Question on a Fire box size

Discussion in 'Reverse Flow' started by bigtimebbq, Jun 11, 2014.

  1. bigtimebbq

    bigtimebbq Newbie

    Hello all new here. I have been reading posts for a couple weeks now.

    I am starting a new RFsmoker i picked up a nice 400 gallon vertical commpressor tank and was wondering on FB size. I was thinking of 36"by 30" im also going to insulate it. does that sound to big or to small. Thanks guys
  2. radioguy

    radioguy Smoking Fanatic


    Welcome to SMF.  This is the place to get great advice on everything smoked.  Please stop by the "Roll Call' area and introduce yourself.  Let know a bit about yourself, what you like to cook etc.   

    As far as your question your minimum firebox volume would be 1/3 of the cook chamber (tank).  400 gal CC would be about 133 gal or a box about 31" x"31" x 32".   This would be your minimum size, most people recommend going a bit larger (10 to 25% bigger) to accommodate for

    ash pan, fire management. 

    There are on-line calculators to get your sizes , dimensions right.  Try and search for Feldons BBQ pit calculator.  There is a lot of help / opinions available here.  Just ask.

    Welcome aboard,

    gril1 sgt likes this.
  3. A 31" x 31" x 36"  is about 12% over the recommended size for a 400 gal tank (according to Feldons)    Recommended Cu In is 30,800

    Your box is 34,956 c i 

  4. bigtimebbq

    bigtimebbq Newbie

    thanks hate to waste steel with prices they are asking for today

    im going to have more questions hope ppl dont mind. will try not to ask stupied ones though
  5. bigtimebbq

    bigtimebbq Newbie

    Morning guys going to start my cooker this weekend i hope.

    so I have a cc old air tank 400 gallons 9'6" round or 36" cirf

    7'6" long dome to dome or 6'8" welded seem to welded seem. 

    think im going with a fb of H34"xW36"xL34"

    calc says fb - cc opening 332.93 sq "

    wondering how high i should make my rf from bottom of tank and does the rf plate end with the same number as the fb - cc opening?

    thinking of an 8" stack that should be somewere around 32"

    Thanks for any help you can offer with advice    
    Last edited: Jun 20, 2014
  6. lendecatural

    lendecatural Smoking Fanatic

    Working backwards from the volume, I estimated the diameter to be about 38", is that a good number? Feldons should be used with a 100% firebox to get the rest of the numbers and for reverse flow should have the recommended opening increased by a factor of 150%. Therefore, the real number you need is an opening area of 370 sq-in, area under the RF plate, at the end of the end where the smoke turns up towards the grates. Using the circle calculator above, you need 14" up from the bottom of the cook chamber for your horizontal firebox to cook chamber cut. The width of the cut (cord across the cook chamber as you look down the length tank) will be 36.6 inches so that should be the width of your firebox so you are really close with the planned width of the firebox. Keep us posted on your progress, that is a really nice tank and should make a great smoker!
  7. bigtimebbq

    bigtimebbq Newbie

    Morning all.

    I have been trying to gather every thing i need to start this build. And ran into a problem with the fire box steele and am having to go a differnt way with the price of a 4x8x1/4 steel the way it is. so i picked this pipe up its 24" round x 5'1" x 1/4". I remember seeing a thread were some one used two pieces of pipe but cant find it now.Any one have any ideas for me?

    Thanks Darrell

  8. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

  9. bigtimebbq

    bigtimebbq Newbie

    I was thinking something more like this pic.
  10. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    With a 400 gallon CC.... the FB "should be" about 130+ gallons for a reasonable size..... 24" diameter pipe is about 1.95 gallons per inch in length...... sooooooooo..... your FB needs to be about 65 inches long to have a "recommended" volume to operate the smoker.....

    I would save that pipe for another smoker and go with the mild steel plate FB....
    Last edited: Jun 25, 2014
  11. bigtimebbq

    bigtimebbq Newbie

    Thanks Dave that is what i was thinking. I didnt think it was big enough
  12. lendecatural

    lendecatural Smoking Fanatic

  13. Hay guys. Throwing together a plan for a down sized smoker to what I see most but using kegs (yes I love stainless lol)
    I have the opportunity to get a half keg, so would there be any downfall to a fb roughly half the size of the smoke chamber?
    Last edited: Sep 16, 2014
  14. smokejumper

    smokejumper Smoking Fanatic


    A larger firebox is OK.
    BTW you should start your own thread instead of hijacking this one.
    radioguy likes this.
  15. Hijacking a question thread??

    But yes will be doing my own build thread.
    Thanks for the reply.

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