Need help with Firebox to Chamber Size

Discussion in 'Reverse Flow' started by sqwib, Jan 28, 2010.

  1. sqwib

    sqwib Smoking Guru OTBS Member

    I am ready to order my thermal plate but need to know if my firebox to smoke chamber dimensions are ok.
    I have done the Pit calc. but can not figure out the cubic inches needed on a circular shape (moon shape).

    The fire box is 1/4' plate steel and the measurement from the bottom of the smoke chamber to the top of the firebox is exactly 5-1/4", so that gives me 5" when the firebox is cut.
    Thatis exactly 1/4 of the tank dimensions (the tank is 20" in diameter).

    Is there a mathematical way to figure out the odd shape?

    Pics to help

    The bottom piece of wood ios where the thermal plate will go and the above piece is the cooking grid level, it will be slightly higher about 4-1/2" from the thermal plate.



  2. 3montes

    3montes Master of the Pit OTBS Member

    Looks like a great build you have going there sqwib. bbally would be the one to ask on this. If he dosen't come by to answer you may want to pm him I beleive he is out of town this week.
  3. sqwib

    sqwib Smoking Guru OTBS Member

    I need 51.84 cubic inches according top the pit calc.
    And from my calculations, if I did it right I will have 78.39, thats exactly 1/4 of 20" diameter.
    Can anyone confirm this


    10" X 10" X 3.1358024 divided by 4 = 78.395
  4. silverwolf636

    silverwolf636 Smoking Fanatic OTBS Member

    I'm not 100% on this formula but I think it's pi x r squared

    This end, my door end is 16 in. across by 16 high:

    the front is 20 in. wide by 16 in. high:

    here's the inside:

    Hope this helps a little.
  5. meat hunter

    meat hunter Master of the Pit SMF Premier Member

    The build is coming along very nice. Looks great so far. I can't help you with the dimensions because I went to public schools LOL. If no one provides you with an answer, maybe shoot bbq engineer a pm and ask him.

    I have question for you. Your cooking grate, does that sit middle of the smoke chamber? At the 10" mark if your tank is 20" diameter?
  6. luvdatritip

    luvdatritip Smoke Blower

    Area of a circle-3.14XRXR

    3.14X10"X10"-314sq in.
    25% of 314 sq in- 78.5 sq. in.

    I'd say you figured it correctly.
  7. smoke_chef

    smoke_chef Smoking Fanatic SMF Premier Member

    This is great stuff... I'm getting close to my first build. Excited and nervous at the same time.
  8. sqwib

    sqwib Smoking Guru OTBS Member

    Ok now were getting somewhere.
    Next question, If I need 51.84, will 78.395 be too much, If it is I could drop the firebox a quarter inch or so. I hate like hell to throw in a damper but will if it needs to be done.
  9. sqwib

    sqwib Smoking Guru OTBS Member

    Good luck and my only advice at this point is read, read read!!!
  10. sqwib

    sqwib Smoking Guru OTBS Member

    Yes the grate will be at center point 10" within a 1/4" but the pic shows the grate level a tad lower.
  11. luvdatritip

    luvdatritip Smoke Blower

    You're only talking about a difference of 26.7 SQ. in. In your original post you were talking about cubic inches which is volume. We're talking about sq. inches which is surface area. From other smokers I've seen and what I've read, it's not rocket science. I'd go with what you have since it's much better to have too much than not enough. You could add a damper later if you felt it needed it. Just my thoughts.
  12. meat hunter

    meat hunter Master of the Pit SMF Premier Member

    In your photos, how far is the bottom edge of the door on the main cook chamber from the center of the tank? Or is the bottom edge at the center?
    I can't offer you any advice of the ratios of the firebox an such, but I would throw caution to one thing, and it may not be an issue at all, but it is in mine, and that is the close proximity of the heat sink to the actual cooking grate. And if I am explaining this wrong, by all means, someone correct me on it, but I have found that in my smoker, my meats are getting done faster due to the convection process rather than using the ambient heat to do the low and slow. On mine, too much heat is being reflected up towards the bottom of my meats. Now I can correct this with the use of a water pan underneath but I wanted to run that by you. And again, if this theory of mine is not correct, please correct me. If it were me, and you had room to move your cooking grates up, provided you are not using 2 racks, I would tack the grate in and allow yourself the means to relocate it if necessary. Just and idea[​IMG]
  13. sqwib

    sqwib Smoking Guru OTBS Member

    Thank you for putting my mind at ease.
  14. sqwib

    sqwib Smoking Guru OTBS Member

    The door is cut at the 11 and 2 position, and the grates will be 4.75" from the bottom of the grate to the top of the thermal plate.
    The picture shows the position of the grate level a tad lower than it will be. Do you think that 4.75" is too close?, that is the exact center point of the smoke chamber.
  15. sqwib

    sqwib Smoking Guru OTBS Member

    The grates will be approximately 3" below the cut out for the door
  16. meat hunter

    meat hunter Master of the Pit SMF Premier Member

    Hey SQWIB, like I said, it is a problem that I encounter on mine. I think it is something to consider. Get ahold of bbally and see what he thinks. Personally, if it were me doing, based on what I have experienced in mine, I would raise the grates up a few more inches, maybe even to the bottom edge of the opening, especially if you don't have a second rack on top. Provided you have room to smoke something larger like a whole chicken or a big pork butt. But by all means, try it the way it is. If you can, maybe tack the grate in, and if you find that it needs to be raised, you can do it. Easier to cut a tack than a weld LOL.
    I am far from being an expert of the designs of these smokers, but I do know how mine works and if given the chance to do so, I would rework my heat sink to grate ratios. Get with bbally and run it by him.
  17. sqwib

    sqwib Smoking Guru OTBS Member

    I cant raise the grates, they are 19-1/4" cast iron grates. they will just fit as is.
    I may be able to drop the firebox a bit but then I have a huge gaping hole to fill.
    I thought I read that these Langs wer about 4" above the thermal plate.
    Now I am worried my is true.
  18. jirodriguez

    jirodriguez Master of the Pit OTBS Member SMF Premier Member

    Used my CAD program real quick to draw a 20" D. circle, threw in a line 5" down from the center point and measure the square inches. It is 61.418 sq. inches, when looked at in flat 2D geometry.
  19. meat hunter

    meat hunter Master of the Pit SMF Premier Member

    LOL, Don't worry buddy. Like I said, check with bbally or lets see what some others take on this might be. If a Lang is built like you said, then I wouldn't worry about it. I just think it's a good idea to hear from others about problems or situations they have encountered in their builds, that way you have more info when building yours, which is looking very nice by the way[​IMG].

    Where did you get your cast iron grates? And how did you get them to be the size of the smoker? I don't think I seen a pic of them, did you post any?
  20. sqwib

    sqwib Smoking Guru OTBS Member

    [​IMG]I Ordered 2 of these

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