Peter van den Berg

Leonardo Bevilacqua wrote:I get the point of not exceeding the ISA determined by the size of the core. But, are there any other proportions that need to be respected for the bell? What about the relation between the size/placement of the core and the size/shape of the bell?

The core could be placed almost anywhere in the bell. Even with one side against the bell's wall will work. The shape of the bell is largely irrelevant, a flat ceiling is better than one with steps, the gases will stream more freely like that.

Leonardo Bevilacqua wrote:Would it work if I make a bell which is long an tall but wide just enough to fit a core inside? Or should there be a minimum space between the core and the bell?

The bell could be deep and tall, no problem. Down to just wide enough to house the core, although in that case there should be at least a space at the back. Size of that space: at the bare minimum, 5 times the cross section area of the stove pipe, more being better. That space is there to lead the gases down to the bell exhaust.

Just a tip: place the core higher in the bell, very convenient for loading. At the same time there will be an empty space below the core, where all the fine dust will settle. And as a bonus: there will be much more options available for the exhaust pipe's position.
The core shouldn't touch the walls of the bell, though, one layer of 12 mm superwool would be absolutely adequate.

OK, I think it would work, also. What you need here is a smooth ceiling, as Glenn suggested. Keep in mind that it will work best with more space where the gases are forced around bends. Looking at it, I would try to fill up the sharp corner where the channel widens, just before the chimney base.

Another way (in a cul-de-sac) to prevent the gases shortcutting to the chimney is this: the entrance to the bench as high as doable, the exit to the chimney much lower. Hot gases are streaming into the bench at the highest possible level, so in order to get the colder gases into the chimney is lowering the entrance the way to go.

There's another effect with this open construction: while starting up cold, the temperature difference between top and bottom in the bench is neglectable. So at first, the gases tend to take the shortest route. Later on, the hotter gases are rising by natural buoyancy and driving the colder gases out. I've seen this behaviour a couple of times, in different constructions. It's rather counter-intuitive to see it happening, made visable by two thermocouples and a digital thermometer. Works like an automated bypass, so to speak.

Hi Leonardo, I received your PM about a month ago, and I was convinced I answered it. As it turns out, I didn't, sorry for that. Sometimes I miss one, this time of year is always the busiest.

A 180 mm system would heat the space. A double skin will hold the heat longer, that's true, although the external temperature will be somewhat lower as compared to a single skin item. Start with a drawing of the bell and how the core is positioned in there. Having the stove pipe at the side of the heater and the connection plus bypass like the heater in The Hague will result in less friction and a heater that will start cold and wet without problems.
The upper part of the bell in refractory bricks isn't necessary for a Shorty core, just the wall where the core exhaust is pointing at will be sufficient.

Regarding your question about coming to Italy and help with the build, I have to decline. I will turn 80 years of age in March, ice and weather permitting. Whether or not I will be able to travel to Italy and act as a build director remains to be seen. But, let's don't get ahead of time, start with planning first.

I fully agree with Julian, his suggestion for placement is much better.

Austin Shackles wrote:If I just build the bench on the floor as-is, the bottom of the chamber inside will be 5 or 6 inches (12-15cm) below the bottom of the outlet ports in the lower part of the stove body.  So the question is this:  is that extra space below the level of the stove going to adversely affect it?  Secondary question, is it actually going to do any good, or would I be better off filling it up to near-stove-base level with, say, some gravel or something?

* FSVO "soon"

I'm not sure what type of bench it will be.
In case it's a dead end bench or cul-de-sac there are a couple of terms and conditions in order to let it work well.
The floor level of the bell needs to be the same as the bench. The opening between bell and bench needs to be as wide and high as the inside of the bench. No thresholds or higher ridges at the top, just the shape of a smooth tunnel is ideal.

Reason for these conditions: while the hot (or warm, for that matter) gases are streaming into the bench it's displacing the colder gases at the bottom. In order to let the process to go smoothly, there shouldn't be a difference in level. The same goes for the top of the bench, against the seat. When the opening into the main bell is lower or narrower, the laminar stream is disrupted and creates a messy pattern. Which in turn will hinder the return stream at the bottom.

Most (if not all) of these conditions aren't valid for a bench that have the exhaust at the far end. Could you provide a sketch picturing what the current situation is?

Benjamin Dinkel wrote:And if the core isn’t within a bell the core surface counts as well. And a barrel obviously counts too.

No, the core surface won't count as part of the ISA. Internal walls and ceiling of the bell is what we are talking about, not the outside of whatever part is sticking out of it. A barrel is also part of the ISA, that's true.

What I use as the simplest calculating method: count all the walls and ceiling as if the core wasn't there at all.
The core, when in the bell will extract heat at first but radiate out again later. When the core is sticking out there need to be something around it, otherwise it will be much too hot. Most of the time there will be a second wall around the core to dampen the heat, in that situation.

