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Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures
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Ryan Jordan
(ryan) - BPL Staff - MLife

Locale: Greater Yellowstone
Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures on 12/04/2013 01:45:11 MST Print View

Companion forum thread to:

Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures on 12/04/2013 05:41:34 MST Print View

Good overview, Ryan. Looking for more!

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures on 12/04/2013 08:59:44 MST Print View

Nice article. I was kind of ignoring it because I don't do group cooking, but there are non-group concepts here.

"This data suggests that upright canister systems for group cooking may have quite narrow ranges of temperature where they can operate competitively with inverted canister systems, and that those ranges do not exist for typical winter conditions."

It seems like the conclusion should be upright and inverted are about the same above some temperature, like 20 F. Below that, upright is so slow as to be not very useful. Especially if you're doing larger quantities of water for a group or melting snow.


Did you repeat each case a couple times to make sure it wasn't just measurement error?

Better to measure temperature of water after heating and calculate temperature change, rather than relying on bringing to boil which is somewhat subjective. But, you can always go back and second guess your measurement technique and come up with some better way to do it.

Mary D
(hikinggranny) - MLife

Locale: Gateway to Columbia River Gorge
Lightweight Stove Systems for Group Cooking Part 2 on 12/04/2013 11:49:57 MST Print View

Jerry wrote:
"Better to measure temperature of water after heating and calculate temperature change…"

I agree! The boiling point of water differs considerably between 7,000 feet elevation (199*F) where I grew up and 300 feet elevation (212*F) where I live now. At what altitude was this testing done? That makes a big difference, especially in calculating the time and amount of fuel needed to reach the boiling point!

Edited by hikinggranny on 12/04/2013 11:53:37 MST.

Mike Oxford
(moxford) - MLife

Locale: Silicon Valley, CA
Skewed data report? on 12/04/2013 12:16:43 MST Print View

I didn't see the "break even" point between the increased weight of the exchange-pot vs fuel consumption so I went a looked it up.
Note again that the MSR and Primus are smaller volume containers than the Open Country.

Open Country 4QT Kettle - 12.7oz ( 360g = +0g)
MSR Reactor 2.5L - 13.8oz ( 391.2 = +31.2g)
Primus 3L - 19oz ( 538.641g = +178.6g)

Mike Oxford
(moxford) - MLife

Locale: Silicon Valley, CA
Re: Re: Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures on 12/04/2013 12:26:21 MST Print View

>It seems like the conclusion should be upright and inverted are about the same above some temperature, like 20 F. Below that, upright is so slow as to be not very useful. Especially if you're doing larger quantities of water for a group or melting snow.

I believe it's slow because the gasses inside boil at differing rates and butane has a hard time below about 32F/0C. When using a propane/iso blend the propane acts as a propellent and helps carry the iso out when close to 0C. When below 0C the butane's a liquid (you're burning the propane) and, assuming that the stove has a preheat loop/tube to get it to the boiling point, will work normally if inverted (the butane is liquid in the tube, boils to a vapor in the preheat and then the gas burns normally at the burner.)

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: Re: Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures on 12/05/2013 06:23:30 MST Print View

"I believe it's slow because the gasses inside boil at differing rates and butane has a hard time below about 32F/0C. When using a propane/iso blend the propane acts as a propellent and helps carry the iso out when close to 0C."

Mike, yes, quite correct. But you don't take it far enough. Fractionating (or distilling is a more common term) will occur. When you are at 32F/0C, the vapour pressure for the butane mix is far less than than propane. Much below 40F the gas starts becomming more propane in the mix. This continues to the freezing point of the butane mix, leaving almost all the gas available as propane. In a pressurized system, like a cannister, the LPG doesn't really boil, except under a few special circumstances. So it looks more like a fairly rapid curve of downward butane pressure and a much slower downward curve for propane. It isn't correct to think of propane acting as a propellant carrying the butane out. Both are fuels.


"When below 0C the butane's a liquid (you're burning the propane) and, assuming that the stove has a preheat loop/tube to get it to the boiling point, will work normally if inverted (the butane is liquid in the tube, boils to a vapor in the preheat and then the gas burns normally at the burner.)"

