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Post by Karl L on Jan 17, 2016 5:56:27 GMT -8
I have a question:
On a batch box system, what temperature do you aim to run your thermal mass up to?
The average temperature, or the range of temperatures, top and bottom.
Thanks, Karl
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Post by Karl L on Jan 11, 2016 13:58:13 GMT -8
I may be able to help with some of this. I am an embedded systems engineer (electronic and software control systems), and I do quite a lot of instrument design. My company ( Micro-Robotics) makes and sells general purpose controllers (like the Arduino, but better ) They are a bit more expensive than the Arduino, so you probably wouldn't want to buy one, but I may be able to help with general electronics and software advice.
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Post by Karl L on Dec 9, 2015 13:37:06 GMT -8
Thanks - did you buy the slabs or did you have to cast them yourself?
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Post by Karl L on Nov 25, 2015 14:26:41 GMT -8
Daniel, what are you using to form the top of your bell?
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scandal
Nov 9, 2015 15:28:41 GMT -8
Post by Karl L on Nov 9, 2015 15:28:41 GMT -8
This is very interesting!
Was your experiment done with the batch box core alone, or with a chimney or flue attached?
If there was a chimney, were the hot gases cooled by a bell, etc, before exiting to the chimney?
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Post by Karl L on Aug 4, 2015 4:43:07 GMT -8
I seem to remember that MatthewWalker guessed that having the main restriction for his secondary air system at the cold end of the tube led to better performance, but he didn't go further with that. (Can't find the page now...)
I wonder if this might be because as the firebox heats up, the secondary air gets hotter and so has a larger volume by the time it reaches the delivery end. This will increase resistance and so reduce the secondary air flow and lead to a richer mixture.
But if the main restriction in the secondary air supply is deliberately situated at the cold end of the system then this effect would be much smaller than if the main restriction is at the hot end.
(Also, perhaps it would be useful to try to meter the primary air when it is hot, so that as the firebox heats up the primary air reduces slightly, tending to control the burn rate a bit. I'm not sure of an easy way to arrange that, though.)
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Post by Karl L on Aug 2, 2015 23:03:41 GMT -8
Having run the core again several times I now understand why the cracks are forming in the riser boards but not in the separate pieces around the port, which probably run even hotter.
Surprisingly, vermiculite board has a large thermal expansion, very close to that of steel. When there is a large temperature differential across the board it bends visibly. Over the length of pieces of the riser this bending can't be accommodated, and it cracks. The smaller pieces around the port experience a similar temperature differential, but these pieces are free to move, and so they bend with no problem.
For some reason, in the boards of the core the bending reversed when they cooled down, but the piece opposite the port now has a permanent bend.
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Post by Karl L on Jul 31, 2015 11:55:06 GMT -8
Thanks, Peter. The lowest part of the riser, where the double vortex was forming, is also made from several pieces of the same board, but not screwed together. (Sketchup 2014 of core attached). I would think that these pieces would get even hotter than the main riser, but they show no cracks. (I just took a closer look at them, and their surface has become uneven, though.) The cracks formed (in the riser) well above where I could see the vortex of flames. I wonder if the problem is something to do with differential expansion? With the inside of the boards being hotter than I've seen before, but the big cracks only forming where the outside of the boards is in contact with cold air? Attachments:S-PortalCore C2014.skp (324.83 KB)
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Post by Karl L on Jul 31, 2015 9:17:15 GMT -8
... by the way, here are some photos of the core:
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Post by Karl L on Jul 31, 2015 9:14:55 GMT -8
I've just tested my new S-Portal 4" batch box core (core-only burn). After a few false starts (not enough wood? Stove needed to heat through?) the burn became very clean - judging from the smell of the exhaust. All well and good :-) However, I noticed that my vermiculite board heat riser had serious cracks all around. This riser had been used a lot, previously, with a 4" Dragon Heaters J-tube core with no cracking at all, in both core-only and enclosed burns. I've used vermiculite board a lot over the years, and seen it withstand a lot of heat. Do any of you know why my riser developed such big cracks this time? Pictures of the cracks attached. Attachments:
