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Post by satamax on Dec 25, 2014 1:25:27 GMT -8
Hi everybody!
Well
In my understanding, the bigger the volume of a bell; for a given isa; the longer the time gases have to exchange their heat with the surounding materials.
So in theory, the best bell would be a sphere? Then a cylinder. Then a cube. Then a cuboid. I'm not sure about the last two difering much!
So is there any intrest in reducing the volume of a bell. Like making a smaller bell with brick ridges to increase it's isa? For example.
Then in bell sizing, we know that a metal bell, shedding more heat, has to be smaller.
So a bigger volume and isa could be atained by making an insulated bell.
Then, where to stop? Ok, max size of an insulated bell, could be given by practicality.
But does anybody has an idea about the point at which it would be counterproductive to insulate; because the radiating energy would reheat the inside gases, instead of letting the energy to be transfered to the outside of the bell.
Any comment guys?
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Post by shilo on Dec 25, 2014 10:18:32 GMT -8
you don't want an inulated bell. you want it to radiate heat to the room
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morticcio
Full Member
"The problem with internet quotes is that you can't always depend on their accuracy" - Aristotle
Posts: 371
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Post by morticcio on Jan 7, 2015 8:29:01 GMT -8
@ Shilo... You may want to insulate part of the bell to limit the rate the heat radiates into the room or to keep the heat inside the bell if you have a heat exchanger in there.
@ Max... The smaller volume bell with the greater ISA is an interesting idea. A larger bell could result in lower exit temperatures causing the stove to stall due to lack of draught.
I don't have an answer to your question about the point at which it would be counterproductive to insulate but there must be a point where the mass gets 'saturated' and it would be more efficient to divert the heat to second bell.
I encounter a similar problem on my solar hot water system. The 2 panels heat the water in the 750 litre tank via 2 heat exchanger coils. There is a diverter valve between the 2 coils to help with stratification, loading the top coil first and then the lower coil. Heating the whole tank up from 15°C to 65°C is relatively easy - two days of decent sunshine (in Italy in the summer, not the UK!). Getting it up to 75°C takes another day. I could get it above 75°C but the higher temperatures stress the system. I would be better off adding another tank and diverting the solar circuit to heat this as and when the bottom of the primary tank reaches 70°C for example.
Using this example you could have 2 identical bells built side by side with the rocket stove in the middle. When the first bell reaches a certain temperature, lets call this the 'saturation point', a manual change-over flap diverts the hot gases from the rocket stove into, & opens the path to the /flue chimney from, the second bell. At the same time the first bell is effectively sealed as the heat source and chimney are closed.
This isn't a double bell, more of a mirrored bell - think RAID1 :-). This way the 'optimal' ISA is maintained and the exit temperature is high enough to maintain an efficient draught.
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Post by shilo on Jan 7, 2015 9:24:55 GMT -8
you can do a double bell with bypass over the second bell. When the first bell reaches a certain temperature, lets call this the 'saturation point', a manual change-over flap diverts the hot gases from the by pass to the second bell/
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gjh42
Junior Member
Posts: 61
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Post by gjh42 on Jan 7, 2015 11:39:43 GMT -8
That sounds reasonable. When the first bell reaches "saturation temperature" it is not absorbing so much heat from the gases, so adding the second bell to the airflow does not increase the effective ISA beyond specs (when sized correctly, whatever that turns out to be with experimentation). I would suspect that the second bell should be smaller in this scenario, as the first bell will still be absorbing some heat.
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morticcio
Full Member
"The problem with internet quotes is that you can't always depend on their accuracy" - Aristotle
Posts: 371
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Post by morticcio on Jan 7, 2015 12:05:17 GMT -8
Maybe we need to define exactly what a 'double bell' is.
It is my understanding that a double bell consists of two bells (or chambers) linked together with the flue gases running through the first bell, then the second bell and then out through a chimney. The ISA of a double bell is calculated by adding the ISA of both bells together (less the floor area of each bell). The second bell is usually smaller. Double Bell Gas Flow Heat source --> First bell --> Second bell --> Chimney --> Atmosphere
What I had in mind with 'mirrored bells' is that each bell is treated as a separate bell/mass but fed from the same heat source. They are not linked together. The gas path flows through either the first bell or the second but not both together. The second bell doesn't need to be smaller or even the same size/shape or on the same level. Both however should be the optimal ISA for the rocket stove providing the heat source. You could have mirrored double bells.
Mirrored Bell Gas Flow Heat source --> First bell --> Chimney --> Atmosphere or (via a change-over flap) Heat source --> Second bell --> Chimney --> Atmosphere
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Post by ericvw on Jan 7, 2015 15:04:48 GMT -8
I call for a sketch, Morticcio! Eric VW
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