I just read the older BPL pieces on canister stoves for winter and Caffin's very technical gas chemistry piece, but there are a few bits of information I would like to make sure I understand properly.
Please correct my assumptions here if I'm wrong:

1. I understand the principle behind the boiling points of various fuels used in a canister; the ambient temperature has to be higher than the liquid fuel's boiling point in order to create gas vapor within the canister. I'm assuming that this vapor created through the liquid to vapor conversion in the canister is not entirely what's creating the pressure forcing the fuel vapor (or liquid, in the case of an inverted canister) to the burner...Correct?

2. I understand that pressures in canisters can change with temperature (just like your car tires, etc.), but I used to think canister stoves wouldn't light in cold weather because they couldn't pressurize enough. But now I understand it's not entirely a matter of pressure, it's also a matter of not having vapor to burn (because the ambient temperature is now lower than the fuels boiling point). Though pressure in a canister will be less at cold temps, there will still be enough pressure to operate- there's just not much gas vapor in it what's leaving the canister and reaching the burner...correct?

3. If I'm correct in #1 and #2, that means I would not have to keep a canister warm in my bag at night providing I am using it in an inverted, liquid feed mode with a stove that has a preheat tube. There would still be enough pressure in the canister to force liquid gas to the preheat tube and burner even if there's little gas vapor in the canister.

Hope I'm making sense...Thanks!

Edited by xnomanx on 01/27/2013 22:51:26 MST.

1) Wrong. The vapor in the canister (created by the boiling liquid) IS entirely what's creating the pressure forcing the fuel vapor (or liquid, in the case of an inverted canister) to the burner. The presence of a pre-heat tube does NOT create any additional pressure.

2) Nope. If the temperature is below the boiling point of the liquid in the canister, nothing will come out, liquid or gas. If the temperature is above the boiling point, vapor (or liquid, if inverted) will come out. To operate the stove, the temperature must be a bit above (10F or so) the boiling point, so there is sufficient pressure to force the vapor thru the jet in the stove.

3) You only need to keep the canister temperature above the boiling point of the fuel. If you use the canister inverted, that will be around 0F for most propane/isobutane mixes. If you use the canister upright, that will start around 0F but rapidly rise as the propane is used up.

Check the "Effect of Cold on Gas Canisters" article if you have not already.

Thanks for the reply.

So I'm totally off on the pressure thing then.

Then this leaves me with the question:

If it is entirely the boiling of the liquid gas into vapor that pressurizes the canister, how then is there still enough pressure at cold temperatures to push liquid in inverted mode but not enough pressure to create or push vapor if using a canister upright?
Wouldn't you then be dealing with a scenario somewhat like trying to use a white gas stove without pumping/pressurizing the fuel bottle...?
What's pressurizing the inverted liquid to get it to the preheat tube and burner if it's too cold to generate vapor in the first place?

This question is what led me to assume it's not entirely boiled gas vapor that pressurizes the canister.

Am I still missing something or thinking about this all wrong?

Hi Craig,

One thing to remember when using inverted canisters is the difference in boiling point of the various fuels in the mix. Butane is around 30F, isobutane around 11F, propane around -43F. Obviously, the fuels with lower boiling points will remain a vapor as the temps cool, leaving fuels with higher boiling points as liquids. These fuels with lower boiling points will of course sit at the top of the canister, above the liquids.

When using upright stoves, the fuels with lower boiling points will burn off first, leaving liquid fuels that won't feed to the burner. When the canister is inverted however, these vaporized fuels remain at the top, providing pressure to force the liquid fuels in to the fuel line, where the preheat tube can vaporize them before they reach the burner.

At least that's what seems like is going on. Roger will be along to either confirm or refute this; nothing like having a resident practicing scientist on the boards ;)

PS: edited for clarity. Thanks Greg!

Edited by cooldrip on 01/28/2013 11:16:39 MST.

Upright stove - pushes gas out of the canister, new gas boils off the liquid, this cools the canister. Works like a refrigerator. When you're operating it in cold but well above freezing weather, the canister will get colder than freezing and water will condense on the outside and freeze.

Inverted canister doesn't have this problem. Only has to boil off enough gas to displace the liquid leaving the container so it gets only a bit colder than ambient. The liquid to gas conversion occurs at the burner that gets heated from the flame.

That's why you want an inverted stove in cold weather.

Adding to the previous post:

If you were using a canister made up of only propane (or only butane or only isobutane), there would be little benifit to inverted vs upright.

Also, for fuel mixture, when NEW (full), there is little difference to using inverted/noninverted.

The benefit comes with fuel mixtures over the life of the canister:
In non-inverted use the gas that boils off tends to be more of the higher pressure gas than the others (eg higher % of propane in the vapor than in the liquid). As you use up the canister the internal pressure drops (not b/c you are losing fuel mass but b/c your fuel mix becoming less and less propane - thereby droping the canisters overall pressure). This causes the usable temp of the canister to drop as it is used up (limit approaching the boiling temperature of the least volatile fuel - typically butane).