Mike, yes, but a strange way of putting it. There is really no "normal" way of operation in an inverted canister stove. To the point of reaching some temperature at which the LPG will expand to ~250 times (to its gas phase), it is liquid. Usually the heat is supplied by a preheat loop. The pressure of the gas expansion goes in BOTH directions: Out to the burner, and, back to the canister. (I am ignoring the valve, jet, line friction, viscosity, etc to keep things simple) The portion going to the burner is easy, it gets burned. The portion going to the canister actually "warms" the canister slightly, if you want to think of it that way. If we increase the pressure on the canister (a closed system) we can interpret this as an increase in heat via Temp=Pressure*Volume where we simply call the volume a constant (1) in a closed system and ignore it. Increasing the pressure, increases the "temperature", in the sense that we increase molecular motion inside the canister.

This heating does NOT occur with "toppers." Drawing off gas will cool the containers by the same effect as above, except reducing pressure. This is, perhaps, the biggest difference between the two modes of operation and accounts for most operational differences.

E J
(mountainwalker) - MLife

Locale: SF Bay Area & New England
Weight of Primus Eta Power 3L? Old Eta Power Pots on 12/06/2013 00:43:33 MST Print View

Thank you for the helpful overview. What's the weight of the Primus Eta Power 3L without the lid or pot grabber, and what's the diameter and the height (with and without the heat exchanger fins on the bottom)?

I think the old style of Eta Power hard anodized pots, a bunch of which are still out there, are a bit lighter, because the new pots protect the heat exchanger fins with a ring of aluminum around the outer perimeter. Think the old 2.1 L Eta Power Pot was lighter than the new current 1.8L. Anyone know for sure?

E J
(mountainwalker) - MLife

Locale: SF Bay Area & New England
Ryan which 2L HE Pot do you recommend for 2P cooking in winter? on 12/06/2013 00:49:35 MST Print View

I've long thought wide hard anodized HE pots were worth it for winter over similarly sized Ti.

Ryan given your results which 2P size HE pot would you choose for winter? It would be great if you could include a note about this in the article or at least comment here in the thread. Choices would include:

-Primus EtaPower 1.8L
-Old Primus EtaPower 2.1L (discontinued but still available)
-MSR Reactor 2.5L
-Primus EtaPower 3L

The older Primus Eta pots seemed to be a bit lighter, at the expense of not having a protective ring around the HE fins on the bottom of the pot.

Both of the last 2 larger pots would of course give you flexibility for a 3P group.

Edited by mountainwalker on 12/06/2013 20:18:19 MST.

Thomas Rayl
(trayl) - MLife

Locale: SE Tx
Fuel consumption to boil snow vs water on 12/06/2013 14:05:14 MST Print View

Specific heat of water (heat energy to raise 1 gm of water 1deg C: 1.00 cal
Heat of fusion of water (energy to melt 1 gm of 0deg ice to 0deg water): 79.83 cal

Heat to raise 1 gm of water from 2deg C (approx avg in tests) to 100deg C (boiling): 98 cal
Heat to raise 1 gm of ice (snow) from 0deg C to 100 deg C (boiling): 79.83 cal + 100 cal

Ratio: (179.83 / 98 ): 1.8


This may account for much of your fuel usage ratio for boiling water vs snow.
Note that this does not account for the fact that you started the snow-melt test with some water (I didn't have the exact amount) which would lower the ratio somewhat.

Great article -- can't wait to read more!

tkkn c
(tkknc) - MLife

Locale: Desert Rat in the Southwest
Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures on 12/06/2013 15:39:42 MST Print View

This is what I am planning to use this winter to solve the propane/butane mixture issues. What will be the impact of running propane instead of the propane butane mixture on the CO output. I will be using the primus eta power for the first trips.

small can

It weighs 104g.

I also suggest a metal sealed cap for the propane bottles, as I have had 3 bottles fail to completely seal when I have removed them from a lantern on winter trips in the past.

Edited by tkknc on 12/07/2013 11:03:03 MST.

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures on 12/07/2013 20:00:55 MST Print View

Using pure propane was discussed sometime back. For toppers, there is likely little difference, except they will require a bit less valve setting; roughly equivalent to an 80F day using butane. For any remotes, it depends. Some have hoses that will easily contain the additional pressure. Some have weak connectors. They could start leaking, or, possibly blow out if you try this at more than 32F/0C. The vapor pressure is about 3 times higher using propane vs isobutane. I am guessing this will work as long as the temps remain below freezing.

The pressures are the reason for the extra thick walls on propane bottles. It is easier, and much lighter to carry an extra bit of water and an extra bowl to put reluctant canister into for using it. For the weight of the propane bottle, you could bring a WG stove and be done with it.

