Go Back   Forno Bravo Forum: The Wood-Fired Oven Community > Pizza Oven Design and Installation > Pompeii Oven Construction

Like Tree5Likes

Reply
 
LinkBack Thread Tools Display Modes
  #21  
Old 04-03-2012, 01:34 PM
Peasant
 
Join Date: Jun 2011
Location: Bowie, MD
Posts: 26
Default Re: Apagios's 42" Pompeii

Well technically if you think of it as controlling the flow rather then restricting it, It' might be easier. I am an aerospace engineer by trade, so I tend to think about things a little differently.

Basically you have a single arch door. You have air going in and air coming out. So some portion of the bottom of the arch doorway is flowing in, while the top portion is flowing out and up the vent.

So think of a wide open arch door way. The two opposing streams of air have to reach a balance, and there will be a little mixing in the middle. Any breeze or obstruction (like reaching you arm in to add a log) will disrupt this balance and the two streams of air have to re-equalize. Also since the bottom stream of cold air has the entire width of the arch, it can move slower the the hot exhaust air which squeezes out the top of the arch. Slower moving air does not mix as well and doesn't reach the back of the oven as well.

By putting a door in place (like jcg31's), you are better separating the two opposite streams of air.

I would venture a guess that the total flow rate of cold air into jcg31's oven is not being reduced at all (proving that would require some level of flow metering though which would be way beyond this discussion).

Whats happening is the smaller holes on the bottom force the oven to "suck" air in, and the smaller holes require the air to travel faster then if it had the entire doorway. This is that "blast furnace" effect. Same amount of air but now it must move much faster through the smaller hole. This really helps the combustion process because faster moving air generate turbulence and mixing as it moves through the logs. And the air is better distributed through the base of the fire and to the back of the oven. When you blow on a fire you achieve a similar result.

Also the faster moving cold air is traveling along the bottom of the archway door, better then if the door wasn't blocking the top of the arch. Because you get a fast stream of cold air along the floor, it in effect gives a little more "buffer" zone between the 2 streams of air as compared to a wide open arch.

The steady fast flow of air should also be much less susceptible to gusts of wind, which upset this flow balance, which you see as smoke sometimes coming out of the front of the oven when the wind blows out the front of the oven.

Hopefully that helped a little. If you think of it more as a control of the air vs reduction in the amount of air or rate of air into the oven it might make more sense.

You could even argue that with the increase velocity of cold air, better mixing and combustion could actually increase the amount of air being drawn. If you think about it, the better the fire burns, the more air it needs to burn, so with the faster moving cold air, there's a chance you are even getting more air being drawn in then if you didn't have a "blast furnace" type door in place.
Reply With Quote
  #22  
Old 04-03-2012, 01:46 PM
Peasant
 
Join Date: Jun 2011
Location: Bowie, MD
Posts: 26
Default Re: Apagios's 42" Pompeii

Ok well just thought of this, the "blast furnace" type door does "restrict" cold air from going in the top of the doorway. so it obviously helps when the wind blows.

...

There are a couple different ways too look at it too. Now that I think about it, if the blast door is in fact reducing the total amount of air in, then it's basically passively giving the fire only what it needs.

Any air going into the oven that is beyond what is needed by the fire is simply being heated and exhausted. This is where you have less efficiency because you are burning wood to heat air as well as your bricks.

By using the blast furnace type door, the fire has to pull in the air it needs. So if it needs less air it'll pull in less, if it needs more it'll pull in more. The more air it draws in the faster the air must travel through the smaller opening, and you get your better mixing and better combustion.

So it's a really interesting question if the door is in fact restricting the air intake down to keep it so that all the air coming is is used in combustion, and no excess air is let in (Stoichiometric Ratio)

I can think of a reasonable way to test this by making a blast furnace door, and having a intake pipe attached, you could yank the Mass Air Flow sensor off of a car and rig it up (calibrate) to measure the amount of air flowing into the oven. You could weigh your wood and make approximate estimations to figure out if the blast door is keeping your fuel/air ratio at the proper Stoichiometric ratio.

Not sure how to measure the basic "no door" configuration though...
Reply With Quote
  #23  
Old 04-03-2012, 01:48 PM
Peasant
 
Join Date: Jun 2011
Location: Bowie, MD
Posts: 26
Default Re: Apagios's 42" Pompeii

Hopefully some of that made some sense!

I know I may have doubled back and contradicted my first response a little. Maybe it's a good debate over pizza & beer!
Reply With Quote
  #24  
Old 04-03-2012, 01:56 PM
Peasant
 
Join Date: Jun 2011
Location: Bowie, MD
Posts: 26
Default Re: Apagios's 42" Pompeii

... yeah the more I think about the more I wonder if the blast door effect is really helping the stoichiometric ratio. This is from the wiki link I posted above:

------------------------
A stoichiometric amount or stoichiometric ratio of a reagent is the optimum amount or ratio where, assuming that the reaction proceeds to completion:

- all reagent is consumed
- there is no shortfall of reagent
- no residues remain.
------------------------

In an oven, you have in effect 2 reagents: Wood (fuel) & Air (oxygen, for combustion)

Fire is very simply a chemical reaction where heat is a by product. "No residuals remain" is pointing at no extra cold air which lowers the efficiency of the oven as excess air is heated and wasted out the chimney.

