Author Topic: Wood Burning pizza oven  (Read 98183 times)

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Offline David

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Re: Wood Burning pizza oven
« Reply #125 on: February 21, 2006, 11:11:54 PM »
David,

I have to respectfully disagree with this position. I think it sounds interesting to use the chimney exhaust to exchange heat with the dome, but I can't see in reality how this will help much. An adequately insulated dome should be more than sufficient. My oven uses a front-mounted brick chimney. The exterior surface of the chimney is barely warm to the touch no matter how hot the exhaust. If the escaping gases were a significant source of heat, the chimney would be extremely hot. 

Bill/SFNM

Bill,
I don't know for certain as i've never seen a Neapolitan oven actually being built,but my comments are based on my desire to be able to build one as I think they are built.Nearly all of the ovens produced ,both hand built and pre fab seem follow the front vent method ,but not the Neapolitan Ovens as far as i can see?I may be way off course here, but I think the Pulcinellas are holding something back. :-XHowever,when i do sell up here and move,i'll build my next home oven following my thoughts and i'll report back.  ;)
If you're looking for a date... go to the Supermarket.If you're looking for a wife....go to the Farmers market


Offline Bill/SFNM

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Re: Wood Burning pizza oven
« Reply #126 on: February 22, 2006, 05:41:39 AM »
Marco,

I appreciate your comments since I have already learned so much from you and look forward to learning more. Debate is a fun way to learn and I would like to continue, if we may, so that I can better understand the role of the chimney and benefit from your vast experience.

I maintain my position that the chimney can't do much to heat the oven via heat exchange, which is based on some facts and some assumptions. Just about every opinion I hold involves assumptions and I am very happy to consider whatever facts you might offer on this point.

Regarding the related issue of airflow: are you saying that the poorly built ovens experienced inadequate airflow resulting in less oxygen to the fuel? And that moving the chimney back improved combustion? If so, I would argue that it was not the path that made the difference as much changing the length of the chimney.  This statement is based on my observation that the volume of air moving through a system is affected by chimney length. It is very clear on my wood-burning offset BBQ pit that lengthening the exhaust chimney had a dramatic effect on the volume of air flow. So is it possible that the poorly performing oven would have benefited from running the chimney in any direction that made it longer, not just back over the dome?

Best,
Bill/SFNM

Offline David

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Re: Wood Burning pizza oven
« Reply #127 on: February 22, 2006, 09:49:19 AM »
Bill / Marco,
                  As I have said my comments are only based on assumptions.Thanks Marco for clarifying that the heat from the Dome was beneficial to promote heat to the chimney more than vice versa.(I had in fact assumed the opposite )I had used information from the following link to bolster my thoughts :

http://muextension.missouri.edu/explore/agguides/agengin/g01732.htm

From comments provided in this article I assume that the fact that more of the flue is enclosed and run over the oven in the Neapolitan style,this would help  to both speed up and maintan the heat of the flue to provide a better draw and also possibly (Due to the curvature of the flue ) reduce the possibility of smoke exiting out of the entrance,caused by such things as downdrafts?My chimney is enclosed in vermiculite and ceramic blanket,so I feel absolutely no heat on the exterior of my oven. Maybe Marco can provide the answer as to the materials used for the flue pipe (Titanium /stainless Steel / Fire Brick) in Naples?Thanks,
                             David
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Offline pizzanapoletana

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Re: Wood Burning pizza oven
« Reply #128 on: February 22, 2006, 03:42:12 PM »
Bill,

Again I never said that the chimney heat the dome (I am saying the inverse). As you notice the chimney doesn't get very hot with the exhaust gasses. A cool chimney doesn't suck the gasses as much as a hot chimney, and that is a fact. Therefore a hot chimney, suck more as well as the the diameter and length is also a factor

Hot air tends to rise up and cool air down.. The chimney right on top of the oven is influenced by this and therefore affects the fuel consuption as well as not producing the necessary heat (the combustions, happen fast but not producing the same heat of a slower, but more intense fire. This is in addition to the optimum air flow /sucking of the heated chimney on the dome...

You can still believe in your theory off course, and if do come back to Naples when I am also around, I can take you the to pizzeria affected and they will talk you about their experience.


