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Post by oxbowfarm on Nov 24, 2015 16:48:42 GMT -8
I am slowly constructing parts for a 6-inch batch box bell bench mass heater for my greenhouse. I've started casting my heat riser in sections similar to PNW Dave did in this thread. The main difference is that I've been unable to locate a source of fire clay, and I am casting my sections about half as tall as David does, mainly because I am using a conveniently shaped flower pot as my interior mold and it was only 6 inches tall, so my riser sections are approximately 5.5in/14cm. I initially tried a straight inner mold but it wouldn't release without ruining the casting, so I needed something with draft. I am happy with how they are turning out, but it occurred to me that the pot I'm using as an interior mold is rather more tapered than David's clever sonotube inner mold. The diameter at the wide end is approximately 6.75 in tapering to 6 inches. So my heat riser is going to get somewhat vertically corrugated internally as these are stacked up. Is that a big deal? Any idea if they should be laid taper up or taper down, or alternating to prevent abrupt transitions? Doesn't matter?
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Post by peterberg on Nov 25, 2015 3:09:47 GMT -8
It would be an interesting experiment to build such a riser, isn't it? I'm inclined to think this slightly widening and narrowing vertically doesn't make much of a difference. What I do know for sure is that abrupt transitions disturbs the flow. So I feel it's best to start with a smaller end at the bottom and alternate taper up and taper down from there. Keep us posted!
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Post by PNW Dave on Nov 26, 2015 11:32:22 GMT -8
Hi, oxbow
I'm looking forward to seeing your progress and how it turns out. This video consistently pops into my mind whenever I think about the interior of a heat riser- youtu.be/WG-YCpAGgQQI had the thought that the tapered interior might have a similar effect. If the sections are stacked with the taper getting smaller at the top, it would create little "shelves" where ash would build up, so taper down was my first thought. My taper was so slight that I had a hard time determining which way was which and now I can't recall exactly what orientation they ended up in. Unfortunately I haven't made any progress on my RMH recently. I need to come up with a solution for the significant amount of condensation that collects inside the bench barrels, which then runs down the sides and even puddles up where the barrels meet the ground, none of the water ever makes it to the drainage ditch running under the barrels. Hmmm...
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Post by oxbowfarm on Nov 26, 2015 15:14:21 GMT -8
Thank you peterberg and PNW Dave. I've been casting sections for the last three days, but only have three that I am truly happy with. Fortunately the crummy ones are easy to crush up and reuse in the next attempt, so not much is wasted. The last two have been the best yet. I had a lot of trouble initially with getting the mix and moisture content right. Too loose and dry a mix and they don't hold together, too wet and they distort before they cure and/or they stick to everything and leave chunks of themselves behind when you move them. One advantage to having such short sections is I am able to fire cure them on a circular metal ring of sheet metal on my barbecue grill. I ruined two sections on the grill before I realized that I needed a hole in the middle of the ring to prevent the sheet metal from warping and cracking or distorting the green casting. That video is very interesting. With the taper on my sections, I definitely could create a noticeable "shelf" if I orient these all in the same direction, as my sections are so much shorter than yours and the taper more extreme I wonder if there is any advantage to laminar vs turbulent flow in the heat riser? From the reading here and on Permies there seems to be a strong theoretical basis for a laminar flow?
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Post by ronyon on Nov 27, 2015 11:29:51 GMT -8
Have you tried using grease or oil as a release agent on your form? How about a layer of foil over the form?
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Post by oxbowfarm on Nov 27, 2015 12:28:26 GMT -8
Have you tried using grease or oil as a release agent on your form? How about a layer of foil over the form? I actually haven't had too much trouble getting the molds to release, it was more having a mix that would hold together, and learning to be much more careful and systematic about handling the green castings till they were cured. My inner mold is a glazed flower pot and has enough draft that it releases very easily. The outer mold is a section of a polyethylene bucket so is very 'non-stick".
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