The Evolution of a Winter Stove - Part 1

Dissatisfied with what was commercially available at the time, the author has been working on the design of a lightweight winter canister stove since 2007.

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by Roger Caffin | 2013-07-03 00:00:00-06

This article is broken up into 4 parts

  • Part 1
  • Part 2
  • Part 3
  • Part 4
  • Introduction

     - 1
    A range of experimental stoves - they all worked.

    Dissatisfied with what was commercially available at the time, the author has been working on the design of a lightweight winter canister stove since 2007. (OK, OK, a bit obsessive, but so what?) Several novel features were required of the design, in the interests of versatility, functionality and safety. These features are explained and the evolution is traced, ending up with the current design.

     - 2
    A range of experimental burner heads.

    Along the way, a lot of research was conducted into just how canister stoves work. Special emphasis was placed on the air flow into the mixing column and the way fuel flows through the burner head. This array is but a fraction of what is in my Reject Bin. Seen in the light of what has been learnt, it is clear many older stoves on the market were designed without enough of this knowledge - and the way they emit carbon monoxide (covered in our lengthy series on Carbon Monoxide Emissions) is good evidence of that.

    Many materials were explored along the way. Brass and steel, the mainstays of the older generation of stoves, have given away to aluminum and titanium. But other materials, such as various plastics, have also been co-opted into the design; often to reduce the weight. Naturally, fuel and heat compatibility have always been considerations. I have had the fun of experiencing a number of 'interesting' failures along the way, including some slightly melted plastic in some commercial stoves.

     - 3
    My CNC machine.

    Finally, to get the precision of machining needed for a reliable design, the author ventured into the fascinating world of CNC machining. While that required a steep learning curve in itself, the use of CNC machining means that designs could be refined in a reliable manner over generations and then reproduced at will, at least to a limited extent.

     - 4
    Selected better experimental stoves .

    Space prevents this series from a detailed examination of the virtues and faults of every model designed, made, and tested. This is probably just as well - otherwise it could be too close to the dreaded ordeal of amateur holiday snaps. In case you are wondering, every stove in this photo has significant differences from the rest. Some readers may even recognize the unit at the top right from an article published some time ago: it was an early venture in this direction, and the start of a long road. Evolving the design has been a lot of fun (?). But it may be worth admitting that not everything outlined in this article was understood in total clarity at the start: it has been a long learning exercise! I will add that while I have been learning, it is clear that one or two other designers at one or two Asian factories have been very active in Stove Development. With regret I have to say I have not seen this degree of learning from the traditional Western companies: they are falling badly behind.

    In this first part we will explore what I wanted to achieve, the major components of the stove-to-be, and a couple of significant technical decisions. In the subsequent parts we will go into technical details for all the bits and pieces and see what I ended up with.

    Required Features

    The starting point for the saga is what I wanted in my stove: the 'required features'.

    Liquid Feed

    The use of a liquid feed out of an inverted canister means the remaining fuel in the canister does not plummet in temperature due to evaporation as fuel is used. This is reasonably well-known to most experienced users of canister stoves and is well documented in our article on The Effect of Cold on Canisters, and is the main rational for the remote-canister liquid-feed winter stove.

     - 5
    A flare-up due to un-vaporized liquid fuel coming out the jet of the Brunton Stove Stand.

    However, the fuel must be vaporized before it reaches the jet, for several reasons. The most obvious one is that spitting liquid fuel out of the jet produces flare-ups, fireballs, and threatens to burn the place down. A less-obvious reason is that liquid fuel itself does not burn. Liquid fuel must turn into a vapor first, so it can combine, molecule by molecule (and atom by atom), with the oxygen atoms in the air. The process of turning the liquid fuel into a vapor is however quite simple in principle: just add some heat.

    Gas Valving

    Along with the need to vaporize the fuel is the need to throttle the flow. Without a throttle it is impossible, for example, to simmer gently food or melt snow. In the past, almost every liquid fuel stove made has done this by throttling the flow of the liquid fuel itself, but this presents problems. Since liquid fuels such as butane and white gas expand by a factor of about 250x when they turn into a gas, it is obvious that the amount of liquid fuel going through the control valve has to be very small. That restriction means that the cross-section through the valve has to be very tiny. This poses a problem because it is easy for dirt to block the flow which makes it hard to get any fine control over the flow. A couple of people have commented that simmering is not something the MSR Simmerlite stove can do very easily. The greater viscosity of liquids over gases does mitigate this relation a bit.

     - 6
    The basic idea for smooth control: valve the gas flow after the vaporization.

