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Post by smarty on Apr 27, 2014 1:04:43 GMT -8
Ok. So following on from the discussion on the thread about my J-tube that chocked itself off on unburned charcoal I have decided to make one in ceramic shell to see if performance can be increased.
Since I am trying to retrofit the rocket stove into an existing solid fuel cooking stove that a few things limit my options. The system size - based on the flue outlet of stove is about 4", the necessary length of the burn tunnel, and the maximum height of the riser.
Together these things mean I need a J-tube that is 1.3 system size. So here are my workings out:
1.3 System Size Where S/S = 100mm
1.3 x S/S = 1.3 x 100 = 130mm
Measuring from top of feed tube to centre of burn tunnel, centre of feed tube to centre of riser, centre of burn tube to top of riser.
proportion of elements: Feed Tube : Burn Tunnel : Riser = 1 : 2 : 4 where 1 = 1.3 S/S = 130
FT : BT : R = 1 : 2 : 4 = 130 : 260 : 520
FT = 130
BT = 260
R = 520
Adjusting these measurements for division into separate element of the stove for the purposes of construction we have as follows:
FT = 130 - 50 = 80 = FT measured top of BT to top of FT
BT = 260 + 100 = 360 = BT measured end to end.
R = 520 - 50 = 470 = R measured top of BT to top of R
Now I'm off to make a cardboard core based on these measurements around which to lay up my ceramic shell.
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Post by peterberg on Apr 27, 2014 7:25:02 GMT -8
Smarty,
You are the first to build a J-tube calculated this way. That is to say, that I am aware of. The development of a calculation system like the batch box specifically for the J-tube certainly is a good idea.
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Post by photoman290 on Apr 27, 2014 14:03:24 GMT -8
this seems like an excellent method of design. i am happier with formulas you can plus numbers into more than direct measurements. if it works ok can someone do a speadsheet. unless there is already one i have missed. been so long since i used excel it would be a struggle for me to do one.
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Post by smarty on Apr 27, 2014 23:22:02 GMT -8
Well, here is my cardboard core. The last photo should be at the start. I made the simple shape first then cut away and added bits until it had as many of the recommended features as possible. So a pitched roof to the burn tunnel and a trip wire as well as a ramp and kick tail. No idea whether I got these details right so any comments appreciated. app.box.com/s/99a3g0li1ln4y7pgxsx7I might try making the core out of ceramic shell because I know that works, and try making the riser using the paper clay method just to see if that stuff is any good for our purposes. Ceramic shell is a bit of a waiting game so it will be a few days before I update with any results. |
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Post by smarty on May 13, 2014 2:26:52 GMT -8
Some pics of the J-tube fresh out the oven. app.box.com/s/j1zqsj5qui2o72kkyzm1Next step is to back up this thin ceramic shell with about a half inch of fiberglass strand mat dipped in the ceramic slurry to add insulation and strength. Getting close to testing the thing though so I'll see soon if it helps with the charcoal build up. |
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Post by Deleted on May 13, 2014 4:43:08 GMT -8
Thanks dor the picture.
How thick is the shell?
How many times did you have to repeat the process?
I would have made round pieces.
Round pieces are less prone to stress of all kind.
The heat loss is smaller too,
as the circumfence for the same CSA is smaller
than with angular shapes.
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Post by smarty on May 13, 2014 8:39:48 GMT -8
Yes good point Karl about the round v square. I think you could be right. I find it easier to cut and stick flat bits of 2mm thick cardboard than roll them though - they tend to get the wobbles and I end up with a correspondingly varying CSA. I could use postal tube or something but it is quite thick and could expand too much before it starts to burn out - thus cracking the shell. Also I wanted to incorporate trip wire and secondary air similar to the dragon burner and it was easier to get my head around using square. I've made square ones before in ceramic shell and they seem remarkably resilient to cracking or stress cracks due to thermal cycling. At the moment they are 5mm thick. This was two coats using sand sprinkled over the slurry coating and then three coats of granules sprinkled over the slurry coating. I was going to back them up today after which they will end up about 15mm thick. the back-up helps to make them tougher with any knocking about they might get and also adds a primary insulating layer. It was raining today, however, so I couldn't resist giving them a trial run. See additional pics: app.box.com/s/ymbmtvbyr78i4ulo6esyIt roared like a good'n I have to say and the coal build up seemed markedly less than in the cast refractory version I made, so it seems like a case of so far so good. It must have got quite hot because the burn tunnel was sitting on a cushion of ordinary rock wool and partially fused itself to it up the feed tube end. There was carbon in the riser but not in the burn tunnel, but then the riser wasn't insulated and was packing out some serious heat. |
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Post by Deleted on May 14, 2014 5:27:59 GMT -8
Looks promising.
