science of how liquid gas works

[Carly Em]

Can anyone out there explain the physics/chemistry supporting the superiority of using unpressurized liquid hydrocarbons in cold weather conditions? I’m no skeptic of their reliability, but it’s my understanding that canister fuels are so effective due to their low boiling points and pressurized states. But kerosene and white gas have much higher boiling points, yet they’re easier to light at lower temperatures? What am I missing here? Again, I’m not trying to draw shade on liquid gas stoves; I’m sincerely just trying to understand the science. Thanks.

[Jerry Adams]

white gas/kerosene go to stove as liquid, get vaporized in tube that’s heated from the flame

butane/propane vaporize in the canister

except inverted canister stoves like Roger’s uses butane like white gas stoves – liquid goes to valve, then gets vaporized in tube heated by flame.

[Jon Fong / Flat Cat Gear]

Carly,  I am not an expert but these are my observations:

It is difficult to get a canister stove to work correctly under about 32 F due to the nature of the gasses/liquids properties.  Thetemperature can be extended by using heat exchangers and/or by keeping the canister warm.

Liquid fuels like white gas and such are actually pretty effective.  One of the biggest knocks is that most stoves require a pre-heat/priming process in order to get the liquid to vaporize before burning.  Even though liquid fuels have a higher energy density, weight efficiency is reduced due to the priming requirements.  Additionally, remote feed stoves loss a little fuel efficiency as you loss some fuel when you separate the pump from the fuel tank.  If you are heating large volumes (>>2 cups) the efficiency will improve as the fuel loss diminishes as a percentage of the whole.

Surprisingly, the fuel of choice on the Iditarod races (Alaska) seem to be alcohol.  It has a lower energy density (hence you need to take more fuel), but is more reliable than other fuels.  It is also used up to -40 F.

Hopefully Hikin’ Jim or Roger will chime in as well.  My 2 cents.

[James Marco]

Most canister stoves are designed to work at >32F or 0C. All stoves require pressure to operate, though, even vapour pressure is enough to get things burning (that’s why alcohol works.)  Butane (Isobutane and mixes) will normally supply pressure by having a boiling point of around -2C or so. Propane plends will cause them to have some pressure down to about -40 or so. But, like any distilling process, mostly propane is boiled off at low temps leaving the canister pretty weak.

So several stoves were designed to get around this problem, usually by adding a preheat tube or heat shunt of some sort to get heat back to the canister gas. This is the SAME design as using White Gas or Diesel fuel.

Canister gas requires about 50-100% more weight in the actual can. WG usually can be carried bulk in a 1-2oz PET (soda/water bottle.) So, for carrying purposes, WG is more efficient.

WG/Diesel fuel is not as compressive as  butane, so the stoves are built heavier to hold the pressure. As was said before, they also require priming. Also, WG stoves are usually designed to work at around 10000BTU output (around 3000Watts.) This is somewhat hotter than a canister stove (around 8500BTU or around 2500Watts.) Often, they do not work well on low unless you get a small stove, the SVEA 123r is only 4700BTU/1400Watts. It also does not have a pump and relies on heat only to pressurize. But, the extra heavy construction to hold the pressure means the stove weighs about 18oz. The cup/pot weighs another 2.5oz. A typical canister stove weighs around 4oz.

For trips up to 5 days or so, I use a canister. For one week to two week trips I use WG. Even though the SVEA is 40+ years old, it works fine with only the needle valve to worry about, as far as moving parts. But it means a lower start weight and lower volume. That’s all I really care about since I will be dropping weight and volume as I go.

[Franco Darioli]

A few years ago I posted this comment on another forum :
Mushers use those stoves for safety because you don’t have moving parts/fiddly bits to deal with.
Roughly ,all other things equal (stove efficiency) it takes about twice the weight in alcohol to boil the same amount as it does with white gas, but again that is not a (big) problem for the mushers.
I have used a short Caldera Cone and a 1.3L pot to melt snow but it would not be my choice if I were out more than two or three days.

So you either have a large stove (those musher’s stoves are VERY BIG) or you need to spend a lot of time melting snow and by that I mean it can take an hour to melt a liter of powder snow into a 1.5L pot.

 
Typical Iditarod alcohol stove :

yes, it is a 5 gallon paint can.