    The solution is obvious when you think about it: valve the gas flow instead of the liquid flow. That is, put the control valve after the part of the fuel system where the liquid turns into gas. In the very stylized and rough sketch here, the inverted canister sits on top of the Canister Connector (CC) and feeds liquid fuel through the Hose to the 'pre-heat loop', where the heat coming in turns it into a gas. Yes, that is a very rough drawing of a preheat loop, but it will suffice. Then the gas goes through the valve to the jet. Like this you get nice smooth flame control. But almost every commercial remote canister stove puts the control valve back at the canister, the easy, obvious and cheap place to put it. It's just not the right place. Clearly, some departure from conventional thinking is required here to get what I wanted.

    Safety

    No one wants a fireball when they are cooking. I exempt users of the MSR XGK stove: the instructions do actually state that 'a brief soccer ball size flame is normal'. I have seen the scars on benches in various huts and on picnic tables from those stoves, although some of those may be from alcohol stoves - maybe. But there is a problem here in the fine details. If the control valve is at the stove end of the hose, then there will be liquid fuel in the hose when the valve at the stove is shut off. When the hose is disconnected from the canister the liquid fuel can spray out backwards into the air. If there is another stove nearby when this happens, or any flame at all - kaboom. Because of the 'reservoir' of fuel in the hose, under normal circumstances there is a long delay between turning the valve off and the flame going out, which can also present problems.

     - 7
    Basic Lindal connector (un-crimped samples from factory).

    A Safety problem exists even with upright canister stoves equipped with the Lindal valve. Many users will have experienced that slight hiss you can get when attaching and detaching an upright stove from a canister. That hiss means that the pin (solid blue line, right-hand diagram) which depresses the Lindal valve plug (shaded red, a bit under the blue pin) is opening the valve a shade before the big O-ring (brown circles outside the valve spigot) seals the connection. The standard solution here is to do the attaching and detaching fast, so that very little gas escapes. At least what you get out is normally a gas, not a liquid, provided you hold the canister upright.

    Why does this happen? There are two reasons. The first is that there is no real 'standard' for the design and length of the actuating pin, so everyone does it slightly differently. This means that a “brand X” stove may mesh cleanly (without hiss) with a “brand X” canister, but that does not mean a “brand Y” canister will work. The second is more complex. The Lindal valve screw-thread insert for all canisters (with the possible exception of some Chinese brands) comes from the Lindal factory. They should all be the same out of the factory (and probably are), but they aren't the same when they get to you. The company which assembles (and fills) the canister uses its own crimping machine, and each machine does the crimp slightly differently. A change in crimping means that, as an example, the Coleman Powermax canister changed the shape of the central nipple by a whole millimeter.

     - 8
    The (blue) valve actuation pin pushing on the (red) Lindal valve.

    A solution to this problem of leakage is to make the valve at the canister into a simple safety on/off valve, not the main control valve. Ideally, this safety valve would never be actuated until the O-ring seal (brown circles) has closed and gas cannot leak out. Then (and only then) the blue push rod is pushed downwards to open the actual (red) valve plug. Only then can the fuel escape from the canister and flow out via the outlet pipe (light blue). The difference between this and a typical stove is that here the blue pin is not fixed in position: it can move up and down. Opening the Lindal valve is now quite separate from sealing the connector to the canister.

    When disconnecting the stove from the canister you first turn off the safety valve, or raise the blue pin so it no longer depresses the red valve, then you let all the remaining fuel in the hose get used up (burnt) at the stove, and only then do you disconnect the stove from the canister. This way there are no gas leaks at all, a fundamental safety requirement.

    Stability

     - 9
    Hoses: stiff, stiff, and flexible.

    A secondary safety consideration is the hose itself. With some older stoves you could use the hose as the handle of a battle-axe; it's that thick and stiff and clumsy. (Allegations that the XGK can be used to pound in tent stakes are unverified.) If the stove weighs a ton that may not matter, but with a very light stove you could find the stove held up in the air by such a hose. You need a much lighter hose for a light weight stove. I believe the reason why the hoses are so heavy is the designer (or the factory) just used stock ° in reinforced fuel line designed for racing cars. This size of fuel line is compatible with most fuels, robust, cheap and not inclined to flap around at 200 mph; just not really suitable for a light-weight stove though. Thinner, lighter and more flexible is possible; as is shown at the right.

