Companion forum thread to:

Thermal Feedback in Upright Canister Stoves

Another good article, Roger, I'll have to digest it a bit more...

I detect a new Caffin product coming.

You have a CNC machine? Nice.

Maybe there could be an upright canister stove with more thermal conduction back to canister? There's an O-ring and the pin pushing the Lindal valve which don't make very good thermal conductivity. Maybe some way to improve on that. Wouldn't want to use it in warm weather which might make it an unviable product from legal perspective.

> I detect a new Caffin product coming.
Who, me ???

> Wouldn't want to use it in warm weather which might make it an unviable product
> from legal perspective.
Have to disagree. You can do a lot by adjusting the windscreen on an upright, and many people already use remote canister stoves in summer. There are even remote canister stoves which do NOT handle inverted canisters.

More to the point, I would argue that you should never operate a stove of any sort without good supervision. Summer, winter, does not matter. make sure the stove is running normally and the fuel container is touchable.


I was chatting some years ago with someone who had two stoves running full bore, side by side, with large pots. No real supervision. Eventually cross-radiation took at least one fuel tank beyond critical, and it blew. They lost their tents, which was minor. Two people recieved extensive third degree burns from the BLEVE over a lot of their bodies. Fortunately they were able to get an airlift to Emergency, so they lived, but the scarring was extensive.

Be careful and be safe.

Cheers

Nice article Roger, which pretty much matches my own experience. Here's an early stove I made which suffered from too much heat conduction down the aluminium mixer tube, which damaged the o-rings in the valve. This was based on a Coleman F1, so this might be a good candidate for an upright stove, were it not for the plasic bits.



My view is direct heat conduction from the burner is too difficult to get right: too much conduction and the whole stove gets too hot, too little means a longer warmup time before you can switch to liquid fuel. My current stove has a titanuim burner and mixer tube to minimise conduction by that route and a conventional vapouriser tube for fast startup.

I agree that valving gas (as opposed to liquid) gives better flame control, but there are practical drawbacks to this arrangement when the stove is surrounded by a windscreen, plus the potential for heat damage to o-rings. Also, I once had a pot of almost boiling water tip off a stove when I was adjusting the valve - now I prefer to keep my hands well away! The difficulty of valving liquid can mostly be overcome by having a very fine thread and a shallow taper on the needle and a low volume tube betweent the valve and the stove. No need to make, if you know where to look.

I'm a stove user, not a designer, but could you make some sort of a hybrid approach to preheat the feed tube? I'm thinking of a small cup that would hold 10-15 ml of alcohol that you would light in very cold temperatures to preheat the tube and then, after a couple of minutes, start the flow of your primary fuel?

It seems to me that this would eliminate the stove startup issues by heating the fuel tube until the stove was generating enough of it's own heat to take over (by which time the alcohol would have burned off).

Hi,

I'm curious about the theoretical lowest operating temperature of a remote canister liquid feed stove ( aka the msr windpro ii ) .

Since the pressure changes depending on how much fuel is in the canister, I would expect this to be a difficult question to answer. Perhaps considering the worst case, where the canister is almost empty, but has been run in liquid feed mode exclusively ( so the proportions of the iso-propane mix remains constant ).

I know I've posted this question before. And I apologize for the repetition. I've been reading and reading about the liquid feed approach. But I'm having trouble finding the answer to this question.

thanks in advance,

-daniel-

Edited by daniel@fishfamilypdx.com on 03/06/2013 09:59:43 MST.

>"I'm a stove user, not a designer, but could you make some sort of a hybrid approach to preheat the feed tube? I'm thinking of a small cup that would hold 10-15 ml of alcohol that you would light in very cold temperatures to preheat the tube and then, after a couple of minutes, start the flow of your primary fuel?"

Actually, Kevin, you apparently are a stove designer. I think you just designed a SVEA 123.

Here's our own HikinJim priming one with denatured alcohol:

http://www.youtube.com/watch?v=TfRRs4x51k8

But the alcohol used is more like 3 ml on a cool day. Maybe two such treatments ( 2 x 3 ml) in snow-camping conditions.

Since no one answered your question:

When I put an iso/propane canister in the freezer, which must be 0 F or maybe -10 F, no gas came out. If it was inverted, no liquid would come out. The minimum temperature you can operate an inverted stove must be warmer than that temperature.

At 25 F I've operated upright canister, but very slow at that temperature. 30 F is good but a little slow.

So, maybe inverted stove is good between 0 or 10 F and 25 or 30 F.

Oh, if you take 1 foot of solid copper maybe #14 gauge, wrap it around canister and up into flame, it will warm up canister enough so it is usable in that range, and maybe even below if you pre-warm it in your pocket.

0 Deg F it is. Thanks so much!

Don't assume it will work at 0 F. It might work there.

I think you can assume it will work at 10 F, but that's just a guess.

Inverted stove should work at 20 F, because I've sort of got upright to work there.

Maybe someone with more than an opinion will respond

Edited by retiredjerry on 03/06/2013 10:47:10 MST.

