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Potable Water - The Third Way.
Ah well, another great idea skuppered by dat old devil science :-) Bruce in Bangkok (brucepaigeATgmailDOTcom) A 32' column of water is a continuous vacuum pump. As long as you put water (salt water) into the column it will pull down and keep a vacuum in the top of the column. The fresh water distills off the top of the sal****er column then migrates as steam to the other side and distills in the fresh water side....also creating a vacuum. You draw off the fresh water on one side and pump salt water into the other side. The salt water side is painted black to absorb sun heat and the fresh water side is painted white to reflect the suns heat. You only need a few degrees difference for distillation and the vacuum creates the boiling at low temperatures...even ice will change state to steam in a vacuum. The idea works. In a practical sense, I would use soft tubing for the sides and a solid "U" shaped piece of copper tubing for the top center with a ring soldered to it so it could be hoisted up the mast of a sailboat. It would take a 30 to 40 foot mast to do the job. The bottom end of the salt water tube could go to a through hull for a continuous supply of salt water and the bottom end of the fresh water tube could go to a small pump to remove the water without breaking the vacuum. |
Potable Water - The Third Way.
"jim.isbell" wrote: Ah well, another great idea skuppered by dat old devil science :-) Bruce in Bangkok (brucepaigeATgmailDOTcom) A 32' column of water is a continuous vacuum pump. As long as you put water (salt water) into the column it will pull down and keep a vacuum in the top of the column. The fresh water distills off the top of the sal****er column then migrates as steam to the other side and distills in the fresh water side....also creating a vacuum. You draw off the fresh water on one side and pump salt water into the other side. The salt water side is painted black to absorb sun heat and the fresh water side is painted white to reflect the suns heat. You only need a few degrees difference for distillation and the vacuum creates the boiling at low temperatures...even ice will change state to steam in a vacuum. The idea works. It works but does it work as well as other methods that are simpler and easier to implement. Also if you have no fresh water on hand to start with there is no way to make it work. I can see someone getting a "Darwin Award" by accidentally spilling all there existing freshwater supply in a failed attempt to get this contraption going. In a practical sense, I would use soft tubing for the sides and a solid "U" shaped piece of copper tubing for the top center with a ring soldered to it so it could be hoisted up the mast of a sailboat. It would take a 30 to 40 foot mast to do the job. The bottom end of the salt water tube could go to a through hull for a continuous supply of salt water and the bottom end of the fresh water tube could go to a small pump to remove the water without breaking the vacuum. That makes no sense. You are going to have a hard time pumping water out of the fresh water side any faster than gravity can deliver it. The salty side OTOH, if you rely only on gravity to feed it, will become a solid block of salt once you have evaporated enough water from it. -jim ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Potable Water - The Third Way.
On Sat, 29 Sep 2007 14:42:17 -0000, "jim.isbell"
wrote: Ah well, another great idea skuppered by dat old devil science :-) Bruce in Bangkok (brucepaigeATgmailDOTcom) A 32' column of water is a continuous vacuum pump. As long as you put water (salt water) into the column it will pull down and keep a vacuum in the top of the column. The fresh water distills off the top of the sal****er column then migrates as steam to the other side and distills in the fresh water side....also creating a vacuum. You draw off the fresh water on one side and pump salt water into the other side. The salt water side is painted black to absorb sun heat and the fresh water side is painted white to reflect the suns heat. You only need a few degrees difference for distillation and the vacuum creates the boiling at low temperatures...even ice will change state to steam in a vacuum. The idea works. In a practical sense, I would use soft tubing for the sides and a solid "U" shaped piece of copper tubing for the top center with a ring soldered to it so it could be hoisted up the mast of a sailboat. It would take a 30 to 40 foot mast to do the job. The bottom end of the salt water tube could go to a through hull for a continuous supply of salt water and the bottom end of the fresh water tube could go to a small pump to remove the water without breaking the vacuum. What you describe is just a still, and a 32 ft inverted U will change nothing. Solar stills are not new. A tall boiler connected to a very tall cond |
Potable Water - The Third Way.
