Rocket stove - hot plate.

I have been experimenting with a P plate, this one is made from 2mm stainless and seems to work.

Fox James wrote:Hi, thank you for your interest:-)
Luckily for me the photo angle is just causeing an illusion because i have made the sections 6x6” .. feed chamber and burn channel are 6x6” the heat riser has a 6x6”base tapering to a 6” ID round.
I found one 3/4 bag of 2000c castable and I mixed that with my standard 1400c castable, total use was approx 35kg.
There are no visible voids and the iinside looks smooth.

So useing the 124 formula my heat riser only needs to be 24” long making a total length on 32” from the burn tunnel base?  

Hi James,

What is the overall outside dimensions of your casing, length, width and height? Also what is the inside height from the base of the burn tunnel to the top of the channel,i.e. Just below the area where you have the little cut out between the square burn area and the round heat riser opening? Thanks.

Sincerely,

Ralph

Hi Ralph, I made the refactory 1.1/4” thick so the outside dimensions would be be 8.5 wide x 18.5” long x 9.5” high and 8.5” over the tunnel area.
The riser started off at 32” total hight from the base of the fire box, but I found that 40” worked soooo much better that I had to dig ou my foundation and sink the whole thing down so my cook plate was not to high!
I have extended the feed box hight to 12” now.
Everything was surrounded with a further 4” of cement vermiculite mix, so it was quite a big piece in the end!

I have a mark two planned for the new year, I have the material but not the time until after Christmas.
So mark two will be made from ceramic board but with a refactory base and feed box, I hope this one will work with a shorter riser as it will use a vortex design.
I also plan a much bigger feed box!

I am still trying to get my head around the best building materials.
I just don’t know enough about the potential of stored heat in the rockets main components.

I can see how low mass ceramic fibre board will heat up the top plate very quickly but for my installation where consistent heat is important, I think a high mass cement engine offers more benefits.

I understand that anything that steals heat away from the flames will slow down the time it takes to get maximums temperatures but what happens when the mass is saturated with heat.
From my very limited experience I have found that changing just one component, the heat riser in my case, really does have an effect!

What would happen if I removed all the mass from my engine, at the moment I still have 50kg of refactory in my firebox and tunnel.

The 5 minute riser does not hold heat at all, the refactory riser holds heat for at least 24hours but perhaps more importantly it gets really, really hot when the fire is working.
The best example I can offer is when burning a full size log, with the five minute riser the log still buns but, will smoke back and not really offer any heat on the plate. With the hight mass riser, the same size log burns without smoke back and still heats the plate to 250c! (Or. Perhaps the pre heated riser maintains heat on the plate)
I wonder, with a full ceramic board engine, how the log would burn?

I think that if there was a bed of coals in a ceramic fire box, then the log would burn but once the coals have gone the log will not burn very well.
In a high mass fire box, the actuall box will be extreamly hot and allow the log to burn?

Or... the refactory will steal heat from the log and it won’t burn?
LOL 😂

Can you mix and match?  Have the ceramic fibre at the base of the riser so that it rapidly comes up to temperature, and allows the secondary combustion to occur, and have the upper half of the riser as refractory mass to hold temperature in the riser?   So, have a full refractory riser, but just line the bottom half with ceramic fibre.

Or do this outside of the low mass heat riser.

i am not sure if having mass on the outside of the engine and riser would have much effect to the burn as it won’t reach the high temps required to combust wood?
I do think having extra mass inside the barrel is a good thing just in a different context.

The outside of my barrel, near the top can get to 350c and 220c at the bottom, so no doubt any mass inside the barrel will heat up and stay warm, but I have a feeling it is when the mass is at 500c + that things start to make a difference.

Generally speaking wood burns best when heated from below and fed air from above, this is achieved when we have a good layer of coals and a good draw of air.
That is fine if you have a good layer of coals, but if you have really hot mass that is holding heat will that be beneficial in anyway or not make any difference?
The same applies at the bottom of the riser, if that is glowing red hot and storing heat will that help second air combust the fuel or offer any other benefits?
I have a feeling that mass would make the rocket more stable and non temperamental but I really don’t know!

