HEEELP!! need assistance figuring out Firebox-to-Cook Chamber Opening

Discussion in 'Reverse Flow' started by 1961shasta, Mar 27, 2016.

  1. 1961shasta

    1961shasta Fire Starter

    I need some help, for the life of me I cant wrap my head around figuring out the size of the Firebox-to-Cook Chamber Opening,

    I have read the tutorial, and it made my brain hurt, I have found the Circle calculator and am not sure which measurements to enter, can someone help me with this?

    I am using a 40 pound propane tank, the internet says it holds 9.4-9.5 gallons, I figured out there are 231 cubic inches in a gallon. the feldon calculator is right on that, My fire box is 10x10 but I cant figure out the Firebox-to-Cook Chamber Opening and on top of that I have read

    the size needs to a bit larger than the calculation doe to friction and using a smaller CC.

    should the opening on the other end of the baffle be about the same size? thanks in advance.

    Last edited: Mar 27, 2016
  2. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

  3. radioguy

    radioguy Smoking Fanatic

    You have to be careful with propane volumes. Some tanks are labled at a lower volume for safety. A gallon of propane may or may not be equivalent to a gallon of water. You may be best to just to measure it and directly calculate the volume. Just Google vollume of a cylinder or volume of propane tank. There are some good calcs out there.

  4. 1961shasta

    1961shasta Fire Starter

    yep, but as I clearly mentioned in my original post, I have already read this,,, a few times.  I cant get the math to work right in my head. and some of you claim to be able to look right at it and see the numbers naturaly, so how about some help.
  5. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    It's written in a step by step.... calculate the first number, write it down... do the 2nd, write it down.... as you go down the page, the previous numbers will fit into the calculations....

    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 / 231 = Volume in gallons

    16 x 16 x 0.7854 x 30" = 6032 cu. in. / 231 = 26 gallons

    Volume in cubic inches X 0.004 = FB/CC opening in square inches
    6032 x 0.004 = 24 sq. in.

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

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

    Volume in cubic inches X 0.33 = minimum volume of the Fire Box

    6032 x 0.33 = 1990 sq. in.

     Don't get confused by the volume of calculations, and just slowly work down the tutorial, it should fall into place....

    If you still have trouble, put your final numbers in here and I will walk through them...
    Last edited: Mar 27, 2016
    1961shasta likes this.
  6. imafishingfred

    imafishingfred Smoke Blower

    Do as Dave says and read the tutorial again. Ask specific questions about your numbers, but ask about them one at a time. When you clear one hurdle in your head go to the next. Trust me, it worked for me. You can read my thread here http://www.smokingmeatforums.com/t/171029/new-guy-and-this-will-be-my-1st-reverse-flow-smoker-build

    and it might help. I ask all the same questions as you and maybe more.
  7. 1961shasta

    1961shasta Fire Starter

    Sorry guys, I get stumped right out of the gate, (Diameter X Diameter X 0.7854 X Length), how can I do this is my ends are rounded? according the feldon calculator my cubic inches will be 2201.43 cubic inches, using 9.54 gallons as the volume I found for a 40 pound propane tank online. so can I use this instead of calculating Diameter X Diameter X 0.7854 X Length?? since that gives you cubic inches, (assuming the calculator is correct with that measurements).
  8. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Use a "best guess" scenario......  this ain't rocket science....    Measure from 1/2 length of the dome to 1/2 length of the other dome.....    or measure from where the straight side ends to the end of the dome...    Guess where 1/2 of the volume of the dome would be to the other 1/2 the volume of the dome would be...      Or, fill it with water 1, 5 gallon bucket at a time and count the buckets....  or fill it with water and measure the water as you dump it out....
  9. 1961shasta

    1961shasta Fire Starter

    well using my best guess, the tank is 9.54 gallons, which equals, 2203.74 inches, x.004 = 8.814 cubic inches for the FB to CC opening,

    I plugged in Chord AB and segment height Ed into the circle calculator and the closest I can get is 9.083 radius, thats the size of the hole in cubic inches right?

    thats 1 inch difference from the feldon calculator which had me at 8 inches
  10. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    What is the length of your tank....   What is the diameter of your tank......  You seem to be not following the steps.


    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..
  11. 1961shasta

    1961shasta Fire Starter

    well I cant fill it with water, I already have the door cut and hinges on it. its total height is 25.25 inches with 23/4 inches of dome on each end,  so subtract one of the 23/4 inches give me 22.5 inches by 12.3 inches, not sure how thick the tank is looks like over a 16th, so best guess would be 22.5 in x 11.9 inches, )Diameter [11.9] X Diameter [11.9] X 0.7854 X Length 22.5 = Volume) =2502 cubic inches,

    which calculates to me needing 10 cubic inches of opening between the FB and CC.
    Last edited: Mar 27, 2016
  12. 1961shasta

    1961shasta Fire Starter

    according to my previous post, this radius gives me 10 cubic inches between the FB and CC right?  I used a 10 in radius because that is the width of the tank where I want my baffle to be at (3 inches up from the bottom) where the tank is 10 inches wide.