jim wrote: "jim.isbell" wrote: Ah well, another great idea skuppered by dat old devil science :-) Bruce in Bangkok (brucepaigeATgmailDOTcom) A 32' column of water is a continuous vacuum pump. 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. As long as you put water (salt water) into the column it will pull down and keep a vacuum in the top of the column. Sorry, this makes no sense. Putting water in does not cause it to "pull down". Yes, you have supply makeup water to maintain column height lost to evaporation. 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. 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. as steam to the other side and distills in the fresh water side....also creating a vacuum. No, this does *not* create a vacuum in the sense you seem to mean. It maintains an equilibrium pressure by lowering the partial pressure of water vapor generated by the 'boiling' process on the seawater side. 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. 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'. You draw off the fresh water on one side and pump salt water into the other side. The salt water side is painted black to absorb sun heat and the fresh water side is painted white to reflect the suns heat. You only need a few degrees difference for distillation and the vacuum creates the boiling at low temperatures...even ice will change state to steam in a vacuum. The idea works. Yes, VERY slowly. You can increase *throughput* by increasing the column diameters, but how practical is that on a boat? It works but does it work as well as other methods that are simpler and easier to implement. Also if you have no fresh water on hand to start with there is no way to make it work. Not quite true...you can seal the 'freshwater' column, using only the column walls for condensation surfaces, until you have sufficient condensate collected to allow the freshwater column to be opened. I can see someone getting a "Darwin Award" by accidentally spilling all there existing freshwater supply in a failed attempt to get this contraption going. It doesn't *have* to be that way, BUT.... :-) In a practical sense, I would use soft tubing for the sides and a solid "U" shaped piece of copper tubing for the top center with a ring soldered to it so it could be hoisted up the mast of a sailboat. It would take a 30 to 40 foot mast to do the job. The bottom end of the salt water tube could go to a through hull for a continuous supply of salt water and the bottom end of the fresh water tube could go to a small pump to remove the water without breaking the vacuum. And what's 'practical' for useability, is impractical for functionality. There are no 'soft tubing' materials I'm aware of that have anything approaching decent heat absorbance, conduction, or emissivity properties, so that will be another very significant rate limiter in the system. That makes no sense. You are going to have a hard time pumping water out of the fresh water side any faster than gravity can deliver it. You actually *can't* pump faster than gravity, unless you want to suck seawater up the column on the other side. The salty side OTOH, if you rely only on gravity to feed it, will become a solid block of salt once you have evaporated enough water from it. Doubtful that you'd ever get a solid chunk of salt (and short of having a bypass circulation loop - cooling the column and further reducing efficiency - I don't see how a pump could even help the situation), but of course as the salinity increases, the boiling point increases, and at some point the process will just stall. The heat input won't be sufficient to boil the brine solution. Then you have to stop, drain, clean, and start over. How quickly this happens will depend on column heights and diameters, but it'll happen at some point. Just another rate-limiting feature. All these rate limiters are natures way of saying that there is no thermodynamic free lunch. A low energy input system will have a low output (in terms of whatever work you want the system to do). Keith Hughes |
Potable Water - The Third Way.
On Sat, 29 Sep 2007 14:42:17 -0000, "jim.isbell"
wrote: Ah well, another great idea skuppered by dat old devil science :-) Bruce in Bangkok (brucepaigeATgmailDOTcom) A 32' column of water is a continuous vacuum pump. As long as you put water (salt water) into the column it will pull down and keep a vacuum in the top of the column. The fresh water distills off the top of the sal****er column then migrates as steam to the other side and distills in the fresh water side....also creating a vacuum. You draw off the fresh water on one side and pump salt water into the other side. The salt water side is painted black to absorb sun heat and the fresh water side is painted white to reflect the suns heat. You only need a few degrees difference for distillation and the vacuum creates the boiling at low temperatures...even ice will change state to steam in a vacuum. The idea works. In a practical sense, I would use soft tubing for the sides and a solid "U" shaped piece of copper tubing for the top center with a ring soldered to it so it could be hoisted up the mast of a sailboat. It would take a 30 to 40 foot mast to do the job. The bottom end of the salt water tube could go to a through hull for a continuous supply of salt water and the bottom end of the fresh water tube could go to a small pump to remove the water without breaking the vacuum. What you describe is just a still, and a 32 ft inverted U will change nothing. Solar stills are not new. A very tall boiler connected to a very tall condenser is all you describe. Why don't you build one and let us know just how it is superior to any other solar still, especially ones without all the windage and topweight. By the way, it takes about 1100 BTU's evaporate a pound of water, and this does not vary with pressure. How much sunlight is your still going to intercept? Sunlight is a maximum of about 1400 watts per sq meter, or, according to the 'calculator that takes no prisoners', the HP48, about one eighth of a BTU /sq ft/sec. Something like half a pound of water evaporated per hour, per sq ft. This assumes that the collector is squarely aimed at the sun, at all times. What you want is the largest possible shadow. A rectangle 1 in by 32 ft is about 2,66 sq ft., by the way. A tube is not a very efficient shape for a solar collector, of course, but it simplifies aiming it, since a vertical cylinder looks the same from every horizontal angle. Your vertical tube will face the sun nice and square at sunrise and sunset. Casady |
Potable Water - The Third Way.
