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DIY wax-powered backpack stove: background

Outdoorsy stuff > DIY wax-powered backpack stove

And now for something completely different. Here’s a homemade backpack stove powered by wax. It’s not just a big candle, which would be weak, smoky, and sooty. This stove supercharges the hot wax vapor with air so it blasts like a regular stove.

The result is a strong, somewhat clean, wind-tolerant flame. Compared to other stoves, it rates better or the same in many ways (and the same or worse in a few others).

As far as I know, nobody’s ever made anything quite like this before. You can make one, too: just click over to the Construction page.

Wax?! What's the deal with that?
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When we think of burning wax, we think of a candle. Wax consists of long-chain molecules that are powerful but do not evaporate easily. A candle uses a wick to convert molten wax to a vapor that then will burn in air. Some of the heat from the flame converts solid wax to molten wax. The liquid wax then travels up the wick, the liquid turns to vapor, and the vapor flames, continuing the cycle. Many other stoves use a similar heat feedback cycle, except that only liquid gets heated to vapor, and no solid phase plays a role. (Most canister stoves [propane, butane, etc.] employ no heat feedback, working only in vapor phase, and so they perform poorly in cool weather.)

A candle, obviously, puts out little power, and its little yellow flame is vulnerable to wind. Blow on it, and it goes out. Place something metal like a spoon over a candle flame, and the candle smokes and quickly coats the metal with soot.

Everything about a candle shows inefficient conversion of fuel to heat. The yellow flame color indicates that the fuel is mixing poorly with oxygen and producing way more carbon monoxide than necessary. The same goes for soot and smoke. Carbon monoxide, soot, smoke and other junk are PICs, or products of incomplete combustion. Just making a candle big, say giving it a huge wick (which this wax stove has, by the way), does nothing to improve its basic efficiency.

In contrast, fuels made of the smallest molecules, like propane and butane, vaporize readily, at room temperature. They spray out of a stove automatically, and then they easily, vigorously mix with air to create a hot, efficient flame. Kerosene and Coleman fuel can also be made to automatically mix with air in a stove burner head once the stove is pumped or primed. The most common stoves using kerosene or Coleman fuel use pumps to squirt the liquid fuel to its vigorous meeting with air. By the way, the older nonpumped, purely primed stove designs work well too and are much beloved by many. The last couple decades have seen a great variety of simple self-pressurizing alcohol stoves emerge. These, too, develop a bit of pressure and jet out their burning fuel for a dynamic mixing with oxygen.

How wax rates as a fuel
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Wax is a fuel that contains about 18,000 to 19,000 Btus of chemical energy per pound. That’s essentially the same as the energy density of kerosene, Coleman® fuel, butane, propane, and similar petroleum-derived fuels. That’s because all these fuels, including wax, belong to a group of chemicals called alkanes. The main difference between them is the sizes of their molecules. For example, propane has 3 carbon atoms per molecule, whereas wax molecules have some 20 to 40 carbons. Propane is made of small, easily-evaporated molecules, and thus propane needs to be stored in a strong container. If a propane canister were punctured, the fuel would boil away quickly, perhaps explosively. Wax, being on the high end of the range in terms of molecular size, doesn’t need a container because it melts and vaporizes only when heated to relatively high temperatures. Wax is quite safe and easily handled.

How about alcohol? Alcohol contains only about 12,000 Btus of energy per pound. Thus, alcohol contains only
the heating power of petroleum-derived fuels. For that reason, using alcohol in a stove leads to inordinately long waits for water to boil and a need to carry greater weights and amounts of fuel. Although alcohol needs to be stored in some type of container, its container doesn’t need to be nearly as strong as a container for propane or butane.

What else is going on in this stove?
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The stove’s stoke port performs three functions. (1) At starting, it provides an easy route for touching a lighter flame to the wick. (2) It lets you insert short sticks of wax into the stove while it’s operating or shortly afterward to prepare for the next operation. (3) The port lets you observe the level of fuel in the stove.



You light the wick through the stoke port. During the first few minutes of operation, most of the flame action is going on in the wick. After several minutes, a good deal of the wax puddle will be melted and hot enough so the puddle itself is burning and contributing much action.

The air tubing injects air right where the flame needs it. The air slots send a blast of air concentrically through the wick’s flame and bouncing off the inner body walls, wax puddle, and flame focuser.

The flame focuser does three things. First, it contains the flame so the wax vapor and oxygen spend more time swirling together. This holding action also helps direct heat toward the wax to help melt and vaporize it so that the stove is not inordinately dependent on the wick. Then, the flame is funneled through a relatively small port, helping the fuel and oxygen mix further, directing it toward the base of your cookware, and from there spreading up around to its edges.

The supercharger sits beside the primary stove unit. This side-by-side layout keeps the stove low and stable versus some other undermount-supercharger layout. The supercharger keeps all electrical components protected inside a self-contained unit. Its shell will shed cooking spills and a light rain.

The stove's advantages
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The stove is cheap (but not easy) to make, is more powerful than alcohol stoves but not as powerful as traditional expensive commercial stoves, is heavier than many commercial stoves but not all, performs well in inclement weather (but then white-gas stoves do too), and performs somewhere in the middle of the pack in many other ways. Use of wax fuel gives it a special distinction. Here’s my attempt to explain the wax stove’s advantages and then, further on down, its disadvantages and areas where it’s "a wash”.

Inexpensive to make. Like all do-it-yourself stove projects, this stove can (pun purposely inserted) be cheap to make. That is, assuming you already have lying around some special tools and materials. See Construction page. The electrical parts will only set you back about $9.

