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So you're out in the Arctic and 4pm will be coming up, however you've run out of firewood; what to do ?
The Didgeridoo Kettle is a metal pipe/plunger combination. Load a cup of tea's worth of snow into the breach and cap it. Now pull the plunger out until it locks, and wait.
In the vacuum, the
snow will sublimate, pulling an immense amount of heat out of the surrounding area via the tube body. Eventually the system will contain enough heat that, when the plunger is let back down, the water inside will be boiling at atmospheric pressure.
Paradoxically the stronger the freezing wind, the faster you can get boiling water.
But wait, there's more: the Kettle can do cold water too ! Perfect for those Saharan treks to the store.
Pour some water into your Didgeridoo and push the plunger in until it locks. After awhile allow the plunger to come back up and shake. Pour the result into a glass or sip straight from the pipe.
Again, the stronger the wind the shorter the wait.
[link]
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How long is the didgeridoo? Compressing water is
not easy. |
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//compressing water//
In the case of making cold water, it's the air that's compressed (thus heated). After the heat has been carried away by the environment, the plunger is released and the air goes back to atmospheric pressure and volume at a much lower temperature. So when you shake the container, the water will absorb the coolth. You wouldn't actually have to have water in there until after you've cooled the air, but it saves a step. |
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In the case of making hot water, no water is compressed (unless you count going from a vacuum to ambient pressure) |
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//how long// that would involve, um... math. |
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Let's do a back-of-the-envelope calculation: |
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Volume of a cup of tea: 250ml |
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Weight of a cup of tea: 250g |
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Latent heat of melting - 334 kJ/kg |
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Latent heat of evaporation - 2,270 kJ/kg |
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I'm fairly sure the latent heat of sublimation is the sum of these - 2604 kJ/kg |
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Specific heat capacity of water - 4.2 kJ/kgK |
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So by sublimating 250g of water you'll extract (2604 * 250) = 651kJ of heat from the environment. |
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Recondensing 250g of water takes (2270 * 250) = 567.5kJ of energy, leaving 83.5kJ to heat the water. This is 334 joules per gram of water which will heat it to (334 / 4.2) = 79 degrees C. |
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As a Brit I can assure you that 79 degrees is nowhere near hot enough to make a decent cup of tea. As a coffee drinker I don't consider this to be a real problem with the idea, so [+]. |
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(Of course this assumes that the surrounding environment is at 0 degrees. If it's colder then presumably the resulting water will be colder too.) |
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"Coffee boiled is coffee spoiled" |
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I'm curious as to the amount of force required to pull the plunger - will it require a very long lever, or some medieval crank-and-pinion device like on a crossbow? |
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With a suitably small piston travelling in a suitably long cylinder, the force can be made arbitrarily small, though the device might become a bit unwieldy. |
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A smaller piston might also make it easier to get a suitably leak-proof seal between the piston and the cylinder. |
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The power-assisted crossbow didgeridoo sounds interesting... |
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//force required to pull the plunger//depends on the area of the plunger and atmospheric pressure I imagine. |
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//back-of-envelope [long involved miscalculation]// |
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I was thinking more along the lines of calculating how big a tube you'd need to hold your 250g of water as vapour at ambient temperature. Generally speaking since hc(water or ice)/degK is nothing compared to the lh(fusion or vaporisation), you'd basically need a length of <x> if ambient was > 0C or <7/6x> if it was <0C. |
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You can probably get it to sublimate at near 0K if you had a long enough tube to hold the molecules far enough apart. |
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Just a dental syringe can be used to make water actually cavitate without a whole lot of effort. The amount of vacuum required for sublimation would be less than this no? |
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yes but once it's evaporated you won't have a perfect vacuum. |
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Actually I didn't even notice your original comment - I was just trying to work out how hot the water would get. |
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Let's have a go at your calculation. Again I'll assume that the ambient temperature is 0 deg C. |
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Molecular weight of water: 18 g/mol |
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So we have 13.8 moles of water. |
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Vapour pressure of water at 0 deg C: 0.611 kPa |
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The pressure inside the tube has to be lower than this for the water to remain in vapour form. How large a tube do we need? By the ideal gas law: |
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V = nRT/P
= (13.8 mol) * (8.31 J/K.mol) * (273 K) / (611 Pa)
= 51.53 m^3 |
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That's a big tube. Lower ambient temperatures will reduce the vapour pressure and make it even bigger. |
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//51.53m^3// .... more sherpas ! |
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I like the idea of pumping heat out into the atmosphere, even if I can't follow the math. |
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But how is this a didgeridoo? A slide whistle, maybe. |
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I was expecting a whistle affixed to a didgeridoo. |
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[+] for lack of bagpipe involvement. |
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[-] for failing to exclude orrerys |
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Now look what you've gone and done ... |
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I was expecting a kettle which would blow steam into a didgeridoo instead of a whistle when it was ready. |
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and I was prepared to point out the need for the didgeridoo player to provide vibrating air, which would complicate the kettle design. |
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Getting a syringe to caviate is not the same as getting the water to sublimate. Also, you never get all the water to cavitate, so you're nowhere near getting all the water to sublimate. |
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