Help understanding air draw and venting?

[Tscarborough]

Re: Help understanding air draw and venting?

It applies to airflow no matter what the purpose. I have the information at work and will post the tables and charts tomorrow.

[brickie in oz]

Re: Help understanding air draw and venting?

That is as close as anyone on this sight has come to cosigning the "old folks method" of preheating a chimney/flu before lighting the fire

Man I can just hear the banjos starting up.......

[azatty]

Re: Help understanding air draw and venting?

It applies to airflow no matter what the purpose. I have the information at work and will post the tables and charts tomorrow.

The formula to calculate "q" (volume of air moved) uses the diameter of the flue and pi, so it's treating everything as a circle anyway. Then with the square flue, you probably have added air friction from the dead space in the corners.

[shuboyje]

Re: Help understanding air draw and venting?

Actually I believe q is based on the hydraulic diameter, which ends up being the diameter when it is a circle but is calculated via 4A/P where A is the cross sectional area and P is the perimeter. Hydraulic diameter essentially converts any shape to the round equivalent.

[Tscarborough]

Re: Help understanding air draw and venting?

That is the way it works. I thought I had the charts electronically but only have them on paper. The only thing that is not common sense is that square and rectangular flues are measured by outside diameter and round flues are measured by interior diameter. Don't ask me why.

[Tscarborough]

Re: Help understanding air draw and venting?

Chimney And Fireplace Sizing - A Discussion

[Sharkey]

Re: Help understanding air draw and venting?

I most definitely plan on having a vent landing area and chimney. I just don't care for the look of a big chimney flue sticking up from a beautiful oven housing. As such, I just wanna know how small/short I can make it.

I would suggest you make a brick flue and it probably doesn't need to be too high. I have yet to finish my oven but have been cooking in it for some months without any problem. At the moment it only has a flue that is two bricks high and this works pretty well - as you can see in the attached photos. I sometimes get a little bit of smoke out the front on startup when it is windy, but that's about it.

[sharptailhunter]

Re: Help understanding air draw and venting?

Thanks fornthe help guys. I understand laminar flow, is that one poiseuille's law? Can't remember, been outta school too long already. What I didn't and still don't understand is how a chimney draws air per se. I do understand the logic behind "warming up" the flue with a smaller hot fire before it really gets going. Nonetheless, I'm thinking of using a round 8" stainless flue, length to be determined later, and boxing it off using steel studs and finishing the exterior with hardibacker and rock veneer. Would that be more efficient than using square tera cotta flue?

[sharptailhunter]

Re: Help understanding air draw and venting?

Ppl tend to over think these things, just build it.

With all due respect, before I drop 2-3k quid on a project that will be sure to absorb a grand amount of my time and will be ever present in my yard, I shall put forth a little thinking and planning so as to maximize my investment and enjoyment.

And correct me if I'm wrong, but I perceive that the Venturi of the oven is in effect related to the inner arch to dome size ratio. As such, I understand the concept of making the inner arch height 63% the height of the dome. What I was trying to understand is how chimney design and size influences airflow through said chimney. It's clear that a round flue is more efficient than a square flue. And it's also clear that a larger diameter of flue will allow for more volume of air flow. What isn't clear to me is how does the height of a chimney effect airflow. An example that I see often in threads on this forum is somebody will write something along the lines of, "with a chimney that tall, it should draw real nice." Yes, I'm paraphrasing but you get the point. Why does a taller chimney draw better?

[Tscarborough]

Re: Help understanding air draw and venting?

Look at the link I provided it, it has the tables. For a given size appliance, a taller chimney allows for a smaller flue, and vice versa. As stated above, however, you do not need ANY flue for the chimney to perform to specifications.

If you choose to use one (which is of course recommended), then you can adjust the size of the flue by adjusting the height of the stack. Or you can simply follow the FB guidelines and be done with it.

[shuboyje]

Re: Help understanding air draw and venting?

Draw is powered by a temperature differential that creates a pressure differential. The flue contains a column of air that is hotter then the surrounding air and is therefore less dense. This leads to the air in the flue being at a lower pressure then the surrounding air. This pressure differential is what creates the flow we call draw, higher pressure air will always flow to an area of lower pressure to try and equalize, it's just the nature of things.

A larger volume of air in the flue will lead to more draw. Hotter air in the flue will lead to more draw.

[sharptailhunter]

Re: Help understanding air draw and venting?

Thanks for the replies. They have been insightful and helpful. That table also has been useful. Thanks again.

[david s]

Re: Help understanding air draw and venting?

More draw equals more oxygen for the fire to feed on. The larger the volume in the flue, the larger the draw. So this can be achieved by making the flue taller or a larger cross section (diam. if flue is round) or both.[/QUOTE]

[Faz]

Re: Help understanding air draw and venting?

Draw is powered by a temperature differential that creates a pressure differential. The flue contains a column of air that is hotter then the surrounding air and is therefore less dense. This leads to the air in the flue being at a lower pressure then the surrounding air. This pressure differential is what creates the flow we call draw, higher pressure air will always flow to an area of lower pressure to try and equalize, it's just the nature of things.

A larger volume of air in the flue will lead to more draw. Hotter air in the flue will lead to more draw.

The reason the longer chimney increases the draw is that this pressure differential has an accelerating force on the air in the flue for a longer time/distance - and so the final speed of the gases out of the top of the chimney is greater and therefore the overall flow is greater.

That's part of it anyway!!

[Gulf]

Re: Help understanding air draw and venting?

Man I can just hear the banjos starting up.......

Brickie,
Now that was funny
But, I have been PieAreSquared slam off of this thread! I think I'll just set here on the porch, pick my banjo for a while, listen to the frogs a while and then maybe I'll hear somesome kayakers coming down the crick D