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Windscreen downdraft


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  • #1314296
    Charley White
    Member

    @charleywhite

    Locale: Petaluma, CA

    For all the, I'll say, "partial" windscreens I see here, I fear I'm missing something obvious. I only have about 25 days on my "weak flame"–alcohol & esbit–stoves, so am newbie. (All prior years a Whisperlite blazer or Optimus 99). I had a problem with draft on the JMT with my hobo alky. Fumed. I concluded had insufficient gap between 700something ml Ti pot and the quart juice can windscreen (pot supported on wires thru can). But I noticed that whenever the least wind hit the pot, which was sticking up, likely as not wind would go downchimney and drive flames out the lower inlets. Yes I would shelter as I could, but…that's what a windscreens for. Back home I solved and tested by hanging a (now narrower heine pot)from the windscreen top via 2 ss wires twisted around the rim bead. Pot top equal windscreen top. Problem solved. Part of me doesn't want to change as I like the idea of a boil pot 100% washed by hot gas and zero % washed by mountain air.

    But part of me thinks I'm missing something big and giving up lots of design flexibility. TIA

    #2081876
    Delmar O’Donnell
    Member

    @bolster

    Locale: Between Jacinto & Gorgonio

    What was said dysfunctional gap, between pot and windscreen?

    And would you classify the stove as a "breather," one that needed lots of air to run?

    I ask, because when I first started playing with SuperCat stoves, I found I needed a really large gap, 1/2"-1", or the flame would starve. But with my current Zelph stoves, I can pull the screen in much tighter and block more wind. They work fine at 1/4" to 1/2" gap and maybe can go tighter, still experimenting.

    #2081881
    Bob Gross
    BPL Member

    @b-g-2-2

    Locale: Silicon Valley

    The standard spacing between the pot and the windscreen is the width of your index finger. That is 3/4", plus or minus 1/4".

    I've seen very few that operated very well when tighter than that. Some will operate fine when looser than that, but the efficiency will drop off pretty soon.

    You want to get your bottom air intake holes to have good area, just like the top exhaust area around the pot.

    –B.G.–

    #2081936
    Charley White
    Member

    @charleywhite

    Locale: Petaluma, CA

    With those standards I was really choked, then. Probably under 1/4.hobo stove/windscreen

    This is a setback for my next stove, a variation on the matthew depan paint can stove, via andrew skurka. I would use it with twigs when possible to melt snow, and esbit the rest of the time. I really like the compact pack of the pot within the can. Choke city though, on esbit.

    Gap with esbit:
    Esbit

    Twig setup (ss foil supra screen):
    Twigs

    #2081941
    Glenn S
    Member

    @glenn64

    Locale: Snowhere, MN

    You mentioned lower inlets, but I'm not seeing very many. Maybe just hard to see in the first pic. If i had to guess at what's happening, I'd say a couple things are going on.

    There's limited air flow, combined with a large flame pattern inside of a small burn chamber area under the pot, so too much fuel is vaporizing from thermal feedback, and not burning off from lack of oxygen. The "downdraft" experience, is actually a backblow of vapor igniting on the interior of the lower inlet holes when some air finally gets to it.

    I don't recommend inhaling the fumes, but when a stove is operating efficiently, the only exhaust should be water vapor. If the exhaust coming up the sides of the pot smell like fuel and burn your eyes and nose, you're getting an incomplete burn from over vaporization and lack of air.

    Kinda just guessing here from pictures and description and my limited experience.

    Maybe some of the real stovie pros can offer more advice and use actual technical jargon.

    #2081985
    Charley White
    Member

    @charleywhite

    Locale: Petaluma, CA

    You got it with the fumes, Glenn. Ice picks in the eyes & nose. Stayed well away. It's hard to tell in the first picture, but all the inlet holes are on the bottom, surrounding where the cat can sits. That metal floor is an upside down stub of a slightly smaller can. Big Vs are cut into the shortened sides of that to feed the bottom holes. The windscreen/can sits on the short screws you can see. That alone provides a surrounding air gap, though maybe only 3mm. Blowing hard on the bottom in bench tests this design was bomber windproof. Maybe this was enough and the choked chimney wouldn't let it draw. Will sacrifice the nose to test with the narrower Heine pot. Fumes then, and clearly not enough inlet.

    The second stove, the "DePan Paint Can" is deceptive because the normal paint can is upside down. The stove bottom is the can lid, with a circle of welded wire mesh the stove sits on–in the grooves–so there's a 1/2" air band around the bottom (or any 1/8" multiple I want to cut.)

    Edit to add: paint can stove on the Skurka site here:

    http://andrewskurka.com/wp-content/uploads/alcohol-and-wood-combined.jpg

    #2082119
    David Gardner
    BPL Member

    @gearmaker

    Locale: Northern California

    Disclosure: I make and sell conical windscreens.

    I've had great success with tight fitting windscreens, but they are all conical so that may be why they work so well. I think the shape may accelerate the gases upward, so the same volume of exhaust goes up but fits through the smaller gap because it is moving faster.

    The First picture shows a more "traditional" windscreen gap of about 1/4", which I went to after discovering it was faster and more efficient than the 1/2" gap I used previously. The second photo shows a tight gap windscreen with about 1/8" clearance, and the third photo shows a windscreen that actually touches the can but has exhaust vent holes. Both the tight gap and the contact windscreen boil about 30 seconds faster than the 1/4" gap, and use about 2 ml less to do it (13 ml vs. 15 ml)

    quarter

    1/4" gap

    eighth

    1/8" gap

    contact

    Contact

    Have never had a downdraft problem with any of these close fitting conical windscreens.

    [edited to add disclosure]

    #2082136
    Charley White
    Member

    @charleywhite

    Locale: Petaluma, CA

    Update edit: added to bottom to not bump this above the fold

    David: "tight fitting windscreens, but they are all conical so that may be why they work so well. I think the shape may accelerate the gases upward, so the same volume of exhaust goes up but fits through the smaller gap because it is moving faster."

    I think that's the venturi effect.

    Your post on your low-rise windscreen is what put this back in mind. Most impressive results. Looking and thinking again at your contact version, I realize it's really a great adaptation of the same principle as the old MSR exchangers: heat channels, but with flue contact.

    My chosen design criteria prevent me from packing any screens in the pots and I retain a preference for the tin-can approach–replaceable from grocery store, so will keep with that for now. Now, though, I will greatly increase inlet ports FIRST and try again.

    OK, hope it's ok that I just grabbed the jpg from Skurka. Fancy feast setup on left:DePan/Skurka stove

    Fume update:

    –I added much inlet capacity to stove one. Better, but still fumey.

    –Then put Heine pot in. No fumes, good draft, head sticking up & lots of hot exhaust escape.

    –Tried my SnowPeak700 in my DePan clone, but now sitting on the cat stove instead of over it (so, outflame i/o inflame). Much slower (8min 2 cups), but no fumes. I see my pot is diameter-bigger than Skurka's. I wish mine were smaller, but I own it. I guess I will also own the crude tradeoffs of the hobo life.

    I'm a tad surprised others aren't more reflexively fiendish about wind prophylaxis (called for or not by their system.)

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