Blue Flame / 30min

This is my second attempt at a cob rocket stove.  I've increased the thermal mass as well as lined the fire box / horizontal tube w/ refractory brick.  The brick was left over from an brick bread oven build.  The clay is that rich Carolina clay. Which means that its laden with iron and holds just under 1/3 sand.  The sand in the cob comes from a local sand bar.  Its rich in mica which makes the cob sparkle.  

The stove pictured was designed to bring a commercial stock pot to a full rolling boil for the intention to make scrapple.  (deer face meat + corn masa).  I've sourced Ianto Evans book Lorena Stoves.  Which holds a lot of insight.  

While active this oven produces a blue flame which disappears half way up the riser tube.  I conclude this is a sign that the gases have ignited.  I consider this a good thing as the bottom of the pot is best heated if the gases have had a chance to combust.  

In the future I plan to create a Cob Rocket Stove with a spiral riser tube.  I'm interested to see how the flame reacts.  As well as how it may effect the draft.

Also I'm a newby at plastering with cob.  The mix I found worked the best was a 1 part Clay to 2 parts sand ratio + heavy on the 2.  with the cob mix being the opposite.
(I'm now using horse dung in my mix)

I learned the hard way that  when plastering. 1 batch is better than 2 for reasons of consistency.  

Great build, how has it been performing?

I built a very similar pot boiling rocket stove out of solely fire brick (had found a very cheap source), I ended up using a camping stove as a pot stand-off and after a few iterations (bricks, stones, other stove grates, etc) I never did find one that was perfect. Other than the pot stand-off the stove worked fantastically with the flames disappearing about 2/3 of the way up the riser and no smoke after just a couple minutes into the burn, which sounds similar to your results. My main changes to my own design would be to build an outer wall and fill the gap with pumice or something similar to act as insulation instead of mass. What do you use as a pot stand-off and is it perfect or what would you change?

What would be the benefit of a "pot stand-off" on this sort of cob stove?

I understand that on a gas stove, the stand off makes sure that the flame isn't smothered. But I thought a rocket stove like this one, would have the air intake near the bottom, not at the pot height?

I also thought from looking at the Lorena they made at Wheaton Labs, that part of the point was to have the heat around the pot sides part way up to increase it's ability to absorb heat quickly - the same idea as a "pot skirt"?

Jay Angler wrote:What would be the benefit of a "pot stand-off" on this sort of cob stove?

I understand that on a gas stove, the stand off makes sure that the flame isn't smothered. But I thought a rocket stove like this one, would have the air intake near the bottom, not at the pot height?

I also thought from looking at the Lorena they made at Wheaton Labs, that part of the point was to have the heat around the pot sides part way up to increase it's ability to absorb heat quickly - the same idea as a "pot skirt"?

From the pictures that I can see unless I am misinterpreting something - this is not a Lorena style stove. This is just a simple J tube and riser - the pot sits on top of the chimney. In a Lorena the pot is placed in a hole positioned mid to late burn, here the pot is post-exhaust and if a pot was placed directly on the stove it would cut the exhaust entirely to nothing, which would be why a properly designed pot stand-off is key to proper functioning. This is valid only if I am interpreting these pictures correctly.

Yes that is what I though, without a pot stand, if you place a pot over the riser, the fire will just go out!

Fox James wrote:Yes that is what I though, without a pot stand, if you place a pot over the riser, the fire will just go out!

It wouldn't go out, it would just use the intake as the chimney making it a smoky and dangerous mess and you wouldn't be cooking anything anymore. I guess the proper way to design the pot stand-off would be to calculate the cross-sectional area of the chimney and maintain that area in the total exit area under the pot and between the stand-off structure, I just never got around to doing it the right way. Also as Jay Angler pointed out a skirt of some kind would increase efficiency as well to keep the hot gases against the body of the pot longer and prevent cross-winds from blowing under the pot. If a pot-skirt was introduced to this system I have a feeling you would need very little fuel to boil a decent pot. My version of this was completely out in the open in the wind and I managed to keep a 3 gallon pot of water boiling for a few hours on one firewood round split down (my intent was to have an easy solution to doing canning outside), adding a skirt to keep the wind off would've increased efficiency massively.

Ezra Beaton wrote: Also as Jay Angler pointed out a skirt of some kind would increase efficiency as well to keep the hot gases against the body of the pot longer and prevent cross-winds from blowing under the pot. If a pot-skirt was introduced to this system I have a feeling you would need very little fuel to boil a decent pot. My version of this was completely out in the open in the wind and I managed to keep a 3 gallon pot of water boiling for a few hours on one firewood round split down (my intent was to have an easy solution to doing canning outside), adding a skirt to keep the wind off would've increased efficiency massively.

OK, so could the skirt be effectively an extension of the "chimney" so long as the gap between the pot and the skirt maintained the equivalent (or slightly larger) cross sectional area? (Area of the skirt cross section minus the area of the pot cross section.)

You'd still need a large enough pot stand to allow the fire exhaust to exit the riser, then it would enter the skirt which is acting as a chimney, and then exit the gap between the pot and the top of the skirt?

Any guess at what the exhaust temps would be at that point?  I'm thinking in terms of either my canning kettle or a large metal garbage can which I use for scalding birds for plucking. My current system is both inadequate, a PITA,* and quite frankly, more dangerous than I would like. However, my canning kettle has handles and although I wouldn't make the skirt go that high, things have to be cool enough not to damage them.

*pain in the ass

Jay Angler wrote:

Ezra Beaton wrote: Also as Jay Angler pointed out a skirt of some kind would increase efficiency as well to keep the hot gases against the body of the pot longer and prevent cross-winds from blowing under the pot. If a pot-skirt was introduced to this system I have a feeling you would need very little fuel to boil a decent pot. My version of this was completely out in the open in the wind and I managed to keep a 3 gallon pot of water boiling for a few hours on one firewood round split down (my intent was to have an easy solution to doing canning outside), adding a skirt to keep the wind off would've increased efficiency massively.

OK, so could the skirt be effectively an extension of the "chimney" so long as the gap between the pot and the skirt maintained the equivalent (or slightly larger) cross sectional area? (Area of the skirt cross section minus the area of the pot cross section.)

You'd still need a large enough pot stand to allow the fire exhaust to exit the riser, then it would enter the skirt which is acting as a chimney, and then exit the gap between the pot and the top of the skirt?

Any guess at what the exhaust temps would be at that point?  I'm thinking in terms of either my canning kettle or a large metal garbage can which I use for scalding birds for plucking. My current system is both inadequate, a PITA,* and quite frankly, more dangerous than I would like. However, my canning kettle has handles and although I wouldn't make the skirt go that high, things have to be cool enough not to damage them.

*pain in the ass

Your description of the calculation for the skirt sizing is exactly what I was thinking. By making the effective chimney area of the skirt slightly larger than the actual chimney (riser) area, you would theoretically be slowing down the speed of traversal, holding the exhaust against the pot longer before exit. Sounds like a great efficiency plan. Others on these forums would have much more concrete data on exhaust temps, I don't think I'm educated enough to even guess. Suffice it to say it would be 'plenty hot', possibly even dangerously hot for the cooking vessel's bottom integrity long term. I would definitely stop the skirt at absolute maximum halfway up the cooking vessel, for ease of handling 1/3 would probably be a nice compromise.

To take things one step further, you could add an insulated lid with a vent, effectively creating a small black oven.