Peter van den Berg

sara ventura wrote:As it is now, I'm at 6m2 ISA and I need to reduce it a little.

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.
Don't worry, please read on.

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.

sara ventura wrote:-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?

Better to have the core not lower than the bench, also construction would be much more logical.

sara ventura wrote:-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.

The slabs for the bench could be 60 cm, sticking out on the outer side. Internally, the bench could then be 50 cm or even less. Making the wall of the bench beefier won't help much, the seat will be the warmest anyway.

sara ventura wrote: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.

Correct.

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. Those columns are there for to support the slabs of the bench' seat, while at the same time the wall of the bell above the bench is resting on the slabs. For how to construct this, see my article about the French Bench.

Gerry Parent wrote:I was under the impression that a person could cheat and use a fan and/or a heater to decrease the amount of time before a smokeless fire is achieved.
If so, wouldn't using such devices be able to prove that these numbers are not fixed but are interchangable as you mention?

I am inclined to say you are right, good thinking. But the combustion system would change from an atmospheric aspirated device (is that the right term?) to a blown combustion system, like a wood gasifier. Food for thought, no doubt about it.

OK, the drawing is ready. I couldn't find a good example of the door assembly, I will look for it. The whole of the core is a little bit larger, although it will run without any problem on a 150 mm chimney diameter.

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

Using hard firebricks seems to be the logical way to go, then.

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 interchangable. Maybe, with an awful lot of turbulence induced both the other two T's, time and temperature, could be smaller.

Actually, I am convinced this could be the case, although I am unable to prove it. A young master heater builder from Germany visited me last year, and he was very surprised my Testo gas analizer showed such low CO numbers after 5 minutes into the burn of a cold Pepper Shaker heater. At the same time, the temperature in the afterburner area couldn't have reached at least 850 degrees C, far from it.
Interesting line of thought, don't you think?

sara ventura wrote: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.

In that case, I've got news for you. This core type can do very nicely without insulation at all.

sara ventura wrote:I've read somewhere that a 20% increase is the usual to add to the amount needed of bricks, is that correct?

That could be correct, or not, depending on what you are building. Better to count the bricks individually when the drawing is finalized.

sara ventura wrote:Also, I've realized while drawing I forgot there are 2 types of brick, not sure that I can lay both between them as I've done? Different expansion rates, perhaps?

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.

sara ventura wrote:I've drawn the top's firebox using the refractory slab G-23, which leaves three joints on the top. Would it be better to have a single piece instead?

Not necessarily, it could be done in two pieces as well, three is also possible.

sara ventura wrote:And the single piece that doubles as top exit riser's port and base of the riser's top, should be from the same material as the rest of the riser? Can I use a cut from the G-23 slab?

Yes, you can, no problem.
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?

thomas rubino wrote:Thank You Peter, from the rocket mass heater community!

<blush on>Thank you for the compliment, glad to be of help.<blush off>

T Cool wrote:In my opinion a pönttöuuni is a 2 bell stove with the second bell very inefficient, being too small!

I fail to see any bell in a pönttöuuni, let alone two bells. It's just an old-fashioned contraflow heater, most of the time with the upstream channel built-in.

T Cool wrote:I read also an opinion of Vladimir Efimovich Grum-Grzhimailo (Владимир Ефимович Грум-Гржимайло)  regarding the transformation of contraflow stove in FMG one, if it is of interest I can post the in link.

According to that article you need to add some pillars inside of Pepper Shakers! More mass practically.

No, I am certainly NOT needing to add some pillars inside the Pepper Shakers. First, there's no space for those, second, no need for it. The pillars are a traditional feature of the Russian heaters, just because they are specifically needed for to close the top of the heater with their large bricks.

Now, you are searching the internet for old drawings and old articles. Once you've found one or more, you are trying to convince me that I should adapt my design accordingly. Let me tell you, I won't.
I tried pillars in a bell as long ago as 25 years, used the dry seams promoted by Igor Kuznetsov, a separation baffle in a dead-end bench, double bells in different configurations and whatnot else. All of those details didn't add any value but complexity instead and some had a negative effect.

I have a briljant suggestion: you are the best person to build these different ideas yourself! Buy one of those expensive Testo analizers and lots of materials, take leave from paid work for, say, 15 years, start experimenting and publish your findings here. Doesn't sound this as a splendid idea?

Cerbu Ulea wrote:hello, I discovered that only the meter that portrudes the roof is 17 /17 cm , the upward chimney is about 190 mm square does that change the math  of the stove? there is no need to go larger , the actual 165 is very  good

This won't change the math of the heater, only the narrowest portion is the one to go for. How's the burn season so far?