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Post by owkaye on Jul 19, 2014 8:09:17 GMT -8
I have not built one yet so I am wondering what might be the maximum temperature I should expect to see in a batch-box rocket mass heater system, and where the highest temperature is likely to occur?
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Post by peterberg on Jul 19, 2014 9:09:01 GMT -8
Different temperatures in different parts. Highest in the riser directly behind the gate, 1200 centigrade. In the firebox between 600 and 900 centigrade, riser end about 900 centigrade.
In Fahrenheit, that would be 2190, 1110 and 1650 respectively.
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Post by owkaye on Jul 19, 2014 19:56:59 GMT -8
Hello peterberg,
Thank you for the prompt answer to my question!
I want to cast both the firebox and riser. Since the highest temperature is 1200°C it seems I should look for castable refractory products designed for stability at this temperature or higher.
Is there a specific type of castable refractory material I should use for the batch box part? The riser part?
Apparently there are many different refractory types available in the marketplace. Should I focus on using one or more of these types? Which one(s) to avoid?
Conventional Castables Low Cement Castables Cement Free Castables Insulating Castables Free Flow Castables
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Post by peterberg on Jul 20, 2014 0:06:17 GMT -8
When you are planning to make the riser and the fire box as separate parts, a high grade conventional castable would do for the firebox and lower riser part. The higher riser part, generally above the firebox, out of insulating castable if possible. Try to cast the walls relatively thin, 30 mm or 1 1/4 inch would do. The reason is to keep the whole thing low in mass with a shorter heat up time as the result.
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Post by Deleted on Jul 20, 2014 5:34:13 GMT -8
With some geopolymer mixtures less than 10mm would be possible
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Post by owkaye on Jul 20, 2014 6:44:40 GMT -8
Peterberg, you suggest using conventional castable for the firebox and lower riser, then switching to insulating castable for the upper riser.
Should I use 100% conventional castable for the lower riser then abruptly switch to 100% insulating castable for the upper riser? Or should I create a 'transition zone' in the riser material by gradually increasing the ratio of insulating to conventional castable over a height of several centimeters?
My thought here is that an abrupt change in material might create a plane of weakness and possible future separation, whereas a gradual transition might eliminate this potential problem. What do you think?
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Post by peterberg on Jul 20, 2014 9:03:12 GMT -8
Should I use 100% conventional castable for the lower riser then abruptly switch to 100% insulating castable for the upper riser? Or should I create a 'transition zone' in the riser material by gradually increasing the ratio of insulating to conventional castable over a height of several centimeters? The lower riser and the higher riser should be different parts. You can't cast the whole stove core as one singular piece, due to temperature differences there will be a lot of thermal stresses. By the way, it's possible to do it like Morticcio but in order to avoid cracks it would be better to devide it in firebox and riser parts.
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Post by satamax on Jul 20, 2014 9:17:48 GMT -8
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Post by owkaye on Jul 20, 2014 19:42:55 GMT -8
The lower riser and the higher riser should be different parts. You can't cast the whole stove core as one singular piece, due to temperature differences there will be a lot of thermal stresses. By the way, it's possible to do it like Morticcio but in order to avoid cracks it would be better to devide it in firebox and riser parts. Hello Peter, thank you for the recommendation for two-part core. Can you tell me how Morticcio did it? Is his approach described in this forum? I tried to search the forum but I do not get the expected results.
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Post by owkaye on Jul 20, 2014 19:49:52 GMT -8
Hello Karl,
Thank you for replying to my questions in this and another thread! I am very new to geopolymers so I am still learning the basic concepts. I have been doing as much internet research as time allows. This is my current understanding:
Mixing certain geological materials and/or coal combustion byproducts in the presence of a strong base and water causes the material to quickly form strong chemical bonds without heat. This process has nothing to do with hydration. The resulting geopolymer material is usually stronger than concrete so it can be poured thinner. And it is exceptionally resistant to degradation at high temperatures. Is this correct?
I am intrigued by your comment that the riser can be made only 10mm thick by using the correct geopolymer recipe. This suggests that geopolymer might produce a lighter yet stronger end product.
