Square vs round stack

[CanSmoke]

Good morning gents.
This is probably talked about somewhere but whad are the pros/cons of a round vs square stack

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[towtruck]

I don't think there is much difference in air flow but I prefer round if I have it.

[dacolson]

I hear that "air don't care" but I've also read that round promotes better draft. I have both and both draft well so ...

[tinspark]

I hear that "air don't care" but I've also read that round promotes better draft. I have both and both draft well so ...

"air dont care " haha.. Here is why it doesn't..
If you think about it, sheet metal workers have been installing round duct and square duct for years both push/pull and draft air just fine.
The restriction happens at either the length of duct or elbows. These restrictions are called losses. Just like it is easier to pull up milk shake from a short 6" straw opposed to a ridiculously long 4' straw ..or trying to push air in that same two straws. In an HVAC system it is a larger fan that used to overcome losses in a longer duct whether it is round or square. It is based on size vs. length of duct (aka volume) In the milkshake and straw illustration it is by sucking a little harder (haha sorry, just saying...) that allows for pulling up more in the straw. . The losses in a duct, or stack is the same with either round vs, square depending on the way they are built. Using an elbow vs. a straight shot up the top dead center of the stack. Yes there is loss in the elbow too. but not as much with a system that pulls as opposed to pushing air.

Square elbows vs. round elbows don't matter either, as long as the radius is correct and not too tight of a radius. The standard radius sheet metal elbow is 1-1/2 times centerline radius of duct. This is what mechanical engineers like installing per SMACNA standards, but they can go up to 7x radius. Not and issue with cookers though, because it is drawing air and not pushing it. So it is a little more forgiving. If someone opted to use a radius throat and heel square elbow out the back of their cooker, with flat cheeks, that would work fine as long as the radius was not too tight, It depends on what someone prefers on the looks, and what tools they have to work with. A straight stack out of either square vs. round makes no difference as long as the volume is accounted for. If we are talking efficiency, both work fine.. If we are talking looks, well...... that is another story and everyone has an opinion.

[dacolson]

Damn Tinspark! I know who I’m calling for my “phone a friend”

[tinspark]

Damn Tinspark! I know who I’m calling for my “phone a friend”

Haha, I like it!! I wasn't even drinking tonight On a roll I guess.

[Pete Mazz]

There is some friction loss when using square vs. round stacks. if you want "close enough" just figure a 4" square stack is equivalent to a 4" round but if you want to be exact....

https://planetcalc.com/6097/

[CanSmoke]

Ok thanks. I just noticed in the pit calculator that it gave you a shorter stack length if you use square. It made me think there might be other advantages.

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[Dirtytires]

No advantages....just what look you want. Size it properly and either will work.

[Pete Mazz]

Ok thanks. I just noticed in the pit calculator that it gave you a shorter stack length if you use square. It made me think there might be other advantages.

The PitCalc only uses stack volume which favors a square shape.

[etotore]

I add my 10 cents, there is less turbulence in round pipes, especially in corners.
Interesting article https://physics.aps.org/articles/v8/94
Does it matter for us, not likely, the quote is the main thing to taken from it.

The shape of a pipe has a large effect on the spreading of particles suspended in the fluid flowing through the pipe. Calculations show that round pipes produce symmetrical spreading along the flow direction, whereas rectangular pipes give an asymmetry

With stack, not sure if draft made by just making the stack height longer is taken into account fully.
Because outside/inside chimney temperature affects the calculations. So it's a lot of feel into it.
The pitcalculator has proven values that many has confirmed to work, still think there will be adjustment over time to it..
You have to use common sense too, you can enter 20" wide and 10" length for a 200 Gallon CC result 10 X 20 X 3.91
I will not think it will give as good result as 45 X 4 X 4.34, that is giving the chimney effect too.
So I think height is important.
See link
https://www.engineeringtoolbox.com/natu ... d_122.html

[tinspark]

its interesting that in the first article that a rectangular duct most resembles a round one unlike the square one, which left the researchers baffled.
The things that make you say hmmm.