I am not trying to re invent the wheel and if I had more time and money I would find out myself with more experimenting, I just want to get the best out of my next build.

Well, i like a little mass in the heat riser. But i will try a five minute riser or two in the future;

To explain about my idea of adding mass outside the riser. On inside the barrel.

I have my lovely workshop heater, you've seen pics now. The heat riser is made of ceramic flue elements from tona. Rockwool, perlite/vermiculite mix. And all of that is dry stacked, held in place by dry stacked pozzolan flue elements.  Let say, i burn for four hours, or five. The heat riser outside layer's heat is still feelable the next morning. 8 hours after the end of the burn. You believe me, you don't. Up to you!

I would say, in your case, it's well worth a try, or/as well as lining the inside of the barrel too.

I will Max...
I will also put another idea to you... I plan to make mark two from brick and with a bell bench.
I thought it might be fun to expose the riser top inside a glass box!
This would be fairly simple to do if I build a square brick bell around the riser, I just need to make a six inch high ceramic glass box on top of the brick.
The idea is you could see some action and movement from the fire, of course it might steam up or go black but it might just work.
I think that even watching a bit of fly ash is better than nothing...

Graham Chiu wrote:Can you mix and match?  Have the ceramic fibre at the base of the riser so that it rapidly comes up to temperature, and allows the secondary combustion to occur, and have the upper half of the riser as refractory mass to hold temperature in the riser?   So, have a full refractory riser, but just line the bottom half with ceramic fibre.

That thought is seriously intriguing...

It is possible the OP is struggling with the subtle, yet crucial, differences between how one would build a rocket stove and how one builds a rocket mass heater.  The concept of building a hybrid of those is usually accomplished with some kind of bell arrangement, but doing so within the confines of the heat riser, itself, is worth contemplating...at least in this use case.

Personally, I like my rocket stove to get as hot as possible, as soon as possible, because of how I choose to cook with it.  Slowing the process down by heating a large mass is contraindicated.  The OP's past experience with building excellent pizza ovens may be influencing his objective of building a rocket stove?

Fox James wrote:

Generally speaking wood burns best when heated from below and fed air from above, this is achieved when we have a good layer of coals and a good draw of air.
That is fine if you have a good layer of coals, but if you have really hot mass that is holding heat will that be beneficial in anyway or not make any difference?
The same applies at the bottom of the riser, if that is glowing red hot and storing heat will that help second air combust the fuel or offer any other benefits?
I have a feeling that mass would make the rocket more stable and non temperamental but I really don’t know!

The fire triangle says that a necessary amount of heat is required for fire to occur but excessive heat is not mentioned as a cause of stopping the fire.  Whereas too much fuel can cool the fire extinguishing it, and too much air (which carries the oxidising agent) can pull the flames off the wood.  So, maybe excessive heat can allow you to put larger pieces of wood into the fire without causing the temperature to drop too much leading to smoke?

Graham Chiu wrote:[quote=Fox James

?
Hey Graham, this NZ gasifier is quite inspiring....

https://m.youtube.com/watch?v=z5YGGDts_tE

Hi Fox

That's a very interesting video on the Waltherm which I hadn't seen before.  It was the first ultra low emissions burner (ULEB) introduced into NZ.
They have a newer model this year which is NZD2000 cheaper, and has automatic shift to down draft mode.

What's interesting to me is that it says it uses 1/3 of the wood a normal wood burner uses but a rocket mass heater apparently uses 1/9 of the normal use.

I bought a similar model this year, and here's a video of my trying an alternative method of lighting it.  You're supposed to incrementally fuel it which is painful so I tried top lighting it and it works.



The Waltherm has emissions of 0.3 g/Kg in down draft mode and mine has 0.2 g/Kg so mine is supposedly cleaner at burning.  But I have a very dirty glass in the bottom chamber as seen in my video suggesting that I've had some dirty burns ( well, the AirBnB guests who were using the house ).  The Waltherm video says that the flue temperature is about 100 deg C but in my video above at 1 hour I'm seeing 230 deg C so I'm losing more heat than they are.  Mine also doesn't have heat exchange tubes that need to be cleaned so I guess that's how they extract more heat out before it gets lost up the flue.

