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Modified pocket rocket

                    


Joined: Oct 23, 2011
Posts: 0
I had this 55 gal. drum left from an old barrel stove so I tried some different things. The stack coming out of the lid is 5'' dia. and the intake at the bottom is 6''. It relay roars but I think an 8'' intake would get hotter.

I tried it surrounding the opening with brick and sticking an 8'' piece of pipe on it. It wasn't air tite enough and started burning up the feed tube.

Another idea would be to notch out a space big enough for a brick burn tunnel.


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paul wheaton
steward

Joined: Apr 01, 2005
Posts: 14163
Location: missoula, montana (zone 4)
    ∞
How hot is the "chimney"?


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Joined: Oct 23, 2011
Posts: 0
The chimney did not get hot enough to burn the galvanized coating off.

The barrel did get hotter with the brick configuration.
brett hunter


Joined: Nov 21, 2009
Posts: 14
how far into the barrel does the burn tunnel go?
                    


Joined: Oct 23, 2011
Posts: 0
Their is no burn tunnel inside the barrel. Just a 6'' flange to attach pipe to.


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Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 682
Location: Okanogan Highlands, Washington
    
  78
woodman wrote:
Their is no burn tunnel inside the barrel. Just a 6'' flange to attach pipe to.



Any particular reason you're doing this modification, instead of the original Pocket Rocket?

I've built and burned about half a dozen of the original version (in sizes from bucket to 55-gal drum), but I've never seen one quite like yours. 

How is it working for you?

What are your goals for it, and is it meeting them?

Here's a sketch of the original Pocket Rocket:


Here's my guesses about the differences in performance.  If you get a chance to make the other kind too, I'd love to hear your firsthand comparison.

I can see where your version allows the wood feed to be lower.  You won't avoid smokeback until you plug all the leaks, but that might be an advantage if you need a short stovepipe for some reason.

In your version you'll have to use shorter wood (the original allows for very long, straight scrap wood about as tall as the barrel). 
And there's not much room for the ash to build up before it clogs your horizontal pipe.  The original pocket rockets have a fair amount of room for ash.

Then your big barrel doesn't get as hot, because the fire isn't touching it as much.  And the flames aren't getting redirected as much, so you're not getting the same degree of mixing for complete burn.

Have you achieved a smokeless (transparent, or evaporating white) burn yet?  If there's blue or brown in your smoke, you're sending unburned fuel out the chimney.

If you want a brick burn tunnel, you're gonna need a heat riser too.  Or a hotter exhaust chimney.  Something's gotta pull that exhaust up and out, or else it's gonna smoke back and not burn properly. 


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Joined: Oct 23, 2011
Posts: 0
I had the barrel already and the added cook surface was the motivator. Different ideas for a Tipi heater. Am going to expand both intake and exhaust to get the barrel hotter.
brett hunter


Joined: Nov 21, 2009
Posts: 14
in my version the smoke is clear or white. zero smokeback, the draft starts up immediately.  i sealed up all of the leaks with high temp fireplace caulk.  most of the heat comes off of the burn tube( 8'' steel pipe), and the barrel gets hot but definitely not hot enough to cook on, though i did keep some apple cider heated in a saucepan on top last night.  my guess is if you had the feed right up against the barrel and the burn tube inserted into it a little bit, the barrel will probably get hotter.  the horizontal part of my burn tube is about 2 feet long,  kind of a mistake, but its made of heavy steel and gets good and hot.  trying to think of how to keep this basic design but make better use of the barrel.


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Joined: Oct 23, 2011
Posts: 0
Brett that is interesting. I had an idea, about putting a divider plate, baffle down in the barrel dividing it in half and extending up to within 3 or 4 inches from the top. Then on the chimney side extend the stack down into the barrel causing the heat to stay in the barrel longer.
brett hunter


Joined: Nov 21, 2009
Posts: 14
i was thinking along the same lines today, dividing the barrel in half somehow.. 
                    


Joined: Oct 23, 2011
Posts: 0
Trying this......put an adjustable air inlet and a plate.




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brett hunter


Joined: Nov 21, 2009
Posts: 14
gonna give this idea a shot,  worried that the draft will suffer greatly, but whatever, im going for it!
brett hunter


Joined: Nov 21, 2009
Posts: 14
so heres what i did,  i added an insulated heat riser inside the barrel as well as extended the exhaust chimney inside down to about 3 or 4 inches above the bottom of the barrel.  it works!  the draft so far seems a bit more sluggish than before but im hoping that im getting a cleaner burn and  more radiant heat in the process.  thanks for the idea woodman!


