Mar 5, 2013 at 3:16 pm #1300043
@maiaLocale: Rocky Mountains
Companion forum thread to:Mar 5, 2013 at 5:00 pm #1961864
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.Mar 5, 2013 at 6:14 pm #1961893
> 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.
CheersMar 6, 2013 at 2:20 am #1962014
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.Mar 6, 2013 at 6:46 am #1962036
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).Mar 6, 2013 at 8:31 am #1962071
…Mar 6, 2013 at 8:57 am #1962084
>"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:
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.Mar 6, 2013 at 9:03 am #1962088
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.Mar 6, 2013 at 9:35 am #1962108
…Mar 6, 2013 at 9:46 am #1962113
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 respondMar 6, 2013 at 9:53 am #1962114
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:
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".Mar 6, 2013 at 10:32 am #1962133
@jamesdmarcoLocale: Finger Lakes
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!Mar 6, 2013 at 11:10 am #1962157
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!Mar 6, 2013 at 11:46 am #1962179
> 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.Mar 6, 2013 at 1:34 pm #1962238
> 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.
CheersMar 6, 2013 at 1:42 pm #1962241
> "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.
PS: your contributions are appreciated.Mar 6, 2013 at 1:52 pm #1962247
> 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.
CheersMar 6, 2013 at 2:21 pm #1962259
> 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.Mar 6, 2013 at 2:52 pm #1962277
@swimjayLocale: Northern California
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'.Mar 6, 2013 at 2:55 pm #1962279
@b-g-2-2Locale: Silicon Valley
I can envision a whole new job field for thermocouple repair technicians.
–B.G.–Mar 6, 2013 at 2:55 pm #1962280
>:"Fair question. I was including the requirement to heat the fuel up from, say, 20 C to wherever it boils."
Got it. While for fuels gaseous at ambient temps (propane, often butane), you primarily need to provide the latent heat, for high-boiling compounds, you need to provide a lot of sensible heat as well. Yes, then diesel would need to absorb more heat to boil than white gas than butane. Also, because the deltaT is less between the burner flame and heavier fuel to be heated and because the heated fuel vapor will have more losses once it leaves the vaporizer section, you'd need an even larger convective or conductive heat exchanger to heat that fuel.Mar 6, 2013 at 6:51 pm #1962436
Hello all, this is my first post at BPL!
I had some thoughts on regulating the thermal feedback to the inlet tube based on temperature. Don't discard your heat shunt concept just yet.
You might be able to achieve regulation (preferably automatic regulation) by either varying the thermal conductivity of some parts of the stove, or by varying the exposure to thermal energy of some parts.
To vary the thermal conductivity of the structure, it would be nice to have a material that has a thermal conductivity with a strong and inverse dependence on temperature. As the temperature of the conducting parts rose, the conductivity would decrease, thereby reducing the thermal energy transmitted to the fuel. Unfortunately, I don't know of any such materials.
But, varying the exposure of the inlet tube and connected parts may be possible. In this case, you would want something to move as the temperature rose. What I imagine here is one of your flat metal shunts made out of bi-metal sheet. As the temperature of the shunt rose, the shunt would bend away from the flame, reducing its exposure and consequently the amount of heat conducted down to the inlet tube. The regulation would vary smoothly with temperature. Alternatively, you might find a shape memory alloy that would bend out of the way at an appropriate temperature. This deflection could potentially be much greater than with the bimetal, but all the deflection would occur over a narrow temperature range, which might be ok or even desirable. Or, you could go low-tech and just put your shunt on a hinge and manually flip it out of the way or adjust it once the stove is warmed up, at least for initial testing. You would probably want the rest of the stove to be designed to contribute relatively little heat transfer to the inlet tube, so that the moving component could have maximum control of thermal feedback.
You might think of other variations on this theme.
-StephenMar 6, 2013 at 7:30 pm #1962457
> 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
Yes, I played around with this idea too, but I had trouble getting it miniaturised and hermetic. It remains possible though.
> a little anaerobic threadlock for good measure.
That bit worried me. Most anaerobic threadlocks are based on methacrylate, which has a limited tenperature range. Maybe a silicone sealant could be used instead?
Maybe I will have another look some time.
CheersMar 6, 2013 at 7:33 pm #1962458
@swimjayLocale: Northern California
Stephen, that's something along the lines of what I was proposing. Great minds think alike.
Unfortunately, so do mediocre ones.Mar 6, 2013 at 7:33 pm #1962459
> flat metal shunts made out of bi-metal sheet.
Novel! I imagine it could work.
But sourcing the material …
You must be logged in to reply to this topic.