Wood gas stove experts, advice please

[Gotte]

I'm making a woodgas stove - I've got a medium paint can with another shorter can which inserts nicely and seals at the top. the can which goes inside is actually a kitchen flower sprinkler from ikea - it has a mesh bottom which will be good for airflow into the burning chamber. When inserted, the burning chamber has about a two inch gap underneath it, so plenty of air under it.

I know there should be a series of holes in the base of the outer can to allow air to be pulled in under the burning chamber, and a series of holes at the top of the burning chamber to allow it to funnel back into flames and reignite (I assume from what I've seen, that that's how it should be, anyway).
Now, how many holes, and how big? Is it better to have a lot of holes, or only a few, and are they better bigger, or smaller? Can they be too big?
Should they only be on one side, as with the Trangia windsheild, or is it better to have them all round so if there is wind, it actually increases air flow?

Thanks

 

[Ogri the trog]

Here are some thoughts from an un-initiated wood-gas stove maker - I intend to soon and I have some cans put aside for it.

From what I've read - and I may be entirely worng - you have to restrict air flow to the bottom of the burning chamber so that the hot wood produces gas and doesn't actually "burn". The resulting gas rises to the level of the top holes and ( as the extra oxygen enters) ignites, burning more cleanly. So the holes, top & bottom, need to balance this air flow which is different for each stove.

You may need to put a plate in the bottom of the burning chamber to hold off air flow - and the number and size of top holes will need to be trimmed and expanded by experimentation.

I'll be interested in how you get on.

ATB

Ogri the trog

 

[Gotte]

Thanks for that. My main concern is whether to add holes to the bottom of the inner combustion chamber or not. There is a good space below it to allow air in from the outer holes, but some models I've seen have holes on the lower sides of the inner tin as well as the inner tin base, but some don't. Not sure which is the best method, and don't want to add them only to realise it was the wrong thing to do. Anyone any ideas?

 

[Podcast Bob]

Building a wood gas stove isn't a quick solution like a hobo stove. If you are serious about it, you'll be having to make at least 10 samples with different variations to get it right. I know I did with the Honey Stove and I still have a wall full of wood gas stoves I'm working on.

Part of the 'restriction' of air flow around the base of the grill is to prevent the wind, increasing the burn. It also allows channeling of warm air to re-ignite the gas from the charcoal. The size of these channels is much more critical than you might think, so I would just say build many and see which works best with the material you have at hand.

Keep us informed as to progress ;-)

 

[Teepee]

The rule for a natural draught Tlud is a hole size ratio of about 3 to 1 secondary air to primary(this will vary slightly from stove to stove) . That means if you have 10 4mm primary holes then you need 30 4mm secondary holes. Its much easier and more effective to use bigger secondary holes than primary and you will need to get the calculator out to work out areas of the holes that you want to drill using the area of a circle= 3.142 x radius squared.

I would work out the total area of incoming air from the gap between the two cans and make the sum area of all holes equal to this value. This maintains a constant area for the air to flow through and will perform better than a drill it and hope approach(my favourite)

 

[Gotte]

Thanks for that. Looks like I'm in for the long haul.

All the best

Phil

 

[Gotte]

I tried it out today, but without having worked out with the 3:1 rule, and it boiled two cups of water in about ten minutes. There was a lot of smoke to start off with, but then that blew off. I could see the inner holes burning at the top, but it was quite a swirly wind, so they didn;t all seem to be firing at the same time. The fire didn;t need much topping up, and the wood rendered down to some charcoal at the end of the boil, and this soon turned to fine ash. At the end of the burn, there was little ash to speak of - very fine cigarette type ash at the bottom with no charcoal left, but that was after I'd taken the kettle off.

I'm not sure if that is a quick boil or not (about the same as my trangia in similar conditions), but as I said, little smoke after getting the fire going, and pure fine ash, so I assume that's a good sign.

Two things I;m not sure of, though. I have a solid tin sheet pot holder on top of the stove, and don't know if this should have lots of holes in it to allow the flames to spread - mine just as a gap for feeding the fire.
And also, and this may sound stupid, but with the formula above from Teepee, are the primary holes the ones in the outside can, or on the inside? most wood gas stoves have the bigger holes at the bottom on the outside, and the smaller more numerous holes at the top on the inside, with some fewer larger holes at the bottom on the inside.
So which are primary and which secondary? I would normally assume primary are on the outside and secondary on the inside, but from my reading of Teepee's post, it sounds like primary inside and secondary outside?

 

[lannyman8]

sounds good to me get some pics up so we can have a look see......

 

[Gotte]

I'll take some images tomorrow and put them up. I was toying with the idea of finding a different tin for the inside - just to try a different hole arrangement.
See, I'm hooked now.

All the best.