Exhaust Stack Heights

Discussion in 'Reverse Flow' started by novasbc, Jun 26, 2015.

  1. novasbc

    novasbc Smoke Blower

    I am looking to install the exhaust stacks today.

    Based on the calculator from DaveOmak, I have the heights of the stack required for various inner diameters.  It seems you want to have this as close to 36" as possible?  I infer this from his comment:
    If that is the case, and I don't have the appropriate sized round or square tube to get close to 36", is there any reason not to run two smaller stacks that add up to the appropriate volume?

    I have a bunch of square tubing, but I'd definitely need to run two stacks.  If I run it through my calculator, with 4" square tubing, I would need a height of 86.54.  I'd need 6" I.D. tubing to run a single stack, and the price goes up a lot as you get to the wider square and round tubes.  However, lets say I have 4" square tubing, and I run two stacks, I only need a height of 43.27", which is a lot closer  The closest would be 3 3.5" stacks.

    How much should I be concerned about being near that 36" exhaust stack height?
  2. novasbc

    novasbc Smoke Blower

    So, I just verified the type of square tubing I already had, and it is 3" inner diameter.  To make 1,384.66 cubic inches with 3", I would need a nearly 13 feet height.  If instead, I choose square tubing of 6" inner width, it would be right at 38.5", which is close to the magical 36".

    Maybe I'll just shell out the money and buy 38.5" of 6" square tubing.  At least it will be precut for me.
  3. radioguy

    radioguy Smoking Fanatic

    You'll have to compensate for additional friction when using square tubing. There are no rules or formulas for this substitution. For a given "round" volume I would add 10-15% to the "square" volume. Adding multiple stacks makes the friction issue worse.

    That additional percentage is only my guess based on what I've read on this forum. Wait for others to comment with actual experience using multiple square stacks.

    That stuff gets expensive. Have you rooted around local junk scrap yards?

  4. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    RG has it correct..... Same size round flows more air than square.... Reducing the diameter by half reduces the area by 75%....

    also:... taken from... .http://www.smokingmeatforums.com/t/...others-ready-to-use-rev5-6-19-15#post_1264161

    Volume in cubic inches X 0.022 0.017 = Exhaust Stack Volume in cubic inches, above the CC.... (ESV)
    (The increase in volume ~30% shows a dramatic improvement in equalized temperatures across the cooking surface... edit 6/19/15 )

    Exhaust Calculation..

    ESV in cubic inches_____________________________ ... = Stack Length in inches (36" +/-)
    0.7854 X Stack Diameter X Stack Diameter

    Adjust the diameter of the stack, until the proper length is achieved... be sure to measure the actual internal diameter of the pipe used..
    this is for round stacks only..

    Last edited: Jun 26, 2015
  5. novasbc

    novasbc Smoke Blower

    That's the formula I used in all my calculations.  Unless there is some magic I'm unaware of, my choices are limited.  I can get 4", 5", 6", and 8" I.D. pipes.  I have to work within those limitations.  With this, and a 6" I.D. pipe, I end up having to go with 49" in length
    Last edited: Jun 26, 2015
  6. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Go with 8" exhaust at the normal height... 34" ish.... If need be, you can install a choke plate on top of the stack...
    Last edited: Jun 26, 2015
  7. novasbc

    novasbc Smoke Blower

    Thanks! Do most people cut the stack flush with the top of the cooking chamber? The easiest is obviously to let one side of the pipe extend down into the cooking chamber.
  8. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Did you look at the picture.... the stack partially inserted into the CC... with a steep angle cut on the end.... that, supposedly, pulls exhaust from a wide range inside the smoker... equally between the cooking grate and the top of the smoker...

    OR, cut it off flush.... OR, extend to about6" from the cooking grate..... OR, install in a plenum on the outside of the smoker....

    I think the plenum is most popular.... gets the stack out of the smoker AND pulls air from a larger area, like the angled cut off stack....
  9. smokejumper

    smokejumper Smoking Fanatic

    Ok,so just to set the record straight...

    Hydraulic Diameter is the term used to describe the flow loss due to friction in a tube of any configuration (square, rectangle, round, star shaped, any other shape).

    It is the equivalent size round tube that would flow the same amount.

    The formula is:  DH = 4A / P

    Hydraulic Diameter (DH) is four times the Area divided by the wetted Perimeter.

    So, for a square duct with sides of length x, the hydraulic diameter is:

    DH = 4x2 / 4x = x

    This means that a square duct with sides of length x has a hydraulic diameter the same as a round duct with a diameter of x.

    So when substituting a square duct for round, use the same size square duct as the diameter of the round duct.

    No further calculations or fudge factors are required.The corners of the square duct are essentially ignored.

    For a rectangular duct, you will see the hydraulic diameter decreasing as the rectangle gets more oblong.

    For example, a 4" x 8" rectangular duct will have a hydraulic diameter of:

    DH = 4(4 x 8)/ (4+4+8+8) = 128/24 = 5.33

    So a 4 x 8 square tube has a hydraulic diameter of 5.33 inches. Meaning it is equivalent to a 5.33" round tube.

    The rectangular tube has a cross sectional area of 32 in2 but it is equivalent to a round tube with a cross sectional area of only 22.3 in2

    I hope this makes things clear for anyone wondering about square exhausts.
    Last edited: Jul 2, 2015
  10. vartz04

    vartz04 Fire Starter

    Not to hijack you're thread but I don't seem to be getting any attention on my build thread

    I have a question about heights... Per the calculator I can use either 2" @ 36" tall or 3" @ 17" tall. I need a 3" piece of pipe anyway for the firebox intake so is there any real downsides to the shorter stack on a small smoker ?
  11. smokejumper

    smokejumper Smoking Fanatic

    The only real downside in this case is getting smoke in your face with a short stack.

    I would recommend using the 3" stack and make it 36" long.

    The calculator gives recommended minimums for most dimensions. You don't want to go overboard on the diameter which could cause your exhaust too cool to quickly and not draft well, but going from 2" to 3" will be just fine.
    Last edited: Jul 2, 2015
  12. inkjunkie

    inkjunkie Master of the Pit

    You are not alone my friend....which is why I will no longer be updating it....if I wanted to "talk" to myself I would just stay out in the garage....
  13. novasbc

    novasbc Smoke Blower

    I went with the 8" pipe, and 34" height.

    I inserted it only as far as I had to. I still need to cut it flush, but it was enough to fire it up.

    Worked great!

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