David,


PS By the way, Da Michele is having the oven replaced, after 40 years....

Ciao




I maintain my position that the chimney can't do much to heat the oven via heat exchange, which is based on some facts and some assumptions. Just about every opinion I hold involves assumptions and I am very happy to consider whatever facts you might offer on this point.


Best,
Bill/SFNM
« Last Edit: February 22, 2006, 04:32:09 PM by pizzanapoletana »

Offline Bill/SFNM

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Re: Wood Burning pizza oven
« Reply #129 on: February 23, 2006, 12:38:53 AM »
Marco,

Thanks for taking time to respond to my post. I hope I'm not being a pest, but I really want to understand this notion of running the chimney back over the top of the oven. I'm still dubious about its value, but am willing to be convinced otherwise and to, perhaps, modify my oven accordingly. This debate has already taught me a great deal by causing me to think more deeply about these matters.

I have no problem accepting the fact that a hot chimney results in better combustion. But facts do not always represents important considerations. If you don't mind, I will explore this fact. The reason this is a fact is pretty easy to understand. The walls of the chimney, which are heated by conduction from the dome of the oven, conduct that heat to the air transitting the chimney. This raises the temperature of that air causing more rapid convection out of the chimney. This draws used, hot air out of the oven which in turn draws more fresh air into the fire resulting in better combustion. Si?   

But this is only a fact if the walls of the chimney are warmer than the air passing through the chimney. If they are the same temperature, no benefit is derived. If the walls are cooler than the passing air, it actually cools down the air and slows down convection up the chimney which is detrimental to the combustion. Clearly, the hotter the walls of the chimney are in relation  to the air, the more the benefit.

So the big question here is what is that temperature differential and what differential is needed for there to be a demonstrable benefit. I'm going to do a little experiment next time I fire up my oven (Friday, I hope). I have a thermometer with a dial gauge embedded in the dome of my oven, about one inch inside the brick. The temperature there will give an approximation of the temp of the wall of a chimney that is routed back over the dome. Then I will measure the temperature of the air just as it enters the chimney. Unfortunately this isn't so easy. My thermocouples (type T) have a maximum range of   
662F. But I have a friend who might be able to lend me some Type J's which go up 1382F. Although my oven isn't a true Neapolitan pizza oven, these two temps should be interesting. At the very least, they will help me see if rerouting the chimney back over the dome would be of any benefit. Stay tuned.

If you are willing, I would appreciate your comments on the fact the chimney length is a very important consideration in the amount of draft (sucking) applied by the chimney. There are formulas that demonstrate this fact, but perhaps it is easier to imagine the following: a chimney that is only 1cm long. It is easy to see that air moving such a short distance before being released into the open produces very little draft. Then imagine a chimney with 1m of length. It should be intuitively obvious that there is much more air moving through the chimney, drawing in more air behind it. And then imagine a chimney twice that length - with twice the volume of air moving through it there would propotional increase in draft force. Right?

Anyway, sorry to have gone for so long. It would be a great privilege if we are back in Naples at the same time to visit the pizzeria to which you refer, especially if their pizza is any good  :D.

Bill/SFNM
« Last Edit: February 23, 2006, 12:41:41 AM by Bill/SFNM »

Offline pizzanapoletana

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Re: Wood Burning pizza oven
« Reply #130 on: February 23, 2006, 05:04:11 PM »
Bill,

I will be waiting to see the result of your experiment... However have you ever put your hands at the end of a two meter chimney and check how hot the air was? I did, it is not even half as hot as when it comes out of the oven mouth... One more thing to think about...


Finally, I would like to share the following story:

A Master Builder was once called to review a poor performing oven with not specified complaint. At the survey meeting, the owner of the pizzeria, the Master and a young pizzaiolo (the one that worked the oven) were present. The Master oven crafter, without having heard the complaint from the young "Fornaio", start summarise his thought on the oven with the following point:

The chimney is too far on the front of the oven, I bet this oven consume a lot of wood and doesn't get as hot as others. Also this... also that... Well the young Fornaio was shocked, as even if he knew nothing about construction, he did indeed work the oven everyday and knew the defects. The Master builder had identified them all....

