Evaporative Heat Loss in Upright Canisters Part 4 – Bombshell
Oct 8, 2017 at 5:40 am #3495424
or How to Use Cheap Butane
Jerry Adams with assistance from Roger Caffin
or Why propane isn’t very useful in an upright canister stove if you want to use the entire canister at low temperatures
Given all the ideas in the previous three parts to this series, one has to ask what limitations restrict the use of upright canisters at below-freezing temperatures. It turns out there some significant restrictions. I took three different fuel mixtures and ran them through the spreadsheet used in the article The Effect of Cold on Gas Canisters by Stuart Robb and Roger Caffin to see what would happen to the propane concentration as the gas was used up. The results were a bit surprising.
The graph here is for canisters at -10 C (14 F). The X axis is how full the canister is: 100 = full canister (at left) and 0 = empty canister (at right). The Y axis on the right is pressure, and 1000 mBar is the nominal pressure at sea level.
The pink line shows how pure iso-butane would behave: the pressure stays constant of course as the composition does not change (it’s all iso-butane). The green line shows what happens if the canister starts with 20% propane: by about the 75% mark the boost from the propane is all gone. It has boiled off.
The pale blue line shows what would happen with a 50/50 blend of iso-butane and n-butane IF you were running it at very high altitude (low ambient presure). The iso-butane would evaporate slightly faster than the n-butane and the pressure would keep slowly dropping. The dark blue line shows what you would get with a propane/iso-butane/n-butane blend: the pressure starts higher but still cascades down to a similar very low end-point. However, and this is important, that would only work at extreme altitude. The pressure at sea level is 1000 mBar, so in practical terms you would never get any gas out of the iso/n canister at sea level at -10 C, and the canister with some propane would terminally die when it was half-empty.
A technical recapitulation: if the pressure is greater than 1 atmosphere or 1000 mBar (the Y axis) then there will be some pressure to the stove and it will work at sea level. Actually, it needs to be a little higher, say 1100 or 1200 mBar, to provide enough pressure for a good burn (corresponding to boiling a pint of water in less than 6 minutes with my stove). If you are at 3,000 m where the ambient pressure is lower, then a slightly lower pressure is enough to get the stove to work – but the ambient temperature may be a bit colder.
Therefore, to use a canister at cold temperature, you want as little N-butane as possible. That really is the crunch point. Whether you can rely on a label which says ‘iso-butane’ – that’s a different question.
In the petroleum refining process, it’s easy to separate propane because the boiling temperature is so much lower than butane, so propane is cheap and widely available. It’s more difficult (= expensive) to separate iso-butane from n-butane because their boiling temperatures are close together. It’s much easier to make a canister fuel that works when the canister is new at low temperatures by just adding back some propane than removing the N-butane. The fact that the benefit does not last is just tough luck.
Could the ‘blends’ we see on the shelves just be what the manufacturers can get out of their available feedstocks at the lowest possible price? Is a blend of propane/iso-butane/n-butane be just what the incoming gas was, without any treatment? And could it vary a bit from shipment to shipment? One wonders.
But note that this is all for upright canister stoves – remote inverted canister stoves don’t have this problem as by design there is no preferential evaporation going on. Adding propane will very helpfully lower the usable temperature for the entire canister. N-butane is okay if balanced by a good amount of propane, but you’re still better off with less n-butane. The Cold Canisters article mentioned above gives results for a number of common fuel blends: use the values for when the canister is full.
or How propane depletion is barely changed at cold temperatures
Let’s look at this from a different direction: how does the temperature affect the depletion of propane from an upright canister? Again, I used the spreadsheet from the article The Effect of Cold on Gas Canisters, this time to see how much propane is left when you start with canister containing a 20% propane / 80% isobutane mixture and use it at different temperatures.
The amount of fuel left in the canister is given at the left. The temperature is the top line of numbers. The rest of the numbers are the percentage of propane left for the given conditions.
The startling result is that changing the temperature from +20 c to -10 C has very little effect on the composition by the time you get down to a half-empty (50%) canister. Running at 20 C there is only 6.9% propane in the liquid remaining in a half-empty canister; running at -10 C there is only 5.2% propane left. This very small difference corresponds to about 1 C (2 F) difference in the boiling point for that canister.
It is commonly said that there is more propane depletion when you run at colder temperatures. This is so because the propane is still (as it were) boiling away but the iso-butane component is not. Well, the data in the table is consistent with this, but the effect is minor. I think it’s better to just accept that propane is going to be seriously depleted regardless of the temperature you run it at, IF you used the canister upright. And keeping the canister warm does not win you much towards the end either.
Once again, none of this applies to the use of an inverted canister stove. There is a reason they exist, after all.Oct 8, 2017 at 9:06 am #3495433Gunnar HBPL Member
Interesting. But when I wanted to dig into the spreadsheet to use it to remind me about the temperature limits at sea level for a inverted canister (lower than -10C), the link led me to something completely different.Oct 8, 2017 at 9:15 am #3495437
Strange are the ways of the web site SW. I have no idea why that happened, but try again please. (I THINK I have fixed it.)
