Peter van den Berg

sara ventura wrote:-I was under the impression that concrete should be avoided on areas of highest temperatures, but here the pavers are right into the path of the gases exiting the core. Will this affect its longevity?

There aren't pavers in the path of the hottest gases, but firebricks instead, five to be precise. In my second or third post I wrote this:

Since the core spews the hottest gases forward instead of upward, only a limited number of firebricks are needed directly above the firebox to protect it.

So your question was already answered, please read carefully.

sara ventura wrote:-Have you calculated the minimum area around the core to ensure optimal gas flow?

Yes, of course I did. Forgot the exact numbers since it's concept but it's sufficient, no doubt about.

sara ventura wrote:Do you think this design would allow down scaling? It's great for a workshop, but in my experience around here, space in workshops is always scarce.

I'm inclined to think the core and bell can't be scaled down any further. The firebox would become rather impractical, only very small fuel could be used.

sara ventura wrote:In Catalan architecture vaults are a thing, so I'm used to thinking that it's a shape that offers long time durability. But it's true Gaudi did never build masonry stoves, it makes sense that with high temps and expansion we're talking of a totally different game.

Catalan vaults are awsome, I've a book in my library (ahem!) about those.
What I recommend is exhaustively tested and proven in daily use. When I am unsure about some aspect, I'll tell you that.

sara ventura wrote:Two first questions:
-Why is there a second door on top of the firebox on both designs?

Both the DSR2 and DSR3 could have a second door above the firebox. Behind that second door is the afterburner space, in the Shorty core this same function is in the short riser, without the possibility of a door.

sara ventura wrote:-In this design, the top of the bell and front wall are both lined with IBF to protect from direct heat, right?

Thomas did go a long way to build his single skin heater containing a Shorty core, out of mostly normal red bricks. Since then, I designed a large 200 mm system heater in The Sculptor's Shop, located in my home city. And I designed a small(ish) heater lined with concrete paving bricks inside. Both heaters only have a firebrick shield at the side where the highest heat is expected, directly in front of the core's exhaust. No insulating firebricks, at all. And as a completely different heater: the French Bench, built as a proof of concept in France.

The Sculptor's Shop build is remarkable, in the sense it is fired all day long, without overheating or excessive chimney temperatures. No problems so far, the same goes for the smallish 900 kg Pepper Shaker in my workshop, which is fired every day once or twice the past four months or so. There's a series production on its way of the latter one.

sara ventura wrote:The other is about the possibility to finish the top of the bell with an arch instead of a flat roof. Leaving aside the technical difficulty of making a vault, would this construction offer advantages on long term durability and performance?

A vault isn't better in long time durability, sorry about that. The short story: every vault is bound to have a horizontal pressure force, most of the time countered by a ring of steel and/or large pieces of wall on both sides. To make matters complicated, applying heat will cause the bricks to expand, so the horizontal force will be increased and decreased following the burning cycle. Not good for a build that is meant to be as stable as a rock, bricks may start to wander.

sara ventura wrote:-With the ISA I gain from shortening the bench, I could make the bell a bit higher. That would allow me to rise the core a bit further from the floor, not as much as to align with the ceiling of the bench, but closer.

Hmmm... What I feel now we are coming closer to a newer design, the Shorty core. This will allow to have a lower bell, higher raised core and aligning the seat with the bottom of the core.

sara ventura wrote:- Having a higher bell, I want to keep it as slim as possible. In order to do that, would it be possible to leave part of the core outside the bell? How does it work, construction wise? Is there any thread around here where this has been done?

It has been done before, not very succesful I have to say. The core itself will get awfully hot, so you have to build something around and over it. Even then, there will be a difference in temperature between the protuding core and the bell's wall. I won't recommend this construction.

sara ventura wrote:-And the last twist in design would be to forget about the cob render and clad the whole build once finished with glazed tiles. The house is poor in natural light and I have to finish the build in light colors. Would refractory mortar be necessary for bonding? This way I could have a render 5cm thinner than with the cob.
What are the contraindications with tiling, besides cracking?

