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# Stack size question

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

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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

Thanks Dave  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

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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