Air intake- exhaust ratio of a cone

[Gary Dunckel]

OK, guys and gals, I need some sound engineering advice (not just guesses, please…). I am trying to come up with a way to simmer a dehydrated meal with alcohol. Some of my concoctions have ingredients that are like BBs and take awhile to soften up. Here’s a photo of what I’ve come up with so far:

I filled a cat can with carbon felt disks, and I placed a titanium disk on top. The ti disk has a 1″ hole in the center to slow down the burn (hard to see in the photo). I made a ti foil screen to match up with my beloved Snow Peak bowl with handles (and a 4Dog lid w/ a swapped-out ‘knob’). The height of the screen was designed to position the pot 1.5″ above the rim of the stove. I think I learned here on a BPL thread that an increased stove-to-pot distance helps to reduce the heat to allow a more subtle simmer.

So here’s my question: What do you think is the optimal ratio of air intake holes to exhaust holes? This would also apply to a Caldera Cone, of course. Do you want them to be equal, do you want more intake holes, or do you want more exhaust holes? More questions below:

#1 If there are less exhaust holes, will that somehow quench the flame and allow an incomplete combustion of the alcohol? Or is the opposite true, that fewer intake holes will cause that?

#2  Will the venturi effect of a breeze overcome the exhaust problem of #1?

#3  Is the amount of air intake dependent upon the rate of exhaust (i.e., is air intake passive, and only as it is needed)?

For reference, there are 15 exhaust holes around the entire top of the windscreen/pot support, and there are 22 air intake holes at the bottom. The intake holes are confined to 1/3 of the circumference, and they would be positioned on the lee side of any breeze. I am wondering if there should be the same number of holes for both the intake and the exhaust.

I have only done one test with this setup so far. I started with 42* F water, and it took awhile to bring it to shrimp eyes. But then it proceeded to reach a nice, moderate simmer, which lasted until the alcohol ran out. It never actually came to a roaring boil, which pleased me. The other thing that pleased me was that the stove ran for 27 minutes on .75 fl. oz. of SLX alcohol–pretty sweet.

So what do you think, do I need to modify what I have so far? All I can think to do would be to add more exhaust holes to try to equal the amount of air intake. But maybe that would just be gilding the lily?

[Jon Fong / Flat Cat Gear]

Gary,

Are you designing your windscreen with the sole purpose of simmering?  The reason that I ask is twofold. 1) If you design it to simmer, you will probably find it impossible to boil quickly.  I have found that the difference between boiling and simmering is about a 4X increase in wattage.  Given a 4X difference, I would extrapolate the air requirements to be 4X as well. 2) If you are designing a simmer only windscreen, the gap between the bottom of the windscreen and the ground/heat shield becomes critical.  Slight gaps and uneven ground will add a lot of variability to your results.

Designing a stove that simmers well can be pretty complex and is really a system level design.  My 2 cents.

[Gary Dunckel]

Thanks for posting, Jon. Your two points are well taken. I was mainly thinking about just using this setup for simmering. This afternoon I started thinking about adding a capability for doing a rapid and efficient boil. I’m thinking of a way to elevate a separate stove burner that would be positioned at the optimal distance from the pot bottom. I doubt that I would be doing any dry baking, or even steam baking, as it would be hard (for me) to control the heat level.

I agree with your comment about sealing the bottom of the wind screen. I always try to do that, which includes a full base plate (and a flat place to set things on). I think the key to a Caldera Cone is the “oven effect” where the heat stays within the cone. This one isn’t quite a cone, but it functions as one.

I’m still unsure about the air intake/exhaust factor, but since my performance seems OK I’ll assume that I’m in the ball park with what I’ve done so far.

[Kenneth Keating]

Being an engineer, I think your question is going to be difficult to get a valid response due to the number of variables such as cone height, air temperature, diameter of holes, volume of cone, volume and mass of items being cooked. etc. I’d be more prone to come up with a design that allows adjustment of the flame throughout all scenarios.  Maybe two ti disks on top with off-centered holes(single or multiple) such that when you rotate the discs the openings either open or closed depending on how much the holes overlap.

[Lori P]

Interesting… I’m more familiar with designing stoves to simmer/boil than expecting the windscreen/potstand to do the work.

 

http://www.backpacking.net/makegear/falk-catstove/

http://hikinghq.net/sgt_stove/om_cat.html

[Jon Fong / Flat Cat Gear]

I would recommend that you design a stove that controls the airflow and not the windscreen.  The difficult part will be that you will have to control the stove output while it is hot (as in an enclosed windscreen).  My 2 cents.

[Roger Caffin]

When you burn alcohol you generate a LOT of hot exhaust gases. They have to go somewhere. If they don’t, the flame stalls and quenches, generating a lot of CO and wasting fuel.

