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Post by Vortex on Oct 16, 2020 3:10:46 GMT -8
Hi Trev, I have been running my stove a couple of weeks now. My bench as I was thought when building is too big. Originally the flue temps were only 50 C. At these temps the stove would form a rams horn vortex but after a short time the air control needed careful management. Still it was testament to your design that it worked at all in these conditions when sometimes the outside temps were in the high teens. It seemed to run best with no exit port then. Since I have blocked off nearly 2/3 of the bench and flue temps now peak at around 75 C it now runs with the exit port. I shall block off a little more of the bench to boost the temps further and was also thinking of making a couple of hinged insulated covers for the hot plate. I have noticed on windy days the flames in the afterburner flux or corkscrew in and out, maybe an anti-downdraft cowl will help? There is no room for a draught regulator. One of my best runs was on a calm dry day where with my umbrella cowl was taken off ( highest flue temp). Delighted with it and thanks. James. Hi James, Great to hear you finally got it finished and running. It took me a while to get mine tuned to the chimney and mass. The double vortex seems to be a balance of forces between too little draw, where it cant spin it up, and too much where it drags the vortex out of the afterburner. Had exactly the same experience when I first started running mine, thought I had too much mass and exit temps were too low. Turned out that BBQ chimney thermometer I was using (and IIRC recommended to you) was reading way under, and the chimney cowl was really restricting the gas flow in the system. The cowl seems to need an exit area of 200% csa, but most available are a lot more restrictive than that, so I ended up making my own by adapting an old anti downdraft cowl I got off of ebay. For the draft regulator could you not make a hole in the chimney breast above the fireplace where the stove is installed and insert a short piece of 6" pipe with one on the end? I'd also like to get hold of one of those old Aga/Raybrun hotplate covers, would be useful for those times when I want most of the heat to go into the mass for later. Trev.  EDIT: If anyone wants the plans for the Anti Down Draft Cowl or Storm Collar they're here: vftshop.com/images/others/Stove/antidowndraftcowl.jpgvftshop.com/images/others/Stove/stormcollar.jpg |
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Post by Jura on Oct 16, 2020 3:26:26 GMT -8
Thanks Trev.
I did searched for his thread before I wrote my question.
The search returned a couple of his posts but no any single thread with a description of his set up.
I know the search forum engine can be messy that's why I wrote a post
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Post by Orange on Oct 17, 2020 3:55:33 GMT -8
Jura do you have the .skp maybe? I don't understand how that slit in the firefox floor doesn't clog up.
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Post by Vortex on Oct 17, 2020 6:37:50 GMT -8
The ash-trap is not on the sketchup as it's not an integral part of the core. It doesn't clog up because there's a large ash-box underneath. It's narrow as it's designed to only let the ash through. It used to be wider with a one bar 'grate' in the middle but I learned that one slit was sufficient, and the metal only lasted a couple of seasons so had to be replaced every few years, so I made it just one slit and did away with the metal piece altogether. Here it is without the threshold in place. The floor is made from a single slab I cut up, and 2 small little triangle pieces at each end in the middle to hold them apart just enough to let the ash through.  |
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Post by Orange on Oct 17, 2020 10:14:58 GMT -8
got it, tnx Vortex!
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cork
New Member
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Post by cork on Oct 20, 2020 12:38:34 GMT -8
How about a castellated top for the threshold? I have also discovered that you can get draft regulators that attach to soot doors or square pipes which may make it easier for my build, it will displace the thermometer though.
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Post by Jura on Oct 20, 2020 14:40:00 GMT -8
Jura do you have the .skp maybe? I don't understand how that slit in the firefox floor doesn't clog up. The *.skp file of the core is being in "statu nascendi". I'm sketching it (in free whiles) along to Vortex guidance notes. Once the skp. file reaches release level it will be passed to Trevor and he's gonna posses all the proprietary rights :-) |
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Post by Vortex on Oct 21, 2020 10:50:38 GMT -8
How about a castellated top for the threshold? I have also discovered that you can get draft regulators that attach to soot doors or square pipes which may make it easier for my build, it will displace the thermometer though. I thought about doing something like a castellated threshold, as it seems to like having air blowing over the edge of it which would cause turbulence. The expanded metal screen over the primary air inlet seems to do a great job of mixing the incoming air and stops any sparks coming out, so I've never got around to trying it out. Maybe I'll give it a go. I spotted one of those old square draft regulators on ebay back when I was researching them. There was only a rectangular door so I wondered what had happened to the rest of it, realised now you say it that it was meant to fit on an outside chimney soot door. Today I decided to go back to basics and set the afterburner back to the original layout; no nose in the port, or sloped rear, or deeper afterburner. Exit port back to 105% port size. Also managed to make the top chamber exit to slightly over 1 CSA. Ran a 3/4 load of pine lit from on top. Strong breeze outside. Stove took off faster than I expected and overfuelled a bit for the first 10 minutes. It couldn't seem to catch it's breath even with the door open about an inch. It looked like the draw was too strong, so I got the idea at 21 minutes to try closing the chimney damper half way. To my surprise it ran very happily between 6% to 8% O2 for the next quarter of an hour. Normally the CO spikes straight up under 8% as you can see in the earlier part of the graph.  |
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cork
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Post by cork on Oct 21, 2020 12:52:24 GMT -8
Too much kindling maybe, did you change setting on the draft regulator? I suppose any lessening of restriction can improve flow or change the harmonics of the stove.
