It's getting chilly and would like to start building very soon.
I have attached a top view scaled drawing and a picture of how it looks like in its early stages.
The batch oven will include a 5 min riser inside a metal barrel, a floor channel (it will be wider than what you can see in the picture), and a slightly tilted front door similar to the 7" batch of the Wheaton's lab post (Thanks for sharing).
Eventhough it's a 7" batch, I am planning to make the riser 8" because it's made of low biopersistent ceramic blanket. My understanding is that it should be 1" wider because this material is not that smooth.
Dry stacked firebricks combustion chamber contained within wide cob. Unsure if I should add perlite/ expanded argile to this cob mix. I would prefer not to use ceramic blanket to contain the burning chamber if there's an alternative that works equally well.
I am planning to use sand and glass bottles as the floor insulating first layer of the benches, contained within a rectangle made of hollow bricks bound to each other with cement. Rich in sand cob on top and smooth cob layer on top of this. Bench walls made of Bricks/ stones embedded in cob.
The exit for the chimney must be through the cold wall just behind the start of one of the benches back seats. I am a bit worried if this design creates a significant ASYMMETRY in the exits of both benches. The exit path is longer for one of the benches. I guess I could make the the shorter path longer... Also I wonder weather this horizontal exit flue would be TOO LONG.
I appreciate your feedback and corrections. This will be my first batch.
I guess I can probably solve the asymmetry and length of the horizontal exhaust pipes if I tilt them soon after they leave the benches and make them converge towards the corner behind the barrel, making them almost equally long. I reckon their length would be close to 2,5 metres ( 8 feet) with a 35% gradient and three elbow joints before they connect with the 10 metre (33 feet) long vertical uralite chimney. The external diameter of this chimney is about 25 cm (10 inches). I don't know the internal diameter.
I haven´t found much info or drawings of the flue/ conexions to the chimney part, neither about the materials and dimensions to allow the hot air go from the barrel into the benches.
I understand that the exhaust pipes must have half of their diameter below the exit/ lowest point of the bench. I don´t know how wide the tubes exiting each bench should be. I assume by the time they merge I should change to a tube with twice the cross section area.
Please correct me if wrong. Any advice/ drawings very appreciated.
I have recently read in a post that it's not that important to insulate the floor of the benches because the smoke will transfer the heat to the upper part of the bench, so I will probably change my floor plan now. I dislike cement for its high carbon footprint and non recyclability, so I would much prefer to use cob in the ground or rammed earth. Still considering embedding glass bottles in it, because we have lots and I guess some insulation won´t hurt either.
I am sorry I haven't learned how to use sketchup or similar software and can just share hand drawings and pictures with you. I hope to find a friend in the area that can help me with this soon.
Thanks a lot for your replies. I have kept building since my last post. All bricks for the combustion chamber (CC) cut and assembled, and also a little base brick ramp for the base of the riser. I rushed, had to return the tool to cut the bricks. Today I should have the floor chanel in. I hope to have it alight in less than a week...
There's a lot of info to take, and sometimes it's easy to overlook some of it. The table quotes a number for the depth, but also says further down depth can be 4 to 5.5 base (I had overlooked this). Thanks fire gurus for not pulling my ears this time. I am putting many hours of research a day, but I am already building, I have to compromise a little bit, otherwise we'll be cold this winter.
I kept the recommended depth at the top and made the base just slightly longer. I wished I had made it significantly longer, but may just let it as it is. I hope the 6% gradient I gave to it may be enough to keep the door stable without hinges and metal frame.
I have plenty of free expanded clay, so I would like to surround the burning chamber with it, and wrap it all up in cob or rammed earth. I am still researching the implications of embedding expanded in that wrapping medium, whether it´s cob or rammed earth. I am a bit concerned about the moisture absortion properties of expanded clay.
Also doing some research about how to protect people from exposure to ceramic blanket. I may be a bit overcautious, I got the low biopersistent kind, but the safer the better I guess, and would be happier not to need a special mask when repairing. Thinking about rigidising the 5 min riser, and possibly wrapping it up in some thermal tape. This could help with the smoothness of the riser walls also, but could be troublesome if it moves/ swivels/ it's bown by the smoke... More importantly also covering the ceramic blanket joints in the CC door. A cheap partial solution would be to wrap the external walls of the 5'' riser in alu foil (saw this in a video).
I thought of something like this https://www.3m.com/3M/en_US/p/d/b40067067/ Does anybody know how hot it gets at the bottom of the riser? I wonder if the tape would take those temperatures at the bottom close to the port.
Batch combustion chamber enlarged at the base, getting a steeper gradient to ensure stability of the unhinged door. Need to cut bricks again, but better design justifies it.
