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Post by martyn on Jun 15, 2022 8:02:49 GMT -8
The two triangles make up a 45mm square, I really need a taller chimney, although the one meter one does actually work, I think I need more hight before I start any experiments with the air supply. I tried to get a 100mm insulated chimney today but our locale stockiest starts at 125mm !
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Post by martyn on Jun 15, 2022 11:41:53 GMT -8
I have a problem! The 4” vortex rocket stove is running far to hot! I need to calm it right down!
I did a few mods today, one was fitting a piece of vermiculite under the chimney with a 95mm hole (the biggest hole saw I have) the chimney stack is 105 x 105 square. I also fitted a step in the ceiling, that can be seen in the last few frames of the video.
I started the stove, top lit with a single fire starter block and the bottom door open, it was smoky for about four minutes and took about 10 minutes to get the vortex spinning nicely. Then at 20 minutes I put the glass in place as the flame was going over the shelf but that did not stop it and the vortex was going like a mad thing. i left it like that, just running on the under floor air for 15 minutes but it was fast and furious running at 450+ on the hot plate! I decided to remove the fire box glass and it really did take off in a crazy if not scary way! Not in the video but I took a hot plate temp reading of 560c …. It was mad hot!! See for yourselves ha ha …..
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Post by Vortex on Jun 15, 2022 13:38:13 GMT -8
It's running nice. A smaller fire would produce less heat, so you could reduce the size of the firebox by adding another layer of vermiculite either side and under the V floor to raise it a bit. If you want to keep the fire viewing as it is and aren't bothered about efficiency then you could add a layer of something under the ceramic cooktop to deflect some of the heat.
Only place I've seen 4"/100mm insulated chimney is where they sell them for pellet burners.
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Post by martyn on Jun 15, 2022 21:51:40 GMT -8
It is all good fun, when I build the next one, I would like to have the unit slightly higher up as I am getting a bad back leaning over while staring at the vortex! I do have a 6” chimney section with an adjustable flap valve that I might try but perhaps bypassing some of the heat away from the hotplate is the answer. Like most experiments you learn as you go along ……
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Post by martyn on Jun 16, 2022 11:38:07 GMT -8
We took a step backwards today! I made and fitted the firebox door and I made and fitted a chimney damper ( a swing plate).
I thought I would try a half load of wood, all was going well although the top plate was still far to hot but, then I decided to add some more wood mid burn.
What happened was the fire started smoking….. then it started smoking ‘big time’ with black acrid smoke coming out of the chimney!!
I opened the fire box door to give it more air and the fire went into overdrive with massive flames escaping out of the afterburner into the top box and then it got worse!… . The glass in the afterburner cracked in two, holy cow I though! I got the temp gun out and checked the top 610c, I opened the cracked afterburner door and the glass fell out on to the patio, I tested the temp inside and it went off the scale, so over 900c.
I need to re think the cooking plate and divert some heat away from the glass top, I might try reducing the vortex slot by 10% and see if that helps.
Basically the fire is to powerful in its current format, the top of my short chimney is showing 180 - 200c that is a lot of wasted heat!
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Post by martyn on Jun 16, 2022 22:25:12 GMT -8
So after yesterdays experience it seems to me that the stove I have built is not suitable for my purpose.
I assume that the insulated fire box and afterburner plus the insulated top box combined with such a short and direct route out of the short chimney and the ceramic glass top simply does not make a good overall design for a hot plate cooking stove.
Perhaps in my case a solid mass firebox and a thick steel cooktop higher above the flame path would work better for me.
No doubt the design works as a heater and a entertainment centre but fails as a cook stove, so I need to give it a re think and redesign a few aspects.
I had a look at the carnage this morning, broken glass, black smoke on the wall and a scorched wooden rafter, not to bad considering the panic I felt during the episode ha ha …
Of course there is another side of the coin and it proves what a powerhouse this tiny stove actually is !
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Post by Vortex on Jun 17, 2022 5:45:06 GMT -8
Wow, never known an afterburner glass to break from heat. I had one crazy burn in a storm where my chimney cowl got ripped off the roof by the wind and the stove went bananas, temp in the afterburner went up to 1268C before the thermometer stopped reading, but the glass never broke.
Reloading is best done after the peak of the burn and before the start of the coaling phase with just one or 2 larger pieces, otherwise you overload the system with too much wood gas all at once.
You could increase the surface area to volume ratio of the top chamber, like I did in the later design, that creates more back pressure and makes it more stable.
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Post by martyn on Jun 17, 2022 8:34:34 GMT -8
I will come up with something, I thought I could make a much larger cooking plate and make the top box double the depth ie 6” from its present 3” and put a diffuser above the entry port like a piece of glass. Perhaps place the chimney exit much lower down so the top box might act more like a bell? The aim would be to get a more even temperature across the hotplate of around 250-300c the problem would be my cook plate would now be 3” above the surrounding work top but I could manage with that. My temperature gun only goes up to 900c so I don't know the true reading inside the afterburner just that it was over 900c. Not that it really matters but I have used exactly two sheets of vermiculite and I did size the top box to get the most from the two sheets however I did initially want a bigger hotplate but that meant buying another sheet …. guess I should of gone with my first plan! The ceramic glass top was a full cm 100 x 55cm before I cut it!…..!! The vermiculite has not changed so far apart from going very hard it is like new.
