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Inverted Downdraft Gassifier- some questions
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Tim F
(kneebyter) - MLife

Locale: the depths of Hiking Hell (Iowa)
Inverted Downdraft Gassifier- some questions on 08/25/2008 11:53:39 MDT Print View

I'm starting my first MYOG project- a woodgas stove that should come close to nesting in my Brasslite 900 pot. I have read the MANY old threads regarding making one of these stoves but still have a few questions.
My materials will be a large Dinty Moore beef stew can, a Armor corned beef hash can, a 12oz Starkist tuna can for the pot stand, and some metal mesh for the bottom of the burner.

Here are my questions:
How far above the bottom of the outside can should the inside can (the burner) be suspended- i.e. should it be above or below the level of the bottom air intake holes on the outside can?

In one of the old MYOG threads someone stated that the ratio of the primary air intake area (the bottom of the burner with the metal mesh) to the secondary air intake area (the holes in the top of the inside burner can) should be 1 to 4. Is this based on some formula, or was it just this persons opinion?

If there is some formula for these ratios, is there also a correct ratio between these inside air holes and the air intake holes at the bottom of the outside can?

Does drilling work well with this type of thin metal, or is there a better way to make clean holes?

Is JB Weld the best adhesive to use? Is it flammable at all?

I know that many of the names that I saw posting in those old threads are still around the forums now. I would really appreciate any information that would give me a starting point. Obviously the materials are not that expensive but I would like to get close the first time so that the needed tweaking is minimal. Besides, even my dogs didn't like the corned beef hash!

Thanks, in advance, for any help you can give me.

Edited by kneebyter on 08/25/2008 12:00:29 MDT.

Derek Goffin

Locale: North of England
Inverted Downdraft Gassifier- some questions on 08/26/2008 03:05:36 MDT Print View

You want the outside holes as low as convenient. The bottom of the burner (where the mesh makes the grate) should be high enough to allow the primary air volume to the grate even with ash build up so it depends how long you want to burn for. I set mine 25mm up.
As I remember the exact ratio of primary to secondary air depends on whether you are burning volatiles or charcoal. So early on you want lots of secondary air later on not so much. 4 to 1 is a good average. I don't remember the exact ratios but as the correct answer is a moving target its not too important. Regular feeding will keep the ratio somewhere in the middle. If I imagine your stove design right the outer holes feed primary and secondary air so you just want big holes at the top of the burner for your secondary air. You should not control the primary air supply by reducing the holes in the grate because the fire will clog with ash. In my design the inner burner stood on the ground, the short outer hung off it so primary air was separate from secondary. I could drill more or less holes in the inner below the grate for primary air. The secondary air was free to rise up the open bottom of the annulus between inner and outer cans and the secondary air holes at the top could be big, limited by not wanting them too low because it makes the firebox theoretically smaller.

There may be a better way but I filled the can with water, froze it, so the ice suppported the thin walls and used a drill stand. Then I had to file off sharp bits.
I did not use adhesive.

Keith Selbo
(herman666) - F - M

Locale: Northern Virginia
Re: Inverted Downdraft Gassifier- some questions on 08/26/2008 09:05:24 MDT Print View

A punch is probably the easiest and cleanest way to get the holes if you have one and the metal is thin enough. Your typical steel can is about .01" thick which may be beyond the limits of hand operated punches.

Twist drills typically won't give you a nice round hole in thin metal but a stepped drill will give you very nice holes if you back the work with a piece of wood. I think one brand name for a stepped drill is Uni-bit.

Beyond being a sexy concept, I'm not so sure what the benefit of a downdraft gasifier is. A well insulated, conventionally drafted firebox with windscreen/chimney combination is very efficient. I measured 69% on one I made. I've never seen any posts of gasifier efficiency measurements, but judging by some of the rough boil times that I've seen from time to time, it doesn't seem as though gasifiers have higher efficiency.

Frankly, I think there's a bigger pay off in a stove design that tolerates wind and rain than in soot reduction.

Jay Morris
(zoonunit) - F
Let's put the "downdraft gasifier" term to rest on 09/11/2008 23:16:12 MDT Print View

I keep seeing people misuse this terminology, so let me try and explain the physics and the history of this term.

Many of the woodgas stoves were based on research done for forced air downdraft gasifiers. It is physically impossible to have a true downdraft gasifier that is naturally convected.

What the original scientists did, is make a naturally convected version of their forced air downdraft gasifier. Because there was no fan, it was really an "updraft" gasifier. I guess in their infinite wisdom, (or as a joke) they called their new design a "reverse downdraft gasifier" which really means, it's an updraft stove, or naturally convected stove.

Just call it a woodgas stove. That's more accurate.

I'd make a few suggestions for an innovative woodgas stove that is light:

1) Only use steel for the inner firebox. If you do not connect the firebox to the outer cover, you won't get heat transfer, and therefore, you can use a light aluminum outer windscreen without the danger of melting.

2) Make your firebox as large as will fit in your pot. You want to be able to load as much fuel as possible to cut down on re-stoking. The heat buildup will also help to encourage total woodgas combustion.

3) Turbulence and heat transfer is the key to efficient woodgas combustion. You want your woodgas to stay in the stove as long as possible so that the combustible materials have time to completely burn. This can be accomplished by using fins to capture heat and introduce turbulence. Think of your outer shell having a cross shaped set of fins inside that the pot will rest on. They not only act as heat "wicks," but help transfer heat to the pot. Do NOT open your pot stand to the open air. You're inviting all your heat and unburned woodgas to blow away in the wind.

4) One of the keys to the Garlington stove, is that the outer shell (not the bigger windscreen) is light aluminum but does not contact the inner steel can. Therefore the aluminum is in no danger of melting. The PROBLEM with the Garlington, is that the outer shell doesn't feed enough air to the primary and secondary sources. (and the firebox has too few holes for efficient air intake)

Try this: Use a LARGE steel firebox so you can load up on fuel. Surround this with an aluminum outer shell stiff enough to hold the weight of a pot full of water. Make sure the outer shell has plenty of big holes near the bottom to feed primary and secondary air, and another row of large holes at the top to vent out the hot combustion gases directly underneath the pot. Enclose the entire structure and the pot with another windscreen made of thin aluminum foil to help protect against wind.