sara ventura

Your concern would be justified in the case of a 1st generation batchrocket build. The Shorty core however, is radically different in this respect. Maximum temperatures won't be at the top of the bell anymore, but opposite the core exit instead. This is better in more than one way, the most obvious one: people tend to live at a lower level than the top of a high bell.

I still struggle to imagine the behavior of the hot air coming from the riser to "open air" inside the bell.
Has it the speed enough to project like a beam to the front wall, and from this first contact with mass ( and heat transfer) bounce and spread upwards?  

Is it l like the air coming from an A/C unit, that you can freeze yourself if you're straight in front of the beam, but move away from there and the air becomes slightly fresh but quiet? I hope I pictured the example correctly : )

Another question, when drawing the airframe and door adapted to my firebox (scaled to fit the size of my bricks), I just have to scale the air inlets accordingly, right?

Have a great spring weekend!

My fault, I've drawn a 20cm exit flue because I'll use a double walled pipe, 15cm exit inside. Also, will have a reducer from 20 to 15 at the base so I simplified the drawing, a lot😅

Noted on the bypass, even better for me, construction wise.
That means the bypass flap will endure maximum temps coming from the core, anyone has experience on how does the flap behave after a long time period?

I'll add a bit more ISA, would you say the best place to add space is between the exit core and facing wall?
To have more space for the gases to move easily to the bench?
Or increase the clear space around exit pipe, wall and core? as it is now I have 4 cm on each side.

Thanks!

Here in jpg so you don't have to download

It's taken longer than expected because spring came early and strong, and the work outside these weeks is keeping me busy.
Plus, thanks to a windstorm, I have wood for the next couple burning seasons, at least!

So, I have my first drawing for the build.
The core will be done according to Peter's drawing, so I've kept it simple on my plan, my drawing skills with CAD are pretty basic.

- Finally, the bypass will be operated through the back wall, because while drawing I realized it's the straightest path.

-As it is now, I can add 2cm more between the exit flue and core and back wall, and I'm at 4,38m ISA.
So I will increase the size of the built to match the 5.3m ISA because I feel it's a little cramped now inside the bell.

-I've extended the heat shield of firebrick to the 4 walls and top, because the different sizes between hard and fire brick would make a nightmare to intertwine.

-The metal frame that will hold the core will rest 5 cm on the opening for the door of the firebox. Four metal legs will support the back of the core.

-Also, I've left the airframe and door out of the drawing to keep it simple.

Do you guys see some possible issues?

I'm gathering materials, and I'll start preparing the field, my idea is to have this built before the summer arrives. Is it necessary to lit the fire to cure the build, or it will be ok if I leave this to the heat that's to come?

Thanks!

I'll add my bit about off-gassing.
I haven't built my mass heater yet, but I've done some research for safety at the workshop.

Any product exposed to heat that produces a smell is off gassing, to keep it simple. So you want to make sure that what you put on top of the bench is completely unharmful for your health. As others have said, stick to natural fibers, not treated with chemicals, because you'll spend long hours on top of this heated cushions.

Around here, you can get untreated wool directly from the shepherds. It's treated like a waste because it has no market value, so you can get it for free, but comes with the work of cleaning it yourself.
Another option you may have available is old wool mattresses.
We're relatively close, so maybe those are options for you too!

How much below the exit riser's port should it be placed? Should the opening of the bypass be facing the floor or the wall?

I used the riser's exit port as reference as I imagine the bypass is best placed close to it, so the hot gases find an exit before going down to the floor and bench.
I know the bypass is placed in the exit pipe, and the shape is a T section, in my case I'll use a butterfly flap that will be operated through the bell's top.

   sara ventura wrote:
   I can squash 20 cm from the top, leave the core aligned with the bench top and lower my ISA to 5'2.
   Will I get away with that ISA and the bypass?


Hard to tell, as long as the chimney is of good quality, it might, since the ISA oversize is now roughly 15%.

From the lowest point of the exit flue to the exterior top: 5 meters with 1 35º turn exiting the bell and another to enter the masonry chimney that the pipe will go through up to  the exterior. Pipe will be double walled.

I am unsure whether or not I mentioned this, but there shouldn't be a wall between the main bell and the bench at all, just a few columns. Try to view the whole of it as a strangely shaped bell.

You mentioned it here earlier, but it's a good thing to keep in mind.
I imagine the inside of the built like the inside of a big cavern in the dark, a large open floor plan and a ceiling at different levels.
In my case there will be a masonry column to divide the 90cm span of the bench and support the bell's wall, and 5 thin metal legs to rise the core.

The accepted bell size for a 150 mm first generation batchrocket is 5.3 m². However, the top combustion rate of a Shorty core is somewhat tuned down, in order to create a much more reliable core. So, for a 150 mm Shorty, that ISA should be 15% less, being 4.5 m².
However... you could have an oversized bell ISA although a bypass is a must in that case.