CheersOct 8, 2017 at 9:29 am #3495439Gunnar HBPL Member
Now its working. Thanks!Oct 8, 2017 at 1:40 pm #3495459
Thanks for posting this Roger, I’ve wondered if part 4 would ever show up : )
This is just in time for winter so good timing.
Except for you upside down people (in Australia)…Oct 12, 2017 at 10:04 pm #3496395
I am still goggling at the way the propane gets used up even halfway through the canister.
CheersOct 13, 2017 at 3:37 am #3496464Mark FowlerBPL Member
I assume that there is a similar faster relative drawdown of iso-butane compared to n-butane so that at the end you are left with n-butane. This no doubt is less dramatic than the propane drawdown but still important if you are counting on getting every btu or joule out of a canister. I expect even canisters claiming to be pure iso-butane will also have residual of n-butane.Oct 13, 2017 at 4:45 am #3496476
You are of course right. The iso-butane will go faster than the n-butane, but not as badly as the propane.
At this stage, I have ceased to believe anything written on the canisters. In fact, I note that many canisters simply specify ‘butane’, which means they are not distinguishing between the two. In practice I suspect that the ratios vary all over the place with negligible controls in place.
CheersOct 13, 2017 at 6:04 am #3496484
SO Roger, which is better in sub-freezing weather?
- upright canisters with Moulder copper strip (or Caffin aluminum strip)
- inverted remote canister
I have both types and a nice Moulder strip for the upright with a Brunton folding burner.
I also have some neoprene cozies to help maintain canister heat. (Big beer cozies are nice.)Oct 13, 2017 at 6:15 am #3496485
I doubt that there is a single ‘best’ solution. Instead I will try to list pros for each one. Doubtless I will miss some things.
Fast to set up in the field
Inverted remote canister:
Will work to lower ambient temperatures
Canister fuel ratios do not change as the canister gets used up
More robust priming and operation
Much larger support for pot so greater stability (Not V1 Caffin stove)
Lower pot, especially with a 450 g canister
PS: if anyone can come up with other key points I will happily edit this posting.Oct 13, 2017 at 1:05 pm #3496498
“I assume that there is a similar faster relative drawdown of iso-butane compared to n-butane so that at the end you are left with n-butane.”
Look at the light blue line in the first plot. That is for a mix of 50% n-butane, 50% isobutane.
You can see the pressure gradually going down (in other words the isobutane has preferentially evaporated a little) for the first 90% of the canister.
For the last 10% of the canister, there’s even less isobutane, just n-butane.
So, yes, there is a drawdown of the isobutane, but mostly it’s just for the last 10% of the canister.
This helps explain why it takes so long to get the last little bit out of a canister.Oct 13, 2017 at 1:10 pm #3496499
since you have both types Eric, which do you prefer?
The Moulder strip in addition to being easy MYOG, is an entertaining project for people to experiment around with different configurations to see what works best. Sort of like alcohol stoves.Oct 13, 2017 at 2:12 pm #3496508Gary DunckelBPL Member
“Will work to lower ambient temperatures
More robust priming and operation”
I’m wondering just what you mean with these 2 points, Roger. I feel that my Moulder Strip performs better at sub-zero (F) temperatures than do either of my remote canister stoves. Also, what is ‘more robust priming and operation’? With either technique one must warm up the canister a bit to get the vaporization needed to get the stove started. The Moulder Strip will soon take over and facilitate smooth operation until the canister is empty (if one so chooses). An inverted canister will still need to occasionally be warmed up a bit to provide enough internal canister pressure to push the liquid fuel through the fuel line. At 0* to +10* F this likely won’t be a significant factor, but when ambient drops down below that my bet is that a remote canister setup will begin to fall behind, while the Moulder Strip system will happily continue to chug along.Oct 13, 2017 at 3:33 pm #3496522Ken ThompsonBPL Member
@hereLocale: Right there
Come on Gary, Roger has turned into what he despises, marketer.
Bob has no money in the race.
Cheers.Oct 13, 2017 at 7:17 pm #3496556
I prefer the remote, inverted canister B/C the burner (MSR Whisperlite Universal) is very stable in compacted snow. I guess I just don’t entirely trust the Moulder strip in very cold weather. (Sorry Bob. I need more experience with it and I’ll get it this winter.)
**For stability in the snow I’ve made a circular 1/4″ plywood base for the burner with turnable screen door hold-downs to keep the burner legs in place on the plywood. The plywood has been painted with several coats of automotive Hi-Temp spray paint, mainly for waterproofing. But then too it’s B/C I use the Whisperlite Universal with white gas as well and the Hi-Temp paint would withstand any spilled fuel that might get accidentally ignited during priming. :o(Oct 13, 2017 at 7:45 pm #3496560
yeah, that’s one thing good about inverted canister, the stove is low to the ground, not on top of a canister
also better wind resistanceOct 13, 2017 at 10:07 pm #3496579
“Will work to lower ambient temperatures
More robust priming and operation”
I’m wondering just what you mean with these 2 points, Roger.
