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Monday, November 28, 2011

Gas Stoves: How Cold Can I Go?

Gas stoves (also called "canister" stoves) can struggle in cold weather.  Just how cold of weather can I run my gas stove in?

Good question.  Do you want the short answer or the long answer?  :)  The short answer is that for the typical upright canister stove (that's the kind where the burner screws directly onto the canister), about 20F/-7C is a good planning number.  Do make sure you have winter capable gas before you try to run your stove in cold weather.  If your stove starts struggling in cold weather, try some of the tricks listed in my Stoves For Cold Weather article in Seattle Backpackers Magazine.  If you need to go colder than 20F/-7C and you want to use gas, you'll probably need to use a stove that can handle liquid feed gas (an inverted canister stove in other words).  See my article Stoves for Cold Weather II for more information on liquid feed gas.

Now, that was the short answer, and it should give you a good planning number as you consider what gear to take on a particular trip.
Note:  I tend to lose some people with the "long" answer.  So, if your eyes tend to glaze over with too much detail, I also offer some short and sweet Cold Weather Tips for Gas Stoves.

Now for the "long" (full) answer:  Canisters typically contain propane mixed with either isobutane or "plain" butane (n-butane).  The boiling points (vaporization point) of each of those gases are as follows:
 Boiling point
n-butane    -0.5C    31F
isobutane    -12C    11F
propane      -42C   -44F

Propane has the lowest boiling point and therefore the highest vapor pressure.  Because of its high vapor pressure, the propane boils off at a faster rate than either n-butane or isobutane.  Toward the end of a canister, you have no propane left, and you're running on just isobutane or n-butane.   N-butane will not vaporize below 31F/-0.5C, so it's a poor choice for an upright canister stove in cold weather.

So, with that little fuel lesson out of the way, here's the answer:  If you take a typical upright canister stove out in cold weather, you can run it on good fuel (no n-butane) to the point where the fuel temperature is about 20F/-7C at sea level throughout the life of the canister.  Yes, isobutane vaporizes at 11F/-12C, but you need a certain amount of pressure in the canister in order to properly drive the stove.  If your vaporization point is 11F/-12C, and your fuel temperature is, say, 12F/-11C, then the pressure in your canister will be so insipid that you can't run a stove off of it.  Generally, about 10F/5C above the vaporization point will give you good operating pressure.  So, if your fuel vaporization point is 11F/-12C, you generally want to shoot for about 21F/-6C in order to have good operating pressure.  20F/-7C is easier for me to remember, so I usually just say 20F/-7C fuel temperature for upright canister stoves using a propane/isobutane blend at sea level.

As you ascend, the higher you go, the lower the ambient air pressure.  As the air pressure drops, so does the boiling point of your fuel, so you can operate your stove at lower fuel temperatures, as shown by the below diagram.  Generally you can operate a stove with fuel that is about two degrees Fahrenheit colder for every thousand feet in elevation gained.  In metric units, that's about one degree Celcius colder for every 300 meters in elevation gained.

The relationship between boiling point and elevation.

Of course, you can modify the stove to divert heat to the canister or add heat by some external means, in which case the fuel temperature will rise independent of the ambient temperature, and your stove will run in colder weather than it otherwise would.  If you divert heat from the combustion, be very careful.  Overheated canisters can and do explode.  A canister explosion is a potentially life threatening event.  ALWAYS  monitor the canister temperature with your (unfrozen) hand.  If the canister feels hot to the touch, turn it down immediately

Also, please note that the colder it gets, the harder it is to keep the canister warm.  If you're going out in really cold weather with a gas stove, think.  Can I really keep my canister warm in those temperatures?  In really cold weather, your life may depend on your stove functioning.  Take no short cuts. 

Main "take aways" from the long answer:
1.  In all cases, it is the fuel temperature which matters, not the ambient temperature.
2.  Propane burns off faster, so you must be able to rely on the other components of your fuel.  Avoid n-butane.
3.  In order to have enough pressure to properly operate a stove, your fuel temperature must be warmer than the vaporization point of the fuel.  Generally, about 10F/5C above the vaporization point will give you good operating pressure.
4.  The higher you go, the colder the weather your gas stove will operate in.
5.  If you heat the canister by some external means, you are not as constrained by the ambient temperature in terms of how cold of weather you can operate your stove in.

OK, so now that you've seen the long answer, is my short answer "planning number" of 20F/-7C any good?  Anyone who has used a canister stove can tell you that the canister gets cold as you run your stove.  Isn't the fuel temperature going to fall below the outside temperature?  And if the outside temperature is already at the bottom of my temperature range (20F/-7C), aren't I going to get screwed?

Good points, and yes, your fuel temperature will fall below the outside temperature.  Recall though that 20F/-7C is for sea level.  In fact, most of us camp well above sea level, so camping above sea level will compensate  for some or all canister cooling, depending on the elevation.  Combine the elevation with the tricks in my Stoves For Cold Weather article, and you should have no problems.  If you still have problems with my short answer, then just use the long answer.  :)

HJ


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4 comments:

  1. Canister gas is really liquid fuel, but gas stoves lack vaporizers, except for those intended for liquid feed. The heat of vaporization comes from the atmosphere and must occur at a rate sufficient to keep the liquified gas temperature up. There are all sorts of trade offs, but I'm just not a canister gas fan. I prefer liquid fuel in most cases.

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  2. Bill, excellent summary. You got it!

    HJ

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