New Build in South Oz

[wotavidone]

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.

[david s]

Re: New Build in South Oz

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.

[cobblerdave]

Re: New Build in South Oz

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

[cobblerdave]

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

[david s]

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

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.

[wotavidone]

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.

[cobblerdave]

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

[wotavidone]

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.

[cobblerdave]

Re: New Build in South Oz

Gudday Mick



Outer arch added later. But flat brick entrance supported by angle iron. No cracks as this is possible cooler than the entrance. Oven entrance is also angle iron and also supports the rear wall of the chimney which is 5 bricks high. This has 2 cracks which cannot be photographed cold probably due to the soot if anything. Hot they can be seen a mm or two wide. They don't appear at the top of the chimney or outside. It think I expected a least some cracking. So they don't bother me and I don't bother them.
Regards dave

[wotavidone]

Re: New Build in South Oz

I think the main problem of steel reinforcing in the refractory is the greater thermal conductivity of the steel compared to the refractory. It means the heat rushes to it and expands it more than the refractory that surrounds it.

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.

http://www.engineeringtoolbox.com/li...ents-d_95.html
http://www.engineeringtoolbox.com/th...als-d_858.html

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.

[david s]

Re: New Build in South Oz

I think the main problem of steel reinforcing in the refractory is the greater thermal conductivity of the steel compared to the refractory. It means the heat rushes to it and expands it more than the refractory that surrounds it. I think this is why stainless needles are used. Stainless so they won't corrode as easily in the hot and moist environment and needles so they can dissipate their heat more easily to the refractory that surrounds them. Steel and concrete in the normal situation have almost identical expansion rates and that's why they are so good for reinforced concrete in bridges and buildings etc, but any heat increase from the weather is slow enough for both materials to equalize their temperature and therefore their expansion easily, not so when heating an oven at 400C/hr. Also presuming brick, or castable have similar expansion rates to concrete.

[wotavidone]

Re: New Build in South Oz

I've found a list of expansion coefficients that says steel expands at around 16 x 10-6 metres per metre per K.
At least, that's what I think it says.
Anyway, if I work this out right, increasing the temperature by 400C gives a 6.4 mm increase in length per 1 metre length of steel.
Anyone reckon that sounds about right?
I'm thinking that expansion over about 600mm is not going to be an issue, since I sincerely doubt the bricks of the outer entry tunnel are going to get anywhere near 400C.

[wotavidone]

Re: New Build in South Oz

Progress report.
Managed to score some 65 mm pavers. These were just what we needed to get around a particular issue.
i.e. with the three inch/75mm bricks sitting on the floor, we had an issue with mortaring the floor bricks under the outer entry. We wanted to do this to retain the floor bricks inside the oven in place, but then the mortared bricks would be higher than the floor bricks. With the 65 mm bricks there was room for a 10 mm mortar layer. So there is now an apron of bricks in front of the oven.

Next problem:
The client who, as supplier of the materials and beer, is always right, wants a rectangular entry, one and a half bricks deep, with a flat top. i.e. not even a shallow arch to stop the roof bricks falling in.

Hmmm. Steel lintels, maybe?

He didn't like the look of that, , and I figured there would be all sorts of movement cracking everything.

However, three Southwark stubbies later, new idea.
We will obtain some house bricks, the ones with the holes through them. We will fabricate the roof of the outer arch in one piece, consisting of bricks mortared together with reo rod running through the holes. Sort of hidden lintels. I reckon, with the reo inside the bricks , it can't get hot quicker than the bricks, so the expansion rates get closer together.

Taper everything to a 7 inch by 7 inch chimney hole in the middle.

Why 7 inches by 7 inches? Because we have discovered that all the old fire extinguishers he has are not aluminium as first thought, but some sort of non-magnetic steel, presumably stainless.

So, we will be having a flue fabricated from 7 inch diameter stainless steel that is about 2 mm thick. Beautiful.

Remember the committee of self employed contractors supervising this job?
It would appear one of them knows a man who has welding equipment capabilities the home handyman can only dream of.

My only concern is whether we can shape everything for good air flow.
Like everything else we've done, we'll cut some bricks and try a few things.

I reckon the old drop saw I bought at a garage sale is going to be fit for scrap by the time we finish. The bearings are getting noisy and it tends to run off course a bit. Still 2 ovens for a $25 investment is pretty good.
When we've finished I'll pull it apart and see if the bearings are replaceable.

[oasiscdm]

Re: New Build in South Oz

Colin,
Do your floor bricks lay straight on the hebel like Cobblerdaves?
I think the layer of bricks protect the hebel from the direct fire, and ensure the hebel heats slowly and evenly.
With all respect to my mate, I do hope to win the debate about the thermal mass/reinforcement.

No bricks sit on calsil board 2" thick then hebel.

[wotavidone]

Re: New Build in South Oz

nice brick work closing the dome tight and symetrical.

Thanks Russell. I do subscribe to the theory that the tighter the bricks fit the less critical the mortar is.
Speaking of management by committee, there was a couple of strong opinions on how the last brick should be cut. Two guys competed to make the keystone.
One by cutting bricks with an angle grinder, one by boring a brick with his concrete borer.
The winner was the angle grinder, the other guy isn't happy with me for not using his perfectly round plug. Unfortunately it was about 20mm too small in diameter.