It's also interesting that the Waltherm can do an extended overnight burn.  Mine doesn't say that but I don't see why I couldn't try it except there's no point at present.  The heat in my living room gets trapped in there by a low opening door ( standard 190 cm high approx I guess with a higher ceiling ) and until I install a fan or a transom window, the fire place isn't going to heat the rest of the house.

It would be nice to be able to install a bell to capture the heat from the flue but my floor has not been designed to carry the weight!

That is a very nice stove you have there!
Just right to sit in front of  when you get one those heavy showers!

It gets too hot to sit in front of it!  So, I move to the sides where the sofas are placed!

So the logs sit on some form of perforated plate that allows air to travel down through the fire and right through the ash as well?

The logs just sit on the bricks or vermiculite board.  There's a cast iron grill that sits in the hole/Venturi between the two chambers.  See https://www.youtube.com/watch?v=LNSe8F9dzDY at 1:39

It's like a crown which I guess is to stop logs from blocking the Venturi.

I also recently noticed that the Venturi has a number of tiny holes around the circumference in the middle of the blocks.  So, I'm guessing that secondary air channels have been cast in the blocks.

Ok sorry I did not watch your vid very well first time!
It looks to me as though you need to pile in a lot more wood to get it really hot before the re burn becomes really effective.

Hi Fox

This might interest you https://www.motherearthnews.com/diy/buliding-an-outdoor-oven-zm0z17amz They use alternating layers of fireclay/sand and ceramic fibre blanket to form their burn chamber.  They claim it keeps the oven very hot on not that much wood.  There's also aluminium foil wrapped around the ceramic fibre which I believe is to reduce infra-red radiation losses ( I read this in one of the Aprovecho writings ).  It may take the same time to get up to temperature on the first burn as a refractory burn/riser but since it's super insulated, subsequent burns may be a lot faster in getting it going.

That link takes me to a clay oven, is that correct?

Yes, the oven is used as the burn chamber.  But the construction methodology might apply to other burn chambers in that not only is it super insulated, it also retains residual heat which the straight ceramic riser/burn chambers don't.

I am not sure about useing foil between the insulation layers because I think it will hold back moisture!
There are lots of reasons why outside dome ovens crack and I try to avoid all of them if possible.

Refactory materials have a nasty issue of absorbing moister from the air and when heated up the moister causes steam which expands and makes the cracks you typically  see inside an average pizza oven!

I build my ovens in component form that allows for expansion but any trapped moister tends to be focused outward away from the heat source.
I think the foil would be a barrier to the steam and cause lots of issues.

In my design, I use 2” of refactory but the mix contains nylon burn out fibre and stainless steel needles. The needles are about 2” long and offer much added integrity, the burn out fibres are micro thin and once heated melt away to leave avenues for escaping moisture.

The refactory is covered in 4” of ceramic fibre, 2” of cement mix and a fibreglass cover, the fibreglass uses  a vent at the top to allow steam to escape.
If the oven has not been used for some time, it will suck up moister from the atmosphere, you can see the steam escape once it is re heated.

My ovens heat to around 500c and re burn any smoke inside the dome before the gases enter the chimney. In the summer the oven holds useful cooking temperature for 3 days, 2 days in the winter.
My own oven is quite a few years old and does not have a single crack in any of the visible refactory.

To get the best smoke free performance, The entry to the oven must be critically sized to allow the right amount of air to fuel the fire and allow the gasses to escape.
What I find so amazingly fascinating, is that there is still evidence  of this exact  formula dating back to Roman times! We have not managed to improve the formula over the last 2000 years! Pretty old technology then.....

Fox, what type of glass are you using on the top of your riser?  And what temperature is it rated for?

