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Cooking on a 4 inch batch rocket stove  RSS feed

 
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I dry stacked some fire bricks to form a 4 inch batch rocket stove, and used mainly kindling to get it going. I could see the dual ram head vortex in the riser but when I put my cast iron plate over the riser supported by a trivet from a gas stove to allow the draft to still function, I didn't really get much heat on my plate.  In the end I gave up and removed bricks from the burn chamber top and placed my iron plate there instead where I was then able to cook. I didn't have any secondary air channel in this set up.

My new, not sure it's reliable, temperature gun said the hot plate over the riser was 120 C, 220 C when over the burn chamber, and the burn chamber was about 500 C.

What's the best way to cook using such a set up without needing to mortar all the bricks together?

https://i.imgur.com/8iDmwNu.jpg

https://i.imgur.com/xhl1tO5.jpg
 
pollinator
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Is that riser just a clay drain tile or similar? If the riser is not insulative, it will sap a lot of heat from the combustion and this may be why your plate didn't get as hot as you wanted. Also, even a "proper" riser enclosed in a barrel probably harvests heat more efficiently than an open air setup, since the sinking flue gases are still pretty hot.

For comparison, I have a 4" RMH with a small cob bench in my glasshouse. The riser is perlite and fireclay (a little too heavy on the clay, so not the greatest) and the burn tunnel is cast refractory incorporating Peter Van den Berg's tripwire and kicktail features. I'm using a 100 litre drum and the top surface normally runs about 250C during a good burn. If I use really energy-dense wood, like a combination of manuka and cedar sticks, I can get it up to 300C, but I regard 250 as more of the sweet spot. I cook on it from time to time...it's great for tortillas and naan. I usually keep a big stockpot filled with water on top, or a few large river rocks, for some extra thermal mass.

I have a feeling that my rig is close to the upper end of what a 4" system can do, so if you're seeing 220 C from your dry stack setup, that's not too bad.
 
Graham Chiu
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I just put the clay pipe there on top of the riser to draw the smoke away my face after I  moved the cooking plate
From over the riser to the primary burn chamber.

If I can get 250 C on top of the riser without needing a bell I'd be happy. I'll try again using Manuka as I was just using mainly pine kindling, and see if I can get some secondary air into the cervix of the batch box. The fact I'm seeing smoke must mean I don't have enough oxygen or heat in the secondary chamber.
 
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Graham, your riser, as it is now is too short . You use dense firebricks all the way, which will suck a lot of heat from the burn. As well, a rectangular riser, this way, isn't my favorite, i would say. I think wider than deep is better. Check Peter's double shoebox secondary burn chamber, for inspiration.  As well, your trivet must let a lot of gases escape from under the pot. Adding a pot skirt could help.

https://www.google.fr/search?biw=1228&bih=599&tbm=isch&sa=1&ei=0p6tW8XBE5GSlwSB5avYBw&q=aprovecho+rocket+pot+skirt&oq=aprovecho+rocket+pot+skirt&gs_l=img.3...105246.128972.0.130112.26.26.0.0.0.0.87.1636.26.26.0....0...1c.1.64.img..0.9.599...0j0i30k1j0i19k1j0i5i30i19k1.0.c7fcuUEEnBk
 
Graham Chiu
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Satamax, yes I can now see that it can be difficult to cook on this without a bell as you need to get the heat under the cooking plate without throttling the thermosiphon. I started on trying to dry stack a DSR last night but ran out of fire bricks. But I gather that design has now been shelved by Peter.
 
Satamax Antone
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Graham, i think it's perfectly possible to cook on a 4 batch, without a bell.

But it has to be a well made batch. Firebrick firebox, insulating heat riser. P channel or floor channel. Proper gap between the pots used and the heat riser. And a pot skirt.

One night, a fortnight ago. I couldn't cook on my little J tube cast iron plancha. Too much wind. 

