Alcohol stoves require a wind/heat shield and base reflector to work effectively.
Most of the time a shield or reflector can speed up boil times in most stoves. But often, a heat shield will limit air supply and slow the boil, or, overheat the stove and cause flare-ups that waste fuel. To be effective, the air intake and exhaust around the pot must be calibrated for the stove and for the conditions - temperature, humidity, altitude, and wind. Some manufacturers use adjustable holes to create a jet or chimney effect to increase efficiency. A heat shield full holes is not a wind shield however and it must still be protected from the wind. Some of the best boil times for the Penny were achieved with no heat reflector or wind shield.
The most common reason they are necessary is because a small diameter pot is used and heat must be funneled to the sides of the pot.
Another reason is that the flame/pot interface is so pour and/or flame size changes so often, that all the residual heat and fuel must be collected and directed around the pot. For example, a flare-up must be directed up the side or unburned gasses must be contained until they can find oxygen to burn.
A third reason is that the pot support is draining heat away from the pot and down to the ground. This heat must be shielded and reflected back up to the pot.
Stoves that burn for the same length of time, on the same amount of fuel, should have similar boil times and efficiency ratings.
I once had a prominent designer/manufacturer flame me and threaten to sue based on this assumption - he didn't believe the Penny performance. The problem with this myth is that many parts of the process are not represented - for example, how well the stove transfers heat to the pot and liquid, and how evenly it burns from the time the match hits the alcohol to the end of a burn.
Most stoves start and end the burn with a low flame and some have a big high flare-up in the middle. Each of these states limits how efficiently heat can be transferred to the pot and how much fuel is wasted. So, most stoves must make a distance from the flame compromise and use other systems to capture heat and save fuel.
Pressure, pop can, and side draft stoves tend to heat only the outer edge of the pot and this limits the amount of heat transfer.
Center flame stoves (Cat Stove) have a single giant flame that limits oxygen mixing with the fuel as it hits the pot, some up the sides, and some may escape unburned - see #1 above.
Small high pressure jets burn hotter and more efficiently and therefore heat faster and use less fuel.
An efficient high pressure blow torch like flames (like a white gas stove or Photon alcohol stove) put out lots of heat and burn very efficiently. However, most of the heat is focused at the very tip of a very small flame and will transfer heat to only a very small spot on the the pot. This limits how fast heat can transfer to the pot and fluid inside. If you have ever welded or braised you know how this works. Small jets also keep oxygen from getting to the center of the burner to supply a center flame that can heat the center of the pot.
In addition, if the flame size increases and laps the pot, it no longer has that hot tip - it's more like a conventional flame but still on a small spot. If flame size decreases, the pot is no longer touching the "hot spot", so again, the torch tip no longer has a special advantage. Also, high pressure stoves are very sensitive to flare-ups and must be tuned to work with a particular heat shield and pot stand. Stoves of this kind also tend to react more to cold and altitude - factors that change flame size and increase the flame/pot interface limitations.
If you doubt the importance of consistent optimum flame size, boil some water on your favorite stove, then lift the pot 1/4" or so and observe the results. Using a jet size that is tuned to pressure regulator weight is the best way hold a consistent flame size.
In this Forbes Conrad photo detail, note how the Penny's laminar flow jets hit the pot surface, creating turbulence, mix with the air, and burn to create seven hot spots - including the central glow from gases released from the penny pressure regulator. Using your imagination and you can almost feel oxygen being sucked up between the jets.
Slow burning stoves take longer to boil but are more efficient than than faster burning stoves.
Again, yes this can be true, but only if the heat is contained and stored. A slow burning stove is usually more effective at transferring heat to the pot and liquid, but it also looses more heat to the environment so much of the advantage can be lost. In fact, if heat loss is more than transfer, it will never boil water. I believe that this is why many alcohol stoves will not boil one quart of water. I think that this myth started because many relatively hot burning stoves were not able to transfer that heat to the pot and/or wasted lots of fuel.
So, there is a theoretical sweet spot, where heat transfer to the pot exceeds heat loss but does not waste fuel. And, this sweet spot changes with environmental factors. In windy or cold conditions, it is easy for heat loss be greater than stove output. The Penny is on the high (hot) side of this sweet spot. Since it's hot and efficient enough to quickly boil one quart of water under normal conditions, it can be effective under almost all adverse conditions. If time is not a factor, and conditions allow it, the simmer ring may lower this sweet spot and increase efficiency slightly. The 1/2 Penny Stove is another option.
Alcohol stoves do not work well in the cold.
Actually most canister stoves don't work at all in the cold - it's suggested that you sleep with the canister - that sound like fun. In his book Annapurna: 50 years of Expeditions in the Death Zone, Reinhold Messner relates how the first party to ascend an 8,000 meter mountain made "tea on the alcohol stove" the night before the "assault". At way over 20.000 ft. it was very cold on that trip - Herzog and Lachenal both came back with critical cases of frostbite. Bill Reynolds believes that "if it is colder, alcohol is more likely to burn" and is testing the Penny Stove to prove his theory.
