Peter van den Berg

sara ventura wrote: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.

Ahhh... I get it now. The intake end of the T to the side, away from the core. Height placement in your case, about the level of the firebox' deck. In other words, somewhat lower than the core's exit opening.

sara ventura wrote: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.

OK, although the bell is oversized for a Shorty core, you can get away with it since you've got an adequate chimney and a bypass. Chances are that you have to run the heater with the bypass open a crack permanently, especially during fall and spring. As long as the build is neatly done, it'll work as it should.

sara ventura wrote: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?

What you wrote there is confusing me. Somehow, I get the impression that you think a bypass is a valve in the core itself. Just to be sure: it isn't, it's an extra opening between the bell and the chimney. Opening the valve in that opening is bypassing part of the bell, so that the chimney heats up much quicker. So, the position of the bypass could be halfway the height of the bell or somewhat higher.
The opening should face the core or the wall.

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%.

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>