BOOH V

[Nutcase]

Okay, so here is the inlet shroud, made from the carcass of an old smoker:



I didn't want to mess with the wire frame, so the stainless box didn't fit. With the top on, air comes in mainly right at the front edge of the fuel port. The smoker cover is less than ideal—too much dead air—but of course it fits. Good enough for now.

The burn went pretty much as I have come to expect. With only paper and small pieces to start, the fire turned easily. it was smoky until the barrel top got to around 250˚ and the fire was well established with some larger pieces of fir. It plateaued at 560˚. The cover eliminated smokeback. When it ran back down to about 425˚, I pushed down what was ready to push down and added two ash sticks and a piece of bamboo, all approximately 1" diameter. The barrel top temp climbed to 750˚. The wood wasn't particularly dry, especially the bamboo, and there were a couple of startling steam explosions. Here's a shot from this phase of the burn. The bamboo has already fallen in, you can see a similar piece at the bottom near my toe. Edit: one of the sticks is actually hawthorn. Not that it matters.



The sticks never smoked, but I put the cover on anyway, some of the time. Water was boiling out of the ends. I did notice some smoke beginning to escape at one point, but this turned out to be loose paint/smoke condensate attached to the smoker that was smoldering. A lot of heat comes back up the fuel port when the barrel top is 700˚+.

Overall, I think the shroud may be a good idea, but there are some details to work out about keeping the amount of trapped smoke and heat down.

[Nutcase]

Today I had another interesting run, working on the shroud and on the viability of high temp stainless for at least temporary structural support.

As a side effect of rotating the wire cage to put the best location for the fuel inlet up, the blanket got rotated so that the ceiling was weak.



So I took one of the heating coils from the sauna heater I've been cannibalizing and spread out the coils so as to provide a support beam for the ceiling. Note that the ends actually stick out into the fuel inlet by a few inches. The lower end is just underneath the burner.



After these shots I reinstalled the end cap and shroud, along with a shroud cover made of fiberock with a cutout to hold this 5"x7" Pyrex baking dish.



Originally I had thought I would put the dish into the hole, but this turned out to be inconvenient, due to the length of the sticks that we used as fuel. We ended up using it upside down, with a piece of wire across the opening to keep it from falling through. There's a reason why manhole covers aren't square, y'know.

[Nutcase]

The burn was generally typical for this system. taking some minutes to stop smoking until the fire built up, the barrel top temp climbing to in excess of 600˚, then going to just under 800˚with some additional wood.



The most interesting point is that the shroud and lid did a good job of limiting both smokeback and firecreep. I think this is because of this particularly favorable inlet geometry. Air is going back along the top of the burn chamber, under the edge of the shroud, and making a quick U-turn when it hits the fuel. It worked noticeably better when the pyrex cap was mostly covering the top opening.



I like this. I think I'll keep it. Not sure about the pyrex dish, though.

[Nutcase]

After the fire began to die down, we started doing a few other chores in the back yard. My wife mentioned at one point that there was an unpleasant chemical smell in the exhaust stream. I didn't wait for the coals to completely burn down before looking into what might be going on. The new element was the heating coil I was using as a roof prop, so let's see:



Well, the end has sagged, so the roof has come down some again. Hmmm. What happened to the threaded bronze fitting it used to have?

Probably something like what happened to the fitting on the other end, except that one was partly protected by the ash. However, it is clear that it has partially melted. and corroded.

[Nutcase]

When I took the barrel off, there was something else interesting to see:



The inside of the barrel and everything else outside of the chimney in the flow of the exhaust has a light coating of a dark blue powder. Copper sulphate? Probable cause of the chemical smell. But why did it become particularly strong during the cooldown? Maybe it's related to a change in how oxidizing or reducing the air was that the copper was exposed to. Maybe it's just that the exhaust wasn't as hot, so more was mixing into the air we had our noses in. Anyway bronze typically begins to melt at 1570˚F.

Fortunately, copper isn't highly toxic. It's certainly interesting that we can get temps like this in the fuel inlet.

[matt walker]

Neat. Your shroud/inlet configuration is looking a lot like my inverted stockpot cover, which I use when the sticks are long and have creep potential. It overhangs a ledge so there is maybe 4 sq. in. Of air intake which has to come up into the pot, then down the feed.

[Nutcase]

I wonder if the three-sided feed shute facing the main air inlet actually makes a significant difference. If it's not inconvenient, I'd like to see a picture of your setup. I'm not sure the picture in my head matches up with the reality. Edit: Well, maybe a picture isn't really necessary. What I envision is air entering more or less vertically away from the fuel port, sweeping the top the pot and then diving into the fuel port. It's dead simple and easy to adjust if the distance to the edge isn't too long, and it's very hard to break. My setup is dead simple in the current context, but maybe it won't be in a more solid and finished state.

[Nutcase]

Matt, would you be inclined to make your burn channel any shorter than 12" in an 8" system? BOOH VI will be harder to modify. For my current system, the burn channel is effectively more like 15", due to how the pieces are jammed together, with some constriction at the transition. It sort of feels like it might be a little long. I think I will end up with an inside channel length of 13" in the next iteration, doing the simple thing, given the dimensions of the material and the construction path I currently envision.

I have one more thing I want to try out before setting this system aside. What happens with a pipe passively delivering preheated air to the burn zone? This sort of thing is unlikely to help during startup and may cause smokeback/firecreep problems, but it should accelerate charcoal consumption. Anyway, it's easy to set up, so why not try it and see?

[matt walker]

I'll try to get you a couple photos of how I run my system/cover shortly here Nutcase.

As for length, I think 12" is just about right. I wouldn't go any shorter, but that's mostly because it gets awkward fitting the barrel. One could offset the barrel, but I think that 12" allows a good combination of the three T's for combustion; time, temperature, turbulence.

As for secondary air, if you research "P channel" on Donkey's board you'll find good info on the current best practice for air injection in a J tube. I'm sure there are other options, but since Peter has the Testo, and has done a lot of research, it's a great starting point. I'd be very careful, as always, to ensure full draft downwards through the feed. That is really crucial, as you are finding out.

[matt walker]

Here's my feed as I normally run it while burning....


Here's how I cover it when going out or not in use...



Here's how I cover it if the wood is long and really dry, or really pitchy, or if I'm just starting with a cold stove and it's finicky...



A profile shot so you can see the air inlet when it's covered...



I had lots of ideas on how to do this, but this simple solution works so well for me I haven't bothered to pursue anything more finished. I use the pot for ash removal as well, so it's just very convenient. No extra stuff, which I like.