Fuel shelf height

[hpmer]

The design principles for the cooking stove suggest that the fuel shelf be placed one third from the bottom of the tube to allow for air to pass under the fuel to be preheated.

It also suggests that air entering above the fuel is less effective since it is cooler.

Wouldn't a shelf placed higher up, say half way, be a better compromise since more air would enter from below allowing it to be heated while still allowing adequate room for fuel, or is there some magic to the 1/3 measurement?

[Donkey]

The folks over at Aprovecho did a whole lot of fiddling to get that stove right. I expect the 1/3 number is golden.
The deal is to get just the right amount of air to the fuel. Too little will choke the fire causing incomplete combustion but too much will actually cool off the fire, again causing incomplete combustion.

I actually don't use the Aprovecho style feed anymore.. When I did use it, I didn't really fiddle with it's proportions any as I had not yet realized how versatile rocket stoves could be.. The fire in my imagination had not yet been lit, as it were...
Since that time I've used the downfeeder method almost exclusively. There are some advantages to it that I just prefer. Also, I live in a forest. While I much appreciate an efficient wood burning experience, I really don't need to save every last calorie of heat.. If the stick keeps burning after I'm finished cooking, well I only need to walk about a hundred feet for another stick.

[hpmer]

Donkey,

Please elaborate on the advantages of the downdraft method. I suppose they're probably obvious (ease of loading, self-feeding etc.) but I'd have to link up another part to insulate the feeder tube, and would like your sales pitch before putting in the effort.

[Donkey]

Ok.
The downdraft method tends to keep the fuel (wood) air and temperature balance constantly in the right place.. Automatically. As the fuel burns out from below, it is replaced by gravity. No plates to fool with, no constantly shoving the wood in, etc.

The possible down-sides and things to watch:
If wood is difficult to get, if you live in the Sahara or around various places near Death Valley and so on, you would need to practice diligence in removing the wood when you are done cooking.

Hardwoods burn and self-arrange better.. If you use softwoods, they will sometimes catch fire a bit too easily, especially around thinner edges and the fire can burn up and out of the box faster than it can burn away the bottom..

You've got to watch side branches and such. They can hang up in the feed, the bottom can burn off and leave a top heavy piece which can roll out of the box and be a fire hazard.

You will need a longer heat riser.. They need to be measured from the TOP of the feed, and the horizontal burn tunnel should not be longer than half the height of the heat riser.
I think it's important to restate: count the height of your heat riser from the top of the feed tube. Even though, likely as not, it sets down a bit from the height of the feed... Still!

... Funny.. That's more downsides than up (numerically)..
But I find that as long as I select the right fuel, the advantage of automatic best fuel/air/heat placement over-rides almost every other consideration.

[hpmer]

Thanks, Donkey.

I was afraid you were going to mention something about the height of the heat riser. With my current configuration the additional component it would make it end up being almost equal (6" burn tunnel, 7" heat riser).

Does this suggest it would not work, or juyst not work optimally?

I am intrigued with the advantages you list, but the thing works really well already and if I'm going to end up with less performance I'll probably wait until I build a bigger, perhaps more permanent stove. Currently I am using a 6 gallon metal trash can as the stove so it is very portable.

[Donkey]

... It would be difficult to light at best..
Nah.. Don't do it.
Wait for the permanent stove and do it there.