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Post by DCish on Feb 5, 2022 17:36:28 GMT -8
Finally caught up on 3 years of posts to this thread. Fantastic developments! Reading it all over just a couple of weeks makes it apparent just how much things have progressed. Fantastic graphs, it's crazy to see the CO drop so low and just *stay* there. And I'm really liking the current tinkering with near zero primary air and dialing in secondary to the afterburner. My 1950s house has no insulation in the walls, and rectifying that would be a giant time and money pit. I may get to is someday, but meanwhile, on colder days the place loses heat so fast that it is convenient to be able to keep a long slow burn simmering along, even with the mass bench in place. Looks like this is a way to get as close to that as is possible with these stoves while maintaining ridiculous efficiency levels. And... biochar anyone? Just lock it down once all the volatiles are consumed (assuming air leakage can be prevented... a bit of a trick on it's own, I suppose).
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Post by patrik on Feb 24, 2022 11:21:53 GMT -8
Hi all! I did a test build last winter and took the burning plates from a Nibe Vedex 3000 to the Port with secondary air. Works good i think. The best part is that I have easy access to buy new ones when they are broken.
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Post by Vortex on Feb 25, 2022 8:50:52 GMT -8
Hi Partik, Thanks for posting the video of your stove. Looks like you need a bit more air. When the vortex flame is going around the shelf up into the top chamber like that the O2 has dropped below 8% and CO will be high. With the reduced firebox size and larger primary air my burn times had drop to around an hour, with this setup it's back up to 2 hours. The chimney temps are lower, it only takes a tiny amount of kindling to light so there's less wood splitting, and a shorter coaling phase. Apart from the initial hassle of making the secondary air setup, I cant see a down side to it. I discovered a downside to that way of running the stove to get slower longer burns using less primary and a lot of secondary air into the afterburner. Even though the testo showed great results and slightly higher efficiency numbers, the lower gas temperatures led to less heat transfer into the mass, and so overall heat output from the stove was lower, as the larger the difference in temp between the gas and mass the better the uptake seems to be. After trying lots of different combinations, I settled on running the stove on 20% csa primary air and only using the secondary if the stove starts to overfuel (which it only does if there's a very strong wind or the fuel is too loosely stacked or small). Once the fire has peaked I open the coal-burn air a tiny bit (~1%), then gradually reduce the primary to whatever keeps the vortex formed in the afterburner. Then full 3% coal-burn air at the end to burn up the coals.
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Post by patrik on Feb 25, 2022 15:24:23 GMT -8
Thanks I will test that when I build the real one this summer
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Forsythe
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Instauratur Ruinae
Posts: 208
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Post by Forsythe on Feb 26, 2022 0:22:38 GMT -8
With the reduced firebox size and larger primary air my burn times had drop to around an hour, with this setup it's back up to 2 hours. The chimney temps are lower, it only takes a tiny amount of kindling to light so there's less wood splitting, and a shorter coaling phase. Apart from the initial hassle of making the secondary air setup, I cant see a down side to it. I discovered a downside to that way of running the stove to get slower longer burns using less primary and a lot of secondary air into the afterburner. Even though the testo showed great results and slightly higher efficiency numbers, the lower gas temperatures led to less heat transfer into the mass, and so overall heat output from the stove was lower, as the larger the difference in temp between the gas and mass the better the uptake seems to be. After trying lots of different combinations, I settled on running the stove on 20% csa primary air and only using the secondary if the stove starts to overfuel (which it only does if there's a very strong wind or the fuel is too loosely stacked or small). Once the fire has peaked I open the coal-burn air a tiny bit (~1%), then gradually reduce the primary to whatever keeps the vortex formed in the afterburner. Then full 3% coal-burn air at the end to burn up the coals. Unless I'm reading/interpreting this incorrectly... this might indicate that a stove running mostly on secondary air would burn cooler and yet still cleanly — and thus a secondary-air driven stove might be better for a cooktop surface, where that type of burn is desirable, no?
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Post by Vortex on Feb 27, 2022 5:12:14 GMT -8
Maybe in a small dedicated cookstove, but it would be at the cost of efficiency. I would think it simpler to just build a smaller stove or put the hotplate farther from the heat source.
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Forsythe
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Instauratur Ruinae
Posts: 208
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Post by Forsythe on Feb 27, 2022 15:09:33 GMT -8
Maybe in a small dedicated cookstove, but it would be at the cost of efficiency. I would think it simpler to just build a smaller stove or put the hotplate farther from the heat source. right, I was thinking along these lines specifically as an adaptation for your 4" dedicated cookstove design.