[Jerry Adams]

“It is difficult to get a canister stove to work correctly under about 32 F”

see topic:

https://backpackinglight.com/forums/topic/98947/#comments

You can go down to maybe 22 F with a full canister, maybe 28 F with a mostly empty canister, much colder with a heat strip, or my theory is a reflector with aluminum foil is even more effective but I haven’t convinced anyone else yet : )

 

[Carly Em]

Ok, so from piecing your comments together with a nifty visual I found on the web (http://zenstoves.net/How.htm#PressurizedPetrolStove), I think I may have a better handle on it now. So, the gas’ ability to vaporize within the cell, if you will, is a factor of both its heat due to its position relative to the flame and its inability to escape the fuel line due to the pressurized environment in the line itself (not the bottle). The pressure is then released from the system as the fuel exits in gaseous form, which is then ignited by the user. Is that pretty much the jist of it? Also, in response to James, what do you mean by saying that “WG/Diesel fuel is not as compressive as butane”? Would you care to elaborate for my sake?

[Jeremy and Angela]

That might be a bit complex, so I’ll take a 2-part stab at it.  Fire is the combustion of vapors that have off-gassed in some manner.  (A fuel in liquid form doesn’t generally burn.)  However, pretty much everything off-gasses to some extent, and that will increase with temperature.  The flash point of a liquid fuel is the lowest temperature at which vapors will ignite, and it’s directly related to the concentration of fuel vapors in the air.

The flash point of diesel is about 126°F, while that of gasoline is about -45°F.  So, if you have a match and drop it into a cup of diesel fuel, it will likely just go out because at room temperature you don’t get sufficient vapor “evaporating” from the diesel.  With gasoline, on the other hand, you will a nice “FOOF!”.  For the diesel to burn you’ll need to warm it up first.  The same principle applies to solids fuels as well; for a log to burn you first need to get it hot enough for vapors to start off-gassing.  (Which is why you need to start with tinder and kindling to slowly get the temperatures up.)

Anyway, the point is that as long as you can supply sufficient vaporized fuel to the burner (i.e. where the oxidation of the fuel to CO2 and H2O is taking place) you should get a solid flame for cooking with.  How that is managed just happens to be different for the various stove types, which follows in part 2….

[Jeremy and Angela]

This is part 2, how different stoves supply sufficient vaporized fuel.

First, as you likely remember from grade school, liquids are not easily compressible while gases are.  All of these stoves, both upright canister or liquid feed (white gas or inverted canister) rely on compressed gas within the fuel container to push fuel out of the fuel container and to the stove.

With white gas stoves that’s accomplished by manually pumping outside air into the bottle.  Since air can be compressed rather easily, you don’t need to pump continuously as the pressure will continue to push on the fuel for quite a while until it gets too low.  When you use an inverted canister stove, it’s just like using a white gas stove where the fuel bottle has been pre-pressurized.

For canister stoves, consider that butane, isobutane, and propane are all gases at room temperature and pressure, but in the canister they’ve been pressurized so greatly as to push them partially back into liquid form.  So, the gas will want to escape, and as it does, more of the liquid form will be able to vaporize and take its place.  There’s no need for a pump as you already have a supply of pressurized gas.

(I’ll stop here for now and wait for any corrections/questions.)

[Bob Moulder]

The problem with canisters and cold is the vaporization temperature of the compressed gases, -44°F for propane (super!), 11°F for isobutane (pretty good!) and 31°F for n-butane (not good).

So why don’t we use straight propane? It requires a LOT more pressure to keep it in liquid state and requires a much heavier container, like those green-colored steel bottles of Coleman propane sold at WalMart. This is why commercial canister blends contain only 20-25% propane and no more.

Given the vaporization temperatures it would seem a stove should run fine down to 31°F. In actual practice, however, a half-full canister can start running crappily (is that an adverb?) with ambient temps as high as 45°F, or even a little higher, because of evaporative cooling as the compressed fuel undergoes the state change from liquid to gas, which causes the frost we see on the outside of canisters, and the fact that propane burns off first, as pointed out above by James M.

Absent an external heat source, as the ambient temperature gets colder and colder a canister stove’s performance gets steadily worse and dwindles to nothing. With a fresh canister with lots of propane remaining, you can probably boil a couple of cups of water with no problem even when it’s very cold. But if you need the stove to run for a long time — as you would if melting snow for all your water requirements — some form of external heat is absolutely necessary to keep the canister warm in order for the stove to run.

There are different ways to supply external heat. Some people use heat reflected from the burner to warm the canister, or a water bath or hand warmers.

There’s also the heat shunt method, and it’s no secret that is my personal favorite (see Jerry’s link above). For the past two winters I and a few others have been using quite successfully and safely my version of the concept, which is to scavenge some heat directly from the burner via a copper strip that transmits heat directly to the canister. Additionally, as testing progressed I found that a cozy over the canister helped to retain more of the heat in the canister, extending the operating range, especially for the larger canister that gave up heat more quickly to the surrounding environment. This might sound dangerous, but I’ve done a massive amount of testing and have never even come remotely close to having an overheated canister.

