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![]() Hmmmm...here's somebody at least taking a shot at analyzing the system. I interpose one or two little comments.... On Sat, 29 Sep 2007 10:04:43 -0700, Keith Hughes wrote: "jim.isbell" wrote: /.../ This is just plain wrong. As a *unit of measure* 32 feet of water column equals about 13.9 psi. Meaning, if you pumped a 40' column up to a 39' height with water, equalized the headspace to atmospheric pressure (assuming 14.7psia), sealed it, then allowed gravity to *drain* the water column to a height of 2', the resulting pressure in the headspace will be about 0.8psia. Now you also have 33' of empty evacuated column. My, my: "it's just plain wrong": he said a column of 32 ft, and you correct him - it's 33 ft. What a loser he must be! :-) But then, you are neglecting to account for the density of SALT water! Not strictly relevant, but interesting to me at least: Joseph Priestley kept a water barometer at his house in Birmingham (before the mob drove him out for his revolutionary sympathies). Guess how high he had to climb to read the water level? The fresh water distills off the top of the sal****er column then migrates Yes, and this "migration" is simple diffusion. *And* you have (in the example above) 33' of column it has to diffuse through on the seawater side, and however many feet of column on the freshwater side it has to traverse prior to condensation. If both columns (fresh and sea) are referenced to the same height, then the evacuated column height on both sides will be the same, and that diffusion path will be up to 66'. That does not happen quickly. Uh? Diffusion of water molecules in low pressure air through 66 feet? Let's say 14 ft, 20 feet even. Now what would the speed be? Hmmmm. Let's see. Would that speed be over 500 meters/second? That's so slow, the time it might take to travel 20 feet, say 6 meters at 500 m/s might be 12 milliseconds? Here's a review of the thermo equation. Just plant the temperature of interest (20 degC say) and the molecular weght of a water molecule (Hint: its lighter than the average molecule that makes up air) in the following calculator http://hyperphysics.phy-astr.gsu.edu...kintem.html#c4 In reality, though, the columns won't be referenced to the same level, with the freshwater column being referenced (i.e. the bottom is opened to) the deck height on the boat. So the freshwater column will be, say 8' higher than the seawater column. The diffusion path is still the same, but the evacuated seawater column would then be 37', with 29' on the freshwater side. Hmmmm...a freeboard of eight feet? Some boat! More boat than I've got, certainly. This relates to the critical rate-limiting feature of the system - maintaining pressure. When you evaporate, or sublime, water into the headspace, the pressure in the headspace increases. The word is "BOIL", not evaporate, not sublime. If it is not quickly condensed returning latent heat, the partial pressure rises quickly sure enough. Better condense it then! I imagine a central cold finger of cool salt water in the fresh column might be effective? (That would however take a hand pump capable of supplying a flow at 15 psi plus. Like a bicycle pump, or better? ) Condensation on the other side lowers the pressure, and an equilibrium pressure will eventually be established. For any given temperature, the evaporation rate is going to be limited by the partial pressures at the headspace/water-surface interface. It's a feedback loop, More evaporation - more water vapor molecules liberated to the headspace - more pressure in the headspace - slower evaporation until the pressure is reduced. And to reduce the pressure, those molecules have to diffuse up to 66'. There you go again - with your really really slow 66 ft diffusion for condensed water in the fresh column..... I can see someone getting a "Darwin Award" by accidentally spilling all their existing freshwater supply in a failed attempt to get this contraption going. It doesn't *have* to be that way, BUT.... :-) Keith Hughes In my experience, the people who talk most about Darwin awards are completely foggy about how Darwinian selection operates. "Accidentally spilling all fresh water" , from a "contraption" Yes, sure. Can you say, "Straw man?" Brian W |
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