Minne"snow"ta castable build....... I think?

[jakeanderton]

My conclusions about the door height to internal height ratio being some kind of magic number for optimum operation differ from that of many members here, particularly after modifying one of my ovens to change the ratio considerably and finding it made no discernible difference to firing or operation. Interestingly the Golden Ratio of 1:1.618 (62%) is pretty close to that which the Canadian study of the average ratio found in a number (don't know how many) of ovens. This is not a scientific study of these ovens operation or efficiency, but does provide some good information about what practical use has determined.
The optimum fuel to oxygen ratio for the most efficient combustion is around 10:1. When using wood as a fuel it only becomes fuel when the wood is heated sufficiently to release its gas. The reduction of the air intake will increase the efficiency of the fuel/air mixture but results in two major drawbacks. Firstly it reduces access to the oven and secondly raises the temperature increase to levels vulnerable to the brick or refractory to cope with the sudden rise in thermal expansion. Far better to allow the oven to increase slowly and absorb the heat gently. It would be the equivalent of a potter firing a kiln at full blast from ambient. The two most vulnerable zones are ambient to 300C, when water elimination is the most likely to cause damage and the 500-650C zone where many of the materials in either brick or refractory have different thermal expansion rates. All potters are well aware of these two danger zones and ensure the rise is low (no more than 100C/hr) for these zones. Most oven owners either ignore or are unaware of this information and fire at around 300C/hr from ambient. Now an oven is not a kiln, and pottery wares are not bricks, but the same principles apply.

For my ovens the ratio is 64% and the modified version 70%

Here's an interesting thread which discusses the issue.
https://community.fornobravo.com/for...g-size#post764

Very informative, david s! I was thinking the larger the door opening height relative to the internal dome height would mean that more heat would escape the internal dome quickly. Meaning more air coming in to burn the wood, and thus going through wood a lot faster. but, with you stating you noticed no difference, at even 70%, leads me to wonder if I could get away with that height, too. I feel that I need as much viewing area as possible if I am cooking 3-4 pizzas at a time.

[Gulf]

I wouldn’t go overboard with this theory. I don’t have any pie R squares to offer you. However, that space inside the dome above the door is where the reburn occurs.

[fox]

There may well be some leeway regarding door opening size due the the inconsistency of fuel type and moister content etc … however there must also be an ultimate size based on averages.
There are other factors like emissions from the chimney and how clean the fire is burning that can only be measured with the proper equipment. (Gas analyzer)

Just by applying basic logic suggest to me there will be a most efficient size opening but…. this might be an individual size to match individual ovens and how they are used and fueled!

If we made the door really huge like half the dome size we know it would not work, if we make the opening tiny, we know it wont work, so logically we know there will be a size somewhere between those extremes that works well.

What is the definition of working well? In my mind apart from the size of pizza you want to fit in, you want a quick start up with as little chimney smoke as possible and there after you want a clean burn … ie a place in the dome where smoke will be held long enough, at a high enough temperature to burn away.

However in the real world we cant accurately define the door opening size because there are far to many variables from one oven to another, all we can offer is a guideline based on average.

A wood oven built with 2” thick walls of high density cement and backed with 4” of high tech insulation is going to work in a different manner to a 4” thick clay brick oven with 2” of low quality insulation?
Chimney size, height and type will also have a considerable effect on how the oven performs, how much air is supplied and how fast is travels are all factors!

[jakeanderton]

However in the real world we cant accurately define the door opening size because there are far to many variables from one oven to another, all we can offer is a guideline based on average.

A wood oven built with 2” thick walls of high density cement and backed with 4” of high tech insulation is going to work in a different manner to a 4” thick clay brick oven with 2” of low quality insulation?
Chimney size, height and type will also have a considerable effect on how the oven performs, how much air is supplied and how fast is travels are all factors!

After my previous build with a 66% opening, I felt that worked well. Sometimes I would have too much flame and it would actually lick out to the chimney. It is all about multiple variables that constitute an oven working well. From my own experience, and what I have read, I feel comfortable with anything between 62-66% of the door height relative to the internal dome height. With a 40" internal diameter, my plan was to have a width of 18". So the dimensions would look like this:
-Internal dome: 40" W x 18-20" H
-Door opening: 18" W x 11 1/2 - 12 3/4" H (going for 64% door opening height relative to the internal dome height)

One open question for me is the chimney diameter and height of the pipe. Previous build was 5' diameter pipe and 36'. It created plenty of draft. I may stick with the same. Unless, someone has other recommendations and can convince me otherwise.