Good article in the second one too dealing with building drafting. Much of what happens in a stack (hot side/ cold side no doubt is the same in principle as a smoker stack. Most want their stacks at least above head height, but lower than a doorway.
I appreciate the calculator on this site, and used it to design my center feed smoker and stack, and it worked great. No reason to deviate from something that is tried and tested several hundred times over.
The losses illustration with a straw and milkshake illustration more resembled the original subject that applied to my GF build with my flameboss attached, where someone noticed that I went square instead of round on my Gravity Feed build thread. (below) which I said "here is why" above. But I obviously should have been a little clearer. I wasn't worried about a deviation from the norm since My fan overcomes losses (heat losses and gains via fan controller) U guys are all over it... ha!

http://smokerbuilder.com/forums/viewtop ... 7&start=25

[CanSmoke]

Many thanks gents.
I am planning to build a RF oul of a 250gal oil tank. my other question then would be, on a tall oval tank like that, would you have the stack draw from the top or come further down to draw from a more central point.

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[Big T]

There have been a lot of smokers on the forum built from that same tank so it will make a great one. You can type in 275 oil tank in the RF search section and a few of them should come up. As for the placement of the stack, there has been a lot of debate on what works best and no rock solid answer. I always mount the stack as high as possible on the CC and I haven't had any issues, it doesn't matter whether it's on the top or the end of the CC.

[etotore]

for normal reverse flow.

For Center feed, I personally think that a low position, is best solution.
Reason for this statement is, lower air flow, and heat is rising in the chamber.
So with low mounting of stack will benefit more uniform temperature and better spread of the smoke in the chamber.
But I have no practical test to reference too, so maybe there are other things I have not thought about

[john_az]

Does diameter for the stack matter as far as draw is concerned? in the pit calculator, If put in a round stack, 6 inch diam, the clac. spits out "x" length....then, if I put in 12-inch diam stack, it just spits out a shorter stack length... but is there an optimal diam. for a stack?

If I had access to any size stack I could dream of, in any length.... is a shorter 12" diam stack better than a super tall 3" diam stack? yes they are extremes on the Diam.... but im just trying to understand how the diam. changes things.... or does it simply not matter at all... just pick any diam. so the height allows you to roll it out of your garage....?

Thanks so much for your help!

[dacolson]

Volume of the stack above the top of the pit is what matters. I imagine there is a diminishing return at some point but most folks look for two things - is the stack high enough to get the smoke out of my eyes? And will the stack fit under my garage door. You can hinge the stack if it's too tall but if you are constantly breathing smoke, I'd increase the height.

[john_az]

Volume of the stack above the top of the pit is what matters. I imagine there is a diminishing return at some point but most folks look for two things - is the stack high enough to get the smoke out of my eyes? And will the stack fit under my garage door. You can hinge the stack if it's too tall but if you are constantly breathing smoke, I'd increase the height.

OK, sounds great. I only ask because i feel like I read somewhere that if it's too tall... the exhaust air cools as its traveling up a taller stack, then slows as it cools, and boggs down the airflow exiting the pit.... not sure if there is any truth to that or not... but that was the reason for the question....

***LASTLY.... what about stack wall thickness.... does a thin wall steel smoke stack perform differently than a thicker wall steel stack? - other than weight, is there an advantage one way or the other?

thanks!

[dacolson]

I’d think you’d have to have a pretty tall stack for the air to cool enough slow it down.

I wouldn’t go under 1/8” - mainly for strength. Never heard of thickness effecting draw or anything.


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[Dirtytires]

Both length and wall thickness are completely up to you. Getting the proper cubic inch volume in your stack is the critical part.

[john_az]

great thanks guys..... another question...the plenum that connects the CC to the stack itself, should that air volume be considered in the exhaust stack, or does that just add volume to the CC.....

I guess I'm asking... when I build the plenum... should I subtract that air volume away from the air volume in the stack? It seems like that would/should be considered in the air volume the pit calc. recommends.... ??

thanks

[dacolson]

Air volume above the CC is what counts. Plenum doesn’t factor into it. That said, I’m all about airflow so I try not to have a restriction under the diameter of the stack anywhere in the plenum. If my stack is 6” ID, then my collector opening is 6” minimum in all dimensions. Just my thought.


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[CanSmoke]

Is there a problem with using rectangular pipe vs square. For example, the calculator calls for 56" 5x5, could i do 28" 5x10?

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[SheffSmoker]

Is there a problem with using rectangular pipe vs square. For example, the calculator calls for 56" 5x5, could i do 28" 5x10?

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Not at all, it's what I did on my last smoker so it would fit in the garage. Doubled up on box section.

[CanSmoke]

Great thanks

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