The bybass was already in my to do list. How much below the exit riser's port should it be placed? Should the opening of the bypass be facing the floor or the wall?

   sara ventura wrote:
   -Space between riser's top and bell's top  ---> I have now 30 cm, could it be reduced to 20cm?


Top gap could be zero, but for safeties' sake, let's say 10 cm or 4" would be sufficient.

That's good news! I can squash 20 cm from the top, leave the core aligned with the bench top and lower my ISA to 5'2.
Will I get away with that ISA and the bypass?

I also like the idea of the overhang of the bench slab, it gives the bench a more comfortable seat if you can bring your heels at a closed angle than 90 degrees from the knees.

So I can move ahead with my plans, will have something drawn this week and keep breaking the building details of the bell as I come across new doubts.

Thank you!

Thanks Peter for the drawing! I'm crunching numbers for the bell again since I realized I was leaving out the top of the bell in my previous calculations...

As it is now, I'm at 6m2 ISA and I need to reduce it a little. Quick questions about internal dimensions :

-Space between riser's top and bell's top  ---> I have now 30 cm, could it be reduced to 20cm?

-Lowering the core 5cm ---> Core 5 cm lower than the bench's top, but still 25 cm above the floor, is it feasible? Would it compromise the gases flow too much?

-Bench height and width ---> Height is now at 30 cm, I can't go lower. Width is at 60 cm, could I make it 50 cm and add this in mass to the exterior to keep 60 cm depth for the bench seat? It could be done either with brick or cob this extra thickness outside? I assume this would lower also the temp of the wall outside, but the idea of keeping the calfs fresher than the rest of the body sounds good to me.

If I can combine these three changes I could lower the ISA to 5'56.

   Peter van den Berg wrote:
   This core type can do very nicely without insulation at all.


Maybe I should explain a bit what happens inside this Shorty core.
In order to obtain complete combustion or close to this state of affairs, the three T's are important. In no particular order: temperature, turbulence and time. Some mainstream knowledge say smoke should disappear as soon as a temperature of 850 ºC (1560 ºF) is reached. There's even a German group by the name of 850 Grad, which is promoting this idea. Of course, nobody specified where that temperature should be reached and accompanied by which amount of turbulence and during which amount of time. Since I've seen numerous times smoke disappearing within 5 to 10 minutes into a burn after a cold start, I started wondering. The numbers doesn't look like fixed figures, it could be that the figures are at least partly interchangeable. Maybe, with an awful lot of turbulence induced both the other two T's, time and temperature, could be smaller.

I get lost with the specifics of the science, but I get the general idea.
So I could go for a core made all from firebrick, without covering it with ceramic blanket to keep the maximum heat in, and still have a clean combustion that guarantees the optimal functioning of the heater.

I used what I thought was the best "insulating fire brick" I could get my hands on in the states. And after a few years these have started to break down, while the hard fire brick has NOT SHOWN any problems. (same stove)

Thanks Scott for your input. It's actually helpful because the main reason I was going with IFB was longevity, the not needing extra insulation outside was a bonus (for me).
And considering IFB are the most expensive type of brick from the three I was sourcing for the build, it's a great experience to hear.

I keep drawing the bell, let's see if I can come up with a functioning bench in the end.

Thanks for all your time and experience!

That's good news, indeed! IFB are 1€ plus per piece than Firebrick😅

I am unsure what type of insulating firebricks you have there. Better to use just hard firebricks for the entire core. The riser is the part that's heat stressed most of all, especially the liner in the lower half. Thin strips of insulating firebrick, I feel scary about its durability.

I have located a high temp materials factory that offers:

Insulating firebrick G-26 -
Dimensions: 230 x 114 x 64 mm
Maximum temperature: 1430ºC
Alumina: 56%
Characteristics:
Excellent insulating qualities.
Mechanical and temperature resistance superior to group 23 bricks.
High purity.
Thermal shock resistance.

Firebrick AL40-
Dimensions: 230 x 114 x 64 / 76 / 20 / 30 / 40 mm
Maximum temperature: 1420ºC
Alumina: 39-41%
Characteristics:
High purity.
Low iron content.
Good mechanical resistance.
Excellent resistance to thermal shock.

I'm trying to avoid ceramic wool as much as possible, I was going with the insulated firebrick for the riser, so I don't have to add extra insulation outside. Keeping the ceramic blanket at minimum when directly exposed to gases.

I"ve had a look at your drawing of the core, and question arose. The depth of the firebox, is that a fixed figure because of the bell depth?

I've drawn 50 cm because the depth is the lenght that has more tolerance, and I'm thinking in the type of wood I'll be using.
Most of the times the horitzontal cuts of the wood are at an angle, length is always irregular. So extra depth to make sure I can fit any kind of cuts in the box load.