- Jan 6, 2011
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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 |
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
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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.
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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
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