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Post by patrik on Feb 28, 2022 13:26:43 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. Thanks
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Forsythe
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Post by Forsythe on Feb 28, 2022 16:17:38 GMT -8
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. +1 on this. The only real downsides to steel or cast iron that I've experienced are A) finding a piece both large enough to span the cooktop area, and (perhaps most importantly) B) thick enough to more-or-less act as a heat sink — by spreading the heat out— to avoid warping, and to avoid spalling in the central underside. A good, thick cast iron top is so hard to come by where I live, that I've even considered melting down brake rotors and pouring cast iron flat bars/plates that could be machined to fit together with lap joints at the edges. I've even tried a scrap sheet of inconel 625 that I got in a barter trade ... but the heat conductance of inco625 is nowhere near what iron or steel would be, and it wasn't thick enough — just a hot spot in the center, basically. (the remainder of that inconel is now slated to become a door and airframe assembly— a role in which it should be far more suited...I think...) That is fascinating that the afterburner bit cracked but the bit above the riser didn't. Did you have part of the cooktop glass (in the afterburner) covered in insulation and part of it exposed to direct flame? I would have thought that being entirely housed within the afterburner would keep the heat relatively even across its surface... but... if part was insulated and part not... that temperature differential could potentially be the same thing which caused Trev's [mullite? mullite-cordierite blend?] kiln shelf to crack along the line between the insulated and the directly-exposed-to-flame areas.
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Post by martyn on Feb 28, 2022 23:39:26 GMT -8
I did make a video featuring some of my experiments including some thoughts about the glass….
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Forsythe
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Post by Forsythe on Mar 1, 2022 0:29:26 GMT -8
Oh, dang... well I feel dumb. I'm sorry I missed that one before. Thanks for re-posting it.
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Post by DCish on Mar 3, 2022 7:10:01 GMT -8
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. How big a difference does it seem to make (that is, how much do you think you are losing with a longer, cooler burn)? The longer burn is quite appealing, and if the efficiency loss isn't too great, it might be worth it. Either way, it's great having this aspect of the burn sussed out. It opens also the option of having a smaller stove and running more batches to get the final heat that I need. My house (1200 sq ft / 110 sq m) is roughly twice the size of your cabin, if I recall correctly, and poorly insulated in the walls. I could start with a 6" build that matches my chimney, and if it puts out too much heat all at once, I could narrow the dimensions (perhaps just do what I think you mentioned somewhere and raise the firebox floor and run smaller batches) while keeping the rest of the system as is. Then I could get the high temps of a wide-open burn paired with longer heat output by refueling as needed.
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Forsythe
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Post by Forsythe on Mar 6, 2022 4:22:52 GMT -8
it's great having this aspect of the burn sussed out. It opens also the option of having a smaller stove and running more batches to get the final heat that I need. [...] I could get the high temps of a wide-open burn paired with longer heat output by refueling as needed. This was my thinking, too, (albeit with a cookstove use-case,) but I just didn't state it as well as you did, DCish . Having the option of fast, high-heat output to bring a large pot up to temp is great — but with cooking / brewing / etc. one also needs the ability to shift the burn profile down to a longer, lower heat output once the contents of the pot have been brought up to temp. If you have a small pot which is light enough when full that it can easily be moved around, then it can be repositioned to a cooler part of a cooktop... but that's not a viable option with a large (40-100 liter or larger) pot. Even if you technically *can* move it, there is still the risk of severe burns from simmering liquid sloshing out onto yourself when shoving around something with that much inertial mass. The Aprovecho Institutional Stove works okayishly at this application, but the need to constantly feed it with small fuel during the latter (lower-heat, simmering) stage makes it a real pain in the neck to do long cooking / simmering times with. TLUDs are