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Post by northriver on Dec 1, 2010 17:13:12 GMT -8
Donkey, thanks for the clarification of the burn tunnel length, it is a little vague in the book. I have been puzzling over the metal insulation sleeve that I will put around the burn tube heat riser.  My initial thoughts on a solution are shown in the above picture. The barrel at the bottom is my intended heat exchange barrel on which I hope to heat tea (diameter of approximately 22"). The barrel on the top is what I am thinking will form my insulation sleeve (diameter of approximately 18"). This means that the gap between the two barrels is 2" instead of the 1.5" recommended by the book. I assume this simply means that the heat exchange barrel will not transfer as much heat and more will go into the flue. Is this a huge deal? Also, the barrel has ribs which will disrupt the gas flow. Again, not sure how big a deal this will be. |
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Post by Donkey on Dec 1, 2010 20:03:16 GMT -8
Totally fine.
I'd say that the measurement in the book is the MINIMUM one, you'd do fine if it were MUCH smaller. Remember to measure the INSIDE of the outer (large) barrel at it's narrowest place and the OUTSIDE of the inner (small) barrel at it's widest. I've forgotten to measure at the riffles before.. Oops..
I'd also recommend that you DOUBLE or even triple the gap height above the heat riser. While the gap size in the book is technically correct, I've had better results with the larger gap.
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Post by northriver on Dec 1, 2010 21:29:36 GMT -8
Donkey, just to clarify your last comment ..
> I'd say that the measurement in the book is the MINIMUM one, you'd do fine if it were MUCH smaller.
Did you mean to say ....
I'd say that the measurement in the book is the MINIMUM one, you'd do fine if it were MUCH larger.
Otherwise, I'm really confused!
Also, if I double or triple the gap above the heat riser I am assuming that will increase time spent trying to boil water on top of the stove?
Thanks for all the comments!
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Post by northriver on Dec 1, 2010 21:30:36 GMT -8
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Post by Donkey on Dec 2, 2010 8:48:08 GMT -8
Donkey, just to clarify your last comment .. > I'd say that the measurement in the book is the MINIMUM one, you'd do fine if it were MUCH smaller. Did you mean to say .... I'd say that the measurement in the book is the MINIMUM one, you'd do fine if it were MUCH larger. Oops!! Umm.. yep, LARGER. Not that I've noticed... Not by much in any case. In fact, I've noticed the opposite on occasion. It seems that there's a sweet spot somewhere in there. Too narrow or at least "regulation" narrow, the top will be cooler than a little higher up. Too tall of course will do as you say. It's a touch and go situation involving a load of factors. My way of getting at it is to run the stove, get it hot, place the barrel over and move it up and down till it runs just right, chalk it up and seal it in. It's important to have complete quiet so I can hear the subtle changes in intensity. This way I can avoid all that tedious mathematical figuring, get on with the thing AND get the best result. Another detail: If you make the upper edge of the heat riser beveled and sharpish, with the bevel sloping inward to the inside of the heat riser, light ash won't build up there (as much). You wouldn't think it, and it's a little surprising but I've seen ash build up on top of the heat riser and block flow. A beveled top makes the ash fall off instead of build up. I like the ash to slide back into the firebox, that's the part that I clean every couple firings. If you bevel it out towards the barrel, you'll be opening your clean-out and getting at the ash drop more often, which can be a pain. |
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Post by northriver on Dec 3, 2010 8:50:42 GMT -8
Another detail: If you make the upper edge of the heat riser beveled and sharpish, with the bevel sloping inward to the inside of the heat riser, light ash won't build up there (as much). You wouldn't think it, and it's a little surprising but I've seen ash build up on top of the heat riser and block flow. A beveled top makes the ash fall off instead of build up. I like the ash to slide back into the firebox, that's the part that I clean every couple firings. If you bevel it out towards the barrel, you'll be opening your clean-out and getting at the ash drop more often, which can be a pain. There is a drawing in the book that shows a detail of this, but I did not read (or do not remember) an explanation for it. Now it makes sense. Thanks! A related thought to this ... Initially, I planned to cut the top off of the barrel and use the bottom of the barrel as the top of the stove heat exchanger. My assumption in doing this was to eliminate the bung hole. Is there some sense in using the top of the barrel as the top of the stove? In other words cut off the bottom of the barrel. You then have a ready-made observation port allowing you to view the top of the heat riser and its state. Is there any convention here? |
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Post by Donkey on Dec 3, 2010 9:30:59 GMT -8
Usually folks cut off the top.. It's more of an aesthetic issue than a function one.
I see no objection to doing it the other way.
Not a bad idea.
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Post by northriver on Dec 5, 2010 22:26:17 GMT -8
Right ... so I stripped the form work off of the footing for the rocket mass heater. The footing looks good; the perlite gives the footing a rough look, almost like having rice krispies in it. We did not do much to smooth the surface of the pour, working the concrete makes it more dense, reducing its insulating ability.
After the form came off the footing we started moving the fire bricks into the earthship. As we moved the bricks we scraped off any old mortar and cleaned the bricks so they would stack nicely as we built the core of the heater.