Doric Swain
(skibug) - M
Heat exchanger/surface area on 12/16/2013 20:22:59 MST Print View

Great report, as expected!

Having looked at several articles and reports over the years, it seems fairly obvious that pots with integrated heat exchangers seem to provide a benefit in terms of fuel efficiency - although with a trade-off of increased pot weight. This trade-off means there is a "break-even" point when the increased pot weight is counterbalanced by the decreased fuel weight.

I've often wondered how much of the effect is due to the increased absorption of heat due to the higher surface area exposed to the stove, and whether a similar (though lesser) effect could be achieved by simply having a "normal" pot with a corrugated, dimpled, or otherwise indented base, increasing the base surface area by around 30 - 50%. Maybe something like an egg-carton shape, though maybe for half sized eggs.

I have never seen such a thing, but also assume, because it's such a simple idea, if it worked, it would have been done. The increased weight of the pot might be around 15 to 30 percent, I imagine.

Any thoughts?

Skibug.

Paul McLaughlin
(paul) - MLife
Re: Heat exchanger/surface area on 12/16/2013 20:55:14 MST Print View

I strongly suspect your idea would be effective, as it is all about surface area. And your weight increase estimate sounds reasonable. If you take a pot that is 6" diameter by 4" tall (dimensions of a 2-qt AGG hard anodized pot that I have), the bottom accounts for about 27% of the surface area - or a little less really when you account for the roll at the lip - so if you double the surface area of the bottom by way of convolutions, you'd be adding let's say 25% to the weight. In the case of the pot I have that would mean 25% of 4.3 oz, or 1.075 oz. And doubling the surface area of the bottom has got to have a significant effect on the heat transfer properties of the pot.

As to why it hasn't been done I 'd bet the answer is that it would be a pot for water only - if you cooked any food in it it would be a pain to clean.

But it sounds like a great idea.

Doric Swain
(skibug) - M
Re: Heat exchanger/surface area on 12/24/2013 18:52:11 MST Print View

Paul Mc wrote:

"As to why it hasn't been done I 'd bet the answer is that it would be a pot for water only - if you cooked any food in it it would be a pain to clean.

But it sounds like a great idea."

Thanks for this, Paul, your weight calculations seem in the ballpark. As for the comment about water only, I agree cooking anything that burns or sticks to the bottom would not be practical, but to be honest the great bulk of my meals on the trail are cooked in boiling water - pasta, tortellini, rice, cous-cous, noodles, soups etc - so I don't foresee much of a problem unless the water boils dry and causes the food to burn or stick. And what ultra-lighter worth the name wastes fuel by letting the water boil off? ;>)

The interesting part of the problem is balancing the surface area/weight trade-off, as well as having a surface that sits securely on the stove arms without wobbles or tipping.

I really hope this gets pursued by someone in the industry, as I can imagine even a 10 or 20 ℅ increase in fuel economy would be greatly welcomed by the community, especially the alcohol stove brigade.

If anyone can think of a way to do this as a diy project, I'd love to hear that, too.

Skibug

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
HE pot on 12/24/2013 20:43:19 MST Print View

Yeah, it's been done. I made about twenty of the little 1 liter ones with various indents in the bottom, finally settling on a radial 5 ridge format. This was about 15%-17% more effective at garnering heat, but looses effectiveness with slower heats. At about 20-25 minutes to boil 2 cups of water it seems the pot radiates heat as fast as it absorbs it. Best time/fuel ratio was about 10-12 minutes with that pot even though it was only about 10-12% more efficient than a flat pot. The actual cooking surface was increased about 20% on the bottom only. (About 1/4" deep ridges makes it look like a "Fresnel" style lens, 'cept made of aluminum.)

Just aneal the bottom then carefully tap around a jig to get it to corogate. Then burnish with a harder metal burnisher (cleaned often to prevent metal/metal sticking.) I continue to use one.

No, it isn't all that difficult to clean. The 5 ridges were sized to a finger so I *can* scrub them out if I cook scrammbed eggs or something elaborate, like a muffin/bread on the trail. A couple teaspoons of olive oil does well at keeping stuff from sticking badly, unless you burn stuff (too much heat.)