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 |
Potable Water - The Third Way.
Brian Whatcott wrote:
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, Uhmmm, no, he said a "32' column of water". Can you see the difference? and you correct him - it's 33 ft. You clearly need to re-read the paragraph whose point you're mangling. The 33' you quote is not a correction to the OP, but a point for further discussion (which you misunderstand later on). What a loser he must be! :-) But then, you are neglecting to account for the density of SALT water! And "about 0.8psia" makes what claim of precision? snip Yes, and this "migration" is simple diffusion. snip 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 Gee, I didn't know you were using 'smart' molecules that travel *only* in the direction you want them too. And they don't bump into each other in the process. Wow, that's really neat, how did you accomplish that? Barring that, of what value, or relevance is the above? Here's a little thought experiment for you: Given the random distribution of molecular velocities and directions (look at the Boltzman and Maxwell discussion on the site you referenced, for example), you can pick a point say 1 foot above the water surface, in the center of the tube (NOTE: 1 foot is an Example). From this point, every molecule has *almost* the same chance of going up, down, north, south, east, west, or any direction in between right? If you say "no", look at your own reference again. Given this, tell me again how the molecular speed is *proportional* to the diffusion rate? (Hint: It's not) 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. Mayhap a definition of "example" could be of help to you? 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", *If* the temp is high enough, yes. This system, in the currently discussed configuration, is most likely to oscillate between boiling and evaporation. I mentioned sublimation since posters continually reference "ice to steam" in this context, and the concept is the same. not evaporate, not sublime. If it is not quickly condensed returning latent heat, the partial pressure rises quickly sure enough. So if you condense it quickly enough, then there is no pressure rise? Right. Look at your own reference above then, and tell me where the energy went between boiling and condensing. That was a GAS law you were citing no? And "quickly condensed returning latent heat" has a name; refluxing. Refluxing = no distilled water. Get the drift? 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..... There you go again with your absurd assumption that all the kinetic energy of the gas translates into motion in one direction only... 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. Yeah, that's likely. I don't think we ever talked about Darwin when I was getting my biology degree... "Accidentally spilling all fresh water" , from a "contraption" Yes, sure. Can you say, "Straw man?" You also seem to need a refresher on how email/newsgroup postings are structured. A casual glance would show that the quote you're mocking was not mine. Keith Hughes |
Potable Water - The Third Way.
On Sat, 29 Sep 2007 13:57:27 -0500, Brian Whatcott
wrote: (That would however take a hand pump capable of supplying a flow at 15 psi plus. Like a bicycle pump, or better? ) Grease guns are, some of them, capable of at least. hundreds of psi. I happen to own a 0-5000 psi gauge. Bought it to check tractor hydralic systems. I forget just what a grease gun pumped it up to, but it was a lot. There is a reverse osmosis watermaker intended for liferaft use, with a hand pump, and RO takes hundreds of psi. That is what you want, if you actually need high pressure. Casady |
Potable Water - The Third Way.
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Potable Water - The Third Way.
On Sat, 29 Sep 2007 12:59:39 -0700, Keith Hughes
wrote: My, my: "it's just plain wrong": he said a column of 32 ft, Uhmmm, no, he said a "32' column of water". Can you see the difference? Hmmm..Priestley certainly could. His water barometer had a water column round 32 or 33 ft high. How 'bout that! :-) Gee, I didn't know you were using 'smart' molecules that travel *only* in the direction you want them too. ..... Keith Hughes Ho, hum: if half of them go in the wrong direction until their first collision, it must take them a really, really, REALLY long time to diffuse through the water vapor/rarified air mix! Brian W |
Potable Water - The Third Way.
