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MSR Windboiler – Completed Review

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Re: °

Postby hikin_jim » Sun Dec 14, 2014 9:03 pm

longri wrote:Image
Thank you very much. May I use this on my blog?

And the dashed line, what does it represent? Is it the decline in temperature as elevation increases per the ISA model? And starting at, what, 25°C and 100 kPA absolute?

longri wrote:The easier thing to do is skip the theory and just go find out what works.
Well, you may ultimately be right. I'm trying to give people some understanding of what's happening, what to expect, and how to plan. Maybe I'll throw in the towel at some point, but I've substantially re-written the Gas Stoves – How Cold Can I Go? post. In reading the post, it became clear to me that I had inadequately described the overall context. It was there, but it needed to be emphasized more. There were also some overly optimistic predictions regarding the effects of elevation (as discussed here earlier) and the pressure needed. I've tried to make what predictive numbers I give more conservative and to put them in context.

God willing, it's far clearer now.

HJ
Backpacking stove reviews and information: Adventures In Stoving



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Re: MSR Windboiler – Completed Review

Postby longri » Tue Dec 16, 2014 12:50 pm

I added that line to illustrate that the ambient temperature tends to drop faster than the boiling point of the fuel.

The lapse rate of the ISA is -6.5°C/km (about 2°F/1000 ft). It's somewhere between the dry and wet adiabatic lapse rates. So in a real atmosphere it's usually higher in winter when the air has less moisture. The ISA is kind of like a broken clock that's only right twice a day. But it's not too bad if thought about as an average. Barometric altimeter watches use the formula. It's what I used to make those charts.

You can look up a description of the standard atmosphere on the internet in about 15 seconds.

Feel free to use my chart to wallpaper your house if you want, just don't credit me. I didn't check it carefully and it could be completely wrong. If I were you I would make my own chart or at least spot check the values in the one I gave you.

One thing I kind of glossed over is the pressure drop that has to happen for the gas to flow. In my at home test I measured the pressure difference vs. flow and then correlated that with the temperature that produced that fuel pressure at equilibrium. But when gas is venting it isn't in equilibrium. Assuming the temperature is constant, there has to be a pressure drop for the fuel to vaporize. That means that the temperature to generate a certain flow rate would actually be a little higher than I was estimating.
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Re: MSR Windboiler – Completed Review

Postby freestone » Tue Dec 16, 2014 1:40 pm

So that's why my canister always poops out or goes empty just before the coffee water is ready to boil on a cold morning, somewhere near the crest.
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Re: MSR Windboiler – Completed Review

Postby hikin_jim » Tue Dec 16, 2014 5:53 pm

longri wrote:I added that line to illustrate that the ambient temperature tends to drop faster than the boiling point of the fuel.
I figured that's what it was, and it's a good thing to add. Some people have thought that I was suggesting to simply climb higher if your stove was conking out due to the cold. Uh, no. The temperature drops at a greater rate than the boiling point.

longri wrote:Feel free to use my chart to wallpaper your house if you want...
Well, it is lovely. :lol:

Hopefully my rigged up chart that I now have on my blog is close enough. It's designed to be quite conservative. It's been 30+ years since I went through physics, calculus, chemistry, etc. I understand in a general sense what's going on, but the calculations are beyond me. I was careful to state that performance "on the ground" was the ultimate arbiter of whether or not pressure was sufficient, letting people that they needed to warm the canister until the desired pressure was produced.

longri wrote:I was measuring flow rate for specific gauge pressures. To be a little clearer, my setup looked like this:

xxxxxxxx canister -- valve -- TEE -- hose -- stove(with valve)
xxxxxxxx canister -- valve ---- \_ pressure gauge
I think I know what the answer is going to be, but if you ever wanted to make a short video showing (via voice, subtitles, or just time into the video correlated to specific psig) the flame at various psig, say 3, 5, and 10 since those are what we've previously discussed, I'd love to post it on my blog. I think it would be helpful to people to understand very clearly the impact of low pressure and why pressure needs to be boosted by warming canister before use and keeping the canister warm during use.

