BOOH VI

[Nutcase]

Inspired by something Manny wrote on the cast core thread, I decided to put my experiments with thin casting on the back burner and move BOOH VI inside. Basically this involved making a "sealed" barrel stand out of another half-barrel and venting the barrel stand into my current woodstove.

Since I had to rebuild to work with this different barrel stand anyway, I made some changes to the system as well, a combination of expedient hacks and some efforts at improvement. This is the main improvement:



Basically, I've taped some aluminum foil together, laid on the blanket material and rolled that around the 8" tube. This is the heat riser. The burn channel is the same, except with another layer. Some of the aluminum will burn off, but so what? it doesn't seem to hurt the blanket, and where it survives, it is a radiation/ convection barrier. Plus it makes rolling multiple pieces more practical.

[Nutcase]

Here is the heart of the system:



This half-barrel is slightly smaller in diameter than the barrel I've been using over the heat riser. So that barrel slides down and stops on the ridge. This is very neat, but it really squeezes the rest of the setup. If I were doing it over, I'd cut above the second ridge and just forget about doing anything with the smaller part. The black duct tape is there as a cutting guide by the way. I haven't done anything to seal any of the metalwork connections or to fill the corners between the circular burn channel and the square stainless. The connection between the burn channel and the heat riser is still something of a hack, but it is an improvement over all the previous hacks

[Nutcase]

The fuel inlet is a total hack, something just sort of thrown together and tacked on. All that's missing are the wire hooks holding the end frame onto the burn channel frame. The mesh and hook system offers a fast and versatile method for holding things together.



Aside from the thrown together aspect, this inlet feels much too shallow for the size of the opening. Fortunately, it's the part of the system that will be most easily modified without tearing down everything else.

[Nutcase]

So here it is, blocked up on the front of the hearth to get the vent into the woodstove, still without the barrel.



It was getting late, so I just did something sort of okay with a Workmate and a couple of wood blocks to get the necessary elevation. I'll do something neater and more level later. With a taller barrel stand cut at the second ridge, I would just have to level the support, and I could have a heat riser a foot taller. This heat riser is 30", previous have been 36" because the wire mesh barrel stand held the barrel 6" higher.



So here it is, fired up for the first small test with some paper, a pitch stick and a handful of bamboo kindling some time around 10:15 PM. Done in a day, sort of.

It started easily and ran up to about 450˚ on this minimal but fast-burning fuel load. However, there was considerable smokeback from this large and shallow fuel inlet without some sort of cap. There was also a bit of smoke leaking from the barrel-barrel stand connection during startup. I'm sure the leakage persists all through the burn, but it stopped smoking in a couple minutes.

This morning we ran it again with some more bamboo, smallish sticks such as you see in the background, and a couple of larger sticks, all in the comfortable working range for my lopper. Which is good, because it really is best to cut this stuff in half before putting it into the fuel port. The system heated up very quickly, on the order of 100˚/min, up to about 750˚.Then it drifted down and plateaued at 650˚± 50˚. I suspect the relatively fast heatup is attributable to the much reduced air infiltration around the burn channel-riser connection and perhaps to the foil. On the other hand, it didn't seem as inclined to run up to 900˚+ as previous systems. This may be due to the shorter riser reducing the draw along with more restriction on the exaust—that 8" vent isn't actually connected to a chimney, so there is no draw to compensate for the increased restriction. Another limitation to the system heating up may be the very considerable heat loss from the fuel channel/air inlet. However, that isn't much changed from earlier systems.

The force available to draw air through of air the system is fundamentally limited by the dimensions of the heat riser, and the air supply fundamentally limits the heat production. I think this is the main thing here. The initial overshoot fits with this theory, as the draw is also affected by the cooling of air in the barrel. Initially, the barrel is cold so the exhaust in it cools rapidly, becomes denser offers less counterforce to the riser.

I wonder if the rule of thumb about equal cross section through the system is really valid with this setup. I suppose I could make a 10" riser and see how that actually works. With the blanket walls, I've got plenty of space inside the barrel to work with.

On the other hand, as a practical matter, without a better fuel inlet, I'm not sure I actually want the system running that hot. So far, my experience indicates that the hottest part of the system is right where the flames are max, which is very close to the inlet. So there's that much more of a smokeback/firecreep potential to deal with.

