Peter van den Berg

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.

Julian Adam wrote:On the dutch batchrocket site (great that you were able to find the time to update, many thanks!), I saw you mentioned the shorty could also be exhausting in the 'roof' of the afterburner. In that case, where is the port located?

Using a top exhaust, the opening should be at the top front side. It is done and measured towards the end of the development phase.

Julian Adam wrote:Are there running references?

Yes, there are, see this link.

Julian Adam wrote:I'm interested because flow-wise this may be more interesting for my own application.

It's one of the possibilities, use those if it fits your purpose. Please, stay within the current specs, it's a sensitive core that's easily disturbed.

Benjamin Dinkel wrote:This is the core lovingly named Shorty?!

Pros: compact, large window, no secondary air channel necessary
Cons: less initial draw

Correct me if I'm wrong please.

The pros could be extended with: very forgiving at reloading, a choice of core exhaust directions, highly resistant to fuel overload.

So, your very brief description is correct, including the lovingly name.

Rico Loma wrote:If I decide to try this paste in March, could you recommend a specific brand, one you have used ?  I can look at Leroy Merlin, a large network that's only 9 km from the project.  Other small masonry supply stores could have similar pastes. I will look at all possibilities there.

The French call this "coulis silicaté", Google Translate makes "silicate grout" out of that.  It's in powder form, need to be mixed with water. No brand name, sorry. It was OK on the firebricks, and absolutely first class on the red bricks. I had to break a lot of that latter bricks the last day, while demolishing the bench and core.

Rico Loma wrote:1, are those 8 large pieces  i see custom cast refractory slabs? I saw Thomas and Gerry build their shorty core last year,nand I understood how and why  they cast those specific pieces. When I viewed Peter's work via Sketchbook the detail of size makes me think yes. Were those hand cast first , before assembly began?

None of the refractory slabs were cast, those are large firebrick slabs, that's all. Also, this was the material that happened to be available.
If I had the choice, I would use large concrete pavers for the bench, top and seat. The core is another thing, I suspect those refractory slabs aren't available in the US. So, you have to hand cast those.

Rico Loma wrote:2, this was made in record time , much of that praise is naturally to the craftsmen involved.  Respect!  Was refractory cement used in place of 1:3 clay and sand mortar, and what was the reasoning behind that decision (longevity, strength, personal preference? )

The whole of the thing was raised in just three short days, several guys working on the core and the bench simultaneously. The mortar used could have been clay/sand, but we used something else, a very versatile paste, not being refractory as such, very thin seams possible. Again, that happened to be available.

Rico Loma wrote:3, was cob used ONLY for the last gasp, i.e., the final exhaust pipe heading skyward?
Thanks for any answers or opinions.  I understand Peter himself is quite busy always,  in winter months especially. I might give this a try, it would be perfect for a project going in the mountains of Portugal 🇵🇹

No cob was used for the entire thing, just bricks, slabs and the mortar paste. This was specific to France, by the way.
The thing would be a very potent heater as such, able to be refilled almost indefinitely. And yes, winter is a very busy time of year, half of my days are spent in front of my computer screen.

Leonardo Bevilacqua wrote:Then, another set of questions. What about the door?

The doors that I made myself in the past 40-odd years are all of the T-profile type.

Leonardo Bevilacqua wrote:How do I calculate the square pipe section needed, and the cuts that need to be made, and their position?

Use the link at the end my last post, all that you'll need is there. The text is in Dutch, admittedly, have it translated into English or Italian by Google Translate. I do that all the time with text that's in a language I don't understand. Links to the SketchUp drawings are there also, that's a standard 150 mm system. You have to start studying yourself, can't be helped.

Leonardo Bevilacqua wrote:Also, which kind of glass is needed?

Good quality heat-resistant glass. There are two brands in existence, Robax by Schott and Neoceram by Nippon Electric Glass. Both are suitable and pricey.

Leonardo Bevilacqua wrote:Where can I find the formulas to calculate the dimensions of the sidewinder core?

At the moment, there isn't a central place where the figures of a Shorty core are summed up, yet. By the way, the numbers for the sidewinder version aren't any different. I don't know which sketchup file you used for upscaling, so I am unable to check whether it is correct.

But here are the numbers for the calculation, all mentioned numbers are internal.
Start with the base figure, written down as B. This is 72.34% of the diameter of the chimney pipe, equal to system size.
Width of the firebox: 2B.
Height of the firebox: 3B.
Depth of the firebox: 4B, upto 5B.
Height of the port: 2.1B.
Width of the port: 0.5B.
Position of the port in a sidewinder: centered in the port 1B from the rear wall, left or right.
Width and depth of the riser: 2B.
Heigth of the riser: 5B upto 5.5B.
Liner in the bottom half of the riser: left and right equal, seen from the port, double that thickness at the port side.
Height of the liner: 2.4B, also crossing over the port.
Resulting floor of the riser: square, each side equal to system size.
Exhaust opening of the riser: in the same wall as the port, width 2B, total square mm of the opening 100% of the chimney pipe's csa.
Heigth of the opening: follows from the calculation of the sentence above.
Piece of wall above the exhaust opening: same height as the opening.

