SMOKE STACK BASICS

Mike rainwater · Jun 5, 2018

ribwizzard said:
Thanks, Ive always used an unwritten formula something like;

CC shape should be like width should be about .35 length

Fire box width= CC width
Firebox height= CC width
Firebox depth= 4/5 CC width
Firebox to CC mounting height 1/3 CC.
And of course stack as mentioned above. Works good for me and sometimes I'll compare it to feldons if its a new size cooker just to see if im close.

Bigger cookers ill downsize the firebox some, and real small ones ill go bigger.

Greetings, I'm building a smaller R V with your suggested ratios. The propane tank is approx. 14 wide by 40 long. You made the recommended FB to CC mounting height to be 1/3. About how far in would you recommend cutting back from the end? I'm a first time newbie. I appreciate any suggestions. Your info has already answered a lot of questions. Thanks so much.
Mike in B-ham

daveomak · Jun 5, 2018

Mike, morning.... Below is a tutorial that will get you a great smoker.... Dave

https://www.smokingmeatforums.com/t...eady-to-use-rev5-6-19-15.172425/#post_1264161

Mike rainwater · Jun 6, 2018

daveomak said:
Mike, morning.... Below is a tutorial that will get you a great smoker.... Dave

https://www.smokingmeatforums.com/t...eady-to-use-rev5-6-19-15.172425/#post_1264161

Thank you.

darkshadow · Jun 6, 2018

Great thread and reference

Good way to look at things! I got it just the way it was laid out.

gary s · Jun 7, 2018

I have been rethinking my smoker stack formula I was using .020 Dave uses .022 What really is working best is up around .030. With your damper open this lits you move a lot of air and smoke and no stale smoke I am leaning more toward bigger stacks all the time.

Gary

daveomak · Jun 7, 2018

Gary's right on..... BIGGER is better...

prestonk08 · Jun 7, 2018

When I made mine I used 6” pipe for my stack because that’s what I had on hand. Mine is too short though, it moves air well but smoke hits me in the face so I’m gonna extend it another foot or more when I get more pipe. You’re right it is a great thread!

Remy Bernabela · Jun 9, 2018

500 gallon. 8" x 52" tall pipe. Works like a dream. Temps are 3 F off without deflector. Here on the island we have a very heavy hard wood and with the door as a intake damper people just don't know this thing is fired up. I saw some other post here where Dave recommended 9" pipe. But the guy ended up using a 8" x 48" tall pipe and was happy with it and he posted some nice thin blue smoke and good temps. My own numbers came close to his. So I went with it and have 0 regrets!!! Last thing I did on it was put a damper on the smoke stack to see if I can improve on something even though I didn't had to. But the only thing that damper does is improve the top to bottom temps dead even. So I'm a believer of the huuuuge stack whenever possible. For this size of cooker anyways.... I think all this science and discussion only applies within certain sized cookers.

Mike rainwater · Jun 9, 2018

Remy Bernabela said:
500 gallon. 8" x 52" tall pipe. Works like a dream. Temps are 3 F off without deflector. Here on the island we have a very heavy hard wood and with the door as a intake damper people just don't know this thing is fired up. I saw some other post here where Dave recommended 9" pipe. But the guy ended up using a 8" x 48" tall pipe and was happy with it and he posted some nice thin blue smoke and good temps. My own numbers came close to his. So I went with it and have 0 regrets!!! Last thing I did on it was put a damper on the smoke stack to see if I can improve on something even though I didn't had to. But the only thing that damper does is improve the top to bottom temps dead even. So I'm a believer of the huuuuge stack whenever possible. For this size of cooker anyways.... I think all this science and discussion only applies within certain sized cookers.

View attachment 366782

Mike rainwater · Jun 9, 2018

Remy Bernabela said:
500 gallon. 8" x 52" tall pipe. Works like a dream. Temps are 3 F off without deflector. Here on the island we have a very heavy hard wood and with the door as a intake damper people just don't know this thing is fired up. I saw some other post here where Dave recommended 9" pipe. But the guy ended up using a 8" x 48" tall pipe and was happy with it and he posted some nice thin blue smoke and good temps. My own numbers came close to his. So I went with it and have 0 regrets!!! Last thing I did on it was put a damper on the smoke stack to see if I can improve on something even though I didn't had to. But the only thing that damper does is improve the top to bottom temps dead even. So I'm a believer of the huuuuge stack whenever possible. For this size of cooker anyways.... I think all this science and discussion only applies within certain sized cookers.

Wrhadftbthr we my

nanuk · Oct 2, 2018

gary s said:
Smoke Stack Basics

I have a friend that has a PHD in Physics ......

<Quote>
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.
<End Quote>

I read this and find it interesting.... it is exactly the opposite of what the Solid Fuel Fire Code (that I got from my local city fire dept) says about chimney stacks.

I can see this in a pipe that is moving air, but with a fire box/wood stove you don't need just volume, you really do need velocity!

In the code book, if you have poor drafting, it recommends to go down in diameter 1" which increases the speed up the chimney, as it has less time to cool.
Granted we are dealing with insulated chimneys, but with short stacks, I don't think insulation matters much on the chimney.

I think there must be a balance somewhere...

Ratrod54 · Jan 24, 2019

Sorry to resurrect an older thread, but I am looking for some clarity. I’m good with the formula. I’d like to confirm that you’ve said larger % of CC is better. I see where you mention you’re leaning more towards .03 now. Still the case?

Also, stack height continues to be mentioned; however, if I have a stack which has an elbow, I’m assuming that the horizontal portion has to be considered when considering volume. So height isn’t length in the vertical sense, right? Does that question make sense?

daveomak · Jan 25, 2019

If it has an elbow, it should be a radiused elbow, not a 90 deg..... The length of the horizontal part does not figure in to the height.. only the vertical measure.... It's height that makes draft... ( CC Volume in cubic inches X 0.022 = Exhaust Stack Volume in cubic inches, above the CC)....
A plenum is preferable over an elbow...

Ratrod54 · Jan 25, 2019

Thank you for the information, Dave. I really appreciate it. I have been looking for information on how to size the plenum. I hadn't planned on adding one, but it shouldn't be hard to do so. I see you say the plenum is preferable over an elbow. I am assuming that the stack right into the CC is the best way to go? I am limited on room but I believe that I can make it work without a plenum. One other question regarding the stack. I see that you have mentioned that it be placed a minimum of 2" into the CC (assuming I am not using a plenum). My smoker is an 80 gal tank. Does the portion of the stack that is inside the CC need to be square or should it be cut at an angle?

Thank yo again!

daveomak · Jan 25, 2019

If you can, flare the end... Probably not.... A flare increase the air flow maximum for the size duct...

If you can see what I'm talking about in this picture....

..

Exhaust stack installs.....

dwaineb · Mar 1, 2019

I read a majority of this thread and find it interesting. Also being new I have read alot of what Dave has posted on Reverse flow builds so I find it extremely helpful.

I am embarking on building a 270 gallon RF smoker with warming cabinet. I picked up a round tank 36 x 61.5 last weekend. in doing all the math and calculations ( which i will ask in the build section when i start it) I will be using a 6" stack but to stick to the 30% rule now should the stack still be roughly 40"??

Also does it make a difference if the stack is positioned 3" above my lower rack, in the middle, 2" below the top or fluch mounted??? I was thinking the lower the better to give more air/ smoke flow in the tank vs coming around, following the contour of the tank and then exiting.

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