[pct-l] Pop can stove

[elee@microsoft.com (Eric Lee (RAT))]

Steve wrote:
>
Please tell us about your design twists on #1 and #2 stoves. I tried
aluminum nuts and screws for the sealed chamber stove. For some reason the
JB Weld didn't stick and the nut fell inside. I'd really like a combo of
the two designs so it has a priming area in the middle, but high pressure.
So I'm thinking of a way to do that but not had anything I'm happy with
yet.
>

Thanks for the web links.  I didn't recognize the Henderson name but those
are exactly the plans I used to build the double-wall, open center stove
(#2).  So no variations there.

My pressurized stove is similar to the Photon stove, but it has some
differences.  The main difference, of course, is that it's build out of
juice cans.  I think maybe the Photon has a few details that are nicer than
mine, but regardless, I'll give a semi-detailed description of the
construction of my stove.  Sorry I don't have pictures!

*** Body construction
I started with two 5.5 oz juice cans.  I cut the top half off of both cans,
leaving the bottoms with walls about an inch high or so.  I cut several
vertical slits in the walls of one can, extending almost down to the start
of the curved bottom, but not quite..  I smeared the inside of the other,
non-slit can with JB Weld then carefully fitted the slit can inside so they
made a sealed container.  I didn't push the two can pieces all the way
together, so my design doesn't have a rim sticking up like the Photon stove
does.  The absence of a rim is important because of the placement of the
burner holes.

*** Filler hole
I bought a "T" nut and a bolt to match.  A "T" nut, for anyone who doesn't
know, is a piece of hardware that allows metal bolts to be screwed into a
piece of wood.  It's a metal cylinder with threads on the inside, and one
end has a flange sticking out perpendicularly with three little teeth
sticking down.  The "T" nut and bolt were bigger than what I needed, so I
used a grinder to cut them down to size.  I removed the majority of the
flange, leaving only a small lip and reduced the length of the threaded
cylinder.  For the bolt, I reduced its length and also cut off two sides of
the head, changing it from a hex shape to a square-ish shape.  I tried to
cut everything so that the bolt would screw into the nut one full turn, with
no wasted metal left over.

I then drilled a hole in the center of the top of the stove, exactly the
same diameter as the "T" nut.  I put a good amount of JB Weld around the
edge of the hole and placed the "T" nut in the hole, with the flange
embedded in the epoxy.

*** Burner holes
I poked a series of small holes around the outside part of the ridge on the
top of the stove.  I believe I made 16 holes in all (though I don't have the
stove sitting in front of me now).  I made the holes by clamping a small
wire brad in a pair of vicegrips so that the tip was just barely sticking
out enough to penetrate the aluminum, but no farther.  Then I could tap the
brad with a hammer and get small holes of a consistent size.

Because the burner holes were punched in the outside of the ridge that runs
around the top of the stove, they point more out than up.  This is important
because it gives more separation for the jets, allowing more air to mix with
the fuel.  (Or that's what I think, anyway.)  When lit, the jets start out
mostly horizontal, and then curl upwards a bit at their ends.

*** Operation
To light the stove, you have to warm up the alcohol inside so that fuel
vapor starts coming out the burner holes.  I made a little pan out of
heavy-duty aluminum foil.  I drip several drops of alcohol into the pan, set
the stove on the pan, and light the alcohol.  The stove usually lights
itself from the priming flame in about 20 seconds or so.

Priming is a tricky art, though - you'll probably use too much or too little
priming fuel until you get the hang of it.  Too little, and the priming
flame goes out without lighting the stove.  Too much, and the priming flame
boils the alcohol in the stove very vigorously, causing a roaring fireball.
If you get the fireball effect, just wait until the priming fuel runs out
and the stove will settle down.  As I noted before, I don't believe this
design can build up enough pressure to be hazardous.  Although, I'm not an
expert so trust me at your own risk.  :-)

*** Performance
I usually test my stove with exactly two tablespoons of alcohol and two cups
of cold tap water in a .9 liter titanium pot.  The tap water is probably
about 50 degrees or so, though I haven't measured the temperature.  I use a
windscreen completely around the pot, and I do my tests in my unheated
garage (meaning fairly cold, but no wind.)

Using this test setup, my Henderson stove will boil water in approximately 3
minutes and 45 seconds.  It burns for a total of approximately 4:45, meaning
that the boiling phase lasts only about a minute or so.  My pressurized
stove, on the other hand, will boil the water in 4:25, and burns for a total
time of nearly 7 minutes, with a boil length of nearly 2 and a half minutes.
(I'm quoting these figures from memory so I might be a little bit off, but
the gist of it is correct.)

*** Theory
I have some theories as to how and why the pressurized stove works the way
it does.  I might be all wet, but here they are anyway.  As I noted before,
I think the Henderson stove runs way too rich.  It dumps out far too much
vaporized fuel for the available air.  The pressurized stove doesn't have
this problem.  In addition, because the burner jets are pointed mostly
outward, each jet has a lot of air available all around it for more complete
burning.  I tried to build my pressurized design out of 12 oz pop cans, but
found that with the jets pointed outward, the ring of flame was actually
bigger than my pot, which was no good.  (Of course, if I used a bigger pot
then I'd be OK.)  The small juice cans allow the design to work for my pot.

I've tried several times to build a stove that has jets pointing upward and
inward, as well as outward, in order to get more flame area and thus a
faster boil time.  I haven't been satisfied with any of them.  In my
experience, trying to bunch the flame area together directly over the stove
leads to oxygen starvation and lowered efficiency.  But maybe I'm just doing
something wrong - if anyone can offer a more technical opinion on this, I'd
love to hear it.

*** Future plans
Steve mentioned trying to build a hybrid model.  That's an interesting
concept.  It should be possible to build an open center well, ala the
Henderson stove, but much smaller.  Rather than having notches at the bottom
of the inner wall, though, it would be sealed.  The filler hole would open
into the space between the outer and inner walls.  The burner holes would be
the same.  The advantage of this design is that it would probably be easier
to prime.  Hmmmm - I'll have to try it and see.

Eric