Vortex Stove

[Karl L]

Jan 28, 2023 11:23:43 GMT -8 Vortex said:

... I find after mid burn I can reduce the primary and add the secondary air up through the ash trap, it seems to work as well as adding it through the sides of the port, but with the added bonus of burning up the embers nicely at the end.

Thanks Trev.

I'm trying to decide whether to include extra thickness between the firebox and the afterburner, in case I later need to put in a secondary air channel to deal with overfueling situations.

Does your 'secondary air through the ash trap' method mean that secondary air injected into the port is not needed?

I seem to remember you saying that sending air up through the fuel led to overfueling, unless it was very near the end of the burn?

Thanks,
Karl

[victorlt]

Hi Victor, don't most vortex stoves direct the gasses down?

Hi martyn. Yes, all stoves at some point directing gasses down, but trying to do this directly from the top box in this system doesn't work properly. As I said I already walked this mile without success and I see the same sooty run in your system. Gasses exit port can be located just on the box top or top right/left corners. When gasses exit the top box, you can direct them wherever you want - this is my personal experience playing with similar configuration like yours.

[Vortex]

Jan 29, 2023 8:06:57 GMT -8 Karl L said:

Thanks Trev.

I'm trying to decide whether to include extra thickness between the firebox and the afterburner, in case I later need to put in a secondary air channel to deal with overfueling situations.

Does your 'secondary air through the ash trap' method mean that secondary air injected into the port is not needed?

I seem to remember you saying that sending air up through the fuel led to overfueling, unless it was very near the end of the burn?

Thanks,
Karl

Hi Karl, I rarely use the secondary air into the afterburner, but you may want to run the stove differently to me, so if it's not going to cause you any other problems I'd give yourself the room so that you at least have the option later.

The bottom air will cause overfueling if you use it too early or too much. I use just 20% primary (12mm x 275mm gap spread across the whole bottom of the door) and leave it like that till the fire has obviously peaked (you can see this because the vortex will start to turn back to a palm tree shape as there is more air than it needs), at that stage I open the bottom air just about 1.5% and start reducing the primary, (so the vortex stays fully formed in the afterburner but not going around the shelf into the top chamber, for that last quarter of the burn this can be as little as 1.5%). That's it - I know 1.5% bottom air doesn't sound like it would make much difference but it has a surprising effect. When the ashbox starts to fill up so there is a pile under the trap blocking it, I notice the fire isn't running right straight away. It keeps the vortex going in the afterburner a lot longer and then burns off a lot of the CO. from the coaling phase, you'll notice the mustardy/greeny/blueish flames burning above the coals almost right to the end.

[Vortex]

Jan 29, 2023 11:06:49 GMT -8 victorlt said:

Hi Victor, don't most vortex stoves direct the gasses down?

Hi martyn. Yes, all stoves at some point directing gasses down, but trying to do this directly from the top box in this system doesn't work properly. As I said I already walked this mile without success and I see the same sooty run in your system. Gasses exit port can be located just on the box top or top right/left corners. When gasses exit the top box, you can direct them wherever you want - this is my personal experience playing with similar configuration like yours.

Hi victorlt, Not sure I understand, are you saying this is because the gasses are too hot and buoyant and don't want to go downwards when they exit the top chamber? In my stove they go across the top then through a large mass gradually going downwards but losing heat all the way.

Your implementation is very different to other vortex stoves. I can see you need a very high gas temperature to drive the heat into the sauna stones as efficiently as possible, so are driving the stove very fast and hot. I found in my experiments I couldn't do that without it producing high CO. levels.

Trev

[martyn]

Karl, apart from possibly supplying secondary air, the bigger gap between the fire box and afterburner makes it a a hell of a lot easier to make an opening top door!

[martyn]

Victor, thank you for that, I will rectify that issue for the next test run.

[victorlt]

Hi victorlt, Not sure I understand, are you saying this is because the gasses are too hot and buoyant and don't want to go downwards when they exit the top chamber?

Hi Vortex, I think you explained it well. My thought is similar - gasses in afterburner are extremely hot and light and by the law of physics they strives to exit box traveling sliding along the top surface. And when you trying to direct these hot gasses down, this seems gives similar effect like trying to sink the ball in the water - they don't want to go down and this gives big back pressure for incoming new gasses and is not allowing effective burning process in afterburner. In your case, when gasses traveling across top plate surface, they are losing temperature and natural stratification happens and they goes down freeing space for new incoming hotter and lighter gasses.
Before implementing my sauna stove,  I was playing few months with the very similar to martyn test stove made from vermiculite boards. I also tested such configuration with downdraft with no success.

BR
Victor

[Karl L]

Dec 2, 2021 13:46:53 GMT -8 Vortex said:

The threshold is still 30mm high and in the same position. I do plan to try it half an inch farther in from the door some time soon.