A nice experiment if you want to try it. I drew a bunch of extended conclusions with it based on about 50 tests. It allows heat to radiate out as fast as in. Good for cooling a pot, too. It performed identically to a slightly larger, but flat pot, leading to the conclusion that heat exhanges only appear be larger diameter pots to a heat source. All were done with a Cone and a 12/10 burner. All pots started life as a standard K-Mart grease pot. Interesting, a single "dimple" in the bottom was only slightly better than a flat one at about 2-3% - within measument tollerance - for a 3/4" dished bottom.) I assumed it was from heat "puddling" in the bottom cavity rather than flowing out smoothly. It just adds insulation, rather than transfering heat to the pot.

2,3,4,5,6,7,8,9 ridges were tried with anything more than 5 becoming impossible to clean if you cooked anything. The difference between 1-2, 2-3, 3-4 was greater than the difference between 6-7, 7-8, 8-9. I settled on 5 as the optimum for cooking(evenings) and boiling water(mornings.) I destroyed a few pots trying to make the ridges deeper (1/2") and settled on 1/4" as the optimum depth. One was 3/8" that worked well but it was difficult to clean, though not impossible. It gets quite thin, though I eventually cut through it while slicing some cheese.

The aluminum can be "stretched" so it does not add weight. Mine weighs about 3.25oz including the top...not bad for a 1 liter pot. Origonally designed for an alcohol stove, it works as well with Esbit, WG or canisters. At about 15-17% better efficiency, it works on all stoves except for very slow heats. I usually set my heat to get around 9-12 minutes for 2 cups and it burns about 4-5g of WG in a SVEA 123r or about the same with canister gas and a Coleman F1 (overheats in the cone) or Caffin's stove. I never chased it down any closer. Works is works...

Go for it...it definitly works. Takes about 30 minutes to make one.

The radial design was just easy to work with. Look up metal forming over moulds. This explains the metalurgy. Other designs are certainly possible.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Re: Re: Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures on 12/24/2013 21:23:41 MST Print View

Hi James

> Much below 40F the gas starts becomming more propane in the mix. This continues to the freezing point of the butane mix, leaving almost all the gas available as propane.

I understand what you are trying to say, but I must point out that you should have written 'boiling point', NOT 'freezing point'. The freezing point of butane is around -133 C I think. (Yeah, COLD!)

The whole subject has been well covered in our article on Cold Canisters:
http://www.backpackinglight.com/cgi-bin/backpackinglight/effect_of_cold_on_gas_canisters.html


Cheers

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: Re: Re: Re: Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures on 12/24/2013 21:34:10 MST Print View

Frozen Butane! Wouldn't that be fun to work with, little cubes of it?

--B.G.--

Paul McLaughlin
(paul) - MLife
Re: HE pot on 12/25/2013 15:00:35 MST Print View

James - great, thanks. Some of this starts to sound familiar - did you have a thread on this a while back? And were the pots you modified just plain aluminum, or anodized?

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: Re: Re: Re: Lightweight Stove Systems for Group Cooking Part 2: Considering Fuel Needs for Cold Temperatures on 12/25/2013 18:37:34 MST Print View

Roger, no. I meant freezing point, though some will argue that even solids have some vapor pressure. All of the liquids I am familiar with have some sort of vapor pressure. This is the same reason you cannot distill 100% alcohol by standard distiliation. About 4-5% still remains water even though it is less than the boiling point of water. Since butane (nbutane & isobutane) is a mix I anticipate some butane in the vapor even at any resonable temp, -20C would be the lower limit for resonable, though there are spots it gets colder.

Yeah, I read the article a couple times. I think it does a pretty good job of explaining stuff. I quote here: "...even a cool pot of water still loses water vapour by evaporation, but it does so much more slowly than when it is boiling. The same applies to propane and butane. It is possible to calculate the evaporative loss using the appropriate chemistry equations, and the technical details of how this is done are given in the Technical Appendix at the end."

I do not agree that there is any real boiling point in a closed pressurized canister, though it is certanly possible to *shock* a system. It depends on the pressures we have in the can and the temperatures at which it will be used. And, how fast it is being drawn off. Though in a pure static system or inverted can the first two are virtually identical. Because of this, we can safely ignore the boiling point of both butane and propane. Boiling implies that volume would be infinite. In tiny volumes, this cannot occur, rather the vapor pressure simply increases, hence raising the boiling point. We are dealing with superheated liquid/gas systems here. Though the superheat is never above 50C (I hope.) With mixtures, pressures simply decline with temperature, though not linearly. The whole discussion of boiling points on mountains in the article just muddies up the article and is not relavent. We can assume a gas pressure *difference* of 1.25Bar as OK pressure and calculate as needed for differing elevations.

