Refractory Chimney vent & Choke

[THX1138]

Hi team from Oz,

I have enjoyed many hours so far looking at the great work going on, WFO's seem to be the biggest thing in Oz since beer was discovered

I plan to cast a refractory dome igloo shortly over a sand mould with a single brick arch at front with an embossed design added

My querey is the vent so I can choke the oven as per many designs around this site fitting in with my layout

I assume I may need to cut the centre top bricks to allow a chimney vent to effectively choke using the oven door, but if so, how do I attach chimney
( stainless steel flue ) as this is only a single brick arch

I can insert a chimney mould creating a seat for the chimney prior to casting but this will probably place the chimney too far back unless I can get it to sit hard up against the arch or build cast higher at front to set the flue

I am attaching a sketch of my dilemma & any advice will be greatly appreciated, I will have the base built over the next few days & plan to cast sometime during the next 4 weeks

Cheers,

Phil

[david s]

Re: Refractory Chimney vent & Choke

When you have the flue outside the oven chamber there is no need to have any kind of choke because your oven door seals the oven chamber isolating it from the flue. It is also possible to arrange the door half open which has a choking effect. You can cast a gallery or manifold in place using your sand technique but you wont get a very good surface on your casting unless you cover the sand with a layer of clay. Hope this advice is of some use to you.

[THX1138]

Re: Refractory Chimney vent & Choke

Thanks Dave,

I know where you are coming from, having the flue outside the chamber is my preference, it is how to make the vent to hold the chimney in place that I need to work out

I should be ok with the casting side of things, I was thinking of maybe making a small form or such at the front of the cast to hold the chimney in place

[david s]

Re: Refractory Chimney vent & Choke

An easy way to do it is to create three tabs that you can bend outwards at 90 degrees, then the flue can slide inside the casting you've made. Surround the base of the flue with vermicrete then your outer shell can go around the vermicrete layer. This way there is a cushion between flue and outer shell which eliminates cracking problems around the flue as the metal expands. Don't use anything but stainless steel for the flue or you'll have corrosion problems down the track.

[dmun]

Re: Refractory Chimney vent & Choke

My guess, based on a look at your drawing, is that your entry in not nearly deep enough. That big hole in a single row of bricks will severely weaken the arch, and not be enough to draw the smoke out during the smoky startup. Is there a reason for such a shallow entry?

Most of us have built an entry that's more than a foot deep at the top, and built a tapered flue opening. There are many illustrations of different ways of doing this on the forum.

You should not, under any circumstances, put the flue anywhere inside the dome. It's bad for pizza making, and makes it impossible to do any retained heat baking.

[THX1138]

Re: Refractory Chimney vent & Choke

Yes, that is correct, that is why I wanted it outside the dome, there are many cast domes over here with flues placed direct into the near front which is why I want bring the flue forward

I noticed 1 x dome on this site with a square brick front with a flange mounted for the flue on top, 2 x lintels support the top layer of bricks, this seems a different & ok way to go??

I have spoken to my steel suppliers who can form an arched steel lintel for me to support bricks, but as you say, will the vent hole be big enough for the draw??

I will try making some models from plastecine or play dough to test the design

Phil

[nissanneill]

Re: Refractory Chimney vent & Choke

Hi Phil,
to complicate matters more, I would not make the flue/chimney less than 8" diameter for an oven greater than 32" diameter.
If you drop to 6", then expect smoke to vent out the front!
I recommend and use rectangular funneled stainless steel vents whith great success.

Cheers

Neill

[THX1138]

Re: Refractory Chimney vent & Choke

Neill.

Is it the diameter of the flue that is important or the vent size?? Do you know the most common diameter flue for cast ovens??

My oven will be @32" internal max

Phil

[nissanneill]

Re: Refractory Chimney vent & Choke

The vent or void should allow the exiting smoke to change direction and be easily exhausted out the chimney. If you have a small chimney, it cannot handle the quantity of smoke produced especially on start up.
If the chimney is in avoid with little or no room to direct the smoke up a flue/chimney, then it will flow through the easiest way out, the front arch.
Check out my vent, it is quite large (when compared to others), but I get absolutely NO SMOKE out my front arch.
Some people have put 6" flues in and get smoke smoke out the front arch and you see others with heaps of smoke stain in front of their ovens even with chimneys visible.
It is really up to you how and what you build as to how it performs.

Cheers.

Neill

[Tim F]

Re: Refractory Chimney vent & Choke

My flue is 6 inch. I do get some smoke out the front but it draws pretty well, and if I hold a piece of board so that it covers the top inch or 2 of the arch every bit of the smoke is sucked back up the chimney. I think that if your design has a large enough smoke chamber - basically some way to block stray smoke escaping before it is sucked back - you can get away with a 6 inch pipe, if it isn't too short.

[david s]

Re: Refractory Chimney vent & Choke

I've found that if the oven chamber is overloaded with fuel then flame will come out and into the vent and sometimes smoke out the front. If you don't overload the chamber then no problems.

[james]

Re: Refractory Chimney vent & Choke

I agree that a 6" chimney should be fine for a 32" oven. My experience is that you need your vent area to be at least 3-4" deeper (front to back) than the vent opening itself, in order to contain the smoke and hot air before it goes up the chimney. Otherwise, your oven will smoke -- even though the vent opening and chimney are large enough. That means that the vent area needs to be 9-10" (1 1/2-2" on either side of the 6" opening).

It's a real design trade-off, in that you have to add depth to your oven without adding cooking space, but my view is that it is well worth doing. You are able to completely close the oven with the door; the vent and chimney draw well and you don't have a huge amount of smoke pouring out the front door. Plus, the area under the vent is very useful as a landing.

My two cents.
James

[THX1138]

Re: Refractory Chimney vent & Choke

Thanks James for your 2 cents, in today's economy that is quite a bit.

I understand the reasoning, like a manifold prior to exhaust,

Thank you all for your points

Phil