    Sometimes you find that the hose has a bit of a curve in it. This could be the result of storage in the factory, or how the stove was packed away in your pot for months on end. Anyhow, in keeping with the prior comments, the hose needs to be able to rotate with respect to both the stove and the canister to get the curve lined up in a convenient direction. Most older-model stoves did not do this (to the user's frustration), although the latest generation do have rotating couplings at one or both ends. The rotation may be a little tight, but it’s quite adequate. Such a rotation at both ends is a requirement, even with the lighter tubing.

     - 10
    Pot supports: long and flimsy vs short and solid.

    Criticism has been made of the pot supports on some stoves. In some cases the pot support is just too flimsy for safety: both the MSR Pocket Rocket and the Primus Micron Ti come to mind. Those two are shown at the left, along with some recent Asian stoves with far more robust pot supports at the right. But this detail has two parts to it. The first part is the long length of those thin arms which support the pot: you don't want them that long. The supports on the Pocket Rocket are notorious for bending, and it is easy to see why. With the current generation of smaller titanium stoves (eg the two at the right), the pot supports come from the burner head and are therefore much shorter. I might add that since there is much less area of metal to worry about in those short pot supports, the designers have been able to make them a bit thicker, and as a result they are even stronger. So the worry about collapsing pot supports has subsided.

     - 11
    The weight of a large pot of dinner.

    There is also the question of the stability of the whole stove: can it collapse sideways under a load? What happens when you are cooking with a large pot on the stove, full of stew, being stirred? (Hey, we get hungry sometimes!) Both the stove legs and the pot supports must be able to take a bit of a load, and this issue has some sneaky aspects we will return to later. Upright stoves avoid the stove legs problem of course by using the canister as a base, but the pot supports remain an issue. Winter stoves however must also have good legs to hold the stove up.

    Versatility

     - 12
    Lindal connectors, with screw-thread, Campingaz and Powermax

    The most common fuel container in America is the screw-thread canister (invented by Epigas, shown on the left). When I started designing in 2007 the Coleman Powermax canister (shown on the right) was still readily available and is very nicely designed for a liquid feed (with an internal dip-tube). However, when one goes to Europe it very often happens that the only re-sealable Lindal-valve canister available is a French Campingaz one (shown in the middle), with the French Easy-Clic connection. This is not the same as the screw-thread connection normally seen in America. As an aside: Coleman makes a screw-thread canisters of course, but they also made the Powermax ones and now own the Campingaz brand as well.

    The Powermax connector at the right has a sort of hexagonal shape near the base, and this hex shape keys into the Powermax connector. It's a rather complex arrangement, and making a custom connector for it requires that you make a hexagonal broaching tool of the right size, and have a press to drive the broach through the material. It's possible, but slow unless you do it by molding. I made a suitable hex broach which worked fine, but it was hard work with a press.

    The common screw-thread connection uses the central spigot as a 'screw', but the thread is atrocious, being barely half there. It tends to rip the thread out of the stove over time, so the stove can no longer connect. That has happened to me in the middle of a long trip, and I had to buy another stove very quickly. It may be cheap and popular, but it is bad engineering. I wanted to avoid the thread.

    The Campingaz design does not use the spigot in the middle except as a housing for the valve and as something for the O-ring to seal against. Even then, the design has a secondary seal in it: the red bit shown in the photo under 'Safety'. You can't see it in this photo: it is black neoprene. This rim of neoprene seals against the sides of the valve actuating pin; the gas goes up a hole through the middle of the pin. The stove is clamped to the canister by means of lugs which catch in a groove under and inside the rim of the Lindal valve. This groove is not shown in the (Lindal-supplied) diagram of the Lindal valve in the photo under 'Safety', but is shown below in the section 'Canister Connector'. It is a very reliable and effective means of anchoring the stove to the canister. This means that if Campingaz is all you can find in a shop when you need to resupply in some little town in the middle of Europe, you have to suddenly buy a new Campingaz stove as well. They are not expensive, but they are rather heavy and primitive things in comparison. Anyhow, there are no Campingaz winter stoves available: they are all solely for upright summer use (which is a bit wierd).

    Some people still have a large supply of the very fine Coleman PowerMax canisters. While we would never recommend you do it, I understand some people even go so far as to refill the empties, just for winter use, because they are really functional. Compatibility with all three styles of connector became a requirement of my stove.

    Manufacturability

    While it is great fun making one or two stoves for personal use, a goal was to be able to make a number of these stoves, all quite identical, so they could perhaps be sold. Hey, everyone needs a cool new stove, right?

