![]() Thermal efficiency, when burning biomass, tops out (so far) at around 55%.Keep in mind that increasing the surface area of the water in a pot also increases the amount of steam, which makes bigger pots harder to bring to full boil without a pot lid.We use 26cm to 30cm in diameter pots with at least 5 liters of water to get closer to 50% thermal efficiency. In the same stove, a pot with 1200cm2 can be expected to result in above 45%. Increasing the area to around 1000cm2 increased thermal efficiency to about 40%. In recent tests of optimized Rocket stoves, a pot with an area of around 800cm2 scored 34% thermal efficiency.Pots have to have sufficient external area to achieve 50% thermal efficiency.The 0.75 cross sectional channels encourage the gases to thin the boundary layer increasing heat transfer.Reducing the constant cross-sectional area channels under and around the sides of a pot to 0.75 of that area helps to keep the gases hot and flowing at highest velocity. Keeping a constant cross-sectional area in the pathway the gasses take through the stove is important.In a Rocket stove at high power, the gases can be around 800C and the velocity can be around 1.2 meters per second.According to Newton’s Law, doubling the surface area doubles the heat transfer when the temperature and velocity of the gases are constant.Illustration from The Smithsonian’s explanation of how a boundary layer worksĪ boundary layer of still air on the bottom and sides of a pot keeps the hot gases from actually contacting the surface and is a dominant factor in heat transfer efficiency. Trying to protect climate requires very clean combustion and we’re working on that, too. Since 1976, ARC has continued to work with local communities worldwide to try to save fuel and protect health. The pot seals into the hole and the smoke flows up the chimney, not into the lungs of the cook and her children. A sunken pot (or pots) sits down near the fire exposing its bottom and sides to the flame. One simple African stove with chimney is seen above. But that is not good enough to protect health inside a home.Īlthough health-protecting chimneys are seen in Latin America and India, it’s rare to see chimneys in Africa. The Rocket stove can be about 50% more fuel-efficient compared to the open fire, so about half the smoke is made. There are now millions of beautiful Latin American kitchens in which the dangerous smoke is transported out of the house, as it is in the USA/Europe. ![]() Īlso, be sure to check YouTube - lots of videos there.Damon Ogle was the Technical Director here at ARCĭamon Ogle and the ARC staff have a long history, starting in Central America and Mexico, listening to folks praising their stoves with chimneys. That's a good place to start to get a feel for how these things work. Here's a good Instructable you can follow to build one out of soup cans. You can use just about any found materials that can tolerate the heat and provide insulation where needed. One way is to light the fire in the chimney, then start adding fuel the regular way. You have to get the chimney hot to make it work. This will create turbulence that will mix the gases and air for better combustion. You asked about principles, and one is that the bend to the chimney should be sharp, not a gentle curve. 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. To create a functioning rocket stove, you need an insulated J-shaped chimney. 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. The cooking pot sits directly over the chimney, so the hot exhaust hits it directly. 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 basic principle is this: use the draft created by hot exhaust in a chimney to suck lots of fresh air in to the fire. 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. 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.
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