Finally, simple, efficient stove/screen, 15ml 2 cup boils with narrow pot, with SLX

[Harald Hope]

Following some invaluable advice from sgt rock, and using his ION stove as a template, and after much testing and debugging, the final bit worked out during a very well timed hot spell here, which forced some redoing on of the screen, here's a fairly easy to make 1/2 ounce slx narrow pot stove/screen setup

.

I had a bunch of wishlist items from using my stoves over the last few years, and also from seeing that sgt rock was able to achieve a wide pot 12ml boil using only SLX fuel, so I used that as my bar to aim towards, but with a narrow pot setup, I like narrow pots, they hold heat better when you eat, and you can fill them more without overflowing, and they pack up super nice, nothing against wide .9 or 1.3 liter pots, in fact I will do the next round of testing on a 1.3 l evernew wide, but this is about narrow pots and getting good efficiency without a lot of tricks.

My goals were basic:

1. at worst, 15 ml to boil 2 cups of water, using a narrow pot, period. My last versions not only got there, they did it in 14ml, but I'm not as confident that number will be consistent, so I'm calling it 15ml to be on the safe side.

2. SLX as fuel, ie, this setup does not depend for its efficiency on using high ethanol fuels like everclear or kleanstrip green.

3. No weird or hard to pack wind screens, like cones or whatever else. Screen should fit inside my pot, that is, without taking up much room. Screen should also not need any unusual ways of connecting the two ends. I use a flat felled joint, so to speak, ie, 1/2" of both ends folded over carefully then inserted into each other, that works very well and is basically idiot proof. It's also a big improvement over my previous screens, which I used 2 bolts/wing nuts to hold together.

4. stove needs no priming prior to placing pot over it, ie, light, put put on, and that's that. The ion stove is perfect for this, my previous testing stove was a modified inpointing penny stove, which is harder to light and has parts you can lose. I like the penny, it's fast, but it's not as efficient as the ion stove/screen setup.

5. As a subset of 1, an 8 ounce bottle, thin and light, carries 8 days worth of fuel, which means you have a very comfortable margin for 7 day trip. I wanted a slim bottle both because it's much stronger than a regular 12oz water bottle, and because I made a slight miscalculation when laying out the pockets for my myog pack, and they are just a touch too narrow, making holding a normal 32 oz bottle and a 12 to 16 oz fuel bottle a bit tight. 8 oz bottles are either very thin or very narrow in general, and are very strong since they don't have a big surface area compared to the volume.

The key area, as noted by sgt ock, is the screen, and particularly, the actual area of the bottom inlets, it needs to be about 2.5 square inches by my current rough calculations to get the stove cool enough to not over-vaporize the fuel, which is always teh cause of inefficient burns, at least so far in my tests.

Besides sgt rock, who designed this basic stove, though his focus and testing were all on the wide .9 l evernew, which is significantly different in terms of the stove / screen combination, and who deserves most of the credit for his very good testing initially, a few key comments let slip during other stove conversations, particularly jon fong, who noted that he did not smell any slx when testing his flat cat gear stoves, which are by the way also efficient, though their screen didn't meet my design specs, but I do admit to testing on a few copies of those during htis process. Anyway, that casual comment finally clicked in my head, and I realized that two things always happen when you fail to get efficient burns:

1. The fuel vaporizes more quickly than it can fully combust as a gas, this results in unburned or partially burned fuel gases, which results in an acrid, eye burning sensation, and a strong smell. So that smell/eye burn, besides being really bad for you, means the stove is not efficient. Jon, I believe, had not noted this smell issue because his stoves/screen setups are very efficient, which it took me a while to connect the dots re the core point.

2. This one is a bit more subtle, and I only noticed it while holding a digital testing thermoter in the various configurations as I tested them, and is almost counterintuitive, until you think about it more: an inefficient stove can result in a much hotter outflow of air rising from the screen and stove / pot setup. This is I believe because the fuel keeps igniting as it rises, but that ignition results in very little actual heating of the pot because it's happening after the gas starts to rise around the edge of the pot.

It took a fair amount of testing to finally connect these two things, and I think it was only because air temp rose to the low 80s during my last weeks of testing, which makes these problems much more visible and much worse, that I finally connected these last dots, and figure out how to work around it.

I don't have a screen how to made yet, which is probably for the best since I tested so many hole patterns before finally getting to the goal of max 15ml slx to boil 2 cups 70F water.

