by Ryan Jordan | 2001-06-25 03:00:00-06
When working under optimum conditions, compressed gas stoves running on propane/(iso)butane mixtures may be the most efficient cooking engines on the market. With the recent introduction of several canister stoves that weigh less than 4 ounces, they have become a viable and highly effective alternative to alcohol and solid fuel stoves for lightweight backpacking.
I used a Snowpeak GigaPower canister stove on an 8-day trip in the Beartooth Mountains of Montana last summer. My cooking requirements included seven hot breakfasts (22 oz water boiled; 10 oz of that was poured into a cup for coffee and the remainder mixed with hot cereal and simmered for 5 minutes) and seven hot dinners (26 oz water boiled; 10 of that was poured into a cup for coffee and the remainder mixed with a pasta or rice dinner that required 5-10 minutes of simmering). In addition, I made six fish meals that consisted of boiling 16 oz of water to poach trout, then replacing the water with cooking oil and spices, simmering for 5-10 minutes. I took only a single MSR IsoPro canister (7.8 oz of net fuel weight, 13.1 oz total full canister weight) and still came home with 1.0 oz of fuel remaining in the canister. That's about 20 hot meals (including 27 pints of water boiled) using only 6.8 oz of fuel. If you only considered the 27 pints boiled (and conservatively neglected the simmering), this results in a fuel efficiency of 0.25 oz of fuel consumed per pint boiled-which is about the same efficiency that I observed when first testing the stove in my kitchen at home. I challenge you to find another stove that meets this kind of standard under field conditions.
Admittedly, these were not harsh conditions. This was summertime, temperatures were mild, and winds were seldom more than breezy (> 5 mph) while I was cooking. The two greatest enemies of canister-stove efficiency are cold temperatures and winds. Improving canister-stove efficiency under these two conditions is the subject of this article.
Cold ambient temperatures are unable to counteract evaporative cooling of the canister that results during combustion, and as the canister cools, fuel volatilization, and thus, burn rate, decreases. One can counteract this by loosely wrapping a piece of thin aluminum foil around the pot/canister assembly and making sure the canister is placed on an insulating surface such as a piece of closed cell foam. The foil acts as a windbreak, but more importantly, provides dead air space between the foil wall and the canister that acts like a heat exchanger to keep the canister warm, using the heat from the flame. There is one caveat to using this technique successfully, however-you need to punch several holes in the lower portion of the foil wrap, not only to provide oxygen to keep the flame burning, but also to prevent overheating of the dead air space that might cause dangerous over pressurization of the canister and the potential for an explosion. One should be extremely cautious using this technique and take responsibility for how hot is too hot (the canister should never reach a temperatures that is too hot to touch with bare skin).
This technique works well in moderately cold environments (with temperatures in the 30s and 40s), but is not likely to provide enough of a boost for subfreezing conditions, where canister stove output can really slow to a crawl (indicated by the rapid formation of ice on the exterior walls of the canister). To cope with very cold conditions, one must resort to a more aggressive heat exchanger.
For years climbers in cold environments have used homemade "copper wire" heat exchangers to keep the canisters warm. The technique simply uses a 1/8" diameter copper wire (flattened with a hammer) with both ends in contact with the flame and the remainder of the wire wrapped once around either the bottom or the sides of the canister. The copper wire provides a small amount of heat to the canister without serious risk of overheating. Often, the canister and copper wire are wrapped with heat resistant tape or closed cell foam lined with foil to improve heat exchange (by providing an insulator) and prevent accidentally touching the copper wire and causing burns to clothing or skin.
One should realize that neither canister nor stove manufacturers will address the validity of this technique, since copper is an excellent conductor of heat and can potentially cause canister overheating with serious consequences (explosion), so any user adopting this technique should use extreme caution.
A safer, but not nearly as effective method, involves preheating a small amount of water in a pan and then placing the canister and stove in the heated water for the duration of the cooking. The heated water provides some resistance to evaporative cooling and allows the stove to maintain its BTU output much like the copper wire or dead air space created by the aluminum foil. The author uses this technique successfully on a 23 degree (F) March morning in the mountains of Southwest Montana (see photo below Title).
A final word of note in using a canister stove effectively in cooler temperatures: the longer the burn time, the more that evaporative cooling will adversely impact heat output. In other words, if you have to boil one quart of water for a meal, you will use less fuel and require less time to boiling if you boil a pint at a time, warming the canister in your jacket or sleeping bag in between pints.
Wind is troublesome for canister stoves. Even those that claim to have integrated "windscreens" (which are really nothing more than small plates that keep wind from blowing out the flame, and do nothing to keep the flame focused on the pot), any wind stiffer than a very slight breeze (2-3 mph) will render a canister stove all but useless. Since many users are paranoid about enclosing the entire stove with a full-wrap windscreen (because of the very real risk of canister overheating), options for a windscreen are pretty limited.
In response, I'd like to provide the plans for a homemade windscreen that includes two parts: (1) a heat reflector that prevents heat loss downward and away from the pot bottom, and (2) a windscreen/heat exchanger that blocks stiff winds, provides heated dead air space around the pot surface, and does not pose any significant risk to canister overheating.
The windscreen can be made using any malleable foil material. I recommend heavy-duty aluminum foil for lightest weight, but for more durability and flame resistance, a thicker aluminum "oven pan" or "flashing" material would be more appropriate.
The heat reflector is simply made from a circular piece with cutouts to accommodate the stove legs and burner head (and piezo ignition, if your stove has one, or alternatively, a small hole to allow access to the burner head with a match for lighting the stove), and the outer lip turned up to provide a platform for the windscreen (Figure 1). The diameter of the heat reflector should be ½" larger than the pot diameter you intend to use with the stove. The windscreen is simply a rectangular piece that is about 1" longer than the pot's circumference with a cutout section to accommodate the pot handle (Figure 2). To assemble the unit, attach the heat reflector to the stove, place the pot on the legs, and wrap the windscreen around the pot so that it sits inside the lip of the heat reflector (Figure 3).
|Figure 1||Figure 2||Figure 3|
The windscreen/heat reflector unit shown in the images was built for a Snowpeak GigaPower stove with a piezo ignition. In a zero wind situation at moderate temperatures, the presence of the unit improves boil times by about 15-20%, due in large part to the effects of the heat reflector and the heat exchanger-like dead air space provided by the windscreen. In windy conditions, I've found that the unit improves boil times by 100% or more and markedly improves fuel efficiency (fuel consumed per volume of water boiled).
The information contained in this article is provided for informative purposes only and describes aftermarket modifications to canister stove systems that are not endorsed by any manufacturer. This information should be used at your own risk, realizing that there exists the possibility of canister overheating or permanent stove damage that could result in property loss, injury or death.
"Homemade Canister Stove Windscreen," by Ryan Jordan. BackpackingLight.com (ISSN 1537-0364).
http://backpackinglight.com/cgi-bin/backpackinglight/00041.html, 2001-06-25 03:00:00-06.