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Post by mintcake on May 14, 2014 7:21:04 GMT -8
Yes good point Karl about the round v square. I think you could be right. I find it easier to cut and stick flat bits of 2mm thick cardboard than roll them though - they tend to get the wobbles and I end up with a correspondingly varying CSA. I could use postal tube or something but it is quite thick and could expand too much before it starts to burn out - thus cracking the shell. I'm about to find out how well it releases from clay+perlite, but I'm planning to use a PVC drain pipe as a form for my riser. 125mm pipe is pretty much dead-on 5inches, which is what I need. I'm thinking that I'll add some thin paper and oil or something along those lines, but not having tried it yet it seems ideal. How rigid or otherwise is your material before you toast it? Do you need something that'll burn out? |
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Post by Deleted on May 14, 2014 8:10:13 GMT -8
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Post by photoman290 on May 18, 2014 2:14:09 GMT -8
i like the idea of ceramic shell casting a burn tube. where do i find the materials in the UK? seems it may be a better option than refractory cement for experiments. saves having to make a vibrating table. my idea would be to make the shell then cover it with ceramic blanket soaked in colloidal silica for a really light burn chamber.
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Post by Deleted on May 18, 2014 6:01:24 GMT -8
where do i find the materials in the UK? You need colloidal silica or waterglass and china clay (kaolin). Zircon silicate flour, which is expensive, may be used for the first layer. All available from pottery supply or eBay. www.bronzecasting.co.uk/4_investment/shellsystem_8.html |
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Post by smarty on May 18, 2014 9:22:30 GMT -8
You can get away without zircon flour in the first coat since you are not trying to cast anything. Molochite flour (-200 mesh) is fine as the basic slurry additive to mix with the colloidal silica. You'll need two coats of slurry sprinkled with molochite sand (50-80 mesh), then three coats with the slurry sprinkled with molochite granules (sugar sized 16-30 mesh). The first coat of slurry wants to be the consistency of single cream, or a little thicker, to get a nice smooth surface, then the slurry for the second coat wants a tad more colloidal silica adding to get it somewhere between milk and single cream consistency to allow it to take to the sand on the first coat. After these two coats you can thicken up the slurry to get a nice build up on the subsequent coats but no more thick than single cream. Each coat wants to be air dried between coats 12 - 24hrs. you can get all this stuff from 'special plasters' here in the uk. Go to their site and the whole lot are there in the foundry supplies section. Ceramic fibre coated in colloidal silica might just be ok for the riser if you can get it into shape but I would imagine that its too fragile for the feed tube area it will be about as strong as weatabix or shredded wheat or something like that.
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Post by photoman290 on May 18, 2014 13:27:50 GMT -8
thanks will check it out. i have been given a few insulative fire bricks to play with. i am slicing them to around 15mm and have lined the burn tunnel with them. i assume the ceramic blanket will be about as soft or even softer. i have a liter of colloidal silica i got for coating the blanket. i still have to get the blanket. i assume the ceramic shell will be harder and suitable for the burn tunnel and feed?
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Post by smarty on May 18, 2014 16:55:00 GMT -8
I'm still playing around with the stuff but so far so good is all I can say. I've made a 6" j-tube and a 4" one and both have had repeated firings with no problems so far. You need to back it up with fiberglass strand mat torn up into hand sized chunks and dipped in the slurry. These can just be dried out quick with a propane torch - provided the shell is already baked to a red heat and the form removed/burned out. you need a few layers of this fiberglass back-up which gives you a final thickness of about 20mm with about 5mm of this being the shell and 15mm being the fibreglass back-up. the final thing then wants baking to a cherry red or hotter if you can for 20 min or so. You'll see how amazingly resistant to thermal shock it is if you lift it out the kiln or furnace at an light orange heat and plonk it down on its side in the damp grass. Any normal ceramic needs to be cooled down slowly but this stuff seems entirely unaffected by such treatment.
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