So the blast door may be keeping the oven drawing in the exact amount of air it needs, and little to no extra (aside from Nitrogen, since air is 78% nitrogen, the fire is technically only using 21% of the air it draws in, air is ~21% O2). Since there's no extra air (oxygen) that it doesn't need (or at least less then a wide open arch with no door) it's heating your oven more efficiently, less waste heat out the chimney. (Heating the nitrogen is an inescapable waste of energy from your fire, but at least some of that heat is convected to your bricks, as it makes it way across your dome and up your vent)

I think the higher velocity of the cold air stream promoting better mixing and penetrating deeper to the back of the oven might be more of a beneficial side effect, that aids in combustion obviously, but not the main reason those blast doors help so much during heat up.

This is probably way more info then you ever wanted to know or think about! Sorry for the rambling musings! See what happens when you poke an engineers curiosity!

Last edited by apagios; 04-03-2012 at 02:08 PM.
Reply With Quote
  #25  
Old 04-03-2012, 03:24 PM
Amac's Avatar
Journeyman
 
Join Date: Nov 2011
Location: Ireland
Posts: 358
Default Re: Apagios's 42" Pompeii

Quote:
Hopefully some of that made some sense!
Actually it makes a lot of sense - it always helps to have some idea of the processes involved. It is amazing the complexity of such a simple object.
Also next time I hold a newspaper across the front of the fire to help start it I'll have some idea what is going on!
Reply With Quote
  #26  
Old 04-03-2012, 03:50 PM
Peasant
 
Join Date: Sep 2011
Location: Valencia, Spain
Posts: 26
Default Re: Apagios's 42" Pompeii

Quote:
Originally Posted by apagios View Post
Sorry for the rambling musings! See what happens when you poke an engineers curiosity!
... or never tell an engineer "this has never been made"
Let me join the theoretical discussion as I started my wood fired oven investigations the same way as your "rambling musings", but the search pointed me to very different conclussions than my starting ideas were.
Quote:
Originally Posted by apagios View Post
Fire is very simply a chemical reaction where heat is a by product. "No residuals remain" is pointing at no extra cold air which lowers the efficiency of the oven as excess air is heated and wasted out the chimney.
In fact this is true with organic fuel gases, butane, propane, metanol... But with wood things happens different.
Wood+O2+heat -> CO2+H2O+CO+C+...
Here comes the complication. Not all the carbon compounds burns perfectly, not at the usual ovens temperature. For get full combustion of CO and C (black smoke) is necessary to reach temperatures over 650 C (1200 F). So, as you pointed, the ballast gases Nitrogen, H20 and excess O2 makes things worst as they make the combustion chamber colder than the desired high temperature environment. Usual wood ovens reach an efficiency of less than 30%. So the blast door can "make up" things a little bit, but still far from the efficiencies of separate fire chamber systems that easily overcome 85%. But here total excess air intake is around 60%, secondary preheated air intake are included usually in the designs and it reaches the "flame" area with proper jet speed as chambers are smallers than the oven cooking chambers.
By the way, in industrial furnaces is compulsory to enrich the combustion with extra air over that stoichiometric amount to avoid explosions.

When I started thinking about building an oven the initial idea was 1.5" thick dome, fiberglass isolation, then I discovered the pompeii oven, then the masonry heaters, then the flow of gases in furnaces theory... and intuition says that yet more things to come for better understanding this fantastic hobby that joins engineering and cooking arts.
Regards

Last edited by Dmendo; 04-04-2012 at 01:10 AM. Reason: grammar
Reply With Quote
  #27  
Old 04-10-2012, 02:22 AM
Amac's Avatar
Journeyman
 
Join Date: Nov 2011
Location: Ireland
Posts: 358
Default Re: Apagios's 42" Pompeii

Quote:
full combustion of CO and C (black smoke) is necessary to reach temperatures over 650 C
Hi dmendo - is it normal to reach the temperatures (>650 C) so no black smoke is issued ? I guess it will take some time to get to that so a lot of smoke at first and later some "cleaner" gases. My knowlege of combustion processes in minimal - but "black smoke" doesn't really sound like wood burning smoke. I would have thought "blue" would be more accurate or is that a technical term?
Aidan
Oh and I forgot to ask someone please recommend an IR thermometer - a pretty good one but for domestic rather than industrial use? I saw somewhere on here that somebody was recommending Extech 42512. That is online at $129 dollars which is within budget.