great for smaller cooking pots, but almost all of them lack the ability to refuel-in-place, so long, continuous stove cycles aren't feasible. (Sidenote: in 2020, this paper was published on designing a refuelable, continuous-burn TLUD... but the modus operandi is one of burning *only* the woodgas, and constantly taking-off the biochar from the bottom... so it only gets ~60% of the energy from the wood used. Worse, still: all the wood has to be in woodchip-to-pellet-size in order to updraft and gasify/char correctly without overfueling. Attached below are some images from that aforementioned paper. Cool concept, but not ideal for all use-cases.) ...So, to my mind, that's why a batch-box which is capable of: A) hot start-up initial burn on mostly primary air, but then also B) running lower/slower on secondary air [somewhat like a TLUD,] yet C) refueling-in-place every ~2 hours during the process of cooking [ rather than every 3-10 minutes as with a J-tube or L-tube rocket like the Aprovecho Institutional Stove] ...is indeed very appealing. I'm also really curious about this, myself. Perhaps this all comes down to the multiple different ways people refer to "efficiency" of a woodburning stove. Trev's absolutely right that it would be less efficient at converting the wood fuel into stored heat in the masonry mass and subsequently delivered to the room as ambient heat — but if the Testo is showing remarkably clean burns when run on secondary air — then the burn *itself* is no less efficient. I'd wager that Trev's cooktop-Vortex- run-on-mostly-secondary-air would burn more cleanly than the Aprovecho Institutional Stove, which loses heat to the pot before the woodgas can be thoroughly burned, and thus it emits at least some smoke. (Those Winiarski / Aprovecho designs are also HELL on the boiling vessel / cookware itself, over time. Aluminum and stainless steel aren't capable of taking the brunt of rocket stove riser heat for more than a few years before they oxidize, embrittle, spall, and burn through. Aprovecho's drive to force every bit of wood heat into the pot as quickly and "efficiently" as possible... comes at the cost of burning through your rather expensive, institutional-sized stainless pots. Which is damnably inefficient in that sense, unrelated to fuel consumption.) ...And... it would probably be a much more "efficient" use of wood fuel in something like a dedicated cookstove than would placing the cooktop farther away from the firebox in a room-heater stove... because in doing so, you'd halve the useful cooking time of each wood-load and re-load, and then you'd need to burn twice as much fuel... thus halving "efficiency" in that other sense, too.
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Post by Vortex on Mar 6, 2022 10:54:11 GMT -8
How big a difference does it seem to make (that is, how much do you think you are losing with a longer, cooler burn)? The longer burn is quite appealing, and if the efficiency loss isn't too great, it might be worth it. Either way, it's great having this aspect of the burn sussed out. It opens also the option of having a smaller stove and running more batches to get the final heat that I need. My house (1200 sq ft / 110 sq m) is roughly twice the size of your cabin, if I recall correctly, and poorly insulated in the walls. I could start with a 6" build that matches my chimney, and if it puts out too much heat all at once, I could narrow the dimensions (perhaps just do what I think you mentioned somewhere and raise the firebox floor and run smaller batches) while keeping the rest of the system as is. Then I could get the high temps of a wide-open burn paired with longer heat output by refueling as needed. It's hard to put a figure on it as it's very difficult to accurately measure. I would guess around 20% when running on mostly secondary from near the start, the longer you leave it until reducing the primary the less it happens. The transition to a slow burn is difficult to do cleanly though, the cleanest is when it's done either near the start or just after the peak of the burn when the fire is naturally slow or slowing down anyway. I find the latter seems to give me the best results overall. Perhaps this all comes down to the multiple different ways people refer to "efficiency" of a woodburning stove. Trev's absolutely right that it would be less efficient at converting the wood fuel into stored heat in the masonry mass and subsequently delivered to the room as ambient heat — but if the Testo is showing remarkably clean burns when run on secondary air — then the burn *itself* is no less efficient. That's correct, if you're not attempting to store heat in a mass then it would not be relevant.
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Post by martyn on Jun 10, 2022 8:00:13 GMT -8
Sorry Trev, I got the forum name confused but I will get it right on the follow up video.