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Post by martyn on Jun 17, 2022 11:35:26 GMT -8
So I replaced the glass and I fixed a piece of vermiculite directly above the port… 8” long and 5.5 wide.
With a safe half load of mixed wood offcuts, the fire behaved itself and gave a nice trouble free burn.
The half load lasted 80 minutes to the coal stage with a nice vortex for about half that time. The fire really does work well, it is a quick and easy start up and puts on a great display for a good 45 minutes.
The top plate above the vermiculite took about half an hour to get to 280c although there were other spots on the glass at 400c within 10 minutes, it was an improvement in the right direction for me.
I used my temp gun to take some reading on the outside of the vermiculite, on the fire box sides it was 250c. So about half the inside temp, on the outside of the afterburner where there are two layers, it was 185 and where there are three layers 135. But the whole area under the top box is 135.
The problem is I cant deal with compromises for long and I will have to get it right or nothing, we will see what I can come up with tomorrow.
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Post by Vortex on Jun 17, 2022 12:43:38 GMT -8
Increasing the depth of the top chamber would actually lower the surface area to volume ratio, ideally you want to increase it while still keeping it 1 csa, to get that you want it wide and shallow. The increase in surface area creates back pressure without a restriction, which makes the burn more stable. It looks like your top chamber is already too big, for the 4"/100mm system it should be about 43mm x 186mm.
If you're burning half loads you could try putting some vermiculite off-cuts on top of the V floor to raise the fire up so that it starts just below the port, you'll get the vortex for longer that way.
Maybe a sheet of steel under the ceramic cooktop would help to even the heat distribution out, would need a slight gap between them so any warping of the steel wouldn't break the ceramic.
My 4" fireboxes were all made from dense firebrick with just vermiculite used for the afterburner and top chamber, so it's possible the insulation of the vermiculite firebox has also made it more prone to overfueling.
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Post by Karl L on Jun 18, 2022 5:37:57 GMT -8
Increasing the depth of the top chamber would actually lower the surface area to volume ratio, ideally you want to increase it while still keeping it 1 csa, to get that you want it wide and shallow. The increase in surface area creates back pressure without a restriction, which makes the burn more stable. Trev, a while back I saw you were experimenting with increasing the surface area/volume ratio by placing pieces of vermiculite board in the top chamber, aligned with the gas flow. Did that work? Given the width restriction in my situation, I'll probably have to do something like that.
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Forsythe
Full Member
Instauratur Ruinae
Posts: 208
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Post by Forsythe on Jun 18, 2022 11:58:08 GMT -8
I’m wondering if the gasification-type solution that Trev stumbled upon would work for martyn ‘s setup — the configuration where primary air is limited to a minimum and more secondary air is provided to the afterburner, providing for a lower-temp fire and slower burn, overall.
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Post by martyn on Jun 18, 2022 12:15:14 GMT -8
Forsythe, I am on the case, I drilled two holes in the back of the afterburner this afternoon, great effect with a tinge of blue added to the display, I also made the fire box smaller and modified the top box. First burn was fantastic but I had some issues with smoke out of the chimney, more experiments tomorrow ….
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Post by Vortex on Jun 18, 2022 12:25:02 GMT -8
Increasing the depth of the top chamber would actually lower the surface area to volume ratio, ideally you want to increase it while still keeping it 1 csa, to get that you want it wide and shallow. The increase in surface area creates back pressure without a restriction, which makes the burn more stable. Trev, a while back I saw you were experimenting with increasing the surface area/volume ratio by placing pieces of vermiculite board in the top chamber, aligned with the gas flow. Did that work? Given the width restriction in my situation, I'll probably have to do something like that. The experiments with high surface area to volume ratio in the top chamber, showed that at the highest levels it creates a very stable low CO. burn, but the fire usually runs out of O2 at some point and then the CO. goes vertical. This happens because the high surface area drag leads to a much slower gas flow, which means less air over the fire. Expanding the air supply only helps up to a point, as the gas flow has slowed to a level where a larger air supply has almost no effect because it cant make use of it. This is how that looks on a typical graph:
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Forsythe
Full Member
Instauratur Ruinae
Posts: 208
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Post by Forsythe on Jun 18, 2022 12:37:42 GMT -8
My 4" fireboxes were all made from dense firebrick with just vermiculite used for the afterburner and top chamber, so it's possible the insulation of the vermiculite firebox has also made it more prone to overfueling. +1 on Trev’s assessment here, too. The black smoke still coming from the chimney —even after increasing secondary air to the afterburner— could very well be caused by too much pyrolysis due to the increased insulation of the firebox itself. There might just be too much woodgas too fast for the afterburner to handle without going into overdrive.
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