Double or single wall wood stove?

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jbutzi
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Double or single wall wood stove?

Postby jbutzi » Sat Jan 28, 2012 11:51 pm

Zelph, your testing of your folding mesh wood burning stove against the bushbuddy and others and your experiment on other stoves seems to show that having two walls does not add to the burn efficiency. I'm learning.
So now I see this and wonder what is going on. A sardine can in a Heiny Pot. It looks like it burns extremely well and with only 1.2 oz. of wood: http://www.hobohilton.com/hwgs1.html

Whats going on there? Looks like gases burning coming in upper inside holes. Not a like of air vents compared to yours. Lastly, I am trying to learn so I can apply it to my recent and ongoing attempts.

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zelph
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Re: Double or single wall wood stove?

Postby zelph » Sun Jan 29, 2012 9:20 pm

That's plain ole air coming in through the holes and forcing it's way into the flames coming up from inside the stove. You should be able to see the same thing in the stove you made. There are many photos here that show my single wall stoves with air shoving it's way into the fire to make it apppear like big gas jets. I say "appear" to look like gas jets. It's just air, no flammable gas. The guy who makes the Bushbuddy even emphasized in an interview that it it not "gas" coming through the upper holes of his stove, just plain air. I'll get a link to the interview and post it with a highlight.
http://www.woodgaz-stove.com/

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zelph
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Re: Double or single wall wood stove?

Postby zelph » Sun Jan 29, 2012 9:51 pm

Here is quote from the interview:

Can you tell us a little about the beginnings of the Bushbuddy stove and how you arrived at the current design?
FHI had been using the Sierra Stove. The fuel efficiency was inspirational, but the need for a battery was a drawback.

I experimented with a number of different wood burning stoves for my own use, which didn’t have a fan. They worked pretty well, but they all had a bad habit of igniting any flammable surface they were set up on. This seriously limited the places the stove could be safely used.

I was trying to find a way to keep the bottom of my stove cool enough that it could be placed directly on flammable surfaces without charring them. Thinking that more cooling air drawn through the bottom of the stove might be a solution to this problem, I put a double shell around the firebox to create an additional draft through the base of the stove. The warmed exhaust air was routed into the firebox near it’s top. When I saw what appeared to be jets of flame entering the firebox, and very little smoke, I knew this was something special. (The phenomena is called “secondary combustion”. The apparent “jets” of flame entering the firebox are not, as some people mistakenly claim, “wood gas” entering the firebox, but warm air, which combines with the invisible, rising, unburned gasses in the firebox). This first experimental stove evolved into what today is the Bushbuddy.

disregard the part where he says "secondary combustion" it is just air pushing it's way into the fire chamber. No secondary combustion. The air coming in is no where near being hot enough to have secondary combustion occur.

Here is a link to the site where the original interview is listed in it's entirety:

http://hikeitlikeit.com/2011/bushbuddy-fritz-handel/
http://www.woodgaz-stove.com/

jbutzi
Posts: 42
Joined: Sun Jan 15, 2012 6:20 pm
Location: SE Wisconsin

Re: Double or single wall wood stove?

Postby jbutzi » Tue Jan 31, 2012 3:15 pm

OK thanks, I understand. So another question: if that is just air coming in the holes at the top then how do the entry holes assist in the combustion process, that is, what is special about the way it enters through holes? Could it just as well come in over the top of the can a little higher? I can supply lots of air at the bottom. If so the stove could be shorter and the fire closer to the pot. I could use a 16 oz short bean can instead of a 14-15 tall soup can. I am also thinking of your folding mesh stove. Its basically more open than closed. It's so open you can't say it has holes. ;)

BTW, I made another 14 oz stove and got it to boil 2 cups in about 8 minutes. But it was just done in time before the fire lost its "force". So there is not a lot of "headroom" for mistakes, wood variation or wet conditions. So what are the options? 20 oz can, but I like to keep it compact; 16 oz short wide can, need to experiment; add wood to the fire mid burn, purists cringe. What do you think. I'm guessing you have covered this ground before. I am looking for a design that is light, and compact that will do the job reliably. Thanks.