Next we placed a 6" diameter straight section of single wall chimney pipe in the space we left in the footing for the ash clean out under the feed tube. This 6" long piece was then embedded in concrete (same mix as before) to hold it in place. I drilled two small diameter holes in the end of this pipe that was to be embedded in the concrete and before placing it put screws in the drilled holes. The ends of the screws stick into the concrete used to embed it in place, and will act to hold the pipe in place. Finally, I put a cap on the pipe. (In order to get a 6" length of chimney pipe I cut down a 1' length that we bought.)
We then placed the base layer of brick down for the core of the mass heater. Before placing these bricks we put a 1/2" layer of concrete (same mix as before) on top of the footing. The goal here is that the bricks will be set and leveled in this topping layer. I am not sure how effective this step will be in the long run ... worst case this topping will crack, but I do not think it will affect the integrity of the heater.
I then mixed the refractory mortar that I bought in Kamloops with water (fairly runny). I placed the bricks for the next layer of the core ... and it was tough. Turns out the mortar I purchased was for repairing cracked fire bricks, and the aggregate was coarse (up to a 1/4"). I used what I could and finished placing this layer of bricks. On Tuesday we are back into Kamloops and I will hopefully track down a more suitable mortar. Ironically, I had originally intended to make my own mortar as we have lots of clay, and access to clean, sharp sand. I did not want to make this job too complicated, so I bought mortar. The job already seemed hard enough!
The core of the stove is shorter than the unmortared version that I built. I will remove 2 bricks from the ceiling of the burn tunnel.
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Post by Donkey on Dec 6, 2010 8:42:45 GMT -8
I don't even use mortar anymore if I can help it.
I just dip the faces in clay slip and put 'em down.
If they are wiggly, THEN I use as little mortar as possible and ONLY on the low side.
Just remember, whatever you do, whatever mortar you use, it's meant to hold the bricks apart, not stick 'em together, so use as LITTLE as possible.
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Post by northriver on Dec 6, 2010 16:38:15 GMT -8
Just remember, whatever you do, whatever mortar you use, it's meant to hold the bricks apart, not stick 'em together, so use as LITTLE as possible. Its hard to remember that. I know the mortar is basically just there to level things up. But I do so WANT everything to be stuck together! Anyway, I hear you and that is what I have been trying to do ... just use it to level the brick courses. Thanks! |
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Post by Donkey on Dec 8, 2010 10:45:31 GMT -8
To be fair, modern mortars WILL act as "rock glue" and stick 'em together. Tends to lead (even good masons) into bad practices. I'm willing to bet that many ancient stone buildings with NO mortar at all will outlast most of the new ones WITH mortar.. You can see the evidence of this everywhere. Stone facings pasted on, shoddy construction beneath, using cement as a crutch to avoid having to do good work or hide poor.. Just one guy's opinion.  |
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Post by Donkey on Dec 8, 2010 10:54:40 GMT -8
Much like firebrick, I get the feeling that high heat mortar is meant to heat up and cool down slowly. Rocket stoves don't do that though. They go through more sudden changes (relatively) and I've noticed that old red brick tends to stand up better (though not always) to the shock.
You want your stove to hold together even if the mortar and bricks crack up. It will if you build carefully using thin, well seated joints and trap it all in with heavy materials (like cob).
Be aware that just about whatever you do, that first brick on top of the burn tunnel, the one right behind the firebox, WILL (almost always) crack in half with use. If it's trapped in with heavy(ish) stuff, it will stay in place and be fine, if it's not trapped and it's allowed to float, it will probably fall in at some point.
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Post by northriver on Dec 8, 2010 22:52:50 GMT -8
We went into Kamloops on Tuesday and exchanged the refractory mortar that did not work very well for a different product. Right away we could tell that this was a much better mortar for the job; there was no large aggregate in it, and it was simple to brush a thin layer onto the bricks. We finished the brick work for the core of the stove on Wednesday. (I’ve included pictures at www.darfieldearthship.com/2010/12/rocket-mass-heater-bricks-finished.html)Donkey, I hear what you are saying about not relying on the mortar to hold the stove together. My original preference was to make the mortar out of material off our site as is talked about in the book. However, we are building this stove under a construction permit with our local building authority so we are using materials (refractory cement, new stove pipe ...) that conform to the building code (I talked a little bit about this at the post listed above). Going this route has definitely made the stove more expensive! Regardless, I am now starting to think about placing cob around the stove itself. The book says that at least the six inches of cob around the stove core should have no straw in it. I assume this is to avoid the straw catching fire and structurally weakening the cob. Is there any value in mixing sawdust into the cob to better insulate the core of the stove? If so how much sawdust would one use? I am fairly certain I read about this, but not sure where now! |
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Post by northriver on Dec 8, 2010 23:16:34 GMT -8
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Post by Donkey on Dec 9, 2010 9:22:51 GMT -8
No sand..
Pearlite with enough clay slip to just make it sticky.
It keeps the pearlite from moving.. The stuff is fiendish in that (if you leave it loose) it WILL find the smallest hole(s) and end up blowing around inside the duct-works. Just a little bit of slip will do the trick.
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