New Oven Flue Size

[david s]

Apart from being easy to cut and shape, polystyrene also breaks up readily. Just attack it with a screwdriver.
Regarding the floor, you can have the dome sitting on the floor or outside it. The advantage of sitting on the floor is that you don't need to be as accurate in cutting around the perimeter because those cuts will be hidden. The disadvantage is that it is very difficult to remove a brick if it has a dome sitting on it.
Regarding your insulation question, it makes no difference, so long as the oven is completely surrounded in insulation.

[MattP]

Hi David, just before I embark on cutting the floor tiles and CalSil board for the oven base could you please clarify the following for best results.
1. Should the refractory tiles be cut along the outer perimeter of the dome castable and flue chamber? - this allows the castable to rest on the floor tiles
2. Should the CalSil board be cut along the outer perimeter of the 50mm insulation blanket?
This is what I have allowed in my design.

[MattP]

David, by using the threaded rod the top portion of the polystyrene mould can be placed at final stage thus allowing a stick to work the mix into place. My immediate issue is how to hold in place the polystyrene vent recess mould while pouring/casting and then be able to remove following the removal (sliding out) of the timber door recess underneath. I thought of using silicone for temporary "glue" which should come off at the time of withdraw. Any comments.

[david s]

That looks like a workable solution, but it may be a bit tricky loading in the mix between the bucket and the inside polystyrene mould. Use a stick to work the mix into the space a little at a time. I've PM'd you my solution

[MattP]

Thanks David. I had anticipated casting issues with vertical/near vertical faces hence I propose some outside formwork. Also I am planning formwork at the front entrance so in effect it will be like casting a concrete structure with formwork surrounding it for the majority of faces. An open plastic bucket is the outside formwork for the flue gallery in combination with the internal polystyrene. I would prefer to cast the whole oven in situ at once. What are your thoughts. Please refer to attached for explanation.

[david s]

Without an outside mould I'd be worrying about trowelling against that mould near the top section without it slumping off. As suggested previously it would be far easier to cast the flue gallery separately. You could lay it on its side with a sand mould for the bottom half. 30 mm is ok, mine is thinner than that, but as you observed I have added buttressing where it needed some more strength.

[MattP]

Hi David, following your advise I undertook to build the flue galley in polystyrene - photos attached including front and door opening timber formwork. I have made the venting recess 40mm high as opposed to 30 in my design. I intend on using a threaded rod to hold the tapered cylindrical recess to aid removal following cast and removal of front arch. The venting recess pieces I can hold but I am having trouble deciding how to remove following the casting - any ideas?
I understand from your previous post 40mm thick castable is quite strong. Is it possible to decrease to say 30mm around the flue gallery/venting recess? Currently I have 50mm vertical at the door and 40mm around the flue gallery and venting recess. I would appreciate your thoughts.

[david s]

It will work, but it would be far more efficient if you make the 135 measurement (front view) much larger at the bottom so there's greater volume there and the shape is more funnel like. At least try to eliminate the sharp corner. Polystyrene is easy to shape with a sharp knife if you are not confident to do it in sand.

[MattP]

Hi David, due to constructability issues related especially to the flue gallery of previous design (tapered/funnel effect) I have reviewed the design again. Could you please refer to attached and please comment on design.
I trust the sectional elevation may make things easier to understand.

[MattP]

Thanks David.
Interesting information re fibreglass mould.
Forgot to mention in my previous post - in order to achieve the recommended design profile of the dome internal top radius, reducing thermal mass near the entrance, and complying with the recommended door height, the door is now approx. 50mm within the arch floor (795mm as opposed to 850mm).
55mm reduction for the cooking surface (in one direction) is not an issue I am just thinking of workability/functionality of the oven.
Any comments?

[david s]

Yes, that's much better. I've made my mould in fibreglass in three pieces, this is way too much trouble for a one off casting. I suggest you cast the dome first, in situ, then build form work with polystyrene and or sand in front of it, then cast the flue gallery, trowelling the mix against the mould. This should not be difficult after the experience of casting the dome first.

[MattP]

Thanks Russell and David.
I have considered the suggestions made and have reviewed the design. Please refer to Rev G attached.
Interesting flue gallery casting David - certainly lightweight (assume approx. 20mm thick?). Also interesting seeing strengthening ribs on the side. Is it in operation currently? In light of your photos I am now considering casting the flue gallery as a precast and the dome in-situ in combination with the precast gallery. It would make life easier to cast however, the challenge will obviously be the formwork required and the joint at the interface with the dome. What material have you used for formwork to cast the flue gallery David? Relevant photos would be appreciated.
I appreciate your feedback.

[david s]

Good point Russell, I didn't check that calculation, just thought it looked about right.You would be better to allow the top of the oven mouth follow the radius of the dome, as the idea is to keep the flames inside the dome rather than allowing them to escape too easily. [ATTACH=CONFIG]n390776[/ATTACH]
Matt, I've attached some pics of my flue gallery to explain what I described about its design. Mine is pretty extreme in reducing weight but the photos explain it better than words.

[UtahBeehiver]

Take a look at your door height to dome radius ratio, typically it should be optimally around .63-.65% of the internal dome radius. Right now it is hovering around 300/425 = 70.6%.It is not a deal killer just something to consider while you are in the planning stages.

As far as the cast portion, you are in good hands with David.

[MattP]

Hi David, I have modified the design again to incorporate a granite/bluestone strip at the front and hence have modified the flue gallery. Could please inform if this will work properly.