A cob finish of 5 cm (2") or more looks like quite a bit. When clay/sand plaster is used, the layer would be much thinner. Applied in two layers, a shrink netting embedded in the first. A finish layer in a light color and you have your white heater.
As for tiles, of course you could do that. But you'll need high-temperature silicon caulk for that, otherwise the expansion of the bricks will cause them to fall off. The brickwork underneath the tiles should be very flat and straight, maybe even plastered thinly.

Did you see the heater of the guy from a valley in nortern Italy? That would be a good one to follow, not the shape as such but the core and the build-up.

sara ventura wrote:I want to start the fire on the days that inside the house is colder than outdoors.

That's asking for trouble, especially without a bypass. In order to have updraft, the temperature outside need to be LOWER than inside.

sara ventura wrote:The floor of the core aligned with the top of the bench? So the temp of the core doesn't mess with the cool gases going down and find the exit, yes?

Partly correct. I was refering to the movement of the gases, not so much the temp of the core. The gases are streaming under the core and into the bench quite freely, coming back along the floor and to the exhaust pipe, provided the core is at the level of the bench.

sara ventura wrote:But that will rise my top of the bell too, thus increasing my ISA again, no? 1m ISA would be a square bench of 50 cm square right?

A square with sides of 50 cm is 2500 cm², equal to 0.25 m², as opposed to 1 m². The seat plus the three closed sides of the bench should be in the calculation, all internal. Make a drawing and calculate the different surfaces, excluding the floor in its entirety.

sara ventura wrote:I'm having trouble imagining any other proportions that would work as a relatively comfortable seat for one person/dog.

I'm assuming you are talking about the outside measurements of the bench, please stick to inside, as long as ISA is in play.

sara ventura wrote:Actually for the core, today my father suggested the possibility to do tongue and groove construction. Is there a reason I haven't seen any so far in the forum, other than the obvious extra work of carving the bricks? We have a wet saw and an skilled brick layer here.

You haven't seen any of this tongue-and-groove in the forum, because it's a redicously amount of work without much benefits, plus you'll need more of the expensive firebricks.

sara ventura wrote:The firebrick we'll be using for the riser will be AL 79-82% 1550ºC,  with wire and a band of superwool for movement allowance and protection I'll be good to go?

I said: some mortar, not without mortar. Otherwise, keeping all bricks dry stacked and applying superwool and wire, you'll probably end up with something that feels like a bucket full of eel, every brick wants to slither somewhere else.

sara ventura wrote:About that, we've opened a can of worms with my dad, will be making new sketches next week with some (probably) crazy ideas, you'all'll let me know!

Sara dear, you speak about your dad as a masonry heater builder? In that case, he on the one hand and me on the other, seems to be involved in a competition. I know how that works, it won't, been there, done that. Better having your dad advising and overseeing the build then.

sara ventura wrote:And to recapitulate, which are the highest temps expected at this points:

- Inside Firebox -
-Inside Riser -
-Inside Top bell -
-Inside Top Bench -
-Exit pipe -
- Optimal fumes temp at exit? -

I've measured some of the above, about others I can do an educated guess, hereafter given as estimated.

- Inside Firebox - 900 ºC
-Inside Riser - 1150 ºC, tops, exit riser 900 ºC.
-Inside Top bell - estimated 500 ºC.
-Inside Top Bench - estimated 200 ºC, tops.
-Exit pipe - Something between 75 and 150 ºC.
- Optimal fumes temp at exit? - 80 ºC, depending on the temperature of the mass to begin with.

Yes, it's been done quite some time ago, and it worked. It would be best to do the masonry in such a way that the chimney is firmly connected to the bell's wall. Failing that, the chimney would crack apart from the wall.
The calculation of the ISA would be the same as compared to a bell without this built-in chimney. And it would be wise to incorporate a cleaning hatch close to the foot of the chimney.

sara ventura wrote:I was hoping to be able to have a hot bench next to the bell. Making some twitches to my current design I'm around 4.7 ISA.
I've read somewhere that by adding a bypass you can increase the ISA, how much are we talking about?
Is it feasible to have a small operating bench like the picture I attach below?

