BOOH IV

[matt walker]

You are starting to get into what I feel is around the max temp territory. My IR gun peaks at 1000*f and when my best stoves are running their hottest, they will peg that one on the barrel top. 700*f a few inches down from the top on the face is about as hot as I've seen. Nice work, you are definitely in the super efficient range.

[Nutcase]

No, looking at some other pictures, that's about it, aside from some rearranging. Here you can see the pretty orange glow emanating from the top of the burn chamber.



In spite of the rapid rise to a fairly high temperature, the system remained intermittently smoky. We didn't see any point in pushing it any further without figuring that out, so we let it burn down. After it cooled off, I looked inside and saw this:



The top of the burn chamber is two layers, and the inner layer got buckled in at some point during the setup. I suppose the narrow channels at the top were where streams of oxygen-free smoke began their harrowing escape from the burn zone. So I pushed the top back up and ran it again.

[Nutcase]

So I pushed the burn chamber back into shape, put things back together and ran it again. Didn't go for max temp, peaked at around 700˚F (on a Mac, the ˚sign is [Option]+k, by the way). This run didn't smoke as much, though it still didn't seem totally clean all the time. I also played around a bit with the orientation of the fuel inlet.



In this configuration, the tendency of smoke to escape from the fuel inlet was much reduced. On the other hand, the tendency of the fire to creep up the sticks was increased. Some of the flames were burning down before they got a chance to burn sideways.

[Nutcase]

One last thing... when I took the system apart again this morning, I found this:



The plywood spacer next to the chimney had caught fire, somewhere along the line. Could explain a lot of the smoke...

It's getting on towards when I should aim for a semipermanent setup.

I'm thinking that the heat riser is okay pretty much as is, though it might be nice to make the cage around it into a better approximation to a cylinder. It wouldn't hurt to insulate it better and/or put it in a sheet metal cylinder, but I don't think that it is the weak link at this point.

What is the best shape for the burn chamber? I'm leaning strongly towards circular. Hey, with a big enough spiral stovetop burner element, you get an elevated grate! Tipping may be an issue, but maybe not.
For this part, I would prefer to use 1" material, since it would be nice to get to 2" thickness anyway and the 1" stuff is more self-supporting. You can more easily cut it so that the edges match up, layer by layer, rather than the spiral wrap I've used with this 1/2" material. At some point I will want to see what a slightly open seam along the top might do. Would the Venturi effect draw air into a stream of particularly hot and oxygen free combustion products, accomplishing something sort of like a p-channel? However, since 1/2" is what I have...

I will probably also line it with something hard, for ash removal if nothing else. I've got a central vac, and I expect to have in-line access to the ashes, but even so, there will be wear and tear that this material won't tolerate for very long.

Maybe I should start a thread on materials where we can discuss the pros and cons of the various options.

[matt walker]

Dang it Nutcase, you just made me spend five minutes trying to figure out how to type an actual degree symbol on a not-mac.

Check it out though, 100°F! Thanks man. If anyone else is wondering, it's the extremely inconvenient combo of ALT and 0176.

I gather my asterisks were offending you! Lol.

I wonder if there are some binders or other things burning off in your materials. Those temps should indicate the wood is burning completely. The charred stick I'm sure added to the smoke.

Square vs. round, I really don't know. I would think round as well, but I'm wrong about this stuff a lot. I too was thinking about cleaning your soft material. I do think you are going to need a "hot face" of something that you can sweep out and not damage. I love the process, this is cool stuff.

[matt walker]

Oh, the open seam...I'd be careful with that, although it would be worth experimenting with. Any leaks between the feed and the heat riser top are robbing you of downdraft pull, which can lead to fire creep. Also, as you found out with the cap over the feed, if you keep heat in there on the top of the fuel, you will eventually creep.

[Nutcase]

Wasn't bothering me, just a tip for those who might be interested.

There are no binders burning off in the materials, for sure.

Maybe I should see if I can make it smoke again, even at high temps, by dropping the top again. It sure seemed like there as a correlation with the sticks breaking and fresh fuel going into the system and therefore a spike of combustables. However, maybe that's influenced by what I expected to see. Seriously, though, you can erase the binder possibility. Wherever it was coming from, it was wood smoke.

Round minimizes the internal surface area, which minimizes the amount of material needed to insulate it. On the other hand, its easier to make the turn in square, at least with the crude and material-minimizing construction methods I've been using. I'm a bit suspicious of upper corners at least as a potential smoke escape path. Maybe not an issue when the system is up and running, but maybe the last part that gets up and the first to get down. Maybe.

At a minimum, I would want to put a hard surface on the bottom third of the burn chamber, just to have something I could run a vacuum wand along without being too careful.

I take your point on downdraft pull. The fire creep issue becomes more of an issue with this extremely hot fire zone so close to the inlet. I know I've leakage around the burn chamber-heat riser interface, and probably a significant amount of air drawn through the insulation as well. Not a bad thing at this stage, actually. If I can get this ugly sucker to run this well, I have reason to be optimistic about how the final (i.e. initially installed) system will perform.

Fire creep...that gives me an idea for a new screen name.

[Nutcase]

After mulling things over and taking another look at the system, I think that the plywood is it. the pulses may have been due to increased flow past that piece as it smoldered when the burn pulsed up rather than unburnt stuff escaping from the burn chamber. The inside of the chimney is clean white to the top, the outside is now quite sooty.

It's interesting that the plywood got hot enough to start smoldering, though it's certainly not startling. Fortunately the mechanical stability of the whole setup didn't actually depend on that piece, it just settled on the base that I had slipped under it.

Next, I'll get to see how the system performs when it is partly wet.

[matt walker]

I think the color of the material is your most telling clue. I am betting it was the plywood as well.

[Nutcase]

After leaving BOOH IV out overnight day before last, the parts not under the barrel got wet from a light rain. Recognizing that I was going to have a bit of trouble getting the system warmed up, I improvised an interesting little grate, using not one but two of the stove burner elements.



It's not entirely clear from the picture, but the elements stand off from each other by about an inch. I like this thing a lot. I also gave my small kindling a minute in the microwave, which also helped.

Even so, needless to say, it took quite a while to drive off enough moisture so that the system could get going. Eventually it did take off, and eventually I got the stove top up to about 750˚F.

Aside from the moisture issue, this was perhaps the leakiest and most collapsed setup so far. In this picture you can see a gap about 3/16" wide at its widest on the top left side. The inner layer of the top was buckled down as pictured for an earlier run. Nevertheless, at temp it didn't smoke, so that issue appears to be settled.



When it was running at temp, the orange glow was visible through the top of the burn chamber and through the back of the heat riser. Matt, I consider your observation about the importance of having the flame make the turn as a given, at this point.

The system runs hottest now when it is eating smallish pieces of dry fir. 2x2 mill ends would be close to ideal.