Stack size question

Discussion in 'Smoker Builds' started by radio, Aug 31, 2013.

  1. radio

    radio Smoking Fanatic

     Reading Munxcub's thread about his build  http://www.smokingmeatforums.com/t/143399/250l-build-now-a-35-gallon-build and his flow issues got me thinking.  I know the calculator has a place to plug in the diameter of the stack you are using and it will tell you how long it needs to be, but I can find nothing for recommended stack diameter sizes or a calculator based on CC size.  Is there a rule of thumb for stack diameter in relation to CC and/or FB size?

    For instance, my New Braunfels SFB is 16X36 and has a 2 1/2' stack and I just don't think it draws like it should.  It does OK, but it's a struggle to get it over 250° and hold it there.  I also get smoke puffing out the cracks around the FB door, so it's obvious it isn't drawing like it really should. 

    Methinks a 4" stack would be better
     
  2. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Radio, afternoon......  The exhaust stack should be around 1.67% volume of the Cook Chamber volume....   and that stack volume should be above the top of the CC.....    

    stack volume calc....  

    .......4" dia stack       4 x 4 = 16  X .7854 =  12.57 sq. in.. or...

    .... 12.57 cubic inches per inch of length...... or....

    .... 151 cubic inches per foot of length......

    Dave
     
  3. radio

    radio Smoking Fanatic

    Thanks Dave[​IMG]   I knew there had to be a formula somewhere, but hadn't seen it.  Will a small diameter stack, but longer,  work as well as a shorter, but larger diameter stack?   I kind of understand the air friction and volume thing, but at what point do you get too much stack and where is the sweet spot in diameter Vs length?  I rather doubt that very few, if any manufacturers go by any mathematical formula when putting a stack on their units
     
     
  4. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Taken from Feldon's calculator........

    Chimney Size
    Enter the diameter of your chimney pipe to find out how long it should be.
      • A chimney that is too short may produce insufficient draft (drawing of air). A chimney that is too long may cause the air to cool before it exits, reducing effective draft and worse, dripping of exhaust materials onto food!
      • Many horizontal smokers have an exhaust between 30-40 inches in length, but there is no hard and fast formula.
      • If you are building a horizontal smoker with a vertical cooking cabinet, realize that the cabinet partly acts as an exhaust, thus you may greatly shorten the chimney.
     

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