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cadyak
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Re: Double or single wall wood stove?

Postby cadyak » Tue Jan 31, 2012 4:20 pm

That is not a bad boil time at all. There is nothing wrong with having to add a few pieces of wood to get it
over the hump.:shock:
With a firebox that small, it is inevitable sometimes unless you have perfect wood. If you can place a layer of thicker pieces at the bottom they can give you a resurgence of flames as the fire gets lower in your fire chamber. Sounds like your having fun with it. I burned my 6 ft tall burning pile this weekend. Lit it with a single beeswax tinder. I built the fire on the very top of the pile and let it burn down.
It was beautiful. :geek:

jbutzi
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Joined: Sun Jan 15, 2012 6:20 pm
Location: SE Wisconsin

Re: Double or single wall wood stove?

Postby jbutzi » Tue Jan 31, 2012 5:50 pm

I made another stove with a shorter 16 oz baked bean can. Again, got it to just boil at about 10 min and ten the flames died out and it was burning embers at the bottom of the can for a few more minutes. I did not put holes in the top of the can at all. I used the same leg design as before and it is quite sturdy especially since the can is wider and shorter. For a pot stand I just stuck three light weight nail through under the rim and back through a small hole 1/2" below that. I bent the tip of the nail to make it go into the lower hole and hold firmly. All together its under 2 oz which is pretty good I think. By removing the nails it fits in the Kmart grease pan with the lid up a little.
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zelph
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Re: Double or single wall wood stove?

Postby zelph » Wed Feb 01, 2012 12:03 pm

Put some big holes near the top. they will help focus the flames to the center under the pot.

Stack dry, split twigs verticle and light from top. Review some of the threads in this forum on top lighting, verticle stacking.
http://www.woodgaz-stove.com/

Wolfman
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Re: Double or single wall wood stove?

Postby Wolfman » Sat Mar 03, 2012 12:34 am

Late to the thread again, but oh well. :)

zelph, I have to disagree with you on this one,
disregard the part where he says "secondary combustion" it is just air pushing it's way into the fire chamber. No secondary combustion. The air coming in is no where near being hot enough to have secondary combustion occur.
I am not sure if you looked at how he built this stove, but I am almost postie that is secondary combustion.

The reason that this stove works the way it dose is the insulation on the outside of the can. This allows the air entering the bottom of the can to get superheated before entering the fire chamber. By superheated, I am saying probably close to 300+ degrees, plenty for secondary combustion. This will not happen on single wall stoves and most 2 wall metal stoves, the heat just dissipates from the outside, yes the air gets hotter, but not hot enough for additional combustion.

Now given that he was burning dried oak chips, not something your going to find in the woods, this may or may not work with normal twigs, but in the link and photos, I am fairly sure that was secondary combustion.

I have tried several times to build a Gas-a-fire stove, and none have worked like that with true secondary combustion, I think the key is the insulation.

Would really like to hear some thoughts on this,

Wolfman

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zelph
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Re: Double or single wall wood stove?

Postby zelph » Sat Mar 03, 2012 6:36 pm

There is a lot of information that I want to give to you but first read this thread on the bushbuddy stove:

Keep in mind there is only 2 inches of fuel space inside the burn chamber. Thats 2 inches of wall surface that will heat the incoming air throught the top holes. Nobody has been able to tell us the temperature of the incoming air. Keep in mind the air is really moving fast over the side of the wall for a distance of 2 inches.

I have taken careful dimensions of the bushbuddy for my tests and know the stove well.

I did some scientific testing of the "bushbuddy" next to the "bushcooker" by fourdogstove.

You won't find any plans here to make "gas-a-fire" stoves. I tested the "beaner" stove that is known to be a gassifier. It was a real pain in the neck to keep burning with twigs. I did not even get to the point of of using wood pellets as suggested by the maker. He wants us to use wood pellets first. After the fire goes out he wants us to bury the charcoal that is left over. Nice guy.


viewtopic.php?f=60&t=4538&start=10

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http://www.woodgaz-stove.com/


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