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Post by Vortex on Oct 22, 2020 14:52:31 GMT -8
Yes, with less restriction from the larger exit port it can overfuel more easier, so it needs less kindling for startup. Haven't changed the draft regulator. Where you have the restriction certainly seems to make a difference to the harmonics of the system. The chimney damper is between the exit of the mass and the start of the chimney.
I repeated yesterdays run as closely as I could this evening. As soon as the fire started to settle and CO dropped down, I closed the chimney damper half way and opened the door about half an inch. Again the chimney temp immediately dropped, followed by the O2. CO stayed low.
It ran between 4% and 8% O2 for nearly half an hour, mostly around 6%. CO around 170ppm. Eff. was up around 94%. Afterburner temp seemed to be a bit lower than normal. Exterior mass thermometer got up to 120C, which wouldn't normally happen on a 3/4 load.
It's repeatable at least, so I'll try varying things and see what happens.
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Post by Vortex on Oct 26, 2020 12:18:20 GMT -8
Since enlarging the exit from my top chamber to just over 1 csa, I've noticed that the core is less inclined to form a good strong double vortex, especially in the earlier parts of the burn.
Today I decided to put the top chamber exit back to 78% csa, and see if closing the chimney damper down to 50% mid burn still has that same effect as with the 1 csa exit. With the top chamber exit set to 78% it formed a good strong double vortex almost from the start. Once the burn was settled in and the afterburner glowing, I gradually closed down the chimney damper to 50%. The drop in stack temp and excess O2 seen with the larger exit was still there but quite a lot less pronounced, and CO ppm rose slightly. The minimum level of O2 at which CO would start to spike up rose from 6% to 7%, and max mass temperature was about 10C lower than with the larger exit.
Conclusion, the average CO ppm is a bit higher with the larger exit, possibly due to being less inclined to form a good double vortex, but excess O2 and stack temp are a bit lower - seems like it's a trade off. I think I prefer the stronger vortex and lower CO.
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Post by fishalive12345 on Oct 28, 2020 9:40:17 GMT -8
Hi Trevor,
very good to see all the latest developments in the world of efficient wood burning.
As always, a question. The afterburner shelf in your stove is very highly insulative like the rest of the afterburner. It's also very thick. I was thinking about using instead, a cordierite (I think) kiln shelf. Obviously not insulative but thin enough to heat up quickly. In this case the total height of the afterburner and topchamber would be some centimetres less. Do you have any thoughts on this? It also relates to Peter's choice of shelf material ie thin castable slab and his plans to try out thin fire brick.
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Post by Vortex on Oct 29, 2020 4:14:53 GMT -8
Hi Brian,
I've never made a version with an uninsulated afterburner, but from over 120 testo'd runs watching this core through all the different stages of the burn cycle, I'm certain the efficiency and clean burn are as much a function of the temperature of the afterburner as it's geometry. I think building the afterburner from thin firebrick or ceramics with insulation behind would be OK, as the low mass would soon be radiating heat back into the burning gasses. A thin kiln shelf for the top of the afterburner would probably be as good as CFB, as it would very quickly be glowing because of it's low mass. A material that can hold some heat and radiate it back is probably better than a pure insulator with extremely low mass, as that could only reflect the heat radiated from the burning gases at that moment.
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Post by patamos on Oct 29, 2020 17:00:20 GMT -8
Hi Trev, I have been running my stove a couple of weeks now. My bench as I was thought when building is too big. Originally the flue temps were only 50 C. At these temps the stove would form a rams horn vortex but after a short time the air control needed careful management. Still it was testament to your design that it worked at all in these conditions when sometimes the outside temps were in the high teens. It seemed to run best with no exit port then. Since I have blocked off nearly 2/3 of the bench and flue temps now peak at around 75 C it now runs with the exit port. I shall block off a little more of the bench to boost the temps further and was also thinking of making a couple of hinged insulated covers for the hot plate. I have noticed on windy days the flames in the afterburner flux or corkscrew in and out, maybe an anti-downdraft cowl will help? There is no room for a draught regulator. One of my best runs was on a calm dry day where with my umbrella cowl was taken off ( highest flue temp). Delighted with it and thanks. James. Cork, i'd be surprised if the issue was bench ISA. I've built a few of the old school vortex fireboxes with waaay more ISA than Trev has in his home unit. (Although it has mostly been with bells chambers) Perhaps a permanent 10% bypass straight to the chimney (or something like that) will help get more heat thru the bench. just a thought |
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cork
New Member
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Post by cork on Oct 30, 2020 11:08:26 GMT -8
Hi Patamos,
I have been running the stove with about 10% bypass recently which can bring flue temps up to 95 C . I have also made a rookie error in assuming that once the flue gases go past the first exit port that once it stays above csa it will be ok . Recent posts have made it clear the second exit is important as well. When I correct this I am hoping that I will be able to keep the bypass closed. There is also a pinch point where the main body of the stove meets the bench which I hope to improve. James.(cork).
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