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

[SmokeRing0909]

This is an absolutely amazing thread! I’m so grateful to stumble up on this forum and the fine people this thread proves are here.

 

[kriols]

Hi - i have been serching all over the internett about calculation when it comes to airintake, pipe, firebox and size of the opning between the CC and the FB.


I have 80 gallon/300L - tank 60x20 inch/150x50 cm as the cookchamber:
The pipe i have is 5,9 in in dim/ 15cm.
I want a Square firebox. have been thinking for size: Width: 20inch/50cm Length: 23.6/60cm Tall: 17.7inch/45cm.
I need help if this is wrong and what should i have ? i will build a smoke
collecter that the pipe will be connected on too.. sorry for my bad english.
I appreciate all the help i can get for this :)

 

[kriols]

Hi - i have been serching all over the internett about calculation when it comes to airintake, pipe, firebox and size of the opning between the CC and the FB.


I have 80 gallon/300L - tank 60x20 inch/150x50 cm as the cookchamber:
The pipe i have is 5,9 in in dim/ 15cm.
I want a Square firebox. have been thinking for size: Width: 20inch/50cm Length: 23.6/60cm Tall: 17.7inch/45cm.
I need help if this is wrong and what should i have ? How to messure it right. i will build a smoke collecter that the pipe will be connected on too.. sorry for my bad english.
I appreciate all the help i can get for this :)

 

[Ramblin2Day]

Newbie here so if I am violating posting protocol, please let me know (nicely preferably)

I posted this question in my build post at

New offset smoker build by newbie - basic design Qs plus ...

Brand new to this whole idea of building a smoker but have decided to jump in and build a small offset smoker using a 30 gallon (new; got from company going out of business) air compressor tank as the cooking chamber. With encouragement of @civilsmoker and @JckDanls 07 , I will post here as I...

www.smokingmeatforums.com

but then realized the question should be posted here instead so here I am.

I am starting to design an offset smoker - direct flow. I am starting with a (small) 30 gallon air compressor tank for the smoke chamber (got a new one for free) so ran the calculations. They ended up with a very small firebox and other dimensions

• Cooking Chamber (CC): 6,930 cubic inches (approx 20" diameter x 22.5" long)
• Firebox (FB): 2,308 cubic inches (20" x 12" x 10")
• FB -> CC Opening: 27.2 square inches (20" x 1.36")
• CC -> Exhaust Stack collector opening: same as FB -> CC opening
• Exhaust Stack Volume (ESV): 153 Cubic Inches (which would be 12.2" tall stack (above CC) with 4" diameter pipe or would be 36" tall stack (above CC) with 2.3" diameter pipe)

So my question is : are the values generated by the formulae intended as minimums or are they supposed to be used exactly to produce a good offset smoker?

Can I build a smoker with a larger firebox (so I have room to put in wood and charcoal) and a larger stack (with baffle plate to run full draft on warm-up and reduce down for normal operation)

I assume I should keep the throat openings as calculated - but please correct me if I am making wrong assumption here.

So what I am thinking is - again, starting with 30 gallon tank as cooking chamber = 6,930 cubic inches:

• Firebox (FB): 8,000 cubic inches (20" x 20" x 20") (3.5 times calculation)
• FB -> CC Opening: 27.2 square inches (20" x 1.36") (using calculation)
• CC -> Exhaust Stack collector opening: 27.2 square inches (same as FB -> CC opening) (using calculation)
• Exhaust Stack Volume (ESV): 535.5 cubic inches (3.5 times calculation) (4" stack standing 42.6 inches above CC - will make foldable with baffle on top to reduce size during normal use)

Any feedback appreciated.

 

[Mechanic of Smoke]

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....

5f7aa7c3_Partsofacircle.jpg

• daveomak
• Nov 15, 2014

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....

The knowledge gathered here is really impressive.

My smoker should have a round CC and a round FB with the same diameter.
Which FB/CC opening is best for this?
Green cut-out area or football shape?

 

[Ramblin2Day]

The knowledge gathered here is really impressive.

My smoker should have a round CC and a round FB with the same diameter.
Which FB/CC opening is best for this?
Green cut-out area or football shape?

If you do have a round firebox and you do cut out the segment as shown, make sure to leave a lip at the bottom to serve as a grease dam to prevent grease from the bottom of the cooking chamber getting into the firebox. This will affext your calculations.
I am a newbie too so maybe you have already thought of this but I had not until I read other posts in this forum.

 

[Mechanic of Smoke]

...make sure to leave a lip at the bottom to serve as a grease dam to prevent grease from the bottom of the cooking chamber getting into the firebox. ...

Thanks for the tip, I hadn't thought of that.
It makes sense that the FB/CC opening needs to be adjusted accordingly.

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

Now I have to go a little deeper into the design of the FB/CC opening.
Dave writes in the tutorial that the opening, the cross-section under the RF plate and the gap at the end must be the same size.
(Because all the values are identical in the three calculations.)

Isn't that very difficult to achieve if the RF plate is not supposed to protrude into the FB/CC opening?
The red area is always the difference between the cross section under the RF plate and the cross section of the FB/CC opening.
They therefore can´t be the same size.

I am probably making a mistake in my thinking or am completely misunderstanding the matter.

 

[Ramblin2Day]

As an added calculator, if you decide to use Pinwheels as your air inlet and want to know how much area in square inches a given Pinwheel design will provide, see the attched Excel spreadhseet.

Enter 3 numbers

• Radius of outside edge of opening
• Radius of inside edge of opening
• Width desire for overlap (on each side of opening) to get seal when closed

And it outputs the area of the opening, factoring for the area lost due to overlap.

 

[JckDanls 07]

Isn't that very difficult to achieve if the RF plate is not supposed to protrude into the FB/CC opening?
The red area is always the difference between the cross section under the RF plate and the cross section of the FB/CC opening.
They therefore can´t be the same size.

Correct.. They (RF Plate and cross section) will not be the same width... The RF plate does not go inside the FB opening... It only comes down to the top of the oval shaped opening ... The RF plate will be wider than the opening ...

What he is saying is that the two openings, FB to CC and the opening at the other end of the RF plate, Be of the same sq. inches... Don't have to be the same shape just as long as they are of the same sq. inches ...

Does this help?

 

[Mechanic of Smoke]

As an added calculator, if you decide to use Pinwheels as your air inlet and want to know how much area in square inches a given Pinwheel design will provide, see the attched Excel spreadhseet.

Enter 3 numbers

• Radius of outside edge of opening
• Radius of inside edge of opening
• Width desire for overlap (on each side of opening) to get seal when closed

And it outputs the area of the opening, factoring for the area lost due to overlap.

Thanks for pointing out the calculator.
But that's another topic.
For the air inlets, I would have used a rectangular slider design.
That way, the inlet is wide and not too high.

Correct.. They (RF Plate and cross section) will not be the same width... The RF plate does not go inside the FB opening... It only comes down to the top of the oval shaped opening ... The RF plate will be wider than the opening ...

What he is saying is that the two openings, FB to CC and the opening at the other end of the RF plate, Be of the same sq. inches... Don't have to be the same shape just as long as they are of the same sq. inches ...

Does this help?

Thank you, that makes it clear.
So I was a bit too precise with the cross section under the RF plate.
The cross section at the end of the RF plate is not a problem.