Dear Richard Casady:
"Richard Casady" wrote in message .. . On Sat, 29 Sep 2007 13:57:27 -0500, Brian Whatcott wrote: (That would however take a hand pump capable of supplying a flow at 15 psi plus. Like a bicycle pump, or better? ) Grease guns are, some of them, capable of at least. hundreds of psi. I happen to own a 0-5000 psi gauge. Bought it to check tractor hydralic systems. I forget just what a grease gun pumped it up to, but it was a lot. But it was a *very* low flow rate. Brian is talking about "a few" gallons per minute, to use cooler salt water in a tube on the "fresh water" side to help carry off waste heat. And it is going upwards a few tens of feet (then back down), perhaps starting at atmospheric pressure. It would be hard work, especially it it had to be kept up for an hour! There is a reverse osmosis watermaker intended for liferaft use, with a hand pump, and RO takes hundreds of psi. That is what you want, if you actually need high pressure. I figure you probably can buy a small hand-held "single-shot" pocket RO unit for just such a purpose... David A. Smith |
Potable Water - The Third Way.
On Sat, 29 Sep 2007 12:59:39 -0700, Keith Hughes
wrote: Gee, I didn't know you were using 'smart' molecules that travel *only* in the direction you want them too. And they don't bump into each other in the process. Wow, that's really neat, how did you accomplish that? I noticed that. Isn't the speed of sound, whatever it is, what it is because it _is _ the molecular speed? They both vary with temperature but not pressure. Casady |
Potable Water - The Third Way.
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Potable Water - The Third Way.
Brian Whatcott wrote:
On Sat, 29 Sep 2007 12:59:39 -0700, Keith Hughes wrote: My, my: "it's just plain wrong": he said a column of 32 ft, Uhmmm, no, he said a "32' column of water". Can you see the difference? Hmmm..Priestley certainly could. His water barometer had a water column round 32 or 33 ft high. How 'bout that! :-) Quite true, which probably engenders the confusion. However, the barometer *starts* with an evacuated column. That's how the atmospheric pressure can push the water 32' up the column - you have 14.7 PSIA to work with to lift the water. Same for a mercury barometer, or a McCleod gauge, etc. Different story than using the water column to *generate* the vacuum. Gee, I didn't know you were using 'smart' molecules that travel *only* in the direction you want them too. .... Keith Hughes Ho, hum: if half of them go in the wrong direction until their first collision, it must take them a really, really, REALLY long time to diffuse through the water vapor/rarified air mix! Half? More likely 99.99999++% of them will not be traveling parallel to the axis of the column. Half of those that *are*, are going in the wrong direction. How difficult this type of mass transport actually is can be seen by looking at flow rates for water vapor from lyophilizer chambers to the condensers (yes, I have done this a lot). Putting a 90° bend in the tube connecting the chamber (where the ice sits on heated shelves) and the condenser roughly (very roughly, given the variability of other design factors) cuts the flow rate in half. That's in a 48" ID tube too! And pulling vacuum through the condenser to maintain 50-100 microbar pressure - i.e. maintaining a significant delta-P from chamber to condenser. And with condenser coils maintained at -65°C. It may seem counterintuitive, but the molecular motion you referenced just *causes* the pressure, while providing little impetus for mass transfer from point A to point B. And once the pressure reaches equilibrium throughout the system, you have to rely virtually entirely on diffusion, which is much, much slower. Keith Hughes |
Potable Water - The Third Way.
On Sat, 29 Sep 2007 17:43:02 -0500, Brian Whatcott
wrote: On Sat, 29 Sep 2007 20:40:57 GMT, (Richard Casady) wrote: There is a reverse osmosis watermaker intended for liferaft use, with a hand pump, and RO takes hundreds of psi. That is what you want, if you actually need high pressure. Casady I looked up an example The Katadyn Survivor 35 hand pumped was formerly called the PUR Survivor 35 RO. At 30 strokes/minute for 1.2 gall/hr - it costs $1500. Calorie expenditure by the survivor(s) could be a problem here. The strokes for this RO unit can probably be performed by devising a simple hydraulic pump to move gears, cams, and levers. The pump cylinder itself would probably need an inverted U tube with legs perhaps 32' or 33' long. An initial vacuum might be applied to the top of the U-tube by using a fitting that can be connected to the PUR Survivor 35 RO. Once the water starts flowing through the vane at one end of the U tube, and the vane shaft is turning the gears, cams and levers will be clacking way, running that PUR unit on auto, good as gold. After that it's all gravy until you have to change the membrane. In the meantime you can spend your time fishing until rescued. --Vic |
Potable Water - The Third Way.
