Vortex Stove

[aleks]

Hi Trev,

Thanks for your kind answers, I will ponder a bit regarding the size of the top chamber vs the battery, my maths skills are suboptimal, so maybe I should just stick with the smaller setup ;-)

just to be completely clear regarding the floor: i get that the V shape is made by the cut up side board, it's supported in the front by the front piece and a similar piece in the back I assume?

And lastly: from the sketchup I can't make out the exact size of the viewing window and I haven't spotted it on the first page either. I guess it depends on the exact size of the bricks of the secondary burn chamber and the top chamber as well.. but do you have any other pointers on that subject?

Kindly,


Aleks

[Vortex]

Hi Aleks,

For the vortex in the afterburner to be optimal there is a correct balance between the push of the fire, the pull of the chimney and the resistance (to gas flow) of the mass. Generally 90 degree bends are to be minimized in a mass, but I discovered that in a very small dense compact mass you can have a lot and use them to your benefit.

If your mass does not have enough resistance you can add more by adjusting the layout of the top chamber and it's end port. If you can get hold of vermiculite board then it makes it very easy to make adjustments, as pieces can just be placed inside or removed until you get the best results. The afterburner glass will show you when it's right.

The firebox floor is supported on a metal plate with a layer of insulation on top. If I was building it now I would make the opening the full length from front to back.





The afterburner window size on mine is approx 9" wide by 6" high. That's the full width of the afterburner and about 1.5" higher, it's good to be able to see up to the top of the afterburner roof shelf where it goes around into the top chamber, so you can easily see if it's over fueling.

Hope that helps,
Trev

[Karl L]

Jul 3, 2023 9:29:08 GMT -8 martyn said:

Refractory casting mix is very dry compared with standard mortar mixes.
I am using large amounts so I use an electric cement mixer but mixing in a barrow with a trowel is fine.
What we want from the vibrating, is a solid dense mass with zero trapped air.
You will just have to do your best with whatever method you can, basically you vibrate until no bubbles rise.
On a hight frequency vibrating table (very expensive machine) it would take about 5-10 seconds with a diy table maybe 60-120 seconds and with a hammer drill, as soon as the bubbles stop coming to the surface?
Its all good fun……

Thanks for all the tips on casting Martyn and Trev.

I made a mould and tried casting one of the firebox sides. Everything went fine apart from the vibration using a hammer drill, which I didn't work as well as I hoped.
I loaded the drill with a chisel and used hammer action only (no rotation). The easiest places to hammer onto were vertcally down on the mould edges (70mm x 45mm timber), all around the perimeter of the mould. It looked like the hammer action did quite a good job close to where I applied it, but that decreased with distance from the edge.
I now have a casting with a visible void near the middle of the bottom face, and I guess there may be a few more voids or bubbles inside.

Is this casting usable? I.e. is a casting with a few voids OK to use?

I do have a very old corded drill, so I might be able to try Trev's method, spinning a U-bolt. What size U-bolt did you use?

Thanks,
Karl

[martyn]

You dont really want any air voids, I made a short video just recently that might help you 

[Vortex]

Here's a pic of my Heath Robinson / Rube Goldberg vibration table I threw it together out of junk about 10 years ago and it worked so well I'm still using it. It sits on top of a plastic fish box, so the drill is under the board inside the box. The molds sit on top of the board to be vibrated. I took out the screws that held the 2 halves of the plastic drill body together and just put some wood screws right through into the board to hold it on there.

Trev

[Karl L]

Sept 25, 2023 10:11:46 GMT -8 martyn said:

You dont really want any air voids, I made a short video just recently that might help you 

I did watch your video before doing the casting - thanks very much.

Is there a big problem if there are air voids - like explosions on heating - or are there other kinds of problem?

[Karl L]

Sept 25, 2023 11:25:36 GMT -8 Vortex said:

Here's a pic of my Heath Robinson / Rube Goldberg vibration table I threw it together out of junk about 10 years ago and it worked so well I'm still using it. It sits on top of a plastic fish box, so the drill is under the board inside the box. The molds sit on top of the board to be vibrated. I took out the screws that held the 2 halves of the plastic drill body together and just put some wood screws right through into the board to hold it on there.

