The Evolution of a Winter Stove - Part 3
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 Roger Caffin (rcaffin) - BPL Staff - MLife Locale: Wollemi & Kosciusko NPs, Europe Re Stuart's comments on 11/21/2013 02:51:02 MST I'm with Stuart here.One of the most common mistakes is to think that the propane in a butane/propane mixture will behave the same way as 100% propane. It won't. Read the article Stuart referenced - yeah, he and I wrote it!Cheers
 Douglas Frick (Otter) - MLife Locale: Wyoming Re: Re: Temperature limits? on 11/21/2013 05:47:01 MST Oh well, back to school...
 Jeff McWilliams (jjmcwill) - M Locale: Midwest Temperature Limit on 11/21/2013 06:09:24 MST So, if I understood the article that Stuart and Roger wrote, it says, "When the canister is used upside-down (inverted) with a stove which has been designed for this mode of operation, the effect of decreasing % propane described above is avoided."Does that mean I could look at Figure 4: Threshold Temp(C) vs Gas Remaining at given Pressure, and assume 100%? At 3000', that would give 20/80 a threshold temp of about -26.5C, and 30/70 a threshold temp of -30C. Is that correct?I like the extra "oomph" of Kovea's 30/70 compared to 20/80, which is what MSR IsoPro Red and JetBoil (I think) contains. Unfortunately, I've not seen Kovea canister fuel for sale anywhere in my area.
 Stuart R (Scunnered) - F Locale: Scotland Re: Re: Re: Temperature limits? on 11/21/2013 06:25:25 MST Learning is good. To be fair, most of the referenced article is about upright stoves, as that is where there are interesting effects that need explanation. However there is a paragraph about inverted canisters. In essence, using a canister inverted preserves the propane % content and so the boiling point of the mixture remains at the "100% remaining" point shown on the graphs. In other words, the minimum operating temperature is preserved throughout the life of the canister, unlike the upright canister situation.
 jerry adams (retiredjerry) - MLife Locale: Oregon and Washington Re: Re: Temperature limits? on 11/21/2013 08:11:48 MST "Jerry, the problem you describe applies to canister-top stoves (canister sits upright, with burner on top) operating in the +12F range, where the propane alone boils and burns off, leaving 3/4 canister of (useless) liquid iso-butane"Oh... Good point. Another reason for using an inverted stove : )Having 20% propane for an upright doesn't make that much difference, although if you just had one or maybe two really cold nights it would be pretty uesful.
 Stuart R (Scunnered) - F Locale: Scotland Re: Re: Re: Temperature limits? on 11/21/2013 08:49:50 MST Iso-butane is not useless. In fact, if you are using an upright stove you want the highest proportion of iso-butane that you can get: MSR IsoPro or Jetboil are good candidates. It's regular n-butane that is useless in an upright stove in winter.On the other hand, with an inverted canister, butane vs iso-butane doesn't matter much, it's a high propane content you want.Reason: with an upright stove, any propane is quickly used up, so it's what's left (butane or iso-butane) that matters. With an inverted canister, the propane is preserved (in % terms), and the more propane the better for low temperature use.
 James Marco (jamesdmarco) - MLife Locale: Finger Lakes Re: Re: Re: Re: Temperature limits? on 11/21/2013 09:07:03 MST Propane is a slightly smaller molecule than butane, so it tends to concentrate slightly near the bottom. So it isn't quite 100%, but clse enough not to worry. Gravitional effects are very slight compared with solutes and usually ignored.Pressure changes (as in a closed canister or one running at constant low power) will dictate that canister temperture will change for "toppers." The liquified gas never really boils in a pressure vessel. Rather, the vapor pressure/gas pressure is maintained at an equivalency, at the temperture it is currently at. (This is why we don't ever hear a canister burping and boiling inside.) By drawing from the bottom, in liquid form, we reduce the effect of cooling on the container, since the liquid has 1/200-1/300 the volume of the gas(given some delta-T.) The heat normally lost through drawing the gas off a topper, is supplied by the heat shunt on Rogers stove. As long as there is enough pressure to force the liquid out of the cannister, through the tubing, valve and and connectors, the stove WILL operate. Though, there will be a lot of fluttering to start with till it reaches a balance. How much is needed? Probably not much. I would guess you could blow water through the system. So maybe 3 atmospheres? This would easily take the stove down to -40 or so, since there is some vapor pressure evan at -40F/-40C(yup, same number...darn COLD) as long as there is some propane in the cannister. Likely closer to -50F(about 4-5 bar in a canister,) since the propane doesn't actually have to boil to hit these pressures, though the stove may not light too easily since the mixed liquid would not gasify too well.EDIT: Sorry, the tables I looked up had both Bars and PSI. The -40F/C temps were based on that. My mistake, it should be about -25C. Edited by jamesdmarco on 11/21/2013 14:04:24 MST.