I think I would like to experiment with the geopolymer approach to making a castable batch box rocket mass heater. Do you have a recipe to create a durable riser only 10mm thick? Or can you suggest a recipe that I might begin with in my experiments?
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Post by owkaye on Jul 20, 2014 20:21:17 GMT -8
Hello satamax,
Thank you for suggesting flue liner tiles. There were lots of fireplaces in the cold northern USA where I lived when I was younger. These liner tiles were very common in fireplace chimneys. But now I live in the 'deep south' where fireplaces (and these flue liners) are rare. Nevertheless I still look for them on craigslist.
My actual goal now is to create a castable batch box rocket mass heater so I can easily reproduce it when I eventually create a good design. This is why I prefer to use castable material.
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Post by peterberg on Jul 21, 2014 0:43:41 GMT -8
Can you tell me how Morticcio did it? Is his approach described in this forum? I tried to search the forum but I do not get the expected results. Yes, this method is described in this forum. Here's the link to "6" horizontal mould".
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Post by Deleted on Jul 21, 2014 4:32:06 GMT -8
Mixing certain geological materials and/or coal combustion byproducts in the presence of a strong base and water causes the material to quickly form strong chemical bonds without heat. This process has nothing to do with hydration. The resulting geopolymer material is usually stronger than concrete so it can be poured thinner. And it is exceptionally resistant to degradation at high temperatures. Is this correct? Yes, that is correct. Fly ash consists mostly of hollow balls with low thermal conductivity and a density below 1g/cm 3. There are light hollow ball fillers made from selected fly ash or by a special fusing of aluminosilicates which an even lower density. The hollow balls will form a strong but light matrix. Hollow balls need very little water for low viscosity. The hollow balls can also be used as an additive to lower the viscosity of other kinds of aluminosilicates. As I could not find a source of fly ash or metakaolin in Gemany I have only experimented with very fine grog and natural clays. About 80% very fine grog 10% lime and 10% sodium hydroxide worked great for me. Can be made very runny with more water, but then will take long to dry. I hope to get some fly ash and highly reactive metakaolin for experiments soon.
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Post by owkaye on Jul 21, 2014 7:38:59 GMT -8
This reminds me of my boat building days. I used 'microballoons' in epoxy as lightweight filler. The microballoons made the mix runny, so sometimes I used fumed silica (brand name "Cab-O-Sil") to thicken and prevent runs and sags.
Fortunately a runny mix will be ideal when creating pourable geopolymer. It will fill all voids in the mold without vibration.
I may try a mix like this myself. How long does it take to dry when extra water is used? Hours? Days? Weeks?
I think I know where to find lime and Sodium Hydroxide but I'm not sure yet where to find fine grog. Hopefully there is a local source near me. If not, maybe I can find an online source with distribution in the USA. Hopefully the shipping costs will be reasonable.
You did not mention Sodium Silicate (waterglass) in your recipe above. I was under the impression that this was a necessary ingredient. Did your experiments prove otherwise? Or is waterglass necessary only when using different raw materials?
I don't know what highly reactive metakaolin is (so I need to do more research) but there are two coal burning power plants near me. I hope to get fly ash from one of them but I suspect difficulty, here's why:
Power plants usually contract with other companies to remove their fly ash in large tanker trucks. They are not set up to sell fly ash to local craftspeople in small quantities. Nevertheless I hope to convince a friendly power plant employee to fill my 1000 liter tote tank with fly ash for free (or at low cost). Then I will have a source for at least one of the main geopolymer ingredients.
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Post by Deleted on Jul 21, 2014 8:33:39 GMT -8
How long does it take to dry when extra water is used? Hours? Days? Weeks? Out of curiousity I have made an extremely runny mix by more than doubling the amount of water. It required warm curing at 70°C to 80°C to harden within a few hours, then it schould be kept in an artight bag for about two weeks before drying. Such an extreme mixture needs very careful and slow drying. In this thread: donkey32.proboards.com/thread/1291/step-recipe-homemade-pourable-refractoryI gave you a source (http://www.blueconcrete.com/) for highly reactive metakaolin, pozzolanic calcium aluminosilicates and cenospheres at reasonable prices. Likely you could not save much if any by cement fine grog(0.0mm to 0.1mm), which may be hard to find.
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