Trev

Thanks - it's very useful to see what it looks like.

[martyn]

That depends if there is escape routes for steam but a surface hollow wont be so bad, it is common to have lots of small voids on the surfaces that touch the mold.
It is very difficult to completely eliminate all the air without ‘over vibrating’ the cement, if you run a vibrating table for too long the cement content travels up with the water and leaves a sandy looking edge at the bottom.
Proper vibrating tools will help stop this happening and a high frequency table will produce very clean work but with most of the DIY options you have to expect some air bubbles.
Trevs idea is pretty resourceful and has worked for him but ideally you want something that offers very fast vibration.
Try to cure the components slowly, just raising the temp slowly over a few days.

[Karl L]

I built a vibration table using an old electric drill and tried doing another casting.

Short story: it didn't work well - I had to go back to using my rotary hammer.

I made the table using a very old Black and Decker drill with a metal body - dating from the 1960s. The U-bolt was a piece of M8 threaded rod, which I bent into a U with about 30 mm between the two centres.
I also added a couple of flanged M8 nuts to the offset leg of the U-Bolt, to give it a bit more weight.
The drill was in high gear - stated as 2400 rpm on the gear control.

I tried it out before doing the actual casting and it felt like it wasn't going to have enough vibration to fluidize the mix, but I thought I should try it anyway.

I was right - it barely had any effect on the mix, so I had to quickly grab the rotary hammer and use that. I vibrated it much longer than last time, so I'll see what effect this had.

[Vortex]

Hi Karl, What did you mount it on? The board has to be able to move about, if it's just screwed to the bottom of a table then that'll adsorb most of the vibration. Mine just sits on top of the plastic fishbox and those bits of batten that are screwed to the bottom stop it from falling off. I've seen other people do the same but using an old car tire instead of the box. It's an M10 U bolt in mine, try using something heavier in place of the U-bolt.
Trev

[Karl L]

The board is made out of 18mm plywood, about the same size as the base of my mould. The edges of the board sit on some bits of 20mm thick packing foam, which bear onto some wooden rails. The board is not fixed down at all.

What is the width of your U-bolt - either between the centres or across the outside of the legs? And the length?

Maybe using a U-bolt with more weight will do it...

[Vortex]

The U-bolt on mine is 7/16" thick, 1-1/2" OD width, 2-1/2" long - must be an old imperial one. Try giving the board a bit more freedom to move and a heavier U-bolt.

[Karl L]

Oct 4, 2023 6:47:27 GMT -8 Karl L said:

... so I had to quickly grab the rotary hammer and use that. I vibrated it much longer than last time, so I'll see what effect this had.

I have to say the casting seems to have turned out very well. Only a few very small air bubbles on the bottom surface.

[Vortex]

Did some work on the stove last month but haven't had a lot of opportunities to light it as the weather's been quite mild. Gave it it's annual cleanout, replaced the vermiculite firebox sides, extended the ash trap back so it's now the full width of the ash box, and rebuilt the afterburner with a 28% CSA secondary air system to the sides of the port.

Afterburner has been reduced to a 5.5" / 140mm system size to match the reduced size of the firebox with the vermiculite sides and steep V-floor.















Initial impressions are that it actually runs better without the secondary air system at all. My theory on this at the moment is that the stove only has so much draw, and when you allow some of that air through the afterburner, it reduces the air intake through the primary, which reduces combustion efficiency in the firebox, so more unburnt smoke and gasses pass through the port into the afterburner, which then requires more secondary air, etc. etc. This was evident by the fact that when the secondary air was open full the top of the firebox had a ceiling of smoke down to about 3-1/2" from the top. The last fire I did I completely closed off the secondary air system and made it as airtight as I could and that was the best burn so far.

The slightly smaller afterburner gets a full vortex easier with less flame but is more easily overfueled. It took me a whole week of my spare time to make these changes and honestly at the moment I think it was better before. Once I've had the chance to use it more and get a better feel for it I'll write another update.

Trev

[Karl L]

Thanks, Trev.

Maybe I missed you saying, but why were you putting vermiculite board on the firebox sides?