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.