Post by mintcake on Apr 19, 2014 13:08:29 GMT -8
Hi,
Over here a while ago I came up with an idea about partitioning a masonry bell, so that the heat would be stored and not
removed by the circulation of cold gasses... I've now been building a couple of them. Results so far don't look good.
I've got a "corner stove" arrangement, with the heat riser from the batch-box in the back corner, and with the bricks on edge, so that there's maximum bell and minimum mass. Maybe things will be different with other formats. For my partition I've been using ytong-brand aerated autoclaved concrete, which is lightweight, insulating and comes in nice big bits. It's all back-garden / yard testing, with a "chimney" which only goes about 1.5 above the batchbox entrance, so the batchbox isn't
exactly in ideal circumstances, but it seems to work.
"#" is heat-riser.
"^" indicates nice hot combustion products going up into a 30cm gap in test 2, and about 20cm (before the bottom of the 20cm high divider) in test 1.
Test one (vertical split):
+--------+
I I I
I I I
I ^ I
I # I
Test two (horisontal split):
+--------+
I I
I I
+---- I
I ^ I
I # I
I # I
Results from test 1: The bricks which make the "back" corner above the riser got really hot, the front didn't.
Obviously the heat is being trapped in the first partition. I'd hoped that it would be almost as hot in the rest of the top, but
this wasn't happening. While the hand-made red-bricks at the back (where I don't really want any heat to go)
got up to 100C or so, the ones in the front were quite a lot cooler. In this test I had some thin firebricks making the top at the front, they reached 100C, but the red-bricks didn't ever get warmer than the red-bricks at the back.
On disassembly the ytong sheet had suffered shrinkage / differential expansion cracks, whereas the heat riser (long, trapesium cross-section, ytong, 8 making an octagon) was perfectly happy. Either this means that the heat-riser wasn't getting as hot as the divider, or the shape of the heat-riser sections have been protecting them from this effect.
Results from test 2:
Even more disappointing, actually. It was a windy day which probably took the heat away quickly, but... yuck.
A lot of heat below the partition (no measurements, sorry), but judging by what's hot I'd guess
that most of the hot gasses were hitting the partition and just going down to the chimney hole rather
than going up to fill the bell. I didn't have fire-bricks on top, just normal red-bricks, but they never got more
than slightly warm where the bricks below the partition at the back were getting too hot to hold.
Testing after the fire was long out, the rest of the bricks never showed any sign of being warmer than the bricks under the partition.
Hesitations / other factors:
1. bricks were not identical, as the above-partition wall-bricks were industrialy made redbrick and the lower ones and the roof were (100year old) hand-made redbrick. I'd be surprised if issues of heat-capacity and thickness would have such an
enormous effect.
2. the above-partition bricks were only mortared in last night, whereas the others had been in the bell for previous firings. Maybe they were still wet?
Conclusion:
1. Another test needed to see if it might have been wet bricks.
2. I need to test the null-case (no partition), but it looks like the only reason for splitting a bell like this is if you want to have uneven temperatures on the outside of the bell, with the maximum close to the riser.
3. (tentative): splitting the bell isn't helpfull
Over here a while ago I came up with an idea about partitioning a masonry bell, so that the heat would be stored and not
removed by the circulation of cold gasses... I've now been building a couple of them. Results so far don't look good.
I've got a "corner stove" arrangement, with the heat riser from the batch-box in the back corner, and with the bricks on edge, so that there's maximum bell and minimum mass. Maybe things will be different with other formats. For my partition I've been using ytong-brand aerated autoclaved concrete, which is lightweight, insulating and comes in nice big bits. It's all back-garden / yard testing, with a "chimney" which only goes about 1.5 above the batchbox entrance, so the batchbox isn't
exactly in ideal circumstances, but it seems to work.
"#" is heat-riser.
"^" indicates nice hot combustion products going up into a 30cm gap in test 2, and about 20cm (before the bottom of the 20cm high divider) in test 1.
Test one (vertical split):
+--------+
I I I
I I I
I ^ I
I # I
Test two (horisontal split):
+--------+
I I
I I
+---- I
I ^ I
I # I
I # I
Results from test 1: The bricks which make the "back" corner above the riser got really hot, the front didn't.
Obviously the heat is being trapped in the first partition. I'd hoped that it would be almost as hot in the rest of the top, but
this wasn't happening. While the hand-made red-bricks at the back (where I don't really want any heat to go)
got up to 100C or so, the ones in the front were quite a lot cooler. In this test I had some thin firebricks making the top at the front, they reached 100C, but the red-bricks didn't ever get warmer than the red-bricks at the back.
On disassembly the ytong sheet had suffered shrinkage / differential expansion cracks, whereas the heat riser (long, trapesium cross-section, ytong, 8 making an octagon) was perfectly happy. Either this means that the heat-riser wasn't getting as hot as the divider, or the shape of the heat-riser sections have been protecting them from this effect.
Results from test 2:
Even more disappointing, actually. It was a windy day which probably took the heat away quickly, but... yuck.
A lot of heat below the partition (no measurements, sorry), but judging by what's hot I'd guess
that most of the hot gasses were hitting the partition and just going down to the chimney hole rather
than going up to fill the bell. I didn't have fire-bricks on top, just normal red-bricks, but they never got more
than slightly warm where the bricks below the partition at the back were getting too hot to hold.
Testing after the fire was long out, the rest of the bricks never showed any sign of being warmer than the bricks under the partition.
Hesitations / other factors:
1. bricks were not identical, as the above-partition wall-bricks were industrialy made redbrick and the lower ones and the roof were (100year old) hand-made redbrick. I'd be surprised if issues of heat-capacity and thickness would have such an
enormous effect.
2. the above-partition bricks were only mortared in last night, whereas the others had been in the bell for previous firings. Maybe they were still wet?
Conclusion:
1. Another test needed to see if it might have been wet bricks.
2. I need to test the null-case (no partition), but it looks like the only reason for splitting a bell like this is if you want to have uneven temperatures on the outside of the bell, with the maximum close to the riser.
3. (tentative): splitting the bell isn't helpfull