Solid fuel. You’ll especially like this stove if you’re married to the idea that solid fuel is better. Wax fuel needs nothing besides maybe a wrapper in the way of containment: no bulky containers to carry around and dispose of. Wax is nontoxic, inert, nonhazardous, nonvolatile, etc. Indeed, wax is commonly added to foods and used to make candy. Molten wax, if spilled, will cool quickly, flow only a short distance, be easily extinguished, and never explode. If you use beeswax rather than paraffin, you’ll be using a renewable resource. (Declare your independence from foreign oil and greenhouse-gas petrochemicals.) Beeswax has a high melting point of 143 °F, which could come in handy in hot weather.

Good performance in cold, windy weather. After priming and once going, the wax stove does well in cold weather. In contrast, canister stoves (using butane mixtures) perform poorly in cool temperatures, and most quit entirely somewhere between 0 and 32 ºF. The wax stove’s battery will function down to 0 ºF or lower, and anyway can easily be kept warm at around 95 ºF in a shirt pocket next to one’s body. The stove’s flame is well shielded from wind.

Disadvantages
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Difficult to make. Cutting, folding, and riveting the supercharger housing can be tricky. Hooking up the wiring requires (the barest of) electrical knowledge. Soldering the wiring and air tubing and brazing the tubes into the main stove body require special skill. Consider it a challenge. See the Construction page.

Slow heating. Tests under standardized conditions show that the stove takes 8½ minutes to bring a liter of water to a boil. That’s after the stove is given a few minutes to fully fire up. That boil time is roughly twice that of a typical commercial stove. That’s not bad, though, and it's good compared to boil times of 12 to 14 minutes using alcohol stoves. The wax stove will serve well for all but the largest meals or largest parties.

Burn hazard. All stoves present an obvious burn hazard. However, in addition, the wax stove produces a pool of hot, molten, possibly burning(!) wax that will cause a nasty burn if spilled on the skin. After use, the molten wax may take several minutes to cool to a relatively safe temperature. Alcohol stoves, however, pose a similar hazard, and butane/propane and white gas are used under pressure, thereby posing (slight) spray, flareup, and explosion hazards if used improperly.

Criteria where it's a wash
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Functional practicality. The wax stove is about as compact, easy to use, and sturdy as most commercial stoves. Most importantly, it is stable, owing to its moderate height of 5 inches and pot-support prongs and legs that span a generous 6 inches. Its fan is designed to run for thousands of hours, and its electrical components are probably about as robust (or finicky, your choice) as the plumbing, O rings, seals, pistons, springs, spindles, and other delicate parts in commercial stoves.

Weight. At about 14 ounces (dry of fuel, loaded with a battery), the wax stove is about in the middle of the pack of commercial backpacking stoves in terms of weight. The battery will power the stove for over 8 hours (conventional alkaline) or 20 hours (lithium), and thus extra batteries are probably not needed. Even so, an extra battery would weigh only 1 ounce (lithium) to 1½ ounces (conventional alkaline).

Thirst. Bringing a liter of water to a boil on this stove uses about 0.8 ounce of wax fuel. Typical commercial stoves do the same task using only 0.4 to 0.5 ounce of fuel. This weight disadvantage of wax is partly mitigated by the fact that wax needs essentially no container. You can pack wax fuel in just a paper or plastic wrapper rather than the metal canisters or tanks needed to pack conventional liquid fuel. So, even when factoring in the metal containers needed to hold petrochemical fuel, wax fuel weighs some 20% more. (Alcohol fuel including its containers, however, will weigh 50% more than wax to do the same heating job.) However, using wax fuel saves you from carrying bulky empty fuel containers on the return-to-civilization leg of your wilderness journey. So, all things considered, weight of fuel is a nonissue.

Cost of fuel. The stove’s fuel can be gathered from home candle drippings, trimmings, and leftovers and then molded into convenient fuel sticks (see Preparation). In such a case, your fuel is free. If you buy wax at say $5 per pound from the local hardware store, then running the wax stove costs about half as much as butane or propane, or about 4 times as much as powering a stove with Coleman
® fuel. Wax, if bought, therefore rates somewhere in the middle in terms of cost. Already accounted for in these figures are (1) rates of fuel consumption (see Thirst, above) and (2) minor cost of the wax stove’s battery power at about 25¢ per hour.

Another new wax-powered stove, from elsewhere
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Googling and browsing on YouTube around 2012 revealed this: A designer, Raymond Gatt, created a backpackable wax-powered stove that he called the G-Micro PSL.

Obviously, Gatt's design benefits from the use of inert, relatively safe, solid fuel — wax, of course — rather than liquid fuels with their various issues (toxicity, explosion potential, etc.). It's also far superior to our DIY project wax stove in many ways:

  • Elegant and well made

  • Extremely light: only 4 ounces

  • An efficient flame, with a relatively high blue-to-orange ratio

  • No electrical components and few moving parts


Most things in life involve compromises, however. The product requires 16 minutes to bring a pint of water to a boil. That time must be at least doubled to allow for bringing a quart or liter of water to a boil. Cited output is a mere 1,100 Btu/hour, which, if you track these things, is about a tenth that of practically any other stove. Also, the stove looks finicky in terms of cookware it will handle.

Still, I say “hurrah” for what appears to be a clever, lightweight, wax-powered stove appropriate for personal or small-party use. There was a website for this product with much interesting information and purchase ($99) details, but it has disappeared as of October 2014.


Disclaimer
Execute this project and use any resulting product solely at your own risk.

Disclaimer

This web page and the information therein have received no input, authorization, or endorsement from,
and the author has never had affiliation with, The Coleman Company Inc., or any other mentioned manufacturer.

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