Help with Rocket Stove for Black Rocket Oven

Hi all,
I am planning to add a rocket stove to an existing wood-fired oven.
I am looking to get the most reasonable design given the limitations I need to live with.
Limitations:
Oven flue exit is 8" (20cm) but it needs a 10" (25cm) flue pipe due to minimum stack height for effective burn in the oven and maximum stack height allowed by construction rules in my area;
Total Rocket Stove hight must be 40" (around 100cm) as that is where the oven floor slab is;
Total volume of the Oven in around 92 gallons (around 350 liters);

My idea was to build an 8" system to have enough heat going in the oven and also to be close to the 10" flue pipe dimensions.
I was thinking about a batch box design, but looking at the dimensions suggested by Peter van der Berg I won't be able to stay below 40" height.
I then thought of a J type stove, but am unsure if that would be effective and how best to dimension it.

Would any of you be able to give me hints on what direction to take?
Thanks very much in advance for your help.

LST

Luca, it may be worth looking at a 'Double Shoebox Rocket'. Peter has been developing a version which doesn't need the height of a 'standard' batch box heat riser. There's some videos on youtube that would be worth checking out.

Hello Satamax, John,
thanks a lot for your suggestion.
Yes I am reading that thread. And it looks like it might be the best way to go.
Can I ask you if you would use the Oven as the top shoebox or would you build the double shoebox so that the floor of the oven is the top of the second shoebox and the flue goes inside the oven?
Thanks again for your help.

All depends what you want to do. If you put the exit of the second shoebox under the slab stones of the oven, and make the gas entering in the oven, on the perimeter of the slab, i think your oven would heat harder, and you would have the mass of the slab heating faster. A bit like a pizza oven. But you would have to bend over a lot to feed the firebox.

Then if you do a side exhaust, the heat would be in direct contact only with the top part of the slab, at the expense of a slower heating. But you would bend may be 20 or 40 cm less than with the other version.

Hi Luca,

I built a rocket stove pizza oven. And we have rebuilt it to fix some problems.

The basic design is to take a barrel type wood oven and on the back arch you extend the arch to create an opening for the rocket exhausts to go into the oven.
I have two six inch batch burn rocket stoves under the oven, one for each side. The risers extend into the oven and go 2" above the floor to both bring in flame and heat and prevent things from falling from the oven into the riser.

The first change was to create a higher burn chamber and to raise the port at the back so the smoke would go to the riser rather than the door when you load more wood, a pizza oven will take many loads. We made sure the top of the door was at least 2" below the top of the port.
That was done on version one.
On version two we have made the wood burn chamber ridiculously larger. I was worried, but it burned well. It is just over 4' or 1.3m deep. We also made the rockets riser on the side of the burn chamber, both original riser ports were destroyed by enthusiastic helpers throwing wood into the burn chamber with too much vigor. Now with the risers on the side and to the rear of the rockets, there is a bit more resistance. And the secondary air comes from below now, the upper one burned out fast, the rocket pizza oven is kept at a high temperature for 5-7 hours so lower secondary air is easier to replace and less steel is exposed to the high heat.

This oven has been used in a restaurant for over a year now and has performed well. The main thing that was needed to make the rocket part work was the door. The first door was a box kind of door, basically a rectangular piece of chimney with a slit on the oven side as low as possible with the area of a 8"/20cm chimney pipe. That lowers the heat layer to bring the temperature of the bricks up to temp quicker. When that box failed and a normal door was put in the oven did not preform as well as before. A new one is being built and it will open guillotine style now via foot pedal.

I can send you pictures if you want?

Good luck

Hello Satamax,
most helpful thanks.
I believe I will go with the second arrangement, with the easier loading of wood, although I hope to scale up the shoebox design so that it won't need refueling on a typical cook.
I seem to remember that Peter van der Berg was not very optimistic about this though. On the original thread, he suggested repeatedly to keep the dimensions as close as possible to the double shoebox. This would change dramatically the dimensions of the top box.
My problem is that I won't have a chance to adjust the design once the rocket+oven is done!

Hello Jason,
thanks for your post: very helpful and very informative.
I would really appreciate if you could send me the pictures. That would make following your reasoning a lot simpler.

Best.

Luca, if you want, you could test this one outside. I haven't had time yet. It's a sketchup file.

https://permies.com/t/38889/a/62582/verticalbatchdsr.skp?download_attachment=true

Made to sit lower, and have a top load. So you don't have to bend down much.  But since it sits on the floor, the riser doesn't go very high.

I kind of based it on the batch rocket, dimension wise, but with elements of the dsr. The air flow crosses the wood. As in the dsr, pretty much. It would be top lit too. So i think hard to overfuel.

Hello Satamax,
thanks for your sketchup.
It looks like an interesting idea, but I am not sure how it is supposed to work.
Would you have to close the front air intake before feeding wood (so that air coming from the top would avoid having flames coming up)?
What parts should be insulated and which should have heated mass (refractory mortar/bricks) to make sure the proper gas flows are established fast?
I take it that this is an untested idea as yet?
Best.

Luca, this is untested core. All of it should be insulated. It is drawn with the firebricks slabs that i have. I dont know yet, if the opening of the firebox would flame back.  If built on the floor. It would be easy to push a firebrick against the air intake with your foot.

Thanks again Satamax,
I will shortly try either your concept or the DSR as per your earlier suggestion.
Best

Hi Luca,


Sorry I didn't get the message there was a new post here.

Here are some pix of the oven.
We still have to add the secondary air feed, so we have to be careful how much wood we put in, but it throws out a lot of heat. The foot pedal is for the guillotine door that helps get the oven really hot.

Hi Luca,

Have you made any progress with your project?

I'm looking at building a rocket pizza oven using an electric oven, this is my thread here if you'd like to have a look, tho no resolution yet:

https://permies.com/t/93073/Black-oven-pizza-baking-build#764933

The DSR seems interesting, I wonder if its possible to make the cooking area the second shoebox. (unless I misunderstand the whole concept)

Jason Learned wrote:Hi Luca,


Sorry I didn't get the message there was a new post here.

Here are some pix of the oven.
We still have to add the secondary air feed, so we have to be careful how much wood we put in, but it throws out a lot of heat. The foot pedal is for the guillotine door that helps get the oven really hot.

Jason,

Any chance you upload some more photos please, or better yet, a sketchup of how your setup work? I can't make sense of it.
Do you get flame inside of your oven?

I'll get some pictures of the older design when I get home. The new burn chamber is too difficult to use. So we are going to shorten it. The old design was 3 1/2 bricks deep and 1 1/2 bricks wide with 4 layers on edge and the port was raised off the floor about 2/3 rd's of a brick height to leave room for the ash build up. The door was below the fourth layer so the smoke would more easily go to the port which was higher than the door. This system was easiest to use, but a volunteer threw in wood too many times and broke the port. Hence the side burn now. I think we will go back to the first version now with some iron cage to protect from volunteer gusto. And yes we get huge flames going into the oven. That is why we load them staggered so the flames can burn better without hitting each other. It is a nice effect, but we get better consistent heat if we let one roar and as it simmers down we load the other and away it goes.

Jason Learned wrote: but we get better consistent heat if we let one roar and as it simmers down we load the other and away it goes.

So the two can't roar together?

I wish I could see this oven in action to get a better understanding how it works. Does it burn clean like a rocket?

Jason Learned wrote:I'll get some pictures of the older design when I get home.

You live very far...