I think that the most common reason for this myth is that most alcohol stoves sit directly on the ground. Many proponents suggest that you insulate them using a foil and/or foam base. And, most stoves don't have the power or efficiency to overcome extra heat loss (see above). Stoves designed in and for cold climates often use legs to raise the stove from cold or wet ground. In Scandinavia these have long been the standard for winter packing. The Penny has a simple insulating base to help solve this problem - one builder reported that it worked well with a wind chill factor of -9 degrees.
Alcohol stoves do not work well in the wind or at altitude.
All brands of stoves need protection from the wind, but most alcohol stoves have major problems in the wind. With an unpressurized stove, even a small gust can distort the undirected flame, keeping it from reaching the pot, and blowing the heat away.
Canister stoves sit up high and have no wind shield. With side burning alcohol stoves wind can force flames on one side to miss the pot completely. Pressurized stoves perform much like gas stoves - the flame shape is more difficult to distort and more likely to reach the pot. When a breeze hits the Penny stove, flames are directed to heat the cup edge and flame size increases to compensate for the wind.
Altitude also increases the flame size because more fuel is released from the jets and the penny pressure regulator. Also, since liquid boils at a lower temperature, altitude and/or a slight breeze will usually decrease boil times. Bob McLaren reported that "it's performance was excellent, even with 50 mph wind gusts (with windscreen of course)." Brad Monsma said that "It worked great in the cold and high elevation..." - the top of 14,491 ft. Mt. Whitney.
Simmering on an alcohol stove is difficult and inefficient.
To produce a simmer, the best commercial stoves block the air supply - either on the stove or the heat shield. This is like controlling the speed of your car by clogging the air filter. It will work, but only because much of the fuel is released to the atmosphere unburned. In a stove however, as less heat is produced, the fuel supply may eventually slow to balance the air supply. But this indirect control cycle makes a simmer very hard to control and can waste a lot of fuel in the process. And, the smaller flame will be far from the pot and much of the heat will be wasted.
A full diameter simmer ring, however, reduces both the fuel and the air supply. In addition, it can raise the smaller flame much closer to the pot for maximum efficiency. The Penny Simmer Ring shields the edge of the Cup from the flame, cools the Cup, and slows the alcohol boil. As pressure is reduced, the penny seals the center flame and jet size is also reduced. The Ring also blocks air to the base of the jets and raises the smaller flame closer to the pot. This provides not only instant control but optimum efficiency and wind protection. Web research shows that the Penny can use 1/3 less fuel to simmer than air adjustment stoves - and do this with enough heat to boil, not just simmer, more than a quart of water.
Alcohol stoves use more fuel so only save weight on short trips.
They do require more fuel (alcohol has about half the btu's of gas), but the stove is much lighter, so most people agree that the break even point is a two week trip for one person. After that, a gas or canister stove may start to save weight. However, the Penny's efficiency can extend this, and, several important factors are often not considered.
Liquid and pressurize gas is toxic and dangerous - even a few drops can ruin your food supply, if not your life. Unlike liquid stoves, alcohol can be carried in a very light cheap water bottle. Unlike canister stoves, you can take only what you plan to use. At the end of an alcohol fueled trip, you are carrying only a few ounces of stove weight and a tiny, recycled, empty, reusable, container. In addition, the Penny will always work. Even if you loose it, you can make another - even on the trail.
Another consideration is that ethanol alcohol is one of the Universal Basics, like water, matches, salt, sunscreen, sun glasses, a hat, and a bandanna. Experience has shown that these should always be in my pack. Alcohol replaces many other products and their associated costs, hassles, and weight. Everclear (95% Ethanol) can be used as a fire starter, disinfectant, *first aid for cuts, blisters & insect bites, a soothing rubdown, a spot remover, emergency frost bite protection or pain relief, and... it makes a great flaming fruit desert or snow margarita. :~)
"A favorite goodie for me is to dehydrate dried fruit in everclear."
* Did I mention the cost of acquiring, protecting, and burning our limited fossil fuel supply? Do you really want to smell gas fumes in the back country?
* When use on or in the body, ethanol should always be diluted with 1/3 water for maximum effectiveness.
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Bill Reynolds theory:
"Basically, the spontaneity of a reaction is determined by the Gibbs Free Energy Exchange. ΔG=ΔH-TΔS where ΔG is the change in free energy, ΔH is the change in enthalpy(heat), T is temperature in Kelvin, and ΔS is the change in entropy. For combustion reactions, like burning alcohol, ΔS is normally negative and ΔH is always negative. The temperature will be left as a dependent variable. When ΔG is negative, the reaction it refers to is spontaneous. So, by increasing temperature, the reaction will eventually reach a point where it will no longer be spontaneous. In other words, at lower temperatures spontaneity comes more easily for combustion reactions. The other side of this argument is that evaporation rates of fuels are lower at lower temperatures, however, once gaseous fuel is being burned above the liquid fuel gas will be drawn out of the liquid at a steady rate. This is simple equilibrium, but has many factors like this are poorly understood. I am going to do a small study on the effect of temperature on the efficiency of a penny stove just to verify the theories I have presented."
Alcohol stoves do not work well in the cold.
© 2007 mark jurey