HJ
Backpacking stove reviews and information: Adventures In Stoving
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Re: MSR Windboiler – Completed Review

Postby hikin_jim » Tue Dec 16, 2014 6:01 pm

freestone wrote:So that's why my canister always poops out or goes empty just before the coffee water is ready to boil on a cold morning, somewhere near the crest.
Pretty much. Your canister will be at its lowest performance toward the end of its life when all the propane has burned off, and your canister will have gotten colder while it was running. Colder + poor fuel = pooped out stove.

HJ
Backpacking stove reviews and information: Adventures In Stoving
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Re: MSR Windboiler – Completed Review

Postby longri » Wed Dec 17, 2014 11:18 am

hikin_jim wrote:It's been 30+ years since I went through physics, calculus, chemistry, etc. I understand in a general sense what's going on, but the calculations are beyond me.

You're probably selling yourself short. The math isn't that hard.

1. Isobutane saturated vapor pressure = 10^(A + (B/(temperature+C))
2. ISA: elevation = D*(1-(pressure/E)^F)

A,B,C,D,E,F are constants

Put equations 1 and 2 together and you can make that chart.

The saturated vapor pressure is the equilibrium gas pressure for a given temperature. You can generate tables for different fuels here: http://webbook.nist.gov/chemistry/fluid/
Antoine equation parameters (the formula I wrote above for isobutane) are in the "phase change data" link for each chemical species, like this one: http://webbook.nist.gov/cgi/cbook.cgi?I ... ermo-Phase

If you want to mix fuels it's easy:

total pressure = x1*P1 + x2*P2 + x3*P3 + ...
where P1,P2,... are the sat. vapor pressures of the fuels and x1,x2,... are the mole fractions

Example: Say you have propane and isobutane, 20/80 by weight at 20°F.

The molecular weights: propane (44 g/mol) and isobutane (58 g/mol).
So the mole fraction of propane is: .20/44/(.20/44 + .80/58) = 0.248, call it 25%.
Thus, the mixture is 25/75 in terms of the number of molecules.
At 20°F the sat. vap. pressures from tables: isobutane = 17.8 psia; propane = 55.8 psia.
(The Antoine values are a little off: 17.9 and 57.5 psia)
So the total pressure = .25*55.8 + .75*17.8 = 27.3 psia.
At sea level: 27.3 - 14.7 = 12.6 psig; at 10,000 feet: 27.3 - 10.1 = 17.2 psig.

With a spreadsheet you can generate endless charts like this.
A monkey could be taught to do it.


hikin_jim wrote:I think I know what the answer is going to be, but if you ever wanted to make a short video showing (via voice, subtitles, or just time into the video correlated to specific psig) the flame at various psig, say 3, 5, and 10 since those are what we've previously discussed, I'd love to post it on my blog. I think it would be helpful to people to understand very clearly the impact of low pressure and why pressure needs to be boosted by warming canister before use and keeping the canister warm during use.

A video sounds like too much work.

Here's another way to think about it though.

At full power my stove is rated at 10,000 BTUs. These fuels contain 46 kJ/g so to generate that rate of heat output requires 3.8 grams of fuel per minute. At 5 psig the fuel flowed through my stove at a rate of 1.5 g/min which means the heat output was 4000 BTUs.

In terms of time, suppose you wanted to melt enough snow to produce 2 liters of water. If the stove were transferring heat at 50% efficiency it would take about 8 minutes at full power to melt the snow. So at 5 psig it would take around 20 minutes. You want it boiled? Add at least another 20 minutes.
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Re: MSR Windboiler – Completed Review

Postby longri » Thu Dec 18, 2014 7:40 pm

I changed my mind. Here's a quick and dirty video: vimeo.com/114935717
I tried to use youtube but they insisted that I cough up my phone number.
Vimeo accepted the video without any drama.

It's not much fun to watch: Zzzzzzzzzzzzz....
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Re: MSR Windboiler – Completed Review

Postby hikin_jim » Sat Dec 20, 2014 7:12 pm

Beauty is in the eye of the beholder. And you made your point.

Would it be all right if I posted the video in some fashion on my blog? It illustrates the point exceptionally well. It's the pressure we're after not a set number of degrees above a boiling point.

HJ
Backpacking stove reviews and information: Adventures In Stoving
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Re: MSR Windboiler – Completed Review

Postby longri » Mon Dec 22, 2014 1:19 pm

The only reason I took the time to make that video was because you wanted to see the flame.
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