BOOH III

[Nutcase]

Well, thank you for the venue.

Since I was reusing the Roxul, it was already affected by heat to varying degrees, and I didn't take a good set of before pictures, so the results are not entirely clear from just looking at the results here. However, the results are clear enough for general orientation.

Here's a shot of the burn chamber with the Roxul top off. That piece of recycled chimney was falling apart due to the binder being almost entirely burnt out, but little if any indication of the yellow melting. However it was already such a mess that it's current condition is not very informative even to me. The sidewalls have not deteriorated dramatically.

[Nutcase]

This is the most informative photo, showing the base. Originally, the discoloration on this piece was more or less uniform from end to end. After the burn, it was partially melted directly under the fuel port. Some of the melted stuff adhered to the wire frame. This led to a poorly defined tear-out hole when the frame was lifted off.



The side walls were more extensively and uniformly fused, probably due to the more direct compressive contact to the Superwool without the intervening wire frame. The top surface doesn't show much obvious change. However, these pieces no longer feel like they can be handled without falling apart, so the binder must be more extensively burnt out inside.



The oxidation on the wire itself shows an interesting color change, from orange to black in the hotter zone. I suppose that might be a temperature indicator.



The bottom of these pieces is also worth a look.

[Nutcase]

Summing up results so far, the Superwool is okay, at least for the sort term, in direct contact with the hot gasses in a small system at the lower edge of a good operating temperature. Roxul is at best very marginal at one inch away from the hot face of such a system. It might work if adequately protected from mechanical stress, but probably not worth it when loose packed perlite or vermiculite could be used instead. It can't be used to provide ongoing mechanical support to the Superwool.

Finally, an interesting video I ran across in my research on the web, a very simple gas furnace made from 1" ceramic fiber blanket, a propane torch and a few firebricks.

[Nutcase]

Matt, I've been thinking a bit about your observation on the length of the burn tunnel. While it isn't long in absolute terms compared to BOOH I, it is relatively long for the diameter, leading to significant cooling before the exhaust hits the chimney, reducing the potential for the mixing to complete the burning. This can be tracked on the Roxul base piece, using the marks from the wire mesh. If you look at the top picture on this page, you can see that the third wire from the left crosses under the burn chamber right at the center of the burn zone, and on the bottom picture you can see that this is right where there the heat damage is max. (What you see under the first wire is a separate piece.) Two wire marks down, the damage track narrows. I imagine this signifies a combination of cooling and stratification in the flow, with combustion more or less over 6" before the burn chamber enters the chimney. Is that how you would read it?

[matt walker]

Well, it's hard to tell scale from the pictures, but that's where I was going with that. The gases need to hit that lower corner while combusting as the turbulence there helps complete the burn. A 55 gallon drum has a radius of 11.5 inches or so. I like to keep the tunnel as short as possible, which usually puts the top of it at around 12". The pattern on the base would seem to indicate that the hottest spot was before the riser, which might mean the tunnel is long. Also, it seems that your feed could only hold a small amount of fuel, don't discount that. To get my barrel top up above 500 degrees typically takes a fairly large fuel load.

[pa_friendly_guy]

You are learning a lot here Nutcase. I was looking into trying to make a Rocket stove oven. There is one on the net that uses 3 55 gal barrels and a smaller oil barrel inside for the actual baking area. Their design uses insulation wrapped 3 times with alum foil and formed into tubes to direct the heat all around the oven. They stuff the foil wrapped insulation tubes in-between the large 55 gal barrel and the smaller barrel to direct the heat in a back and forth pattern and then put a blanket of insulation over the top of the 55 gal barrel. One barrel is made into a base to hold the fire box and support the oven. The 3rd 55 barrel is cut and spread open to cover the insulation layer that lays over the oven to hold the heat in. It looks like a very nice design. I was really wondering about fiberglass insulation standing the heat though, even with the foil wrap to protect it. It sounds to me like the 2nd produce you used would hold up a lot better. Because the flames may not actually touch the insulation, and because it is protected by the Alum foil it maybe OK to just use regular insulation, but in my mind, why take the chance.

[Nutcase]

Interesting, pa_guy. Could you post a link? I'm not building an oven, but I'm not building a metal melting furnace either...

Matt, thanks for the clarification. The wire mesh is 6", so that sets the frame of reference, more or less. The system is taller than wide, due to how it was scrunched into the existing 14" wire frame.

I'm sure you're right that this system would have been cleaner if the burn chamber had been shorter. Obviously, in the original 8" square Roxul system, the 2' burn chamber wasn't too long...However, I'm less than certain that it makes sense to have a barrel top routinely running at temps in excess of 700˚F indoors. If 500˚ is a better target, then more heat loss that bypasses the chimney may make sense. However, that's fine tuning in the context of where I'm at now.

As I've indicated from the outset, I've been inspired by your observations about the great consequence of insulation and thermal mass in the burn chamber, and it's predictable that these low mass systems would come up fast and, when adequately insulated, run relatively hot for their size.

Next, I think I will try to make an 8" diameter burn chamber insulated with loose perlite outside the blanket. I probably will put the fuel inlet closer to the barrel, as you suggest.

[pa_friendly_guy]

I posted a link on my Rocket Stove Cooker thread. They show several designs there. I liked the last one a lot. Matt has a Rocket Stove Cobb Oven design that he also posted there. I have always like the idea of a Cobb Bake Oven, but I also like the fuel efficiency and quick heat up time of the Rocket Stove.