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Post by Karl L on Sept 9, 2014 8:02:08 GMT -8
Imagining a heat riser in a bell, you could make one side (or top) out of metal and point the heat riser at it for max heat transfer. Then you could pull a rock wool blanket over it when you want to keep the heat in to be absorbed by the thermal mass. Thanks - yes, I had been thinking about a bell with one metal side and three brick sides. Pointing the heat riser at the metal face is an idea to explore. I've been wondering if it is workable to direct the flow of gases with 'gates'. They would either be made from refractory material to take the heat, or situated at the low temperature end of things. Alternately you could use more mass instead of a blanket. For example, I live near lots of granite countertop installers who pitch their scraps. A few small pieces would be easy to throw on a metal surface to soak up heat. That sounds like a lot of mass to be swinging around if its anywhere near the mass of the other brick sides! Karl
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Post by Karl L on Sept 8, 2014 11:19:25 GMT -8
Satamax, Thanks very much for your comments and calculations. I'll think some more. Maybe I should lay out what I am trying to achieve (which may not be possible...) I am thinking that if I just use a standard bell built from bricks, there will be quite a long time between lighting the stove and much heat coming out into the room. I'd like a system that can both heat up a cold room very quickly, and then, when the room is hot, be able to switch to a different mode, where it outputs low power into the room, storing most of the heat in a thermal mass. In 'heat the room' mode, the hot gases would first run through a steel section of the bell, sending lots of heat out into the room, and then the cooler gases would run through the (brick) heat storage bell, extracting the remaining usable heat and pre-heating the storage bricks. In 'store the heat' mode, the hot gases would mainly run through the heat storage bell. ... and all of this in a compact size Is there any way to do that, do you think? Karl
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Post by Karl L on Sept 7, 2014 7:55:27 GMT -8
Hi Peter and others, I've been trying to think how to create a very compact 'bell', while keeping a relatively high internal surface area and a relatively low impedance to the gas flow. The sketchup model shows this. (I've put it here temporarily as I got a message saying the formum attachment limit had been exceeded: HozFeed 2v0.skp) Fire bricks are stacked on-edge to form tall fins 76mm (3”) thick. The exhaust flows roughly horizontally between the fins. I am hoping that there will be some degree of stratification, i.e. that the exhaust will tend to flow through near the top of the fins, and then tend to flow through lower as the tops of the fins heat up. I am thinking this will happen because a) the hot gases will take the highest path they can, but b) the hotter gases have higher volume, so there will be greater flow resistance though those gas paths where the gases don't cool so much. Later, heat is regained from the mass through convection flowing up through the hot fins and down the steel outer skin. Has anyone tried anything like this before? Can you see any problems with this idea? I also have another question: in the batch box calculator, the heat riser is defined by its diameter. I will probably use a square cross section riser, but I guess I can't just use one with the same CSA, as there are probably 'corner effects'. Is there a known scale factor for this? Thanks! Karl
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Post by Karl L on Sept 7, 2014 5:57:50 GMT -8
Thanks, Peter!
That's very useful information. I'll carry on with my design...
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Post by Karl L on Sept 6, 2014 2:16:04 GMT -8
I've been reading the threads on horizontal feeds and bells, and haven't been able to find definitive answers for this set-up:
I am trying to design a small batch box Rocket Mass Heater with heat storage. A 4" (100mm) system is all there is space for, I think.
The bell is likely to consist of fire bricks.
I have some questions:
1. What is are the suggested maximum/minimum internal surface area for a bell fed from a 4" batch rocket?
2. Do the dimensions in PeterBerg's spread sheet for calculating Batch Box calculator spreadsheet scale down to 4"?
3. Is there a ever likely to be a problem with CO leakage in the following situation? The wood has burned down to embers, the ballast air cools the air moving up the heat riser to less than the interior of the bell, so there is no flow up the heat riser, so the exhaust from the burning embers rises up the P-channel/tube and out into the room.
Thanks in advance for any help, Karl
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