SMOKE STACK BASICS

Discussion in 'Reverse Flow' started by gary s, Oct 13, 2015.

  1. Smoke Stack Basics
    There has been a lot of discussion on smoke stacks, how long, what diameter, how far to extend into the cook chamber.
    I would not attempt to tell anyone what is correct or not correct, just what works for me and some good basic information, to help folks understand how and why the stack works.

    The smoke stack is a very important part of your smoker and has to be sized properly in order for it to work well. It also needs to look good. You have spent a lot of time working on and building your smoker, besides working properly it needs to look good. When it comes to the stack hold it up there, sit or tack it on, step back and take a look. The most important thing is that YOU ARE HAPPY, it’s your smoker.

    The stack plays a very important part of your smoker’s ability to perform. Too big an opening you lose heat and smoke to fast, to small an opening you retain to much heat and smoke creating stale smoke, so what is the magic opening (or stack ) size? Well let’s figure it out.

    The old rule of thumb (Feldon’s is 5% of the Fire box volume) not trying to discredit Feldon’s they have been a very useful tool and a pioneer in helping people figure out what they need.

    I am of the opinion that Fire box volume has nothing to do with stack volume. I think the Cook chamber is a better factor, after all that is where all your smoke and heat are accumulating.

    I played with different numbers, started out with 0.015 % then went to 0.017 %, 0.018% and now at 0.020 % of Cook chamber volume.

    Example:    Say you have a Cook Chamber that is 22 in. diameter and 60 in. long that equals 22,808 cu. inches, almost 100 gallons

    Cook Chamber 22,808 Cu. In

    THESE ARE EXAMPLES ONLY  JUST SHOWING DIFFERENCES  

    I Never Use anything Less Than 4"  I Like 4" to 6" Just Seems to work the best

                                                                                                   ID Pipe Diameter

     % of                                                  3”          3.5”         4”          4.5”        5”          5.5”        6”                                   

    Cook Chamber         Cu. In.                                   LENGTH OF STACK

    0.015
     
    342.1
     
    48.40

    35.56

    27.22

    21.51

    17.42

    14.40

    12.10

    0.017
     
    387.7
     
    54.85

    40.30

    30.85

    24.38

    19.75

    16.32

    13.71

    0.018
     
    410.5
     
    58.08

    42.67

    32.67

    25.81

    20.91

    17.28

    14.52

    0.02
     
    456.2
     
    64.53

    47.41

    36.30

    28.68

    23.23

    19.20

    16.13

    0.022
     
    501.8
     
    70.99

    52.15

    39.93

    31.55

    25.56

    21.12

    17.75

    0.025
     
    570.2
     
    80.67

    59.27

    45.37

    35.85

    29.04

    24.00

    20.17

      As you can see stack lengths are all over the place depending on the stack diameter and what cook chamber percentage you use.

    I have a friend that has a PHD in Physics and is a fanatic on smoking, I have asked him many questions over the years to help me understand and improve on the smokers we build. (My son and I try to build one or two a year when it gets cooler and his work allows) I emailed him this question two or three years ago and here is his response:

           , good morning

    I wrote you a few weeks back about insulating fireboxes, I have another set of questions. As I told you my son and I try to build a new smoker or two every year and want the next one to be better than the last.

    There has been much discussion on the size and length of the smoke stack on various BBQ sites. Using Feldon’s pit calculator you can size your chimney.  I have been kicking around stack size/length I am thinking that I had much rather have a shorter, larger dia. stack rather than a smaller, longer one. Here is my reasoning.

    Let’s just say you have a CC that is 20" dia and 36" long over 11,000 cu in, if I am going to try to get that amount of air/smoke out as quickly as possible a 4 or 5 inch pipe would evacuate quicker than let’s say a 3 or 3.1/2" pipe, Like trying to push a fat man through a narrow door vs. a wide door.

    I get that, if you have a 3" pipe that's 26.5" long and a 4" pipe that's 15" long they both have 188.4 cu in vol. But the 4" will move more vol. quicker. What do you think?

    I am starting to think the smallest dia. on a smoker should be 4" and go up from there, and unless you are building a really small smoker then 3" would be alright. I asked this question to several different people but really never got an answer that made complete sense to me. Please shed some light on this dilemma.

    Thanks

    ­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­----------------------------------------------------------------------------------------------------------------------------

    Gary-

        This is a bit tricky because the vent is used to control combustion as well as adjust the temperature. But the pit calculator is incorrect and your intuition closer to the mark.

