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-   -   firebrick arch vent question (http://www.fornobravo.com/forum/f6/firebrick-arch-vent-question-6191.html)

mrgweeto 02-26-2009 12:59 PM

firebrick arch vent question
 
Getting ready for the Spring thaw in sunny CT to start the oven. Have been looking at several projects that have had pix from start to finish. The thing that I noticed in one was that the inside arch was at the same level as the outside arch which allowed the smoke to partially exit outside the front opening as was evident by the smoke stain on outside of the arch above the door opening. Shouldn't the inside arch be slightly higher than the outside arch so the smoke in essence will bank off the back of the front arch and billow up the chimmny? Sorry for the long question and I hope it is understandable.
G

dmun 02-26-2009 01:12 PM

Re: firebrick arch vent question
 
I don't think the relative position of the two arches is critical in this regard: if you want to avoid smoke on startup, you need a taller chimney, or a draft door to channel the smoke up the chimney. Once the fire's going, any sort of chimney will do, there's very little smoke at full heat.

Les 02-26-2009 01:15 PM

Re: firebrick arch vent question
 
G,

You are correct, some are taking that into account. I made my inside and outside arch the same height. I dropped my decorative arch down about an inch to help keep the smoke from exiting the front.

Les...

james 02-26-2009 01:26 PM

Re: firebrick arch vent question
 
Hi guys,

I moved this to getting started. It seems like a good spot. I also left a redirection notice at the original spot.

Keep going.
James

david s 02-26-2009 01:43 PM

Re: firebrick arch vent question
 
If your outer arch is lower than the inner one isn't that going to create problems getting the door to fit?

Les 02-26-2009 02:01 PM

Re: firebrick arch vent question
 
Quote:

Originally Posted by david s (Post 51519)
If your outer arch is lower than the inner one isn't that going to create problems getting the door to fit?


Kinda - I had to make my door 2 inches thick instead of 4, so I could tilt it to get it out.

Les...

mrgweeto 02-26-2009 03:01 PM

Re: firebrick arch vent question
 
David,
Good thought. Never thought about that. Of course the smoke can also be a result of how one and where one starts the fire. In the case of starting a fire in the fireplace I have always lit a peace of newspaper and held it right under the damper to get a draft going and that eliminated any puff backs or smoke inside my room. Don't see why that wouldn't work in this case. Also not trying to put 2 weeks worth of paper and a cord of wood at one time would help out as well. Like I mentioned, or maybe I didn't, my brother and I are pretty handy having been involved in building several homes and doing masonry projects. This is a new experience and I welcome all the advice. We are very coachable. Thank you for the input so far. Oh did I mention my nephew is going to the CIA to become a bread and pastry chef and just finished his bread making course. Needless to say we are quite anxious to get this project going once we get warmed up here.
G

wlively 02-26-2009 03:47 PM

Re: firebrick arch vent question
 
I doesn't matter much as to how high the arches relative to each other. It is more importantt to get the dimesions and "flow" of the hot gas correct.

As a precaution I made my outside arch 1/2" lower than the entry, but it turned out insignificant. IMHO you want the chimney opening in the arch as wide as possible and no sharp corners. You want to make it easy for the smoke to flow out and into the chimney, so corner cut or round off the hot side of the chimney entry opening. The chimney opening in the arch should be larger area than the area of the the chimney itself as you want to increase the gas velocity which decreases air pressure to increase draw. You want to make a smooth path for exhaust flow, sharp corners creates turbulence which slows flow. The taller the chimney the better the draw, but not by a large number in our application.

You can also just set a pipe over a square hole and it will work. But if you decide you want to have the best drawing chimney you can build, then like the rest of the oven it will take some planning and work as well.

gjbingham 02-26-2009 09:44 PM

Re: firebrick arch vent question
 
I made my outer arch an inch lower than the interior angle iron. I do get smoke out the front of the oven every time I throw on a new log. Once it gets burning well, the smoking stops. The inch difference in heights didn't matter at all.

I think the problem in my case is a combination of a couple of problems:

- The angle iron at the top of the dome opening allows smoke to spill out across the entire surface of the iron. I assume that an arched opening would result in most of the smoke exiting the dome at the height of the arch instead of across the entire width of the opening, and would direct the majority of the smoke directly into the vent opening.

- My vent opening, while built according to the plans, seems too narrow for the width of the dome opening, which allows smoke to spill out and run along the sides of the entryway ceiling, all the way out to the outer arch before the draft of the chimney starts to pull it back toward the vent. Invariably, some finds its way out the front. I think that a cast vent place atop vertical entryway walls would be a much better choice to combine with an angle iron type dome opening.

Regardless, the pizza's are great! I'm happy!

david s 02-27-2009 04:24 AM

Re: firebrick arch vent question
 
After firing my mobile oven at a market where I was next to a stallholder selling baby clothes, I learned that it is quite possible to fire an oven with virtually no smoke. Th trick is to be extremely vigilant to make sure there is always a flame, start with very little paper and only add sufficient fuel for the flame to consume. The natural tendency is to load the chamber with much more fuel than the fire can cope with. Potters call this a reducing atmosphere and it produces unburnt fuel (smoke) The resulting fire is actually not as hot, like a car running on a rich fuel mixture. In practice who really has the time and vigilance to monitor their fire this closely ? But it can be done. The top down method of lighting also produces very little smoke for the same reasons.


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