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Post by Vortex on Aug 1, 2022 0:54:31 GMT -8
I think I spoke too soon as regards the steel cook top as it has now deformed quite a bit! I will post some picture in the next video but, the top is quite massive with lots of angle iron welded in place, it is so heavy I can barely lift it! The top has been stable for the last weeks of cooking but, yesterday I used some smaller offcuts of softwood and the increased heat has warped the top making it impossible to straiten back into a flat top! Interestingly my other stove in my partyhouse has a 8mm steel top that has been in use for about 5 years and is still functioning perfectly, this is because it is round and the heat source is in the middle. This means that although the top does deform it is deforms equally around the edge so it still sits on its gasket seal, the middle in fact conclaves but always returns back to flat. The vortex stove has a rectangle top and is heated near the front edge making the outer corners lift up even with its massive construction. Unfortunately 8mm is not thick enough. I had a 5mm hotplate on my original stove, welded all around the edges with 2" / 50mm angle iron, it bowed up badly in the center above the fire and twisted lengthwise. It was a real battle to keep it down in place. Here's an old pic: When I built the new stove I went for 15mm hot plates, 10mm with 5mm slightly larger pieces welded on top, they sit in the holes of the steel frame, which allows them to expand and contract without affecting the rest of the stove. Here's a pic: (left hand hotplate removed and up-side-down). I think if I was building it now I'd go for 20mm - two 10mm slabs welded together on top of each other, as at maximum temperature the hotplate above the firebox still curves slightly from front to rear so the edges raise a few millimeters. It's not enough to cause any problems just annoying. You could try welding large angle iron vanes on the underside like they often have on cast iron hotplates, but this may cause other issues as it would change the surface area resistance of the top chamber.
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Post by martyn on Aug 1, 2022 8:21:23 GMT -8
For the time being I have fixed two cast iron BBQ plates under the glass with a 5mm gap, works ok.
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fuegos
Full Member
not out of the woods yet
Posts: 177
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Post by fuegos on Aug 2, 2022 1:12:13 GMT -8
I remember our old Rayburn stove had a round lift out plate directly over the firebox, cast steel that was a fair bit heavier in construction than the surrounding top.I wonder if that prevented the top warping , being able to expand & contract at a different rate to the rest of the top?
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Post by martyn on Aug 2, 2022 3:34:21 GMT -8
Yes I am sure that is one of the reasons and that is exactly how my firehose stove is set up. What I have discovered with this little vortex stove is the top box seems to work much better at 75mm deep or higher (up to 170mm so far tested) than with a low ceiling. So now with the steel in place underneath the glass top it is operating with 65mm ceiling hight and does not work as well. It takes longer to start up, smokes far more during start up and does not produce anything like the same vortex display. It does work though and once going burns cleanly for an hour or so. I wonder just how high I can go with the top box as the higher it goes the more even the heat distribution becomes….. Next up will be finding a way to speed up the coaling stage as this is an issue for me as refueling has not worked very well so far, it seems to needs to be top lit to work properly and putting in extra wood just sends it into overdrive.
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Post by martyn on Aug 2, 2022 11:16:21 GMT -8
Not the best video but here you go…..
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Post by martyn on Aug 4, 2022 8:29:40 GMT -8
The inside view and check over, you can hear how the vermiculite has sintered into a hard surface and notice just how small the unit is!
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Post by Vortex on Aug 4, 2022 12:45:51 GMT -8
It's looking great Martyn. The optimal height of the top chamber seems to be determined by the length and quality of the chimney and resistance of the mass if there is one. With a short chimney and no mass it seems to work well with higher top chambers.
Have you tried refueling with just 1 or 2 large pieces that fill the rest of the firebox, after the peak of the burn but before it starts to go into the coaling phase? That's the only way I can get it to refuel without it causing problems.
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Forsythe
Full Member
Instauratur Ruinae
Posts: 208
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Post by Forsythe on Aug 4, 2022 22:53:05 GMT -8
For what it’s worth: cast iron has a higher max. temperature range (925°C) than carbon or mild steel (roughly 800°C, depending on the specific alloy.)
But perhaps most critically: steel is inherently more flexible than cast iron, and begins to soften at a much lower temperature — nearly 400 degrees lower than cast iron.
Steel also has a larger coefficient of thermal expansion than that of cast iron, largely related to the reasons above.
That’s typically why woodstove cooktop hobs are made from cast iron rather than steel. A steel cooktop would need to be considerably thicker than a cast iron cooktop of the same surface dimensions to avoid flexural warp at operating temp.
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Post by martyn on Aug 4, 2022 23:42:37 GMT -8
For what it’s worth: cast iron has a higher max. temperature range (925°C) than carbon or mild steel (roughly 800°C, depending on the specific alloy.) But perhaps most critically: steel is inherently more flexible than cast iron, and begins to soften at a much lower temperature — nearly 400 degrees lower than cast iron. Steel also has a larger coefficient of thermal expansion than that of cast iron, largely related to the reasons above. That’s typically why woodstove cooktop hobs are made from cast iron rather than steel. A steel cooktop would need to be considerably thicker than a cast iron cooktop of the same surface dimensions to avoid flexural warp at operating temp. However mild steel can work. As I have mentioned before … on my other rocket stove, I use a steel oil drum with a 8mm mild steel top. The steel top had a removable 4” plate just off centre, when the fire heats the plate the whole thing becomes slightly concave but the edges still sit perfectly on the gasket seal. Furthermore the concave plate now allows any cooking juices to flow toward the centre and burn off. So in fact mild steel is the perfect material to use under the right circumstances. In this case with the little vortex stove the heat source is way off centre and the plate is rectangular so the effect is the the far corners raise up dramatically.