Last edited by Amac; 04-10-2012 at 02:37 AM.
Reply With Quote
  #28  
Old 04-10-2012, 09:16 AM
Peasant
 
Join Date: Sep 2011
Location: Valencia, Spain
Posts: 26
Default Re: Apagios's 42" Pompeii

Hi Amac.
Of course, I can't imagine anybody burning tyres or plastic stuff in a cocking oven .
Black smoke refers to the black component of the smoke. Perfect combusted gases are colourless and you can see only white condensing water vapour leaving the flue. And yes, the starting wood combustion, no matter wich hi-tech device is always smoky, the difference with traditional fires is that well designed fireboxes need some minutes to reach the optimal working temperature and becomes smokeless.
Regards.
Reply With Quote
  #29  
Old 05-09-2012, 06:54 AM
Lburou's Avatar
Il Pizzaiolo
 
Join Date: Dec 2010
Location: DFW area, USA
Posts: 1,110
Thumbs up Re: Apagios's 42" Pompeii

A remarkable oven Dave! Great job, it shows the musings of an analytical mind.....

I'm happy to see the hemispherical arch with the diagonal cuts on the back side of the arch bricks becoming 'typical' in the construction of the pompeii oven. Well and truely done
__________________
Lee B.
DFW area, Texas, USA

If you are thinking about building a brick oven, my advice is
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

Our One Meter Pompeii Oven album is
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

An album showing our Thermal Breaks is
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.


I try to learn from my mistakes, and from yours when you give me a heads up.
To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
Reply With Quote
  #30  
Old 06-04-2012, 04:14 AM
Laborer
 
Join Date: Jan 2012
Location: Connecticut
Posts: 89
Default Re: Apagios's 42" Pompeii

I have a question. For the arch, it looks as if a full brick was used with a 4 inch diagonal cut to match the half dome bricks. Is this true? Thanks

Quote:
Originally Posted by apagios View Post
Hi Dave,

So yeah I didn't exactly capture every step! I did start with all standard 4.5"x9"x2.5" firebricks. There was no magic really to figuring out the cuts, I originally was thinking something much more complex, but ended up laying it out so that the angle on the top of the arch would roughly match the angle of the course of bricks at that height. Basically the first full course of bricks above the arch would sit on the arch and load it evenly.

This is not the best picture because I only happened to catch it in the background of another shot. But you can see I laid out a sample arch representing my dome. Then I laid out a smaller arch representing my 13" arch doorway inside. If you look close in the pic you will see a pencil mark on the brick with a diagonal cut line that approximately matches the angle of the dome bricks:



I was originally thinking I'd have to match the angle at each level up, but it turns out you can cut the last bricks in each dome course to fit in the slot between the arch and the rest of the course. There will be some dihedral angles to cut there, but once you have a whole course done except for that last brick it's easy to see the shape you need and custom cut it.

Incidentally that diagonal cut on the arch bricks should be a length approximately equal to 4.5" since that's the size size of the 1/2 bricks used to build up the dome. That way the first course of brick sitting on your arch is entirely supported by the arch!

I would HIGHLY recommend building the doorway arch right after your soldier course is done, and before you do the 1st course on the dome. That way as you get higher you simply cut the last brick in each course to match the door arch, and since the door arch will already be set you have a nice visual reference.

Plus building arches you will come to find is easy, once you build the form you just mud the bricks and slap them up and the form holds them, It's good practice

I did spend some time laying out that bricks as you see in the pic and just trying to visualize in my head the 3-D shaping of the entire oven. It helps a lot. But if you cut that angle on the arch bricks like I mentioned, the first full course of dome bricks above the arch will sit on the arch, it makes for a very strong build IMHO.

If you look right next to my arch in this pic you can see the two smaller "keystone" bricks I cut to fit into the space between the arch and the 1/2 bricks of the rest of the course:



One other nice part about the arch being done and set up before building your dome, is that once you put those little custom cut bricks in, your course is now self supporting. It let me build pretty high before needing a form since the courses "lock" in to the arch, once that smaller brick is wedged in it's physically impossible for any of your other course bricks to slide down before the mortar sets.

And best of luck with your build! Just wait till your dome and chimney are done and you are doing curing fires. You'll feel like your done but there's still a ton of work to insulate and build the shell/weatherproof enclosure! I am thrilled with how mine turned out, but was definitely tough to get back to laying brick for the red brick bar top and bullnose bricks! I was mainly motivated by the impending frost, and my refusal to not get it done before winter!
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
May be starting my 42" Pompeii in NYC this week... NYC Pompeii Oven Construction 123 05-27-2012 10:04 AM
Insulating boards for a 42" Pompeii . . . Cheesesteak Getting Started 5 04-20-2011 11:59 AM
42" Indoor Pompeii w/ room addition spidersden Getting Started 11 02-05-2011 11:22 AM
Island Pompeii 42" in Ontario Food With Legs Pompeii Oven Construction 12 07-14-2010 01:27 PM
Damian's 42" Tuscan Style Pompeii WFO Damian Introductions 3 09-21-2009 10:41 AM


All times are GMT -7. The time now is 10:08 PM.

Powered by vBulletin® Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Search Engine Friendly URLs by vBSEO 3.6.0
2006/10 Forno Bravo, LLC