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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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Post by peterberg on Sept 7, 2014 1:03:04 GMT -8
1. What is are the suggested maximum/minimum internal surface area for a bell fed from a 4" batch rocket? Hello Karl, I've noticed this is your first post, welcome to the boards. At this time, there isn't concensus about the size of a bell that could be served by a 4" batch box. However, I'm inclined to view it as a scalable dimension related to the riser cross section area. We know what the bell of a 6" system would be like, due to the work of Klemen in Slovenia we know that is just short of 6 sq m, or 64 sq ft. The 6" riser area is 2.25 times as large as compared to the 4", so a bell to the same proportions would be 2.6 sq m or 28.4 sq ft. Mind you, this an educated guess only. 2. Do the dimensions in PeterBerg's spread sheet for calculating Batch Box calculator spreadsheet scale down to 4"? Yes, several of those has been built with good results. 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. This same question is creeping up from time to time. No, nothing will come out of the p-channel. Simply because you are wrong about the flow in the riser. When the chimney is warm, there will be a draw which in turn keep the flow in the riser going. The ballast gases won't cool the air to less than the interior temperature when there are embers, however few, in the fire box. Even when the stove is completely black there still is a small draw maintained because of the underpressure the chimney stack generates. The only time that I've noticed smoke was coming out of the p-channel was at the few times I've experienced a chimney stall. Besides that, the Testo analizer sports an environment CO-detector which never let out a single beep. |
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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 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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radek
Junior Member
Posts: 94
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Post by radek on Sept 7, 2014 8:55:51 GMT -8
hi Karl to match round 10 cm heat riser,the sides of your square riser has to be 10cm.Yes the cornes dont count.
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Post by satamax on Sept 7, 2014 11:31:41 GMT -8
Karl, i'm not too keen on your idea. It will gather heat, but transfer it may be too slowly to the room around.
There's two contact points, the top one and the bottom one, this is slowing the heat transfer. While convection movements in the bell will tend to transfer heat to the air drafted by the chimney. I think it would be better to make all of thoses fins contact the front of the bell. So it would radiate more heat. And i'm afraid your ISA is way too high!
You're already at 3.67m² just with the three "fins" and the endwall! And i haven't accounted for the rest of the walls, the top and the edge of the fins!
Way too much!
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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 satamax on Sept 8, 2014 12:58:49 GMT -8
May be with a cast ion oven in the bell?  |
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Post by DCish on Sept 8, 2014 12:59:30 GMT -8
Hmmm. 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. The down side to this is that you lose the ISA of the insulated area. 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.
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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 DCish on Sept 9, 2014 12:34:03 GMT -8
I like the idea of directing the flow, sounds fun.
As for mass, I was envisioning a couple of, say, 6" x 18" x 1" pieces. They'll be a bit heavy, but manageable (I have a couple now) -- less mass than bricks, but it still takes a good half hour to get warm, longer to get truly hot. If you warm the room first, then throw on mass, you'll still likely get yourself to the end of the burn it takes to charge the bricks.
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Post by dpottier on Sept 12, 2014 0:39:46 GMT -8
At this time, there isn't concensus about the size of a bell that could be served by a 4" batch box. When I submitted the reply it vanished. Will repost later. |
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