    Last edited: Mar 27, 2016
  13. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    If the diameter of the tank is 11.9", the radius is 5.95" or 6" is good enough....    You don't put the baffle somewhere.....  You calculate for it...

    2500 ci. x 0.004 = 10 si  I'd use 22 si under the RF plate and the FB/CC opening

    2500 x .33 = 825+ ci FB

    2500 x .001 = 2.5+ si ..  3" below and 2" above is what I'd do...

    2500 x 0.022 = 55 ci..  55/.7854/3/3=8'" tall... Knowing small exhaust stacks don't suck well, I'd use a 3" stack about 32" tall..

    So your 3" guess is right on...   good guess....  [​IMG]  .....

    That 3" will be the green area or ED on the colored picture...  That is the shape of the cut out for the FB/CC also....

    The reason for enlarging certain areas is friction loss...   A long narrow passage creates a lot of friction and when you have a "natural draft" unit, friction is a killer....

    I don't know how to copy and paste the circle calculator...   Put 6" radius and 3" segment height ED in it and those above numbers are what should come up....
    1961shasta likes this.
  14. 1961shasta

    1961shasta Fire Starter

    So what you are saying is my top cut should be 3 inches below the center of the tank, and my bottom cut will be 2 inches from the floor or bottom of the tank?

    how wide does the top cut need to be, I dont under stand the circle calculator, from what I am reading the top cut should be 10.39 inches. right?
  15. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    First of all, have the tutorial open on a page you can reference... 

    Look at the picture....  the red circle is the CC......   ED is the segment height..3"......  once you measure up 3" from the bottom of the CC, and draw a line across the CC, it should be Chord AB which should be 10.39" long....    Then you will REMOVE the GREEN area from your CC and slide the FB up to that HOLE you just cut into the CC...   Mark the cutout on the FB and remove that area so when it's all welded together, you will end up with the FB/CC opening...

    2500 x .001 = 2.5+ si ..  3" below and 2" above is what I'd do...

     The above has NOTHING to do with the CC/FB opening....  If you look at the calculation steps, it refers to the air inlets for the FB...

     Volume in cubic inches X 0.001 = 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.... approx. 20% upper and 80% lower...
  16. 1961shasta

    1961shasta Fire Starter

    OK, I can finally see it now, thanks for all the help!!!, looks like I got a small issue, my fire box is only 10 inches not quite 10.39, I finished my fire box today, well almost, still have to put a vent in the side.
    Last edited: Mar 28, 2016
  17. 1961shasta

    1961shasta Fire Starter

     on another note about the FB vents, the guy who gave me the steel cut me a 3 opening vent into the door without me asking with his CNC plasma cutter, according to what I have read it was too big, and with my small box it was almost going to draw air in under the grate, which I read produces ash on your food, so I blocked most of the bottom portion of 2 openings off when I was adding some bracing to the inside of the door, it's on 1/8th inch. and I will put a 3 inch in the side below the grate.

    Last edited: Mar 28, 2016
  18. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    and with my small box it was almost going to draw air in under the grate,  which I read produces ash on your food,

    Well, That may have happened ONCE to someone but it's a bunch of BS....   doesn't work like that...    maybe from a gust of wind but not from operating the smoker....    OR someone was thinking that would happen but it won't...  Ribwizzard has built hundreds of smokers and has his air inlets in the bottom of the FB....   He has a business selling smoked meats etc.....    It's amazing how stupid ideas get spread around and people believe them....  and it's amazing how folks look for information on how to build a smoker, and think the information I give them is WRONG... as if I want them to build a smoker that covers their food with ash.... 

    I'm sorry you listened to that particular idiot that gave you misinformation ...  hundreds of smokers have been built as pictured below...   The drawing on the right, is from a reputable forum....   the picture on the left is from a member on  this forum...


    Be sure to let me know, in the future, where I'm deliberately trying to steer you wrong so your smoker is a failure...
  19. 1961shasta

    1961shasta Fire Starter

    I didn't mean it that way, and what you described is right and what I am doing, [​IMG]what I had read said if you put a vent in the front door to feed the fire and push smoke into the CC, you don't want it too low or if will push ash into the CC. but you are right, not everything people post is correct. thanks for all the help, I am almost there. unfortunately it will be about a week before I have time to continue working on it.
  20. Another thing I noticed was the expanded metal in your fire box. I would build an angle iron frame and put the expanded metal in if.  If left like it is when it heats up it bow and sag, and you will be replacing it.

    Here are a couple of FB doors we have made 

    This is my smoker, we added the top vents after the fact  Really Makes a Difference

    Here is one showing the FB to CC opening and the baffle (Heat Deflector)  plate

    1961shasta likes this.

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