Glenn Herbert wrote:A J-tube with natural draft not needing the chimney warmed to burn well, I believe, could work fine in a larger bell while taking longer to store as much heat. The ISA of my bell is irrelevant to the early functioning, when any size bell would still be cold.

Forgive me Glenn, for not mentioning why the above statement won't hold, so here we go.
Of course you are entitled to believe whatever you like.
But... there is a certain effect that is firmly based on physics, the kind that won't be influenced by faith. That effect is mostly referred to as "chimney stall". About +/- 20 minutes into the burn, the exhaust gases into the chimney need to be warmer than 60 ºC (140 ºF), otherwise the chimney draw will cease to exist and all smoke will stream into the house. What I mean with temperature measurement, is done in the very center of the chimney pipe, where the stream has its highest temperature and velocity.

I stumbled upon this phenomenon many years ago and it took a lot of time to understand what the hell was happening. As you may know, combustion of woody material will produce heat (obviously), CO² and water vapor. Quite a lot of the latter, about half a liter of liquid water for every kilogram of bone dry fuel. Translated in imperial measuments: 30.5 qubic inches of water for every 2.2 lbs of dry fuel. As such, it is a by-product of the combustion process, much like natural gas. When the fuel wasn't as dry to begin with, this water content will be added to what is going into the chimney.

For now, we concentrate on the water vapor. This will be sent into the chimney and when the temperature is low enough, something between 40 and 50 ºC (104 and 122 ºF), the vapor will condensate on the chimney wall into liquid water and runs down. Lower in the chimney it's warmer, so the water evaporates again and is added to the vapor that's already there. So it rises into the chimney, but since the gasses are more saturated with water vapor now, it will condensate in an earlier state and lower in the chimney so it runs down again. This process will be repeated over and over again, consuming more and more heat, until there's no more heat to carry the vapor to the outdoors and the chimney will reach the state what we call "stall". No more draw, all smoke and water vapor is streaming into the house.
Sometimes, the stall can be deminish by itself and the draw seems to be restored. But in almost all cases, within minutes the chimney stall shows up again.

If you like to check the above explanation, extend your bell by 100%, start the thing up stone cold and watch what happens.

Hmmm... In October 2015, I started out with a 6" batchrocket and a bell of 64.6 sq.ft (6m²). The chimney being 6" (15cm) diameter, insulated and straight up. It was a complete disaster, the darn thing refused to play ball for two weeks straight. I brought it down to 53.8 sq ft (5m²) and it was managable, it was willing to work. Just before this 11th heating season I brought the ISA up to 57 sq ft (5.3m²), not accidentally the accepted maximum ISA, and it still worked. Although I had to be very careful not to rush a cold heater into full burn. This particular heater doesn't sport a bypass, by the way.

So, you are saying that your 8" J-tube is coupled to a 50 sq ft bell and works well. I think you are right here, this could be a little bit larger and still running well, in my opinion. But I highly doubt this same J-tube core would run as well with a bell with an ISA that's double what you have, 101 sq ft (9.4m²) to be precise. That's the maximum ISA size an 8" 1st generation batchrocket could serve without running into problems.

What I meant to say is this: in about 13 years, time and again the J-tube proved to be about half as powerful as the 1st gen. batchrocket for the same size. This isn't speculative, or an opinion, just hard numbers and the result of literally hundreds of experiments.

Glenn Herbert wrote:A J-tube the same system size as a batch box will eventually heat up the same bell fully. How long it would take depends on the specifics of the situation.

Not quite correct, sorry about that. Having a J-tube of the same system size as a batchrocket doesn't mean they have the same power output. Which means the batchrocket bell is grossly oversized when combined with the J-tube. Without a bypass of some kind, the J-tube won't come up to clean burning temperature at all.

Been there, done that, learned from it.

Scott Weinberg wrote:Please read-  This is only my experince, I am not suggesting or implying your results will be the same.  But without gages, recording, and study, it would only be a great working warm stove.  now it is a on going experiment daily.  

I fully agree. Point is, the guys and girls of the shop have other things to do than to try to get "the most out of it". The place isn't mine, the heater isn't mine, as soon as the work was done it was out of my eager hands. That said, I built up a lot of goodwill, and I am very welcome to bring visitors with me to show the heater, offer them coffee and a chair to "sit by the fire" with me as the proverbial storyteller...

tony uljee wrote:so i can only hope that this will be noticed by other interested companies/customers----who can get Peter to build another stove ---or at least under his guidance-----thankyou  Peter.

Your welcome. Tony. In the mean time, there is a another heater built under my guidance, I am convinced you didn't miss that one. See the  following heater report, which obviously (or at least I hope it does!) bears my signature. This guy did the design himself, I only did steer him in the right direction a number of times.
https://permies.com/t/364721/Transforming-Fireplace-Splendid-Rocket-Mass