31"/800mm in Eindhoven, the Netherlands: Design critique and build topic

[Toiletman]

What happened at the top of the cast?

As mentioned and shown in posts #71/#72: I mixed the concrete too wet and it was slumping down, so flowing out of the top part of the cast. I now have more thickness at the bottom than planned, and had some gaps at the top. I filled them in again, and they're now damp curing... my feeling is that it's good enough to hold up and provide sufficient surface for the chimney connector to sit on.

I am a bit perplexed when going back over this thread. Post#65 shows pics of the top of your flue gallery arch. With your casting sitting on top of it, the slot in the top of the arch becomes redundant and a restriction for smoke flow. I was under the impression that your casting would replace the brick arch. I think it will still work ok as you propose, but not as efficient as it could be.

I did consider a fully cast gallery arch initially, but since the gallery is so wide to accommodate the barbecue area and the arch has such a shallow curvature I thought it would be very challenging. Having a small thickness would probably not have enough strength, and making the cast as high as the brick gallery would make it very unwieldy to manufacture and transport. I also like the look of the brick arch face a lot.

You are right about the shape of the bricks becoming a restriction. Not fully a restriction, as the the smallest surface area is always the chimney pipe, anything upstream is wider. However, the widening of the brick arch will cause a negative pressure gradient which might cause some recirculation. Overall though, from the cross section I think it's still smooth enough. Especially if you consider the lengthwise cross-section: the vertical path in the middle of oven is very straight and smooth, it's only at the sides that the recirculation might cause a decrease in draft.

Attached some pictures of the cross sections, as well as an alternative design I considered fully covering the brick arch with a casting --> in the end I felt it would only add to the complexity of construction and make the casted part less robust --> the only material that's really needed is the material to sit around the smoke opening. My main worry was about the brick arch being able to support the weight of the casted part, that's why I thought of extending it to be supported on the gallery walls. In the end my feeling is the gallery arch should be able to support the weight, as well as that once the vermicrete plaster is in, it will provide a nice stiff surrounding shell, with a lot of height giving it also a lot of stiffness, probably eliminating the problem completely. Feel free to chime in with your thoughts...

[Toiletman]

The casting should've finished its damp curing by now, so recently I spent some time getting the plug out. See the progress below in pictures. The 'repair' area cracked while doing this and I discovered some more voids. I'm thinking of just filling the voids and adding a little cement here and there. As the whole thing will be loaded in compression and then encased in vermicrete I don't think recasting the complete top is warranted. Let's hope that is indeed the case.

Now it's waiting for temperatures sufficiently above freezing so that I can put the casting onto the gallery arch. In the meantime I'll see if I can prepare the sheet metal connections as proposed by david s in post #53.

[david s]

Difficulty removing the plug may have been caused by the lack of a slight taper (called a "release" in mould making parlance).
Also as the casting sets it shrinks slightly, especoally if there's slightly too much water in the mix. Removal from the mould at around 48hrs usually works best so the casting has sufficient strengrh, but a reduced amount of shrinkage.

You have in effect a gallery on top of a gallery, so the smoke flow will not be as efficient as it could be.

[Toiletman]

Difficulty removing the plug may have been caused by the lack of a slight taper (called a "release" in mould making parlance).
Also as the casting sets it shrinks slightly, especoally if there's slightly too much water in the mix. Removal from the mould at around 48hrs usually works best so the casting has sufficient strengrh, but a reduced amount of shrinkage.

You're fully right. That's quite stupid of me, because 3D printing would basically have given me the taper for free... ah well, we live and learn.

You have in effect a gallery on top of a gallery, so the smoke flow will not be as efficient as it could be.

Yes, a fully open path directly into the chimney would be more efficient. Luckily the actual profile is not as bad as in your sketch. The width of the gallery arch is much larger than the width of the vent arch. The chimney inlet width is roughly 90% of the vent arch width --> so no significant restriction. Depth-wise the position of the chimney does present a restriction: as a result of the very deep gallery and having the chimney centered on the gallery there is quite an offset from the face of the vent arch. For the smoke to exit the front of the gallery it would have to travel much further as well, because the gallery is so deep. Therefore I'm not too concerned at the moment, but we'll see how the actual performance is once the fires start burning.
Btw, I've seen several of the ovens I looked at as examples have a similar design. Also the kit from for instance the firebrick co. has a similar design. The big difference is that my oven's gallery is much wider and deeper --> the typical one has roughly the same width as the vent arch, and depthwise might even use the vent arch as the back of the gallery arch.

Luckily there's also some actual progress on the chimney connection to be reported. See the pictures.

Based on your sheet metal adapter I've gone the same way: thanks! I've cut slots into a piece of sheet metal, then bent it into a cylinder (I did it by hand/hammer, I'm assuming you have proper tools david s ?). Once it fit the chimney pipe, as well as the smoke inlet, I fixed some wires around it and bent the tabs inwards and outwards. It's already quite sturdy like this, even without any surrounding vermicrete.
For final fitting I'll have to put some cardboard sheet around the chimney to give it some space to expand, but I'm confident enough in it that it's now basically down to waiting for good weather so that the stuff can go onto the actual oven and I can get away from boring CAD pictures .

[david s]

The sleeve I make for accepting a removable flue is made from 0.55mm 304 stainless. Being thin it is easy to shape by hand. As shown in post #53 there is a space so that it does not complete a full circle. This helps in providing some room for expansion as well as providing a space to accomodate the lapped joint of the flue pipe that fits into it. To make the pipe fit inside it I wrap some plastic around the pipe, then fit the sleeve over it and tighten the wires up so they are just firm, but not really tight. The plastic allows for easy enough removal as well as providing enough of a gap so the pipe is a nice sliding fit.
Also added a couple of pics showing the fitted sleeve and the weather cap I use when the flue pipe is not in place. Hope this helps, maybe too late.

[Toiletman]

The sleeve I make for accepting a removable flue is made from 0.55mm 304 stainless. Being thin it is easy to shape by hand. As shown in post #53 there is a space so that it does not complete a full circle. This helps in providing some room for expansion as well as providing a space to accomodate the lapped joint of the flue pipe that fits into it. To make the pipe fit inside it I wrap some plastic around the pipe, then fit the sleeve over it and tighten the wires up so they are just firm, but not really tight. The plastic allows for easy enough removal as well as providing enough of a gap so the pipe is a nice sliding fit.
Also added a couple of pics showing the fitted sleeve and the weather cap I use when the flue pipe is not in place. Hope this helps, maybe too late.

Thx! I'll also have to think about how to make a nice edge and a cap around the final product. The thinnest sheet metal that I could get was 1mm, so bending it in shape is a lot more difficult than 0.55mm!
Don't worry about the gap: instead of a gap I left an overlap, so that the metal can easily slide on itself when expanding.

Besides that, we have some non-freezing at night temperatures, so I woke up from hibernation and mortared the chimney connection in place. See attached pictures. After mortaring it I put wet cloths around it, so hopefully by tomorrow there's a nice strong connection. I'll re-wet the cloths again for the coming days, and then it's praying for properly good weather to start working on the insulation.