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Standard Vertical Riser V.S. Double Shoebox  RSS feed

 
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I've reached a crossroads in my design deliberations regarding an 8 inch BBR with masonry bell and attached thermal mass bench.  While my design and build space constraints allow for a vertical heat riser, although a bit on the short side at about 43 inches, I am intrigued by the idea of using the double shoe box design and forgoing the vertical riser.  Would it make any difference if the double shoe box core was encased in a masonry bell?  I've read just about all I can find on the subject, but the 2Xshoebox in a bell just doesn't come up in my searches.

Using the 2Xshoebox would allow me to make my construction considerably shorter and maybe even have a cook top.

Has this construct been worked with enough to predict an outcome?

Thoughts?
 
pollinator
Posts: 1944
Location: Cincinnati, Ohio,Price Hill 45205
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forest garden trees urban
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Go check out Donkeys rocket stove Forum.
Peter spends a lot of time there,and he is probably the most knowledgeable on both of these types of stoves.


That being said, the original calculations for riser height have been amended.
4.1 times base, or roughly 3 times diameter has been shown to provide complete combustion.

An 8" batchbox should be fine with a 24" tall riser.

The shoebox is still in development, last I checked,but you should also check out Matt Walkers cookstove designs.
Matt is a fearless innovator.
I believe his almost riserless design pushed the development of the shoebox.

rocket stove science is moving so fast, it's hard to pin down a design.

Ive built and fired a J core/riser of perlite and rapidset cement but I'm abandoning it for a cast batchbox  with a "5 minute riser"
At least, that's what I'm doing for now😏

I still want to explore the water cooled feed tube tgat kinda worked on that core.
Innovation can be addictive,and distracting!
 
Thomas Tipton
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William,
When you say 24" tall riser, do you mean as measured from the floor of the burn chamber?  Peter Berg has offered some advice on this build already and his suggestion was to go 46", with the caveat that some have gone as short as 35" and still managed a properly working batch rocket.

Kinda got me wondering about what a "water cooled feed tube" would be good for......

I know I intend to sandwich some copper pipe or  pex between two layers of stone on my mass bench top to draw off hot water and keep the bench from getting too hot, but I'm sure that's not what you have in mind.
 
William Bronson
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Posts: 1944
Location: Cincinnati, Ohio,Price Hill 45205
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forest garden trees urban
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I did indeed mean from the floor of the burn chamber.
The precident was established back in 2015 in this thread:
http://donkey32.proboards.com/thread/1845/scandal?page=1

It was Peter approved back then, so I'm surprised to see him suggest a longer minimum  riser.

My reason for water cooling a J tube feed tube is to allow super long/tall fuel to be used.
Without some intervention the fire burning sideways at the bottom of the feed tube can burn the wrong direction when long +fuel is used.
A water cooled collar puts a stop to this.
So far, my resoivars of water have proven to  be too small.
 
Thomas Tipton
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Thank you so much for sharing that thread.  I had no idea.  You probably just saved me $500 in firebrick.  I am intrigued by this water cooled feed tube problem.  My first instinct is to suggest that it could never be make 100% safe, in that any failure of the mechanism may allow for uncontrolled burn up the feed tube.  But I'm sure you've already considered that. If I can assume you are using a tube within a tube to create a water jacket,  is it your intention to use a pump to circulate the water in the jacket, or rely on thermosiphon and circulate through a larger reservoir?  Again, I would think no matter how cool you could make the sides of the feed tube, the fire will always have an inclination to ascend up through the center of the fuel stock.

I can imagine a mechanized system whereby the fuel is loaded horizontally and is constantly under a slight tension to drive it forward into the burn tunnel as it is consumed.  Of course this would create new problems with preventing the fuel from being pushed into the port and blocking airflow entirely. 

So many possibilities.

Thank's again and good luck.  Please keep me posted with your adventures.  I'll be documenting my build here.      https://permies.com/t/76265/Solar-Water-Heater-Rocket-Mass

 
William Bronson
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Posts: 1944
Location: Cincinnati, Ohio,Price Hill 45205
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forest garden trees urban
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Be certain to check with Peter about  his current thinking regarding the shirt riser.
He might have changed his  mind after more study.

The water cooled feed kinda worked for me until the water boiled.
I underestimated how much heat I would be battling.
My first iteration was two  steel tubes,the space between them filled  with hydraulic cement and sealed with silicon caulk/thinner.
My next go at it will consist of a small stainless steel  stock pot, set in one end of a steam table pan.
The other end of the pan  would  have room for a float valve.
I would leave it open to the air at first, though a loose  cover to capture steam and either cool it or use it is not out of the question.

Why bother with any of this?
Well,  I would like to be able to drop a sapling in the feed tube and walk away.
Clearly that would need a cage to contain the upper end of the wood, an addition that by itself reportedly causes draft, and encourages burning in the wrong direction.

None of this is suitable for use in my house,  but it's interesting to me , so I persist,off and on.

Your idea for a horizontal feed is not new, but is still worth pursuing.



