Here is Peter van den Berg doing a dry stack test. Note that there is no vertical riser - that is the big difference in the double shoebox design. Instead, the roof of the batch box box has a slot in it which leads to the rams horn mixing.
Trying a little cooking:
A bit more design for what will be built at allerton abbey:
here is some video that fred took of what we have so far at allerton abbey. We will try to take some better video when the project is complete:
I'm not clear on if there is an insulated spot for secondary burn/reburn of wood gases, etc. I tend to think about vertical risers, not horizontal ones, so I'm confused by that turn of phrase. Is it insulated under the glass cooktop? (Also curious how you cut a glass cooktop.)
He has another thread that is about the general design called a very different batch core, no riser at all. For years, ernie, erica and i have laughed about people that make "a freakshow of flaming death" and mistakenly call it "a rocket mass heater" - and the default example we go to is the time that somebody made the riser horizontal. After all, how can the heat rise if the riser is horizontal. And, yet, here is Peter doing exactly that. Only E&E&I are not scoffing - because, of course, it is peter! Further, peter has tested it and the numbers are solid!
Next up: there have been a few problems with batch box systems. And those appear to be getting solved a lot at this year's jamboree. The double shoebox solves one: wood getting shoved too far to the back and plugging the channel to the heat riser. Now that the opening is on top of the batch area, it cannot be plugged!
Another big plus with the double shoebox: lots of glass to see the fire.
Here is a video of the prototype before it was put into allerton abbey:
And then some detailed design:
And here is the build:
It looks like the outside can be built with ordinary building bricks. I am familiar with buff colored fire bricks, these are required for high heat locations like the core of a rocket mass heater. The lighter colored bricks are protecting the outer plain bricks from heat, along with a piece of thinner white material. I just don't know much about different grades of firebrick yet.
Julia Winter wrote:I'm not clear on if there is an insulated spot for secondary burn/reburn of wood gases, etc.
The space above the firebox is the reburn area. The sides and bottom of that area are built of insulated fire bricks, the top happens to be a glass cooksurface. I know, it sounds opposite the ideas about secondary burn areas but glass and even this ceramic type is actually a bad conductor, much so as compared to a cast iron cooktop. So beneath the glass it need to be awfully hot before you would be able to cook on it and a lot of heat is confined to the space beneath it.
Julia Winter wrote:(Also curious how you cut a glass cooktop.)
This has been done with the wet saw for bricks at base camp. In two runs, very slowly while the glass plate was supported by a piece of plywood. The glass needed to be cut twice and I was quite shaky when I got that done...
In all, quite a lot of thinking out of the box (pun intended) was involved and assuming certain processes were going on. It turned out I got away with it.
Julia Winter wrote:I'm with you on the coolness of seeing the fire. This double shoebox design is a lot closer to the masonry heaters I've seen. Are the buff and the pink bricks (in the drawings) both firebricks? [Edit: after following your link above - yes, I think they are both different kinds of firebricks. Maybe the pink ones are more insulative but not as strongly fired as the lighter colored ones? You need the hard-fired bricks for where you load the wood, but can use a different sort of fire brick for the upper fire box.]
You've got that right Julia, the pink ones are of a radiacally different material and are much lighter in weight. The idea was that the lower firebox built of hard firebricks could serve as an oven.
I hope the all-important secondary air tube isn't blocked. In fact, this tube is providing about half of the air supply in this design.
Official name from this day forward: Double Shoebox Rocket or DSR for short, just like the way Batch Box Rocket can be abbreviated to BBR. Max made the remark that DSR could be pronounced as Désirée, a girl's name in France which means "the long awaited for".
With such a romantic name, it's high time I pick up development again. Today I ordered a box containing 25 insulating fire bricks so next week I will be able to change the development model several times if and when I feel the urge.
Désirée, the desired one. Also, by extension, le long awaited for.
He also contends that the "dead end box" contributes to the ability to slow the burn right down after the box has come up to full heat. Which he contends a normal rocket stove is less able to do. He says he gets 4-5 hour slow efficient burns. He cooks on it and in it and it heats his 2000 sq. ft. house. 2 burns per 24 hours.
