Oct 3, 2013 at 9:56 pm #1308340
I have a traditional self-made aluminum flashing windscreen for my SuperCat stove, and on a recent windy morning I could not keep the stove lit due to wind. My alum flashing windscreen has relatively small 1/4" holes along the base of the screen, about 5/16 from bottom edge to center of the 37 vent holes…in other words, relatively small holes that are pretty close to the base of the windscreen. I'm thinking that should be reasonably effective in breaking up direct wind, and errs on the side of holes that are too small for a SuperCat, if anything. (SuperCats need loads of air.) The screen is 2-1/4" tall so it nests inside a Grease Pot.
I've been pondering my windscreen problem.
I was reading some of Zelph's old posts. One in particular, where he noticed how screen door mesh really cut down on a brisk wind. Has anyone tried just plain mesh for a windscreen…if so, what happens? I imagine you'd lose a lot of the reflective nature of the windscreen with all mesh, but does an all-mesh windscreen tame the wind, with its much much smaller holes?
I've also considered somehow affixing a strip of screen just around the holes of my alum flash windscreen, to baffle the air even further.
I've also considered making a plain, short (1" high), unperforated inner ring of flashing, sitting perhaps a quarter inch inside my existing windscreen, whose job it is to simply break up any direct wind coming through the 1/4" holes.
Another fix, which I don't want to contemplate, is making a taller windscreen that doesn't nest.
Ideas? Are my descriptions clear, do I need to post photos?Oct 3, 2013 at 10:13 pm #2030712
Can you estimate the distance from the top lip of the cook pot to the windscreen?
–B.G.–Oct 3, 2013 at 10:17 pm #2030714
When pot on SuperCat, there's 2" of pot visible above the top of the screen. If I make a taller screen, no more nesting. :-(
Are you thinking wind coming in over the top of the screen, and blowing down, was the problem? That would indicate cinching up the windscreen more tightly around the stove. SuperCats are "big breathers" so you don't get the option of cinching it up too tight, but typically I have 1/4" between the screen and the pot, all the way around. Not good?Oct 3, 2013 at 10:31 pm #2030722
I was a little surprised to see this.
Some people use a short windscreen like this, and it keeps the wind off the flame area and that's all.
I always use a tall windscreen, and it comes up to the height of the cook pot on the burner. That keeps the wind off the flame area, and it also keeps the wind off the cook pot. That seems to be much more efficient, heat-wise. It holds more exhaust heat around the cook pot.
The space between the top of the cook pot and the top of the tall windscreen varies, but it is generally equal to the size of my index finger or maybe one centimeter.
–B.G.–Oct 3, 2013 at 11:00 pm #2030727
I'm certain you're correct, Bob, a taller screen would add efficiency. I cut it as you see it so it would nest inside the pot, under the pot's strainer. It's maximum height under that constraint. As is, I'm not complaining about efficiency, the 30 hole SuperCat is giving me 4-min boils for two cups of water on 3/4 oz alcohol, with flameout at 6 min. Part of the reason is the Grease Pot's wide circumference…the flame must dwell along the bottom of the pot a long time before going up the sides.* Other stoves can do better, efficiency wise, but I'm not complaining. I AM complaining about flame stability in wind, and keeping it lit.
One other possibility I'm considering is making myself a "SimmerCat," same stove with only one (top) row of holes. Such a stove would need less air, allowing a tighter windscreen around the circumference. Also the vertical distance between the holes of the windscreen and the holes of the stove would be greater. And, on top of that, I've been reading that slower stoves are generally more efficient. So maybe I need to make myself a new cat stove to get a more stable and dependable flame.
*Off topic, one "problem" of the grease pot, IMO, is that it's too tall, with too much capacity for solo. It's 4-1/2 cups. It would be nice to be able to re-roll the lip of a grease pot about an inch lower, to get a squat, wide pot. Can't beat the weight, though. These grease pots are lighter (and more conductive) than Ti. (Not to mention, $7 at Kmart).Oct 3, 2013 at 11:08 pm #2030729
"These grease pots are lighter (and more conductive) than Ti."
So far, that hasn't gotten you very far.