Right now  I'm preoccupied with the idea of boiling a large volume of water off in the form  of low pressure steam, in order to move heat to the inside of a house.
A 8"  batch box would reliably produce a crapload of heat,  so suddenly I'm into something "practical"!

I find no insulative  bricks locally, and the dense firebricks are expensive,so I am planning to cast slabs of perlite/rapidcast/rockwool fibers , for the fire box.
I plan on using the   5 minute riser which can be seen here:

https://m.facebook.com/media/set/?set=a.10154992139891974.1073741834.559871973&type=1&l=c6e07d689b

Along with the rest of Doug's build.
It's just ceramic fiber insulation inside of a piece I round duct. Briliant,cheap, easy.

I will probably coat the inside of mine,to prevent the escape of  tiny fibers during burning.
No evidence that that is  happening, just  an  abundance of caution.
 
Thomas Tipton
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So you like to solve unsolvable problems.  I get it. 

You led me back to Matt Walker's stuff, which I was aware of, but didn't really understand all too well.  Now that I have had a better look at it, I have a better grasp of what he's doing, but I would need to size that up to an 8 inch box, as his fuel box is rather smallish.

Seems his most recent iteration is using ceramic fiber board instead of fire brick.  Going that route may enable me to shrink the size of my monster considerably. 

Just a thought, but you might want to consider smearing a bit of 3000 degree masonry cement on the inside face of that ceramic blanket. I was going to do that with the riser I was going to make out of Sodium Silicate and loose ceramic fiber.   My intention was that the hi temp cement would give added structure to my cured, ceramic fiber cylinder riser and smooth out the inside face, while giving it a "hot face".  I was also going to coat the outside of the riser with the hi temp cement for added structure and rigidity.

So, in short, thanks to you, my build ideas are all over the place.  LOL.
 
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Location: North Alabama
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William Bronson wrote:

I find no insulative  bricks locally, and the dense firebricks are expensive,so I am planning to cast slabs of perlite/rapidcast/rockwool fibers , for the fire box.




You might want to re-think the cast slabs mentioned above. Rapidcast will not live long in the environment of the burn chamber. Look for refractory cement instead, it comes in premixed tubs or in powder form. Rapidcast is Calcium Aluminate and is only good to about 900F, after that the water in the crystalline structure begins to break down and evaporate and the structure will flake apart. Refractory cement has enough Calcium Aluminate to set hard at room temperature and the primary binders begin to fuse at temps above 1000F and are good to somewhere higher than 2400F. 1400F to 1600F is the standard temps inside a rocket stove burn chamber. Refractory cement is somewhat more expensive but your project will last many years instead of a few months.
 
William Bronson
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Location: Cincinnati, Ohio,Price Hill 45205
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The calcium aluminates that are important  in calcium aluminate cement  are Monocalcium aluminate,which  melts at 1390 °C.(2534 °F) and Dodecacalcium hepta-aluminate melts at 1,400 °C (2,550 °F)

While CementAll is  far from perfect, I have used in a ratio of 1 to 6 with perlite, with good results.


I can buy a proper refractory cement , rated at  2,550° F. It costs about 3 times as much and has similar ingredients:



[url=https://www.menards.com/main/items/media/TWINC003/SDS/1891185AKrefractorymSDS.pdf]
Medium-Duty Refractory Mortar
© Akona Manufacturing LLC.
Ingredient Typical Percentage CAS #
Portland Cement ......................................20-40% 65997-15-1
Calcium aluminate cement .........................0-55% 65997-16-2
Silica Sand (as quartz) .............................60-75% 14808-60-7
Lime .............................................................0-7% 1305-78-8
Calcium carbonate .......................................0-4% 471-34-1
Polymeric binder ..........................................0-4% *
*Specific chemical identities and concentration withheld as trade secret. They are available
upon request to health profession[/url]




By contrast , CementAll contains the following:
[url=https://www.ctscement.com/wp-content/uploads/2014/02/SDS_Cement-All_2014_1007_1.pdf]Calcium Sulfoaluminate Cement 40-60%
Silica Sand,quartz 40-60%
Sodium Sulfate 0-4%
Methanal 0.0-0.1%
[/url]


Rutland Castable Refractory Cement is another refractory.
It contains Silica, Quartz, plaster of Paris, Portland cement and  Alumina Silicate, depending on the specific formulation.

If these other formulations are any indication, I'm missing Portland cement and plaster Paris..but I think these are less heat resistant ingredients.

There are other reasons I'm not concerned is my intended usage.
Those high temperatures that where cited occur in the riser, and I intend to use these slabs to form the firebox.
My riser will be ceramic fiber inside of a piece of duct.
Further, the stone wool fibers should act like the straw in cob, redirecting tiny cracks that develop, so they cannot become big cracks.

I might even swap out the perlite for clay, or clay soil, for better wear resistance and lower expense.
Fireclay is expensive here, but if Im doing slabs, I think the shrinkage that is found in lesser clays might not be an issue.



 
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