Gear: The Alpine BOMB

Or, more aptly, how to modify a cannister stove to work in really cold weather. This isn’t my idea. It was published years ago in Mountain Review magazine (issue 7 page 74). It also appears in Mount McKinley Climbers Handbook, Glenn Randall (page 32). The version shown here is much simplified and not adapted for hanging. I also use a wind shield with it (not shown).

A few years back I was climbing in Southern BC with a friend of mine. We were staying at a hut but it was during a cold snap in late November. The end result was it was really cold even inside the tiny building. My climbing partner had brought the stove a cannister stove with a remote cannister, similar to an MSR WindPro. Getting the stove to work gave us much needed additional entertainment in the windowless hut. It also prompted me to dig out that Mountain Review article when I got home and start tinkering.

Cannister stoves don’t work well at low temperatures because if the temperature of the liquified gas falls to below its boiling point then none of the liquid can boil and turn into gas so the stove stops working. In addition, when a liquid turns into a gas it cools the remaining liquid – physicists call this the “latent heat of vaporization”. All very interesting, what does this mean for your stove? Even if the gas in the cannister starts off warm enough it will cool down during cooking as gas boils off! This is why cannisters always feel cold after use even though they’ve been sitting under a hot burner.

Typically cannisters are filled with propane (boiling point-40C), n-butane (boiling point -0.5C), isobutane (boiling point -12C), or a mixture of these gases. Pure propane camping stoves aren’t popular because propane liquifies at a much higher pressure and so requires a much thicker cannister. Imagine carrying one of those big propane BBQ cylinders up a route – no thanks! Pure n-butane isn’t going to work at low temperatures due to boiling point being around freezing.

Manufacturers now fill their cannisters with mixtures of gases, MSR’s IsoPro for instance. IsoPro is a mixture of 80% isobutane and 20% propane. It may also contain some n-butane as pure isobutane is difficult to manufacture. This produces a gas mixture with a lower boilding point. There is an odd side effect here. The liquified gases don’t boil off at the same rates. In cold conditions the more volatile liquid will boil more readily. What this means it that a half full cylinder will contain a mixture with a higher percentage of isobutane because the propane boiled off faster. So you might end up with a 95% isobutane mix in a half full cylinder. This will make the effective boiling point of the remaining gas higher.

A little bit of heating will make a big difference. You can pre-warm the cannister under your clothes. This helps getting the stove started but that’s about it. The main problem is that as the gas vaporizes it has a cooling effect (that “latent heat of vaporization” thing). So a pre-warmed cannister will quickly cool off.

What’s required is a way to continuously warm the cannister. The simplest way to do this is to place the cannister in a pan of warm water. This works great! If you’re in a nice hut with a table to mess about on and you happen to have two pans and a bit of time to kill. Of course if you happen to be stuck on some tiny ledge half way up a route then this trick might start to seem a bit less impressive.

What you need is a way to redirect some of the heat from the burning gas back to warm the cylinder. A windshield will do this to some extent. MSR sells one or you can make one out of the bottom of an aluminum turkey tray. A far more efficient solution is to build a “Bloody Outrageous Mountain Burner” or BOMB, as described in Mountain Review. The one described here is a much simplified version for use in the moderately cold temperatures of a Pacific Northwest winter.

All you need to do is a scrap of closed cell foam from an old sleeping pad and cut it to fit snugly around your favourite brand of gas cannister to form an insulating cup. Glue the foam together with a contact adhesive and then cover the whole thing in duct tape to make it a bit more durable. Next get a short length of 1/4″ copper piping (available at Home Depot) and flatten the bottom section to give a nice tight fit against the cylinder.

When in use one end of the copper tube sits in the flame while the other is flat against the side of the cannister. This acts as a heat exchanger to warm the cannister. You only need to warm the cylinder not heat it up. These stoves are designed to work just fine at room temperature.

WARNING: No stove manufacturer will recommend this! If the cannister overheats it will explode. Do not use in anything but cold temparatures. Do not allow the cannister to get hot, both the heat exchanger and wind shield may cause the cannister to overheat.

Use at your own risk and fully consider the implications of your actions before attempting. The author disclaims all responsibility implied or expressed as to the safety of this modification.

Useful Links:

Liquified Gas Stoves – Lots of good stove information
The nature and behavior of mixtures of fuels – the physics behind liquid gas cartridge stoves
Air Liquide – Physical Properties of Gases – lots of useful data on the properties of gases

One Comment

  1. fanat1c:

    Why does not alpine climbers use liquid fuel instead? It works good at high altitudes and low temps.