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My first build, help me please! reverse flow

post #1 of 71
Thread Starter 

Please review my work before I start cutting things up!  I also have a few questions if you do not mind.


Currently I have a big green egg and a weber smoky mountain.  I like the egg for grilling but I am not loving it for smoking (I know it can be a great smoker, but I prefer traditional).  the WSM is great, but I want to try an offset and I like building things.  Thought about buying one, but don't want to spend the money for the quality I want and I don't want to waste my money on a cheap unit.    I only need enough space to cook for the occasional bbq and for family.  The 33 gallon tank is about the same size as my weber smoky mountain so I figure it is the perfect size for my needs.  Can you review my work before I start?  Ive never built a smoker before!



33 gallon craftsman air compressor, will use for cc.

firebox - 1/8" steel.   15" x 13" x 13"

air intakes - 5" x 1.5" on bottom 4" x 1" on top

fire box to cooking chamber - 25 - 27" square inches of opening



1. is 1/8" steel going to be ok for the firebox?  It seems pretty stout.  I don't really have the tooling to work with 1/4" steel.  My welder is a 140 miller.  Obviously it might be better to bump up to 3/16 steel but I already have the 1/8 on hand so buying 3/16 adds another cost. 


2. I thought about building the firebox a little bigger.  Is it ok to go up to 120% on the calculator?  maybe 16x14x14 instead of 15x13x13.  Are there any advantages/disadvantages of going too big?


3. for the air intakes.  I understand one on top of fire and one on bottom of fire is best . I assume it is OK to go too big on these as they will be closeable.  Correct?




Ive actually never even smoked on an offset smoker so not only do I lack experience building them, I also lack experience cooking on them to know their nuances.  I have smoked on the wsm as well as the bge though.


Thank you!

post #2 of 71

Hello and good luck. 

I can give some of what i went through on my build and hopefully some of it helps. in terms of the over sized inlets, be careful,  i thought the same and had to redo my fire box, i was unable to get temps below 300f.

As for the steel 1/8 will be ok if your running charcoal in a basket type of heat, but wont last that long, 3/16 or 1/4 would be best. 

remember, these things are like tuning a carburetor, needs to be just right. 

I hope some of this helped.


post #3 of 71

May I suggest you use the above RF Smoker building tutorial for your build..... Feldon's has some inherent problems.....
post #4 of 71
Thread Starter 

Dave, that helps a lot.  Using that, here is what I come up with.  It is pretty close to the other calculator as well.


Volume = 7696

FB/CC opening = 30.78 square inches

area under RF plate = 30.78 square inches (is this then the actual size of the plate?)

area at end of RF plate = 30.78 square inches

firebox = 2359 cubic inches = 14 x 13 x 13. 

air inlets = 7.7 square inches  ill do 4.5 on bottom, 3.2 on top (sound ok?)

3.5" stack at 13.5" long or


Thanks again.

post #5 of 71
You must follow all the steps.... Continue on down the tutorial... The circle calculator provides some very necessary numbers....

This circle calculator is really cool... It does it all.... Just fill in the necessary blanks...
......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....

Circle Calculator

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..
post #6 of 71
Thread Starter 

Dave, you the man.  So essentially, I come up with 3.2 for line ED.  So if I am understanding this correctly, my RF plate will sit 3.2" high from the opening into the CC or another way of stating it is the widest part of the opening will be 3.2".  Then whatever 3.2"in height corresponds to will be the width of my RF plate, which is line AB (width of plate).


Another question.  Is the opening at the end of the RF plate then the same exact size as the opening for the FB/CC?

post #7 of 71
The opening between the CC and the FB should look EXACTLY like the green area in the picture of the circle calculator...

Line/Chord AB is displayed in the circle calculator..... The numbers displayed are NOT the numbers for your smoker... Line/Chord AB is the width of the RF plate... All dimensions are inside dimensions...

post #8 of 71
Thread Starter 

Radius= 8.75

Segment  height HD  = 3.2


CHORD AB = 13.529

Apothem OE = 5.55

Central Angle = 101.27

ARC AB = 15.465

circumfrance = 54.9

segment area = 30.116

triangle area  =37.

sector area = 67.59

total circle area = 240.53


So in this example, the width of the RF plate is 13.529, correct?  The opening from the FB to CC is 30.116 square inches, correct?