Ciao

Offline Bill/SFNM

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Re: Wood Burning pizza oven
« Reply #131 on: February 24, 2006, 02:50:10 AM »

However have you ever put your hands at the end of a two meter chimney and check how hot the air was? I did, it is not even half as hot as when it comes out of the oven mouth.


Actually, yes, I have since the height of my chimney is about 2 meters. The exitting air seemed extremely hot, not like you describe. Perhaps this is the key to our disagreement. My chimney is extremely well insulated with the same materials that insulate the rest of the oven. And the first 12 inches or so of the oven is built of the same brick as the dome. See this diagram: http://www.earthstoneovens.com/viewdetails.html#90 . In both my scenario with a straight-up chimney and yours with the chimney passing back over the top of the dome, the chimney is part of the thermal mass of the entire oven. Anyway, later this morning after my oven gets up to temp, I'll try to measure the air temps both at the bottom and near the top of the chimney.

Bill/SFNM

Offline Bill/SFNM

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Re: Wood Burning pizza oven
« Reply #132 on: February 24, 2006, 08:25:42 AM »
Here is a side-angle view of my oven so you can get a better idea of the position and insulation of the chimney. The chimney in the foreground is from the pizza oven. The one behind it is for the BBQ pit.

Bill/SFNM

Offline JPY

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Re: Wood Burning pizza oven
« Reply #133 on: February 24, 2006, 11:37:31 AM »
So having a metal flue (chimney) would work better than a brick one because the metal will heat up faster. and create a stronger flow.   Also you do not want the dome too low, the wood will not burn properly, you need some headroom.  If you use gas only then you can bring it down some. Nice oven Bill...
« Last Edit: February 24, 2006, 11:44:19 AM by JPY »
-JP-


Offline Bill/SFNM

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Re: Wood Burning pizza oven
« Reply #134 on: February 24, 2006, 11:44:05 AM »
JP,

An uninsulated metal flue will cool off quicker also. My brick chimney transitions to a metal flue which is surrounded with insulation.

Bill/SFNM

Offline JPY

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Re: Wood Burning pizza oven
« Reply #135 on: February 24, 2006, 11:48:52 AM »
We usually reccomend the "grease duct" style pipes.  Which is a double wall stainless insulated duct.  You know the codes in the US.  They want the pipes liquid and air tight so grease will not escape.  B-vent style would work too, however in case of a flue fire the grease duct will keep it contained in the pipe where B-vent may leak.  Of course you can always weld some duct together.
-JP-

Offline Bill/SFNM

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Re: Wood Burning pizza oven
« Reply #136 on: February 24, 2006, 12:33:01 PM »
JP,

Yes, I am using the double-walled stuff. Not sure how a grease fire would ever be a problem with these systems.


Bill/SFNM

Offline Bill/SFNM

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Re: Wood Burning pizza oven
« Reply #137 on: February 24, 2006, 02:03:44 PM »
Marco,

OK, I took some measurements, but not all the ones I want. I wasn't able to borrow thermocouples from my friend. He works out our freindly neighborhood weapons laboratory and they aren't too keen on lending out instruments.  ::)

So I wasn't able to take any air temps. And also, I got a late start firing up the oven, but the pies still came out great (see post on lardo pizza I'll be putting on the Neapolitan section after I'm done with this post).  Had I time to allow the whole thermal mass to stabilize, I know for sure that the probe embedded in the dome would have been higher. Here are the measurments I took:

Ambient temp: 45F
Interior surface at dome: 995F
Probe embedded inside dome: 890F
deck: 855F
Exterior of dome: 50F

It would have been very useful to get a reading of the temps of the interior surface of the chimney but the opening was far too narrow to get an IR reading.

Anything useful here? Well, the fact that the exterior of the dome was barely effected suggests almost all the heat that might have been applied to a chimney running back to the top of the dome was going to chimney anyway (see photo above - easy to see this). So, if we're going to accept the fact that a heated chimney is beneficial, then perhaps it doesn't matter whether the chimney comes to the heat or the heat comes to the chimney. And it it is still undisputed that the length of the exhaust path is a major consideration.

I'll probably order a few new thermocouples so I can better understand the heated chimney theory. I'm still having fun learning about this.