OK, explanations needed.
A nominal propane/butane canister will generate enough pressure to drive an inverted canister stove at -20 C (-4 F) without any further assistance. And there will be negligible cooling of the canister while an inverted canister stove is running, so again, no further assistance is required. Operation is robust even at such low temperatures.
However, it is not difficult to keep a canister a bit warmer than that in your pack while you are traveling: your water bottle does not freeze in your pack as it is kept ‘warm’ by heat from your body. So using a canister out of your pack when the ambient is -30 C should still work easily.
With either technique one must warm up the canister a bit to get the vaporization needed to get the stove started.
That is wrong, when above -20 C with a nominal propane/butane canister, for any stove. The difference is whether the fuel is subsequently evaporating inside the canister and cooling it, or at the stove.
Perhaps more importantly, starting an inverted canister stove at -30 C can be done with little more than a bit of water in a bowl, followed by some radiation back from the flames to the canister. That is reliable. It is not difficult to keep the canister above -20 C. Again, note that the fuel is not evaporating inside the canister and does not cool down there.
You could in principle also start an upright stove with a Moulder Strip like this, but I think a little more care and skill might be needed (imho). Given the observed delay in getting heat back down the metal strip to the canister, there is some risk of the fuel cooling too much before the heat gets there. More skill needed.
You can get much faster feedback to an upright canister with an aluminium foil reflector, but that may take a bit more skill. The same principle applies to an inverted canister getting some radiation from the flames immediately after the stove has been lit.
Roger has turned into what he despises, marketer.
A decidedly LOW blow!
I plead guilty to promoting the use of canister stoves in general.
CheersOct 13, 2017 at 10:12 pm #3496583James MarcoBPL Member
@jamesdmarcoLocale: Finger Lakes
Gary, Boyles law more or less says the inverted canister in a remote system will raise in temperature as it is used. This approximates the temperature rise of a canister using the Moulder Strip.
Since the pressure at the heat exchanger on a remote stove will travel back down the tube to the canister (due to warming) AND out the regulated jet, it will force the volume of the gas to warm in the canister. Since the expansion of the liquid to a gas (the base cause of evaporative cooling) is confined to the heat exchanger, all it can do is warm up a bit and force more liquid out to the heat exchanger. Volume=Pressure x Temperature. (The reverse is also true. Temperature = Pressure / Volume.) This means that supplying more pressure to an essentially closed system will warm it. Heat is an illusion, it is actually a bunch of molecules pushing against each other. Soo, packing them closer means…more heat and higher pressure. IR is the only true heat. (Our perception of heat is not correct because both types feel the same.) Anyway, when using a remote stove, there is no need to rewarm the canister. Do not forget that the Moulder Strip needs to overcome evaporitive cooling whereas the remote stoves do not (well around 15 times less, anyway.)Oct 13, 2017 at 10:27 pm #3496587
Sorry James, but I am going to have to disagree a bit.
The pressure inside the canister won’t change except in response to the temperature of the bulk fuel in there. It’s that temperature which controls everything.
The vaporisation of the fuel at the stove can not increase the pressure inside the canister unless it is done by sending heat energy back up the fuel line or by forcing fuel back up the line. Given the flow rate down the fuel line, this is extremely unlikely. I have watched the fuel flow down the fuel line many times in the lab.
CheersOct 14, 2017 at 12:15 am #3496621Rex SandersBPL Member
Would a Moulder strip stove be less prone to clogging from waxes and dirt in cheap fuel canisters, versus inverted?
— RexOct 14, 2017 at 12:41 am #3496630
In a word, yes. That is one of the major virtues of any upright stove, given how the fuel in the canisters now has more dust etc in it. The dust gets left at the bottom of the canister.
CheersOct 14, 2017 at 7:39 pm #3496713
POSSIBLE DUMB QUESTION:
Would insulating the flexible fuel hose from an inverted remote canister to the burner help conserve some of the fuel’s heat in sub-freezing temperatures? (This is given that the canister has been warmed a bit and has an insulating “cozy”.)
** P.S. I’m beginning to realize that what’s happening here with all my canister questions is that I seem to be re-thinking my anti-canister bias for winter camping.
I may even become a convert to the “Inverted Canister persuasion” as I try it this winter with my MSR Whisperlite Universal stove. Halleluia Brothers! AMEN! (If I begin buying titanium cookware please do an intervention for me.)Oct 14, 2017 at 8:41 pm #3496723
On the one hand … but on the other hand …
Top of the head opinion, totally unsupported by any data – I don’t think the effect would be noticeable.
In fact, one might equally suggest that radiation from the stove might warm the hose and the fuel in the hose as much as anything else. But again, the effect would be insignificant.
I THINK that what would matter would be how much energy you manage to pump into the canister via radiation and possibly hot air from the stove. Warm the fuel in the canister by a couple of degrees and the pressure in the canister will rise. That will force fuel down the hose to the stove, regardless of how cold the hose gets. Once the fuel hits the stove body, which could be above 50 C or as high as 100 C, who cares what temperature the fuel came in at?
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