Introduction: Build a Low Tech CFV Stove
The most popular alcohol fuel based DIY backpacking and camping stoves utilize aluminum beverage cans to provide a laminar or pressurized vapor fed flame. However, they have limited fuel storage and thermal generating capacity, durability and adaptability. Most require an extraneous pot stand or holder and have additional construction, storage, startup and usage needs.
While high tech CFV (Capillary Force Vaporizer) burners represent an advance in technology they are unfortunately too high tech to be offered to DIY stove builders as a solution.
Fortunately a low tech CFV stove can overcome the limitations and disadvantages of both the high tech CFV stove and the alcohol fueled beverage can stove by using low temperature evaporation of alcohol fuel fed to the flame at the perimeter by an encompassing pad wick made of almost any absorbent material. In fact, it might (except for the flame) be mor properly called a Capillary Force Evaporator stove.
While being easy to construct and simple to use it can be made to meet virtually any fuel storage, thermal generation capacity or durability requirement in addition to making replenishment of fuel during operation possible and startup hassles a thing of the past.
A basic low tech DIY CFV stove can be built quickly in an emergency to meet most any fuel or thermal need but with more time and materials it can be adapted and built to accommodate virtually any backpacking, camping or other need.
Step 1: Gather the Materials and Tools
You'll need some aluminum sheet. This can come from most any source to meet whatever requirements you have for durability.
To construct the test stoves I used aluminum flashing, since it is about the minimum thickness that can be secured with 1/8" pop rivets. Brass, stainless or aluminum machine screws can be used in place of aluminum or stainless pop rivets. I used aluminum pop rivets since I will most likely not be taking the stove apart. In the event I do need to take it apart, aluminum rivets are the easiest to remove. You'll need a pop rivet gun if you go with pop rivets as well as 1/8" by 1/8" pop rivets plus the length of the thickness of the stove you want to build - usually 1/4" long rivets will allow adequate space for fuel. Be careful not to over compress them as I have done.
Next you'll need some aluminum door screen. The dimensions of both the aluminum sheet and the aluminum screen as well as the number of pieces is the same. You will dimensions big enough to cut circles about 1/16" to 1/8" greater than the pot you plan to boil water in. For a frying pan you will want dimensions that are half the diameter of the frying pan. I used kitchen shears to cut both the screen and sheet.
You will need a straight edge,ruler,scribing compass,a power drill and 1/8" bit and some absorbent material. I used regular kitchen paper towels but cotton or fiberglass cloth would certainly be more durability. The choice of absorbent material is yours with the exception of synthetics and plastics, which melt. Update: I've now tried fiberglass angle hair, mat and asbestos cloth and I'm not really happy with either. The hair is extremely absorbent and for that reason the fuel can not be added before the pot is placed on the stove else half will spill. The fibers of the mat are too straight and stiff and will come loose and break into splinters. I have tried to seal them on the edges and sides with a blow torch but the glass can break and expose the fibers again. Asbestos cloth is best so I imagine fiberglass cloth would work at least as well.
I recommend alcohol fuel only since oil, mineral spirits and kerosene need to be heated inside a closed vessel until they become pressurized or they will burn incompletely and produce excessive carbon monoxide and soot.
Step 2: Decide How Big to Make Your Stove
You want the diameter or perimeter of your stove to be a bit larger than the pot you are going to use. Your pot does not have to be round either. You can use a tea tin that is square if you want and only cut the stove slightly larger so your tea tin pot will sit on top of the stove with minimum protrusion of the stove and maximum setback of 1/8" of the pot from the edge of the stove to give not more than 1/8" clearance.
The first stoves I built were for 12 oz and 24 oz soda cans. I used the outside dimensions of the bottom foot since there is a taper from the foot of the cans running up to the sides for the flame to follow. Since these dimensions were 2-3/16" and 2-1/2" I added 1/8" to get a cutting diameter of 2-5/16" for the 12 oz can and 2-5/8" for the 24 oz. can.
