Science behind smokestack size?

[fourthwind]

Ok so I know via the excell sheet that my stick burner is going to need a 36 inch exaust stack.. My question is what drives the length and diameter? I have looked but cannot seem to find a real answer. Does the length of the pipe help create back pressure and regulate flow, or does it have to do with pulling air through the system with heated air, or none of the above???? somone has got to know the reason behind it all.

Also should I put a butterfly valve on the stack? everyone seems to say that you should just let the stack run full open and control flow from the fire side, but I see about every large scale stick burner out there has a valve. ???

 

[rj2316]

I'm no expert, but I do have a degree in mechanical engineering and am in the process of building a smoker also. My understanding is that if your smoke stack is too long, the smoke will have a hard time leaving the cooking chamber. This will result in stale smoke in the chamber which would cause creosote to build up on your food. My guess is if the stack is too short the smoke would leave too soon and it would be hard to maintain temperature and get any smoke flavor on your food. Again, I'm not an expert by any means, but this is what I think. I came up with this by comparing water flowing through pipes. I won't go into explaining that logic, but that's what I used. I hope this helps. The people that know will be coming soon and will surely point you in the right direction.

 

[mikey]

I'm not so sure that that type of logic works in this case. Heat & smoke naturally rise. Water flowing through a pipe vertically upwards without the aid of a pump is impossible. Not a good apples to apples comparison, imo.

 

[rj2316]

Not really how I meant it. I probably shouldn't have mentioned it. I'm not going to try and articulate what I was trying to say. The other stuff I'll stay with, bit forget the water example. It's my head,but that doesn't help anyone here.

 

[wavector]

http://webpages.charter.net/tomchism...alculators.htm

 

[fourthwind]

I have the calculator thanks.. I was hoping for more detail on what was going on, but I think what rj2316 is saying is that an exaust that is to long creates back pressure on the system, and the smoke will have a hard time flowing creating creasote. (bad eats). It actually may explain why you see a lot of butterfly valves on smoke stacks. If your stack is by design shorter than it should be for proper flow and heat retention, then putting a valve in would help regulate the short stack. Any other thoughts?

 

[mikey]

What would you base flow out of the exhaust on? CFM? And more importantly, how would you measure it while adjusting the valve? Since the "calculator" only gives pipe diameter & length of the pipe, maybe the "calculator" is correct. On Edit: I would look closely at either a Lang or a Klose, since these are the "premier" pit builders, and see if you see a valve on the exhaust stack.

 

[pineywoods]

My Lang has a valve/flapper on the exhaust stack and i use it to keep big bugs out

Actually I have heard that some pros do adjust the exhaust they never close it but they open it at different angles to hold smoke in longer or let it out faster. The science I don't know mine stays open and I adjust heat with the intakes

 

[mikey]

The science I don't know mine stays open and I adjust heat with the intakes

That's probably what most folks do. My Bandera has a flapper on it also, which I keep closed when it's not in use. After plugging in all the dimensions of my smoker into the "calculator", my exhaust is approx. a foot short. Might change it out or just leave well enough alone. Nice topic for discussion.

 

[fourthwind]

when you examine the calculators, the only conclusion I can come up with is the flow rate based upon the size of the fire box, the inlet into the fire box, and the outflow from the fire box to the smoke chamber all have a direct affect on the length of the stack. The length of the stack must have to do with back pressure to keep the flows at the optimum level for heat and smoke levels. Hopefully an engineer type that understands this can confirm or shoot down our idea's. The curious part of my brain just wants to know. I have seen some of the competition smokers around here use these big water main type valves where they can make really small adjustments to the flow.

 

[lightfoot]

My smoker has a 4" I.D. stack and stands about 2 foot tall. I got tired of the smoke in my eyes (every time i got near) and bought a 6 foot long metal dryer vent extension, to get the smoke up away from me. With the extension in place (it's removable), the stack is over 8 ft in the air. I've used it both ways, it has not affected the way the smoker performs at all. It cooks exactly the same with or without the long pipe. It may depend on the smoker-i dont know-some may say it has an effect on their smoker, on mine it does not. My stack does have a butterfly valve inside it, but i leave it wide open, with or without the stack extension in place.
I do know that my eyes dont burn like they used to, and i don't smell like i've been to a fire. so i'm sold on the tall stack.
Just my .02 worth.