        Hot air is less dense than cold air. So the combusted air in the smoker wants to rise. This motion is impeded by the friction of the smoker and mostly the chimney. Curiously, friction in a pipe depends on its length and diameter and the air velocity itself. Making calculations difficult. But you can make a few generalizations.

        As you double the length of the pipe the air velocity reduces by the square root of the length, or 0.7x. When you double the diameter, the velocity increases by the square root. or by a factor of 1.7x

        In a fireplace or smoker, you care less about the air velocity than the flow rate- e.g the volume of air per second exhausted. The flow rate “F” is the air velocity times the area of the pipe. So F is proportional to (D^5/L)^.5 , where D is the pipe diameter and L is the pipe length. Yikes. Not simply the volume.

        For example, if you start with a 3” diameter, 6 foot long chimney and compare to a 4” diameter, 3.4 foot long chimney (both have the same volume), the flow rate is actually 2.75x higher in the larger diameter chimney. Even though the volumes are identical.

        For a large diameter pipe (5” and above say), the chimney volume has no effect on the flow rate- the damper completely controls the flow rate. For a small diameter pipe, its diameter and air friction matters. Some people like to use a chimney with enough friction that it acts as a damper and self-limits the flow and combustion. Others prefer to fiddle with the damper (and firebox vent).    A 4” pipe is a good compromise.

     ----------------------------------------------------------------------------------------------------------------------------

    Now taking all that into consideration, let’s size a stack for our smoker.

    We know we have 22,808 cu. in. of Cook Chamber, what will look good and work properly

    Let’s use the 0.02 of the cook chamber volume (Which I favor) and go with a 4 in. ID pipe we will need it to be a little over 36 inches long.

    Now if you like it, it looks good, go with it. If you think it’s a little long let’s look at 4.5 or 5 in.   A 4.5 id  would need to be a little over 28.5 in. and a 5 in. id would need to be about 23 ¼ inches, they all have the same amount of volume and will work, but which one will look the best? It’s totally up to you.

    As stated above when you get into 5 – 6 inch id and above, the volume has no effect on flow rate, which is good.

    I would not put a 16 in. id x  6 in. stack on this smoker, just wouldn’t look right so if you wanted the 6 in. you could extend it to 20 or 24 in with no effect  on the flow rate. Try to size the stack according to smoker size; after all you want your smoker to both work and look good.

    Like I said earlier there is no way that I would ever tell someone, my way is the only or correct way, but I have done a fair amount of research, trial and error and it works.

    I like helping folks, I am not trying to be a know- it- all or a smart ass, just passing along what I have learned from experience and help from people who know. 

    Now, as how far to extend the stack into the cook chamber, I don’t know. There all kinds of opinions and views on this one and I have not tested it to make a useful comment.  I do like the plenum type mount for the stack as well as half in and out on the end and just a top mount.

    The smoker I use all the time is a Reverse Flow that has a top mount stack that extends about 2 inches into the cook chamber.  If you have seen my post you know I smoke a lot of everything and can take my smoker from 100° up,  in 10° increments with no problem.

    Lastly, I recommend nothing less than 4 inches for your stack, again depending on the size of the cook chamber. Just my recommendation.

    I hope this helps and answers some questions.

    Thanks

    Gary S
     
    Last edited: May 6, 2016
  2. c farmer

    c farmer Smoking Guru Staff Member Moderator OTBS Member SMF Premier Member

    Thanks a good read, I will have to save this for when I start my build.
     
  3. prestonk08

    prestonk08 Smoke Blower

    Great info! My stack is 6" by 16 and I think it's a little short so I may extend it. Great info for me to know since I'm 3/4 of the way complete with my smoker.
     
  4. inkjunkie

    inkjunkie Master of the Pit

    Great info, will be printing it and saving for next time...
     
  5. bearcarver

    bearcarver Smoking Guru OTBS Member

    Great Thread, Gary!![​IMG]

    I know I'll never build my own smoker, but I'm betting this thread will help an awful lot of guys!!! [​IMG]

    Very easy to read & understand, even for a Bear!![​IMG]

    Bear
     
  6. Thanks Guys    I felt a full explanation on how the stack works is a whole lot better than saying "Just do it this way"

    Again thanks

    Gary
     
  7. inkjunkie

    inkjunkie Master of the Pit

    I have a SERIOUS problem when someone tells me they are right.....but other than their limited experience they can't provide any real proof. I frequent a car site. A member there is a Lighting Engineer. Folks will often talk about how great their elcheapo HID conversion headlights are. Dan will come along and just recite his lab findings. The other folks will try to argue saying that their PERCEPTION is the HID conversion is great...to bad the scientific end of the argument is the polar opposite...
     