I was particularly struck by sgt rock's use of slx in reaching his 12ml boils, because it has a signfificantly lower energy content than high ethanol alcohols like kleanstrip green, which means his efficiency is even crazier than the average test that people report here using high ethanol fuels. I was also previously using the m ore expensive and harder to find klean strip green fuel to get to the 20 to 17.5 ml efficiency with a penny stove, so the increase in efficiency using the ion / screen method is really quite large.

All measurements are via digital thermometer, home made and well calibrated graduated cylinder for fuel measuring. I've logged most of them, having forgotten only what proved to be the true key, air temp around stove, outside air that is. Wish I'd logged that as well, I'd have even better data, I did use barometric pressure and humidity but saw no real pattern there, I believe the actual air temp is the key, at least that's my current working assumption.

The design here can be further optimized by at least 10% to maybe 15% because when I tested the screen prior to adding the slot holes I got stunningly good results by lifting it with paper clips up 3/16" all around, so I know there is further room for improvement. That boiled to a full 212.3 in about 8 minutes, maybe 9, and boiled for at least 2 minutes, maybe more, using 15ml. But that design is not real world practical so I decided to just use what I have now while still working on it now and then. I believe all further work will focus on the screen. SLX is also a lot cheaper, and you can buy it in gallons, and reflects very well the fuel you could expect to actually locate if you need to buy it somewhere, or at a trail town, so I think it's a very good source. Higher ethanol fuels will just be a bit more efficient, along with a bit of sooting. Pure methanol like yellow heet should probably require about 1ml more fuel, give or take, you'd have to test it to see.

As to why I found it a good idea to do this much testing etc, well, sometimes in life circumstances out of our control keep us from spending as much time hiking/backpacking as we'd like, this past year unfortunately was one such time for me. Besides, playing with stoves is fun, I've learned all about alcohol ethanol/methanol efficiency in this process as well.

This stove/screen is ridiculously easy to use, fill, light, and setup, it's really almost absurd, in terms of fiddle factor, I made it 4x easier than my earlier penny stove versions. My hats off again to sgt rock, he did all the heavy lifting on this in terms of serious testing, if there is 'an answer' to the question, I'd say the ion is it, more or less. Because it's easy to use and make, hopefully we can use the 1/2 slx as a base amount of fuel when comparing other fuel and stove types, in the past people have used as much as one ounce of fuel when comparing alcohol to other fuel types, which is not fair since it's not accurate in terms of what efficiency you can actually get without a lot of work. And 1 oz alcohol weighs .8 oz, remember, also a key factor in comparing fuels and stoves.

Do of course heed all regional fire warnings and stove restrictions, as being talked about in another thread, unfortunately, the prediction I made a few years back here re alcohol stoves and them getting banned in fire season due to bad practices and bad designs has proved to be sadly true, which is another reason by the way I did not want a stove that does not use a pot stand, just another spot for someone not too aware to fail and start the next fire.

Weight of stand, screen, and stove, heat reflector disk, 44 grams, I almost forgot. That's for a snowpeak 900 ml setup. I've tested this a few times on a 600 ml 10cm pot I have, steel, very light, lighter than ti in fact, and it is not quite as efficient, but does boil at 15ml fairly comfortably. I don't know about narrower pots, I'm guessing they won't work super well, but they might, you'd just lose efficiency is my guess.

Thanks to all the stove people here, I read the threads though I don't usually comment on them, so I've learned a lot over the years seeing how other people do it, and also in seeing how I want it to turn out re screen/stove/stand. This is about as neat, clean, and simple as I can get it, everything is very durable, nothing flimsy or awkward to use or store or carry, everything fits in the pot, along with all your other kitchen stuff, small cup, spoon, tea spices etc.

The space of the 900ml tall pot packed is why I didn't want to go with a wide pot, plus it holds the food heat so well in the cold, having a smaller top surface area to radiate heat out while the body is surrounded by the cozy as I eat, so I really wanted to see if I could get a good easy to do narrow pot setup that equals or betters most of the stuff out there.

I'm putting this in gear forum because I think this kind of stove thing involves a lot of general interest areas, re efficiency, methods, screen types, stove types, etc.

Happy trails, I'll have the rest of the how tos done in a while, one for a very well constructed cozy, one for the screen, and maybe one more for another pot stand using the ace hardware materials. All materials here by the way come from Ace Hardware, 4" aluminum flashing, 0.055" music wire, aluminum tubes, drill bits for the holes, plus the two 7.5" soda cans.

By the way, if you see on the wood, those black rings? those are from stoves that burn openly without using a base, that's how hot it gets under the stove, for example a tea light candle stove, or any stove that burns the fuel basically openly in the stove. That is why you should always use a stove base, and include it in the design of the system, along with the bottom heat reflector of course.

