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-   -   captain. what size vent (http://www.fornobravo.com/forum/f8/captain-what-size-vent-1342.html)

captain 12-19-2006 08:55 AM

captain. what size flu
 
ok I have only the last few brick in the dome every thing is realy comming together, I am taking pictures as I go and keeping a full jurnel of each step. I will post the photos next week. what size flu should I use for a 36" pompaii 17"s high with an opening 18.5" wide and 11" high. Thanks for any Info on this
Ronnie

Marcel 12-19-2006 10:57 AM

In flue size, bigger is usually better, .... I think.
 
(M) Dear Captain Ronnie,

(C) "what size flu should I use for a 36" pompaii 17"s high with an opening 18.5" wide and 11" high. Thanks for any Info on this
Ronnie"

(M) I use a 6" metal flue but I believe I would have less smoke, initially, had I gone to a larger size. I think that even a larger size would initially cause some seepage of smoke in the baker's face.

(M) Once my fire is HOT I get no smoke out the front, and very little out the chimney. Because I use metal that has been insulated with Perlcrete, my flue gets hot quickly and that helps produce a desireable draft. Fot those who use clay, concrete, or some kind of ceramic, their heat up time is likely greater since their added chimney mass needs to become hot before the fire draws well.

(M) If a size larger than 6" for your flue is not feasible, consider looking at the solution I used:

http://www.fornobravo.com/forum/phot...index.php?n=35

Ciao,

Marcel

captain 12-19-2006 12:05 PM

captain
 
thanks for the info I was intending on using terracotta flu 8" do you think it will be ok.

Marcel 12-19-2006 12:18 PM

8" flue for a 36" Pompeii should be fine.
 
(C) "thanks for the info I was intending on using terracotta flu 8" do you think it will be ok."

(M) I think that will work fine. You may still have some initial smoke exiting the front when starting your fire. That smoke should subside to almost nothing when your fire is going well.

Ciao,

Marcel

CanuckJim 12-19-2006 02:45 PM

Flue
 
Marcel, Ronnie,

Eight inch flue tile should work just fine for the size of oven in question. My AS 4x3' oven uses 7" round flue tile, and the oven draws like a bear, with no smoke out the front at all. The arch bricks are as clean as the day I laid them.

Jim

captain 12-20-2006 06:06 AM

captain
 
Thank you guys for that info, jim does round work better than squar flue .

jengineer 12-20-2006 06:56 AM

Fluid Dynamics/Aerodynamics
 
Yes air and cimbustion products are often considered fluids. You are looking ot direct and force/draw these products out of the cobustion chamber. One way to make an engine more efficient and i my case a ticket monger is to port and polish the heads.

In gas turbines the exhaust nozzles or nacelles are smooth up to the exit plane. In fact there are thrust and drag penalties for stuff that sticks out into the exhaust stream.

Similar principles should be applied to the exhaust ducting of your oven. If you are using bricks in your transition at the throat then little things like chamfering provide for a smoother transition as the flow will not "trip" causing more turbulence.

http://mha-net.org/graphics/wild06/oven/oven57.jpg

photo linked from MHA News - 2006 Meeting - Backyard Oven with Peter Moore

As has been stated in previous posts a Rumfod design of the throat which is a restriction followed by a larger chamber also porvides for draw.

Applying jet engines and rocket design this is similar to a con-di nozzle. Convergent-divergent where the exhaust plume is first squeezed into a convergent opening (like a funnel) and then allowed to expand out a through a convergent opening like the horn of a trumpet.

Or I could be blowing smoke

CanuckJim 12-20-2006 08:28 AM

Round vs Square
 
Captain,

It's tough to say whether round works better than square, but there are some advantages to round. First, the round tiles are lipped so they fit together snugly; square tiles aren't. Second, I've found in various applications that 7" round has almost as much area as 8" square, they're easier to install, and you can build a smaller "box" with air space included around them. Third, without the square corners there's less opportunity for creosote build-up, but this isn't a consideration with high-heat ovens, as it is with wood-burning stoves/fireplaces. Fourth, as joengineer hints, there is less turbulence with round tile. If you do decide to go with it, as je hints, make sure you corbel in (step in) the bricks, course by course, to reduce the vent gradually until you are ready to install the tiles. In my case, I mortared in two pieces of half inch rebar to sit the first piece of tile on. More a precaution than anything else.

I'll post a photo of the throat in the Gallery. If you look closely, beyond the soot, you'll see the corbelling and the round tile. The camera was pointed upward, with the flash, so I couldn't really see what I was doing. Hope you get the idea.

Jim

Hendo 12-30-2006 04:57 PM

Re: Fluid Dynamics/Aerodynamics
 
1 Attachment(s)
Quote:

Originally Posted by jengineer (Post 6939)
If you are using bricks in your transition at the throat then little things like chamfering provide for a smoother transition as the flow will not "trip" causing more turbulence.

This sounds entirely feasible. Going a step further, I imagine that radii (as in bull-nose bricks - see thumbnail) in lieu of chamfers would be yet more efficient. And why stop at the oven throat - surely they could also be used on the inside face of the vent arch to enhance the draw of cold air from in front of the oven?

On an associated matter, many pictures and oven designs (including those at Pompeii) show a vent arch higher than the height of the oven opening. Surely this would encourage combustion products from the oven to exit through the vent arch, rather than solely via the chimney? Would it not be better for the height of the vent arch to be the same as the oven opening? Of course it cannot be lower as we need to have sufficient access to place a door over the opening, but surely the same height would create a more efficient draw?

Marcel 12-30-2006 05:33 PM

Re: captain. what size vent
 
(M) Hendo, with the bricks shown in your illustration above, I believe one could thread all of your arch Holy bricks with all the re-bar(s) needed for your arch (calculating enough space for your mortar between those bricks, drill re-bar size holes in your hearth slab and then bend the re-bar in a semi-circle so that the short exposed re-bar would fit into the holes. That would make your arch very strong and preclude the need for a temporary support. You could lay it out first on a piece of plywood on the ground for a "dry fit".

Ciao,

Marcel


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