 Stuart R (Scunnered) - F Locale: Scotland Re: Re: Re: Re: Re: Temperature limits? on 11/21/2013 09:32:55 MST As usual, I don't understand a word you write James.Propane is a slightly smaller molecule than butane, so it tends to concentrate slightly near the bottom.Really? At the bottom of what? I rather think the thermal energy of the molecules will ensure a pretty even distribution in something a small as a canister on planet earth.The liquified gas never really boils in a pressure vessel. This is why we don't ever hear a canister burping and boiling inside.Take a cold canister, put it in some warm water and listen - you WILL hear it fizzing inside.there is some vapor pressure evan at -40F/-40C(yup, same number...darn COLD) as long as there is some propane in the cannisterSure there is some vapour pressure due to the propane, there is even some due to the butane at -40F/C BUT the combined vapour pressure is LESS than atmospheric pressure (unless you are on the summit of Everest:-) so when you open the valve, air will go INTO the canister: gas will not come out.
 Douglas Frick (Otter) - MLife Locale: Wyoming Re: Re: Temperature limits? on 11/21/2013 11:56:26 MST Jerry, you might want to ignore the details in my explanation, after I got schooled on it. :-) An (important) correction: I checked my stove log, and found that the coldest I'd operated the Coleman Exponent stove with Powermax fuel UNDER THE CONDITIONS I STATED (fuel actually at ambient air temperature) was -25F, not -30F. I've used the stove at -30F and lower, but on those trips I always did something to keep the canister warmer than ambient.So, I've been trying to reconcile my experience with the operational limit of my Coleman Exponent stove being -25F, with Stuart's comment "you can expect the stove to work with an inverted canister down to 0F." However, after looking at the charts, it seems both of our statements can be correct (after mine was corrected ;-). The left chart of Fig. 4, typical canister mix at sea level limit is -23C, +5 degrees to get some working pressure, so -18C = 0F. The right chart, Powermax fuel (30% propane, 70% iso) at 10K altitude (where I use the stove) limit is -36C, + 5, so -31C = -24F. Assuming I haven't screwed up again, that explains why my stove operated (poorly) at -25F.Thanks, Stuart and Roger, for reminding me about your excellent article. I forgot just how much difference mix and altitude make; I'll try to remember that when I'm outside Wyoming. Also, thanks for clarifying the mixed-gas boiling science.To the original question, perhaps the most relevant point is that, while all canister stoves stop operating at some low temperature if you don't do anything to keep the fuel warm, you can run these stoves regardless of the air temp if you simply keep the canister at, say, +32F. As mentioned in this thread (and in the article), that can be done with a pan of liquid water. The bottom cover for my original JetBoil is has a diameter just slightly larger than a small fuel canister, so I set the assembled stove in the cover and fill it with water. After a while a skim of ice forms on the water, but it runs just fine. For my Coleman stove, I usually work-pack the canister in snow to keep it 'warm' and prevent it from chilling in the overnight air. Edited by Otter on 11/21/2013 13:34:51 MST.
