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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 Jul 31, 2015 9:17:15 GMT -8
... by the way, here are some photos of the core:
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Post by peterberg on Jul 31, 2015 11:34:55 GMT -8
Most of the burning in a Dragon Heaters J-tube is taking place in the tunnel and just now and then a bit in the riser. In a batch box the situation is quite different, there's no tunnel to speak of and the lower part of the riser just behind the port will get awfully hot.
Some people go as far to say a batch box isn't a real rocket stove, but a category on its own. The thermodynamic process in there is very different when compared to a Ianto Evans J-tube. Even the Larry Winiarski rocket stove behaves not the same way but it's probably a closer relative.
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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 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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