[USA Duane Hall]

I'm about to leave for work, so had no time to follow your links. What was the starting temp for your water? Was it realistic? At least for Sierra water sources or tap water?
Duane

[Harald Hope]

The links starting the first posting have further links to explain water temps. My tap water if I let it run comes in between 68, when it's cold, and 71, when it's warm. However, fuel consumption, as I learned when researching the actual physics of heating water in terms of joules required per pound per degree F, is a direct and linear function, that is, if 15ml heats water to a boil from 70, that means it raised the temp 140 or so degrees, plus the time it boiled, during which time the water temp would continue to increase at about the same rate if you started with colder water, that is, it's irrelevant what the starting temp of water is as long as it's liquid, x ml will still raise that water y degrees F.

So the starting temp of the water is not actually that important, what is important is how much fuel is required to raise the water x degrees. This would not be different on any stove from what I can see, a canister will require x grams to raise the water y degrees, so will a whisperlite, so will any alcohol stove.

If you say 14ml to raise water 140 degrees, that is precisely the result all stoves will get relative to each stove's fuel and construction efficiencies. This alcohol fuel efficiencies article has the raw data, which I need to update now that I got a consistent 15ml boil, I was a bit conservative there. The appendixes in that article cover altitude, water temp, etc.

One of the real eye opening things for me researching that article was that if you're going to cook in cold with cold water, you're well advised to warm up the water with body heat before you start cooking, 1 pint in your backpack next to your back for example, because it's always a function of I think roughly 1kj to raise 1 pound of water 1 degree F. It's a constant, that is. So a stove that gives you a starting temp 70F 15 or 14ml raise in water temp of 140F will give you that increase no matter what starting temp you have, except for ice or snow of course. This is why, unlike many things in our world, stove efficiency, ml/grams to boil 2 cups from 70F, is actually a pretty solid metric. Had I taken more college chemistry and physics I wouldn't have had to relearn such elementary principles but better late than never I guess.

The one significant variable in the sgt rock ion testing/design process is using a standard fuel source, SLX, to achieve these efficiencies, something I can assure you I did not believe was possible until I actually got a solid strong boil with 14ml myself.

The reason this type of design works so efficiently is that somehow it avoids that flareup stage most stoves go through, and, once it blooms, it maintains a fairly steady burn rate until a little bit before it dies, maybe 10 to 15 seconds.

The one thing I have not yet tested is burns in the cold, ie, in winter, it's possible a wind / heat screen with fewer holes would give optimal burns in that scenario, this system functions very much like a carburetor in a car, except you only have control over the air flow. I know however that when I was doing my initial testing of the screen design in spring, during cooler times, I was getting 15ml boils using only about half the area of air inlets, which is actually why I put off putting this data online, it took me a while to figure out what had changed. So my initial guess is that a summer and winter screen/heatshield would probably yield best results, this can be easily tested by simply sticking aluminum foil over half the holes to see what happens in the cold.

[Harald Hope]

To put this a bit more empirically, now that it is summer, with strong sun, if you are hiking in the mountains and come across a cold snow melt stream and take a quart of water for cooking, if you stick that quart bottle or bag directly in the strong sun before boiling it, top of your back, whatever, you will save fuel. If you can get it to 80F or 85F, which is not at at all unrealistic for strong sun, you may get 13ml boils, for example.

Or, if you are hiking, and grab a pint instead and stick it somewhere on your pack where it gets direct sun, it should be much warmer by the time you cook dinner. As with fuel, solar power works pretty well in a predictable way in terms of heating quantity x some number of degrees y per hour. Hint: cover the bottle with black fabric or plastic, see how hot it gets over 10 min, in fact, I will run some solar heating tests now that I mention it, ie, black surrounding bottle, bottle on black patch of fabric, bottle on stone/brick. I wouldn't be at all surprised if a black wrapped 1 pint bottle can be raised to over 100F using solar power, the question is, how long, I'll get back to you on that.

If you take 14 ml and divide 140 degrees F by it, you get the convenient 1 ml heats 10 F, which is roughly what it will be. Drop water temp 20F, to 50F, and you need 16 ml, give or take. It's not fully linear but it's close enough.

