Minnesota Reverse Flow Smoker Build

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Original poster
Jun 25, 2015
Hello all, 

I have gotten a fuel barrel offered to me for a reverse flow smoker build and found this site with lots of input and information. Below is the rough information for the barrel in the Calculator. Let me know your input. Hope to have pictures soon. 

Calculations for a standard design, reverse flow smoker

38” D x 60” L Fuel Barrel

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...Outside Dimensions Currently Used**

Diameter 38 X Diameter  38 X 0.7854 X Length 60 = 68,047.06 Volume in cubic inches

68,047.06 Volume in cubic inches X 0.004 = 272.19 FB/CC opening in square inches

68,047.06 Volume in cubic inches X 0.004 = 272.19 Area under the RF plate in square inches

68,047.06 Volume in cubic inches X 0.004 = 272.19  Area required at the end of the RF plate in square inches

68,047.06 Volume in cubic inches X 0.33 = 22,455.53 minimum volume of the Fire Box

            * Does this firebox look like a good size? * 

                          26" H x 36" W x 24" L 

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~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...

68,047.06 Volume in cubic inches X 0.001 = 68.05 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....

68,047.06 Volume in cubic inches X 0.022  0.017  = 1,497.04 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      

0.7854 X Stack Diameter X Stack Diameter

     1,497.04    = 52.96”      OR      1,497.04  =  29.78” Stack Length in inches (36" +/-)
.7854 x 6 x 6                           .7854 x 8 x 8

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..

Circle Calculator:
......click on this link......

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....

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..

Radius: 19 

Segment Height: 11

Chord AB: 34.467

Segment Area: 272.3

Thanks in advance for the input and advice. 
Thanks Dave.

What gauge metal is recommended for the fire box as well as the RF plate? 
I need to check the barrel gauge yet. I currently only have outside dimensions. I was thinking 1/4" which I can get from work. Thanks for the response. 
Quick question. If my RF plate is 34.5" wide the opening at the opposite end of the firebox should be approximately 34.5"wide x 8"deep? Would this be sufficient? 
Quick question. If my RF plate is 34.5" wide the opening at the opposite end of the firebox should be approximately 34.5"wide x 8"deep? Would this be sufficient? 
Yes, that is sufficient.

Area at the end of the RF plate is at least as big as the FB/CC opening.

Your math looks good here.
Thanks for the reply SmokeJumper. This is my first build so just wanted to confirm i was on track. Hoping to get some pictures uploaded soon. 
Well I didnt get pics during the build. Too busy getting it done but here is the finished product. New wheel/tire combo will go on when new fenders arrive with inside skirts to deflect heat. Did one run so far on some pork butts and maintained 225° quite easily.  

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