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: HE pot on 12/25/2013 18:48:11 MST Print View

Yeah, maybe. I first posted several years ago on the Backpacking light list at Yahoo, now pretty much defunct. It was discussed here several times. The pots were just "Plain Jane" aluminum. K-Mart (Stanco) grease pots. Anodizing means some sort of hard mineral coating. This, invariably, does not do well with forming.

Doric Swain
(skibug) - M
Convoluted pot base design for increased heat transfer. on 12/31/2013 22:22:26 MST Print View

Hey James,

Most interesting: "At about 15-17% better efficiency, it works on all stoves except for very slow heats"

I would have thought this is of major significance - a 15% efficiency improvement means around 30 grams, or about an ounce, weight saved for a medium sized gas canister. Even more for alcohol. On long, or group, or cold expeditions, many would consider this significant, and worth some money - look at how much people will pay for (eg.) torches and titanium pots that result in smaller weight savings.

Furthermore, I can only suppose that a large company with an R&D budget (or someone like Roger Caffin) could possibly turn this into a 20 - 25% improvement with the most efficient designs/shapes and materials.

Thanks for your work and reply, I genuinely admire your effort to think and experiment outside the square - let's hope some manufacturers are prepared to do the same!

Skibug

David Thomas
(DavidinKenai) - MLife

Locale: North Woods. Far North.
Turbulence is our friend on 01/01/2014 00:09:02 MST Print View

Dimpling or pressing in ridges in a pot bottom isn't necessarily just about surface area. Something that creates turbulence generally improves heat exchange.

I played a little with vortex generators on the sides of a beer-can pot and it definitely helped, especially with a very full pot (full pots have more wetted vertical sidewall for heat exchange). Also, because it wasn't in the flame proper, it wasn't so hot there, so I managed with foil tape - very cheap and fast to fabricate, I could tuck the materials in my wallet like a business card if I wanted to modify a found beer can on site.

I'd suggest (this just occurred to me), that rather than tweaking the bottom of the pot, the pot supports could be arranged so as stir up the hot gases. All the pot supports I can think of are strictly radial. If they had angled vanes to mix the flow a little, that energized boundary layer wouldn't be so insulating. Another cool thing - twisted pot supports needn't weigh much more at all. And they would benefit any pot you put on the stove whereas HX fins only help on one pot.

This addresses one of those blind spots we develop when we focus on "base weight" = lighter stove, lighter pot. But HX fins on the pot, a better fuel efficiency on the stove, and insulation to let you seep your pasta all save fuel which DOES weigh something on your back.

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Turbulence is our friend on 01/01/2014 08:34:26 MST Print View

Yeah, turbulence has an effect too. It seems that on the pots, the totals I get with a 12/10 stove include both, though. I never tried to seperate them. Even using the baffles on the stove doesn't really seperate the two.

There are several designs for doing exactly what you describe on Zen Stoves. Anyway, it appears that the side mounted fins increase angular flow, while decreasing vertical flow, leading to an increased heat density of the gasses surrounding the pots. It would help a bit, but, I never tested for any of that.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Turbulence is our friend on 01/01/2014 09:51:46 MST Print View

It seems like what you don't want is layer of hot gas next to pot, another layer further out - there will be less heat transfer for that layer to pot

So, what you want is vertical turbulence, not sideways. A vortex will produce sideways turbulence, although there will also be vertical so maybe it doesn't matter.

The simplest structure would be a ring of wire. Maybe halfway out from the center. Positioned in the middle of the hot gases. With the right diameter wire to produce good turbulence.

Or, the burner could have some jets that point more vertical, and some that point more sideways, combined you'de get vertical turbulence.

If the pot was bigger diameter, there'de be more surface area on the bottom (good) but it would be less deep on the sides so there'de be smaller area absorbing heat. Maybe the pot above the water level would absorb heat that eventually gets into the water. Playing with pot diameter may yield efficiency.

Fins on side of pot may be more important than on bottom, for example:

windscreenhx

Maybe bigger or smaller channels would be better. This also keeps the hot gas next to the pot better. Maybe some dimples or whatever would increase turbulence in the channel.

I know this is doing something, because my oatmeal sticks to the inside of the pot where the aluminum touches the pot. I measure maybe 10% or 15% efficiency, but that's right at the edge of my measurement accuracy.