I suggest you should probably have one row of holes at the bottom and three rows of holes at the top. You might be able to do this just by inverting the windshield/pot support.

My 2c
Cheers

[Gary Dunckel]

Thanks for the responses, each of you. Kenneth, your idea of a second disk is very interesting. That would require far less titanium foil than making a new windscreen. I’ll try that out today. What I’ve noticed thus far is that the stove puts out a relatively greater flame at the beginning, then tapers off as some of the fuel is consumed. I might be able to take advantage of this by bringing the water close to a boil, and then placing a second disk with a smaller diameter hole to slow the burn even more. This likely will result in a longer burn and a more gentle simmer. We shall see…

As for the exhaust, I guess I could add a second row of holes at the top, Roger. I’ll see how the next set of tests goes. If the slower burn works well, then maybe I already have enough exhaust capability.

I do think that one setup won’t quite work for both simmering and rapid boiling, but if I can figure out a way to make it sort of work, I’ll be satisfied.

Thanks again, folks.

[R]

You might consider using aluminum for your prototyping rather than ending up w/ too many holes in your ti.  Oven liners are cheaply available at most grocery stores.

+1 to assigning throttling tasks to the stove rather than the wind screen.  A screen that can let a rapid boil breathe won’t interfere with a simmer.

This is one solution to adjust the simmer inside a cone:

http://adventuresinstoving.blogspot.com/2012/01/featherfire-alcohol-stove.html

http://packafeather.com/stove.html

[Gary Dunckel]

I tried to find that recent thread that dealt with simmering with an alcohol stove, but I couldn’t. So I will piggy-back my latest project onto this thread.

First off, Ken had the best solution to simmer for the purposes of THIS thread. I simply made a second titanium disk, with a smaller diameter center hole (.56 sq. in.) than the fixed disk in the cat can (1.35 sq.in.). By using .75 fl. oz. of alcohol, I get a near-boil in 9 minutes. I then remove the pot and place the second disk with the smaller hole  on top of the fixed one. This reduces the flame intensity dramatically and puts it into simmer mode. The simmer will last for another 12-13 minutes. If things get too hot, I just remove the pot lid for a minute or so to let things cool back down. I also made a “flame snuffer” for shutting the stove off. Here’s the setup:

By the way, I doubled the number of exhaust holes for the top of the screen, and the performance was very good. I’m a happy boy.

Then I came up with yet another way to do some simmering, this time with an Evernew EN-400 cup (for smaller quantities like soup) and its dedicated ti Caldera “clone” and base plate. This morning I poured just 3/8 fl. oz. of alcohol into the “big hole” cat can with felt, just enough to almost bring 1.5 C. of water to a boil. Then I removed the burner and replaced it with a 3-wick bee’s wax candle that I made yesterday. The candle brought the water to a mild boil within 5 minutes, and it would continue to simmer for as long as I wanted. The weight of the 2 “burners” (cat can/felt/ti disk + cat can wax candle) + ti snuffer is 110 grams, or 3.88 oz. Add to that a 2-oz. bottle of alcohol, the pot, and the cone/base, and it becomes a full-service cooking system for a one-night outing that weighs 9.25 oz. Sure, there are lighter setups, but none that I have that will do a pure simmer. The major weight penalty with this one is the 2.8 oz. bee’s wax candle, but man does it burn nicely with no soot on the pot. Maybe one of Zelph’s simmer stoves could do the job at a lower weight?

OK, I’m done. Thanks for indulging me on this. And thanks to you, Ken, for suggesting the flame “step-down” idea.

[Kyle]

Quick, off-topic, question:

Where do you guys get the silicone for the pot handles?

Thanks,
-K

[Gary Dunckel]

Kyle, we have a mega hardware store here in Boulder (McGuckin’s) that stocks several diameters of silicone tubing that I buy by the foot. I also picked some up at a local aquarium store once. I haven’t seen any at Lowe’s or Home Depot, but you might try your local Ace Hardware store.

[R]

Also available as fuel hose for RC hobby craft.

[Roger Caffin]

Also try an aquarium shop. It’s used for air feeds.
And there is always my favorite engineering shop – called eBay. You can get it in pretty colours too.

Cheers

 

[Kyle]

Thanks guys!

[DAN-Y]

In this video made years ago, I show how bottom intake air has an effect on the performance of the starlyte stove. There is a lot of shared technology on my website, it’s free, no advertising ;-)

http://bplite.com/viewtopic.php?f=2&t=5580&sid=6666dce93213877a613ea30504a2fa93

Adjust the amount of air intake holes so the column of flame is smooth. Do this in your garage or outside on a calm day.

[Roger Caffin]

Hi Dan

You can’t do genuine alky research with paper clips! Especially as they are steel paper clips, not titanium ones. Disgraceful!

Actually, how very elegant. Even without audio, the flames make a very graphic demonstration of air flow. My compliments.