Good question... based on what I know about these things, I'd say you might go up to 6m² ISA. Mind you, when fired just half a load per day, the bypass need to be open a crack all the time. Having said that, it looks like a small bench is a possibility.

sara ventura wrote:I'm almost certain it's an IA picture, but it's the one I've found that looks closer to what I have in mind for my build (Minus the oven).  In case this was possible, would all the face next to the bell be all open wide inside to allow the gases to flow freely?

I'd think you mean the opening between the bell and the bench need to be as high and wide as the bench internal, the answer is yes. However see the remark about a bench lower down.

sara ventura wrote:The exit pipe from the stove is currently 2cm and 15cm from the corner, so I have to move it anyway. If it's centered at the back of the bell, gases can flow from 3 diferent directions?

That's correct, so the pipe's position, relative to the floor, could be lower.

sara ventura wrote:Left, right and from under the core, is that correct?

Yes.

sara ventura wrote:And the section of the pipe can be parallel to the floor?

Yes again.

sara ventura wrote:Being at 15 cm from the floor, is it convenient to rise the core a bit more than that to not have them both at the same level?

The pipe could be lower than the 15 cm, and even lower again when you are going for the following recommendation. The stream profile into the chimney could be improved by mounting a 18 to 15 cm reducer at the bottom end of the pipe. In order to ensure free flowing of hot gases in and out of the bench, I highly recommend that the core would be lifted to the level of the bench' seat.

sara ventura wrote:I was planning to buy double wall insulated pipe for the masonry chimney to avoid condensation, which has always been an issue as it can be seen in the pictures of my corner. Can this same pipe be used inside the bell? and the section inside the house be simple wall?

Yes, the same pipe could be used inside the bell. A short piece of single wall pipe inside the house isn't doing any good, better to continue with the double wall pipe.

sara ventura wrote:

It escapes me what you mean with wires to reinforce the heater core?  

Sorry, I got lost in translation, I meant rods. As seen in the pictures of the Thomas Rubino's new build, I meant the internal frame that braces the core, and the wires to reinforce the riser.

Hmmm... so you want to use the same construction as Thomas', a compression frame and no mortar? Can be done, if you want to.

sara ventura wrote:Then, probably just coating the wires from the riser that face the highest temps, and the rest of the metal inside the bell will be protected by the wood tar coating?

In my mind, it would be sufficient to use some mortar for the riser and have insulation around it, held together by the wire.  The wire is out of the highest zone and the wire is coated automagically.

sara ventura wrote:If I go with the double wall tube, will this be protected enough? or giving a layer of Vitcas on the upper part won't hurt?

It won't hurt, but the double wal insulated pipe is probably stainless steel anyway.

sara ventura wrote:My idea to crown the bell was to make double-skin only the last row and the top, as in the sketch below*. Should this apply to all the FB rows?
Even with cob finish instead of double skin bell?

Yes, and also every piece of wall that's above the riser's end, not just one row. The cob finish won't  work the same as a double skin bell. It is what is called, a finish, not the same as a construction. Please, don't under estimate the thermodynamic forces that are in play here.
The firebrick liner inside the higher portions of the bell could be split bricks, probably 3 cm thickness. This would help to simplify the build. Position of the bypass: just below the riser's top end, to keep it out of harm's way.

The bell's ceiling in the sketch doesn't need to be done like this, better lay the bricks dry, a layer of superwool and another layer of dry laid bricks on top, for ballast. The heater will be high enough to have the top out of sight anyway, so a cob finish is some overkill there.

sara ventura wrote:First questions raised by these drawings:

-Can the depth of the portal be 6.4cm? And the height of the riser 105cm instead of 108? 6.4 is the width of my firebricks.

Yes, the port could be deeper than suggested, so 6.4 cm won't be a problem. The riser could be anything between 10B and 8B. Shorter isn't recommended. Top gap, above the riser, AT LEAST the chimney's diameter. More is better.

sara ventura wrote:-Between the back of the firebox and the back wall of the stove I have left 21cm( x 61cm width). Is that enough?

This should be enough by itself, but there's another opening, at the right side of the firebox. Together they provide plenty of space. It is important that the gases are able to stream under the firebox, and from there to the chimney.

sara ventura wrote:-Would it make sense to draw the riser on the left, place the exit pipe on the right corner and have a much shorter lever for the bypass?