Vic Smith wrote:
On Sat, 29 Sep 2007 17:43:02 -0500, Brian Whatcott wrote: On Sat, 29 Sep 2007 20:40:57 GMT, (Richard Casady) wrote: There is a reverse osmosis watermaker intended for liferaft use, with a hand pump, and RO takes hundreds of psi. That is what you want, if you actually need high pressure. Casady I looked up an example The Katadyn Survivor 35 hand pumped was formerly called the PUR Survivor 35 RO. At 30 strokes/minute for 1.2 gall/hr - it costs $1500. Calorie expenditure by the survivor(s) could be a problem here. Oh yeah, right. Now you want to survive also. Geez, what next? :-) The strokes for this RO unit can probably be performed by devising a simple hydraulic pump to move gears, cams, and levers. The pump cylinder itself would probably need an inverted U tube with legs perhaps 32' or 33' long. An initial vacuum might be applied to the top of the U-tube by using a fitting that can be connected to the PUR Survivor 35 RO. Once the water starts flowing through the vane at one end of the U tube, and the vane shaft is turning the gears, cams and levers will be clacking way, running that PUR unit on auto, good as gold. After that it's all gravy until you have to change the membrane. In the meantime you can spend your time fishing until rescued. Sounds like perpetual motion to me, but I'm having a hard time envisioning what you're describing above. Keith Hughes |
Potable Water - The Third Way.
On Sat, 29 Sep 2007 19:44:35 -0700, Keith Hughes
wrote: Sounds like perpetual motion to me, but I'm having a hard time envisioning what you're describing above. Of course you are, since it is basically nonsense. No mention of where the energy comes from. Casady |
Potable Water - The Third Way.
On Sat, 29 Sep 2007 20:38:17 -0500, Vic Smith
wrote: The strokes for this RO unit can probably be performed by devising a simple hydraulic pump to move gears, cams, and levers. It comes with a simple hydraulic pump. I fail to see where adding complicated machinery, with no power source whatever, will be of any benefit. Casady |
Potable Water - The Third Way.
Dear Richard Casady:
"Richard Casady" wrote in message ... On Sat, 29 Sep 2007 19:44:35 -0700, Keith Hughes wrote: Sounds like perpetual motion to me, but I'm having a hard time envisioning what you're describing above. Of course you are, since it is basically nonsense. No mention of where the energy comes from. Links were provided. "waste heat" (from what process?) and / or "solar heat" have been cited so far. All the vacuum does is move boiling temperature closer to ambient. Making more common materials suitable for this application. David A. Smith |
Potable Water - The Third Way.
N:dlzc D:aol T:com (dlzc) wrote:
Dear Richard Casady: "Richard Casady" wrote in message ... On Sat, 29 Sep 2007 19:44:35 -0700, Keith Hughes wrote: Sounds like perpetual motion to me, but I'm having a hard time envisioning what you're describing above. Of course you are, since it is basically nonsense. No mention of where the energy comes from. Links were provided. "waste heat" (from what process?) and / or "solar heat" have been cited so far. All the vacuum does is move boiling temperature closer to ambient. Making more common materials suitable for this application. David A. Smith Sorry David, I think you lost track of the posting train here. Richard was responding to *my* response to the following post from "Vic": "Calorie expenditure by the survivor(s) could be a problem here. The strokes for this RO unit can probably be performed by devising a simple hydraulic pump to move gears, cams, and levers. The pump cylinder itself would probably need an inverted U tube with legs perhaps 32' or 33' long. An initial vacuum might be applied to the top of the U-tube by using a fitting that can be connected to the PUR Survivor 35 RO. Once the water starts flowing through the vane at one end of the U tube, and the vane shaft is turning the gears, cams and levers will be clacking way, running that PUR unit on auto, good as gold. After that it's all gravy until you have to change the membrane. In the meantime you can spend your time fishing until rescued." Which appears to be a reference to a perpetual motion machine with no energy source. Nothing whatsoever to do with the vacuum distillation discussion. Keith Hughes |
Potable Water - The Third Way.
