double-walled pot

[cwayman1]

yo!
so my plan of the moment is to construct a double-walled pot. I am wanting to use two coke cans, but every time that I attempt it all i get is either two crushed cans from the pressure or one can slightly crinkled inside the other. I thought it might be a bit easier by drilling a hole in the bottom of the outer can to relieve pressure but that doesn't get the cans MUCH farther.

the reason a want a double-walled is because I think that one aluminum can is just too thin to retain enough heat to bring the 12oz of water to a boil-- and i really don't need much, if any more than 12 oz.. i can boil 2 cups in about 7 min with ~15mL in my grease pot, but cannot get the same amt of fuel to do 12 oz in a coke can

thanks for any help

cwayman1

[Knotty]

Your problem may be more about flame pattern. The grease pot has a wide bottom allowing for a lot of flame contact. The small diameter coke can may not have enough contact, especially if you're using a side burner stove.

[Many Walks]

Seems like the air space between the walls would act as an insulator rather than a conductor. I agree with RonM, the flame spread and bottom contact is probably the key.

[Petr]

It's been about 6 years since my last physics course, but I think that I disagree with Many Walks. The insulation between the two walls will (again, I think) favor heat retention due to protective effects against wind chill. Your idea might be an elegant way to do away with the awkward wind screen. An outer can could preserve the heated air surrounding the inner can thus making your boiling more efficient. I guess it depends on whether the wind screen predominantly affects the heat generated by your stove or the heat surrounding your pot. RonM/Many Walks point about flame spread and contact is still valid, but I think that your idea is worth pursuing. Perhaps trying a traditional soda or Red Bull can housed inside of the wider Heineken can would result in some benefit. I haven't experimented much with this stuff, but oxygen supply to the inner can might be a limiting issue in terms of optimum burning, so you might want to vent the outer can at the base in order to allow enough 02 to reach the fuel. Give it a try and let us know.

[zelph]

yo!
I think that one aluminum can is just too thin to retain enough heat to bring the 12oz of water to a boil-- and i really don't need much, if any more than 12 oz.. i can boil 2 cups in about 7 min with ~15mL in my grease pot, but cannot get the same amt of fuel to do 12 oz in a coke can

thanks for any help

cwayman1
Modify your stove to burn slower and with a narrow flame pattern.

what stove are you using?

[Franco]

Double wall pots
Three possible scenarios that I can think of.
1) No air space in between
I cannot see how you could push one inside the other for more than an inch or so. I am not aware of two cans that are "almost" the same size.
2) Air space and sealed at the top
The cans will expand and explode.
3) Air space but not sealed
The outside can will buckle and collapse.
Of course I could be wrong...
Franco

[The Old Fhart]

Petr-"It's been about 6 years since my last physics course, but I think that I disagree with Many Walks. The insulation between the two walls will (again, I think) favor heat retention due to protective effects against wind chill...."It's not physics, it's thermodynamics. Air is a far better insulator than metal which is why they don't use metal for insulation in houses or sleeping bags. They already make double-walled containers called thermos bottles (or Dewar flasks in scientific circles) with a vacuum between to prevent heat transfer. The less dense a material is, the better its insulation ability generally is. A double-walled container used with an immersion heater would work though. Now to get power for the immersion heater....:-?

Also (and we already went through this once before) by definition wind chill only affects living beings, at least south of the Canadian border.:D

[cwayman1]

I tend to agree with Petr. I know that dead air is one of the best insulators (physics between any down/synthetic sleeping bags... they trap the air/heat) and that that would help retain heat, rendering a faster boil time, but i don't really think that there is enough room for air to play a factor. i think it is a matter of a thicker wall retaining heat better. i have gotten one "made". the outer wall is smooth but the inner is slightly crumpled...

http://www.whiteblaze.net/forum/vbg/showimage.php?i=34994&c=516

this pot boils water with a bit of fuel to spare (starting with ~15mL) and keeps a boil for bout another 1.5-2 min. so a thicker pot def helps... i think

zelph, my primary stove for this idea is just a tealight tin. nothing special. i figured that it would be the best BECAUSE of the small surface area of the coke can and the central burn of it.

[beakerman]

double walled pots actuallly use more fuel for the same shape and size in comparison to single wall pots. It's not about thermal mass (thicker walls) it's surface area being heated. The more contact area be better the heat transfer to the pot.

Old Fhart goes into this.

however wind chill does effect inanimate objects--the actually use it in a sort to calculate relative humidity with a device called a swing hygrometer. It is basically two thermometers in the same clamp but one has a little sock over the bulb that they make wet. They then twirl it around and the wet bulb is almost always cooler than the dry bulb because evaporation is a cooling process. It can be the same in 100% RH but never warmer unless you screwed it up real bad.

[bullseye]

The name of the game is heat transfer. A double walled pot with an air space will insulate the inner pot from the direct heat generated against the outer wall. The best heat transfer is going to come from a single wall design.

Also, the double pot doesn't negate the need for a windscreen simply b/c it's 1st purpose w/ an alky stove is to act as a windbreak. Any extra generated heat or performance is merely a secondary effect.

Zelph, and the others are correct that concentrating the flame pattern is most important when using a soda/beer can pot. Also, don't overlook the need for a lid, as this helps facilitate a quicker boil.