In your case, the heat riser is too short, i think. Not insulated. No door on the firebox, tons of un insulated mass in those bricks.  I use the top of the firebox for cooking, but my batches are 6" and 9"

 
Graham Chiu
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I made the riser taller, and sat a kettle on top with a side opening as the chimney exhaust. I was getting 500 degrees C in the primary burn chamber but the kettle never boiled. I think it finished at 27 deg. The cast iron cooking plate reached about 220 degrees again.  The width of the fire box is 5 inches, and the riser is 5 inches square.

I had problems with smoke. So I think I would benefit from lining the riser with ceramic board, and adding a proper chimney. Is there any point insulating the primary burn chamber??



My aim is to create a dry stackable cooker with minimal custom components.  I don't have secondary air source as I don't have a way to work metal yet.

@Satamax, if you're also using the primary burn chamber for cooking, what do you do with the heat from the riser?
 
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Graham, just a few points:
There's way too much mass in your setup, the bricks are taking up lots of heat. Also, since it's a small system, the ratio between volume and wall area is very unfavourable. Scaling down of the original test bed means the wall thickness should be scaled down as well.
Because it's dry stacked, there will be a lot of small crevices, this could together easily be more than the whole of the inlet.
The air inlet is way too big and it's closed at the bottom.
The exhaust opening is smaller than the csa of the riser.
All these factors contribute to the sluggish running and low temperatures.

Recommendations:
Use split firebricks if possible, insulating firebricks of whatever thickness would be close to ideal.
Mix a clay slip and use that between the bricks when you stack it. A running bond is always better to get the darn thing airtight.
Make sure that the air inlet is at the bottom of the firebox opening. About 25% of the riser's csa should be plenty.
Enlarge the exhaust opening to 150% of the riser's csa, the gases aren't willing to go through sharp corners so they need room to flow freely.

When you've done all that it should go much, much better. Even the steel plate on top of the firebox would get much hotter, especially when the air inlet is low in the firebox and the top end is closed.

edit: the riser seems to be quite short. For such a small system, it should be at least the recommended length. Let's see for a 4"system that would be around 29", measured from the floor of the riser/firebox (which should be the same figure).

edit2: Listening carefully to what you are telling in the video, I'd get the impression the port is up to the top of the firebox and lifted from the bottom. This is utterly wrong, the thing need to be lower in order to work properly. This design is the result of lots of work experimenting. It's highly optimized, as such it is a very, very tight design. Any deviation from the recommendations means you are almost certainly doing something that I've tried already and ticked it off my looong list(s) because it didn't add to proper running, rather the opposite.

Please follow the design as close as you can and it will do what it says on the tin!
 
Graham Chiu
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Thanks Peter. I rebuilt the thing using the 5 inch measurements from the spreadsheet on http://batchrocket.eu/en/building as much as I could using dry stacked bricks but the performance was the same.  So, I guess the issue really is the fire bricks are sucking the heat out.  I don't think I ever exceeded 500 deg C in the primary burn chamber.  And locally "split" bricks are 5 times the cost of the fire bricks I'm using which makes it really not viable (NZD40 per brick).

What temperature should one expect to see in the primary burn chamber when using a cast refractory?
 
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You’ll need to insulate around the outsides. if you do it will eventually get up to temp. Can you cut your existing bricks yourself?
 
Peter van den Berg
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Graham, you are talking about the bricks as mass. That is definitely too much, yes. What about the other recommendations, did you address those?
 
Graham Chiu
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Hi Peter, yes, I matched the measurements as much as I could to the spreadsheet values, placed the port level to the firebox but didn't have any clay to seal any cracks not that there any of note, and shrunk down the air inlet.  When it smoked it only really came out of the deliberate openings.

I'm not sure insulating outside is going to make much difference since most of the bricks don't get particularly warm
 
Graham Chiu
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Just for interest this is what I saw in one Muslim restaurant in Urumqi when I was on holiday a couple of months ago.





