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Post by rocketman86 on Dec 10, 2018 17:22:11 GMT -8
Hi There, Pleased to have come across this forum, Im looking to build a heater for my static only im thinking of using red brick and hardie backer cement board instead of flue pipe for the loop and instead of using a barrel. Ive uploaded some design pics, links below, Would really appreciate your thoughts on the design. Brick channel forming loop and barrel, Approx channel size 6"x6" Topped with cement board Mass storage area for stone & cob Extractor heat reclaimer box added for 100mm extractor ducting. Thanks for viewing, look forward to your thoughts. Jon
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Post by satamax on Dec 11, 2018 0:32:42 GMT -8
Remove few bricks from the inside wall at the U turn at the end of the bell. You will free the gas movement.
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Post by pinhead on Dec 11, 2018 5:58:42 GMT -8
You can greatly simplify the design by eliminating the inside wall and putting the chimney exit near the floor. This will form a "bell" which is much more efficient at both heat extraction as well as mass flow.
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Post by pianomark on Dec 11, 2018 7:43:49 GMT -8
I believe the Hardie backer cement board is a portland cement based product. It will not stand up to the high temperatures in a properly functioning rocket. Especially above the riser as you seem to have pictured.
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Post by rocketman86 on Dec 11, 2018 8:52:57 GMT -8
Thanks for your suggestions guys. You can greatly simplify the design by eliminating the inside wall and putting the chimney exit near the floor. This will form a "bell" which is much more efficient at both heat extraction as well as mass flow. Ahh ok yeah that makes a lot of sense and like anything that makes it simpler. Remove few bricks from the inside wall at the U turn at the end of the bell. You will free the gas movement. Ok thanks, Think I like pinhead's suggestion of removing the inside baffle completely but to clarify do you mean here: I believe the Hardie backer cement board is a portland cement based product. It will not stand up to the high temperatures in a properly functioning rocket. Especially above the riser as you seem to have pictured. Ahh, I knew it had a a1 fire rating and is widely used to support chimney pots so assumed it might have been ok but was unsure so I was hoping someone would raise concern if the case, You having said this prompted me to check and turns out it only has a temperature rating of 100 Celsius. With an a1 rating would I be correct in thinking it would not catch fire but deteriorate under the heat? In which case could it still be used but only as a former to top with reinforced concrete left in there so when it does deteriorate its not a problem? Anyone know if the temperature in the bell is likely to exceed 100 Celsius? Other alternative im thinking would be using steel which would improve heat transfer in to the 'mass'. Anyone think of any issues using sheet steel like this? Tia, Much appreciated Jon
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Post by rocketman86 on Dec 11, 2018 9:39:46 GMT -8
Turns out the hardie backer board has a higher resistance to temperature than standard concrete which is just 80c, fire cement based concrete is an option but expensive. Maybe steel is the best way to go.
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Post by josephcrawley on Dec 11, 2018 15:58:19 GMT -8
You can use brick to close the top just turn some inward in the next to last course and make sure you remember to build a few vertical columns from the floor up as you build. Most brick yards carry a commercial sized brick which is a foot long which would make it easier.
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Post by peterberg on Dec 12, 2018 1:46:53 GMT -8
Anyone know if the temperature in the bell is likely to exceed 100 Celsius? The inside of the bell will exceed 100 ºC almost every time the heater is fired. Using a medium sized batchrocket it's quite easy to run it hot enough to get 900 ºC at the mouth of the riser. In my view, anything above the riser need to be refractory material otherwise I expect it to detoriate in short order. It should be noted that temperature of the bell's top to some extend depends on the distance between the riser's mouth and the ceiling of the bell. The greater the distance, the more the heat is spread out. Hardie backer board isn't simply up to these circumstances, as is any portland cement based solution. So reinforced concrete is out of the question for three reasons: the steel reinforcement will expand at a greater rate than the concrete, breaking it apart. And the portland cement isn't up to this level of temperatures plus the christal structure of the silicon in the filling materials like sand and gravel are changing shape while expanding a great deal. So any material that's unable to resist about 500 ºC is off the list for safety reasons. Steel could resist such temperatures but will warp quite badly unless it is a very heavy gauge. A barrel's top is held together by its walls so when it's expanding it could buckle in. The steel of a barrel is much thinner, it will shed heat quite fast. There's a limit to the heat such a thin walled item can shed. When the temperature of the inside and the outside of the steel sheet is the same or nearly so it doesn't get hotter even when the gas inside is higher in temperature. Heat travels through the steel by conduction, when there's no difference in temperature (delta T) between inside and out conduction stops until delta T is larger than zero again. I would recommend you buy the Rocket Mass Heater Builders Guide by Erica and Ernie Wisner, and read the website batchrocket.eu/en/ thoroughly. Especially the quite extensive parts about bells and their construction.
