Windscreen downdraft

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.

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.–

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

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:

Twig setup (ss foil supra screen):

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.

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

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)

1/4" gap

1/8" gap

Contact

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

[edited to add disclosure]

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:

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.)