I guess that's what one is trying to improve upon.
 
Satamax Antone
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Graham, per my latest experiments, you don't need much clay.

I live on what was once the bottom of a glaciary lake.  It also has been the river bed at some point. And the mud is a mix of pebbles, sand, grit, a little earth, some schist particles, and may be a smidge of shistic clay. I sieved the mud from the gravel. And used that mud to seal a prototype of vertical batch. Several heavy rains after, it's still holding together.

So dig a bit of your garden/backyard or else, sieve it, and stick your bricks with that.
 
Graham Chiu
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Satamax, I've got some clay coming. It's too wet at present to be screening dirt right now.

Have you got a picture of your 6 inch cooking setup? And an idea of what temperatures you are reaching?
 
Satamax Antone
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Six incher is a prototype.

 
Graham Chiu
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Satamax, that's some impressive fires you have there in your collection of videos.  And you're using a lot more wood than I have managed so far.  But the trouble is, the only advantage I see from an ordinary wood stove as in the pictures I posted above is that you're getting a clean smokeless burn, but the energy from that burn is literally going up the chimney unused.

It's a pity that the double shoe box design proved to be unreliable as that seemed to be the most direct way of capturing the heat from the combustion of the wood gases for cooking purposes.
 
Satamax Antone
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It is just a prototype, the thing is meant to be inserted in a bell and bench setup. Oven will go on the side of the heat riser, and obviously cooking plate will be left where it is.

See the door there



I have done this to assemble the heat riser in the workshop stove.

http://donkey32.proboards.com/thread/1817/starting-build-220mm-rocket-double

But i didn't think this could be used as a black oven at the time. Next one will have the door to the side of the riser. Still permitting to assemble or maintain. But will also double as a oven.
 
Graham Chiu
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Looks like you've removed most of the images in that board's thread?  Well, at least the first few pages I looked at.

I can't make much of the geometry from your video
 
Satamax Antone
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Bleemin hostingpics is gone! :(

It looks like this.

 
Satamax Antone
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Well, i fitted a new cooktop today.  I absolutely love it. 87*45 cm 14mm thick steel.





Already cooking cabage saussage with taters deglazed with fischer three hop beer.
 
Graham Chiu
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What's the difference between using 14 mm steel and cast iron?  And does a metal radiator there affect the fire box temperatures or drop efficiency much?
 
Satamax Antone
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It is pretty much the same as cast iron. Big difference tho, i Can weld on it. And this let me make the back lip that joins the "barrel"  and has some rockwool on it, to airtight the thing. It was given to me, of the right dimension. So, who am i to complain.
 
Graham Chiu
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I just noticed that my new temperature gun maxes out at 500 deg C which is why I was only ever seeing this value!  At other times I was seeing ERR which I presume it means it was outside the upper range.

Is there much to be gained by getting a gun that goes up to 950 deg C?  (Does wood gasify at over 700 deg C?)
 
Satamax Antone
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Well, theoretical wood burning maximum temperature, is 1980c°.
 
Graham Chiu
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Well bricks aren't lego but they're close enough.  It seems clear to me that a batch rocket stove is for heating and any cooking is just incidental and fortuitous.

So,  I tried rearranging these fire bricks into a single layer box rocket stove.



which looks promising to me.  I ended up with cooking temperatures similar to my first experiment cooking over the burn chamber of my malformed batch rocket.

And yes, I should not use a plastic chimney.
 
Satamax Antone
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Graham, i'm sorry to say, but this is not a rocket.  You're cooling the combustion gases too early.



Without insulation, you won't go anywhere, as well as dry stacking bricks.  Well, you can dry stack, if you have insulation outside, which prevents air leaks.