It would make sense, although not for the bypass. The door is further away from the wall as a left-hand sidewinder, I would see that as an advantage. Thought: you are building a 1st generation batchrocket. Provided there is an adequate chimney, this model doesn't need a bypass. So, what will the chimney be like?

sara ventura wrote:-Is 3 or 2 cm space enough to leave between the pipe and the internal walls? The exit pipe will have an angle as it is because of the thickness of the wall of the stove,    so I may well place it wherever I want.

This internal chimney pipe should be insulated somehow, otherwise the heater's exhaust gases will be higher and efficiency would be quite a bit lower. Insulating of the pipe could be done with 1" superwool around it, fixed to the pipe with chicken wire.
Another thought: since your chimney pipe is in a corner, about half of the opening won't be used by the downward streaming gases that need to go through a 180 degree change of direction. Stream profile should be about twice the chimney's cross section area, in order to avoid friction spots. Calculating... that internal pipe should be at least 15 cm from the floor, assuming afor-mentioned pipe is shoved in the corner of the bell.

sara ventura wrote:-If I rise the core with a metal structure, would a vitcas paint coating 1750º/3180º or similar be ok to protect it and the wires to reinforce the riser and core?

Of course you could use that Vitcas coating. But I doubt it will be necessary, because during the running-in period the heater will produce quite some soot. All the cold steel inside will be quitely coated with a layer of wood tar, no corrosion expected. It escapes me what you mean with wires to reinforce the heater core?

Yet another thought: the top of the bell should be double-walled (bricks on edge), with thin (1/2") superwool between it. See the Mallorca or the Brussels build. The reason for this is that the top of the bell will get hottest during the burn and the bricks will expand because of that. The inner skin will be able to expand in the above construction, without pushing the outer skin apart.

Leonardo Bevilacqua wrote:I have a question though. I was examining your sketchup model of the door. In your model you built the airframe with a 60x60x3 mm square pipe, which has an internal section of 54x54mm which is 2916 mm2.   If I follow the instruction on the page you sent, this section should be 15% of the chimney pipe section. The chimney pipe section for a 150mm system is 17671,5 mm2, and it's 15% would be 2650mm2, not 2916 mm2. I don't understand why measurements don't match. What am I getting wrong? This is only one example, but also the main slot opening doesn't match the requirements. Am I calculating something wrong? Or, should I just use your model, scale it up 1.2 times to match my 180mm system, and copy your measurements?

If you are aiming for 15% of the chimney's cross section area, that would mean there aren't any square or rectangular tubing that are exactly that size. So, what you should do is searching for a size that is close to what you want, preferably a little bit larger, not smaller.
As you may be aware of, the air supply is only just large enough, it isn't  the limiting factor during the burn, the internal proportions of the core are. During development I had the air inlet on anything between 30% and 45%, it turns out the 30% figure is enough and not too skimpy.

The measurements are what to aim for, the model is the result of the aim.

Hi Matt, I was waiting and expecting for somebody bolting an oven to the reversed Shorty core, and you did. A white oven like yours is probably less complicated as compared to a black one, I realized while I was viewing your pictures.

The deviation from the airframe design might play up when the core is really, really hot. The air box as the lower door frame member means that the air, going through two changes of direction could pose a friction point for the air stream. Maybe you could widen the intake opening even more, so that the air streaming doesn't need to make sharp bends. That the air intake is larger doesn't make any difference, the proportions of the core are the limiting factor, combustion-wise.

In my design, the air box under the door is also functioning as a threshold, so the ash won't fall out when the door is opened. And in a recent implementation I added a tiny slit high in the inside of the air box, and it appeared to be working as a boost when the core need to heated up. The slit is thin as a normal cutting disk, and only 4" wide. Done in the prototype of the Pepper Shaker.

All some random thoughts.

Port depth isn't really part of the specs. The thickness of the liner is tied to system size, but the thickness of the wall between firebox and riser is not. As long as you use standard firebricks on edge, thickness between 50 mm and 65 mm would turn out being within reasonable tolerances. I don't have experience with even beefier firebricks but I fully expect those will work as well.