Once you invert you send liquid fuel toward the vaporizor tube and boil off the mixture entirely - thereby keeping the fuel mixture proportions relatively constant. So one big benefit is you keep the canister pressure (at a given temp) constant throughout it life.

Another important nuance btw the two:
In upright use every gram of fuel that is burned has to evaporate in the canister. This evaporation steals heat from the mixture to drive the evaporization (just like sweat does from you skin). This can lead to the canister temp being noticably colder than ambient air temp (this of course leads to a drop in canister pressure).
In inverted use some fuel still has to evaporate (have to make up for the lost liquid volume with vapor volume). But the amount of vaporization is much lower b/c a gram of liquid fuel occupies much less volume than a gram of fuel vapor).

Hope this helps,
James

This makes me wonder about another possible force at work here. I wonder if the vaporization of fuel in the preheat tube and the subsequent ignition at the burner creates a vacuum in the line, thus drawing luquid fuel from the canister? Not sure about this, but maybe another mechanism at work?

nm

Edited by greg23 on 01/28/2013 12:40:58 MST.

Scott's point (lighter compounds boil off first in vapor-feed mode) is correct, but is the smaller factor, typically.

Often, even when your upright canister starts off well above the boiling point of the fuel, the following happens: At first, with a warmish canister, there is sufficient pressure due to the fuel boiling in the canister. But WHENEVER and WHERE-EVER a liquid changes to a gas, it absorbs heat from its surroundings. This is how sweating cools us. It is why more volatile alcohol cools us off more quickly if spilled on our skin. If the liquid to vapor transition is in the canister, the canister is cooled. With little to be gained from the cold environment, the canister temperature can fall to near the boiling point and the flame goes to almost nothing. Not a problem with on a hot day or with a higher-pressure fuel (although in some of my non-BPing pursuits, I vaporize propane so fast that it can form ice on the outside even on a warm day, and eventually lack sufficient pressure.

However, in an inverted, liquid-feed canister, the liquid to vapor transition ISN'T in the canister - it's in tubing near the heat of the stove - sometimes, like in a white-gas Coleman stove, right in the flame. That provides the needed heat and leaves the canister at its original temperature. You need the canister to be warm enough to start and in cold climates you need to keep it from cooling due to snow, cold air, etc. But the canister isn't rapidly cooling itself by vaporizing liquid into gas. And therefore, inverted canister stoves are good to a significantly lower temperature and requires less elaborate schemes to reflect stove heat onto them, etc.

In a pinch, a tea candle under a canister will warm it up pretty quickly and provide the pressure needed to run a stove. Never let the canister get more than warm to the touch.

With upright, vapor-feed, you need to keep putting heat into the canister. That can be from a warm day, heat reflected from the stove, a pot of warm water, or the candle scheme. What constitutes a "warm day" varies with different fuels as others have noted.

>" creates a vacuum in the line, thus drawing liquid fuel from the canister?"

Nope. Can't happen. For there to be flow in the line, the pressure must be higher in the canister than in the preheat tube. And since the preheat is upstream of the burner and the burner must be at atmospheric pressure, the canister must be above atmospheric pressure for any fuel to flow. As noted by others, it must be enough above the fuel's boiling point to provide some additional pressure for frictional losses in the valves, tubing, and jets. Household natural-gas devices operate on 1/4 psig (7" water column - sorry, metric users!). Higher output propane devices (weed burners, 100,000BTU/hour cookers, etc) have regulators inline maintaining 1 to 5 psig.

You could create a partial vacuum in a canister by cooling below its fuels boiling point. Using fuel won't do that because fuel flow stops when the canister reaches atmospheric pressure. But leaving an n-butane canister out on one of our fine, -25F nights would. Then you could suck a little air INTO the canister if you opened it.

Really obscure point: Scott, maybe you are wondering if the high velocity of fuel vapor in the jet creates a partial vacuum? Yes and no. Not the way you are imagining it, not in any way that would suck more fuel down the line or otherwise help fuel flow. The high speed vapor does has a lower "static" or "bursting" pressure - if measured perpendicular to the air flow as the static port of a venturi does. Or the upper surface of an airplane wing (preventing you and other passengers from plummeting to a fire crash 31,000 feet below). The "total pressure" of a packet of vapor = static pressure + velocity pressure. Total pressure always goes down along the flow direction. You can shift between static and velocity pressure with piping constrictions and expansions, but you can't create more total pressure without a pump. In airplanes, (the velocity pressure is increased so that) low static pressure is created by a longer path over the top of the wing. In stoves, (the velocity pressure through the orifice is increased so that) low static pressure is created and combustion air is sucked in and mixed with fuel vapor. A "pre-mix" burner has an efficient blue flame, not the cooler, yellow flame of unmixed fuel and air like in a candle or many alcohol stoves.

Thanks everyone, you've cleared this up for me.

I'm going to be ditching the fussy, messy white gas stove (currently an Optimus Nova) for winter and purchasing a canister stove that can invert (and is less likely to burn my shelter down), likely an Optimus Vega.

Just wanted to be able to properly wrap my head around what's going on in that canister....

I've never used inverted canister, but people report that the valve can get gummed up. And inverted stove is heavier and more expensive.

