Woodburning camp stoves

[mrcharly-YHT]

How much heat radiates downwards?

A lot of campsites won't allow disposable bbqs or stoves that sit on the ground, due to scorching of the grass.  I do like my kelly kettle but it has the problem of burning the ground.

[pcolbeck]

An officious French campsite manager gave me a right shouting at for "destroying the beauty of the campsite" years ago for using a bbq that sat on the ground. If we had had it on grass I could have understood but this was on Mont Blanc and I had the bbq resting on a a slab of igneous rock that must have weighed 200 tonnes, no way was a disposable BBQ going to make any impression on that. Indeed next morning when we chuck the bbq in the bin there wasn't a mark on the rock. We had however bent most f our tent pegs trying to get the tents even slightly fastened down.

[mrcharly-YHT]

Camping and bushwalking in WA gave me a profound preference for tents that don't require pegs; the soil in the Darling Scarp is often only a few inches deep over solid rock.

[Charlotte]

How much heat radiates downwards?

A lot of campsites won't allow disposable bbqs or stoves that sit on the ground, due to scorching of the grass.  I do like my kelly kettle but it has the problem of burning the ground.

I have one of these:



It's a small glass fibre heat resistant mat, made for plumbers to use to shield floor joists and the like when soldering pipes together with a blow lamp.  It rolls up and goes in with the stove kit and makes sure that you don't scorch the grass.

[clarion]

I always use a plumbers mat for my stoves.  Not sure how a woodburning stove differs - good or bad - from a gas one in this respect.

[Vince]

Camping and bushwalking in WA gave me a profound preference for tents that don't require pegs; the soil in the Darling Scarp is often only a few inches deep over solid rock.

Sounds just like the Kent Show Ground

Back OT. Am I right in thinking that the air rising in the jacket is pulling air down through the bottom of the burning pot? Therefore the ratio of size/quantity of the holes around the bottom of the outer jacket compared to those in the base of the burning pot is critical and closing off the holes in the outer will increase the air draft down through the fire?

[Kim]

Am I right in thinking that the air rising in the jacket is pulling air down through the bottom of the burning pot?

That's how many people describe the process, but I don't see how that can possibly happen without some sort of fan; the hot air in the burning pot (which is never going to be cooler than that in the jacket) will rise, and draw air in through the bottom, as well as through the secondary combustion holes at the top.  I don't believe the jacket is essential to the process, except perhaps by pre-heating the air for the secondary burn.  It does cool the outer wall somewhat, which is handy.

Thought experiment: if the air is being pulled downwards through the fuel, where is it coming from?  Which way are the smoke and flames going?

[Vince]

I would like to think the smoke is being pulled down into the hotter part of the fire and combusted, making it cleaner.
Thinking about the jacket and simplifying the model by losing the holes in the outer jacket. The air in the jacket is being warmed and will expand, becoming lighter than the air outside the jacket so will exit the jacket at the top. This leaves a low pressure area which can only be filled by air either coming back down the jacket or being drawn throught the fire. The former I suspect would pulse, the later give the draw down effect described.
Of course this is buggered up by the big holes in the outer jacket which will allow air to be drawn from the outside up through both the jacket and the fire.

(My experience of fires is limited to setting scouts alight helping scouts light fires on camp!)

[Kim]

I would like to think the smoke is being pulled down into the hotter part of the fire and combusted, making it cleaner.

The smoke is mixed with fresh air above the fire and combusted, making it cleaner.  You can see it happening.

Thinking about the jacket and simplifying the model by losing the holes in the outer jacket. The air in the jacket is being warmed and will expand, becoming lighter than the air outside the jacket so will exit the jacket at the top. This leaves a low pressure area which can only be filled by air either coming back down the jacket or being drawn throught the fire. The former I suspect would pulse, the later give the draw down effect described.

Yeahbut:

The air in the pot is being warmed *more* and will expand *more* and rise, this leaves a low pressure area which can only be filled by by air coming back down the jacket.  This air will contain rather a lot of combustion products, and it will either reach a smouldery equilibrium or go out (indeed, you'd have the same problem if it were being drawn down through the fire).

Of course this is buggered up by the big holes in the outer jacket which will allow air to be drawn from the outside up through both the jacket and the fire.

Which is how they seem to work in practice.  The experiments to do are to try running one without the jacket in place, and to draw off a sample of the air inside the jacket and test for combustible gasses.

[turista]

"I would like to think the smoke is being pulled down into the hotter part of the fire and combusted, making it cleaner."
I think this is correct - and is an important feature when larger versions are used in peoples' homes in developing countries. That and the reduced amount of fuel they require.
I find that the lower part of my stove stays surprisingly cool. I cook on top of a collection of pie dishes, I'm much more likely to scorch grass with the bottom of my pan than from the wood  gas stove.

[clarion]

That's true.  At Mildenhall, there was just a pie dish (no pie) between Turista's stove and a wooden picnic table.  I thought there might be some scorching, but there were no marks.

[turista]

You can put water in the pie dish if you want to be double plus careful. I have a little pile of the pie dishes (collective noun needed!) - I really ought to keep one out to put the hot saucepan onto, then I wouldn't scorch the grass!

[Wobbly John]

Not exactly a camp stove, but Colin Furze has used the wood gassification to fuel a lawnmower! 

https://youtu.be/FK2qK-NCQH8