BOOH VII

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BOOH VII

Postby Nutcase » Sun Apr 07, 2013 9:47 pm

So I've started to experiment with putting a solid refractory layer on the inside of at least some parts of the system. The material I have started working with is mizzou.

I selected mizzou on the basis of it being on offer from High-Temp Inc. and because of something I read on a forge thread somewhere about protecting blankets "with a mizzou slurry worked into the blanket." This was second-hand info, i.e., the person commenting had seen and had worked with a forge based on this construction, but it wasn't his work, so he didn't have any details. So anyway, I got a 55# bag.

When I opened the bag, I was surprised to discover that it had fairly coarse aggregate in it, up to at least 3/16". Unrealistic to work pebbles into the blanket, even little pebbles. Hmmm.... nevertheless, I weighed out 5#, added enough water to get a somewhat spreadable consistency, put it on a 1'x2' piece of blanket and rolled that onto a letterbox form. It broke immediately when I took it off the form, but the blanket held the pieces together well enough to stand it up, dry it and do an initial firing in the oven, then use it as a burn tunnel for one test run. This was actually back between BOOH V and VI. I just wanted to see if the added mass did anything drastic to the way the system performed with bare blanket. Basically it seemed to somewhat slow and smooth out the ramping up of the system temp.

Recently the stars have again aligned so that I had time to work on this project again. The aggregate is too coarse for what I want to do, so I sifted it out. Using a collander with roughly 2 mm metal mesh, sifting pulled out 40% by weight, pebbles along with a certain amount of refractory fiber lint. The sifting mats this stuff up, so it probably isn't worth the trouble to try mixing it back into the 60% cement/fine aggregate.

Cut to the chase. I molded another letterbox piece, using 4# of the sifted material:

Image

This piece is 8" wide and tall, and 12" long. Based on a density of 140 pcf, the hard layer is about a sixth of an inch thick on average. Obviously it is weak against wishboning, and by the time I got it stabilized in the build, it had two longitudinal fractures. However, the blanket keeps it together and once in place with the sides buttressed, it wasn't in imminent danger of collapse.

I stuck another flat test piece in as a fuel landing pad and otherwise more or less rebuilt BOOH VI around this new burn channel. The heat riser is unchanged.

This build is still pretty much of a hack, so the details aren't interesting. I did manage to get the connections comparatively tight.

So far we've run this system twice. Of course it draws better with the roof not collapsed. It heats up at about 50˚/min, the way we have been running it, and it is possible drive the temp up to 900˚. However it "wants" to run more in the 650-750˚range.

The air/fuel inlet is now the biggest problem. It is just way too shallow and it's somewhat awkward to choke. Plus I have to reduce all the fuel to 10" or less to minimize the smokeback problems.


Image
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Re: BOOH VII

Postby Nutcase » Mon Apr 08, 2013 2:22 pm

Yesterday I extended and narrowed the air/fuel inlet. I also put the shroud back in place, but this of very little functional consequence:

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The inlet is now 5"x5" at the top and about 9" to the top of the port into the burn channel, 14" to the bottom with the current accumulation of ash. We've also begun using a cheap steel 8" baking pan as a choke.

Image

This port turned out to be a big improvement. The fire was easy to turn, and barrel top temps rose at 100˚/min until the thermometer maxed out. Since we were using fuel already cut down to 12" or less, it wasn't really challenged at all with regard to smokeback/ firecreep.

Then my wife set the pan as you see, and she took to adding wood every half hour or so, three times over the course of the movie we were watching. Each fuel load was no more than fits comfortably into the port, maybe 3 pieces of 2" ash or the equivalent. Fueling at that interval, the fire was down to coals, but the fresh wood of this size caught fire promptly without any further aid. We didn't track the barrel top temps.

It's interesting that the system ran so nicely while completely choked. The air/fuel port is still quite leaky, but there are no individual openings amounting to more than a fraction of a square inch.
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Re: BOOH VII

Postby matt walker » Mon Apr 08, 2013 3:44 pm

They run surprisingly well choked down, and I imagine yours has enough spots in can draw air in no matter how much you try to seal it. I wonder how much the HT Superwool breathes?

As for feed shape, I'm starting to believe that there can be some improvement by having an internal shape to the feed, in profile, like this....

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With the choked feed offset away from the riser, and a fire box that's a bit taller than the tunnel. This seems to add another mixing point, as well as a pretty effective smoke trap in the little void between feed opening and tunnel. My system sort of developed this by wearing away the front of the tunnel over time, and looking at Peterberg's batch burning design, it pretty much replicates the same shape. The firebrick models I have built which didn't have this seemed to suffer for a lack of that little secondary mixing trap there. I have no hard evidence, but it might be worth trying to achieve this shape.

I'm sure it's on your radar, but what about a little cast feed/firebox that you couple to the rest of your system/systems?
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Re: BOOH VII

Postby Nutcase » Mon Apr 08, 2013 8:08 pm

I like the smoke trap idea. Right now I just have the sides sort of angled in at the top, plus there is a bit of a lip hanging in front of the burn channel.

The things going on in the air/fuel inlet are a bit complicated, but the general idea of trapping the combustion zone below a bottleneck has to be right.

The blanket does breathe, more or less like you would expect. I don't think that it's as much of a factor as the leaks at connections at this point, but it's going to be a factor as the construction becomes less of a hack. The aluminum foil mitigates where it survives.