[USA Duane Hall]

Bump. I hope you get more responses, good info. When I melt snow in the winter for water, I set my bottles on something, bark, rocks etc. to save on fuel to melt snow, good point to do something about preheating water if possible. Something I think about when out. I'll follow the links tonight as I'm on my afternoon break. I prefer alcohol on vacations over gas now. Gas and kerosene I use for my old, collectible stove use on weekends.
Duane

[robert van putten]

This looks like a very easy to make stove!
It is not a double wall stove or filled with fiberglass or anything like that at all?
It looks like just an open burner with a relatively small central opening and some holes around the edge.
I might have to make one of these and give it a try.

I agree about the wind screen being the most important part of any alcohol stove setup.
The 2.5 sq. inch inlet area is an interesting bit of data. I wonder how that much open area would work out in the field with moving air, and how it compares to my current setups ( some of which can overheat a Pepsi can stove in still air ).

I'd like to see how the wind screen was made.

[Harald Hope]

Thanks for the mild nudge, I just tested solar water heating, it's hot today here. Yes, I'm avoiding some work projects, heh.

Temp in sun, 100, temp in shade, 90.

I was unable to take the temp of the stone /brick surface I used, but it's hot, almost too hot to touch, much as darker stones would be in nature.

Water temp to start, 75F. I used two 1 pint bottles, both lying on their sides on a board, to emulate roughly having your bottle on dirt. One I wrapped in a black plastic bag, to see how that changed it.

I checked temp 3 times, at 15, 30, and 45 minutes.

The black plastic wrapped was much slower to warm, to my surprise. I believe you'd need a painted black or very dark bottle to gain advantage there, a covering does not work, in fact, it would keep the bottle cooler in direct sun than having the bottle uncovered. Who would have thunk?

On the clear bottle, it raised to 85F in 15 minutes, 92 in 30, and about 97 in 45, ie, it looks like the water temp was approaching air temp in sun. For the last 15 minutes I placed the bottle that had been in the black plastic bag directly on the darkish bricks, hot to touch, and it warmed right up, as you'd expect. So my conclusion is, find a hot rock and put your water flat on that, an evernew / platy bag will probably warm more quickly I believe. 15 to 20 minutes should do it unless the air is very cold, maybe you can check that if you hit a cold day in the mountains that is very sunny?

So you'd get that pint up to 85 very quickly in the direct sun, unless the air is very cold I assume.

[Harald Hope]

robert van putten, I will update my requirements list to include no fancy methods or weird ingredients in the stove, like fiberglass, double walls, etc. Sgt rock went through every possible iteration of the designs you listed, and discovered, almost by chance, that none of that was required, the stove is precisely as it appears, nothing extra is required, this is part of the brilliance of the design. It wasn't an accident though, I believe the development occurred over a period of time on whiteblaze forums, so it's not a random thing in general, and sgt rock did excellent testing, he spent a long time on this back in 2005/2006.

Your question re the air holes is exactly on point, in fact, all my earlier tests used only half the screen with holes, and only when outside air temp rose to 80 and over did I realize that it needed a lot more air.

Basically this is not really a wind screen in my opinion, it's actually a cylinder wall, sort of, like in a piston engine. However, it's not hard to shield a stove from moving air outside the screen, a tent, tarp, a big tree, a big rock, a foam pad stuck up with sticks, there's a lot of options. And, to be fair, during earlier trips on windy ridges with a penny stove and a 6" screen with only holes on the non windward side, performance was still dismal, from what I can see, as soon as you get moving air crossing the top of the stove screen that's about it, end of story.

I've tested some earlier versions of these very short screens by the way in a creek floor with a decent breeze coming down stream, and I found that as long as the stove/screen was behind a rock it was just fine. But that was not this type of screen.

Once I lock down the actual air hole size and dimensions, I will play with other screen things to see what matters and what does not.

Because the by far best results I got was when I lifted the entire screen up 3/16 I realized that two things happen with small holes, one, simply not enough area, second, turbulent air through the holes, vs a thin sheet of air entering all around the edges, that's why I decided to go with the joined 1/4" holes, that emulates that a bit better. And that worked right away, not quite as efficient as the clean gap at the bottom all around, but pretty well.

I have still not done any work on a simmer ring, my first try will cut a hole in the spare can top I have left from the stove creation, and see if that works.

I took this last night, it's hard getting a shot of the pattern but this shows pretty well why it's so efficient, notice that the flame edges barely go up the sides of the pot? Opening the screen of course significantly changes how it burns, but this gives an idea.

[Harald Hope]

"I'd like to see how the wind screen was made."

I could have sworn I took pictures of the screen construction but I didn't, so I'm going to make a new one for a 750 pot i tested with and use that for the how to, maybe also one for the 1.3 l evernew I have. Ace hardware sells 4" aluminum flashing, that's the way to go, it's very strong and allows joining the edges with the flat felled connection, that's what I call it anyway since it's the same as you'd do on a felled seam in sewing.