Cheers

 

[Kyle]

cool demo! its a whole other can of worms when you introduce wind into the equation, though. then you aren’t necessarily optimizing for the same things…

[Mario Caceres]

Nice video Dan, I wonder if the results would be the same if you were using your “starlyte simmer” where the inflow / air requirements are lower.

[DAN-Y]

Thanks Roger :-) I’ll have to contact Gary the “grille guy” and have him make me some titanium paper clips. That video has been viewed in excess of 17,000 times so it has some interest.

My motto has been “never deny your stove oxygen”. Most of the time, I would not release a stove if it did not boil 2 cups using 1/2 ounce of fuel and have an integrated pot support. With alcohol stoves, most of the time, it’s been a mechanical device that would limit oxygen to the stove in order to make it simmer. The Trangia and my Starlyte with slide shut-off valve are the exceptions.

Mario, if a simmer Starlyte was used, I think the windscreen in the video would have sufficient holes and the diameter of the screen was reduced. Air will want to follow the path of least resistance ;)

Kyle, the introduction of wind is something we don’t want to think about when designing the basic stove and windscreen. After we get the basics perfected then we take it outside and do what we need to do to make our set-up as efficient as we can. Use something to block the wind

[Mario Caceres]

For windscreens I have been using a MYOG Caldera “Clone” and a “Vari-vent” from Trail Designs.  I think I like the Vari-vent better as you can use it with different diameter pots and also you can adjust the air in-flow ports according to conditions and requirements.  i.e if i’m simmering I can close down some vents, If I boiling I can open them up. I like also you can change the way you open the vents to accomodate in which direction the wind is blowing

I just started to play with the Kovea Spider stove which is a remote canister stove.  I modified one of the-intake ports of the vari-vent so I can pass thru the hose of the remote canister.  Works great.

[Gary Dunckel]

Your video was enlightening, Dan. Are you suggesting that one can achieve the same smooth laminar flame with proper intake holes (number, size, and location) as when the screen is elevated 1/2″ above the ground? When it gets dark tonight, I’ll have to see how my various wind screens and cones behave.

As for titanium paper clips, yes, I have made some with leftover scraps of rod. I’ve even passed them through a stove flame to put a patina on them, to make them look like “Grateful Dead” paper clips. Then I gave some to friends as gag gifts. The thing is, they don’t work very well. You see, the rod is leftover from when I make my grills, and the metal is CP-2 and rather soft (and easy to bend). This works for the usual paper clip functions, but it likely won’t be so good for holding a windscreen up off the ground. You need something that is stiffer, like what you used. I did try to make some from 6AL-4V titanium, which would work well, but I couldn’t bend the rod into tight loops without it breaking. Roger will be disappointed of course, but maybe you can impress him by using long pastel colored standard paper clips.

[Roger Caffin]

Pastel paper clips – how very … um … nice.  :-)

Yes, bending 6Al-4V alloy wire is tricky, but it can be done. I bend 2.4 mm 6Al-4V welding wire into bends with a radius of about 6 mm on a routine basis. 1 mm wire can bend even smaller – enough for a paper clip even. The secret (I think!) is to do it with a former, so the bending is really distributed. I use a proper bending jig I made which wraps the wire around a hard steel post, but some large needle-nose pliers will work with 1 mm wire. Using those pliers with 2.4 mm wire requires a somewhat Herculean effort! Using ordinary engineering pliers (or a vice) is nevery successful: too sharp a bend.

There’s a tiny roller out of sight driving the wire around the post.

Cheers

 

[James Marco]

Well, molar values are 4 input to 5 output.

1 ethanol + 3 bivalent oxygen’s

With the composition of air (oxygen specifically) approximately 20% oxygen, you need around 5 moles for each of the 3 input oxygen moles, or roughly 15 moles of air plus 1 of ethanol…a total of 16 moles input.

On the output side, we have 2 carbon dioxide’s plus 3 molecules of water, plus the 12 moles of inert “air.” Or, a total of 17 moles.

So, the ratio for in/out is 16/17. Note that the heat produced means a somewhat faster flow rate though at a lower pressure.  I believe they are roughly equivalent. I usually err on the side of caution and add extra holes on the input side, roughly 20/17, to insure minimum carbon monoxide production.

[Roger Caffin]

Hi James

As a molar ratio, yes. But I think you forgot a couple of things. The first is the amount of nitrogen in the atmosphere, although that by itself is not all that significant.

Far more importantly, you haven’t allowed for the HUGE expansion ratio due to taking the incoming air at (say) 20 C and heating it up to about 1000 C at the flame. As an extremely rough guide, the expansion ration would be about 5:1. That means the effective flow out is about 5x bigger than the flow in.

If you want low CO emission, you also need to make sure you don’t choke the flame. A further increase in hole sizes, top and bottom, may be helpful.

Cheers

 

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