  8. jckdanls 07

    jckdanls 07 Master of the Pit Group Lead OTBS Member

    So you have me confused with your percentage of the cook chamber numbers... where did these numbers come from.. were they just pulled out of a hat ?? What i'm gathering from you is that I can use any percentage number that I would like ??
     
  9. I just took some random measurements for the example.

    Example:    Say you have a Cook Chamber that is 22 in. diameter and 60 in. long that equals 22,808 cu. inches, almost 100 gallons

    If you are wanting to figure stack vol. for your smoker, just figure your Cook chamber Cu. in.  and go with whatever percent you like.

    I like the 0.020         Your Cook chambers Cu.In. x 0.020  will give you the stack volume.

    If you need help figuring the length just give me a shout

    Gary
     
  10. is2326

    is2326 Fire Starter

    Man Dave really got to you... Great post and will be very helpful to many. Dam I went with that 36'' +/- LOL
     
  11. joe black

    joe black Master of the Pit OTBS Member

    Great read with a ton of information in a very understandable format. The only problem is that now I'm standing in front of my smoker with a tape measure and a calculator. I'll let you know how it comes out.

    Thanks, as always, for your efforts to benefit the craft, Joe
     
  12. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    they all have the same amount of volume and will work, but which one will look the best? It’s totally up to you.


    Gary, morning......

    Help me understand your thinking....

    Are you saying, if I use a 3" dia. stack, any length is fine.... or if I use a 6" dia. stack, any length is fine.....

    I'm confused as others probably are...
     
    Last edited: Oct 14, 2015
  13. There may be a few people who are confused because they are new and don't fool with smoker building all the time. I think you understand what I am saying completely, or at least you should because of all the advice and calculations you have given,

    For example, if you start with a 3” diameter, 6 foot long chimney and compare to a 4” diameter, 3.4 foot long chimney (both have the same volume), the flow rate is actually 2.75x higher in the larger diameter chimney. Even though the volumes are identical.

        For a large diameter pipe (5” and above say), the chimney volume has no effect on the flow rate- the damper completely controls the flow rate. For a small diameter pipe, its diameter and air friction matters. Some people like to use a chimney with enough friction that it acts as a damper and self-limits the flow and combustion. Others prefer to fiddle with the damper (and firebox vent).    A 4” pipe is a good compromise.

    No  I am not saying any length is fine    But you already knew that

    You know Dave, it would be much easier to get along, put our heads together and come up with the best calculator and information anywhere.

    Gary
     
  14. I have always believed that the diameter is more important than the length, and only consideration I put into what length to use was how it looked and if it got the smoke above eye level or tent level.
     
  15. bearcarver

    bearcarver Smoking Guru OTBS Member

    Thanks Gary!!

    That's amazing----I got the same thing when I read your original post, but I wasn't sure I got it right----It seemed too easy to me.

    However, now you confirmed my understandings from your original post were right----And I didn't know Jack about chimneys before reading this thread.

    Now if I was a bit younger, and of better health, and had any metal working talent, I could make my own Smoker!!! [​IMG]

    Bear
     
    Last edited: Oct 14, 2015
  16. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Gary...... So, what size and length chimney would you recommend, if a member asked, for a 22,808 cubic inch Cook Chambered smoker...

    There are 42 choices on your example.... They all can't be equal....
     
  17. Thanks Rib, My point exactly

    Gary
     
  18. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    Gary, don't you want to answer my question ?? Pretend I'm a new member and want to know what you recommend....
     
    Last edited: Oct 14, 2015
  19. smokin218r

    smokin218r Smoking Fanatic

    Nice post Gary!!

    Saved for future reference when hopefully, someday, I might have time to try a build!

    When that day comes, I know where I'll look for help........
     
  20. Good Question Dave, glad you asked, Like I said I really Like helping folks who don't know

    So ---------  We have a cook chamber that is 22,808 cu. in  about 100 gallons

    And you are right they are not all equal,  If you notice there are different percentages to use to figure stack Volume,

    I favor the 0.020    I think you like the 0.022   so lets go with the 0.022

    So that will give you 501.8 cu. in.

    A - 4" stack would need to be 40 inches long   ( that would be too long for me)  Remember step back and take a look !!

     A - 5" stack   25 inches    Hmmm    What do you think ?

    A  - 5.5" stack  ---  21 inches

    A -- 6" stack  ----  17 3/4 inches

    I would probably go with the 5 inch or 5.5 inch   for a uniform look, but that's just me

    Glad I could help Dave,   PM me any time if you need help

    Thanks

    Gary
     
    bearcarver likes this.

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