I used to use a beer can pot, but I have to say it's worth the extra 2 or 3 ounces for a good UL cookpot. I more than make up for the "extra" weight in fuel weight savings and quicker boil times.

[The Old Fhart]

beakerman-"however wind chill does effect inanimate objects--the actually use it in a sort to calculate relative humidity with a device called a swing hygrometer."To be correct, it is a type of hygrometer called a sling psychrometer. I worked at the weather observatory on Washington in winter and used an official NWS sling psychrometer often. The wind affects inanimate objects but wind chill does not because it is a 'feeling' of relative coolness.


beakerman-"...the wet bulb is almost always cooler than the dry bulb because evaporation is a cooling process...."'Almost always' is correct. When the wick on the wet bulb goes below freezing the water goes from liquid to solid the "latent heat of fusion*" causes a minor jump in the wet bulb temperature. Once the wick is frozen, evaporation no longer takes place, but sublimation take over and the solid water (ice) goes directly to a gaseous state (vapor).

*"the amount of heat required to cause a change of phase from solid to liquid, or the heat released when the phase change is from liquid to solid; in the case of melting snow, the phase change from ice to water requires a significant amount of heat—160 times that required to raise the temperature of the same amount of ice by just 1 degree Celsius; until the required amount of heat is supplied to completely melt all of the ice being considered, no further increase in temperature will occur."

[beakerman]

To be correct, it is a type of hygrometer called a sling psychrometer. I worked at the weather observatory on Washington in winter and used an official NWS sling psychrometer often. The wind affects inanimate objects but wind chill does not because it is a 'feeling' of relative coolness.
'Almost always' is correct. When the wick on the wet bulb goes below freezing the water goes from liquid to solid the "latent heat of fusion*" causes a minor jump in the wet bulb temperature. Once the wick is frozen, evaporation no longer takes place, but sublimation take over and the solid water (ice) goes directly to a gaseous state (vapor).

*"the amount of heat required to cause a change of phase from solid to liquid, or the heat released when the phase change is from liquid to solid; in the case of melting snow, the phase change from ice to water requires a significant amount of heat—160 times that required to raise the temperature of the same amount of ice by just 1 degree Celsius; until the required amount of heat is supplied to completely melt all of the ice being considered, no further increase in temperature will occur."

Yeah that's it. I couldn't remember the exact name and well to be completely honest I was too lazy to look it up but I know how it works and what it is used for...it's been over 20 years since Ive even seen one...that gives me partial credit right?

[Petr]

I'll let the big boys discuss this one.

[mister krabs]

yo!

the reason a want a double-walled is because I think that one aluminum can is just too thin to retain enough heat to bring the 12oz of water to a boil--

cwayman1

You don't want the pot to retain heat when you're boiling, you want it to transfer (conduct) it from the flame to the water. Aluminum does this well, copper does it better. We use aluminum because it's a good compromise in weight/cost/conductivity

Try a small heineken 12oz barrel can, it's sturdier than a pop can.

[zelph]

yo!
so my plan of the moment is to construct a double-walled pot.

the reason a want a double-walled is because I think that one aluminum can is just too thin to retain enough heat to bring the 12oz of water to a boil-- thanks for any help

cwayman1

Suggestion......change your plan of the moment. Use a thicker single wall aluminum bottle. I've used a "snapple" energy drink bottle with success as a water boiling pot.

Here are some photos of the bottle and a link to a stove making thread showing a stove/pot combination using the snapple bottle.

You also can use a Budlite beer bottle.

http://img.villagephotos.com/p/2006-4/1174156/snapple003.jpg

http://img.photobucket.com/albums/v228/obijiwa/spookcamp034.jpg


whatchamacallit alcohol stove (http://www.whiteblaze.net/forum/showthread.php?t=18201&highlight=snapple+energy+drink)no potstand required:D

.

[cwayman1]

yeah, i actually came across a couple of 12oz. heini-cans while i was taking care of some recycling. I used it last night... and it worked! but one thing i noticed was that air bubbles (that's the technical term i believe) were only coming up from where the flame was directly hitting. it makes sense, i guess, that the thin aluminum doesn't transfer ample heat to the water unless the flame is hitting it directly...? but i WAS excited that it boiled! and you have to be committed if you get really excited about water boiling!

and "retain" i suppose was the wrong word. my thinking being that a thinner pot would radiate too much heat uselessly to the surrounding air, while a thicker pot would ... well... yeah... retain more heat and transferring more of it to the water.

also, is there any validation to this idea of adding "heat exchangers" to the heini-pot?
http://www.youtube.com/watch?v=44y7EUug1Xg
i'm gonna try it and see what happens. i mean, i've got a few cans to throw around with :)

[Allen1901]

yo!
so my plan of the moment is to construct a double-walled pot. I am wanting to use two coke cans, but every time that I attempt it all i get is either two crushed cans from the pressure or one can slightly crinkled inside the other. I thought it might be a bit easier by drilling a hole in the bottom of the outer can to relieve pressure but that doesn't get the cans MUCH farther.


Cut the concave disk out of the bottom of the outer can just like when making a pepsi can stove. Then cut off the top of the can.
Leave the inner can full and unopened until You finish pushing the cans together.

Cheers!