Dual Chimneys and it's effect on draft

Discussion in 'Smoker Builds' started by gmh313, Jun 14, 2016.

  1. Good Morning!

    I am new to this forum, but have been cooking for about a decade now; parties ranging from 10-150 people. I'm currently working on a 500 gal propane tank, reverse flow smoker. 

    I used feldon's calculator, and based upon it's calculations, I'm requiring a 6" exhaust at 66". Given this is a trailer rig, an exhaust that tall is not possible or practical. I'm inclined to cut it in 1/2 and run dual 6" exhausts at 33". This seems to fit within the "rule of thumb" parameters from what I've seen, and will of course look pretty cool.

    My question is; early this morning when I should have been sleeping, it dawned on me that two 6" stacks might be the same as one 12" stack. Again, reaching back to Feldon's, a 12" stack would be only 16" long, which seems like it will cause an extremely lazy draw. I want to avoid this since the reverse flow will already slow things down.

    Unfortunately I couldn't find much, or any, information about people running dual exhausts and it's effect on length and diameter. Can someone point me in the right direction please?

    PS- i will post pictures this evening.
  2. Update:

    After a little more research, I'm gathering that chimneys 6" or larger, the friction effect doesn't really come into play, and length is less important. That being said, dual 33" stacks @6" diameter should fit the bill just right.

    Can someone confirm? 

    Thank you
  3. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

  4. murphy625

    murphy625 Fire Starter

    When it comes to smokers, I have no idea what I'm talking about so take this for what it's worth.

    In the wood stove world, stack height is used to create vacuum inside the wood stove.   In fact, most installers won't even install a wood stove unless they can get at least 10 feet of stack.  

    It has nothing to do with diameter..   As heat rises in the stack, it must suck in more air from below.. the taller the stack, the faster the heat want's to rise and the stronger the suction at the bottom.

    In fact, the effect is so significant, some green energy engineers has once conceived of a solar heated stack system 1000 feet tall.. they wanted to put wind turbines inside and flare out the bottom over several acres. Much like a nuclear reactor cooling tower but narrower at the top and spread over acres of land at the bottom..  As the sun heated up the air at the bottom, it would rush up the stack.  They estimated something like 45 to 55 mph upward draft while simultaneously creating a naturally air conditioned space in the several acres below.  (IE: a market or shopping center under the stack).

    My point is that stack height means a lot more than stack diameter.

    Why not make a fold down stack for your rig?  When you're driving, just fold it down and secure it in place?
  5. @DaveOmak  Thank you for the heads up, There's definitely some variation in my calculation now. Fortunately my firebox didn't change too much from my current plan, but the inlet/RF opening/fire box to cook chamber dimensions did.

    @Murphy625  I actually remember reading something about an uplift turbine theory when I was in college! You have me intrigued about the collapsible exhaust idea. [​IMG]
  6. joe black

    joe black Master of the Pit OTBS Member

    Just from pipe volume alone a 6"x10" rectangular stack 32" tall would be the same as a 6" round at 66". I also think that a rectangular stack would look cool. However, I'm not a smoker builder, I just am a pipe fabricator.
  7. rectangular sounds cool for sure! I'll have to swing by my steel guy's shop this afternoon and see what he's got.

  8. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    EDIT....  boy did I screw that up....
    Last edited: Jun 15, 2016
  9. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Taken from Feldon's calculator.....   You need to size the exhaust stack about 35" tall above the CC....

    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.
  10. joe black

    joe black Master of the Pit OTBS Member

    Dave,As you know, I'm not a builder. I was just trying to offer an option that would give the same volume. I also assumed that he was building a horizontal smoker. I hope that I didn't cause any inconvenience for anyone.

  11. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

     No problem....  GMH sounded as if he was heading off to buy stuff...    He, as well as many others, often times forget what they read about important points of a build...   That could end up costing money and time...  I don't have either to waste so I wanted to head off an error at the pass...  

    Not a big deal....   It's a learning experience for anyone that reads this thread...   so, many will benefit....

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