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Post by rocketman86 on Dec 12, 2018 5:29:09 GMT -8
Anyone know if the temperature in the bell is likely to exceed 100 Celsius? The inside of the bell will exceed 100 ºC almost every time the heater is fired. Using a medium sized batchrocket it's quite easy to run it hot enough to get 900 ºC at the mouth of the riser. In my view, anything above the riser need to be refractory material otherwise I expect it to detoriate in short order. It should be noted that temperature of the bell's top to some extend depends on the distance between the riser's mouth and the ceiling of the bell. The greater the distance, the more the heat is spread out. Hardie backer board isn't simply up to these circumstances, as is any portland cement based solution. So reinforced concrete is out of the question for three reasons: the steel reinforcement will expand at a greater rate than the concrete, breaking it apart. And the portland cement isn't up to this level of temperatures plus the christal structure of the silicon in the filling materials like sand and gravel are changing shape while expanding a great deal. So any material that's unable to resist about 500 ºC is off the list for safety reasons. Steel could resist such temperatures but will warp quite badly unless it is a very heavy gauge. A barrel's top is held together by its walls so when it's expanding it could buckle in. The steel of a barrel is much thinner, it will shed heat quite fast. There's a limit to the heat such a thin walled item can shed. When the temperature of the inside and the outside of the steel sheet is the same or nearly so it doesn't get hotter even when the gas inside is higher in temperature. Heat travels through the steel by conduction, when there's no difference in temperature (delta T) between inside and out conduction stops until delta T is larger than zero again. I would recommend you buy the Rocket Mass Heater Builders Guide by Erica and Ernie Wisner, and read the website batchrocket.eu/en/ thoroughly. Especially the quite extensive parts about bells and their construction. Very informative thank you. All makes complete sense. I'm now thinking of casting the entire heater from refractory cement and firebrick grog in sections and gluing together with fire cement. This method makes a lot of sense to me as I can utilise my cnc to create the formers, the entire heater will then be upto withstanding 1100c and like you say no mixing of materials that can expand at different rates. It will hopefully look nicer in the living room too which for me is important. Here is what im thinking, sticking with pinheads suggestion of single channel bell lowering the flue pipe down inside the channel near the floor instead of a loop. But your right, I should indeed buy a book on it before committing to the design, will look into your suggestion, thanks. Please feel free to scrutinise ** Just spotted the cuttout to let gas into the bell is missing, need to determine the best cutout size there, The burn tunnel is 5"x5". Flue pipe also 5".
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Post by coastalrocketeer on Dec 14, 2018 11:15:55 GMT -8
That “cutout” to let gasses into the bell from your construction over the riser is the manifold, and should normally be larger than system CSA. 1.5x System size cross sectional area, I believe.
It is because the gasses streaming down have to take a “hard right angle” turn. Most people find a “tapered cone” works, but anything enough bigger than system csa should work (could be a wide low opening, or a rectangle.
If there is any chance of fly ash building up in that area, then give some extra space so that even with a lot of ash collected at the bottom, 1.5x system CSA is maintained.
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Post by coastalrocketeer on Dec 14, 2018 11:19:47 GMT -8
Another important “clearance for gas flow” area is the gap at the top of the riser. As long as you go with “plenty” then no calculations or testing required, but I believe 1/2 system size(diameter) is an absolute minimum. And fly ash build up on top of the riser walls can restrict this as well.
Others will correct if my memory is wrong
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Post by rocketman86 on Dec 15, 2018 11:37:56 GMT -8
That “cutout” to let gasses into the bell from your construction over the riser is the manifold, and should normally be larger than system CSA. 1.5x System size cross sectional area, I believe. It is because the gasses streaming down have to take a “hard right angle” turn. Most people find a “tapered cone” works, but anything enough bigger than system csa should work (could be a wide low opening, or a rectangle. If there is any chance of fly ash building up in that area, then give some extra space so that even with a lot of ash collected at the bottom, 1.5x system CSA is maintained. Thanks for answering that one, Big help. One point my mate pointed out was removing the divider that formed the loop to the flu pipe in original design's bell as per pinheads suggestion, and moving the flu pipe away from the riser, there would be no heat in the flu to start the thermos syphon in the flu. So not sure how youd get around that to stick with the single chamber bell as in new design. Ive just installed Autodesk cfd which does thermal simulation including radiation, Thinking im going to try having a go a simulating the design and also going to try a more compact version with two loop sections going around the riser each having an area filled with 'mass' above each ring, Bringing the flu pipe back in next to the riser. Does anyone feel that casting the entire heater from refractory cement and firebrick grog, that this type of concrete may not be thermally conductive enough to perform properly? Tia
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Post by rocketman86 on Dec 15, 2018 13:13:53 GMT -8
Here is what i'm now thinking, Two loops around riser carrying exhaust with Cob above each individual loop. Opinions? Tia Hopefully I can get some results from the CAD thermal simulation to see how it performs before building.
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Post by coastalrocketeer on Dec 15, 2018 13:32:48 GMT -8
For that people usually do what’s called a “bypass” in the main/first bell. Search bypass on the forum and you will find MuCH discussion of varoious ways to do it. Another idea I’ve seen suggested is to have your chimney leave the second bell near the main bell and travel through it vertically some distance. With stoves having a barrel, just having the first few feet of (non insulated) stove pipe run vertically next to the hot barrel is often a solution. This heats the pipe early, but allows heat loss after the burn, in the case of ones running inside a mass like a masonry primary bell.
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Post by coastalrocketeer on Dec 15, 2018 13:34:53 GMT -8
There are non-isulative castable refractory products that should accept and release heat as well as a standard masonry mass.
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