New Oven Flue Size

[MattP]

Hi All,
I am new to this forum from Melbourne - Australia, and hope to share some information and also learn from other enthusiasts.
I am in the process of finalising the design (own design) for my wood-fired oven.
It will be a cast refractory concrete 850mm internal diameter, with flue extending past the protective all metal roof structure.
The flue total height is approx. 2m.
Here is the dilemma/question for the experts.
What is the most suitable flue size (diameter) for my application?
I have been told 6" diam may be too large and may draw all the heat out of the oven and perhaps 4" or 41/2" diam may best suit.
A section of twin skin flue will be used at the roof penetration.

What do you think?
Please refer to the attached design.
Matt

[UtahBeehiver]

As far as casting, David S is our resident expert. He can give you good insight on cast ovens. IMHO, 850mm = 33.5 inch diameter oven should use a 6" flue, The FB plans recommend 6" on ovens 32 - 36" ID.

The floor and particularly the dome insulation seems light, 40mm (1.5") CaSi board should be a min of 2" or abt 50mm, Same with the dome, 25 mm pretty light. View one does not show the floor insulation under the dome, view 2 does so just making sure you have the dome insulated away firebrick or the concrete hearth or these act as heat sinks.

OasisCDM is from Werribee so he can give you insight on material sources

[MattP]

Thanks for your prompt reply UtahBeehiver.

I guess 6" is the way to go. I was told that it may be too big for my application of approx. 2m height.
I will take on your suggestion/recommendations re. CaSi board and dome insulation.

View 1 represents the front elevation/entrance and in particular the decorative tiles I intend to use.
View 2 is a cross section of the oven which shows the 75mm thick casting, 25mm insulation blanket and finally the render.
The base, starting from the concrete slab, consisting of CaSi board, 40-50mm refractory tiles.

I hope this explains my design.

Matt

[oasiscdm]

see private message

[david s]

Hi Matt,

Apart from Russell's suggestions which I endorse I think your entry could do with some redesign. It will work much better if you funnel it to the base of the flue pipe. 80 mm in diam is way too much of a restriction where it meets the pipe. As is it will cause lots of smoke to escape out the front on light up and won't start to draw until the flue gets hot. If you increase the flue size to 150, as suggested, you could make it 125. It will act like a venturi at that point if you design the restriction with a taper. Because you don't want lots of thermal mass outside the oven which acts as a heat sink you can reduce the thickness of the entry lots, this also has the advantage of reducing the depth of the entry. Making the entry more shallow means less of a tunnel to have to work past. Suggest you get some melt extract fibres (stainless steel needles) for reinforcing the castable. You can get these from Antec Engineering.

[MattP]

Hi David,

Thanks for your reply and suggestions.

I initially designed the oven with a 150dia flue, but was advised to reduce it to perhaps 100 or 125 in view of its length and draw (approx. 2m).
Your technical suggestions seem to make sense and hence I have redesigned the flue and associated gallery.
I am unsure if I require the recess for venting purposes in the flue arch or is the taper opening sufficient by itself.

Please refer to latest design and I welcome any comments.

Matt

[david s]

Yes, that's much better, but I still think you've got too much mass in the flue gallery.

[MattP]

Thanks David.

I don't fully understand what you mean by "too much mass in the flue gallery".
I would appreciate a sketch or a marked up of my design in order to be clearer.
Matt

[david s]

Having a lot of thermal mass in an area away from the inner oven means it acts like a heat sink. This means more fuel and time to heat it, but also more heat loss from the inner oven via conduction. Because the front of your entry is not insulated it will be bleeding more heat to the atmosphere. Some builders make a thermal break between the inner oven and flue gallery to reduce this heat loss by conduction. Others (me) insulate between the flue gallery and the outer decorative arch. Neither of these are really necessary, but reducing the thermal mass of the flue gallery would be a good idea IMO. Building in brick tends to constrain the design because of the limits of the unit sizes and joints required, not so with casting which more easily adapts to compound curves and thinner walls.
By reducing the thickness of the casting at the entry and around the flue you reduce the thermal mass. Castable is plenty strong enough at around 30 mm thick.

[MattP]

Thanks again David.

