Batt out of hell...

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Batt out of hell...

Postby Nutcase » Sun Jan 20, 2013 3:32 pm

So this week I finally got the rockwool batts and did my fast and ugly build.

Image

So now I'm trying to figure out how to upload an image...let's see...Okay, got that sorted out.

So this is three 3 1/2"x23"x47" batts of Roxul rockwool cut in the more or less obvious way to make an 8"x8" rocket.

I'd say that cutting the rockwool with a bread knife is a piece of cake, except that would be overstating the difficulty. It's nice to work with. Not dusty, not itchy.

Doing a small burn here in the garage just to get the most preliminary feel for the thing. Obviously I need more than gravity and duct tape to hold this thing together.
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Re: Batt out of hell...

Postby Nutcase » Sun Jan 20, 2013 4:31 pm

Here it is outside, now with a concrete reinforcing wire half-cage instead of duct tape.

Image

The top plate of the fire chamber has the corners notched to form a tab that supports the narrow (and short) front wall of the heat riser. I've used a couple pieces of copper wire to hold the chimney up. I was able to carry it easily in this configuration. The material is 2.5 pcf, so the whole thing weighs about 20 lbs.
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Re: Batt out of hell...

Postby Nutcase » Sun Jan 20, 2013 5:46 pm

Here is the setup with a barrel added.
Image

The barrel is supported by a partial ring of crw. With the up-pointing free ends bent in and out alternately, the barrel is self-centering and fairly stable.
I've modified the top and back of the burn channel to make a better fuel port. You can see changes in coloration in the batt from earlier small test burns. The organic binder burns off. in the garage burn, the smell was noticeable but not particularly strong or unpleasant. I've started what will be a larger burn here. the system is still smoking back. The heat riser is kinda big for the barrel, and the system is cavernous for the very small startup fuel load. Anyway, after a minute or so the fire began to draw well, more fuel went in and the barrel began to smoke as paint began to burn off.
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Re: Batt out of hell...

Postby Nutcase » Sun Jan 20, 2013 6:12 pm

The total fuel load amounted to the equivalent of about a foot of not particularly dry 2x4. However, my impression is that this is an unusually low thermal mass/conductivity burn chamber, so it got pretty hot pretty quick. The woodstove thermometer I had on the top quickly indicated in excess of 700˚F.

Image

Problems soon became apparent. In this shot you can see that the top of the burn chamber has sagged. Inside, a big piece came loose and fell into the chamber, largely blocking it. The left side almost burned through, and there were indications of near burn-through at the base of the chimney. A little flame is escaping through the fuel port. I let it burn out and left it for the evening.
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Re: Batt out of hell...

Postby Nutcase » Sun Jan 20, 2013 6:38 pm

The next day, I took the barrel off, then took off the front of the chimney and the top of the burn chamber.
In this shot you can get some sense of what happened inside. The bottom is pretty much slagged. The back and sides show a fused yellow layer on top of a very fragile fiber layer with the binder burnt off, with deep fissures running almost to the outer surface. To the left you can see the chimney piece. A dog has stepped on it, creating the pit.

Image

here is the burn chamber top. Shortly after the burn got up to temperature, the broken-off piece fell on the chimney end but remained up on the fuel end, so it largely obstructed the channel. Nevertheless, the system burned merrily away.

Image
Last edited by Nutcase on Fri Mar 01, 2013 4:01 pm, edited 1 time in total.
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Re: Batt out of hell...

Postby pa_friendly_guy » Sun Jan 20, 2013 6:56 pm

Hi Nutcase, I have never seen or used Roxul Rockwool. I thought it was just an insulating produce. What was the purpose of your experiment to use it as the entire burn tube, fire box and up-right burn chimney? Was this experiment just to see how it would hold up under these extreme conditions? Do you feel that if you had a solid fire box and burn tube and chimney that the Roxul would have worked well as an insulating wrap around the outside? Or might it be best to shred the Roxul and mix it in with the clay and straw as an additive to the cob mix inside the fire chamber and burn tube? I really have never seen or used the product, so I am just asking questions about its best uses in this type of application.
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Re: Batt out of hell...

Postby matt walker » Sun Jan 20, 2013 7:11 pm

Holy cow, what an awesome experiment! I'm grinning here, I love it. That furnace cement I've been mixing with clay can be watered down a long way. I wonder how it would go if you dipped those bats in a wheelbarrow of furnace cement slurry first? So cool Nutcase, keep it up.
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Re: Batt out of hell...

Postby Nutcase » Sun Jan 20, 2013 7:28 pm

Lessons learned...

The Roxul is nominally rated to melt at about 2000˚F. It's obvious that this system easily exceeded the melting temperature, whatever that is.
Next time I think I will put in some pieces of copper and see what happens. Pure copper melts at about that temp.

The heat riser is too bulky in this configuration. It would be easy to make a suitably sized hexagon that would give better clearance and could be contained in an easily fabricated cylindrical wire cage. However, what's the point if its just going to get destroyed like this.
There really isn't any practical reason to make the system this well insulated. I'm not actually trying to smelt copper, though I'm a little bit curious about how high you can get something like this.

I also tossed a small chunk into my current wood stove. It gets up to a nice orange glow next to an ember and is not much affected.

A system with thinner walls and mineral fiber with a higher temp rating might work well.

I wonder if I just happened to accidentally hit on a configuration with minimal excess air flow at peak burn rate.

Anyway, I'm going to put the pieces back together today and see what it does, burn-throughs and all.

Hi pa-guy,

Roxul isn't intended for this use. Of course, concrete reinforcing wire isn't intended for barrel stands, either.

I wanted to get an up-close and personal feel for how a rocket stove works without investing a lot of time or money. This was very quick and cheap as these things go. I'm at about $20 total on materials actually consumed. I know a lot more now.

I doubt that I will find a use for this material in something built to last. it's not the right stuff to mix into cobposit. Too friable and thickly matted. Makes it nice to cut, though. great stuff for what its for, insulation between studs on 24" centers.
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Re: Batt out of hell...

Postby matt walker » Sun Jan 20, 2013 7:37 pm

Regarding your comment on the fuel/air mix, yep. A well insulated J tube does an amazing job of passively metering the mix. It's very hard to beat that set up as a passive carburetor. Very cool experiment, on to the next one!
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Re: Batt out of hell...

Postby Nutcase » Sun Jan 20, 2013 9:09 pm

Thanks Matt.

I also got a tub of refractory cement. I smeared a little on a chunk of the Roxul, then discovered as you have that you can thin it out pretty much as necessary to get it to stick to the surface. I threw it in the wood stove without any preliminary drying at all. When I took it out, I had refractory pita bread! Part of the smear, on the order of half a square inch, had lifted up as a bubble. The smear was thin and roughly potato-chip fragile.

I will probably do something like you are suggesting. Maybe even if the temps behind the hotface burns off the roxul, the hotface might still survive long enough to backfill or some such thing.

I don't have a feel yet for how robust this passive carburation effect is. At some point you have to get an overdraft if the output impedance is low enough and there is some limit on the rate of burn, like when I did the small test in the garage. I could hold my face in the output. Need more experience! So I need a test bed that is a little more robust than what I have now...
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