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

Discussion in 'Reverse Flow' started by daveomak, Nov 15, 2014.

1. ### novasbcSmoke Blower

Thanks, that helps a lot!  I guess it would be helpful to include in the description what the baseline is, that you need to watch for a 15% deviation.  I went and measured my father-in-law's tank, and while it was different than mine (my ends are much closer to flat than his), the diameter of the tank was less, the cylinder section of it was almost identical in length, so the difference is made up in the end caps.

Last edited: Dec 23, 2014
2. ### novasbcSmoke Blower

By the way, if it would be better for me to start a new thread rather than adding to this one, let me know, I don't mind.  I figured I'd discuss it here because this is all based on the calculations in post #1.

3. ### lendecaturalSmoking Fanatic

Mark, you are good posting here about calculations and development of a tool within which they can be simply executed. The questions you raised may well be on someone's mind when they start the design process and are things to ponder when pen goes to paper. I think it was Rib that said to draw it, have a beer or your favorite beverage, and think about how it should work and if it will fit your needs as one of the most valuable steps in the process of creating a smoker.

Since I came from an actual rocket science background, paralysis from analysis is a real problem in some cases; however, taking advice from a wise sage like Dave will save time and money.

Last edited: Dec 23, 2014
4. ### smokingunnyMeat Mopper

I have to say that using these formulas, plus input and guidance from Dave, Len and Rib, you can't go wrong if you do the math correctly. Considering I didn't know what I was doing and what I had to work with in materials and budget, I'm happy mine turned out as good as it did.

5. ### novasbcSmoke Blower

It's easy to overanalyze, especially for me, I like to do things as right as possible the first time.  However, this is a lot of firsts for me, first pit to build, first time welding, cutting metal, etc.  I ended up just buying the metal to finish up the frame, and some expanded metal to put on top.  I'll get the opening for the firebox cut, and then see about the RF plate.

When something is completely new to me, I just don't have a good feel for how much fudge factor is OK.  I don't understand at all why the formulas for the airflow/smoke areas work, so for me its definitely useful to know if the results should be fine, or if you are going to be off, be off in a particular direction (bigger, smaller, etc), and a baseline.
That's definitely true, especially if you know you are calculating the right things!

6. ### storeman0913Smoke Blower

Hi,

I'm about to begin an RF build using a old USA-made 24" x 69" 3/16" thick air compressor tank, rounded on both ends.  Tank placard says 120 gallons.  I plan to fabricate a rectangular fire box of whatever size is indicated.  I'm considering an insulated build.

Entered rough numbers in the calculator just to see if the answers seemed to make sense, and they do.  Thanks for your efforts.

My question is:  Does the calculator assume a cylinder (flat ends) or a tank such as mine?  If a cylinder, do I need to adjust for the volumetric loss of the rounded ends?  Thanks for your help.

Jerry

7. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

Just gallons, cubic inches what ever volume....  round tank, round ends is the norm...
If you read up on builds... there are a few pits to fall in...   doors that spring when cut.... welding on hinges and having them "pull" as the metal shrinks so the door doesn't fit...  The threads answer all the questions but....  not in one thread...   come on back with questions a pictures are a must....

Start a new thread on your build...  That will be a good place to answer questions and for our members to follow along.....   Dave

8. ### storeman0913Smoke Blower

Will do.  Thanks.

Jerry

9. ### sdanderNewbie

I have not started my build yet.  Couple of years ago I made a simple smoker out of two 55 gal. drums but now want to try something a little bigger.  I realy like Dave's formula and since I am a moderate Excel geek, I put the fomulas into a spreadsheet.  Assuming your are using a round tank, all you need to do is enter the diameter and length of the tank (I have example dimensions for LP tanks i found online) and it calculates the rest.  Then enter the diameter of your exhaust stack, again assuming it is round, and it will calculate the lenght it needs to be.

You will still need to go to the circle calculation website that is linked to in the thread to calculate the actual dimensions of the CC/FB hole and RF Plate.  If I can figure out how to calculate that as well, I will add it to the sheet.

10. ### novasbcSmoke Blower

Got past a busy period at work, hoping to be able to post a version of this that you can access from my webserver in a few days, so that folks without Windows can try the tool.

I do have the visual representation part done, so it shows you the dimensions of the tank, to give a better idea of what you are doing when you go to measure and make cuts.

Last edited: Jun 26, 2015
11. ### scoobNewbie

I cannot figure this calculator out.. Math is my enemy. Any help would be appreciated

12. ### gary sSmoking GuruOTBS Member

*

Last edited: Mar 29, 2015

Try this one

14. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

Scoob, morning...... It's a step by step..... Do 1 step at a time......

Calculations for a standard design, reverse flow smoker..

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

Diameter X Diameter X 0.7854 X Length = Volume in cubic inches
24 x 24 x 0.7854 x 60 = 27,143 cu. in.

Volume in cubic inches X 0.004 = FB/CC opening in square inches
27,143 x 0.004 = 108.6 sq. in.

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

Volume in cubic inches X 0.33 = minimum volume of the Fire Box
27,143 x 0.33 = 8957 sq. in. minimum

If you do the rest of the tutorial step by step it will all fall into place.... read each step thoroughly...

15. ### scoobNewbie

Oh i see, you multiply the same diameter two times.. that was throwing me off a bit. Thanks dave

16. ### the1pearsonFire Starter

I hope this helps. I'm a visual learner and used this when I was doing calculations on my build.

17. ### bkleinsmidSmoking Fanatic

Morning guy's......

Do these calculations work for an offset smoker as well?

18. ### jcbiglerSmoking FanaticSMF Premier Member

Can we get this thread stickied tot he top of the Reverse Flow build subforum? It would make it much easier to find when I need to come look for it.

jgunne and joe ny like this.
19. ### smokin' joe smokerNewbie

Hmmm.  There seems to be a discrepancy between BBQSmokerRatios2 spreadsheet and Feldons BBQ Calculator.  My chamber is round, 24" diameter, 56" long and has flat ends.  Both calculators show the same desired firebox capacity (approx. 8440 cu in - which makes my firebox 20.5" x 20.5" x 20.5") and the same chimney capacity (430 cu in which makes my chimney 4" diameter, 34" long).  However, when it comes to the firebox-to-cooker opening, Feldons says 68 sq in is desired and this one says 101 sq in.