are the calculations that I used for my design. Haven't started the build as COVID has delayed my ability to get into the workshop.

I got these from daveomak

JC

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....

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.....

FB-CC relationship round to round.jpg

daveomak

Dec 24, 2014

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: Apr 26, 2020​
Wow thanks so much for that info! That's really helpful

#### civilsmoker

##### Master of the Pit
OTBS Member
daveomak data is the real deal based on actual builds so it is a very good guide. The only additional comment I have is that its really easy to reduce your fire box intake opening by sliding a gate but its impossible to open up the exhaust stack if it is to small (diameter or length). Having a little more diameter and length for the stack on a scratch build would be my recommended. It won't degrade the function at all.

Last edited: