# 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.  ???


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## 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.


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## 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.


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## 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.


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## wavector

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


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## 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?


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## 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.


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## 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


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## mikey

Pineywoods;352739 said:
			
		

> 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.


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## 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.


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## 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.


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## 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.


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## doyne

SmokeMaster85 said:


> 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.


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## daveomak

SmokeMaster85 said:


> 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


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## 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.


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## 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....


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## 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


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## daveomak

SmokeMaster85 said:


> 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......


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## smokemaster85

Side Fire box with tuning plates


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## 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....


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## cmayna

Is that formula only for a certain type of smoker such as a charcoal one?  What about Electric and propane smokers?


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## cmayna

Question:  Does type of smoker (charcoal, propare, electric, etc)  dictate different forumlas?


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## ccjammer

ccjammer here,i have a 20 inch x 48 inch long cooker with a  18 x 20 square fire box offset

it has a 3 inch dia. x 30 inch long stack on the side top and was wanting to replace with a 4

inch dia. that is 4  foot long,is that sufficient enough or do I place it towards center of pit,any

.answers will be very appreciated.


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## pootin pig

Trying to determine exhaust stack size. I'm wanting to do dual stacks. I've taken an old pig cooker that is 60" longX30" deep, and 30" high. I built a firebox that is 24x24x30 with an opening that is 30x 51/4. The firebox is mounted on the lower back of the cooker and I have a plate inside of the cooking chamber to spread out the heat. I want to do dual stacks on the top at the rear corners. the only calculators I found give you single stack only. I'm thinking like 2- 3" stacks around 32" tall should do it. What do you think???????? Would that be the same as 1- 6' stack?


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## daveomak

No... not the same....   actually exactly half the size needed....   4 - 3" stacks would be the same...  I would go with 2 - 5" stacks...


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## bitterroot mt

I know this is a old thread but based on the 15%

and 1/3 theory

if my tank is 92 Long

it is 38 wide

My stack should be  6in wide and 31 Tall.

I am building a firebox from 2 55 gallon drums w with doors at either end like drum stoves.

23 wide 70  long

I realize it is a little undersized .... all I could scrape up.

.

2  6 inch inlets between the firebox and cook chamber .

Set up like a diamond plate Pit Mod.

Anyway.. Could I use   two   3 inch or 4 inch stacks  with dampers.on this and mabey a little shorter   say 24-26 inches tall

and be ok ..

MY thought is that it would hold in heat longer and  still let out the correct volume.... am I correct here?

Thanks


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## daveomak

Are you using this tutorial for your build ???

http://www.smokingmeatforums.com/a/reverse-flow-smoker-how-to-calculate-build-tutorial


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## dave schiller

I used to heat my house with a wood burning stove and from all I read and learned, the draft effect of the chimney is caused by heated air being less dense (lighter) than cold air.  The heated air in the flue therefore rises, inducing (pulling) cooler air into and through the heating chamber.  Since the heated air rising up the flue is gradually cooled by contact with the flue, I guess it's possible that if the flue is too long, the heated air will drop to the temperature of the ambient air, thus eliminating the draft effect.

Conversely, if a fan is used to force air into the heating chamber, as with a BBQGuru or similar system, then it wouldn't matter if the flue gas is too cool to induce draft through the system.

My guess is that the optimum length for a chimney on a smoker will vary with the ambient air temperature.  That is, during hot summer days, longer flues can be used.  But on very cold winter days, a long flue may result in too much cooling and reduce or eliminate the draft effect.


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## Ron Burg

Looking to do a vertical smoker. 
50 high x 23 wide 18 inches deep.

Do I need 2 smaller stacks on each side...
1 larger one in back 
Ranger uses no stack, on one model  just open vent in bac k.

Thanks


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## anstissk

daveomak said:


> 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....



Dave, so just because I am curious...I have seen on a few builds where the OP did the calculations in the tutorial and you mention that you would make the stack longer. In this example, you mentioned you would make it 10" longer just because....
Is there something driving that decision, do you like taller stacks in general? Just curious


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## daveomak

Natural draft requires volume in a stack to reduce friction and pressure differential to facilitate draft...    Pressure differential can be from height of stack and temperature differential of gasses in the stack and ambient temperature.......


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## anstissk

daveomak said:


> Natural draft requires volume in a stack to reduce friction and pressure differential to facilitate draft...    Pressure differential can be from height of stack and temperature differential of gasses in the stack and ambient temperature.......


Makes sense. So typically just adding some additional length as it most certainly will not hurt anything.


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## offset1945

This topic keeps going and going .....
In the real world this is Flu Stack Science and you can have a job where this is what you'd care about.
I know people, myself included have looked for the hard science here and I can tell you that you will not exactly find your answer in the BBQ world, it is all about opinion and 'feel'.

I can say that the calculator I 'feel' is spot on with its CC to FB ratios and that as Dave suggests needs to allow for slightly more volume on the stack calculation.

Now I am talking 'volume' .... so if Dave is suggesting 10" more for a 4" inner diameter stack, I feel that is spot on and I am in full agreement.

The calculator is doing this for an FB that is 20" wide and 24" long ( a VERY popular size) based on one saying they have a 4" stack diameter:
Volume =  πr2h= π×22×30= 120π= *376.99111843078 inches3* 

It is saying your stack "volume" should be 376 cubic inches and therefore the stack should be 30" long based on opting for a 4" stack diameter.

The number to care about there is the 30 ... that is what the calculator solved for .... 

The 376 cubic inches comes from the notion of 5% of the firebox volume should be the stack volume.
The calculator takes what it feels is the correct stack volume and then based on your input for stack diameter give you the length of the stack.
Google 'Volume of a Cylinder' and you will find plenty of calculators.

What Dave and I are saying from this calculator perspective is that the % of firebox volume used to derive stack volume should be higher ... or perhaps not based on FB volume, but that is getting sideways.

So to say it again, it is all about volume and if Dave suggests adding about 10" and if he is taking about a 4" diameter stack I fully agree.
(.i.e. you would not add 10" if your stack diameter is 6")

Cheers to all!


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## MikeFromTexas

Considering the air just outside the inlet of the firebox and the air right at the top of the smoke stack and energy balance can be made. An energy balance between these to points takes into account all forms of energy losses & additions between these two points. Energy losses can be broken down into what’s called minor losses and major losses. Minor losses are things like sudden contractions (like between the firebox & cooking chamber; also from cooking chamber to exhaust pipe), bends (like the 180 deg turn of a reverse flow), sudden expansions (end of exhaust pipe). Minor losses are directly proportional to the square of the velocity locally where the loss is occurring.  Major losses are those associated with friction. Friction derives from the viscosity of the air and is directly proportional to the square of the velocity and also the length-to-diameter ratio. The flow in a smoker is what’s called buoyancy driven flow or also known as natural flows. These flows are driven by a temperature difference. The air right outside the firebox is cool and has high density. The fire in the firebox heats the air inside the firebox. As air temperature increases, the density decreases. In the presence of gravity, the lower density will rise. As the lower density air rises, the cooler air from outside the firebox is moves in to fill the space of the air that has moved from the firebox into the cooking chamber. So all the magic of a smoker is really about achieving the proper flow rate of air which is influenced by the above listed factors. The energy losses hinder the amount of flow.
The use of a damper is really an obstruction to the flow and as such is a minor loss used to control the flow rate. This limits the amount of air that can then enter the firebox. The exhaust pipe diameter to length ratio is most important, not diameter or length alone. The friction energy loss developed by the exhaust pipe walls is not adjustable.


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## Timbo77

daveomak said:


> Natural draft requires volume in a stack to reduce friction and pressure differential to facilitate draft...    Pressure differential can be from height of stack and temperature differential of gasses in the stack and ambient temperature.......


I am making a smoker out tank that is 5ft long and 33" wide barrel. My firebox is mounted on one end. The heat is coming into barrel 7" from bottom.  The firebox is 26"W x 20"L x 24"H. My question is my stack is at the other end of smoker so what size and long does it to be? And how high so it need to be?


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## daveomak

Picture please..  Maybe a 6" stack 40" above the CC...
Firebox is not used in the calculation...  Only the CC volume....


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## Timbo77

daveomak said:


> Picture please..  Maybe a 6" stack 40" above the CC...
> Firebox is not used in the calculation...  Only the CC volume....


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## daveomak

I'm trying to figure out the size of the opening between the FB and CC...  Ususlly it's a 1/2 moon shape following the curve of the CC..  and to the top of the FB.....

Read this tutorial...  It will help explain stuff...






						Standard Reverse Flow Smoker Calculator... by DaveOmak and others... Ready to use..  rev5.. 6/19/15.
					

Here's to "Alien BBQ" and the folks that first put the calculator together and "1728 Software Systems" for this great "Circle Calculator" and all the folks at "SmokingMeatForums", with a special shout out to Ribwizzard, that contributed their ideas and feedback while this thing was coming...




					www.smokingmeatforums.com


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## Timbo77

daveomak said:


> I'm trying to figure out the size of the opening between the FB and CC...  Ususlly it's a 1/2 moon shape following the curve of the CC..  and to the top of the FB.....
> 
> Read this tutorial...  It will help explain stuff...
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> Standard Reverse Flow Smoker Calculator... by DaveOmak and others... Ready to use..  rev5.. 6/19/15.
> 
> 
> Here's to "Alien BBQ" and the folks that first put the calculator together and "1728 Software Systems" for this great "Circle Calculator" and all the folks at "SmokingMeatForums", with a special shout out to Ribwizzard, that contributed their ideas and feedback while this thing was coming...
> 
> 
> 
> 
> www.smokingmeatforums.com
> 
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> 
> View attachment 446364
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> View attachment 446361
> View attachment 446363
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> View attachment 446362


I was wanting to build a offset smoke not a reverse flow


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## daveomak

You still need air flow ?????
The numbers in the tutorial work perfect for either type of smoker....


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## Timbo77

Timbo77 said:


> I was wanting to build a offset smoke not a reverse flow


Ok. I was making sure and I agree with you. Just trying figure out what I need to do because it has a 4in stack on it now and I'm losing heat out of the stack. So I'm just wondering if I need a bigger stack or if they put the stack to low or what I need to do


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## daveomak

This would be closer to a Jambo designed smoker....   They work very well.....






       A 6x25 opening between your FB and CC...  and the exhaust plenum installed just below the food grate with a 6" exhaust stack....  



T
his is smokin-aces version of a "Jambo" design...     It works well....






						First Big Smoker Build. Advice welcomed!
					

Hey guys,  I am starting my first "big" smoker project, and of course big is relative. I have built two 55 gallon double barrel smokers in the past (one this May and one a few weeks ago), but I decided to step into the big leagues and make a trailer smoker.   The smoker I am making is a tank...




					www.smokingmeatforums.com
				




The key is air flow...   Large enough openings and stack to allow for free air movement without restriction....

...


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