     - 13
    Manual lathe and manual mill

    I did some early manufacturing with a manual mill and a manual lathe, but it proved too easy to make every unit unique: just slightly different in some dimensions. Making parts by hand to reasonable tolerances turned out to be very slow, even after the first couple of dozen models were made and abandoned. Granted, I don't have a lifetime's experience as a toolmaker. (I just learnt from two very good ones.) I needed to change how it was done - which is in fact the history of production engineering.

    I looked at getting parts injection molded, but there are two problems there. The first is the cost of the molds. I refused to use a soft low-temperature plastic such as polyethylene here: many parts have to be either a hard plastic such as Acetal or aluminum. Additionally, 1 mm (0.04 in) accuracy such as you might get from a MakerBot is simply not good enough. Some parts had to be made to better than 0.05 mm (0.02 in) accuracy to be reliable - especially for O-ring seals. After some research I found that each mould would cost between $2,000 and $5,000 to be made for me on a CNC machine. Molded parts (made in China) would then be quite cheap of course. This might be acceptable if I had an absolutely final and perfect design, but I didn't; I was still in the development phase. So any change would need a different set of molds, made on a CNC machine. You can imagine how the cost goes up.

    An added complication is ensuring that the Chinese molding factory did not promptly go into pirate production for its own benefit, but the advice there was to use several factories and do the final assembly myself. I discussed making the molds myself with a CNC machine, but a friend (who knew more about CNC machining than me and actually had his own CNC machine) suggested I make the first 'few' stoves with the CNC machine instead, and get the design right. This was obviously the right way to go.

     - 14
    Adept CNC machine: combined mill and lathe

    So I bought a small CNC machining center. It has a full 3-axis CNC mill with an option for a 4th axis (a rotating bed), and it also has a CNC lathe which I have fitted up as a partial Swiss design. I also spent some time getting it going and learning how to program it, but that's a different (and rather long) story. I will simply say that has been lots of fun, with a few anguished moments along the way. Broken milling cutters are not unknown in the trade, and they cost money. A common saying among CNC users is that a CNC machine allows you to make many identical, very accurate parts at high speed, but all equally wrong. This is true. But what did come out of the CNC experience was a slight change in design thinking, so that as much of the stove as possible could be made in multiple units in parallel on the CNC. Once you get into making ten at a time, things really start humming. Injection moldings can come later. Hopefully, the design will be final by that stage.

    In Part 2 we will get down to some hard details; in Part 3 we will look at the final result.


    Citation

    "The Evolution of a Winter Stove - Part 1," by Roger Caffin. BackpackingLight.com (ISSN 1537-0364).
    http://backpackinglight.com/cgi-bin/backpackinglight/caffin-evolution-of-winter-stove.html, 2013-07-03 00:00:00-06.

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    The Evolution of a Winter Stove - Part 1
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    Mike Cecot-Scherer
    (mikescherer) - F - M
    Re: CO on 07/04/2013 14:53:41 MDT Print View

    Hi Roger,

    It's hard to know what way to write this. I was pretty sure I said that carbon monoxide results from incomplete combustion CAUSED by the fuel-to-oxygen ratios being shifted to too rich by low oxygen exhaust mixing into a stove's intake. You seem to be disagreeing with this most basic first-order stoichiometric chemistry in favor of a second-order effect flame quenching argument. Let me first point out that these two effects would not be mutually exclusive and one would therefore expect that there would be times when one predominates (see below for a tragic real-life example).

    Exhaust recirculation is a a primary cause of carbon monoxide generation in many accidents – people die all the time from this. Cars don't produce CO until they're run in a closed space and the exhaust goes back into the intake. Ice fishing hut heaters work great until the occupant wants to stop loosing that last bit of precious heat and closes the vents - making the exhaust go into the intake. HVAC engineers have learned to be very very careful with exhaust.

    Flame quenching is a second order effect. Congratulations on finding it. But I must point out that you have no actual data as to where CO is generated and destroyed in the flame. For all you know the reason that CO production drops with raised pots is exactly because recirculation currents are effectively stopped. Or goes up when the pot is lowered because of strengthened recirculation. A recirculation explanation seems to fit your data quite well.

    I know recirculation can cause CO generation because I have made CO with a camp stove or two in just this way. And I've found that wind screens are a terrific way to cause these recirculation currents. Put them on: significant CO generated. Take them off: low to no CO. Isolate the intake from the exhaust and then put on a windscreen: again, low to no CO (all done with a pot of water on the burner). Seems pretty convincing.