Thanks again.

post #9 of 71
Yep...... You got it......
post #10 of 71
Thread Starter 

Next question.


When calculating the size of the exhaust stack.  Does the part that hangs down into the CC count as part of the length or am I only considering the length that sticks out from the CC?  Also, should the stack be on the end (im using a compressor, so it is round, not flat) or can it stick out the top?

post #11 of 71
In the calculator there are diagrams of stacks and how to measure them.... Above the CC is the calculated length...
post #12 of 71
Thread Starter 

Next question as I am about to place an order for the firebox steel.


Can the firebox be TOO big?  What is too big?  150% on the calculator?  125%?  Is it better to go bigger?  Id like to be able to put standard cut wood in my box but my tank calls for a smaller firebox.  I was thinking of making it slightly oversized.


Thank you.

post #13 of 71

Too big is not a problem at all, if you are using Dave's calculator, it is based on the cook chamber so make the firebox to fit your needs and it will work the same.

post #14 of 71
Thread Starter 

I have a few more questions on the calculations as well as exactly what each calculation pertains to.  On my picture, (sorry for the crappy drawing/pic) I have coorelated A, B and C to each area.  Am I accurate as to what each calculation is measuring? 


My biggest question is B, the area under the RF plate.  Is this total volume or just the distance between the RF plate and the compressor wall)? 

A. Volume in cubic inches X 0.004 = FB/CC opening in square inches

=7715 x .004 = 30.86


B. Volume in cubic inches X 0.004 = Area under the RF plate in square inches

=7715 x .004 = 30.86


C. Volume in cubic inches X 0.004 = Area required at the end of the RF plate in square inches

=7715.004 = 30.86


post #15 of 71
Finish reading the "tutorial calculator".... It describes the "green" area in the circle calculator.... THAT is the area under the RF plate.... That area is in square inches.... the circle calculator figures it out for you.... gives the height ED.... the width of the RF plate AB

post #16 of 71
Thread Starter 

Thanks dave.  My post is confusing.  I am not really sure how to refrase it other than I am just trying to figure out what my calculations relate to in regards to stuff inside the CC.  my "A, B, C" have nothing to do with the calculator, just three questions but I think I have A and B figured out.  Confusing...I know. 



Is the following correct as it correlates to my picture below?


A. Volume in cubic inches X 0.004 = FB/CC opening in square inches

=7715 x .004 = 30.86


B. Volume in cubic inches X 0.004 = Area under the RF plate in square inches

=7715 x .004 = 30.86


C. Volume in cubic inches X 0.004 = Area required at the end of the RF plate in square inches

=7715.004 = 30.86



Edited by busmania - 3/18/15 at 3:31pm
post #17 of 71
YEP !!!! 30.86 square inches opening from the FB to the end of the RF plate... that gives a smooooooooth air/heat/smoke flow....
post #18 of 71
Thread Starter 

I know everyone likes pictures and I've been boring everyone with questions so I finally made some progress.  Hoping to have it done by mlb opening day (april 10) so I can tailgate proper.  Got most of the motor brackets and feet cut off and ground down.  Realized I was out of cutting disks before I could get the last one cut.  Taped up the door just to get an idea.  Still trying to decide how big to make it and on which part of the tank (seam on bottom, top, back etc).




post #19 of 71
Myself.. I made my FB the same width (I.D.) as my RF plate.... in your case it would be 13.5"
post #20 of 71
I always put the seam somewhere between the bottom & back side of the CC.
If you think of the end view as a clock the CC door would be 12 - 3+1" and the seam qould be somewhere between 6-9.
The only other thing I would consider would be the location of your holes in the endcap of the tank, if you can get rid of most of the holes by planning your firebox cutout its less work for you in the build.
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