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Joined: Oct 23, 2011
Posts: 0
Brett has the wood burn time slowed down, and does the barrel get hotter?
paul wheaton
steward

Joined: Apr 01, 2005
Posts: 14163
Location: missoula, montana (zone 4)
    ∞
I wonder if the feed tube is "too tall" - as in, maybe it messes with drawing in the air.  If this turns out to be the case, I wonder if it can be mended with some holes in the tube.



paul wheaton
steward

Joined: Apr 01, 2005
Posts: 14163
Location: missoula, montana (zone 4)
    ∞
I think the addition of the insulated combustion chamber is a large positive step.  I would think that would do a lot to make for a much cleaner burn and to really push the exhaust.

The next thing i see as a point worth tinkering with:  getting more heat out of the exhaust before it leaves. 
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 682
Location: Okanogan Highlands, Washington
    
  78
paul wheaton wrote:
I wonder if the feed tube is "too tall" - as in, maybe it messes with drawing in the air.  If this turns out to be the case, I wonder if it can be mended with some holes in the tube.



Don't be poking holes in a perfectly good feed tube. 
Our experience has been that any extra 'air intakes' tend to increase smokeback - you need the air to come rushing past all the fuelwood, and sucking any smoke along with it.

I agree that the feed tube is too tall in proportion to the heat riser.  The system must rely on the heat of the chimney for draft, like an ordinary woodstove.

I'd shorten it by swapping the feed tube for a shorter pipe of the same size.  If it doesn't work, you can always swap it back.  Or just keep using the hot chimney to provide the necessary draft.

That means that you can't scrub any more heat out of the exhaust without giving careful consideration to how to replace the lost draft.  If that exhaust cools down too much, there will be no draft to speak of and the thing will smoke back like a monster.
charles c. johnson


Joined: Dec 02, 2009
Posts: 369
I agree that the feed tube is too tall in proportion to the heat riser.  The system must rely on the heat of the chimney for draft, like an ordinary woodstove.

is there rule of thumb for feed tubes like 23% of heat riser's hight? also would cold air from out side in a insulated pipe  cause more draft?
                    


Joined: Oct 23, 2011
Posts: 0
If an adjustable air intake is located toward the bottom of the feed tube then the feed tube heigth can be tall . Just put a lid on it after loading.
paul wheaton
steward

Joined: Apr 01, 2005
Posts: 14163
Location: missoula, montana (zone 4)
    ∞
woodman wrote:
If an adjustable air intake is located toward the bottom of the feed tube then the feed tube heigth can be tall . Just put a lid on it after loading.


Uh .... my impression is that the concept of "adjustable air intake" does not go with any of the rocket stuff.  You want the hottest, fastest burn possible and nothing less. 

I could be wrong, but I think you must now prepare yourself for Erica to come here and slap you silly.

                    


Joined: Oct 23, 2011
Posts: 0
Well on another forum referring to air intake:
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Bottom intake
« Thread Started on Oct 14, 2009, 1:05pm »
(Hi all, so I built a pocket rocket to see what all the hubbub is about. I'm hooked!

Anyhow, after two burnings I thought the fire would burn better if the air got preheated by the coals. So I popped a hole under my ash pit and put in a grate. It was a very noticeable improvement! Besides the preheating, the stack is made longer by not having the air pull down the feed tube.)
-------------------------------------------------------
Re: Bottom intake
« Reply #5 on Oct 16, 2009, 9:01am »
I can't really speak to the modifications on your pocket rocket..

I can say that I've noticed that rocket stoves (seem to) run best with about 1/2 - 1/3 air intake to system size ratio..
So-- Good observation.

It would be nice to pin it down a little tighter than all that. Though roughly right IS better than precisely wrong.
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Re: Bottom intake
« Reply #6 on Nov 17, 2009, 12:40am »   


So I figure it's time for an update:

Before I start, It does seem that each and everyone's stove has it's own personality (no two are alike).
I've probably had about a dozen fires now and I feel comfortable making some observations.

Mine is a five inch system with the burn tunnel/base cast in perlite and cement. At the bottom of the feed tube is a grate. Below the grate is a removeable ash drawer. I can control the bottom air intake by how much this drawer is open.This whole base is wrapped in thin sheet metal for protection. The burn tube is 5 1/4" long and square. My riser is steel tubing filled with perlite. 33" high, 5"ID, 12 1/4"OD. The radiating barrel is 14" wide and gives 2 3/4" space above the riser. I have a duct comming of the bottom of this.