He seems to very happy with the design. Others have been building to his design as well.
Can't remember which stove forum I followed that design thread on. I suspect some one here knows that. I have a feeling the so-called "double shoe box" has been used in masonry stoves a lot longer than is supposed by some.
Look at the pics. The secondary burn area is not at all the same. In the DSR. Flame goes up, and gets mixed with secondary air in a rather big chamber. While on Trev's stove, gases goes into the port. Then get bent 90 degrees. And the secondary burn chamber is smaller. Which leads me to think that gases might not have as much time to burn, as the double shoebox.
barney fife wrote:This appears to be a variation on the "Vortex Masonry Stove" which in essence is a double shoebox.
No it isn't, IMHO.
In the vortex stove, the expansion chamber is on top of the firebox as in the DSR and they're both burning wood as fuel but that's where the similarity ends.
The vortex has it's throat at the front above the door as you mention, but that's about as wide as the chimney size. Furthermore, the expansion chamber is much wider than high and the throat is parallel to the door. Secondary air is fed through the door or door frame, as far as I can tell close to the throat. Streching the burn for 4 or 5 hours sounds highly suspicious to me, I would be interested to do a full-blown test with a gas analizer to establish whether it burns clean that way or not. As far as gas path goes and where the throat is situated, the combustion core of the Vortex stove looks more like a brother of a Dutch design called Tigchelkachel. Which is a much older design and has been in production for decades.
The double shoebox rocket on the other hand has it's throat or port at the back and perpendicular to the front plus it's much smaller than the chimney size. The expansion chamber (in the development model) is as high as it's wide and gas stream direction is to the front instead of to the back. Furthermore, the secondary air is fed in low at the front through a steel rectangular duct, leads horizontaly to the rear of the firebox, 90 degrees vertical and ends halfway in the port itself. The port is a restriction in the gas path and as such it's a venturi. Burning in the double shoebox can't be slowed down and stretched out for hours. The glass will stay clean despite the fact that the flame isn't drawn to the front.
In my view, those combustion cores doesn't look much alike.
The guy who came up with the Vortex Masonry Stove conceived of it on paper after studying many older masonry stoves and did only one revision after it was built. He wasn't an inveterate tester / improver.
His only real revision was a raising of the height of the upper box roof and found that improved certain of the burn parameters, exactly which one(s) I cannot recall.
I found his contention regarding the stove's alleged ability to slow the burn down, but still gassify the most interesting. He claims that the lack of cross flow over the burning mass, as found in rocket stoves, allows for this slow down. I find his so-called "dead end box" effect and port placement the most interest part of the stove.
However the design is still a double box, secondary fed ceiling port no matter what the semantics used; and I doubt that he was the first to employ it.
The introduction of secondary air on the Vortex while not as refined or as effective as your scientifically refined variation is still secondary air feeding an upper port.
I have no doubt that the Vortex Masonry Stove can be further refined with small dimensional changes with regard to the port size/shape and upper box format to make it that much more efficient.
His design in no way detracts from your own accomplishment where efficiency is concerned.
Is this stove in regular use? What are the temps you are experiencing on the hob?
How much do they fluctuate during a burn?
How is the firebox working as an oven? Is it satisfactory?
Does it cool too much if the embers are allowed to burn out before starting to cook? Or do you cook with the embers still glowing? And if so how do you sheild the base of what is in the oven from burning?
I am following the DSR development on Donkey's forum and am very keen on hearing how you are finding this itteration.
Many thanks for any answers, Esbjorn
Esbjorn Aneer wrote:I am following the DSR development on Donkey's forum and am very keen on hearing how you are finding this itteration.
Many thanks for any answers, Esbjorn
I wanted to post exactly the same words!
Some follow up would warm my heart...
This same effect is also found in the normal batch box, sidewinder and p-channel or floor channel versions. There seems to be a need for space behind the air inlet. Also, when there's a piece of fuel sticking into the inlet the thing starts smoking just like when there's something solid in the port.
It looks like this DSR can be a viable alternative to the earlier implementations using batchrocket technology although there's a lot of more experimenting to be done.