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Post by shilo on Feb 14, 2016 13:06:21 GMT -8
why we used to calculate the proper ISA for a given system size relatively to the CSA of the riser? why not calculate it relatively to the cubic volume of the box? (the amount of wood) |
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Post by peterberg on Feb 15, 2016 4:51:37 GMT -8
Quite simple really, found out by trial and error, or to put it a little more sophisticated, found out by empirical method. My own heater functions properly now with a little bit over 5 m2. This is complicated a bit by the fact that the exhaust opening is quite a bit lower than the firebox. So maybe a bell with the firebox at the same level as the exhaust opening could do with 5.3 m2.
The CSA of a 15 cm system's riser is 177 cm2, the csa of a 20 cm riser is 314 cm2. The ratio between those two is 1.774 and assumimg the ratio of the bell system is the same, a 20 cm system should be 1.774 x 5.3 m2 = 9.4 m2 bell ISA. That are the ratios that appeared to work during the workshops in Montana and North Carolina for a 20 cm system. So, this method could be a good way to calculate the respective internal surface areas. The volume ratio of the respective fireboxes is much larger: about 2.3, the diameter ratio of the risers about 1.3 is much smaller. Take your pick, but I think the riser csa ratio is a simple method to go by. And since I like to keep things simple...
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Post by shilo on Feb 15, 2016 5:17:10 GMT -8
but we don't want the nicer or simpler calculation. we want the real one (if there is a big different). why use 1.3 end not 2.3?
the proper isa is proportional to the energy output of the core. the energy is proportional to the amount of wood that burning at a given period of time.
so if 8" core burn a different amount of wood but do it at the same time, so we need to calculate with the 2.3 factor or we will end with a poor efficiency stove.
were is my mistake?
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Post by peterberg on Feb 15, 2016 5:49:51 GMT -8
You can use whatever method you like or see fit. However, in two instances as I told above, the 9.4 m2 was about the maximum ISA a 20 cm batch box system could drive reliably. A larger ISA could be alleviated by a bypass but that doesn't necessarily result in better efficiency. During the warming up of the chimney the efficiency is disappointingly low. As of now, there isn't a more real calculation until someone (maybe you?) devises it.
By the way, my own 15 cm system is doing between 90% and 95% efficiency on average for a large part of the 103 test runs to date. Single bell, no bypass, not even a shut-off valve. Sounds like the right ISA, isn't it?
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Post by shilo on Feb 15, 2016 6:49:32 GMT -8
it is. I just ask about the other sizes.
and if empirically you find that the CSA is the proper method, I steel wonder why. what is the logic explanation.
and another question- do you think the same is true for energy output calculation?
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Post by matthewwalker on Feb 15, 2016 8:42:56 GMT -8
Because the firebox is the fuel tank and the riser is the cylinder displacement, to use a rough analogy.
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Post by peterberg on Feb 15, 2016 9:13:47 GMT -8
A logic explanation? Someone with better educated brains than mine should tackle that problem, I am in the dark here.
Energy output calculation is quite another matter, that should be done with the weight of the fuel. There is one reason I could think of why the bell doesn't scale up the same way as the energy output which is much higher for a bigger system by the way. That reason could be the volume versus surface area phenomenon. Those aren't distinct factors although it looks like that. The volume will allow the gases to slow down so the wall area is capable of absorbing energy. When the wall area is doubled, the velocity of the gases due to the larger volume is slowed down more than in the smaller bell. That implies the extraction of heat in the larger bell could be much more efficient. Which might be the explanation why the firebox volume is 2.3 times larger and the bell ISA only 1.77 times while keeping the efficiency of heat extraction at a sufficient level.
Just rambling...
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Deleted
Deleted Member
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Post by Deleted on Feb 15, 2016 9:41:36 GMT -8
As I have stated elsewere the CSA of the riser determines the maximal performance of the system at a given time.
The performance is determined by the CSA of the riser and the maximal speed of the gases,
which together determine the maximal mass of the gases that can pass and be burned at a given time.
The possible speed of the gases is determined by the temperature differences within the system
and in the stack to the environment and the stack hight, assumed the resistance in the system is neglible.
Thus it makes sense to calculate the heat extraction area based on the CSA of the riser
and not based on the storage capacity of the firebox.
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Post by shilo on Feb 15, 2016 11:01:04 GMT -8
hmmm
do you think the depth (4B-5.5B) have effect on the proper bell ISA? and on the energy output?
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lawry
Junior Member
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Post by lawry on Feb 15, 2016 12:09:10 GMT -8
But karl a 6" j-tube and 6" batchbox throw out different amounts of energy. The air flow is different. Rate of energy output from the firebox is different. My point being: conditions in the riser are dependent on the conditions in the firebox/feed tube... So the riser just moves the gasses. The riser CSA/batchbox size relationship ensures optimal performance of the machine as discovered by PvdB. I tend to agree with shilo... Energy input (mass of fuel) should determine the sizing of the rest of the heater/cookstoves. Dont ask me how?  Someone needs to tie all the pieces together. |
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Deleted
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Post by Deleted on Feb 16, 2016 6:13:37 GMT -8
A batchbox can create a higher temperature difference in the system
and creates a significantly lower resistance to the gas flow.
If a 8", 10" or 12" batchbox can burn the whole load of wood in exactly
the same time as a 6" batchbox, then the heat extraction area
should be adjusted to the load not CSA, but only then.
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Post by shilo on Feb 16, 2016 9:25:59 GMT -8
"If a 8", 10" or 12" batchbox can burn the whole load of wood in exactly
the same time as a 6" batchbox, then the heat extraction area
should be adjusted to the load not CSA, but only then."
o.k. that my point.
the load burning time is similar
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