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  #31  
Old 01-21-2014, 01:47 AM
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Default Re: New Build in South Oz

Thanks Dave.
I've had a look at your pics. We are in the process of building an entrance very similar to yours, except it is not connected to the oven at all. I used a piece of hardiboard to space the whole thing off the inner arch, as my mate wanted a heat break. So there is about a 7-8 mm gap. He has some of that fibreglass rope seal that they sell for replacing the door seal on a domestic oven. I just wanted to seal it by putting a layer of perlcrete over the outside, bridging the gap, but he is adamant the fibreglass is insulating and wants to use it. Can't hurt much, might do some good, he got it free, and the customer is always right.
The kicker is my mate, who happens to have some serious angle iron that'd hold up anything, just can't visualise how they'd work. I reckon I'll show him yours. He did want to use some flat steel he's got that's about 3mm thick and maybe 3 inches wide. He can't see why not, because it is fairly stiff when we lay it there, but I reckon flat steel isn't real strong, especially if you heat it. At work we specifically won't run steel fans in gases over about 270C, because the steel softens and then the fan stretches and hits the housing.

Last edited by wotavidone; 01-21-2014 at 02:07 AM.
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  #32  
Old 01-21-2014, 02:25 AM
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Default Re: New Build in South Oz

Gudday Mick
An 8 mm air gape sounds good. I like simple.
I'm not that convinced that a heat gape is that important thought. If you think about it it takes 2 good hours of firing to get the heat to saturate though 100 mm of firebrick and 25 mm of cladding. So its really not that fast. It would take it quite a while to heat up the whole entrance. Most of the heat on the internal surfaces is radiant heat from the fire anyway. Before I buttressed my entrance you could hardly feel any warmth much less heat on the external brick.
Still ,if you can include a heat break especially if its not time consuming and expensive why not.
Regards dave
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  #33  
Old 01-21-2014, 02:31 AM
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Default Re: New Build in South Oz

The client is always right.
Heat break he wanted, heat break he got
I reckon it would make a bit of difference, but not all that much.
I'd have preferred to build the entry right up against the arch so as to give some support, but I reckon it'll all hold together anyway.
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  #34  
Old 01-21-2014, 02:39 AM
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Default Re: New Build in South Oz

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Originally Posted by cobblerdave View Post
Gudday Mick
An 8 mm air gape sounds good. I like simple.
I'm not that convinced that a heat gape is that important thought. If you think about it it takes 2 good hours of firing to get the heat to saturate though 100 mm of firebrick and 25 mm of cladding. So its really not that fast. It would take it quite a while to heat up the whole entrance. Most of the heat on the internal surfaces is radiant heat from the fire anyway. Before I buttressed my entrance you could hardly feel any warmth much less heat on the external brick.
Still ,if you can include a heat break especially if its not time consuming and expensive why not.
Regards dave
I agree with you Dave, but if the heat break is made of a flexible material like vermicrete that can compress, it acts not only as insulation, but as an expansion joint. Ovens without this often have cracks in their outer arch or where the outer shell joins the outer decorative arch. If the inner parts of the oven are made so that they can expand independently and freely from the outer shell and decorative arch then the expansion problem is relieved.
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  #35  
Old 01-21-2014, 03:28 AM
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Default Re: New Build in South Oz

Gudday Mick
I recon angle irons pretty strong stuff.
A mate brought this box trailer and conned me into helping him fix it as I had a baby stick welder. ( beer will do that)
It was pretty crook. Basically folded steel box welded at the corners flat steel floor it strength at the back a simple fold in the metal. It wasn't that someone had broken that fold by backing it into something it was more that then filled it it bricks or gravel or both. The welds in the tailgate which had probably been the major strength at this stage must have broken at speed as was evident by the condition of the tailgate. All in all one pretty tired looking piece of kit.
We set about fixing it. Removing the axle resulted in tin box twisting ever worse. So we set about massaging the floor back to level with a sledge hammer and levering the side back to straight with wood and car jack. We cut a series of floor braces from the angle and with me mate sitting on one corner and his wife on the other I tack welded them into position. When they stood up it actual held together and pretty straight in spite of my welding skills. Refitting the axle and made a tailgate finished the job and that trailer lasted for years.
In spite of its being completely twisted just 4 pieces of 40 mm angle gave it back its strength. It survived quite a few more years of hard service never buckled again regardless of the loads.
Regards dave
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  #36  
Old 01-21-2014, 04:17 AM
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Default Re: New Build in South Oz

