Word of the day: Frustum-a portion of a solid that lies between two parallel planes cutting the solid.

Our inverted lampshade-like pot skirt is a frustum of a cone, or so we discovered when we tried to construct it from sheet metal. It’s not as easy as it seems; you can make a cylinder from a rectangle or a cone from a semi-circle, but a frustrum is constructed from a “rainbow”-shaped template whose dimensions determine the height and diameters of the part’s top and bottom surfaces.

Because we didn’t know that our pot skirt was formally called a frustum, figuring out how to build it was an adventure. Matt (a former teammate) and I had a friendly competition: he tried looking for the equation on the Internet while I made a barebones paper frame of the shape we wanted and then unfolded it to get a rough outline of the template. I like to say I figured it out first, but he found a neat little frustum dimension calculator that proved very useful for trying different pot skirt heights and widths!

I mentioned in my last entry “Skirting the Issue” that most pot skirts are actually cylindrical. So why in the world did we go through all this trouble to make such a tricky shape? There is an ideal gap for efficient air flow (about ¾”) between the pot and the pot skirt. When the team was in Myanmar, we saw that the women there cooked with an astounding range of pot sizes , with the smallest one having about a 5″ diameter, to the biggest one about 16″. To make matters more complicated, women often cooked on a wok as well. Thus, our pot skirt had to be designed to create the same ideal gap for multiple pot sizes. The “inverted lampshade” shape allows smaller pots to sit lower and bigger pots to sit higher on the skirt, creating a reasonable gap for air to pass through.

However, one of the difficulties with this design is that women have to reach further inside the pot skirt to take a small pot out. Since the air between the pot and the skirt is VERY hot, this poses a significant burning hazard for women. This safety issue is one reason why we are still hard at work at improving our initial concept!

A year ago, after we had just completed a prototype of a stove design we liked (it was literally made from an old metal trash can!) we decided to try adding a “pot skirt” to see how much it would improve our stove’s efficiency. We had read about these heat shields in other stoves before; the basic idea is to surround the pot on all sides with a thin metal shield that creates about an inch-gap for hot air to pass through. The pot skirt thus prevents heat from the stove from escaping to its surroundings, keeping it close to the walls of the pot and increasing the heating surface.

The idea sounded interesting, so we set to work building a prototype and testing it out! (My next post, “Frustrated by Frustums” will explain why our prototype is the shape it is-most pot skirts are actually just cylindrical.) The result was… WOW.

Adding the pot skirt cut down our rocket stove’s boiling time almost in half. The rocket stove itself was already able to boil water in maybe half the time it took an open flame, but with the pot skirt, we were boiling water at incredible speeds: up to a third of the time it takes to boil water on an open flame. Whatever we expected, I don’t think it was as drastic as this.

As great as it may sound though, our pot skirt idea still needs a lot of work. Adding that much material to our stove makes it more expensive and difficult to manufacture, not to mention clunky looking. Because it traps hot air, the skirt metal gets really hot too, posing a potential burn hazard to our customers. Our first prototype didn’t allow the cook to see the flame very well either (we fixed that issue by cutting out holes into the skirt).

Right now, we’re starting to explore new skirt designs, as well as some completely new concepts. One of the things we’d like to try is something similar to a “heat exchanger” which traps hot convective air at the bottom of the pot. The Jet Boil camping stove uses a mechanism like this. Such a design wouldn’t increase our heating surface area, but it will at least increase the concentration of hot air, which may be enough.

If you have any ideas about this engineering challenge, we’d love to hear them!