For this instructable I decided to build a stove able to boil 1 liter of water in a 404 x 700 bean can and to boil 2 liters of water in a 602 x 602 coffee can
To meet these specifications i chose a diameter of 4-1/4" plus 1/8" to support the bean can using only a flat surface or the coffee can and to support the coffee can using the bean can for support.
Thickness, or Height
I used a bit of math and chemistry to help guide me in selecting the thickness or height but once you have calculated the basic requirements you can use trail and error to fine tune.
To boil 2 liters or 4.41 lbs of water from 60 F I needed to add 152F to raise the temp to 212F.
I BTU is required to raise the temperature of 1 lb of water, 1 deg. F so I needed 4.41 lbs times 152F or 670.2 BTU.
91% Isopropyl alcohol contains 20.73 BTU/g, so I needed 32.33 grams of Isopropyl fuel to bring 2 liters of water to a boil from 60F.
As life would have it there are thermal and evaporative losses and in actual practice it takes 50 to 60 grams of fuel to boil 2 liters of water.
If I use a container to store the stove to retain any unused fuel then I can make the stove thick enough to hold excess fuel. However this has a tendency even with the weight of the pot to increase the burn rate so I am testing another method which is to use the top of the lid of the bottom can as a small fuel reservoir which the stove can tap via a string running through the center of the burner. In practice this helps but if the reservoir fuel gets hot enough to escape under pressure then if will increase the burn rate further and start producing yellow flames. This is somewhat less efficient but boils the water nonetheless.
Since isopropyl weighs 0.785 grams per cubic centimeter I'll need to make room for at least 4.66 cubic inches of Isopropyl fuel to boil 2 liters of water.
For a stove diameter of 4.375 I'll need a height of 0.31" or 3/8" thickness or height.
Since paper towel is only about 40 percent efficient I've used a piece of string to feed the burner from the reservoir in the top of the bottom of the bean can for the 2 liter boil.
Therefore, I'll need a 3/8" thick stack of compressed paper towels 4-3/8" in diameter .
Step 3: Cutting and Assembly
Next I used a layout compass to scribe a 4-3/8" diameter circle in two pieces of aluminum sheet for the top and bottom halves of the stove and then cut out the disks and drilled a 1/8" hole in the center
I then placed two uncut pieces of screen on each end of the stack of cut paper towel and then placed the two aluminum disks over the screen in a two slices of bread sandwich fashion.
Next I used a drywall screw to temporarily hold the assembly together by screwing it through the center of the assembly while compressed.
I then drilled three 1/8" holes equidistant from each other, about an inch from the center and installed the pop rivets through them to hold the assembly permanently together.
Last, I trimmed the screen to match the diameter of the aluminum disks and paper towel, removed the drywall screw from the center hole and inserted a one inch piece of string in its place.
Step 4: Usage
The 2 liter boil
An empty bean can is turned upside down and filled about halfway up the ridge with Isopropyl. If you want a larger reservoir then you can use a rubber mallet to bash the end of the can into a deeper depression than is provided by just the ridge of the bean can. I was able to fill this reservoir with 40 grams of fuel after the first stove turned out to hold only 20 grams of fuel.
Now the stove is either soaked in a dish filled with 91% isopropyl alcohol or if you have found a leak proof container for the stove the container is filled with isopropyl and the stove soaked in it.
Next the stove is placed on top of the bean can so the string can soak up the fuel as it is burned.
Next the coffee can is filled with 2 liters of water and the outside thoroughly dried. failure to dry the outside may contaminate the fuel.
Next the coffee can is centered on top of the stove.
The bottom of the coffee can will extend out over the stove and the bean can by about an inch.
1 liter boil
The coffee can is placed upside down, the stove is loaded with fuel, centered on top of the coffee can and the bean can filled with 1 liter of water and centered on top of the stove.