    Pooling of warm stove exhaust at the top of a tent is easy to verify. That pool may or may not mix well depending on conditions. Just saying it mixes well is not an argument. If you hang a gas lantern high in a closed-up tent and let it run for a while you will see CO production. Probably not a heck of a lot, I've seen 70-100ppm, but definitely not the "theoretical" amount one would find with a false premise and quite enough to give thinking people pause (and to make one worry what might happen if someone, for instance, ran a stove below the lantern).

    Your pooh-poohing the advice of a VERY experienced combustion engineer is entirely unsupported and not at all the kind of attitude that the buying public wants to see in their would-be stove designers.

    And then there's your "faulty expert" example which brings no information to the table (unless, perhaps, you were intending irony). Nobody knows how many experts that lawyer had to interview to find one that said what he wanted to hear. Doesn't matter either.

    I can tell you this: that combustion engineer I referred to and I were asked by a lawyer to figure out why two people died in their tent. They were found in their sleeping bags, pink skinned, their faces laying in vomit - obvious carbon monoxide victims. Their tent was zipped shut but there was a non-closable vent right at the top. They had mis-assembled their lantern (very easy to understand if you saw it) so that the exhaust was being forced down toward the intake which resulted in a plume that registered 500-1000ppm CO. That's a monstrous amount of CO - enough to actually knock someone out. Their tent did not/could not ventilate enough. And we were shocked to find that a wind - which one would have expected to aid ventilation - could actually block outflow. So don't be so nonchalant about adequate ventilation. What happened next? Nothing. The lantern company had gone bankrupt. The wives and children of the victims got nothing.

    Looking at the whole outdoor stove configuration, it's kind of preposterous that it normally works so well. I mean, here's an open flame blasting away within a few inches of the intake which is sucking air for all it's worth. The flames and exhaust mostly go up and out - no question - but it's hardly a failsafe design.

    Mike,
    (just Mike)

    jerry adams
    (retiredjerry) - MLife

    Locale: Oregon and Washington
    Re: Re: CO on 07/04/2013 15:28:37 MDT Print View

    You're saying that CO is produced when the intake air has a reduced Oxygen level?

    Roger Caffin
    (rcaffin) - BPL Staff - MLife

    Locale: Wollemi & Kosciusko NPs, Europe
    Re: Fuel mix/type on 07/04/2013 15:48:10 MDT Print View

    Hi Eric

    Yes, 100% propane would be much better, but the pressure is much higher and you have a safety issue with the Powermax-style Aluminium containers. This problem is not insurmountable, as you can buy such canisters with 100% propane in Europe. I forget the brand right now, but they are allied to Primus. I have tried to get some, but they can't be bothered shipping them to either Australia or the USA. DoT regulations etc etc. SNARK!

    I suspect that they have simply made the containers with a heavier wall thickness. That should be enough to get the higher tensile strength to handle the pressure.

    Cheers

    Nick Gatel
    (ngatel) - MLife

    Locale: Southern California
    Re: Re: Re: CO on 07/04/2013 15:48:52 MDT Print View

    CO is produced when there is incomplete combustion of any hydrocarbon fuel. Perfect air/fuel mixture produces no CO, but this is almost impossible to do. With a backpacking stove there are too many uncontrollable variables.

    In a car, perfect combustion would be an air/fuel ratio of 16:1, but that would create excessive HC and NOx emissions. So car manufactures compromise with 14.7:1 ratio and use exhaust gas recycling and catalytic converters with oxygen sensors to provide constant feedback and adjustments to keep all emissions within "acceptable levels." There is no free lunch :)

    jerry adams
    (retiredjerry) - MLife

    Locale: Oregon and Washington
    Re: Re: Re: Re: CO on 07/04/2013 16:02:01 MDT Print View

    So in a car if you have something less than 14.7:1 air to fuel it will start producing CO?

    Roger Caffin
    (rcaffin) - BPL Staff - MLife

    Locale: Wollemi & Kosciusko NPs, Europe
    Re: Re: CO on 07/04/2013 16:16:35 MDT Print View

    Hi Mike

    A couple of things. First, I have followed the literature on flame chemistry and combustion engineering for some time now, and I do understand the basic chemistry and thermodynamics. Second, I have also followed the literature on CO deaths in small enclosures, including tents and huts for some time.

    Can one kill oneself by using a stove in a moderately (or even partly) sealed enclosure such as a tent? Oh yes, absolutely, and people have done just that on a number of occasions, around the world. We have had that happen here in Oz in a snow cave, with heavy snow fall blocking vents, wet snow stopping air flow, and a well-known white gas stove. The four bodies were found in and out of sleeping bags, with the stove set up, valve open and tank empty.