Bill/SFNM

   

Offline JPY

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Re: Wood Burning pizza oven
« Reply #138 on: February 24, 2006, 07:24:56 PM »
One thing it could be is the oven has extra insulation over the dome, thus reading the cool exterior dome temp.  :)  I'm not sure if the ovens that Marco has been referring to has an insulation layer over the bricks, besides the mortar that is used.  I was talking to Maurice about this and he was comming to the same conclusion as Bill.  But again I can try to bend the flue over the dome with one of the ovens and see what changes.
« Last Edit: February 24, 2006, 07:26:51 PM by JPY »
-JP-

Offline Bill/SFNM

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Re: Wood Burning pizza oven
« Reply #139 on: February 25, 2006, 12:31:15 AM »
JP,
It would be great if you could run that experiment in your test kitchen and compare performance with vent running across dome vs. vent running some other direction so that the length is the same.

I was thinking about placing a variable speed electric motor in the flue so I could control the amount of draw out the chimney. You must deal with this kind fan all the time in your commercial indoor installations. Can you point me toward a source and specs for such a fan? Thanks!

Bill/SFNM

Offline JPY

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Re: Wood Burning pizza oven
« Reply #140 on: February 25, 2006, 11:31:58 AM »
One of the fans we deal with is the Exhausto brand.  www.us.exhausto.com  The RS-009 should be good and it does have a speed control.

http://us.exhausto.com/files/pdf/ProductInformation/3932001.pdf

This fan mounts on top of the stack.
-JP-

Offline Bill/SFNM

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Re: Wood Burning pizza oven
« Reply #141 on: February 25, 2006, 12:33:15 PM »
JP,

Thanks. I notice the temperature rating is 575F intermittent and 482F constant. I suspect I'm already operating at higher temps and this fan could make them even higher. Maybe this isn't such a good idea. Ain't broke - don't fix.

Bill/SFNM
 


Offline JPY

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Re: Wood Burning pizza oven
« Reply #142 on: February 25, 2006, 09:29:14 PM »
I think when we did UL testing the duct temps were about 225-300F.  This was done with firing the oven at an abnormal high temperature.  They did do it with and without the fan. I can look up the report. They had about 100 probes all over the oven, the exterior of the oven, vent shafts and the exhaust hood (for commercial use). Of course the fan will be at the end of the stack so the air will cool down by the time it reaches the fan.  Also the hot air that comes out of the oven does mix with the cooler exterior air which will cool the exhaust off a bit.  I don't think the air temps would reach 500F in the chimney.  But your last statement is correct. :)

But when you have an oven in the house, you may need a fan just to fight off other ventilation in the same room. For example this kitchen has a strong fan over the cooktop, and when you turn that fan on and run the wood fired oven at the same time the smoke tries to go to the hood over the cooktop. I don't have a fan for the oven, but I do need to open a door to neutralize the pressure in the room
« Last Edit: February 25, 2006, 09:31:12 PM by JPY »
-JP-

Offline Bill/SFNM

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Re: Wood Burning pizza oven
« Reply #143 on: February 26, 2006, 12:28:21 AM »
JP,

What a gorgeous oven! Big difference between between that one and mine is that I have a great deal of insulation around the chimney. I also have no guns hanging on the wall, but I would have them much closer to the oven in case of attack.  >:D

Bill/SFNM

Offline Pete-zza

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Re: Wood Burning pizza oven
« Reply #144 on: February 26, 2006, 12:33:48 AM »
Bill,

The guns are used to kill pigs to make lardo ;D.

Peter

Offline chuckh

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Re: Wood Burning pizza oven
« Reply #145 on: March 19, 2006, 11:01:02 AM »
hello, I have finally finished my backyard oven (barrel type out of fire brick) and have fired it up three times, achieving a floor temp of over 700 degrees .  I have not insulated it yet and i believe it will approach 900 when I do.  My question is about my flue on top.  I have an 6X6 clay flue, 48 inches long, just sitting on top of my dome, and I am going to fix it in place, BUT, from the posts there seems to be discussion on running a flue straight up vs running them back at an angle.  My gases coming out are over 200 degrees, would it help to insulate the flue with ceramic blanket, and cut it so it leans backward at an angle coming off the dome?" 
Thanks, Chuck