I have some spare bricks lying around that I'm interested in using to build a rocket stove for cooking. What are the key principles I need to understand to build a rocket stove?
Is there a maximum or minimum size? Does it matter how tall should the chimney part of it is? Should the air/fuel inlet be a smaller gap than the chimney?
I am by no means an expert in rocket stoves, but just in case we don't have a rocket stove expert here, I'll offer what I know.
While rocket mass heaters have become popular in the USA as an efficient way for hippies in cob houses to stay warm with very little fuel, they originated as a way to cook when wood fuel was scarce.
The basic principle is this: use the draft created by hot exhaust in a chimney to suck lots of fresh air in to the fire. The chimney is insulated and gets very hot, which helps burn more of the gases of combustion, producing more heat from the same fuel.
The cooking pot sits directly over the chimney, so the hot exhaust hits it directly. You can also add a skirt around the pot, so the exhaust gas is held close to the sides of the pot, further improving heat transfer.
To create a functioning rocket stove, you need an insulated J-shaped chimney. Like this:
You can add a second 90-degree bend to make the whole thing U-shaped, but the chimney must be much longer than the fuel end, so it will draw the exhaust in the right direction.
You asked about principles, and one is that the bend to the chimney should be sharp, not a gentle curve. This will create turbulence that will mix the gases and air for better combustion.
You have to get the chimney hot to make it work. One way is to light the fire in the chimney, then start adding fuel the regular way.
You can use just about any found materials that can tolerate the heat and provide insulation where needed. Here's a good Instructable you can follow to build one out of soup cans. That's a good place to start to get a feel for how these things work. http://www.instructables.com/id/How-to-make-a-Rocket-Stove-from-a-10-Can-and-4-So/
Also, be sure to check YouTube - lots of videos there.
I consider the Rocket Stove design as described below.
| |
| |
| |
feed | | heat riser
| | | ^ |
| * ====== | |
|* * --------| |
================
* fire, - air flow
The feed is in an upright position(1). The air flow is being created by the extra combustion of the solid particles released at the feed, in the heat riser, thus creating an under pressure. Of all the videos I've seen, I estimate a feed to heat riser ratio of 1:5 or 1:6
The distance between the heat riser and the feed can't be too far apart, as the temperature will be too low to efficiently burn the remainder of the un-burned particles. If it is to short, the air flow will be too much which will increase an in-efficient burning of wood.
All is to be build with fire resistant materials.
For casual use, have the feed come in at a slope to the burn chamber allows the stove to self feed as the ends burn off.
The burn chamber has to be large enough for the heat to start pyrolizing the wood chunks. If the chimney is the same diameter as the feed tube it's harder to get a balance of air speeds that will keep the fire from moving up the feed tube, and still get enough heat up the chimney.
Having the chamber have some depth below the feed tube entrance gives you a place for ash, so that stove does't need to be cleaned as often.
Ball park guesses: Feed tube should be 4-8 times the diameter of the typical fuel inserted. Fire chamber should be 1.5 to 2 times feed tube diameter. Feed tube should come in 1 diameter above base of chimney.
Start work using a 6" to 4" Y duct?