What is your goal ? Just cooking ?

http://aprovecho.org/?paybox_id=88




In my idea, you really need to make at least one real working J tube and batch, or several, before starting experimenting, and calling sub par results; deviating from the  usual builds; a "rocket stove"  I know this is not that nice to hear. But, a rocket stove has worked for thousands of others, why wouldn't it work for you?  I can cook a steak, few taters, or sausages, with two handful of sticks, in my J tube. And the heat riser is even made out of insulated metal.

So, it doesn't take much to make a working cooking rocket stove.


I cooked again yesterday night on my workshop heater (well it heats the whole building. 635 M3 ) It took 20 minutes before it was up to temp for cooking, and took a little crate of offcuts. 40cm x 30cm x20 cm. or so.

Keep at it, and you'll have a result.

But remember, to be a proper rocket, it has to have at the very least an insulated chimney. be leak proof. And you don't rob the combustion energy. Except on a batch, where you can get away with the top of firebox plate.

Your latest attempt, i will not even comment.

But on this one.



You don't have a door. Normal that your cooking plate doesn't get hot. You have cool gases being forced underneath.

The port for the primary air has to be at the bottom. 

As Peter stated before, your opening at the back of the riser is too small. If you want to boil that kettle, raise it two three inches above the riser, with anything which doesn't impede with the gas flow.

And if possible, skirt it.   Remember, no restriction from the  CSA  (cross sectional area of the heat riser)  If you have a 90° turn, increase your gap by 1.5 times the right figure to keep the CSA.

Go and dig some earth in the back of your garden, screen it, and wet it to make mud. It's all it takes to stick the bricks together. It's infinitely reusable and cleanable. Find yourself a piece of batt insulation, at least. To wrap that heat riser.

If you put your griddle over the heat riser, it will work better, faster, because of the big surface to extract heat from the gases.  In any heat exchanger, it's either mater of surface, or time.

I think, stick to that batch, you will have better results.

HTH.

Max.




 
Satamax Antone
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Nother one.
IMG_20181006_212727.jpg
[Thumbnail for IMG_20181006_212727.jpg]
 
Graham Chiu
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Max,

Thanks for the feedback.  I'm primarily interested in occasional outdoor cooking using the scraps of wood that I get from the garden.  I'm not interested in heating such being the nature of building codes here.

I'm happy to just experiment at present, and if I go wrong I can look up the spreadsheets to see exactly what I've done wrong.  But I can take the most promising experiments and then I can decide on spending more money on adding insulation and getting a metal shop to build me a secondary air channel.

As for stealing heat too early from the second combustion chamber, if I don't see smoke, aren't I going in the right direction?

My effort from yesterday was modified to a L tube feed.  A J tube is for auto feeding people who want to heat rooms. I just want to cook so I guess it's not a major design consideration.

 
Satamax Antone
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Graham, you still don't have the hang of the "gap"  i would remove that brick you have in the middle of the cooking plate. Your fire is very sooty because of this.  If it's a full brick heightwise, break it in two, so you have a half brick gap under the cooking plate.

Your thing is getting close to the lorena stove.

https://www.google.fr/search?q=lorena+rocket+stove&tbm=isch&tbo=u&source=univ&sa=X&ved=2ahUKEwiiwKO0_PPdAhUQ6RoKHcIwBToQsAR6BAgFEAE&biw=1228&bih=599

And the ecofogao.

http://www.ecofogao.com/english/
 
Phil Stevens
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Graham -

Have you built a simple, unmodified J-tube rocket with an insulated riser? I ask because if you did this, and observed it in a range of burning scenarios, you'd see that you will get a lot of heat out the top of that riser. The whole point of the refractory burn tunnel is to create an environment where the combustion gases can mix with high turbulence, and the insulated riser is a conscious design feature to keep the heat *inside* the system to make the combustion as complete as possible.

At the top of an insulated riser, the gases coming out are still really hot (1200C would be the low side of normal and I'm sure lots of garden variety builds approach 1800-2000C). But they've also finished all the oxidation that they are able to do and are mostly N2, CO2 and H2O. This is the best possible time to harvest that heat, and by putting your grill over the riser you get the simplest place in the bargain.