Welding job of floor channel nearly completed. Does everything seem right to you?
Has anybody tried vitroceramic glass as either the door or the top of the combustion chamber (cc)?
I can not find refractory bricks/ tiles long enough to make the roof of my cc. Would it be ok to glue refractory bricks to a large hollow brick to enclose this area? If so should I use refractory cement for this purpose or other cement would do?
It's not clear to me how to measure the height of the vertical section of the floor channel. What's the cut-off point? The opening of the mouth? The tip of hoodie? The sides? I guess it's not critical.
Amazed by the heat shield concept in a previous post. https://permies.com/t/204327/Batchbox-RMH-projet . Studying its implications for final distances of barrel to the two walls I have. Thinking of making them 50cm (20") and 20cm (8").
If I understood correctly the latter would create a heat shield and a current of air between wall and barrel, I guess ideal to dehidrate/ dry, while the former could be used to make an enclosed hot camera to bake at low heat...
I had some personal problems and project got delayed.
I got a bit slowed also down to have clear the interphase area allowing for maintenance holes. I learned how to use sketchup in the mean time.
Also unsure about other issues:
Is it safe to use vitroceramic as the cover of the combustion chamber? I saw some video of it being used on a second chamber on top of the combustion chamber (DSR?)
Has anyone tried a gas/ electric oven door as a door?
There are no refractory bricks long enough around to make the top of the combustion chamber. Would it be OK to use (refractory) cement to bind refractory bricks together to make this top? Any tips on this?
I attach the sketchup file of this project. You can add or remove layers for a better understanding. Interphase layer is still work in progress.
If vitroceramic is a cfb board, it will work until the wood abrades it to nothing.
I know, I tried cfb on two different batches, and even being careful you will still abuse the roof.
EDIT) I read the previous posts. I see Vitro ceramic is glass.
Ceramic glass has a max operating temp of 1450F Your batch roof will be running 1800 plus.
Do not attempt this as a roof.
I use ceramic glass in my doors but always in a metal frame.
Attempting to use refractory to hold 9" bricks will fail probably spectacularly!
I defiantly would not attempt it.
On my 7" batch I am currently using a large thick cast iron griddle as a roof.
My 7" is an extreme burning stove. Once lit it gets full loads back to back all day.
The cast iron has cherried out (1800 F) and sagged an inch or so in the middle but is holding there just fine.
If they are wide enough Kiln shelves have been used.
They do make 12x24 x2.5" firebrick tiles but shipping them is very cost prohibitive.
I am using one on my 6" batch and it is working great.
A gas oven door works great a 450-500F, I suspect it would melt if exposed to the inside of an 8" batch!
I am very sorry, but the drawing doesn't make it clear how you want to build the benches. What I see, is a drawing that consists of many flat sufaces that are clad with a brick veneer. In order to produce a usable drawing you should draw the whole of it brick by brick, just what you would do in real life.
I know, this sounds daunting at first although it's in fact quite simple. Draw one brick of the size that are available to you. Group that one brick and copy that again and again with copy/paste. This does work just like in a text document, Ctrl-C and Ctrl-V. Stack those virtual bricks so you'll get a drawing that exactly mirrors real life.
I could do a bit for you, but I don't know the size of your bricks. Please let me know what size of firebricks and red bricks you have there. Otherwise, I can't do anything for you.
Also, since you are in Spain there's a manufacturer of firebricks and so on in Andalusia. It's called Refractoria Andalusia, their bricks are marked with the letters RA. It seems to be a good idea to contact them.
thanks a lot fire gurus for your last replies. They were very helpful. Good to know best to forget some materials/ techniques to stay safe in this specific build/ expected temperatures.
I have spent a couple of hours looking for LARGER REFRACTORY PIECES. There seems to be a supplier in Italy that makes pieces as large as 50x50x6 cm. And I found two Spanish ones that can supply 33x33x4 cm (13x13x2").
So for a 7" batch, 259 cm wide cc, I could mount these 33x33x4 on TOP OF THE COMBUSTION CHAMBER, resting 3,5 cm on each side (1,37") .
That would leave 1,5 cm (0,6") until the edges of the brick on each side. I imagine combining a refractory ceramic layer and a thick cast iron sitting on top of it, with ceramic blanket strips on the 1,5 (0,6") edges . I guess will make it safer to cook on it, the iron won't get as hot or warp as much.
As far as the BENCHES go I have quite a few different materials to use as filling (sandstone rocks and scavanged solid non-refractory bricks of random lengths, standard 11cm widths and heights of 4,5 and 5 cm)...