Upright stove - just gas goes to valve so no problem

Inverted - liquid goes to valve. There's stuff in the liquid that can foul the valve. With upright, that stuff just stays in canister.

You can get upright stove to work in colder temps - put in bowl of warm water, you have to re-warm it occasionally.

Or, what I've tried that's better, is to wrap a solid #16 copper or aluminum wire around stove and up into flame. Heat is conducted down to canister. Maybe you have to warm canister in your pocket just to get it started.

That would be simpler than any other stove.

"Or, what I've tried that's better, is to wrap a solid #16 copper or aluminum wire around stove and up into flame."

I used to get good results out of a much larger copper wire, like 10 or 12 gauge. Also, to help the heat transfer, I would slightly flatten the wire using a hand sledge. If your wire gets too big, then it is too heavy for what it accomplishes.

--B.G.--

#16 is lighter

good idea about flatening for better heat transfer

after you've experimented and fiddled and found working solution, then it's a simple and easy

(and I'm not saying inverted canister or anything else isn't a good solution also - many ideas for people to consider and use what works for them)

I enjoy it if i can understand it (this time I did), thanks David.

One question that has been in the back of my mind while reading/watching this thread-
Stuart said "To operate the stove, the temperature must be a bit above (10F or so) the boiling point, so there is sufficient pressure to force the vapor thru the jet in the stove" and "You only need to keep the canister temperature above the boiling point of the fuel. If you use the canister inverted, that will be around 0F for most propane/isobutane mixes."

With an inverted stove
1. does the different gases burn off at a different rate, assuming one is heavier than the other?
2. if so, which one will burn last?
3. does the last burnt gas have an effect on pressure at lower temp's (which gas and what temp)?

I know this is for conversation only and the real world adds many variables to the mix.

I've used upright canisters down to ~15 degrees F successfully by keeping the canister in my bag at night and in a breast pocket inside my jacket when hanging around camp.

Aside from temp issues, the main reason I want to go with an inverted, liquid feed is for more stability for larger pots for group cooking and the benefits of being able to use a well-fit windscreen.

When winter camping high in the southern rockies I always look like a pregnant kangaroo; tons of shit(water, snickers, battery powered stuff, fuel) under my warm puffy layers once the temp drops. Size outer layers up a size to compensate.

For fuel, and sorta rev to this thread, I use the following technique:
First, 'belly-heating' stuff sucks heat from me, so I try to be judicious. But the fuel always goes under covers during the eve/ nights that are generally teens or way lower. This ensures that the jetboil will always light on minus five degree mornings. Insulating the canister from the direct contact with snow with will help keep that heat through a boil.

But super cold does happen or at the end of a average ski camp afternoon with fuel in the pack getting cold, I'll warm the canister fuel by:
1. putting only a cup or so of water into the pot so it heats quickly with the struggling stove output.
2. When the water feels warmish I turn off the stove and disconnect it from the pot. Wearing gloves I slowly lower the fuel canister (clean bottom before trip) still attached to stove into the pot with warmed water. Immediately you'll hear the liquid gas starting to boil inside the canister.
3. Reconnect the stove to pot and relight. The output will be like normal and it seems much more efficient at low/slow mode on the jet boil. Repeat if needed but I rarely find it necessary above zero Fahrenheit at 8-11k elevation.

Granted this technique is with a solo capacity jetboil stove system, I hope this helps.

> With an inverted stove
> 1. does the different gases burn off at a different rate, assuming one is heavier
> than the other?
Yes, although the fuel 'weight' is not a direct factor. Propane boils much more easily than butane, so it boils off faster. This doesn't matter much at +20 C, but it really does matter at -20 C.

> 2. if so, which one will burn last?
Since the propane boils more easily, it's the n-butane or isobutane which is there right at the end.

> 3. does the last burnt gas have an effect on pressure at lower temp's (which gas
> and what temp)?
As above. Yes, with mostly butane left at the end the canister pressure is much lower.

Others have mentioned putting the canister in a bowl of luke warm (not hot!) water. This is a very good idea and worksd very well, with both uprights and remote canister stoves.

Check: the Optimus Vega does (visibly) have a preheat tube, OK. Farewell to messy white gas...

Cheers

i wonder if the copper wire in the flame could increase co production. not that i would ever use my canister stove in a tent (insert wink here), but i always think of proper ventilation when lighting up in a confined space.

Thanks Roger,
I guess my question was- if one gas was heavier than the other, the heavier gas would be the first to burn off in the inverted position. Depending on which one burned off first it might make things better or worst depending on the weight of each gas. When the heavier gas burns off some type of gas needs to replace it in the canister, which I assume is the lighter gas, being on top. If its Propane; that's good, because of its lower boiling temp. If its Butane, not so good, because the stove looses pressure at a higher temp.
This is more for a point of discussion and doesn't matter really in actual use.

I really don't worry about it, because I use my Xtreme in the winter (always liquid fed) and in the Northwest it doesn't get cold enough to push that stove to its limits.

Edited by bestbuilder on 01/28/2013 23:12:19 MST.