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Originally Posted by david s View Post
I agree with you Dave, but if the heat break is made of a flexible material like vermicrete that can compress, it acts not only as insulation, but as an expansion joint. Ovens without this often have cracks in their outer arch or where the outer shell joins the outer decorative arch. If the inner parts of the oven are made so that they can expand independently and freely from the outer shell and decorative arch then the expansion problem is relieved.
Gudday
I see what you mean about the entrance and the dome being free to move separately due to the different expansion rate. I lucked out I recon with that one. I recon I butt joined the entrance to the dome due to using different bricks rather than trying to tie them together.
Question . Do you think its more important to include a heat break on a smaller oven such as yours if only because being a smaller mass any heat loss is more telling. And slightly off that point would a heat break be more important in colder climates due to the bigger difference in temps between inside and out?
Regards dave
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  #37  
Old 01-21-2014, 12:08 PM
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Default Re: New Build in South Oz

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Gudday
I see what you mean about the entrance and the dome being free to move separately due to the different expansion rate. I lucked out I recon with that one. I recon I butt joined the entrance to the dome due to using different bricks rather than trying to tie them together.
Question . Do you think its more important to include a heat break on a smaller oven such as yours if only because being a smaller mass any heat loss is more telling. And slightly off that point would a heat break be more important in colder climates due to the bigger difference in temps between inside and out?
Regards dave
Q1. I think any advantage of a heat break would be the same regardless of oven size. But not really sure, it could make a bigger difference for a smaller oven. With kilns the rate of heat loss is much the same for a small one as a large one, which would indicate that mass doesn't matter that much. However as a general rule the higher the temperature the greater the rate of heat loss.

Q2. Yes, that would be a logical conclusion, but the difference would be small because from freezing to pretty hot ambient is only around 30 C difference and the inside oven temp maybe around 400 C. But it could be the small extra difference that creates enough stress to produce some cracks.

Last edited by david s; 01-21-2014 at 10:53 PM. Reason: More thought
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  #38  
Old 01-26-2014, 08:37 PM
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Default Re: New Build in South Oz

We've made a bit of progress. Built outer arch/entry one and a half bricks deep, rendered the entire dome with chicken wire and a high lime render (so now I'm happy with both the strength and the thermal mass), installed the top on the entry.
My mate found the roughest bricks in the universe to make the roof of the arch. He set it up the other night while I was sleeping off the Feast of Santo Nino, so it was a done deal, and anyway the client is always right. The roof is also two bricks deep - he is definitely in need of looong handles on his oven tools. His plan is to face the two vertical walls of the arch with the cut ends of the super rough bricks, which will bring the walls out as far as the roof. Two bricks deep just to reach the inner arch. His arms are definitely too short!

Mate started fitting the hebel block walls. the dome will be surrounded with hebel, with the voids filled with loose perlite.
The heat break between the inner arch and the entry will be filled with woven fibreglass rope seal that they use for oven doors, but I'm not sure exactly how we'll mate the hebel to the dome around the arch/heat break area, I reckon he needs something to fill the gaps - can't really see us cutting hebel to such a close fit that there isn't a gap to fill.
Guess we'll work it out.
The flue is sitting in there just for a test fit.
Attached Thumbnails
New Build in South Oz-2014-01-26-23.05.47.jpg   New Build in South Oz-2014-01-27-12.16.10.jpg   New Build in South Oz-2014-01-27-12.15.51.jpg  

Last edited by wotavidone; 01-26-2014 at 09:23 PM.
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  #39  
Old 01-27-2014, 03:01 AM
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Default Re: New Build in South Oz

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Originally Posted by wotavidone View Post
The steel will be encased in the brick. If the brick has a lower conductivity than the steel, how will the heat rush to it?

BTW, many stainless steels have a higher thermal expansion and the same thermal conductivity.