    > They had mis-assembled their lantern
    User error. Unfortunate of course, but the responsibility rests with the user. A bit like driving a car at 100 mph on a wet road.

    > a plume that registered 500-1000ppm CO. That's a monstrous amount of CO
    True, I agree. MSR seem happy to sell the Reactor stove which emits (by my measurements) around 2,000 ppm. User responsibility to take the necessary precautions.

    On to techie details.

    > Flame quenching is a second order effect.
    I am not sure what you mean by 'second order' here. I don't think the term means anything. What I am sure of is that flame quenching does happen in practice, as I have run enough enough experiments under controlled conditiosn to verify this. That is, I have put cold steel and titanium in and out of a flame and monitored the effect on the CO levels. It happens.

    > A recirculation explanation seems to fit your data quite well.
    We will have to disagree on this. It does not fit the data at all. See next.

    > but it's hardly a failsafe design.
    Nothing in this world is failsafe. However, experimental data from actual measurement shows that one can have a stove emitting under 10 ppm of CO in the exhaust stream, sometimes as low as 2-3 ppm. If the hazard from recirculation was that severe, you would not get that result. the recirculation theory fails the experimental data. So I don't think it is all that preposterous.

    > the kind of attitude that the buying public wants to see in their would-be stove designers
    Me, I go for experimental results over theory every time. Sure, the theory is valuable in helping you design something and understand what may be going on, but only real measured data tells you whether you got the theory right! I have the data.

    Cheers

    Nick Gatel
    (ngatel) - MLife

    Locale: Southern California
    Re: Re: Re: Re: Re: CO on 07/04/2013 16:17:07 MDT Print View

    Less than 16:1 produces CO. Cars use catalytic converts to reduce the CO because the A/F ratio is under 16:1. At 16:1 the combustion temperature is too high, which creates too much NOx. Lower combustion temperatures are critical for NOx reduction. By recycling exhaust gases into the combustion chamber we can reduce combustion temperatures, which even at 14.7:1 would be excessive.

    Also at 16:1 too much HC is produced.

    So cars run at 14.7:1 because we are concerned about HC and NOx too. Too much NOx is very bad for our atmosphere. Here is what my 200,000 mile SUV currently produces out the tailpipe:

    CO2 = 15%
    O2 = 0%
    HC = 1 ppm
    CO = .01%
    NOx = 5 ppm

    ppm = parts per million

    Allowable emissions in Calif for my year vehicle are:
    HC = 98 ppm
    CO = .55%
    NOx = 978 ppm

    There is no standard for CO2.

    Edit: this is response to Jerry's question.

    Edited by ngatel on 07/04/2013 16:19:25 MDT.

    Michael Gillenwater
    (mwgillenwater) - M

    Locale: Seattle area
    Re: The Evolution of a Winter Stove - Part 1 on 07/04/2013 20:43:44 MDT Print View

    Roger,

    I'm excited to see your series on this coming out. As I have posted before, I am eager to see someone better optimize a design for a winter canister stove. Keep it coming and I want to be an early customer.

    Tanner M
    (Tan68)
    Re: Re: CO on 07/04/2013 22:45:25 MDT Print View

    Are you suggesting that the base of a pot could create a circulation pattern that could lead exhaust to the intake ?

    I wonder how much exhaust would have to enter the mix chamber for appreciable amounts of CO to be created.

    There was an article a little while back that explained how the bottom of a pot could work together with the burner to create a roiling effect. This particular stove created very little CO and is actually designed to create this effect. The description just seems like a good illustration of what I believe is your point.

    You might have to jump to 'Fire Maple FMS-300T' at: http://www.backpackinglight.com/cgi-bin/backpackinglight/2013_developments_canister_stoves.html#.UdZKUsUXJv8

    I don't get the idea you exclude flame quenching as a source. I guess that is a secondary effect because it happens after combustion and exhaust gas recirculation is a primary effect because it happens to/during combustion. I don't think you call it 'second-order' in an attempt to minimize its contribution.

    If I put cold metal in the flame and that cold metal is a pot, I could contribute CO generation to quenching but that doesn't mean some exhaust gas didn't recirculate. I could isolate the intake to remove the possibility and test one variable at a time. I suppose if I just put cold bar stock into the flame there wouldn't be much likelihood of exhaust gas being redirected... I guess it would still be a good practice to isolate the intake.