In short, although I think you've got an interesting conceptual direction here, a little grounding in the basics would probably give you a lot of lightbulb moments and help you arrive at a workable design.
 
Graham Chiu
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Max - in my first test in a different version I used a 3/4 brick under the metal plate but I didn't get the temperatures I wanted so I thought the gap was too big and not forcing the gases under the plate.  So, I switched to using firbreglass rope.  I could try it again.

That Ecostove, aka Justa Stove looks very interesting.  It looks doable with just what I have though it needs 70 bricks which is a lot more than the 15 or so I'm using at present.

Phil - no I haven't built anything insulated.  I do have a full piece of ceramic fibre board that I bought from Forman in Wellington but I'm waiting to find the most promising design before I cut.  The stuff's too expensive to throw away after testing!

Cheers,
 
Satamax Antone
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Graham, just build yourself a J tube or a batch, with your plate over the heat riser. I tell you, i really wish i could cook at lower temps sometimes. It often sticks because of too much heat evaporating any fat i put on the plate.

I often cook with this one



https://permies.com/t/35569/Range-retrofit

And it takes time to reach the temps you want, but it can do it.

But in the summer, i cook mainly with this one.  With a cast iron plate and two metal spacers to rise it above the metal rim.

But you really need to insulate the burn tunnel and heat riser, to have real smokeless results.

WP_000808.jpg
[Thumbnail for WP_000808.jpg]
 
Phil Stevens
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Perlite and fireclay slip is an easy and inexpensive material choice for an insulated riser.You could use a cardboard tube as the inside form and a couple of paint tins for the outside.
 
Graham Chiu
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Phil - that sounds like a fun project for the Xmas holidays.  BTW, do you know of anyone who sells those light insulating firebricks that don't cost an arm and a leg?  And you can easily cut without using a wet saw.

Max - I was reading about the Justa stove which looks very doable with only primitive tools.  But I see some users are having to submerge the bottom of the pot into the fire stream itself as otherwise pots don't get hot enough.
 
Graham Chiu
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What exactly is fireslip clay?  And what are the proportions of this in combination with perlite?  And are there grades of perlite?  Or does one just use the stuff from garden centres?
 
Phil Stevens
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Graham - I ordered bags of coarse perlite from this place https://www.easygrow.co.nz/34-mediums and it worked pretty well. The fireclay came from Southtile in Invercargill https://invercargill.southtile.co.nz/refractory (they also have a good selection of firebrick, ceramic blanket and refractory cement). You might find what you need there. I haven't ordered any firebricks yet.

To cast a riser with fireclay and perlite, you make slip and mix the perlite into it so that it is coated and sticks together but still has a "dry" consistency. My mistake was to use too much of the clay slip relative to perlite, which made the casting slump and take a long time to dry. It also means that it's not as insulative as it should be. When I tear the RMH apart to put in a new core I will probably cast a new riser while I'm at it.
 
Satamax Antone
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Well, lot of people have had problems with perlite clay mixes. I would say a five minute riser is may be easier to get right the first time.

https://www.facebook.com/photo.php?fbid=10154992096951974&set=a.10154992139891974&type=1&l=3e4ec1844f&theater
 
Graham Chiu
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There's a guy in another city who is selling ceramic blanket for a really really cheap price.  But he won't ship.  Such is life.
 
Graham Chiu
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Looks like I'm now the owner of a ridiculous amount of ceramic fiber blanket ... once I manage to get it shipped to me.
So I should soon be able to build my insulated 5 minute riser.  And, it's back to building the 4 or 5 inch batch rocket stove.
Just need a way to figure out on how to harness the heat coming out of the riser for cooking.

PS: Is there any need to buy some rigidiser to spray onto the blanket?
 
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