Armchairing and maybe building a rocket heater

[Cramer]

Not entirely. A double bell system is about 15% more efficient as compared to a single bell setup. So the total ISA of the bells should be smaller.

Does the surface of the divider between the 1st and 2nd bell contribute to the ISA then or only the exterior walls?

[peterberg]

Yes, the divider wall or ceiling/floor between two bells do contribute to the ISA. It's all mass inside the stove, fed by the surface that's exposed to hot gases. Eventually, all the heat inside the bell will radiate out, seeking equilibrium with the surroundings. The divider wall will be "full" earlier though, because it's exposed to the heat on more than one side.

[Cramer]

Yes, the divider wall or ceiling/floor between two bells do contribute to the ISA. It's all mass inside the stove, fed by the surface that's exposed to hot gases. Eventually, all the heat inside the bell will radiate out, seeking equilibrium with the surroundings. The divider wall will be "full" earlier though, because it's exposed to the heat on more than one side.

Excellent! I believe my single bell 6 inch combination just became a double (or maybe triple) bell 8 inch! Since temperatures are successively lower from bell 1, to bell 2 and then finally to bell three there will be variances in the surface temperatures which I can put to use in my application!  Is a three bell combination for an 8 inch burner feasible or would a double bell be the limit for a J Tube?  If a J tube will only support a double bell then using previously learned parameters I would assume a batch box would easily support a triple bell.

To further understand the concept of ISA, if (as in the attached) there was a freestanding wall open at the top instead of at the bottom so that the gasses could stratify around the mass, would both sides of this wall figure in to the ISA calculation?

freestanding wall.skp (16.47 KB)

Previously it was mentioned that a double bell was approximately 15% more efficient than a single bell.  This sort of bolsters my thought that ISA is connected to adsorption (a surface phenomenon) rather than absorption (which would concern the entire mass of a material).  If that is indeed the case then it would simplify things greatly. The 15% increase in efficiency would then translate to the surface area of one side of the dividing wall between the two bells, which would be the side facing the chamber where the hot gasses originate but in the case of something like the attached drawing BOTH sides would be considered in the calculation of the total ISA.

[peterberg]

Dec 21, 2013 6:00:45 GMT -8 Cramer said:

Since temperatures are successively lower from bell 1, to bell 2 and then finally to bell three there will be variances in the surface temperatures which I can put to use in my application! Is a three bell combination for an 8 inch burner feasible or would a double bell be the limit for a J Tube? If a J tube will only support a double bell then using previously learned parameters I would assume a batch box would easily support a triple bell.

A batch box is able to support a triple bell system, definitely. Don't know about a J-tube though, I've never built such a large one.

Dec 21, 2013 6:00:45 GMT -8 Cramer said:

To further understand the concept of ISA, if (as in the attached) there was a freestanding wall open at the top instead of at the bottom so that the gasses could stratify around the mass, would both sides of this wall figure in to the ISA calculation?

OK, I'd view the file now. That deviding wall belongs to the ISA calculation, yes. However, it's not necessary to have a devider like that open at the top. Usually, such a wall is a support for the top plates. As long as the exhaust opening is very close to the floor the whole bell will get warm. Even when the exhaust is quite close to the inlet opening provided there's enough space around. I'm assuming the round pipe at the bottom of this bell is the inlet side of things, but where's the exhaust located?

Dec 21, 2013 6:00:45 GMT -8 Cramer said:

Previously it was mentioned that a double bell was approximately 15% more efficient than a single bell. This sort of bolsters my thought that ISA is connected to adsorption (a surface phenomenon) rather than absorption (which would concern the entire mass of a material). If that is indeed the case then it would simplify things greatly. The 15% increase in efficiency would then translate to the surface area of one side of the dividing wall between the two bells, which would be the side facing the chamber where the hot gasses originate but in the case of something like the attached drawing BOTH sides would be considered in the calculation of the total ISA.

Probably adsorption is playing an important role here but I don't know enough about these phenomenon to say whether it's crucial or not. The ISA calculation however, should be done in a fairly straightforward manner. Calculate every area except the floor of the bell, together these should be a little less than 6m2 (64.6 sq ft) for a 15cm (6") system. When the system does consist of two (or more) bells distract 15% of that 6m2 for every bell extra. These are not hard numbers, there ought to be some tolerance in there. Variables like chimney quality, elevation above sea level and the used materials do play a role here.

You are asking tough questions, sometimes I'm not sure I understand those completely.

[Cramer]

However, it's not necessary to have a divider like that open at the top. Usually, such a wall is a support for the top plates. //  but where's the exhaust located?

The drawing was intended as a tool for further understanding of the concept, I would not see actually doing such a thing would have any real practical purpose. except maybe for shrinking a bell to achieve a smaller footprint while still maintaining a single mass of stratified gasses.  I did not locate an exhaust outlet because in this case it did not matter where it was... and yes, the round bit was intended to represent the inlet side of things . 

You are asking tough questions, sometimes I'm not sure I understand those completely.

Actually, I believe you are understanding them quite well as you have provided the answers I was seeking!  Thank you sir!