    Mike Cecot-Scherer
    > I know recirculation can cause CO generation because I have made CO with a camp stove or two in just this way. And I've found that wind screens are a terrific way to cause these recirculation currents. Put them on: significant CO generated. Take them off: low to no CO. Isolate the intake from the exhaust and then put on a windscreen: again, low to no CO (all done with a pot of water on the burner). Seems pretty convincing.

    Stuart R
    (Scunnered) - F

    Locale: Scotland
    Re: Re: CO on 07/05/2013 08:56:34 MDT Print View

    > carbon monoxide results from incomplete combustion CAUSED by the fuel-to-oxygen ratios being shifted to too rich by low oxygen exhaust mixing into a stove's intake.

    In a stove the fuel-to-air ratio of the pre-mixed gas is rich in any case, without any exhaust recirculation. Complete combustion relies on secondary air mixing with the flame.

    > But I must point out that you have no actual data as to where CO is generated and destroyed in the flame.
    The light blue part of the flame is where all the CO is generated and the dark blue upper part of the flame is where this residual CO burns in the secondary air.

    > I know recirculation can cause CO generation because I have made CO with a camp stove or two in just this way. And I've found that wind screens are a terrific way to cause these recirculation currents. Put them on: significant CO generated. Take them off: low to no CO.

    Yes, if you restrict the availability of secondary air (or quench the flame) then you will get CO. Wrapping a windscreen tightly around a stove, or using it in a sealed up tent are both good ways to restrict the availability of secondary air.

    jerry adams
    (retiredjerry) - MLife

    Locale: Oregon and Washington
    Re: CO on 07/05/2013 09:05:02 MDT Print View

    Is CO only an issue if you run stove in closed tent, room, snow cave, or whatever?

    If it's raining, I run stove under one side of tent right next to door which I leave open. And I just boil water so I don't run it long. I don't think I have to worry.

    If you're creating CO does that mean you're not running the stove efficiently? Or does making stove more efficient tend to result in CO? Or are the two independent?

    Damien Tougas
    (dtougas) - BPL Staff - F

    Locale: Gaspé Peninsula
    Needle valve after pre-heat loop on 07/05/2013 10:53:03 MDT Print View

    Very cool, I am looking forward to the rest of the articles!

    Having the needle valve after the pre-heat loop, is there any danger of excessive pressure building up in the fuel line due to the high volume of expansion that happens when the fuel vaporizes?

    Mike Cecot-Scherer
    (mikescherer) - F - M
    Re: Re: Re: CO on 07/05/2013 11:17:35 MDT Print View

    Hi Tanner,

    Exactly! a circulation pattern (not necessarily symmetrical or uniform) that brings exhaust back to the intake.

    My understanding is that combustion engineers will always shoot for a slightly lean mix (including secondary air entrainment) as a safety feature even though it decreases the efficiency due to the heating of extra gas. Lean flame becomes surprisingly inefficient.

    I definitely don't want to be seen as trying to exclude flame quenching as a source of CO. I refer to recirculation as primary because it messes with the most basic part of the combustion reaction - the fuel/air mix, because it's the dominant effect in many accidents, and it's able to create incredibly high CO concentrations for long periods of time.

    That roiling pot/stove combo is very interesting. Seems like they made the circulation strong enough that it entrained fresh air and isolated the intake - very cool! A nice example of the fractal nature of reality - loosely speaking, a bad thing that becomes good after a certain point.

    After thinking about it I think I can offer a couple of characteristics that should be not-unusual for recirculation caused CO in some stoves:
    — uneven CO distribution around a centered pot that has maximums in-line with the intake ports.
    — a burner that fires more upward than outward might show high CO because of what I'm going to call "flame bounce" off the pot; essentially driving exhaust downward. Stoves with more outward directed burners should be more resistant to recirculation. A Pocket Rocket might fit this.

    Best,
    Mike

    Edited by mikescherer on 07/05/2013 11:29:19 MDT.

    Mike Cecot-Scherer
    (mikescherer) - F - M
    Re: Re: Re: CO on 07/05/2013 11:26:15 MDT Print View

    Hi jerry,
    Yes, exactly. But the reduced oxygen I'm referring to is the percentage of oxygen, not the absolute up or down variation from altitude. Altitude is another thing. Maybe Roger can explain some of the ins and outs in his next article.
    Best,
    Mike

    Nick Gatel
    (ngatel) - MLife

    Locale: Southern California
    Re: Re: CO on 07/05/2013 11:42:35 MDT Print View

    Roger's testing under controlled conditions show that some stoves emit more CO than others. If I remember correctly, MSR Pocket Rocket among the worst and Snow Peak GigaPower among the best. With proper ventilation you can safely cook in a vestibule. The problem with CO poisoning is that it is a silent silent killer. You are dead before you know there is a problem.