–B.G.–Oct 3, 2013 at 11:10 pm #2030730
What do you mean, Bob? The pot (and stove and screen) have functioned excellently for many dozens of meals. I'm complaining about one windy morning I couldn't keep it lit, which I attribute to the windscreen and stove (and perhaps my technique), rather than the pot. I'd say the pot's gotten me pretty durn far. But the pot is a side issue; I'm trying to fix my flameout problem, with an eye toward more windy days in the future. But let's not go so far as to assume abject failure, here.
(I should add, I DID get my breakfast that windy morning. I used my CCF sitpad as a windbreak, and all was well.)Oct 3, 2013 at 11:19 pm #2030732
I use a 1.8 ounce titanium bowl that holds 20 fluid ounces. It is thin enough that thermal conductivity is not a problem, but I use it primarily over an Esbit burner. I have four or five alcohol burners as well, but they don't hold much advantage for me. The 12-10 alcohol burner seems the best of the bunch.
If your windscreen is aluminum flashing metal, then there is a break in the circle. Try opening up the break so that there is a gap to let in more air. Either that will make it better or make it worse. That will tell you if it is an input air problem.
–B.G.–Oct 4, 2013 at 12:18 am #2030741
If a taller screen is needed (probably the case), then a nesting one can still be made. Just make it in 2 pieces – i.e. add an extension top half which you can sit on the existing screen. Join them by cutting matching slots/castellations in each piece. Sounds like you will only need the top piece occasionally.Oct 4, 2013 at 5:13 am #2030757
I'm not sure about the singular focus on the height of your windscreen as the source of the problem. Maybe, but what about the simple possibility that the intake holes facing the wind are letting too much in? I'd suggest trying the windscreen upside down, with the side facing the wind flat to the ground and using some small twigs or pebbles to prop open the bottom around the sides and back. Or, just use a bit of aluminum foil to block the holes facing the wind. If that works then you can investigate a way to block some of the holes more permanently.Oct 4, 2013 at 5:42 am #2030763
I would guess it was more windy than normal. Just one of those things that happen when out in the field. Use your pack and your body to block the wind on those exceptionally windy days.Oct 4, 2013 at 8:35 am #2030796
Hi Dan! Thanks for weighing in, hoped you would…what's the scoop on simply using a fine mesh screen as a windscreen?
PS: Recently ordered my first Zelph stove, a SimmerLyte (or whatever you are calling it), very excited. Thanks for the special BPL offer on those.Oct 4, 2013 at 9:29 am #2030809
I bet is isn't the height of the windscreen.
I use a cone type windscreen. With the first one I made, I could blow out the stove inside the cone. The next iteration used paper-hole-punch sized holes and is much more difficult to blow out.
You could try to attach a strip of screen around your windscreen with a paper clip or maybe a run of shock cord/large rubberband.Oct 4, 2013 at 9:43 am #2030814
Dan also made a video showing how raising the bottom of the windscreen 1/2 to 3/4 of an inch off of the "ground" aided in flame stability. This would also allow for more air flow to feed the stove's flame.
Is it at all possible that your stove was being starved for air despite the windy conditions?
Newton ;-)Oct 4, 2013 at 11:37 am #2030850
@dwambaughLocale: Pacific Northwest
I would build another screen with no holes on one side and experiment with holes on the other. That way you can orient it to suit the wind direction. Or just add foil tape to yours.
The physics of alcohol stoves and pot size, air gaps, and air holes is a mystery to me. It seems like a lot of trial and error is needed.
+1 on using your pack, foam pad, etc as wind blocks.Oct 5, 2013 at 3:14 am #2030979
@brooklynkayakLocale: South West US
Screen does reduce wind gusts, but still allows good air flow.
It is common to use pantyhose/nylons stretched over a wire frame around microphones to reduce wind noise for outdoor recording.
Aluminum screen would work, specially double with a space in between, but at what weight?
I have used my sleeping pad formed into a big wind block tube during extreme wind.Oct 5, 2013 at 2:09 pm #2031059
@cfrey-0Locale: US East Coast
My first blush response would be to agree that the problem is not enough air.
Wind rushing past those small evenly spaced holes in your screen may be setting up something akin to a venturi siphon, sucking the air out of your system. Add in the fact that the lingering combustion gases across the bottom of your wide pot are looking for an exit and … well who knows. I'm just spitballing. I don't think a taller screen is the answer though. I think it was Tony Beasley (here or on BPLITE) who showed that as combustion gases rise and cool, if you entrap them too long they can actually steal heat from the sides of your pot. I'm just repeating (poorly) what he said though, with no proof one way or the other.