 

[smokemaster85]

I Build smokers for a living an your stack should be 1/3 of the length of the smoker , and the diameter should be 20% of the tank diameter.

 

[doyne]

 
I Build smokers for a living an your stack should be 1/3 of the length of the smoker , and the diameter should be 20% of the tank diameter.

I am not sure that would hold true for every situation.  Your smokers are probably very good and that holds true for your design but there may be some exceptions.  For example my Oklahoma Joe has a Chamber/tank length of 35" and the diameter is 17",  so the stack should be 11.6" x 5.25"??  The above referenced calculator says my stack should be 25.7" x 3".  The stack on mine is actually 17" x  3".  It works great.  But I may add some length to get the smoke away from the face like Lightfoot did to his.

The thing about smoke and heat is that it rises.  So if your stack is 10" or 30" in length it shouldn't make that much of a difference at the temperatures we usually smoke at.  However, I believe the diameter does make a difference in the amount or volume of smoke that is allowed to escape the chamber.  The baffle is used to regulate how much volume you want to allow to escape. ie. changing the diameter of the pipe end.  Obviously if your stack is to small in diameter you will have issues.  A 2" dia. stack would not be good.  I think that the diameter probably should be around 15% the dia. of the tank.

With that said, the damper or baffle, as we all know, is also used to control the heat.    The fire chamber needs air to burn the fuel so the baffle there is used to regulate the air intake thus the temperature.  So the two work in tandem to control the heat and the length of time the smoke stays in the chamber.  

I personally don't believe the length of the stack really makes that much of a difference.  The diameter is what determines the amount of smoke and heat released.  The temp control is critical and I find that the fire box baffle is the primary way to control that.  However, if you completely close the stack baffle you won't have air flow and your fire will die eventually.  

The real secret to a great smoke is to know your smoker and learn what it takes to get the results you want.  A few mods can mean the difference between great meat and outstanding meat.  In my humble, uneducated opinion, the science is simple.  Heat and smoke want to go up and they will find a way to do that.  Determine the route and velocity you want to allow that heat and smoke to take and put your meat in its path.  

  That is just my 2 cents worth and about all it's worth.

 

[daveomak]

I Build smokers for a living an your stack should be 1/3 of the length of the smoker , and the diameter should be 20% of the tank diameter.

How did you arrive at that number......

Dave

 

[smokemaster85]

The engineers where I work came up with that formula for what works best for there design . and 15% would be good  They like 20% so incase of overheating you have enuff stack to evaquate heat quickly with a wide open stack valve. I have been using a 1 1/2" to 1' on my own tanks and it works grate 2 foot tank 3" stack as far as hight I question there logic I made my wife a smoker with a 4 Foot stack on a 24"X 48" tank to keep smoke out of her face and it works just as good.  I just posted as a guideline to start out with. good luck building your smokers.

 

[daveomak]

OK..... so I ran the numbers for a 48" x 20" CC and my calculator comes up with a 20" tall stack X 4" diameter...... I would make it 30" tall just because.... Pretty much what your 16" tall x 4" diameter...... Your engineers probably copied us.... HAHAHAHAHA...... our members make some darn good smokers....

 

[smokemaster85]

 my intent was not to ofend you I was just trying to lend some geathered knowledg . I dont know where thay got there science from, Im wanting to expand my knowledg base so I joined your forum.

I m curantly building my wife a smoker 24"X48" and I will use a 3" X 20 " stack on it reverse flow design

 

[daveomak]

I Build smokers for a living an your stack should be 1/3 of the length of the smoker , and the diameter should be 20% of the tank diameter.

One more question.... Is that stack number for a Side Fire box with tuning plates OR a Reverse Flow Side Fire Box......

 

[smokemaster85]

Side Fire box with tuning plates

 

[daveomak]

OK.... that makes sense...... For a reverse flow, you need to enlarge the exhaust stack..... diameter by a little and height by a little.... The friction loss from the increased surface area and 180 flow direction change at the end of the RF plate.....

that being said, I like the formula for the chimney using the CC.... easy....