    jerry adams
    (retiredjerry) - MLife

    Locale: Oregon and Washington
    Re: Re: Re: CO on 07/05/2013 11:43:49 MDT Print View

    Yeah, Roger hates Pocket Rockets : )

    Mike Cecot-Scherer
    (mikescherer) - F - M
    Re: Re: CO on 07/05/2013 12:04:24 MDT Print View

    Hi Jerry,

    The vast majority of stove users have found no issues with CO in well ventilated areas. Your description of how you use your stove sounds pretty safe. It seems to match the protocol of the Rainier Guides. You could always check things with a little CO monitor...

    If the burning device causes concentrations of CO over 500ppm it could literally knock a person out so my personal opinion is that there is no safe way to use such a product.

    (BTW I should probably mention here, given what's been written here in BPL, I have a production MRS Reactor, love it, and mine produces no CO.)

    That said, I believe that hikers and climbers that are struggling to acclimate to higher altitude are probably routinely being impaired by low levels of CO and, in the case of sealed-in stove users, by lowered oxygen in their tent. My 2¢.

    Best,
    Mike

    Edited by mikescherer on 07/05/2013 14:09:08 MDT.

    David Thomas
    (DavidinKenai) - MLife

    Locale: North Woods. Far North.
    Enclosed areas. on 07/05/2013 13:01:06 MDT Print View

    Jerry: Being in an enclosed area has several effects.

    For the stove: if you're in such a tightly sealed environment (unventilated tent, snow cave without vent holes or in which vent holes have been covered by snowfall) that the percentage of O2 falls and CO2 rises, then the stove will see less oxygen in the pre-mix air and hence the flame will be richer in fuel (i.e. prone to make more CO) - the effect Mike is discussing, although he is also arguing for smaller-scale recirculation within a windscreen, for instance.

    For you: not only may the stove be making more CO per minute, but all the CO (and CO2) being generated is being retained in your breathing space and increasing in concentration the longer you run your stove.

    Usage pattern makes a difference. If, in a very-tight house, someone bakes a batch of cookies, they end up with a batch of cookies. If, however, they attempt to heat their very-tight house by running an unvented stove continuously, they can end up dead. Similarly, your brief usage of a stove, near the entrance, just to boil water is well within my personal safety limits. While running a stove in an attempt to heat up a sealed tent or a snow cave is a bad idea. If the area is sealed enough to retain heat, it is inadequately ventilated to run combustion equipment.

    All stoves make CO. Some make a little, some make a lot. If you use it in a way that you are venting that poisonous gas away, fine. If you are in an environment in which the stove's heat or water vapor effluent (two things you can detect) are being retained, you need more ventilation.

    Nick Gatel
    (ngatel) - MLife

    Locale: Southern California
    Re: Enclosed areas. on 07/05/2013 14:33:57 MDT Print View

    Jerry,

    We have a furnace in our camper with a vent to the outside, so we can keep the camper closed up when using it. But most often we use a catalytic heater because it uses about 75% less propane and no electricity. When using the catalytic heater, the camper must be vented otherwise the heater will deplete the oxygen, which then cause more CO. Some catalytic heaters have oxygen depletion sensors that will turn the heater off. Most of these heaters with the depletion sensors don't work above 7,000 feet.

    That is my blue-collar explanation since I am not a scientist. Sort of my translation of what David Thomas said :)

    Roger Caffin
    (rcaffin) - BPL Staff - MLife

    Locale: Wollemi & Kosciusko NPs, Europe
    Some simple facts on 07/05/2013 16:38:34 MDT Print View

    Let's have some measured facts here.

    Measurements show many stoves emit quite low levels of CO (<30 ppm). This is incompatible with the recirculation theory.

    Measurements show that a stove emitting a very low level of CO can be made to emit a whole lot more by inserting cold steel into the upper part of the flame and quenching the combustion process. Removing the steel from the flame drops the CO level back.

    Measurements show that a simple windscreen placed 3/4 of the way around a stove does not increase the CO emitted. A totally enclosing windscreen might bve very different - don't do that.

    Measurements show negligable amounts of CO in a tent when using a good stove in a ventilated vestibule. (Actually, very often the readings are down around 2 - 3 ppm, in the 'noise'.)

    Basic physics/chemistry also says that an incomplete combustion process gives off less heat than complete combustion.

    Cheers