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rocket stove variation

tel jetson
steward

Joined: May 17, 2007
Posts: 3082
Location: woodland, washington
    
  52
I'm wondering what parts of the rocket stove are critical to it's efficient operation.  particularly, I'm wondering if the downward flowing section after the burn chamber contributes to the efficiency of the burn.

is it the horizontal obstruction above the burn chamber and the consequent turbulence?

would a horizontal obstruction followed by an upward flow still do the trick?

I'm not so much concerned about the efficiency of the heat exchange just yet, but only the efficiency of the burn.

I'm visualizing something similar to what I found called the "Picasso Stove" on Aprovecho's website (pages 39-41 of this pdf), only with the typical J-tube intake and burn chamber.  I'm wanting to heat a room quickly, but not for a long time (a couple of hours at most), so heating a thermal mass doesn't really make sense.  this will be a kiuas (sauna heater), if you're curious.

I don't know if something like this would technically still qualify as a rocket stove, but that doesn't really concern me.  will it work?


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Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
the barrel in a rocket stove is the place where the smoke burns. so yes it is necessary if you want very high fuel efficiency. but if you want just radiant heat several other stove designs are better at this. smoke is lost fuel so this is where you take the hit for most of the radiant stove's; the other thing is the smoke load on your community might not be a good thing.

Erica and I try to asses the best least polluting stove for the application sometimes this is a RMH sometimes it is a pocket rocket sometimes its an aprovetcho stove but we always lean to this stoves that are as fuel efficient as possible.  this means less wood used and often less work keeping the stove fed. in a sauna you are going to be heating the thermal mass of moisture in the air so you might want to compute how much of that the stove needs too heat. steam is a good model for the calculation.


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Ernie and Erica
Wood burning stoves, Rocket Mass Heaters, DIY,
Stove plans, Boat plans, General permiculture information, Arts and crafts, Fire science, Find it at www.ernieanderica.info


tel jetson
steward

Joined: May 17, 2007
Posts: 3082
Location: woodland, washington
    
  52
thanks a lot, Ernie.

Ernie wrote:
the barrel in a rocket stove is the place where the smoke burns. so yes it is necessary if you want very high fuel efficiency.


does the smoke have to be traveling downward to burn?  in the Picasso Stove, the smoke would still be traveling through the same barrel as the standard rocket mass heater, but it would be traveling upward though it.
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
Hi Tel

Ok The answer is "that depends"

rule of clean combustion is time, temp, and turbulence.

time= how long the gasses are exposed to heat for combustion.
temp= the amount of heat in the area with the gases to make them combust.
turbulence= the mixing of the gases (smoke, O2 and heat (kinda))

what the barrel gives you is the time and turbulence elements. the gas hits the flat of the barrel and splashes causing mixing ( I am greatly simplifying) the down in a rocket stove barrel provides you time for the combustion and allows for the radiant heat surfaces to work effectively.

as long as the TTT is satisfied you can have a system that is vertical. in my experience however; there are very few radiant heaters that accomplish this. most with an up part dont have sufficient O2 for a complete re-burn and try to get around it by injecting outside air into the system at the re burn points (Dampers and not sufficient cross sectional area are a leading cause of this). usually this is badly done and add to much O2 so you have other problems.

anyhow thats what i know about it.
tel jetson
steward

Joined: May 17, 2007
Posts: 3082
Location: woodland, washington
    
  52
great.  that helps me a lot, Ernie.  thanks.

I'll leave the damper out and try to make sure my cross section is large enough.  I may also add a small amount of pre-heated secondary air at the exit of the burn chamber, but only if I can work out a simple way to do it.

hope to get working on this before too long.
Emerson White


Joined: May 02, 2010
Posts: 1206
Location: Alaska
No, you just need an L or J shaped tube and plenty of O2. I stacked up some cinder blocks and was able to use that to burn some brambles and rotting wood (some of which was dug out of the ground the day before) with no smoke, however I lost my heat to the sky. The barrel is key for moving hot gases back down to near floor level with out a blower. All you really need for rocket action is a good insulated flu pipe and burn box, something to add turbulence if you have a big fire going is a good idea too.
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23

if you where not getting steam you where getting smoke. even off a j tube there lots left to burn
Emerson White


Joined: May 02, 2010
Posts: 1206
Location: Alaska
I smelled it, no smoke. The whole thing was square and stacked, which helps with turbulence, and there was plenty of airflow. Secondary combustion was taking place in the stack itself, I suppose I may have lost some CO (no way to test that) but nothing condensed out of vapor form from the stack, presumably because it was able to mix with the outside air and keep the partial pressure of H2O low enough not to form steam. I did get condensation on the cement block I used to deflect the flow my way to see what it smelled like, but only for a moment. When I reduced the number of blocks in the chimney it became slightly Smokey, also when I added fuel too fast and restricted airflow.

Maybe you need a taller J tube for a stronger draw, or a smaller burn box to prevent creating too big a fire? I was also working primarily with small twigs (though I dropped some big pieces of wood down the chimney, to burn them out of the way) and the extra surface area as well as the radiation hitting the twigs may also have contributed to the clean burn, so maybe for a J tube the answer would be additional splitting of the wood to create something more akin to kindling in order to get a hotter fire and have secondary combustion out of the way sooner rather than later.
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
cool; its not been my experience with any height heat riser.  I always get smoke unless i have a good re-burn  (most times the smoke is invisible but its always there) . what wood where you burning?
Emerson White


Joined: May 02, 2010
Posts: 1206
Location: Alaska
The wood that was dug out of the lawn was old fence posts, at least 46 years old based on the fence line (the lot was changed after the 1964 Good Friday Earthquake) and had fairly wide strong grain, so maybe dug fir or pine The had rotted or been cut off at surface level then stopped, little round around the edges but still workable. Additionally I burned parts of some birch limbs cut to 16 inch sections 3-4" in diameter, some of which had lost about 15% of their mass to a wood conch of some sort, then lots of twigs from the birch and cotton wood and willow and cherry trees and wild rose brambles (which I cut off to approximately the height at which there was still living growing tissue). Temp was in the low 60's (which is why I didn't mind loosing the heat to the sky) and humidity in the 50's, together this does mean that the O2 in the air is more concentrated and that the chimney will draw harder, all with out adding too much thermal mass in the form of water vapor or making it too easy for the partial pressure of water to climb too high and produce steam. I did not take enough measurements (draw strength, wood consumption exhaust temp, etc.) to give you a calculation to theoretically prove my observation true, but I'm working on building an L tube out of cob and a piece of stove pipe, and with any luck I'll be able to take pictures of that once it is completed.
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
pretty much the type of stuff i test burn with. cool deal get some measurements if you can of the height of the riser from the bottom of the burn tunnel and length of the burn tunnel from the front to the back brick please.
Emerson White


Joined: May 02, 2010
Posts: 1206
Location: Alaska
Its made from cinder blocks, so thats an easy one. 48" from bottom to top, 40" of it relatively enclosed (air gets in through cracks between and now in the blocks, and the burn tunnel is 13" long. I started with a gutted double block to form the burn tunnel, put another double block on top of it as the first flu segment and fuel feed tube, and added 4 singles as the rest of the flu. Once the cinder blocks were blackened on the inside the fire didn't get hot enough to clean out the top 3, but it didn't deposit any more creosote either ( I changed some out and shifted some) unless I did something stupid (like clog the entry) Pretty much right away the burn chamber split longitudinally  and the double I had on top of it split after a few heating cooling cycles and maybe 8 hours of use, so that's no fun.
 
 
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