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Post by manuel on Nov 27, 2016 11:05:31 GMT -8
Hello all.
Trying to better understand how bell systems work I've read again the info on batchrocket.eu and the sheets I've been compilating. And I do have lots of doubts, but let's go now with mass topic.
I read somewhere on the forum that "350kg of mass for every kw output of the core is good practice".
On a 6" batch box expected to deliver 18kwh it could go up to 6300kg.
On a bench system (flue covered by cob, about 50+50cm) we can expect about 365,5 (Kg/m linear) * 15 (máximum bench without elbows) = 5482,5 kg. Pretty close to the optimum.
But on a hybrid system semi-exposed barrel + bell covered with about 20cm cob I only get 1600 to 2000kg on my calculations.
Any thoughts? Should we expect to need for more firings on the lighter one than on the heavy one? Is there any way to add mass without increasing bell's ISA appart from adding it to the cover? How much cover width could we add before it gets nonsense?
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Post by peterberg on Nov 27, 2016 12:32:53 GMT -8
I simply have my doubts regarding the 350 kg thing. In my view, the output of the heater is dictated by the average temperature of the surface and the area that is emitting radiation. A simple rule of thumb: 0.5kW per m² per hour is generated by an average surface temperature of 55º C. The mass behind it is dictating for the biggest part how many hours this is maintained. A double skin heater (10 cm + 10 cm) is able to store more heat longer because of the break in conduction between inner and outer skin. An old Austrian rule says 100 kg active weight for every hour of emitted heat. A double skin bell of 2500 kg will be nice and warm after 24 hours in normal room temperature.
And then, you can't make heat output out of thin air, twice a day 5 kg is more or less the same as 10 kg in one run, the difference is storing capacity.
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Post by manuel on Nov 27, 2016 13:18:56 GMT -8
I simply have my doubts regarding the 350 kg thing. In my view, the output of the heater is dictated by the average temperature of the surface and the area that is emitting radiation. A simple rule of thumb: 0.5kW per m² per hour is generated by an average surface temperature of 55º C. The mass behind it is dictating for the biggest part how many hours this is maintained. A double skin heater (10 cm + 10 cm) is able to store more heat longer because of the break in conduction between inner and outer skin. An old Austrian rule says 100 kg active weight for every hour of emitted heat. A double skin bell of 2500 kg will be nice and warm after 24 hours in normal room temperature. Thanks a lot Peter. Just too many things I simply don't know about thermal transfer and accumulation. I've been reading different post for hours now... And your answer takes me down to the floor. I'll keep with the Australian rule for now. Yes, that was one point: how much of a cob single skin should be optimum for a good balance between storing and radiating . Let's figure: (i.e. just gessing and I don't know the real numbers) 10cm can keep storing heat for, say, 5h and then the exit flues go up and up because of that. Don't know if I'm saying nonsense here... or I could make the walls so wide it could get to a point the radiation to the house is silly small. By the way, now I do hink I posted the thread on the wrong place of the forum. Don't now how to move it, please, feel free to relocate it where needed. Let's keep reading and thinking about the design..
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Post by ericaus on Nov 28, 2016 11:44:35 GMT -8
Don't give us Australians too much credit here manuel. Peter was referring to an old "Austrian" rule.
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Post by manuel on Nov 28, 2016 13:37:44 GMT -8
Don't give us Australians too much credit here manuel. Peter was referring to an old "Austrian" rule. OOOOPPPPPPPSSSS thanks for the correction ericaus
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