The 1/4" gap between stove and screen is also from sgt rock, he tested this extensively and found that was the sweet point.

As work / time permits over this week I'll do a cozy and screen how to. Just check back to the linked how to page and the top links that current say it's not done will be live once I get those done. I'll probably do the cozy first because that's easy.

[robert van putten]

Heh, my testing is a little more basic –

I realized my wind screen didn't have sufficient airflow when it my stove setup left scorch marks on my wooden porch. And that was with a foil covered cardboard disc below the stove for a pad.

So, I punched enough holes to eliminate scorching the surface the stove was placed on. I wonder what the actual area is? I think I'll have to count holes.

[Harald Hope]

If you use a 1/4" hole punch, it's about 52 holes, give or take a few.

That's why I connected the holes instead, that gives smoother airflow I believe, 1/4" holes are going to create little turbulent streams of air, not one smooth river, if that makes sense.

This is a very new discovery for me so I don't want to claim it's right, it just works and solved the problem.

area of circle: pi r ^2, ie, pi times half diameter, or 2.5 / ( 0.125" x 0.125" x 3.14) = number of holes required. 0.125" x 0.125" x 3.14 = 0.05 sq in., so you need about 50 of those.

Two joined 1/4 inch holes centered 1/2 apart forms a space about 11/16×1/4 = 0.17 sq/in, so you need about 15 of those.

Using 5/16" gives much more area, but it's hard to find punches that make that size hole, I was drilling those out in earlier screens however, but you can also punch carefully to just make the 1/4 a bit bigger too.

[Harald Hope]

Added how to build your own alcohol (or esbit) stove wind / heat screen. This uses a creased folded edge connector between the sides, stores in pot, is very strong and solid, yet very easy to store, pack, and assemble, all key requirements for the system when I first thought it up.

That shows two, one for 900 ml tall pot, and one for a 1.3 L evernew wide pot.

Full pics and construction directions are in the page I linked to.

Since the screen is such an important part of the stove, this was a missing piece, I had just forgotten to take pictures on the previous one.

I don't have a good punch to make those type of holes, or rather I didn't at the time, so they are a bit ugly because I was playing with the air flow, starting small and expanding the air slots until I got the best efficiency, ie, equal to having no holes and the screen raised 1/8 to 3/16" inch, or, for a wide pot screen, about 3.5" of air holes.

Note that using a 1.5" or so high strip of thick aluminum to put in front of the windward facing holes seems to protect them enough, though of course, you want your alcohol stoves out of direct wind because any moving air is going to significantly compromise efficiency, whether over top or bottom of stove.

[Bob Gross]

What is the thickness of your aluminum?

–B.G.–

[Harald Hope]

Ace hardware sells many widths of aluminum flashing by the foot, this is the 4", so it's the thickness of aluminum flashing minus a touch because I sand it down to get rid of the plastic coating.

I don't have a micrometer so I can't guess the gauge, but it's thick enough to hold a curve, to hold the bent over slotted ends, at least 2x thicker than stove pan aluminum. And it's a stiffer grade of aluminum I believe, though I'm not certain about that.

I really like this material for screens, it holds the roundness, it has enough spring in it to not make storing inside the pot an issue, and it holds the circle without anything more than the folded over ends.

I used to use bolts/wing nuts to hold the edges together, which is rock solid, but I knew I'd end up losing those eventually, even if I carried spares, so this was my version 2.0, simple, idiot proof, solid.

thicker aluminum also lets you form a better seal against the ground, ie, it doesn't bend in or out. You can also skip the pot stand by punching in holes and using ti stakes or something to hold the pot if you are so inclined, though I tested that method and found the pot stand a lot easier to use, and lighter too, at 7 gm.

By the way, I tested the hole size by finding first the optimal efficiency when I was testing 4 cup boils with a penny stove (8 minutes or so, 30ml), and I just increased the slotted size of the holes until the efficiency matched the optimal I had gotten with no holes and the screen raised 1/8 or so inch off the ground with paper clips. I would guess every stove type has a sweet spot for air hole area and most efficient boil, but it takes a lot of work to test each variable so I felt this was good enough. What is shown here will yield consistent 15ml 2 cup boils at 70F water temp, and 30ml 4 cup boils, which is good enough for me, and as good as any other stove system I am aware of using SLX as fuel.

I have a suspicion that colder weather would benefit with smaller holes, less air hole area because of the colder air entering the burn chamber.

[Author]

Posts