The first thing I will say is it is very easy to light. after placing my kindling on the grate, I cover the feed tube and light it from below the grate. There is nowhere for the smoke to go but up the riser! Once the kindling is going, I remove the feed cover and throw bigger chunks in to get a coal bed started. I have my bottom intake about 3/4" open. After about 20 min. when there are good coals, I close this down to about 1/4"-3/8".

The standard intake pulls air down the feed tube so only the tips burn. Most of the smoke and heat is at the bottom of the burn tube. It has been my observation that very top (1/2"-3/4" of the burn tube is pulling only air. By pulling some air (hot air) from the bottom, we are moving the heat up slightly. This is placing the burn higher up. I am burning more than just the tips of the wood and getting better mixing. I feel that I am preheating the wood a little more and can burn with higher moisture content. If I open the bottom air too much, or I get too much of a coal base, it starts to flame out the feed tube.
If the coals don't fall through the grate automatically, I just need to wiggle my sticks a little. I like that I can clean out the coals/ashes while its running.

This picture I found shows what apears to be an outside air intake.


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Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 682
Location: Okanogan Highlands, Washington
    
  78
woodman wrote:
If an adjustable air intake is located toward the bottom of the feed tube then the feed tube heigth can be tall . Just put a lid on it after loading.


Yes, you could do that.

My experience has been mostly with strictly J-tube systems.  We often want to load them again after they've been burning a while. 

If a lid is used to close off the top, and air is let in through the bottom, you have a fancy L-tube situation.  The lids tend to get hot, so opening it again to load them creates a temporary smokeback.

We do use a cover on ours toward the end of a burn, when there's not enough wood to keep the air flow adjusted.  Our 'lid' is a chunk of stone tile that we can slide on with a variable gap.  That is basically an adjustable air intake, and the area that works well seems to be about 1/3 of the system size.  But that's usually when we're down to 1 or 2 small logs, or 1/2 to 1/3 the wood the burn chamber can hold when it's full. 

I don't tend to slap people for adjustable air intakes.  There are too many commercial devices out there with this feature - what most people don't know is that modern commercial stoves with 'adjustable' air intakes don't let you adjust the air below a certain amount, so there's always (theoretically) enough for a clean burn.

On your J-tube rocket stove with air intake, if you want your stove not to smoke back when you take off the lid to re-fuel, I'm guessing you simultaneously shut down the lower air intake, and crack the lid.  Gradually opening the lid and letting the air clear the smoke before you open it all the way.

The ratio of height of feed tube to height of heat riser seems to be about 1:3.  Well-insulated systems with everything else in their favor can sometimes cut it as close as roughly 1:2.  Bigger differences are better as far as draw goes; but since people tend to want both feed and barrel top at a convenient height for handling, they don't often settle for a bigger difference.
charles c. johnson


Joined: Dec 02, 2009
Posts: 369
WoodGas Stoves


Image modified from:
A Wood-Gas Stove for Developing Countries
T. B. Reed and Ronal Larson

If wood is heated up and gasified, it has the potential of burning more completely, efficiently and with little or no smoke and heavy pollutants.  These stoves tend to be a bit more complex than a tin can with holes punched in it and therefore tend to weigh a bit more, but may make up for extra weigh if you are packing in wood pellets or where fuel is more scarce.  For more information on wood gasification, check out Wood-Gas Stoves for Developing Countries  and Testing and Modeling the Wood Gas Turbo Stove.

The picture above depicts a stove made from coffee cans and is more or less self-explanatory except for the "Gas Wick."  The gas wick is a smaller diameter can suspended by wires inside the larger coffee can with its closed bottom up.  The wick become very hot and draws gas and air up along its sides, and prevents air from going down the chimney.

It is also important to note that fuel is ignited from the top down for the gasification process to work.


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paul wheaton
steward

Joined: Apr 01, 2005
Posts: 14163
Location: missoula, montana (zone 4)
    ∞
I think I read somewhere - maybe in the book - that if you get a certain kind of micro-barrel, it has a "bung hole" that is just the right size for the air intake. 

As I was wrapping my head around it - I thought about how something like that would make sure that the air flow coming to the fuel was exactly above the fuel - in a strong directional flow - thus assisting in preventing smokeback.

 
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