I give up. My Masters degree in Physics is of no value here. My
Bachelors degree in Math is of no value here. My 20 years with the university (retired) means nothing. Someone with an opinion (however false) instead of facts of physical science, seems to be more able to swing the belief of the uninformed. I will try to explain it again. The vacuum will hold the column of water in the tube. Dont believe it? Test this statement, take a simple soda straw stick in in a glass of water put your finger over the end and lift it out. The 8" column of water stays in the straw because of the vacuum in the top of the straw. Now remove your finger and the water drops out. So, it doesn't TAKE a 32' column of water, but that is the tallest column of water that will be suspended, a simple law of physics. Thats why a lift pump like the old rocker handle pitcher pumps have to be replaced with either submerged or Jet pumps in deeper wells. A lesser column WILL work however. At the top is a vacuum. If its 32 feet, thats the greatest vacuum you can create. There is salt water on one side and fresh water on the other. The salt water will boil earlier because of the salt content. Now test that statement. Put a pot on the stove and then before it comes to a boil add salt. Voila, it begins to boil. The fresh water column is sealed at the bottom and fresh water, as it builds a higher column, can be drawn off WITH A PUMP. You cannot open the bottom of the tube to get the water out or you will break the vacuum. As you draw fresh water off WITH THE PUMP you will draw salt water into the bottom of the other end (which is under the surface of the salt water the boat is floating in) to replace the salt water that has been boiled off. Obviously you will have to be careful that you don't pull off enough fresh water to cause the sal****er column to overflow into and contaminate the fresh water column. Also, I make no representation as to the efficiency of such a system, only that it WILL work. Now, I have nothing more to say on the subject as I don't have the time to waste. I didn't realize I would have to go into such miniscule detail. I have been casting my pearls before swine and I dont have the time for that. If someone with an appropriate education and who has done the above experiments as I outlined, would like to contact me off list I would be willing to discuss it. BUT...if you are supposing, without knowledge, using feelings for facts, DONT bother me. On Sep 29, 12:04 pm, Keith Hughes wrote: jim wrote: "jim.isbell" wrote: Ah well, another great idea skuppered by dat old devil science :-) Bruce in Bangkok (brucepaigeATgmailDOTcom) A 32' column of water is a continuous vacuum pump. 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. As long as you put water (salt water) into the column it will pull down and keep a vacuum in the top of the column. Sorry, this makes no sense. Putting water in does not cause it to "pull down". Yes, you have supply makeup water to maintain column height lost to evaporation. 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. 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. as steam to the other side and distills in the fresh water side....also creating a vacuum. No, this does *not* create a vacuum in the sense you seem to mean. It maintains an equilibrium pressure by lowering the partial pressure of water vapor generated by the 'boiling' process on the seawater side. 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. 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'. You draw off the fresh water on one side and pump salt water into the other side. The salt water side is painted black to absorb sun heat and the fresh water side is painted white to reflect the suns heat. You only need a few degrees difference for distillation and the vacuum creates the boiling at low temperatures...even ice will change state to steam in a vacuum. The idea works. Yes, VERY slowly. You can increase *throughput* by increasing the column diameters, but how practical is that on a boat? It works but does it work as well as other methods that are simpler and easier to implement. Also if you have no fresh water on hand to start with there is no way to make it work. Not quite true...you can seal the 'freshwater' column, using only the column walls for condensation surfaces, until you have sufficient condensate collected to allow the freshwater column to be opened. I can see someone getting a "Darwin Award" by accidentally spilling all there existing freshwater supply in a failed attempt to get this contraption going. It doesn't *have* to be that way, BUT.... :-) In a practical sense, I would use soft tubing for the sides and a solid "U" shaped piece of copper tubing for the top center with a ring soldered to it so it could be hoisted up the mast of a sailboat. It would take a 30 to 40 foot mast to do the job. The bottom end of the salt water tube could go to a through hull for a continuous supply of salt water and the bottom end of the fresh water tube could go to a small pump to remove the water without breaking the vacuum. And what's 'practical' for useability, is impractical for functionality. There are no 'soft tubing' materials I'm aware of that have anything approaching decent heat absorbance, conduction, or emissivity properties, so that will be another very significant rate limiter in the system. That makes no sense. You are going to have a hard time pumping water out of the fresh water side any faster than gravity can deliver it. You actually *can't* pump faster than gravity, unless you want to suck seawater up the column on the other side. The salty side OTOH, if you rely only on gravity to feed it, will become a solid block of salt once you have evaporated enough water from it. Doubtful that you'd ever get a solid chunk of salt (and short of having a bypass circulation loop - cooling the column and further reducing efficiency - I don't see how a pump could even help the situation), but of course as the salinity increases, the boiling point increases, and at some point the process will just stall. The heat input won't be sufficient to boil the brine solution. Then you have to stop, drain, clean, and start over. How quickly this happens will depend on column heights and diameters, but it'll happen at some point. Just another rate-limiting feature. All these rate limiters are natures way of saying that there is no thermodynamic free lunch. A low energy input system will have a low output (in terms of whatever work you want the system to do). Keith Hughes |
Potable Water - The Third Way.
