BOOH VI

[matt walker]

NC, when using my cover (the pot inverted), once it goes over some long sticks, it creates a situation where the sticks will be smouldering up high. The pot captures every last bit of smoke, but what I'm saying is, once it's on there over a fuel load, I have to leave it on. Typically, long sticks uncovered will be fine and not smoke back at all, but occasionally I get a really pitchy one which the fire can climb, or a stick with a really dry outer layer and a wet core, so it burns up the stick before it drops down into the feed. What I'm trying to say is, you may be able to burn some longer sticks without the pot cover, but once it's on there any long fuel will start smouldering so it needs to stay on until they drop. So use it when you need it, not by default, would be my advice.

[Nutcase]

Thanks, Matt. I had pretty much come to that conclusion, but it speeds things up for me to get your pointers. I think my setup smokes back with virtually any long stick at this point. Maybe this would change if the opening was set in a plane surface, and of course if the system was overall less leaky.

I'm learning a lot, doing things in the most expedient way I can think of while taking steps toward a better result. It's an approach that can really give you a feel for things, once you get something that is at least marginally functional. I've gone beyond that, all the way to marginally useful!

[Nutcase]

This morning, after running the system for a total of 7 to 10 hours in three main sessions, I took things apart enough to see what there is to see inside. We've been running it pretty hot, with the barrel top temp generally over 700˚ and maxing out the thermometer frequently when wood was added. We're almost at the end of our heating season here, so there's no reason to hold back. Plus it just seems to want to run hot. Anyway, there are some interesting things going on in there.



Here's the end cap/inlet piece pulled off. The grate shows significant corrosion, but it's still pretty much intact. However, the piece of blanket partially covering it has shrunk a bit, fused to adjacent material and become quite crisp. While it lacks resilience, it still has some strength. I was able to carefully invert the unit without dislodging the grate. Here's a closer, albeit slightly out of focus look:



According to the manufacturer, this material contains glassy silica that devitrifies, i.e., crystalizes, when exposed to temps in excess of 1000˚C=1832˚F. This is an oversimplification, as this sort of phase change is a function of both time and temp. Even with the devitrification, it isn't supposed to shrink more than 4% at temps up to 2300˚F — in carefully defined test conditions. the practical working range is supposed to run up to 2100˚, but even that depends on the chemistry of the high temp environment, not to mention the mechanical challenges right under the fuel inlet on top of the grate. In any case, this piece ain't what it used to be.

[Nutcase]

Looking down the burn chamber, we see that the inner layer of the roof has sagged. The aluminum foil has burnt off



It has probably been like this for most of the time. It's not as crispy as the piece on the grate, but it has stiffened and shrunk a little, enough so that it won't be easily persuaded to stay up. 1" material would probably be more self-supporting. The near edge may be down in part because of snagging on fuel. The roof has a hinge-like high point at the top of this part. Some of the pale crust on the bottom left appears to be from aluminum foil, probably from foil that was once between the first and second layers near the front edge of the roof. The inside of the channel didn't start out with a foil layer. Much of the foil between the first and second layers may befully oxidized, but this isn't easy to assess without unrolling.

On the bottom left, you can also see some thinning attributable to the weight of fuel off the grate and the loss of resilience. From touch, the changes in the roof increase toward the tunnel, whereas the changes in the floor decrease. Then things look and feel more like the original condition immediately past the roof sag.

[Nutcase]

Looking down the heat riser, we see how things change at the turn:



The gray stuff is what remains of the heat riser's foil wrap. It is apparently less completely oxidized than the foil from the burn channel. In the picture, the blanket looks crusty where debris has accumulated on the bottom, but on closer examination it seems basically unchanged. Everything that is visible is past the constriction caused by the roof sag. the gray sheets mark the base of the riser as a structural element.

[Nutcase]

BOOH IV has been functioning quite well with a fairly severe bottleneck and some bypass flow over the top of the sagging roof. Interesting. I'm going to put in that sauna heater so as to limit further roof collapse, maybe tweak a few other things and run it another ten hours over the next few days, or until something fails, then reassess.

[mannytheseacow]

Hi Nutcase,

I've been on the road for a while and just getting caught up with your project here. It looks awesome! Great work! I know there are many threads on the BOOH project and I haven't had time to read them all... what made you decide on this material design? Is it just what you have on hand? It certainly must be significantly lighter than cast blocks. I think it's interesting how you've rigged it up to your current wood stove. It's great that you can fire it inside and get a feel for how it burns and the heat produced before actually doing a full installation!

[Nutcase]

Hi, Manny,

Basically, I was inspired by Matt's example. He pointed out the importance of a relatively low mass—low thermal conductivity burn chamber, and ceramic blanket seemed like a good way to get that in a simple, cheap way. From there, it has just been an interesting evolution. No, I didn't have any refractory material when I started.

Like most of us, I've done a lot of prowling around on various online forums, hunting for hints on many, many things. This seemed like a good spot to pay back a little.

I'm pretty happy about getting my setup inside before the end of the heating season here. The bare ceramic blanket isn't gonna hold up in the burn chamber, but it looks like it really is good for at least tens of hours in the heat riser. Probably good for hundreds of hours and possibly thousands. And the blanket plus wire mesh weighs less than ten pounds, took maybe an hour to build and utilizes materials that cost me under $20. I'm free to make a lot of mistakes with this stuff because the builds are so quick and the amount of material actually used up is small. If nothing else, this could end up being a good trial system for other folks, something that you can actually try to live with for a while without doing any significant remodeling.

[Nutcase]

I've moved on to a new build, so this will be my last bit on BOOH VI.

The roof support failed more quickly and completely than I had anticipated:



The consequences were predictable. The draft was weak, so the the smokeback/firecreep problems were strong. However, it was still possible to get a very hot fire going. The heat riser has accumulated a lot of soot on the outside, but the inside remains clean and seems essentially intact.

[matt walker]

Looks like you were still having fun destroying things in it right up until the end. I've been paying closer attention to temps over these last few weeks, I gotta say, your ability to regularly hit 900°F on the barrel face with these burners is pretty amazing. That's a REALLY hot burning core. My typical temp at the hottest point on the face of the barrel is more like 400°F, and I consider 700°F+ extremely hot for a typical system. Am I right, you are getting those temps on the face of the barrel, or are you measuring the top of the barrel when you are giving system temps?