6" batch box core details- advice needed

[anotherluke]

Hi All-

First off, thanks to everyone for contributing to this forum. It is a fantastic resource. 

Last year I began a collaboration with Matt Walker building his 6" J tube and offering them for sale at rocketstovecores.com.

I'm hoping to begin offering a 6" batch box in the near future. My goal is to offer something that is reasonably affordable and is easy to assemble. My current plan is to build the core completely out of ceramic fiber board and to include enough ceramic fiber blanket for the customer to build a 6 minute riser. This will add some more work for the customer but I think it is a worthwhile tradeoff in terms of affordability. I'm hoping to design it in such a way that a customer could source some heavy duty fire brick splits locally and replace a few of the stove pieces to make a more durable core. Alternatively RA330 sheet metal could also be purchased and used as a lining material (pricey, but I've heard it works well). 

Dimensions: The table at Batchrocket.eu recommends 8 5/8" w x 12 15/16" h. The depth can be 4 to 5.5 x the base, resulting in 17 1/4 up to 23 3/4". I am planning to go with a depth of 18" because it is divisable by 4.5" (the width of a standard fire brick split here in the states). Would enlarging the width to 9" cause an issue? That would make lining the floor with firebricks fairly straightforward (except for around the secondary air).

Port: Could the port be built out of full size fire brick (4.5" x 9" x 2.5"). It is supposed to be 9 1/2" tall, 2 3/16" wide, and 2" deep. With full size fire brick it would be 9" tall and 2.5" deep.

Secondary Air: I plan to cut a channel under the ceramic fiber board floor that is 2 1/2" wide and 5/8" high". Then send this up through a cylinder of  RA330 that is held in place by a small casing cut in the top of the ceramic fiber floor (see images below). What should the internal diameter of the tubing be? How tall should the tubing protrude from the floor? My goal is to eliminate the need for a metal pipe down the center. This will, however, reduce some of the benefit of pre-heating the secondary air. Thoughts?

Six minute riser: Have people had success using HVAC pipes from a big box store? Have they held up well over time? Something like this?

Super duty dense fire brick splits: I've seen 'super duty' dense fire brick splits listed that are built to withstand temperatures of up to 2700° F (~1500° C). Does anyone have experience lining a batch box with these? Do they hold up well? 

I have included a photo of the design (without the secondary air stub, just the hole where is should be place) and two close ups of the secondary air port (below and top view).

Thanks!



 

[belgiangulch]

Good Morning Luke; Your secondary height of 5/8" is to small. 2.5" x 1.5" for a 6". I would use a 2" RA 330 for your stub.
Changing the dimensions of the box itself (other than the depth) is not a good idea. Peter went thru extensive testing to come up with the perfect sizes.
Fire bricks even super duty ones will still crack over time.
Now the port... I do know some who built and went with 9". It seems to work but I have to think that efficiency sufferers.
So many of the specs on a batch were fine tuned with a testo. Just looking at the chimney to see smoke is not a good indicator.

6 minute risers) With experimentation it appears that creating a short doorway that your riser sits around is better than the original which butts up to the port and uses cfb to seal it up. We have had some metal pipe edges deteriorate if the blanket shifts.
By creating a short (less than an inch) doorway with brick that sticks inside that is eliminated.

The five minute riser that sits on top of a box can be made with hvac pipe no problems.

[josephcrawley]

Have you considered making molds and selling a cast core? The riser could be cast from light weight castable. This would be by far the easiest to assemble and would eliminate the need for adding fire brick. Also with the high price of CFB it may even be cheaper.

Just a note of caution in my experience firebrick sold a refractory suppliers and pottery stores are 9x4.5 but the ones sold at brick yards are 9x4. So far I have had no issues with spalling in the stoves I have built with these narrower fireplace duty brick.

Good luck with your venture.

[peterberg]

Hi Luke, welcome to the boards.
You prefer to use standard firebricks in order to minimize cuts. That can be done.
The dimension table on the site is done in steps but actually the thing is fully scalable. Which means you could enlarge the base number with a random percentage. On the site is explained how the base number is used to calculate all the other dimensions.