Hi Trev,

I'm thinking about door and primary air arrangements.

I haven't been able to work out the exact positioning of your threshold relative to the primary air inlet slot. 

E.g. is the top of the primary air slot level with the top of the threshold?

How far back from the mesh is the threshold?  (It looks like the mesh is tipped 'back', to angle the airflow slightly upwards, so maybe the average distance?)

Thanks,

Karl

[Vortex]

Hi Karl,

The top of the primary air slot is 14mm above the top of the threshold. The front of the threshold is 70mm back from the inside of the primary air slot. The mesh is just attached at the top by a few twists of copper wire so it might appear angled back in the pictures but it's meant to be vertical.

The threshold and mesh were all designed to deal with the issues that the primary air blowing on the bottom of the fire sometimes caused. The 275mm wide primary air slot right across the front directing the incoming air down towards the floor, probably reduces the importance of the size and position of the mesh and threshold, but that's just a guess at the moment.



This is how much it needs to be open for 20% csa primary (I use the tip of my index finger as a guide), so you can see how it directs the air downwards allowing it to then flow evenly up across the whole fire.

[martyn]

I am still experimenting even though I am not really getting very far it is still interesting to me anyway! 

[solobird]

Jan 26, 2023 0:16:50 GMT -8 martyn said:

Solobird, vermiculite is fantastic for certain parts of the stove and to be fair it will last very well in most circumstances.
Most rigid refractory products will crack under extreme conditions, vermiculite is no different.
Small non structural cracks are to be expected, as a rule the smaller components can last for years but the bigger pieces are prone to cracking.
It has its uses and nothing else comes close when experimenting due to how easy it is to work with.

Martyn, from what I remember from your videos, you applied waterglass to some vermiculite and zircon paste to other. How did these do in time compared to the raw board ?
Best I could get where I live is either waterglass or try some Kalsilite from Kaolin clay and Potassium hydroxide.
Have some left over insulating fire brick (950 C rated) from a Walker riserless core and would either coat it (if it works) or use it at insulation behind hard firebrick splits.
Bricks in the horizontal part that leads to the riser got fluxed, cracked and expanded. Almost closing off the channel.
The ones in the firebox are ok, apart from the abrasion.

Jan 26, 2023 11:37:30 GMT -8 Karl L said:

I've used Vermiculite board a lot, rated at 1100C.

One thing to bear in mind is that its coefficient of thermal expansion is similar to steel - i.e. not negligible.
So if it's very hot on one face and cold on another it will bend. And if it's a big piece and it's restrained from bending then it will break.

I never have a problem with this because I divide large surfaces into smaller pieces and allow them to move a little.

I've seen it fixed with screws and/or something like skamol glue. Biggest piece inside a 100mm (4") vortex is 300x260mm (12x10") - top of firebox - largest piece exposed to high temperatures. Best to not screw them then ?

[Karl L]

Feb 3, 2023 11:10:12 GMT -8 solobird said:

I've seen it fixed with screws and/or something like skamol glue. Biggest piece inside a 100mm (4") vortex is 300x260mm (12x10") - top of firebox - largest piece exposed to high temperatures. Best to not screw them then ?

The problems come with long pieces and large temperature differentials. Here's the thread where I found this out the hard way:

Vermiculite board cracks in S-Portal test

If the Skomolex has high temperatures on both sides (as does the top of a Vortext firebox) then it will bend a lot less.

I tend to build the Skomolex box inside some other material (e.g. steel), and then design the pieces so they hold themselves together like a 3D jigsaw, and then use pins made from 3mm steel fencing wire into drilled holes to lock some peices in place.

But Martyn seems to have glue and/or screwed his 4" firebox together, and it seems to be working pretty well.

[martyn]

I have noticed that the Skamol board is a lot more robust than the original vermiculite board I was using years ago, I think it has been developed for the latest generation of wood burning stoves.
In my local stove store the vast majority of log burners are lined with Skamol board and have secondary air supply.

The Zircon sets hard like stone, it is still going in my J tube with no serious deterioration but I can see lots of tiny cracks so who knows how long it will last?
Water glass might help but not a lot as far as I can tell.
I think the Zircon would work at its best on soft fire brick.

[martyn]

I think I might be getting somewhere now, I tried loading fresh wood onto a burning fire and it pushed through without over fueling,
unfortunately not all of my video came out and I did not manage to record the second re load but it went pretty well!

[Vortex]

That's excellent martyn, well done!

IIRC Peter discovered way back during the early development of the batchbox that the sharp edges of the port were really important for some reason.

I look forward to see your results with more heated secondary air, remember the more you heat the secondary air the more it expands and the less O2 it supplies, I never tried more than 10% though so maybe with 30% it'll make up for it. I used old pieces of stove glass for the afterburner floor where the secondary air feeds into the sides of the port, they've been going for a year and haven't broken yet.

Trev