Attached please find Rev B design.
I have modified the casting thickness at entry and around the flue. Also I have reduced the dome casting thickness to 60mm in light of your comments regarding the cast strength and have increased the render thickness by the same margin (15mm).
My previous dome design was based on an even inside radius of 425mm (dotted circle) hence the extra thickness at the entrance however in order to address the "thermal mass" issue this is my attempt.
Do you think the venting recess is necessary in combination with the flue gallery or is the flue gallery alone sufficient.
I have found a supplier of CaSi board with 50mm thickness and also refractory tiles of 50mm thick for the floor. These late changes are not reflected in this drawing.
Please feel free to make whatever suggestions as this is my first attempt at oven design. I enjoy other people experience and knowledge.
Matt

[david s]

Yes, that's better, but as Russell pointed out 25mm of blanket is not really enough insulation over the dome. If you don't want to go for the expense of a second layer then 25 mm of 10:1 vermicrete which you can mix yourself would give you the extra insulation required. It has the advantage of smoothing out the bumps and lumps of the blanket and provides a firmer substrate to render against. The downside is that it takes up lots of water which needs to be removed before rendering. Your planned extra thick render layer is a waste IMO because it's only an outer shell to hold the insulation and to keep the weather out. I only do around 15 mm thick and it seems quite sufficient unless you plan on parking your car on it, then make it thick.
Also for the reasons given in my last post you should insulate around the flue gallery as well as the dome (your drawing shows it uninsulated).

[MattP]

OK thanks again.
I have revised the insulation issue and modified the render thickness as well as the cast dome thickness.
As you can see I am unable to insulate at the very front of the flue gallery (apprx 1/4 of the total circumference) hence it is only cast construction.
Some supplier has stated Perlite to be a better material than Vermiculite for render mix. Any comments?
Could you please review REV C and indicate if I need the recess for venting or simply the tapered hole for the flue to work.

[david s]

I don't use any perlite or vermiculite in the outside render at all. I use blanket then vermicrete then a straight render. Vermiculite or perlite reduces the strength, but add it in if you wish. I do find that a mix of 50/50 vermiculite and perlite makes a more workable mix than either of each alone though. Perlite contains fine dust that is an irritant if you breathe in whereas the vermiculite doesn't. (At least for the stuff I use)
Your plan looks much better now. Retain both the recess and tapered hole.

One more thing I noticed from your plan is that it is hard up against a wall or fence at the back. If this is so then you will be cursing when you get finish the last layer at the base because of the poor access. Try to leave a min of 200 mm there if possible.

[UtahBeehiver]

I see you added additional floor and dome insulation. Wise decision. I would second David S comment on working space if the cover over the oven is existing. If being built after the oven is done then working space at the rear is a non-issue. One of the Brisbane members recently posted he had surplus SS needles for cast oven and only wanted postage to cover cost.

https://community.fornobravo.com/for...-steel-needles

[MattP]

Thanks David and UtahBeehiver.
Working space is not an issue as the structure is a metal shelter consisting of metal trusses and colorbond steel roof which is supported by corner posts - access not an issue.
I assume vermicrete is a mix of vermiculite and cement?
If so please indicate mix ratio.
In terms of perlite this is what a supplier has written.

Why not Vermiculite? Vermiculite particles are relatively large and flaky, Perlite particles are smaller and less easily crushed. Both Perlite and Vermiculite are usually used in hydroponics as they hold water, which is not good for making concrete. Because they absorb water, you’ll have to use quite a lot of water to wet out the mixture, which dilutes the strength of the cured mixture. This is why you’ll find a lot of people complaining that their ‘Vermicrete’ is weak and crumbly, even after a week of curing. A local company here in Victoria make a product called ‘LiteFill Perlite’; the Perlite is treated, each particle is coated to prevent it from absorbing water. This makes it terrible for hydroponics, but brilliant for making lightweight, insulating concrete!
Even using the LiteFill Perlite, this render layer is porous due to small voids in the structure of the render itself. You’ll need to apply a waterproof layer over this render, however you should wait until the oven is fully cured before you do so. Acrylic texture coatings work well, as they are almost 100% waterproof, and they usually have a latex content which makes them slightly elastic, so it will bridge small cracks

Are you able to comment on the above as I do not have any experience.
Further to your suggestion of incorporating both the recess and tapered hole, would I need to make the recess the same depth as the hole (180mm) or can I reduce to something less?