Flue pipe/gallery integration and size (single or twin walled)

[Phartog]

I am currently ready to work on the flue gallery and I have some questions regarding this. When planning I always considered to use a 125mm flue pipe (see attached image), and I wanted to use the david s method of connecting the flue pipe to the casted flue transition. You can see it in this post:

I've recently taken on the task of building a flue gallery and outer decorative arch from a customer who attempted his own solution which failed. My approach is rather different than building in brick units which have three big drawbacks. Firstly the problem of the alignment of vertical brick joints and secondly the thickness of the brick units contributing to a large thermal mass acting as a heat sink drawing valuable heat from the oven. Lastly it is almost impossible to create a smooth compound curve with brick units and therefore a smooth smoke flow. However casting enables all three of these shortcomings to be avoided as the attached pics show. Also the labour involved is considerably reduced. The first stage was to create a mould plug from 8:1 vermiculite and casting plaster. Then use a suitable form to create the inside diameter for the flue gallery casting.So far only 3 hrs labour.

But I currently have a 120mm flue pipe lying around, would it matter to much if I would use this 120 instead of 125mm, I know 125 was kind of the minimum required because of the 80 cm internal diameter of the oven.

Alternatively I was thinking of buying a twin walled flue pipe (125mm-180mm). I thought about doing this for multiple reasons:
1. I read the isolated pipe would improve draw/draft, maybe this could be a plus with my rather shallow flue gallery/entry
2. I think the bigger diameter (180mm) would fit my oven design a bit better
3. It would give me the possibility if I would want to build a small roof over the oven later, it would also be sufficient for that

I would really want to know what your ideas on this is.
When I would go for a twin walled flue pipe, I was thinking on how to construct and connect this to the flue gallery, and it would be difficult to follow the exact method of david s.
My idea was that I can maybe just slide the twin walled connection part/adapter into the casted flue gallery (transition) and fix it by using high temperature silicon, so it accomodates for expansion/shrinkage. The other method would be to integrate this adapter/connection part directly into the cast, I think Macrinehart did this recently:

So today I made the first attempt to repair my flue gallery. This time I purchased a single-wall to double-wall pipe coupler. This coupler seals the interior of the two walls and it has a 3" deep and 8" diameter flang on the base which I felt would make an excellent coupler to the flue gallery.

to start I re-filled my gallery with wet sand. Then I centered this coupler on top and filled it with wet sand. Next I cleaned and dampened the already dried concrete-mix I previously molded in place. Using a concrete bonding adhesive, I made a slury of sand, concrete and adhesive per directions on the bottle and spread that around the top and exterior sides of my existing concrete-mix. Then I mixed up more of the casting home brew and used that to build up the flue gallery around the pipe coupler.

My experience at this point is that the wet concrete wanted to slowly slump. After messing around for a while trying to mitigate this without much success, I finally decided to wrap the damp concrete in plastic cling wrap with the hope that a little pressure from the outside could hold everything in place while it slowly dries. Finally, once everything was satisfactory I pulled the coupler out.

My hope is first that I get a reasonable bond between new and existing concrete. I presume if that works then once the material is set up I'll have to do a little work to get the coupler to fit. If it doesn't work, then I'll have to knock this gallery down and redo it.

Any tips and comments are very welcome, this part of the build is really holding me back

[david s]

An 800 mm ID oven requires a150mm diameter pipe. I’m all for using what you have to hand, but you are likely to end up with an oven that won’t breathe well. Draw can be increased by making the pipe taller, but this would require you purchasing a second pipe anyway. Diameter increase has a far more powerful effect on draw than height anyway. A double flue is only required if you are penetrating a roof. A single pipe gets hot, but it’s high enough at the base ((adult eye level), to be well away from small kids.
The 8;1 vermicrete/perlcrete plug that forms the mould for the casting, is easy enough to carve and sand so you can get a good form and smooth surface. Cover with strips of wet newspaper which act as a release agent.

[Macrinehart]

Cover with strips of wet newspaper which act as a release agent.

Wish I had that advice before casting my gallery.

With respect to my solution, I have a 42" (107cm) diameter oven and the minimum diameter chimney pipe is 8" (~200mm).

The chimney pipe vendor I used had an 8" single-wall to double-wall transition piece that I purchased for my second attempt. The first attempt I built a sand mold, and then formed the flue gallery directly over the sand mold. The opening at the top was too small after drying, and also not level. That was partly because I tried to shortcut the job by building the flue gallery before my chimney pipe arrived, and I discovered that the od of my double wall chimney pipe was 10.25" instead of the 10" od I planned for.

I used an angle grinder to cut off the top and level it, but removed too much material leaving gaps around the chimney.

Take 2: I ordered the single-wall to double-wall transition piece, and used that as a guide for building my sand mold. I also got a concrete bonding agent and applied that over and around the original flue gallery. Finally I extended that gallery to match the diameter of my transition piece.

After drying, I used the angle grinder to level the top and remove a little material from the inside of the gallery so that everything fit together properly. this time it all worked!

[Phartog]

Thanks david s for the reply. Based on your feedback I decided to go for the 150mm flue pipe, I already ordered it. I don't want to risk putting in a smaller flue pipe and have bad results.

Macrinehart Thanks for the reply. Your build looks incredible and I think your second attempt for the flue transition looks good. Eventually I will "copy" as much as possible the method of David, described in the first post.

[david s]

If you use the flue pipe itself to create the mould to cast against, be sure to wrap some cardboard around it so tits diameter is about 3mm greater than the pipe. This allows for some expansion of the stainless pipe which will expand first against the casting. I cracked the second and third oven gallery casting before realising that this was the cause. I’ve never had the problem occur with any subsequent ovens. Also wrap some plastic around the cardboard spacer for easy removal of the mould/pipe.

[Phartog]

Thanks david s for the tip! I are casting against a 3d printed cylinder (instead of the flue pipe). So I make the 3d print 3mm greater. I ordered the new 150mm flue pipe, but unfortunately it's in backorder, so I have to wait until it's in stock again.

I have another question about the flue gallery/transition. When i eventually cast or make the flue transition from castable refractory (I will use the homebrew with the addition of the fibres), so I cast it against the brick dome, or should I first lay down the isolated blanket (fixated with chicken wire) and then cast the flue transition? I made 2 drawings explaining what I mean, the drawings are a bit exaggerated, the casted flue transtition won't touch the brick dome so much.

And also something I am not sure if I should do it. Should I isolated the flue gallery from the brick dome with a heat break (red line indicated in the drawings), I would use some kind of fibreglass hollow gasket, I have seen other people use. I have the feeling it makes the build of the flue gallery more complicated and I am not sure it has a big advantage?

[david s]

A heavy gallery can act as a heat sink drawing heat away from the hotter dome. Also, because it will be cooler than the dome itself thermal expansion will be less . Creating a gap to reduce heat travel by conduction then creates structural weakness because the gallery is an independent structure. Using a fibreglass flexible insulating material, or similar, in this position also runs the risk of fibres falling into the food. My solution was to make the gallery thin, by casting it, shallow and therefore light, to minimise the heat sink effect, but an integral part of the dome as well as being covered with the dome insulation to minimise radiant heat.loss. Other solutions by manufacturers us a stainless gallery to reduce mass. I’m sure there are other solutions.