Coefficients of Linear Thermal Expansion
Thermal Conductivity of Metals

It really depends on what steels you are comparing, but I reckon the only real reason to use stainless is the corrosion resistance, if you need it.
A more thermally conductive material will get hotter than the material that surrounds it. A couple of examples of this are 1. On my first oven iset in a dense rock with the word PEACE on it above the arch. We found that this rock got so hot that you could not hold your hand on it when the oven was ready for pizza, yet the outer shell around the rock was still cool enough to hold your hand on it.
2. The steel rails underneath the Hebel supporting slab on my mobile get hotter than the Hebel that sits on top of them.

The hotter a material gets the more it expands.

There are many different stainless steels, but they all seem to have less thermal conductivity than mild steel.

The preferred reinforcement for refractory is stainless needles presumably because they perform better than mild steel. Not saying your mild steel won't work, just use what does work.

The tables you referenced do show a big difference between the thermal expansion of brick and mild steel though. This could create a problem for you. Did you leave a little gap at the ends of the steel so it can expand without forcing the bricks apart?

It also shows quite a difference between the thermal expansion of concrete and brick that I was not aware of, I thought they were much the same.

Last edited by david s; 01-27-2014 at 03:17 AM.
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  #40  
Old 01-27-2014, 03:59 AM
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Default Re: New Build in South Oz

Quote:
Originally Posted by david s View Post
A more thermally conductive material will get hotter than the material that surrounds it. A couple of examples of this are 1. On my first oven iset in a dense rock with the word PEACE on it above the arch. We found that this rock got so hot that you could not hold your hand on it when the oven was ready for pizza, yet the outer shell around the rock was still cool enough to hold your hand on it.
2. The steel rails underneath the Hebel supporting slab on my mobile get hotter than the Hebel that sits on top of them.

The hotter a material gets the more it expands.

There are many different stainless steels, but they all seem to have less thermal conductivity than mild steel.

The preferred reinforcement for refractory is stainless needles presumably because they perform better than mild steel. Not saying your mild steel won't work, just use what does work.

The tables you referenced do show a big difference between the thermal expansion of brick and mild steel though.
1) Can't quite get my head around your reasoning. We aren't talking about a kilo of brick and a kilo of steel sitting side by side in a kiln with the steel absorbing energy quicker than the brick due to different conductivity.
Even then they can only reach the temperature the kiln is set to.
A piece of steel fully encased in masonry can only absorb heat at the rate the masonry conducts it to the steel. And surely, since I presume we are not talking about any phase changes in the steel or the masonry, the steel can only get to the same temperature as the surrounding masonry.

Thermal dynamics wasn't my major at school, but I can't see how masonry that gets to say, 300 degrees, can raise the temperature of any steel encased in it any higher than that.

Especially since the steel runs the full length, one would think that, if anything it would assist in transferring heat from the hottest part of the masonry to the coldest part, thus evening things out a little.

2) Don't forget, if two materials are the same temperature which is hotter than your hand, the one with the highest conductivity will feel hotter as it more easily transfers the energy to your hand.
If they are cooler than your hand, the one with the least conductivity will then feel hotter as it retards the loss of body heat from your hand. All depending on which way the heat is flowing.The only way to really tell which is actually hotter is to measure each with a contact thermometer.
So, did you actually measure the temperature of your steel rails vs your hebel, or just feel it with your hand? Ditto the hot rock v the cooler rock? How much hotter did the rock feel? Possibly just a much better conductor than the other rock?

3)The tables I referenced show an even bigger difference between the thermal expansion of masonry and every stainless steel mentioned except one.
The one with the slightly lower expansion rate is 410 grade. Its a hardened steel with lesser temperature and corrosion resistance that is unlikely to be the one they supply for your application.
In fact my research has turned up fibres for refractory made from 304 and 310 grade stainless, both of which have higher expansion rate than steel.

I reckon the only reason they use stainless fibres rather than steel to reinforce refractory is superior corrosion resistance.
In the unlikely event that my mates reinforcing corrodes (since it isn't inside the oven getting really belted, we only have to protect it from water infiltration), we'll be able to replace it easily.

Last edited by wotavidone; 01-27-2014 at 04:08 AM.
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