15

When melting snow to obtain drinking water, what is the most efficient strategy if one needs more water than results from one pan of snow? Most efficient for me is using a minimum of fuel, not necessary the fastest way.

I can think of multiple strategies:

  • Squeeze snow in pan, melt fully, pour water in container and repeat.
  • Squeeze snow in pan, melt fully, pour partly and leave some water in pan, add new snow and repeat.
  • Squeeze snow in pan, start melting, pour water out and add new snow as snow melts.

Or am I overthinking this and doesn't it really matter?

Assume that the water is not intended for immediate cooking, as in that case it looks obvious to continue heating until it boils.

14

For maximum efficiency (i.e. melted water per used fuel) make sure the following things are always true:

  • Always have some water in the pot.

  • Never have only water in the pot.

Having water increases the thermal conductivity between the pot and the snow/water. With just snow you have a smaller contact area.
As long as there is both snow and water in the pot, the temperature stays at the lowest possible (i.e. melting temperature in the prevailing conditions). All energy goes into melting snow. When there is only water, energy will go into raising the temperature of the water. This means it is best to remove water and add snow in regular intervals.

Edit regarding having/opening a lid:
As pointed out by Paparazzi the benefits from having a lid when melting snow is probably negligible. At melting temperature water will hardly evaporate at all (the vapor pressure is at only 0.006atm at 0degC while it is at 1atm when boiling).

Unrelated to handling snow/water but still very relevant:
Only based on experience the most important factor for efficiency is something else: Heat exchange between the flames and the pot itself. That is why devices like Jetboil (heat directed around a pot with "heat exchanger") are very fast at boiling water (usually at the expense of weigth to size ratio). For any burner this means use a wind shield. Preferably one that is as high as the pot and with just a bit bigger diameter than the pot. Also lowering the flame will increase efficiency at the expense of speed.

  • 4
    I wonder why this had a -1. – whatsisname Jan 8 '17 at 7:39
  • Convective heat loss is pretty small to negligible at 32. The vapor pressure of water at that temperature is 0.0060 atmosphere. Remove water with snow present is problematic as snow floats. Remove all water will result in temp rise during the process. – paparazzo Jan 8 '17 at 8:10
  • @Paparazzi Thanks for the hint, one should always think instead of mindlessly applying a rule (in this case "always use a lid when heating water"). Well we are not heating water... And I am supposed to be a physics student?! – imsodin Jan 8 '17 at 11:25
  • 1
    When you want to drink it immediately, heating it a bit above freezing may not be completely wasted, in particular if it's cold outside (which it probably is if you have snow but can't find meltwater). – gerrit Jun 30 '17 at 18:38
1

You want to have some water in the pot/pan, as otherwise you will scorch it. It doesn't have to be too much, just enough to cover the bottom. Then you would just want to keep stirring it.

  • Any more than that, and you would probably be wasting energy heating already melted water As long as you keep stirring, the melted water is at exactly 0 celsius, because it's in thermal equilibrium with the now. You're not wasting any energy heating the water, because it sticks at 0 C until all the snow is melted. – Ben Crowell Jan 7 '17 at 21:44
  • @BenCrowell That makes sense. – Charlie Brumbaugh Jan 7 '17 at 22:27
  • @BenCrowell Wait a minute, the more water is in the pot, the more energy it will take to raise the temperature. If I remove the melted water, wouldn't that be more efficient? – Charlie Brumbaugh Jan 7 '17 at 22:49
  • @CharlieBrumbaugh: the water temperature won't raise at all until all of the ice is melted. Additionally, having a good amount of water is beneficial as it will maximize the heating surface area to the snow. – whatsisname Jan 8 '17 at 7:38
  • You ultimately have you pump in at least 333.55 kJ/kg of water to melt it, no matter whether you do it a gram a time or all at once. – whatsisname Jan 8 '17 at 7:46
1

A small flame compared to the size of the pot means less heat escaping up the sides. This will of course slow things down. Further minimise this with a pot skirt if at all possible. I had a skirt for my MSR stove that was just very thick foil.

If you have two pans you can put snow in the larger one and use it as a lid on the smaller one when cooking, or even when melting snow.

-2

The heat to melt is the same so will not make a lot of difference.

A larger pan will have more surface area and be more efficient.

Continually add snow and ladle out water with a small cup.

Unless it is really cold out you can put snow in a clear container and leave it in the sun.

  • You want to have some water in there from the start, in order to get good thermal contact between the snow and the pan. The heat to melt is the same so will not make a lot of difference. Not true. If there is poor thermal contact between the snow and the pan, then you're wasting more heat by convection and radiation. – Ben Crowell Jan 7 '17 at 21:43
  • "a larger pan will ... be more efficient." This isn't necessarily true. A larger pan can also serve as a radiator and provide cooling opposed to your heat source. – whatsisname Jan 8 '17 at 7:41
  • 1
    @Paparazzi: you totally missed the point. Surface area of your pan can just as easily conduct heat to the cold air as from the stove. You want your pan to be sized just to your stove and no bigger. Using a turkey fryer pot on a camping stove is going to perform worse than a camping pot. – whatsisname Jan 8 '17 at 7:51
  • 1
    @whatsisname So now this is a turkey pot to melt snow for a water bottle? Good day. – paparazzo Jan 8 '17 at 7:54
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    @Whatisname if the flame is as big as the pan, the hot air beyond the flame that carries a lot of heat just goes up the sides and is lost. The bottom of the pan near the edges should be cool if you heat gently so heat loss will be low. But if the ambient temperature is very cold even cool surfaces lose heat. – Chris H Jan 8 '17 at 8:28

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