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...
Last Edit: Feb 15, 2016 4:53:22 GMT -8 by peterberg
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?
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?
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?
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.
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.
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.
"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
maartenmartens: beste Peter, ik heb je eergisteren een mail gestuurd via het contact formulier op je website , heb je die goed ontvangen ? mvg Maarten Martens, architect - geobioloog (mail betreffende de bouw van onze eigen RMH)
Jun 20, 2018 13:21:57 GMT -8
martinm: any one here with info \ experience with heat (from a mass heater) distribution with ducts throughout two storey house ?
Sept 8, 2018 22:58:52 GMT -8
padica: Good morning, this is a wonderful subject, please someone can help me with the theme of double and triple combustion, how it is achieved, theory and design, thank you
Sept 15, 2018 7:13:40 GMT -8
wiscojames: I'm afraid you won't get a response to such a vague question - I suggest reading through some of the threads related to your questions before asking for an explanation. People will be very generous with their knowledge if your question is more specific.
Sept 18, 2018 4:48:00 GMT -8
daniel: in my experience as I am working now on something of that nature, I have thought of making a heat exchanger and distributing the heat through vents using a slower fan. Now after a few years I realize that mass heaters give out primarily radiant heat, for
Nov 30, 2018 12:48:38 GMT -8
coastalrocketeer: Anyone posting in this shout box... if you have something you want people to see and respond to, create a thread in the appropriate forum section... this is not the place...
Dec 16, 2018 18:10:49 GMT -8
vesuvius: High Temperature Glass options,
Jan 4, 2019 16:28:17 GMT -8
vesuvius: Do any of you have experience with using the glass from home oven doors on a rocket stove? I'd like to have a viewing port on my stove but don't want to fork out for new ceramic glass. Any thought as to whether it would take the heat of a rocket stove?
Jan 4, 2019 16:30:40 GMT -8
yaya: you dont need the blowair for that temp..
Jan 13, 2019 16:43:57 GMT -8
flybywire: Glass window from an old washing machine is designed to take high temps. Cheap solution to your need.
Jan 27, 2019 0:10:58 GMT -8
coastalrocketeer: Anyone posting in this shout box... if you have something you want people to see and respond to, create a thread in the appropriate forum section... this is not the place to have a discussion... it is for short announcements
Feb 7, 2019 0:11:23 GMT -8
coastalrocketeer: you won’t likely get replies to questions here, and it is not a spot that makes holding an ongoing discussion possible...
Feb 7, 2019 0:12:22 GMT -8
TexasGonzo: Sooooo glad I found this site! Its always rewarding to find such a super group of folks! To any and all, feel free to PM me anytime. Thanks for having me!
Mar 11, 2019 18:56:41 GMT -8
jlmtech: GADGET: CONSIDER USING A JET PUMP INSTEAD OF A BLOWER FAN TO INDUCE DRAFT; NO CLOGGING.
Mar 26, 2019 8:19:28 GMT -8
michaelegan: i am unable to open the sketchup files on my mac. I used sketchup a few years back but apparently the company now requires a subscription. does anyone have any advice/instructions on how to use the program or how to view pictures without spending money?
Aug 20, 2019 18:41:48 GMT -8
mannytheseacow: michaelegan: download AutoCAD student version for free... import .SKP
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topbaza: hi everyone, been searching all over net and this is were i need to be i think!!
Sept 28, 2019 6:16:25 GMT -8
anounaki: Hi, why I cant upload photos when I make new tread to this forum?
Feb 28, 2020 2:09:40 GMT -8