I like the article and the helpful overview it gives. I did however tweak on the sentence, "The higher the boiling point, the more energy is needed to vaporise the fuel." and not just because someone replaced the "z" in vaporize with an "s".

I'd accept that "The higher the boiling point, the more TEMPERATURE is needed to vaporise the fuel."

But when I look at Hvap for different fuels, as the boiling point goes up, the heat of vaporization (in kJ/kg) goes down. When I factor in the slightly decreasing heat of combustion for higher MW fuels, the normalized Hvap still goes down. When I include all the conversion factors, I think I end up with a new dimensionless number (stand back Mr. Reynolds and Mr. Prandtl!) the meaning of which is:

The fraction of a liquid fuel needed (to be burned) to vaporize itself:

propane 0.0102
butane 0.0094
hexane 0.0091
decane 0.0068

Yes, they are all close to 1% which is shoot-from-hip, ballpark, reasonable. But higher MW fuels need a smaller fraction of their available BTUs to vaporize.

You may reference this as the "Thomas Number". But if you convert to Australian by turning it upside down and firing the stove on Foster's beer, then it is the "Caffin Number".

Roger,good article. Looking forward to part II.
Yeah, the problem is balancing the thermal feedback to safe levels with canisters and "toppers." It is generally easier to insure that your fuel is hot or hotter than needed and avoid heating the canister at all. This simply devolves to the simplistic aproach you took with remote canisters. This also allows the use of cone type wind screens...you really don't care of the valve and burner get hot, just the fuel line needs to stay cool.

Good job!

Nice article Roger. I am an engineer too and I always enjoy reading your articles about stoves. Well done and I look forward to reading more of your articles in the future!

> I'm curious about the theoretical lowest operating temperature of a remote canister liquid feed stove ( aka the msr windpro ii ) .

Theoretical lower limit would be about -40C as long as you had some propane remaining. You'd need to do some fudging to warm the canister a bit to keep it going well.

You may find this article interesting.

> theoretical lowest operating temperature of a remote canister liquid feed stove
This depends very much on what fuel is in the canister. Read our article Temperature Effects on gas canisters to get a good understanding. To be sure, you can get fuel out of most blended canisters down to -20 C or further, and the higher you go in altitude the lower you can go in temperature - which is convenient. Pure isopropane works below -10 C as well.

But this ignores some convenient practical points: the interior of my pack is never that cold (because my back keeps it warm), and a little liquid water will warm a canister up nicely. I have poured water over a canister in the cold and heard the gas inside boil! And I have stored the canister under my quilt overnight in the snow.

So in practical terms you can use a canister stove at significantly lower temperatures than -20 C if you are smart.

Cheers

Edited by rcaffin on 03/06/2013 14:54:39 MST.

Hi David

> "The higher the boiling point, the more energy is needed to vaporise the fuel."
Fair question. I was including the requirement to heat the fuel up from, say, 20 C to wherever it boils. Obviously, it takes a lot more energy to heat diesel to 300 C than hexane to 69 C. But no, I did not do the calculations to justify that.

S and Z - flavour to taste.

Cheers
PS: your contributions are appreciated.

Hi Stuart

> suffered from too much heat conduction down the aluminium mixer tube,
Yeah, problems there. Make the walls of the tube thinner? Air holes elliptical?

The problem with vaporiser tubes is sealing them. That's why the base block on the FMS-118 is heavy brass rather than Ti or Al. We await a good solution to that one. Not easy.

> there are practical drawbacks to this arrangement when the stove is surrounded
> by a windscreen
I really do agree, but I don't have that problem as I never completely surround my stove. I always have a decent gap downwind with the valve handle sticking out there. In addition to giving access to the valve it ensure plenty of oxygen. That has always worked for me.

I will add that having a secure stable base for the stove is important, and often ignored. I always spend a little time preparing the place where I put the stove base so that it is stable. More on that in a future article.

Cheers

Hi Roger

> The problem with vaporiser tubes is sealing them.

At the stove end I drilled a hole thru' the centre of a brass M5 bolt and then brazed the SS vaporiser tube into the hole. The bolt with tube then screws into the aluminium base, with a little anaerobic threadlock for good measure.
The fuel line is attached to the other end of the vaporiser tube with a crimped on brass ferrule.
No leaks so far.

If we work backwards, it seems that the ideal is to maintain a short stretch of fuel line, occurring before the control valve, at a given temperature, the temperature required to vaporize the fuel traveling through the line. Once the stove is going, the heat available to do this is the cooking flame, and Roger has tried various conductive paths-- extra metal of various types, the metal of the stove itself--to achieve this. What about a more active conductive path, one that was self-regulating?

Just as a thought experiment, imagine that we could use the heat of the stove to generate electricity, then use that electricity to heat the tube, with an intervening active thermostat-like circuit that would maintain the tubing temp at the correct point-- we'd be done. Environmental factors, wind screens, would all be taken care of.

Not practical yet. But perhaps there's some kind of thermocouple-like solution possible, one that would open or close a heat path as needed?

Just sayin'.

I can envision a whole new job field for thermocouple repair technicians.

--B.G.--