[david s]

A 5” diameter pipe is way too small for your oven size. You need at least a 6”, probably an 8”
64
Most home operators end up cooking one pizza at a time because it only takes 2 mins and usually longer than that to prepare one. If everyone shares whatever comes out of the oven it’s easy. Multiple pizzas cooking at once introduces more difficulty in managing their cooking.

Regarding the oven opening, heat travels in three ways, convection, conduction and radiation. Circulation in the chamber is governed by convection. As heat rises by convection the circulation can be improved if the flame is pulled sideways. This requires dgreater raw from the flue than an updraft system like a fireplace. In my kiln, which is a down draft system the flame enters the chamber from below, travels up to the hemispherical crown, then is pulled down to floor level where it exits into the flue. Control is made by adjusting the gas intake, and air controlled via a sliding baffle in the flue. A downdraft system provides more efficient circulation and better fuel economy. The cross draft system of our ovens is somewhere in between that of an updraft fireplace and a downdraft kiln.

[jakeanderton]

A 5” diameter pipe is way too small for your oven size. You need at least a 6”, probably an 8”
64
Most home operators end up cooking one pizza at a time because it only takes 2 mins and usually longer than that to prepare one. If everyone shares whatever comes out of the oven it’s easy. Multiple pizzas cooking at once introduces more difficulty in managing their cooking.

Regarding the oven opening, heat travels in three ways, convection, conduction and radiation. Circulation in the chamber is governed by convection. As heat rises by convection the circulation can be improved if the flame is pulled sideways. This requires dgreater raw from the flue than an updraft system like a fireplace. In my kiln, which is a down draft system the flame enters the chamber from below, travels up to the hemispherical crown, then is pulled down to floor level where it exits into the flue. Control is made by adjusting the gas intake, and air controlled via a sliding baffle in the flue. A downdraft system provides more efficient circulation and better fuel economy. The cross draft system of our ovens is somewhere in between that of an updraft fireplace and a downdraft kiln.

Ill have to review the linked thread.

RE: Cooking one pizza at a time. I understand that the cook time is so short, and the inability to stretch dough, top it off and get it in the oven in under 2mins. I plan to be running 5-6 pizza peels at any given time. 3 pizzas in the oven, 2 more already stretched, loaded and ready to cook, while the other 3 peels have their dough being stretched and topped off by someone else. Ideally, I would be cooking 6-10 pizzas total for a gathering. Even when its just my family and I at a meal, this would be 4 pizzas (my 5yr old loves cold pizza for breakfast the next day). But, if a child's birthday party, I could see 10-15 pizzas of the 8" variety as "personal pizzas" for the kiddos. At least, those are my aspirations.

[fox]

Ill have to review the linked thread.

RE: Cooking one pizza at a time. I understand that the cook time is so short, and the inability to stretch dough, top it off and get it in the oven in under 2mins. I plan to be running 5-6 pizza peels at any given time. 3 pizzas in the oven, 2 more already stretched, loaded and ready to cook, while the other 3 peels have their dough being stretched and topped off by someone else. Ideally, I would be cooking 6-10 pizzas total for a gathering. Even when its just my family and I at a meal, this would be 4 pizzas (my 5yr old loves cold pizza for breakfast the next day). But, if a child's birthday party, I could see 10-15 pizzas of the 8" variety as "personal pizzas" for the kiddos. At least, those are my aspirations.

That will be quite a feat, please take some video

The two minute pizza is only possible with a very hot oven and a very thin pizza but most folk like to load up their pies and use a cooler oven so they cook through but, even then, a well loaded pizza will normally need to be lifted up towards the top of the dome to crisp the top.

We have all seen the pros on youtube cooking three at a time but for me that is nothing like fun just a load of stress!

From my own personal experience, we prefer to cook one at a time using a three man team, one loads the pizza, one cooks the pizza and one cuts and delivers the pizza to the table.
People then share the pizza while number two is being processed……. Again just from my own personal experience most guests like to spectate and watch the process or join in rather that shout “ where is my pizza”

In any case it is all good fun and I am sure you will develop your own style once you are all set up and going….