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Forsythe
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Instauratur Ruinae
Posts: 208
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Post by Forsythe on Feb 27, 2022 15:30:06 GMT -8
part of my thinking, too, was that the slightly lower temps would help the Neoceram / Pyroceram / Robax glass cooktop last longer, since those ceramic glasses have a limited number of service hours at the hottest rocket stove temps before eventually developing enough thermal stress to crack.
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Post by Vortex on Feb 28, 2022 1:39:26 GMT -8
I have never had a piece of robax or neoceram crack from thermal stress and I've had the rear of the afterburner up to 1268C.
I'm even using two pieces of old robax for the rear afterburner floor (either side of the port so it forms the top of the secondary air channel), the surface of them has gone all flaky but they have not broken.
Stove glass usually breaks from people trying to close the door on wood that is too long, or from corrosion of the mountings that hold the glass into the door.
I would imagine the ceramic cooktops get more stressed than stove glass by people putting cold pans on them when they're really hot. I think steel is a better option for the cooktop though.
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Forsythe
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Posts: 208
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Post by Forsythe on Feb 28, 2022 1:58:42 GMT -8
That's fair. I have heard of the ceramic glass cooktops failing after a few years of very high heat, though I can't say whether the additional shock of cold pans on the hot surface was a factor. Crystalline (ceramic and borosilicate) glasses do accumulate physically- and thermally- induced stresses over time, though, even without being subjected to huge, undue temperature gradients like from cold pans. Considering that Robax publishes these finite service lifetimes at specific temperature thresholds, it's no wonder that they would fail when fired exceedingly hot, that's all. Agreed that cast iron or steel might make a more lasting cooktop under many circumstances.
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Forsythe
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Posts: 208
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Post by Forsythe on Feb 28, 2022 2:44:27 GMT -8
A large "cooktop" piece of ceramic glass —spanning the flame path (in the middle) and the far-cooler firebrick (around its perimeter) — with underside to fire and topside to ambient temps— will also undergo a lot more thermal stress than a much smaller piece mounted vertically in a window, or relatively-evenly heated while mounted entirely within the firebox or afterburner, and no surface exposed to a much lower external [ambient] temperature gradient.
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Post by Vortex on Feb 28, 2022 3:13:54 GMT -8
Good point. Martyn noticed this downside to using ceramic cooktops: one other issue I need to address is the ceramic cooker hob glass does not conduct any heat so the centre strip was around 350c but only 60 c 200 mm away on the edge .. again I will have to wait until I get it working better but I think steel might be better fo me.
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Post by patrik on Feb 28, 2022 8:58:10 GMT -8
Hi Partik, Thanks for posting the video of your stove. Looks like you need a bit more air. When the vortex flame is going around the shelf up into the top chamber like that the O2 has dropped below 8% and CO will be high. With the reduced firebox size and larger primary air my burn times had drop to around an hour, with this setup it's back up to 2 hours. The chimney temps are lower, it only takes a tiny amount of kindling to light so there's less wood splitting, and a shorter coaling phase. Apart from the initial hassle of making the secondary air setup, I cant see a down side to it. I discovered a downside to that way of running the stove to get slower longer burns using less primary and a lot of secondary air into the afterburner. Even though the testo showed great results and slightly higher efficiency numbers, the lower gas temperatures led to less heat transfer into the mass, and so overall heat output from the stove was lower, as the larger the difference in temp between the gas and mass the better the uptake seems to be. After trying lots of different combinations, I settled on running the stove on 20% csa primary air and only using the secondary if the stove starts to overfuel (which it only does if there's a very strong wind or the fuel is too loosely stacked or small). Once the fire has peaked I open the coal-burn air a tiny bit (~1%), then gradually reduce the primary to whatever keeps the vortex formed in the afterburner. Then full 3% coal-burn air at the end to burn up the coals. Hi, Do I need more primary or secondary air. Or both?
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Post by Vortex on Feb 28, 2022 11:43:02 GMT -8
That depends if your top chamber has the correct amount of resistance, if not then giving it more primary air will just make the overfueling worse. If it is correct and your stove was already running with 20% CSA primary air, then 10% CSA secondary air into the afterburner should stop the overfueling. You'll be able to see when it's right as the Vortex flame will be fully formed but stay in the afterburner and not go up around the shelf like that.
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Post by martyn on Feb 28, 2022 12:49:16 GMT -8
I dont like to use ceramic glass for a cook top surface, it does not suit my needs at all. It might be ok for a small cooktop but I find steel or iron much better as it holds the heat and allows the heat to spread out. Apart from feeling very fragile the glass just allows spot heating and very quickly cools down. I have found it does go opaque quite quickly when used directly above a heat riser but will still last for ages. When I was experimenting with the vortex stove, I also lined the afterburner floor with a old ceramic cooktop glass and that cracked on the first burn!
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