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Permsteading.com • View topic - Muddy Box 2

Muddy Box 2

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Muddy Box 2

Postby 4seasons » Thu Apr 03, 2014 7:59 pm

After the first attempt of a batchbox crumbled I redesigned and built another. This one is also a 4 inch made of local clay, wood ash, sand, and straw. Rather than off set the riser to create a single cyclone this time I centered the riser and used a twin cyclone idea to try to maximize the "ram horn" effect. The idea is to create more turbulence in the riser than my first design where I tried to speed the gasses up the riser. Today I got it put together and fired it up.

I think my cobbing is getting better as this one turned out smoother than the first one (which you can see the crumbled remains to the left) and I didn't get the mix anywhere near as wet this time. You can see the port is just a few holes drilled in the inner mold so it wont come up to full power until it burns out.

As you can see it immediately came up to speed and burned smokeless, oh wait.....
By the way I used the same heat riser from the first version just cobbed up some cracks and sealed it up to the new core with cob.

Most of the form was burned out by this time and the first "ram horns" appeared. The whole thing is still pretty soft and wet and I haven't put the P-channel in yet so maybe I can get some performance on the next burn.
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Re: Muddy Box 2

Postby matt walker » Thu Apr 03, 2014 8:52 pm

Nice man, love seeing these burn vids. These short lived cast mud builds are a great way to get acquainted with the things, I wish more folks would do it. Really good stuff 'Seasons.
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Re: Muddy Box 2

Postby Rhett » Fri Apr 04, 2014 10:40 pm

Well, this thing sure looks killer, but dang that's a lot of smoke. What's the problem there? Is the cob not a good insulator? Or is the riser too short?

Can cob withstand repeated burns or is it "doomed to failure"?
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Re: Muddy Box 2

Postby matt walker » Fri Apr 04, 2014 11:24 pm

I'm pretty sure it's smoking like crazy because the wooden mold is still inside and the passageways and ratios are all out of whack until it burns completely away. That can take a long time as the wood is in contact with soaking clay and it will kinda do the boiling water over the campfire in a styrofoam cup trick.

Cob/perlite cores can actually be pretty dang good. I've had risers last indefinitely, I take them apart before they fail typically. The wood feed area doesn't do so well. Natural mixes tend to be really prone to crumbling and/or just turning to powder once completely dry, so any wood banging around in there will quickly deteriorate a mud core. That's totally dependent on your area's clay though. I suspect there are places where you could go dig in your yard and cast a really tough core. Texas might just be that place, judging by all the old mud forts and stuff.
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Re: Muddy Box 2

Postby Rhett » Sat Apr 05, 2014 12:44 pm

Oh I see,I didn't realize there was a wood lining. That definitely makes sense.

As for dimensions being off, are they really that important? I'm going to say this now - I think a lot of the math people are throwing around is pseudoscience. I know those guys have built hundreds of stoves and have a ton more experience than I do, I just think their formulas imply a certain mathematical rigor that actually isn't there. And more to the point isn't needed. I saw a formula for computing the CSA of the exhaust port that was like double this, multiple by pi, add twice this dimension, divide by that. It was almost comical, and it is one of the most referenced threads on the topic. My stove is not the final word on rocket stoves, and I haven't implemented the bell yet and that can change everything I get that, but it doesn't obey a single ratio of any of the published stuff and burns great. It just makes me question all the numbers being bandied about.

Come to think of it, maybe it burns great but could burn greater? What would you say are the telltale signs that a stove is efficient or not efficient? Clear vapor at the exhaust? No or little smoke smell? Very little ash production? Good draft? Roaring sound? Maybe we can think our stoves are burning great and superficially they are, but there are deeper signs of problems or inefficiencies.
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Re: Muddy Box 2

Postby 4seasons » Sat Apr 05, 2014 2:22 pm

The dimensions of this one are off where it matters. The port is undersized compared to the heat riser because rather than having a 1.5 in wide by 6 in tall port it started burning with only four 1 inch holes drilled into the wood form. By the third video most of the port had burned out and that is when the ram horns appeared. From my first build I learned that until all that wet plywood burns it is going to smoke. Also with a little 4 inch batch box your wood needs to be very small and short. Any wood that get pushed back into the port opening will cause it to smoke. So it would seem that the port dimensions to CSA ratio doesn't have much room for fudging.

I now have the mold burned out and put the P-channel in so I will try to get some more videos of it today. I am starting to think that my heat riser may need to be rebuilt to match the shape of the core though. I have a feeling that the transition from the oblong shape that I used to maximize the ram horn is not transitioning into the round riser very well and really slowing everything down.
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Re: Muddy Box 2

Postby matt walker » Sat Apr 05, 2014 3:18 pm

There are some dimensions that matter more than others, but in my opinion they do matter when building a true mass heater with one of these as the front end. We ask the combustion chamber to do a whole lot to drive the gas through the mass and still behave, and little changes in the dimensions can add load it can't push, or slow things down to where they cool and things get plugged, etc.

In a core only or a no mass system, for sure there is a lot of leeway. As for clean burn, one of the most important things I learned this last year interacting with stove designers and scientists in DC was that the absence of smoke does NOT indicate a clean burn. There are a whole lot of stages beyond clear exhaust before we get to truly clean burning. You can do a pretty good job with your nose once you have some experience, but you do need to have some good clean burns to know what you are looking for. This is why I'm not a fan of metal rockets. If it isn't hotter than the metal can handle then you aren't burning off all the stuff, and it's going to burn dirty. You'd never know though, the exhaust would be clear, but full of CO.
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Re: Muddy Box 2

Postby Rhett » Sat Apr 05, 2014 4:04 pm

That's amazing. So is the gold standard in testing a co2 and co emissions test? Or maybe temp? Ie if you are get to X temp then you are probably burning almost everything. But then that probably depends a lot on your fuel source...
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Re: Muddy Box 2

Postby matt walker » Sat Apr 05, 2014 4:41 pm

Getting outside my knowledge with regards to what's in there and what the gold standard is. My understanding is CO levels are a good indicator of a clean burn. Not sure if it's the most important stuff to burn off or not. One thing I picked up in conversation was that it was easy to measure compared to, say, CO2, so that plays a part in why it's used as an indicator.
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Re: Muddy Box 2

Postby 4seasons » Mon Apr 07, 2014 1:29 pm

After getting the mold burned out and putting the P-Channel in I am still not getting a smoke free burn. My next step is going to be to build a heat riser that matches the shape of the core. I know that people have used a square to round transition and got away with it before but I think this transition is just to extreme and is really slowing things down.

Even though you can see the ram horns spiraling up the riser I am still getting smoke and not throwing flames out the top of the riser like the single cyclone did.

I guess it is time to mix up some more mud and form up a better riser.
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