 James Marco (jamesdmarco) - MLife Locale: Finger Lakes Re: Re: Re: Re: Re: Re: Temperature limits? on 11/21/2013 14:00:01 MST "Propane is a slightly smaller molecule than butane, so it tends to concentrate slightly near the bottom.Really? At the bottom of what? I rather think the thermal energy of the molecules will ensure a pretty even distribution in something a small as a canister on planet earth."Yeah, very small, ignore it."Take a cold canister, put it in some warm water and listen - you WILL hear it fizzing inside."Yeah, that can happen. But this doesn't effect the over-all steady state pressure as you use the stove, well slightly decreasing. You just need to calculate at the new temperature. In your example, once it hits around 32F/0C it should stabilize again, within a minute or two, I would guess. Boiling is simply the explosive release of vapor pressure. In a closed system, this cannot happen, except as you say, by greatly changing temperature/pressure conditions. The valves(lindal and control valve) *limit* the flow of gas and/or liquid. There needs to be positive pressure in the canister for the stoves to work. The pressire also acts on the liquified gas varying the vapor pressure AT the boiling point of the new system. Not above it or below it. (Actually as you burn the gas it is slightly decreasing, drawing more gas out of the liquid state, but not really boiling, vapor pressure is usually enough.) Like boiling water in a closed vessel. Unless you actually flash heat it, no boiling happens till it actually explodes, then ALL the superheated water (at the new lower pressure) will try to boil till it reaches a steady state again.The data was from: http://www.engineeringtoolbox.com/propane-vapor-pressure-d_1020.htmlYour right, I just realized the tables were not all in bars, the F temps are in PSI. Sorry about my mistake. Looks like -13F/-25C will supply about 2 bars. Again, my apologies. I will correct it in the above note. Anyway, from this it is possible to conclude that the interior surface of the LP gas inside a canister may be more important than we previously guessed. The concave bottoms, once exposed inside, probably cause lower volumes of gas to be available for use...perhaps the last sixth or so of the canister.
 Stuart R (Scunnered) - F Locale: Scotland Re: Re: Re: Temperature limits? on 11/21/2013 14:11:25 MST I'm pleased that we are both right Douglas :-)My statement "you can expect the stove to work with an inverted canister down to 0F." was beng conservative, an inverted canister can work at lower temperatures depending on your circumstances whch I was unaware of.You say you were using a Powermax canister, which alters things. I thought that in the US they contained 35% propane, 65% n-butane? (I believe they contained 40% propane in the rest of the world, Roger will correct if I am wrong here) If the first assumption is correct, then the boiling point of this mixture is -15F at sea-level and -31F at 10k feet. So if your ambient was -25F then this would work, just. Edited by Scunnered on 11/21/2013 14:14:55 MST.
 Stuart R (Scunnered) - F Locale: Scotland Re: Re: Re: Re: Re: Re: Re: Temperature limits? on 11/21/2013 14:25:04 MST James, that table is for 100% propane. You need to look at these tables for propane/butane mixtures:http://www.engineeringtoolbox.com/propane-butane-mix-d_1043.htmlNote that this table is refering to the % gas mixture, which is different from the % liquid (weight) mixture shown on canisters: a canister containing 30% propane in the liquid has about 70% propane in the gas mixture, which according to these tables has a positive pressure (Psig) down to around -25FThe concave bottoms, once exposed inside, probably cause lower volumes of gas to be available for use...perhaps the last sixth or so of the canisterYou lost me again...Edit - correct link Edited by Scunnered on 11/21/2013 14:41:51 MST.
 Roger Caffin (rcaffin) - BPL Staff - MLife Locale: Wollemi & Kosciusko NPs, Europe Re: Powermax on 11/21/2013 14:36:04 MST As far as I can tell, the Powermax canisters did not state the fuel composition - and I suspect it changed over time too. But they had the right design (for winter)!Cheers
 Andrew Stow (AndyS) - F - MLife Locale: Midwest USA Re: Re: Re: Re: Re: Re: Re: Temperature limits? on 11/21/2013 14:51:55 MST "Anyway, from this it is possible to conclude that the interior surface of the LP gas inside a canister may be more important than we previously guessed. The concave bottoms, once exposed inside, probably cause lower volumes of gas to be available for use...perhaps the last sixth or so of the canister."If you're saying what I think you are, that since in upright use the dome will expose leaving a donut of liquid surrounding a shallow metal dome, with the liquid surface generating vapor pressure and the metal surface generating none... no, pressure does not work that way. The gas volume will stay at the vapor pressure of the remaining liquid until no liquid is left. The wetted surface and its shape are not relevant.