500 gallon smoker build - Page 3
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Don't forget the added friction... 8" square has 32" "circumference".... where 8" round ha has 22.6" circ.... big difference.....
According to Feldon's 5.5" needs to be 82.64 " tall Way to tall, 2 - 5.5" about 42" still pretty tall, 500 gal is a pretty big smoker besides working properly you want it to look good, especially after you put hours of work into it. That was a good deal on the 5.5" It it were me I would keep checking and find some 8" One that size smoker it will look and work better. The problem with real long smoke stacks besides looking funny is cooling, As the smoke travels up a long stack it cools, cooler air is heaver and you don't get the flow that you really need, not saying it won't flow, just not as efficiently. Large smokers need to flow a lot of air, heat and smoke, you don't want stale smoke staying in your cook chamber. A 500 gal smoker needs 1,964 cu. in vol.. The problem is length you can get the same volume from 2" if it is 635" long . My opinion from the smokers we have built, we try to go with a stack that is proportional to the cook chamber, and as Dave said around 30 to 35" now that being said 8" would need to be 39", according to the calculator But when you get to that diameter the length is not as important, you can easily make it 30 to 35 " with no problem
I wouldn't do that I'd just use the 5.5" how long did you decide to make them? I have been doing some research on stack size and length
One opinion I read was when you get to the 5" , 6" and above that the length becomes irrelevant . it doe's not need to as long as what Feldons calculates , Feldons takes the cubic inches and converts them to whatever diameter pipe you put in to that length,
Here is one of the answer I got back ;
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. Other prefer to fiddle with the damper (and firebox vent).