You might want to play with varying the hole size and height of your vents relative to one another. Good Luck. I'd be interested in hearing how you make out.Oct 5, 2013 at 5:26 pm #2031098
Thanks for the interesting leads, folks. I had not considered "air starvation" on a windy day as a possible cause.
I tried to repeat the effect in my home kitchen by setting up an 18" Patton fan (no small fan) and set the fan on medium (no small wind) and placed it 7 feet from my stove. This gave me a blustery test area. Turned off the overhead lights so I could see the flame. Used the same windscreen as before, but set it a little tighter, maybe 1/4" space all around. (I believe I had set it at maybe 1/2" all around when I got my failure.) The flame danced about plenty, but did not extinguish. So, puzzled, still. In previous testing, the SuperCat has been very sensitive to "screen too close" so my impulse is to leave the windscreen relatively loose around.
I'm curious about the comment on the even spacing of the holes. That makes a difference?? Is it better to have holes randomly spaced? Oval?
What about even more, even smaller holes? In effect, making my own screen?
Regards the idea of holes in one side of the windscreen and none on the other: that occurred to me too, and I made a screen like that. It was a failure; the flame would burn brightly only on one side of the stove (the side with the screen's holes), and I got an anemic weak flame on the side of the windscreen with no holes. A severely asymmetrical flame. Clearly this SuperCat wants to breathe, and breathe heavily.Oct 6, 2013 at 5:50 am #2031184
Here is the link to the video that Dan / Zelph made that shows and explains the idea of raising the windscreen much better than I could.
FWIW here is a picture of my version of a MYOG aluminum flashing windscreen in which I incorporated Dan's idea of a raised windscreen.
A windscreen like this will allow air to feed the stove flame but you may still need to use your pack and or foam pad as a wind blocker on those blustery days on the trail.
Newton ;-)Oct 7, 2013 at 5:44 am #2031450
Thanks for posting that video link John. I am going to have to redesign my windscreen now.
CheersOct 7, 2013 at 7:01 am #2031461
Yes, interesting video.
Are there "rules" regarding good windscreen design for an alcohol stove? Or is it all controversial, trial and error?Oct 7, 2013 at 1:04 pm #2031546
The fire triangle consists of fuel, heat and oxygen to support combustion.
Different stoves like different carburetors on engines, (I'm and old guy) require "tuning".
You can adjust the size of the "jets" on a stove and you can adjust the airflow with the windscreen.
Add in to the equation that when the stove achieves optimum temperature things work well if the airflow is right. Let the stove overheat and efficiency is lost and fuel is wasted with a rapid burn and flame out.
Short answer is Trial and Error. ;-)
NewtonOct 7, 2013 at 2:30 pm #2031566
@rcaffinLocale: Wollemi & Kosciusko NPs, Europe
> Are there "rules" regarding good windscreen design for an alcohol stove?
Yes. They do NOT like any wind.
CheersOct 7, 2013 at 2:35 pm #2031568
"They do NOT like any wind."
However, there needs to be a steady intake air supply.
–B.G.–Oct 7, 2013 at 7:11 pm #2031682
A few other things to consider based on my use of a super cat stove:
– Cold Fuel. I recently had a similar problem, on a windy day barely above freezing. I think one of the biggest problems was that both stove and fuel were about 35F. It's probably a good idea to keep both in an inside pocket if wind chill temp drops below 50F, in fact I think this may be the biggest problem below that point.
-Placement. I find it's better to brush off combustible materials and place the stove in a hollow in the ground, than it is to place it on an elevated flat rock. Wind speed decreases exponentially towards the ground, so even a few inches makes a difference. Plus, it will effectively make your windscreen taller.
-Cat can hole size. The original instructions suggest 3/16" is more wind resistant than 1/4". I find it boosts efficiency too (I estimate 15%). However, a good 3/16 hole punch is probably about $7.
-Black. Painting the stove and the bottom/lower sides of the pot with black barbeque paint increases efficiency (I estimate another 10-15%). It also allows the stove to absorb additional heat, thus forcing the gas out with greater pressure, thus making it more wind resistant. I find scotch tape makes a nice run-free mask (better than masking tape) to make a beautiful black edge that ends just where the bottom of the pot starts curving, then tape newspaper on to protect the rest of the pot. BBQ paint is about $5.
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