Now here's the trick: take the width of the firebrick that's available to you and use that as the base number. This should result in a core that's ever so slightly larger than 6".

The firebrick size you refer to is 9" x 4.5", so the base number would be 4.5". The bricks I used during development were 214x107x63 mm, which translates as 8.43" x 4.21"x 2.48". As you can see, the width is very close to the ones you have, so de scaling factor is very small. The size of the port is an issue, this won't be solved by this trick.

In the original development model the port was 72% of riser cross section area, while there was an overhang of the overhead p-channel. The resulting opening of the port should be 70% of system size. Roughly calculated this appears to be correct, the port could be 9" high and 2.2" wide.The downside is that the likelyhood of getting smoke out the door when opened is greater.
Depth of the port might be an issue in case it's above 50% above recommended value. So your brick thickness is well within tolerances.

The secondary air channel used by Matt is about the same cross section area throughout. Diameter of the vertical part is 2" in a 6" system size core, your planned channel feels a bit cramped. Matt's secondary air tube is almost but not quite as high as the port if I remember correctly so it's best to check with him. Normally, there's a generous layer of ash over the horizontal channel so most of the heating of air is done with the vertical part in my opinion.

I'd think the 2nd air tube is too far away from the port in your drawing, both sides can be as close as half the width of the port, from corner to tube. Check with Matt as well.

[anotherluke]

Thanks Tom, Joseph, and Peter for your follow up.

Tom- thanks for your suggestions about the stub. I will be in touch about sourcing them. How tall of a stub should I use? I still need to get the exact details of the 6 minute riser figured out- in particular the transition zone that you mentioned. Perhaps the port could be an inch wider on each side and the 1” which ceramic fiber blanket from the 6” riser could but folded out and attached in some way to the port? I will try and mock something up and send a photo

Joseph- you mention that you haven’t had any issues with firebrick splits spalling. That is very helpful to hear. Is this in a batch box stove? With the ‘duty’ grade high heat bricks, or just a generic off the shelf fire brick? Have they lasted for multiple years so far? I can get firebricks from a nearby masonry supply store but they are probably just rated to around 2100. I’m considering purchasing heavier duty bricks that are rated to 2600 service temps.

I haven’t looked into a cast core but am open to considering it. I haven’t researched it much. Would the castable be dense enough to withstand abrasion from the wood? Would it ship well? The appeal of the CFB is how light it is to ship and how it is relatively easy to cut. I also like the idea of firebricks that can be replaced by a customer when damage inevitably occurs. For now I’m going to pursue a ceramic fiber board core but am definitely open to other possibilities.

Peter- thanks for all the knowledge you have shared, including the plans for the batch box and other stoves. I have found your site very helpful. I like the idea of using 4.5 as the base instead of 4.32. In my current plans I have designed the base to be 9” wide and the height to be 12 7/8 when built with just ceramic fiber board and 11 5/8 high when lined with firebrick splits.

I want to ship an all ceramic fiber board core that can be assembled and used straight out of the box, but that could also be lined with uncut fire bricks splits if the user wished. This constraint leads to some challenges. My board is 25mm thick (1 1/16”) and firebrick splits are typically 1 1/4” thick.

Please see the two attached photos for my current ideas. They present some compromises that I’m uncertain about. It is fairly easy to replace the inner side wall of ceramic fiber board with a slightly thicker brick. The challenge for me is the floor. I’m wondering how significant of a difference it will make in performance to have the floor lined with 1 1/4” bricks. As mentioned above, this would reduce the box as shown from 12 7/8” tall to 11 5/8”.

Lining the floor with bricks could also potentially reduce the size of the port. My plan is to construct the port of ceramic fiber, just like the rest of the core. If a customer wishes they could replace it with something made of firebricks, although it won’t be quite as straightforward as lining the sides and floor with uncut brick. The port that I build from ceramic fiber board will be 2” thick, 9.5” tall, and 2 3/16” wide as called for in your plans. I am wondering about having the port raised 1 1/4” from the floor of the box so that if the box is lined with firebricks they won’t reduce it’s size. See the drawings below. What do you think of this solution?