I prefer this method because there’s virtually zero fiddle factor and the stove runs full power down to the last molecule of fuel. Even better, it doesn’t matter what fuel blend is used. A few weeks ago I did some tests using straight n-butane at -5°F ambient temperature and got identical results. Of course, with a vaporization temperature of 31°F for n-butane I had to warm the canister up just to light the stove burner, but very quickly the thermal feedback loop was established and the stove ran fine. With a commercial blend containing propane, the stove can be started with no pre-warming of the canister as long as there is a small amount of propane remaining.

Photo of MiniMo (left, with n-butane) and BRS-3000T (using Primus “Summer” lol gas) running just fine at -5°F

What the basic setup looks like

[James Marco]

Jerry, yes, they can be made to work down to still having pressure in a can to start a flame.. There are a LOT of tricks to make canister stoves go (including turning them upside down for a second or two to get a large fireball to warm the canister a bit.)

Carly, WG is normally a liquid. Considering that it is usually a liquid at room temps it is not compressible. Butane is normally a gas at room temps meaning it is highly compressible.

In a closed system, a canister, it will maintain its same state well beyond its “normal” room temp state. There is simply no room for “boiling.” We are dealing with a closed, pressurized system. ALL fuels need to be vaporous to burn or explode, as was said. But, in use, WG requires more pressure or a higher temp to be maintained to reach the same flame requirement for burning. IE, it is less compressible than canister fuels. Pressure=Temperature x Volume (roughly speaking.) Basically you cannot get a canister to boil, since it implies an explosive compression (bubble) in a closed system. Rather, it just increases in vapour PRESSURE off the surface. IE it is more compressible than WG at most temps above the boiling point.

WG stoves require a certain amount of priming to reach vaporization pressure. They usually have some sort of pre-heat tube around the burner to let the fuel heat and vaporize. Since this is needed even at normal room temps, it is a more or less standard method.

The SVEA actually uses the flame spreader and bowel to vaporize the fuel just below the jet. This means no tube, and no pump. The heat means expansion occurs fine, but in BOTH directions. This usually means the stove will actually sound a bit funny, a rapid “put, put, put,” increasing tank pressure, also. Again, in a closed system, this is equivalent to heating the tank, so the whole stove gets hot. Adding pressure means heating something. You can see Temp=P/V, but since volume remains the same, a constant (call it one in a closed system,) T=P.  Note, the SVEA WILL overheat, and, it weighs a LOT since it is designed to take the extra pressure/extra heat involved. A splash of water on it will cool it off, some, though.

You can “shock” a canister, but it will revert to a closed system again within a few seconds. Vapor pressure is what is used for burning, since ALL fuels need to be in gaseous form to burn. Try dipping a canister at 0F in a bath of water at 40F. You can feel the shock effect as it boils off to meet the new equilibrium. But, this is a rarity and is not in the normal room temp ranges.

Using a topper, this means that you are reducing pressure. In a closed system, this means reducing temperature. So, a canister gets colder as you use it. They actually perform less well at 40F since using one to boil a liter of water will drive the temp down to close to 32F or lower. Often, you can simply shake one to increase vapor pressure some and get it to work a bit better for 30 sec or so. Shaking will actually increase the heat available for vaporization so shaking a cold canister will work well at near boiling. Moulder’s heat shunt for toppers works, but you need to have pressure and you still have the problem of the fuel fractionating in the can as it starts in colder temps.

Using an inverted stove, you only need a heat shunt from the flame to the gas.  See Roger Caffin’s articles for a rather full explanation of this. But they also need priming. Often heating the stove/heat shunt connection with a lighter for about 10secs will give enough heat to get things going. Again, as it heats, it vaporizes pushing in BOTH directions. So it sort of warms up the canister faster than needed once it gets going. Again, this may take up to a minute or two to reach good operating temp.

[Jerry Adams]

The other technique to use at lower temp is to put canister into a container of water.  As you’re boiling water, put a little bit into the container to keep it warm.  All you have to do is keep it from freezing.  Theoretically 1% of water boiled.

Or turn canister upside down, fill bottom with water, put on a lid that exactly fits, turn right side up and use.

Previous threads on this back in history.

But someone will eventually try the 12″ square aluminum foil reflector at cold temp and find this is the best technique – least fiddly, 0.1 ounce.  Loosely fold up to reflect heat from burner back to canister.

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