[pa_friendly_guy]

Safety First Nutcase, Safety First, Moving the unit indoors is a huge step forward, but you still have a jury rigged out fit there, Safety First.

[matt walker]

I love it NC! I know you've run that thing enough outdoors to have a good handle on temperatures all around it, and how it behaves in general. Any chance you can get a "T" in the wood stove's flue and couple that chimney a little better to help the draft?

[Nutcase]

Okay, so it's not exactly to code. Nevertheless, it's only marginally more dangerous than any other rocket mass heater. Just smokier. That lashup is actually quite stable, and the barrel is boxed in by the stove and the elliptical. Anyway, like the old saying goes, good judgment is the product of experience...and experience is the product of bad judgment. Do you see any specific risk?

Anyway, this morning I did revise the base and got the shroud around the inlet. I should still do something to prop up the particle board under the inlet end.



I also used a piece of fiberglass batt from an old diswasher to make a cover for the stove door. It has couple magnets holding it more or less snug, or at least in place:




Edit: Matt, I'd rather not do anything hard to reverse until I absolutely have to. I think the seal to the stove I've improvised could be improved, but I'm not sure even that makes the current cost/benefit cutoff. The chimney now works like a chimney, and the stove actually picks up some warmth from the exhaust. However, it still doesn't draw well enough in this configuration to suppress smokeback. I do still have several square inches of open channel between the burn chamber and the stainless box, something that can't be helping the draw, plus minor leaks between the stainless and the barrel. The leaks will have to be dealt with and the inlet deepened and narrowed. Edit: this includes the smaller but possibly very important leaks around seams in the inlet. Then we'll see what we see.

[matt walker]

[Nutcase]

Matt, I'm not sure I'd go that far, but thanks for the vote of confidence.

There are the rules, and there are the real rules. Guard rails and cliff edges. The guard rails are very useful when you aren't paying very close attention to what you are doing.

We've been running the system for a couple of hours now, keeping it in the 700˚± range with spikes up to about 850˚ when a new piece of wood goes in. This is actually the first time we've run it more than 40 min, not counting the burndowns. It is supported by a sheet of particle board. This is definitely outside the guard rails. We have a 4 quart Revere Ware pot covering the inlet, and that is in the 4-500˚ range. Need to rig up a better lifter. I've gotten some small burns, and the hot pads are getting dinged up too. It's fortunate that the way I have built things with the wire frame, etc. limits the accumulation of heat on the bottom. Otherwise, I'd have to stop running it because I would notice that the heat was building up and then I would have to do something about it.

It's really no more dangerous than driving, if you think about it. No less dangerous if you don't think about it too.

[Nutcase]

Last night I made a better lifter for the pot:



This morning I made some tweaks on the inlet, narrowing it somewhat and closing off the worst leaks below the stainless channel that the pot rests on. The combination reduced smokeback but not firecreep. That is to say, any long piece sticking into the pot smoldered, but as long as the pot was in place, the smoke got sucked down. The system also ran pretty hot. Using moderate amounts of dry hardwood, something equivalent to two 1 1/2-2" sticks in the vigorous flame stage at one time, the barrel top maxed out the thermometer.

I opened the top of the stove when we were up and running. The chimney continued to completely inhale the exhaust stream and the draw into the fuel inlet was still adequate. I didn't look closely to see if there was an effect one way or the other, but it certainly wasn't dramatically bad. I stuck my hand into the exhaust stream. It felt much like what you feel with a dryer vent stream—the air felt warm and humid, but not uncomfortable.

The pot handle worked well. The wire is bent so that the pot remains securely inverted. The handle is high enough so that it never got uncomfortable to grip. It might work better with one side closed off, so more like Matt's arrangement, but that's not easy to arrange with this setup.

We now have some sticks on the stove, warming by convection from the stove and radiation from the barrel. I'm tempted to build a drying rack. That might be a step too close to the edge, though.

Bamboo makes great kindling. Some crumpled up paper stuffed into the burn channel, some split bamboo, easy to get going with very little smoke, even with the leaky seal between the barrel and the stand.

I still have a leak into the barrel stand approximating the area difference between a 12" square and a 12" circle, running through a channel length of 9". This can't help, but it doesn't seem to hurt much, so I will leave it alone for now.