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-   -   The vent and oven draw (http://www.fornobravo.com/forum/f6/vent-oven-draw-3440.html)

james 02-29-2008 09:59 AM

The vent and oven draw
 
I just lit my first fire in our Artigiano with the vent and chimney pipe attached. I know you already know this, but wow! What a difference. I've been curing and cooking with the oven for some time without the vent/chimney and after the oven gets hot, it does a so-so job of venting smoke and hot air out the top of the opening. But I lit a couple of damp concrete bags for my vent test (to make sure no smoke was leaking out of the assembly) -- and it was great!

The top half of the oven opening was sucking up the smoke up and out (it was full) and the bottom half was drawing in cold air (it was completely clear).

It's so cool seeing these ovens work properly. Eccellente.
James

asudavew 02-29-2008 10:07 AM

Re: The vent and oven draw
 
Way to go James!

Pics...............:confused:


;)

Dutchoven 02-29-2008 12:12 PM

Re: The vent and oven draw
 
I have had so many guests comment on that when they see it...the clear air at the bottom and the smoke and gases out the top...could really explain to the kids the reason for crawling out of a burning building
Dutch

jcg31 02-29-2008 05:46 PM

Re: The vent and oven draw
 
James, probalby been asked elsewhere but I can't locate it. Are there "best practice" dimensions for vent and flue"? My vent opening is 19" x 4", nearly the entire width of the arc and reduces down to accomodate an 8"x12"x24" flue. At this point (two days of curing fires) I am not particularly impressed with my draw. Certainly not the show you describe above. I am wondering if extending the flue (by adding another 24" length), reducing the size down to 8"x8" or both would help the draw. Any thoughts?

Thanks for any help.
Jim

RCLake 02-29-2008 05:55 PM

Re: The vent and oven draw
 
Quote:

Originally Posted by jcg31 (Post 25371)
James, probalby been asked elsewhere but I can't locate it. Are there "best practice" dimensions for vent and flue"? My vent opening is 19" x 4", nearly the entire width of the arc and reduces down to accomodate an 8"x12"x24" flue. At this point (two days of curing fires) I am not particularly impressed with my draw. Certainly not the show you describe above. I am wondering if extending the flue (by adding another 24" length), reducing the size down to 8"x8" or both would help the draw. Any thoughts?

Thanks for any help.
Jim

I'm also very interested in this question. My vent area is almost 7x7 with a 24" vent. I have another 24" section if needed, but am concern about the weight, though it seems real sturdy now.

Dutchoven 02-29-2008 06:31 PM

Re: The vent and oven draw
 
When thinking about oven draw you are trying to create a constriction in space to create an area of low pressure...generally speaking I think most try to reduce the space by half...in square inches...so jcg 4x19=76sqin...8x12=96sqin...you should see if you can change flue tile to a 6x6...although 8x8 would work...taller is better also if you have the right ratio in the reduction...as Dmun said in a post in another thread "serious boilers have very tall chimneys because they supercharge the firebox"...
RC if your vent opening is indeed 7x7 you should look to reduce to a 4 inch flue...or make the vent opening larger
hope this helps
Dutch

wlively 02-29-2008 06:34 PM

Re: The vent and oven draw
 
By your listed dimensions the opening is 76 sq inches and it transistions to an opening of 96 sq inches. Thanks to Bernoulli's law we know that when a moving fluid slows down pressure increases and when it speeds up pressure falls. Your vent by transitioning to a larger opening slows down the air at that point and raises pressure. What you want is the opposite to happen, so it would be better to reduce the area of your flue to a value less than the area of your opening. That will cause the air to increase speed and the pressure in the flue to be less than the vent opening. The 8x8 you suggested would seem fine.

My flue is 24 inches (8inch duravent) and draws very well, so you don't need more for good draw.


Quote:

Originally Posted by jcg31 (Post 25371)
James, probalby been asked elsewhere but I can't locate it. Are there "best practice" dimensions for vent and flue"? My vent opening is 19" x 4", nearly the entire width of the arc and reduces down to accomodate an 8"x12"x24" flue. At this point (two days of curing fires) I am not particularly impressed with my draw. Certainly not the show you describe above. I am wondering if extending the flue (by adding another 24" length), reducing the size down to 8"x8" or both would help the draw. Any thoughts?

Thanks for any help.
Jim


gjbingham 02-29-2008 09:14 PM

Re: The vent and oven draw
 
Jim,
I'd give it time. I love the physics utilized in the the last two posts, but every oven is different, if not only for the reason that the archways and vents we build are not made from a cookie cutter.

Cool fires will put off more smoke and will not draw as well out the chimney. Deal with the smoke and the stain on your brick for the time being. Cover the brick with masking if its killing you to watch it get black. Once you get the dome to burn clear is when you will know if you need to work on your vent. If the oven is 800 degrees and your still getting smoke out the front, then you will probably want to do something.

I built 6 X10 inches for my opening on the inside of the arch. I widened it with a grinder to about 8 X15 with a slope towards the oven openng so that smoke spilling out of the oven would be given an immediate chance to rise and move up, rather than out.
Reality check: How's your wood? Really well cured? Wet wood produces much more smoke than seasoned.

jcg31 03-01-2008 09:05 PM

Re: The vent and oven draw
 
Thanks folk, I went with constricting the flue a bit. For now I just lined the 12"x8" with some Kaowool and that had an immediate and positive impact. What I will do next weekend is either install a pipe within the current system, or cut down the existing 12x8x24 flue and install an 8x8x24 in its place. Either way it will be fairly easy and cheap to fix. Thanks again for the direction!

Jim

RCLake 03-02-2008 04:00 PM

Re: The vent and oven draw
 
1 Attachment(s)
Quote:

Originally Posted by Dutchoven (Post 25378)
RC if your vent opening is indeed 7x7 you should look to reduce to a 4 inch flue...or make the vent opening larger
hope this helps
Dutch

This whole vent/chimney and draw thing is quite new to me. As you can see in the phote my vent area, like an inverted funnel, in my second row (which is 6.5" wide) goes from 9.3" at the bottom of the row to 6.5" at the top of the row. The support for my flue goes up another 2" and then I've added my flue liner which is 6.5" x 6.5". So far it's two feet high but I have another 2' section, Plans were to place it on top and see if there was any difference.

So if the bottom of the vent should be twice the top in area, should I tamper the sides of the second row so that opening is about 9" wide? :confused: If 2:1 ratio is suggested, what distance from the top to the bottom is recommended?

I guess not understanding all of this and flying by the seat of my pants, I'll try stuff and see what works. Any advice will be considered! Maybe the e-book should be expanded to help future(present) builders.


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