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Post by Vortex on Jun 11, 2022 9:26:19 GMT -8
Hi Martyn, Glad to see you're experimenting again. I just started playing with my 4" cookstove again yesterday. Is yours going to be for use as a cookstove? Where's the chimney going to connect? Trev
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Post by Karl L on Jun 11, 2022 12:08:57 GMT -8
At some point this summer or Autumn I'm going to convert my 125mm (5") DSR2 into a Vortex-style stove.
Because I'm converting an existing stove, the new design has to fit in a narrower space than ideal. I hope that won't compromise the design too much.
I think the available space can support a Vortex design with a system size of 115mm (4.5").
Part of the reason I'm doing this is to remove all of the ceramic fibre board from the stove and use vermiculite board only.
Another reason is to get rid of the steel secondary air tube, which burns away and needs replacing too often.
I am guessing it may be difficult to find/achieve the correct top chamber resistance.
Also, it may be hard to figure out the size and positions for the various air inlets, given I want to use as much of the pre-existing metal work as possible.
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Post by martyn on Jun 12, 2022 7:33:19 GMT -8
Trev, I have taken a few pictures and some video and I will do a better video once I have it finished.
The exhaust exit at the right hand side back and at the moment just goes up a heath Robinson cement board 1 m high chimney! It will be a hot plate cook stove with a 22” x 16” cooking space.
I actually fired it up about two hours ago for the first time with just a stick holding some glass in place for a makeshift door.
Surprisingly even with a half load of offcuts it was working straight from the off with a nice vortex and no smokey glass.
The only air supply at the moment is coming from underneath the angled floor pieces and that seems to be working fine but there’s plenty of scope and time to experiment.
The half load of wood lasted one hour and 10 minutes with the vortex showing for about 50 minutes which might suggest there’s not enough air.
There has been some shrinkage in the vermiculite but I have not fixed the top box to the fire box as yet as I predicted there might be some movement.
So basically the first burn was a success but I now need to install it in situ and make some doors…. However I am back to where I was a few years back while experimenting with ceramic glass for the top!
I have a piece of glass in position but as before it is spot heating above the flame path, showing 400c directly above the flame but very quickly dropping to 100c just before the chimney. More to the point is that just to the right of the flame path (the path runs left) is only 120c just a few inches away! So I will need a steel top …..
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Post by Vortex on Jun 12, 2022 9:01:55 GMT -8
I found the dedicated cookstove style top a lot more effective at putting the heat into the hotplate, was there a reason you didn't go that route? ( donkey32.proboards.com/post/36456/thread ) I'd recommend a 10mm thick steel hotplate. Was there any issue with the left side of the vortex getting distorted by the higher draw on that side? I had problems with that on an early design that exited to the left front side. If the glass stays clean then it's running well, look forward to seeing the videos. Trev
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Post by martyn on Jun 12, 2022 13:38:24 GMT -8
It might pull to the left but my test was in bright sun outside so to be honest I could not really tell! Yes there are reasons why I did not use the afterburner roof as a hot plate. Firstly I am expecting the afterburner roof to be far to hot for a ceramic glass top (might not apply now) secondly I want a bigger top window for cosmetic reasons and thirdly I think I cam make my system work. I will progress forward and make everything air tight and secure and we will see what develops…….
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Post by martyn on Jun 14, 2022 12:24:46 GMT -8
I have made an updated video. I am surprised at how well it works considering the short and uninsulated chimney! I want to try a 2m tall chimney, a steel top plate and some air in the afterburner. Temperature inside the afterburner with the door open was 690c.
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Post by Vortex on Jun 15, 2022 5:10:51 GMT -8
That's excellent Martyn, well done! Is the air feed from under the sloped floor pieces just exiting at the front, what size are the 2 triangular channels it comes through, and do you have an air control for it somewhere? How's she run through the rest of the burn? Trev
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