Originally Posted by razrbakcrzy
The whole effect of heat rising and creating a draw up through an exhaust stack is thermo dynamics and fluid dynamics at work. If you think its absurd, your absurd....
I have personally experimented with diferent length exhaust tubes from the barrel flat top and I can with the exact same inlet openings, change the temp by as much as 50degrees with a longer exhaust tube. I currently use a 6" exhaust but with a 12" stack the drum will run 50 degrees hotter than the 6" with the exact same setup....
with the normal offset location of the bung hole on a flat top drum the heat on the side oposite the exhaust will rush up against the top of the flat lid and turbulate causing a high pressure zone(turbulent flow), where the heat that rises adjacent to the exhaust opening will rocket straight up and out the 2" exhaust (laminar flow) causing a vaccum(low pressure) and thus a positive flow or a draft in the upward orientation....
With a domed lid the heat all rushes up toward the center exhaust and out, thus the webber lids usually can get by with a much smaller exhaust because of this effect. (the eight circumferential holes would eliminate the turbulence since the hole are all equidistance from the center thus requireing less voulmetric flow)
If you notice the exhuast on a weber usually comes out with a fairly high flow rate and a nice amount of force. Conversely try looking at a drum with an offset exhaust, where the exhaust comes out a failry low flow rate.... but with a higher volume. It's all about volumetric displacement, how much air can you move thru the inlets up through the coals across the food grates and out through the exhaust..
Now, Im done with this. This place was not meant to argue or point finger at each other and call each others ideas absurd, Instead we should be considering all the things each other have to say in the intrest of our mutually shared hobby and passion... This was not meant for the person who simply wanted to discuss this topic
, have a nice day..
Heres a little light reading for you...
I'm not going to drag this out and argue over a 2" exhaust but I will make some points. I am an engineer both by education and profession, please don't throw around thermo and fluid dynamics to justify your point. In this case with a vertical smoker, you are building heat and exhausting it in the same concentrated area and getting some benefit of convection. You are not trying to draw it to one side like you would an offset smoker which would require a larger volume of air moving through the cook chamer to accomodate. In addition to your comments regarding the longer pipe cooking at hotter temperatures, that is absolutely correct. The length of your pipe serves to add restriction to the exhaust, effectively reducing the air going through the system. In the end you can adjust the flow of exhaust like weber does or any other pits that have variable openings in the exhaust or you can add or remove length to the exhaust. The length of pipe is the same thing as how much vertical head a pump can pull - it is distance dependant when horsepower remains constant.
In the end if we are talking about smoking temperatures without any type of stoker system four 3/4" exhaust holes are enough. If you might want to cook hotter than 350 then I would say you would need to add additional intake and exhaust.