[jakeanderton]

That will be quite a feat, please take some video

The two minute pizza is only possible with a very hot oven and a very thin pizza but most folk like to load up their pies and use a cooler oven so they cook through but, even then, a well loaded pizza will normally need to be lifted up towards the top of the dome to crisp the top.

We have all seen the pros on youtube cooking three at a time but for me that is nothing like fun just a load of stress!

From my own personal experience, we prefer to cook one at a time using a three man team, one loads the pizza, one cooks the pizza and one cuts and delivers the pizza to the table.
People then share the pizza while number two is being processed……. Again just from my own personal experience most guests like to spectate and watch the process or join in rather that shout “ where is my pizza”

In any case it is all good fun and I am sure you will develop your own style once you are all set up and going….

Yes, everyone develops their own style and that is half the fun! When we have gatherings with people, eating one pizza amongst 8 people means a single slice per person. Waiting on making another pizza, then cooking it means a very long time between pizzas and more than an hour of cooking. Just doesn't make it great, to us. Hence the impetus to cook more pizzas at a go. Even if I can get 2-3 slices per person, and it takes 6-10min between batches, that is far better in my mind.

[jakeanderton]

Still working on the perlcrete insulation layer between the cement pad and the firebricks. I ran far short of perlite on my first mold into the template. Just finished the last one I need. I am making these into 2 separate slabs, giving me a natural control joint. I pulled the other one (the drier one in the top of the pic) after it cured for 5 days. It was still very wet on the underside, and this will help accelerate the dry time. This leads me to believe that if I had done this on top of the pad, I would have been looking at 2, 3 maybe even 4 weeks of cure time. Very happy I am going about it this way.

[jakeanderton]

I’ve been sidetracked putting up a privacy fence and finally getting back to this. Of course, now that it has turned cold here.

somewhere in there is the perlcrete with the firebricks on top of them. Then the sandcastle mold on top and around. The mix was a 10:1 sand to clay, with water. I am shocked at how many bags I went through. I thought 6bags would be fine, and ended up using 22. Next step is either Saran wrap covering or paper mache before refractory cement over that.

Any recommendations on the best layering method between the sand and refractory?

[david s]

You could cast directly onto the sandcastle, but sand grains are likely to stick to the inside of the casting. Strips of wet newspaper works ok, but in windy conditions tend to blow away.some PVA or flour mixed with water would help make the strips stick better. Thin clingwrap tends to slide out of the way.

[UtahBeehiver]

Based on the pic of the sand mold. it looks like the cast portion of the dome will sit on the concrete hearth rather than pcrete or the brick floor on pcrete. If this is the case, the concrete hearth will act as a heat sink robbing the BTUs from the dome. The dome, like the floor, needs to be isolated from the concrete hearth. Now is the time to address this issue.

[jakeanderton]

You could cast directly onto the sandcastle, but sand grains are likely to stick to the inside of the casting. Strips of wet newspaper works ok, but in windy conditions tend to blow away.some PVA or flour mixed with water would help make the strips stick better. Thin clingwrap tends to slide out of the way.

I like the idea of the paper mache route. We have decent wind here all the time, and that seems to be a heavier option to stick to the sand than the clingwrap. Thank you!

[jakeanderton]

Based on the pic of the sand mold. it looks like the cast portion of the dome will sit on the concrete hearth rather than pcrete or the brick floor on pcrete. If this is the case, the concrete hearth will act as a heat sink robbing the BTUs from the dome. The dome, like the floor, needs to be isolated from the concrete hearth. Now is the time to address this issue.

Hmm. I never thought about that. My goal is to fire the oven for pizza making, and maybe, very occasionally, bake bread with leftover heat. So, in using it this way, I am not too concerned about losing some of the BTUs. Or do you think I will lose a significant amount of BTUs as to robbing the pizza making time?

[jakeanderton]

Unfortunately, with the MN temps turning colder I am on pause. I talked to the manufacturer of the castable refractory, and they advised that with my current temps, and inability to cover and heat the material while its being casted, that i would have a very weak bond. Something I do not want. So, I am stuck until spring comes around... Anyone have any good plans for forms I could build over the winter in my house, then assemble in the spring?