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Forsythe
Full Member
Instauratur Ruinae
Posts: 208
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Post by Forsythe on Aug 5, 2022 1:04:34 GMT -8
For what it’s worth: cast iron has a higher max. temperature range (925°C) than carbon or mild steel (roughly 800°C, depending on the specific alloy.) But perhaps most critically: steel is inherently more flexible than cast iron, and begins to soften at a much lower temperature — nearly 400 degrees lower than cast iron. Steel also has a larger coefficient of thermal expansion than that of cast iron, largely related to the reasons above. That’s typically why woodstove cooktop hobs are made from cast iron rather than steel. A steel cooktop would need to be considerably thicker than a cast iron cooktop of the same surface dimensions to avoid flexural warp at operating temp. However mild steel can work. As I have mentioned before … on my other rocket stove, I use a steel oil drum with a 8mm mild steel top. The steel top had a removable 4” plate just off centre, when the fire heats the plate the whole thing becomes slightly concave but the edges still sit perfectly on the gasket seal. Furthermore the concave plate now allows any cooking juices to flow toward the centre and burn off. So in fact mild steel is the perfect material to use under the right circumstances. In this case with the little vortex stove the heat source is way off centre and the plate is rectangular so the effect is the the far corners raise up dramatically. Oh, for sure. The shape of the steel plate and its position over the heat source make a huge difference in its usability as a cooking surface, because those factors will dictate the orientation and degree of deflection. It’s probably also worth stating that because cast iron is less flexible than steel in general —and less soft under heat load in specific— this in turn means cast iron is more brittle, and thus can be slightly more susceptible to thermal-shock cracking than steel. (…If, for example, your particular stove design and method-of-use entails rapidly superheating the cooktop up from an ice-cold start, or if it has something extremely cold placed upon it once it’s hot.)
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Post by martyn on Aug 12, 2022 9:46:37 GMT -8
While playing with the fire today, I slid the top off just after I had finished cooking my chicken. The cast plate was at 280c whereas the glass above the same spot, was only 160c so quite a difference ! The hottest I have seen on the glass was well over 600c but with the cast iron in place it reaches only 320-40c. I am going to try without the secondary air next time, I am pretty sure it gets quite a bit hotter with the secondary air but I need to check that… I can see blue flame, just a tiny amount to be honest and I cant get it to show on film but definatily happening. I am only guessing but I assume the blue flame flashing off the top of the secondary air means something?
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Post by Karl L on Oct 19, 2022 11:05:45 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. I finished converting my 125mm (5") DSR2 core into a 115mm (4.5") Vortex core a few weeks back. The weather has been too warm here to run it much, but it seems to work well. I wanted to see if the Vortex core would work with an unlined (~6m high) chimney, and it seems to be fine. I'll have to wait for the cold weather to really test it. Because I was worried about the lack of draw from the chimney I left the limiting primary air path restriction (the mesh) at 42%, as I could always reduce the inlet if necessary. Compared to the DSR2 I've found: - It's much easier to light
- Doesn't over fuel so easily (even with exactly the same 25mm Skamolex wall firebox)
- Burns for much longer
- Takes up only 75% of the volume
- Leaves firebox glass much cleaner
It's attached to a temporary bell -- a tall welded steel box with 2.0 msq internal surface area, surrounded by 250Kg of dry stacked firebricks.
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Post by Karl L on Oct 19, 2022 11:28:32 GMT -8
The next thing I want to do is improve the flue a little by insulating some of the length -- I can only do the first 2m -- and then add a bigger mass with 3.0 msq ISA and maybe 600Kg mass.
Because of the poor flue I guess it will have to be a bell of some kind.
I think I will have to avoid building the bell in front of an exiting fireplace opening, just in case the stove produces enough soot, etc, over a long period, that I need to sweep the chimney.
Or maybe I can design the mass so it can be easily deconstructed after the first season, to check/sweep the flue?
Trev - for that second option I am interested in how you built your mass. It looks like steel sheets and steel angle without any gas- or water-tight seals at the edges -- is that right? I think you would have said if you had any exhaust leaks or condensate leaks.
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Post by Vortex on Oct 19, 2022 12:40:14 GMT -8
Hi Karl, Glad it seems to be working out OK for you.
6 meters is a good length chimney, mine is only a bit under 4 meters from the bottom of the stove to the top of the cowl. I never get much soot out of my chimney when I sweep it, usually not even enough to cover the bottom of a bucket. Could you put a clean-out door in the chimney somewhere?
Yes the mass is steel sheets and steel angle without any gas or water-tight seals at the edges. I've never had any exhaust or condensate leaks. I thought about putting fire rope seals in between the steel sheets and angle when I was making it, but the thinnest I could find were 2mm and the steel was very clean and flat, so the two flat surfaces tightly together was as good as a fire rope seal. I could have used silicone but wanted it to be easy to take apart again.
Trev
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Post by Karl L on Oct 20, 2022 5:41:53 GMT -8
6 meters is a good length chimney, mine is only a bit under 4 meters from the bottom of the stove to the top of the cowl. Thanks - is your chimney lined or unlined?
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