Ralf
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Posts: 42
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Post by Ralf on Nov 6, 2016 22:42:34 GMT -8
Hello to the experts,
I´m about to construct my thermal battery for my rocket mass heater with an already finished combustion unit on wheels (6-incher). Since it´s situated in a rented workshop I want to be able to dismantle it if necessary, so I planned it as a hollow bell bench built directly on the concrete floor slab and made of concrete steps (as front side) and multi-layered flagstones (as top), with tiles on the surfaces, all mortared with cob.
Concerning this build I have two questions:
1) The slabs I will use are exposed aggregate concrete / washed conrete so I thought to put this on the inside to have a bigger heat exchange surface. With this bigger "rough" surface of half-embedded gravel, do I have to change the calcuation of my maximum ISA for heat exchange ?
2) The bench sits along a wall that doesn´t need to be heated, so I thought of making the backside of the bench with blocks of insulating air-entrained concrete (Ytong). Does an insulated internal surface still count in a calculation of max. ISA ?
And if you think any of those ideas might cause problems please let me know.
Thanks, I´m very grateful for all the great Information and generosity on this board !
All the best, Ralf
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Post by peterberg on Nov 7, 2016 1:50:40 GMT -8
Interesting, never thought about rough surfaces and their implications. At first I would think the ISA should be smaller because of the increased surface area. For example, when gluing two surfaces together it is benificial to have it sanded and increase the area that way. But, in aerodynamics it is all about streaming fluids, which is radically different. A shark's skin for example is very rough, like number 40 sandpaper. The net effect is that a thin film of water is practically at a standstill at the surface. Friction of water to water is less as compared to water to skin.
In a bell the gas velocity is low and the boundary layer effect as on the shark's skin is quite small. So, I would think both effects are in play here and honestly, I think the net result would be neglectable.
The surface of the ytong will take up some heat but the entrapped air will prevent conduction into deeper layers. It's effect isn't zero but again neglectable, in my opinion. So please ignore those surfaces in the ISA calculation. For safety's sake, keep the ISA on the small side but I think minus 5% would be enough.
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Ralf
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Post by Ralf on Nov 7, 2016 1:58:36 GMT -8
Tanks Peter for your reply !
Do you think a J-tube will service less ISA than a batch box ?
If I calculate the planned ISA without the ytong I´m at 3,75 sqm, the bench is 4 meters long and will be 35 cm high on the inside of the bell.
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Post by peterberg on Nov 7, 2016 2:16:27 GMT -8
A j-tube is able to deliver about half of the power of a batch box of the same size in any given time. But I investigated solely the batch box for this aspect, I can't possibly say anything about a J-tube of what it can serve in terms of ISA.
It's nice to give some dimensions, but what is the width of this bench and what is the rest of the system?
A 150 mm (6") batch box system is able to serve a total of 5.3 m² bell ISA, given a good chimney stack and very little restrictions in the gas path. Gives you plenty of slack I would say.
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Ralf
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Posts: 42
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Post by Ralf on Nov 7, 2016 2:56:21 GMT -8
Ok, I like experiments ! As for the rest of the dimensions: My combustion unit is a 6-inch / 15 cm-System on wheels, j-tube is made of insulative firebrick lined with thin (1 cm) cordierite plates on the inside of the burn tube. Heat riser is just short of 1 meter made of cast very light insulative refractory I mixed myself, giving more of a 1:2:4 ratio of the j-tube proportions. The manifold area is constructed as an intersection of the vertical 40 cm "barrel" tube and the 15 cm horizontal tube, creating lots of space for a smooth direction change of the exhaust gases. The unit functions very well in standalone mode as can be seen here (back then the interns were still humid and now it had a summer to dry out completely) donkey32.proboards.com/thread/1994/combustion-unitThis mobile unit will be coupled to the bench that sits in an adjacent room. The exhaust gases will be delivered to the middle of the bell by the delivery duct. Bell internal will be 4 m length x 35 cm hight x approx 50 cm width. The walls will be 15 cm thick on the front and approx 20 cm thick on the top, ytong at the back is 10 cms. I will also install a shortcut valve for cold starts, chimney runs inside an old brick one and will be 13 cm diameter stainless pipe backfilled with perlite and about 3,5 ms high, opening 60 cms above an almost flat roof. I hope the information is now complete. I´m documenting everything by photographing and filming, but I will only publish it after at least one season of operation, so as not to claim that it works before it´s proven. ;-)
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Deleted
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Post by Deleted on Nov 8, 2016 22:39:40 GMT -8
Hi Ralf, I did a 180 mm rocket with a single bell whose ISA is approximately 9 sq. m. It's outside and the chimney stack in 4 m of uninsulated steel duct. It works very well but the exit temperature is very low (approx. 40°C above ambient temp. in the middle of the burn) causing excellent efficiency but also condensation and occasionnal backdrafts. I have a picture here webcloud.zaclys.com/index.php/s/v14ccRvaO543B0pFrom that experience I would say that it is OK to use the recommended ISA for your J-tube. But if you chimney stack is uninsulated, take 10% less.
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Ralf
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Posts: 42
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Post by Ralf on Nov 9, 2016 1:50:56 GMT -8
Hi Yasintoda,
thanks for your reply & encouragement, I was looking for typical duct lengths of 6 inch systems in Erica & Ernies Book yesterday just to get a feeling what might be right, and the bell system has less friction, so plus the estimates of Peter I´m pretty sure I´m on the safe side. Chimney will be insulated, it´s originally an old square brick chimney and I´m going to put a stainless chimney duct inside and backfill that with loose Perlite.
All the best, Ralf
PS: Calculated that the bench will weigh about 2 tons, pretty curious how log it will stay warm.
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Deleted
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Post by Deleted on Nov 9, 2016 22:46:35 GMT -8
Hi Ralf,
It's perfect for the chimney. Don't forget to add a closing valve. It's very important because there will be a residual draft that will take away most of the heat.
All the best,
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Ralf
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Posts: 42
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Post by Ralf on Nov 10, 2016 4:13:33 GMT -8
As I read many times most experts strongly advise against usage of a chimney damper with a RMH. If the system is air tight it should be enough to cover the feed tube completely once the fire´s done to stop heat loss through the chimney, or am I missing something ? Since I have a sturdy T tubing to install the bypass shutoff valve I could easily install an additional valve towards the chimney, question is whether this makes sense. It would surely require strict operation rules, f.i. only close it once the fire´s completely out, but how you determine that ? No glowing coals ?
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Ralf
New Member
Posts: 42
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Post by Ralf on Nov 14, 2016 15:34:56 GMT -8
Hi Folks, I think I got my last question answered over at PermiesChimney dampers in RMH´s are almost as doomed as metal. For good reason. Cheers Ralf
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Post by satamax on Nov 15, 2016 1:05:31 GMT -8
Dampers are dangerous, if they aren't done well or used well.
They should be dampers, not vanes. In France, you can't have a damper which would let less than 20% of the gases through. Which means you're always open.
Then, if you make yourself some. Closing it should only occur when the fire is completely off. And it would be best practice to make it foolproof, so the damper can't close by itself.
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