It’s trade name is NeoCeram, I have managed to get a good regular supply from my local wood stove shop.
Just off cuts but handy sizes, it is rated to 600c but I find it goes opaque at around 500c after a few hours.
The piece over my fire box stays clear and clean it is the piece over the riser that seems to sort of melt just on the surface, a sort of grainy effect but it doesn’t seem to crack!

Hmm.  I was under the impression that the top of the riser would be higher than 600 deg C.  Is that with your ceramic fibre blanket riser?  And 3 inches or more above the riser?

Both risers offer about the same max temp, the hottest I have seen was 547 c on the glass.
If I shine the gun down the riser without the glass in place it shows around 800c.
(4” gap)

I think it might of  been possible to get higher temps if I had used better insulation around the fire box.

I used 4” of vermiculite mixed with cement but maybe some heat still leaks through into the surrounding soil.

Perhaps pre heated primary air would add a little more.

Howerver I have watched hundreds of videos and read as much as my time allows and I don’t see any other J tubes going much higher than 500c  on the barrel top.
I would think that a batch box with its much bigger load of wood should go up higher inside the riser but they normally have a large gap above the riser.


For the hot plate I used two rings of steel bolted together in order to avoid to much distortion but even then the plate takes on a very noticeable concave shape when the temps go up high! I think a single piece of steel would deform too much!

I have learnt huge amount by experimenting with this stove and I am sure I can improve on the basic  design when I build the next one.

Peter says he sees 900 deg C at the top of his batch box riser - see https://permies.com/t/93424/Needing-Feedback-design-sidewind-batchbox#798102

Wow 900c is amazing!
Although I am interested in the batch box design, I don’t see it has much use for my purposes.
They have been designed to heat mass very quickly with short intense burns, I want a constant heat and the fun of feeding the fire while I cook.
I think an 8” J tube would be closer in performance but I don’t really need any more performance than a 6” can offer.
What I would like is a shorter riser and gasifieing option with a visual experience thrown in.

I'm not sure that's the case. What the batch box does is deliver heat for a defined period if you don't keep feeding the fire.  So you control the heat by the amount of fuel. The duration of the burn stays the same probably because heat transfer to the wood is not affected by the amount of fuel being loaded. This means that you have time to cook between loading the fire box. And you can have a fierce flame visible in the batch box door.

For cooking, i like the speed of a small 6 J in the summer

And absolutely love my workshop batch. With the added oven and range top.

I already have an amazing wood oven and I just don’t think I would enjoy a batchbox as much as a top loading J tube.
I do have quite a few people interested in smokeless outdoor cooking plates though, so I need to build my new design and take it from there... if someone wants a batchbox I will be only to happy to build them one.

It's hard to beat the entertainment value of watching a fountain of fire hitting a cooking plate in the double shoe batch box rocket stove!

Graham Chiu wrote:It's hard to beat the entertainment value of watching a fountain of fire hitting a cooking plate in the double shoe batch box rocket stove!

Ha Ha well maybe I will find a corner somewhere to build one

Preparing for our Christmas Eve party, I will have the pizza oven heated up, the rocket stove glowing and my washing machine drum flared up too
Cooking all sorts of seasonal fayre for around 40 guests.....

Fox James wrote:
Cooking all sorts of seasonal fayre for around 40 guests.....

Got room for one more?  

Come over next year mate!
As it happens my best mate is in NZ, he has a house in the Bay of Islands but I am not so keen on warm humid Christmases but I might come and check out you rocket stove next time I am in NZ.

Fox James wrote:Come over next year mate!
As it happens my best mate is in NZ, he has a house in the Bay of Islands but I am not so keen on warm humid Christmases but I might come and check out you rocket stove next time I am in NZ.

I'd love to but where exactly are you?  It's not in your profile.

As for my rocket stove ... well, I finally have access to the back yard.  The builders have finally gone for Xmas and left the new house without water, gas or power.  So, it looks like I need to build something quick for my daughter to cook with!

Well I am just about as far away from you as could be possible...
I live on a small island called Guernsey

Fox James wrote:Well I am just about as far away from you as could be possible...
I live on a small island called Guernsey

We drink milk from cows that came from your area, that and Jersey!