I will have to figure something out for the rear piece of firebrick on the floor that interfaces with the stub (it will need a whole cut out of it). I might supply this myself and let customers source the rest of their firebrick locally.

It looks like the p channel in your plans is only 2 3/16” x 5/8”? Do you know why Matt’s is bigger? I have adjusted my plans to reflect the larger channel but I am curious about the difference.

I will post updated photos in a following post.

Thanks again!

[anotherluke]

[anotherluke]

[anotherluke]

One other question I have is whether 2300° board should be adequate or if I should source 2600° board?

[josephcrawley]

Feb 27, 2021 10:34:32 GMT -8 anotherluke said:

Thanks Tom, Joseph, and Peter for your follow up.


Joseph- you mention that you haven’t had any issues with firebrick splits spalling. That is very helpful to hear. Is this in a batch box stove? With the ‘duty’ grade high heat bricks, or just a generic off the shelf fire brick? Have they lasted for multiple years so far? I can get firebricks from a nearby masonry supply store but they are probably just rated to around 2100. I’m considering purchasing heavier duty bricks that are rated to 2600 service temps.

I haven’t looked into a cast core but am open to considering it. I haven’t researched it much. Would the castable be dense enough to withstand abrasion from the wood? Would it ship well? The appeal of the CFB is how light it is to ship and how it is relatively easy to cut. I also like the idea of firebricks that can be replaced by a customer when damage inevitably occurs. For now I’m going to pursue a ceramic fiber board core but am definitely open to other possibilities.

I have used both splits and regular fire brick generic from the brick yard with good results and no spalling after 4 years in service.

I have not worked with castable but my understanding from others on this site is it has the durability of fire brick.

I feel that the most difficult part to produce is the metal work for the stove and that would seem to me the most marketable and helpful kit. If someone is going to build a stove based around a kit core they are probably cable of laying the brick for the firebox. Just my two cents.

[anotherluke]

Thanks Joseph. Good to know about the firebrick. Agreed about the metal work being one of the most difficult parts of the stove for most people. I'm hoping to offer a core that is affordable and reduces a lot of the head scratching involved in regards to both the metal work and the brick work. For some people a brick core made on location is the way to go but I have heard from folks who want a pre cut ceramic fiber option and I'm hoping to provide something. 

Peter- if you read this and have a chance to comment about some of my questions regarding dimensions and the port I'd really appreciate it!

Thanks again
-Luke

[anotherluke]

(Edit: there was a question about numbers for the secondary and primary air. I had my math wrong)


Is the primary air always at the very bottom/ground level?


Also- any creative solutions for reducing sparks? I have seen the photos on Peter's site with stoves that have a hinged piece of material facing up which lets both primary and secondary air in. And of course Thomas's door. I'm hoping to make something simple out of CFB that could be modified by the end user. It might be up to them to figure out how to deal with the sparks. But I'd like to hear of any other ideas... 

Thanks!

[peterberg]

Luke, regarding your questions about lining the firebx floor with split firebricks: of course you can do that, no problem. Think always from inside out, what the outside dimensions are is not important, the fire won't know anything about that. In case you want to rise the port slightly above the floor in order to line it with firebrick: that's fine. But in that case you need to raise the ceiling as well so the piece of rear wall above the port remains the same.

Matt's secondary air channel is more akin to my floor channel. The p-channel is an overhead device, quite unlike the other options. My floor channel arrangement is a bit different from Matt's in the sense that the horizontal part is twice as large as the vertical part. In this way, the vertical part will act as a venturi in itself, providing a better cooling function. The function of both is quite the same, it's up to you which one to use.

[peterberg]

Mar 10, 2021 18:21:33 GMT -8 anotherluke said:

Is the primary air always at the very bottom/ground level?

Yes, at the time I tested a multitude of air inlet arrangements and this one appeared to yield the best consistent results.