Has anyone ~actually~ measured boil times of wide vs. narrow pot?

[James Marco]

"James, I agree that surface area exposed directly to the flame is the biggest factor. Unfortunately increasing it via spirals or ridges would likely cause the price to increase by orders of magnitude. Vortex generators are interesting. That might be easier to apply. I wonder how crude they can be and still have a significant effect."

Eugene, Sorry about the delay, must've missed your post. Actually, it is realtivly simple with standard annealing and forming. I made several pots with deep ridges in the bottom and acheived an easy 15%-20% decrease in boil times, or, 15% less fuel for the same boil. Trail Design has those numbers. (My computer crashed and the new one lacks a proper interface for my backup tapes…someday…)

I simply heated the bottom of an aluminum grease pot, then pounded a series of ridges in it. To do it neatly would require tooling and a press, of course. I think I did something like 30-40 runs and and came up with an average, so the 15% is a pretty solid number. This works about the same in a cone as a larger pot. IE, the surface area of the 5-3/4" pot is about the same as the bottom surface area as a 6.5" pot, just concentrated.
2.875^2*pi=25.9672*1.20=31.1607
3.25^2*pi=33.1831

This is all a heat exchanger does, BTW. I call the the ExPot…Exchanger Pot.

For a simple vortex generator at home, a ballpeen hammer around the outside will work. A series of smallish dents (about 1/32" or so) spiralling up the sides will work as well. The thought is to create turbulence without pushing the heat away from the sides to badly. I believe that you will find the cooler air layer is only about 1/16" thick, thickening as it goes up the pot. Perhaps 1/8" at the top. So you really don't need vortex generators, a simple dimple will work, spaced evenly about 10-20 dimples around the pots circumference. A small peen will work about the best for hand work. A continuous ridge will cause dead spots behind it, good and bad.

However, the vortex generators could also be used as heat exchangers around the sides and bottom of the pot.

More elaborate would involve a press twisted into it causing a slight helical ripple in the metal. Easily done with production tooling. Extraction might cause some difficulty, but a two step stamp, heat, twist/stamp should form it OK. Labour is cheap for this. Tooling is expensive. Anyway, the resulting pot, a cone/windscreen should be about $1USD or perhaps bit more to produce, rather than the current ~$.75.

Trail Designs rejected this stuff, soo, feel free…this is a patent free design.

[Jon Fong / Flat Cat Gear]

Maybe you can convince Dan to make some custom Ridgelines with your proposed vortex pattern. He may be able to imprint a pattern in a cost effective process. Jon

[James Marco]

With alcohol stoves, there are compounding factor. As the pot gets smaller so does the windscreen and therefore the enclosed space around the stove decreases. I have found that the same stove burns hotter and faster with smaller diameter pots. I believe that this is due to trying to push out the same wattage through a smaller air volume. I have found that my 1.3 liter system is way more fuel efficient than my SP 700 system (15 ml to boil 2 cups compared to 17 ml). Additionally, my 1.3 liter has a faster time to boil (6:30 verse 8:00). For my uses, wider pot is far more efficient than a small diameter mug. My 2 cents – Jon

Jon,
You are likely encountering the "chimney" effect. A smaller outlet vent will slow things down a bit.

[Jon Fong / Flat Cat Gear]

It is not due to the chimney effect as the pot to windscreen gap were the same as well as the vent sizes. I measure the air temperature in the burn chamber and can see significant differences between big and small systems. I measure the outlet air temperature and see the same thing. I measure the windscreen body itself and see the temperature difference. I can take a large pot windscreen and add a donut shaped shield to reduce the burn area volume and that will also raise the internal temperature. Best regards – Jon

[James Marco]

Jon,
I don't think that is what I was talking about. Basically, a taller column of air will rise faster given the same heat input. Well, within limits, of course. The increased gas exchange can cause a smaller stove to overheat, and/or run hot. Anyway, just a guess…

[David Thomas]

In spec'ing chimneys for wood stoves or fireplaces in a home, 3 things can be wrong and result in not enough draft:

1) the chimney can be too short (duh!)

2) the chimney can be too narrow (frictional losses in a small pipe)

3) the chimney can be too wide (the hot air gets diluted, cooled and you have less chimney effect).

Additionally, the chimney's outlet can be to close and low compared to the ridgeline of the roof and catch a downdraft when on the downwind side.

The chimney can NOT be too tall. Tall is always better if you want draft.

Since draft gives you more vigorous flows and that would allow you to have the additional baffles, bumps, dimples, fins, and VGs that help HX, a very efficient stove would tend to have a tall pot/windscreen combo.

I realize that weighs more and is harder to pack. I'm just talking about exchange here.

Time to start saving some beer cans and doing some tests!

[Author]

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