So I was thinking of using sandstone at the bottom and bricks in the upper layers, as compact as I can minimising spaces in between them, filled with straw-less cob. Maybe while building I realise some other disposition fits better.
Because of their heterogeneity sketchup modelling may not match the real build. Do you still think it's worth modelling it brick by brick, Peter? Thanks a lot for your offer to help. I want to treasure your support Peter and don't want to abuse it.
From what I have understood the internal cavity of the benches (and all the areas where smoke travels through) should be smooth, smooth cob finish.
So in the sketchup bench layer I emphasize the internal cavity. If you remove all other layers and see them close to the axis intersection you can see they are hollow inside with a supporting wall creating two cavities. The idea of this internal wall is to support the weight of the back seat bench and increase the SA to come closer to the ISA.
If I have done numbers right this time, internal surface area inside each bench would be 1,88 m2 (3,76 m2 combined) and 1,6 m2 for the barrel 5,36 m2 total. That leaves 1,84m2 slack to reach 7,2 ISA, but I have not estimated the interphase areas yet.
It came to my mind it would be cool to have the back (specially kidneys) warm, so I want to warm the back bench without compromising on its structural safety (heavy load).
Note there's 10cm seat overhang to allow room for feet underneath.
I should be completing the drawings including the smoke chanels in next 2 weeks while I am away in Holland.
David, your plans how to implement the benches would mean those won't act as bells. Too shallow inside, the floor of the bench void won't need lots of mass, in a bell system the floor is the coolest spot in the system. The dividing wall in the benches will disrupt the natural flow of gases as well, hot gases in along the ceiling and cool gases out along the floor.
The clue of a bell system is to recognise how the gases would flow when left to themselves and provide room to facilitate that behaviour. A piped bench works radically different and you can build it like that, of course. A batchrocket system on the other hand, is very, very picky about restrictions in the smoke path, much more so as compared to the average J-tube system. You might end up fighting a system that fights back every time you are trying to run it.
I have put quite a few hours in the sketchup file. The cavities in the benches are ammended as suggested in the last feedback post.
Rough estimation of ISA is 7,1 m2 (chart critical ISA for 8" system is 7,2m2) and I plan to introduce a manifold/valve before the benches.
I am struggling with the interphase between 1. benches and barrel and 2. benches to chimney.
1. I can´t quite visualise how to build that bench-to-barrel gradient without cement and without limiting the opening of the bottom of the barrel. I am still not happy with my design for this part.
2. I had originally imagined one of the benches-to-chimney passages to go under the core, to make the exit path short and straight. This complicates a bit the ground support of the former.
So far the main idea has been to use ONE wide gate into and out of each bench. No ducts; stratification in hollow benches.
I am willing to consider other options. I am a bit concerned about the pickiness of the stratification systems and my design delay: not sure how much more time I need to sort the design of these interphases.
I also thought of
- sacrificing one of the benches + adding an extrabarrel
- reducing the length of the benches + using an exit gate at the end of them + conecting the bench smoke duct to a duct behind the backseat. If so, I guess it would be better to feed the smoke from barrel to backseat and then as it cools into the bench cavity (I think easier to build, rather than 1st bench 2nd backseat.
I attach the latest sketchup file.
would it be better to get the 5min riser right next to the barrel or should I leave a very little gap as I did in the drawing? (I know smoke won´t run through it) I am a bit unsure how hot it will get on the other side of the ceramic blanket. I guess leaning it against barrel may help slightly help with the stability of the riser and clearing some extraspace in the front.
likewise, is cob a good choice for the triangular sections between riser and the containing brick box?
today when I went to the scrapyard I thought of using a metal frame instead of bricks to support the barrel and connect it to the bench with a metal mesh on it to be embedded in cob. Hopefully that could work. I was struggling with the bulkiness of bricks and the gradient problem and it seems I just needed to leave sketchup and go to the scrapyard to get some inspiration...
You would want ‘castable refractory’ I think the one rated for 20mm max is a different product maybe for laying bricks.
You do not want to use any metal inside a refractory casting, it will just expand and crack, fiberglass is ok but most good quality casting refractory will already have fibers in the mix.
Thanks for your feedback James. So no metal embedded in cob, got it, different heat expansion properties to that of cob. What you call castable refractory I now understand is called hormigon refractario here, different indeed to refractory mortar.
I attach the latest sketchup file that emphasize the exit path of the smoke towards the chimney. I have tried to clean up the file. Sketchup seems to be a wild creature and there are errors that I don't know how to get rid of. But the important aspect in this drawing is the intended stratification and exit path.
I hope to weld legs to support the flying side of the barrel tomorrow and will be puting the bricks and the cob for real this week.
So anything else I should correct/ do differently pyrofellows?