It might make sense to cast at least some part of the fuel inlet, or even make it out of bricks, for durability and for making the bottleneck into an optimally effective firebreak. A heat sink. A thick, wide collar of cast iron, with soapstone on top! Back to reality. I am still early in the learning curve with learning how to handle the castable refractory. While I'm sorting that out I can also mess around with the geometry—once I get clear of all that urgent spring gardening stuff.
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Re: BOOH VII

Postby Nutcase » Tue Apr 09, 2013 1:00 am

With the stove thermometer at 4" down from the rim, we've been running at 450-550˚ most of the time when it's up and running today, with one excursion up to 620˚. This appears to be pretty comparable to what you report about your systems. Similar system sizes, similar power when revved up, since neither system is lame. The variable variables have more to do with acceleration, stability, ease of use, safety, durability, etc., etc. Next up, redoing the front end to allow axial access for cleanout and for starting, eliminate some of the leaks and play around with the inlet geometry.

It has been interesting, by the way, being able to open the top of the woodstove and just flat out feel how low volume the exhaust flow actually is and how little of the heat is escaping in it. Well, more than it might seem, given how much is tied up in the evaporated water.
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Re: BOOH VII

Postby Nutcase » Wed Apr 10, 2013 4:54 pm

Matt, mulling over your smoke trap, I wonder if it works by facilitating a standing vortex. Vortices can work like roller bearings for the air flow. A vortex at that location would tend to guide inlet air down and directly into the burn zone. Obviously this is an idealization, what with random sticks in the flow path. The roller bearing analogy for standing vortices applies with flow at the top of the barrel too. Anyway, where would you suggest starting with the dimensions? HxL 4"x4"? 2"x6"? Length subtracted from the burn channel or added on? Of course I've got a lot of other factors to consider, other things to fiddle with, so I probably won't do exactly what you suggest—or even what I plan on.
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Re: BOOH VII

Postby matt walker » Wed Apr 10, 2013 5:39 pm

That's how I picture it NC, a little roller right there. There are times when you can see it, particularly on start up when the fuel is starting to pyrolize, and the draft isn't strong enough to suck it directly horizontally. It will try to come up, get hit by the downdraft there, and sorta tumble into the tunnel.

As for size, well, mine started as the feed. So, from my cast core plan, the "feed box" is slightly larger than 7" square, and about 12" deep. Restricting the feed opening across the top closest to the barrel creates that shape naturally, and I've settled into an opening that is somewhere around 4" x 6", or a variation of that CSA since I'm working with clay rather than the shape constraints of brick. Not really added on to the tunnel as much as subtracted from the feed, if you will. The feed already has the oversized CSA that allows that shape.

Nice to hear your temps are right in the same range. I was really wondering there for a bit if you had created some kind of supercollider or something! After messing with countless iterations of these things I can tell you without question that yours is a well running system if you can hit 600°F+ on the face. There are times with some of them that it's a struggle to maintain 300°F.
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Re: BOOH VII

Postby Nutcase » Thu Apr 11, 2013 4:44 am

"Supercollider." LOL. I think that if you saw it all, you would see how it fits right in with your other experience. Same physical laws, just a hotrod setup. I'm not driving the exhaust through a thermal mass, and that makes a big difference. On the other hand, I could add a foot to the riser, which would make a big difference too.

It might be convenient to have a piece that hangs from the top of the feed box so that one can change the inlet size. Sometimes it's convenient to have a big opening, but most of the time it's better to have it choked down quite a bit.

We've been burning some larger sized pieces of ash today, and with the system pretty well choked the face temp is the 300-350˚ range. To go higher one must burn a larger number of thinner pieces. Anyway, in this climate at this time of year, it's nice to be able to run it like that.
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Re: BOOH VII

Postby Nutcase » Thu Apr 25, 2013 10:32 pm

Various things have been interfering with my playtime, but I will have something interesting to report soon about what's possible with the mizzou and the ceramic fiber blanket. Meanwhile I have a question about firewood. One option I have is "oak" recycled from shipping pallets. 3"x3"x4'. If it's like the stuff I got a few years ago, some of it is actually oak and all of it is dense. Does anybody have any experience with burning very dense, very dry hardwood in that size range? Well, actually I would expect to cut it down to manageable lengths, but would it have to be split? I've gone through some ash up to about that size, but nothing denser. Oh, and Matt, how was your wood consumption with your rocket overall compared to previous years?
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Re: BOOH VII

Postby matt walker » Fri Apr 26, 2013 4:11 pm

I too have been occupied such that I haven't had much time for updates here. I hope to get some photos and post what I've been up to here this weekend.

I've burned plenty of wood even larger than you describe there NC, some of it Madrone or Maple. Not sure exactly how that compares to oak, but I can't imagine you'll have to split. All of them behave differently and depending on moisture content you may have to cut them short enough to be down in the feed, or may be able to leave them full length. I took down a large old big leaf maple that was standing dead for the last year. That wood is dried so nicely that I've been using branches up to around 4" diameter and putting two or three in the feed, and leaving them at 5' or so long! You don't want to walk away when there's sticks that tall sticking out, but it's nice to just load it the one time for a few hours' fire each evening.

Overall I'm thinking I've used somewhere around 4 cords this winter, down from up to 10 with my old box stove. It's a bit hard to judge though, as my above example illustrates. I started the season with 2 cords of fir cord wood, and have been burning scrounge from my property for the last two months without feeling any stress about wood. In the past if I didn't have 8+ cords of good cord wood in the shed at the start of the season I was in trouble. It's been a much more comfortable winter than I've experienced as well. There is absolutely no comparison to hanging out on the warm mass. It's wonderful, and I'll never go back to a metal box stove. They seem so primitive to me now, robbing the heat from the burn the way they do.
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