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Firebox to Cook Chamber opening??

Discussion in 'Side Fire Box' started by BigMiike, Dec 5, 2018 at 7:09 PM.

  1. BigMiike

    BigMiike Newbie

    Hey guys

    I'm from New Zealand and we don't have 250 gallon tanks available here. I have built my own from 2 upright tanks, cut one end off each and welded them together for the cook chamber. I have made the firebox from some 6.125mm (or 1/4 inch) steel and I am abut ready to put them together.
    I am struggling with the throat size, between the firebox and cook chamber. I have put my numbers through Feldons, and the smokerbuilder calculator (all results identical through both) and it has said I need a 142.5 square inch opening from firebox to cook chamber. I have tried to use the circle calculator but cant figure it out.
    Can anyone tell me what size the half moon opening should be? The fire box is 25 inch wide so the opening can be upto that length, I just don't know how deep, or what shape. (i.e is it a half circle or is it the bottom of a circle if you know what I mean)

    Here is the link to Feldons - Link to BBQ Pit Calculator

    One more question, what is the effect of over sizing this opening? Being to small would be no good but could it be bigger? Over size firebox and exhausts are used regularly, do people oversize the firebox to cc opening too?

    I will try and upload some pics soon.

    Thanks in advance.
    Last edited: Dec 5, 2018 at 7:36 PM
  2. BigMiike

    BigMiike Newbie

    20181110_161809.jpg 20181117_212914.jpg
  3. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Mike, evening I need the dimensions of the Cook Chamber in inches... diameter and length...
  4. If you plug in your needed volume, it comes up with a 19 1/16" wide half moon. Height is the radius which would be 19.0625 divided by 2 for a radius of 9.53125" at the apex of the arc. You could always play with numbers to still equal the given area needed. Don't discount an elliptical cut out either. Google area of a ellipse and plug in your numbers to get the area you want. A little bigger would be fine, like 5-10%.
  5. BigMiike

    BigMiike Newbie

    Hi Dave - Cook Chamber is 82" long to the tip of each dome end and 30" diameter.
    Firebox is 25" wide x 25" higher x 28.5" long
    Exhaust is 6.125" diameter, not yet cut to length.

    In the Feldons link above I have entered the tank length as 78" to compensate for domed ends. I don't know the actual volume of the dome ends.
    Last edited: Dec 5, 2018 at 8:18 PM
  6. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member



    30x30x.7854x74 (-3" for each dome) = 52308 Cu. In. volume in the CC....
    52,300 x 0.004 = 209 sq. in. FB/CC opening and 52 sq. in. for the FB air inlets 40 sq. in for the lower and 10 upper..
    52,300 x 0.022 = 1,170 cu. in. for the exhaust stack volume...
    With a 6.125 exhaust in needs to be 40" above the cook chamber... + what ever goes to the plenum or however you connect it... I recommend a plenum that is open to the CC from just above the lower food grate to above the upper food grate...

    The opening for the FB/CC....


    We are calculating the green area for the opening...
    Segment height is ED... 10"
    Chord AB.. 28.28" is the RF plate width..


    So measure up 10" from the bottom of the CC... Straight across is where the RF plate goes... allo a bit for metal thickness...
    If you set the FB up against the CC and have the top of the FB 10 1/2" up, that should give you the 10" necessary for the opening.. That's with the 28" wide part against the CC....
    If you decide you want the 25" width of the FB against the CC, using the same numbers will be fine... Not much area in the last 1 1/2" on each side to screw anything up...
    I would inset the FB into the CC past the weld seam on the dome ends.. about 2"... that will make fit up for the RF plate easier.. you won't have to screw with the weld joint... The usually have a backer plate on the weld joint..
    The RF plate can be welded directly to the top of the FB after the FB is inset into the CC and welded up...
    For the door, this diagram shows a good door placement....

    CC door  opening height.jpg

    If you are building a Side Fire Box, these numbers will work fine... We will have to go over how to build and install the tuning plates if that's the case...


    The weld seam on a propane tank....

    Propane Tank weld joint INSIDE.jpg

    You can tack the FB in place after the cut-out of the opening... The weld it from the inside after you cut the door....
    It has been recommended to cut the FB opening and tack it in place BEFORE cutting the door to reduce CC warping...

    If I screwed anything up or you have any questions... I'm here...

    Smoker Exh and Intakes 2.jpg
    Last edited: Dec 5, 2018 at 9:02 PM
    fivetricks likes this.
  7. BigMiike

    BigMiike Newbie

    Thank you Dave
    Legendary help! I really appreciate you putting in this time.

    I should have said it will not be reverse flow, it will be standard flow offset, and it will be the 25" wide part of firebox being cut and mated to CC, the 28" will be the length.

    Thanks again Dave
  8. BigMiike

    BigMiike Newbie

    Oh Dave, 1 other thing.

    When I put the measurements into the smoker builder calculator, it gave me an opening of 142.5 sqaure inch opening. Yours was 209? Is the bigger number better for reverse flow?
    What is optimum for standard?
  9. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    You used Feldon's.... It has problems... We here at SMF worked for about 2 years to fix it's problems...
    Build your smoker by our numbers and you will be happy....
    One member smokes cheese on his smoker at 80F....

    Smokerbuild said:
    Hi Woody, does your smoker have any flap that covers the halfpie cut out in the fire box to control the smoke and heat? I saw one like that. Thanks, Pat

    Hey pat. I just wanted to add to my last post. Right now I am smoking some cheese

    Now I am truly convinced you don't need a flap or dampener between the FB and CC. It's holding temps steady at 80 deg left to right. I'm a true believer in DaveOmak's calculator.
  10. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    You will have to scribe the domed end of the CC to fit the 25" width of the FB... Elevate the CC so the top of the FB matches up with the 10 1/2" height above the bottom of the CC.... use straight edge to mark the cut for the sides of the CC.... Triple check all that stuff.... have a beer or two while you look at it and think about the cuts... sleep on it and come back the next day to cut....