The above is about a normal sidewinder version, a reversed sidewinder version is slightly different.
The above information should be sufficient to check the upscaled drawing you have there.

Leonardo Bevilacqua wrote:It would be very nice to have a place where anyone can find tables, and perhaps spreadsheets, with all the dimensions for the different types of core.

I am working on it, a Dutch description of the Shorty is online in preliminary form, English will be next. But don't hold your breath, winter is a very busy time for me. See https://batchrocket.eu/ontwerpen#shorty

Peter van den Berg wrote:Could be done, although there are compromises in there, flow-wise.

Leonardo Bevilacqua wrote:Could you please expand more this point? Could you tell me what the compromises would be, or why you don't like my solution very much?

The question is simple, the answer isn't.
Try to imagine the hot stream from the core's exhaust. It will travel more or less horizontal to the front and then up. OK so far, from there along the ceiling and the walls down to the large opening behind the core. Since the core is elevated from the floor, there's space underneath, you would like to have the stream going underneath the core and just above the floor to the back, where the exhaust of the bell is situated. It's obvious that the hot stream could choose to take the easiest route and going straight down, instead of the more complicated route through two changes in direction extra. Just my interpretation of what the preferred route could be.

Leonardo Bevilacqua wrote:So, just to understand, do you think we would have problems with my design? If so, I would build a sidewinder, as you suggested.

The problems lie in the fact that the heater's door, during burns the hottest spot of the entire heater, is pointing to everyone that is walking past the heater through the door opening right beside it. The chances that somebody, especially children, will burn themselves is actually built-in like that.

Leonardo Bevilacqua wrote:I just calculated roughly the measurements of the opening, trying to come up with a rectangular opening that had the same hydraulic diameter as the exhaust pipe. Is that right?

No, it isn't, due to part of the gases coming from the top. Just because in order to allow for a 180 degrees change in direction, you'll need a lot more than just the hydraulic diameter. I'd say about twice the cross section of that hydraulic diameter. The whole point is to eliminate most if not all of the potential friction points. Even it looks like it's spaciously enough, try to imagine that the gas stream is expanded by a factor of say, two and you see that it could be a potential pinch point.

Leonardo Bevilacqua wrote:You mean 30cm from the top of the bell to the center of the pipe, or to the top of the pipe?

The top of the pipe that's housing the bypass.

Leonardo Bevilacqua wrote:When you talk about a "right-handed sidewinder" you mean one that has the riser on the right of the core?

Yes, that's correct. Since there's room left and right of the combustion core, you won't have the possible shortcut delemma.

And to answer your question from another post: please stay away from IFB's, You'll never know how it will fare inside the hottest (or close to the hottest) parts of the heater. Use hard firebrick, and insulate that at the outside, a much safer route.

Leonardo Bevilacqua wrote:Thank you all for your thoughts. I finally had time to work on a first draft of the project.
I attach a few pictures of what I came up with.
I also attach a not very detailed sketchup project. The measurements are correct, but I didn't put in a lot of details.
I tried to make the bell as slim as possible, due to space costraints. Tell me if it could work like this.

It would work, but I am not overly fond of this. To me, there might be a better solution.

Leonardo Bevilacqua wrote:To you, Peter, I calculated the area behind the core and, if I'm not mistaken, it is well above 5 times the CSA.

Yes, it is, 9 times to be precise.

Leonardo Bevilacqua wrote:As I was drawing I came up with a couple of questions:
1) Is it ok if both the long sides of the core touch the bell? There would be 12mm superwool in between.

Could be done, although there are compromises in there, flow-wise.

Leonardo Bevilacqua wrote:2) How can I design the exhaust port at the bottom of the bell? How do you determine shape and size?

What you have drawn is in the right direction, starting low at floor level and quite wide. This construction makes it possible to sweep the chimney from the roof without dismantling anything. Is the roof accessible?

Leonardo Bevilacqua wrote:3) How high should the bypass pipe be installed on the bell?

You've drawn it very high, could be lower. Say, something like 30 cm from the top.

Leonardo Bevilacqua wrote:4) Any thoughts on cleanout ports location?

Two at the front and one at the right, where the chimney starts.

The firebox is placed quite high, that isn't really necessary, 65 cm from the floor is usually enough.

Now for something different. It seems to me that the front of the heater, where the door is, is facing the wrong direction. Built like this, you won't be able to sit by the fire with friends and family and see what is happening. With that in mind, please give the following a thought.

Instead of a straight core, you could build a right-handed sidewinder. The firebox door would be in the wide front of the heater then, facing directly into the rest of the room. Roughly calculated, that would mean the heater's front is protuding 14 cm more into the room. The bell will be obviously less wide, although not overly so. The whole of the thing will be more in proportion, no flow compromises .