In rec.boats.cruising jim.isbell wrote:
:I give up. My Masters degree in Physics is of no value here. My Not if you think salt water has a lower boiling point than fresh, no. |
Potable Water - The Third Way.
Dear jim.isbell:
"jim.isbell" wrote in message ups.com... I give up. My Masters degree in Physics is of no value here. My Bachelors degree in Math is of no value here. My 20 years with the university (retired) means nothing. Someone with an opinion (however false) instead of facts of physical science, seems to be more able to swing the belief of the uninformed. Someone with this much experience must know that the ignorant will always trample the carpet of wisdom. Why do you waste your time responding to them? The entire comedy is misinterpretation of wording, and argument about strawmen. Relax and have what is left of a weekend. For they (in this case) are as right as you are... just about different things. David A. Smith |
Potable Water - The Third Way.
Wow, Pearls! Let this swine take a look and see if he can root them out...
jim.isbell wrote: I give up. My Masters degree in Physics is of no value here. My Bachelors degree in Math is of no value here. My 20 years with the university (retired) means nothing. Someone with an opinion (however false) instead of facts of physical science, seems to be more able to swing the belief of the uninformed. I stand (sit actually) abashed in the light of your professed achievements. I will try to explain it again. The vacuum will hold the column of water in the tube. Pearl #1? Hmmm, no. No one claimed otherwise. continued rooting sound snip irrelevancies related to above statement Thats why a lift pump like the old rocker handle pitcher pumps have to be replaced with either submerged or Jet pumps in deeper wells. A lesser column WILL work however. At the top is a vacuum. If its 32 feet, thats the greatest vacuum you can create. There is salt water on one side and fresh water on the other. The salt water will boil earlier because of the salt content. Pearl #1? Hmmm, alas no. Physics relating ionic strength and boiling point of water must have changed since your education. continued rooting sound Now test that statement. Put a pot on the stove and then before it comes to a boil add salt. Voila, it begins to boil. Pearl #1? Hmmm, no dice here either. It's called *nucleation*. Ever notice the water *stops* boiling as soon as the salt is fully dissolved? And doesn't boil again until the new, higher, boiling point is reached? Try it. continued rooting sound The fresh water column is sealed at the bottom and fresh water, as it builds a higher column, can be drawn off WITH A PUMP. You cannot open the bottom of the tube to get the water out or you will break the vacuum. Pearl #1? Hmmm, not yet. Of course you can pull water from the BOTTOM of the freshwater column - if its end is below the freshwater reservoir level. Just like the seawater side is. Elevate the freshwater reservoir, decant to maintain column height. No pump needed. How hard was that? continued rooting sound snip Also, I make no representation as to the efficiency of such a system, only that it WILL work. Pearl #1? Hmmm, well, maybe a tiny, dull one - well, maybe not, no one said it wouldn't work. And if you think the 'perpetual motion' reference was to this system, then learn to read. That post was about *RO* and using this system as a hydraulic pump mechanism. continued rooting sound Now, I have nothing more to say on the subject as I don't have the time to waste. I didn't realize I would have to go into such miniscule detail. I have been casting my pearls before swine and I dont have the time for that. If someone with an appropriate education and who has done the above experiments as I outlined, would like to contact me off list I would be willing to discuss it. BUT...if you are supposing, without knowledge, using feelings for facts, DONT bother me. OK Mr. Oyster. Oh, and by-the-by, *you* choose to be bothered or not, we don't do that for you. Have fun in bivalvia... Keith Hughes |
Potable Water - The Third Way.
On Sep 30, 9:13 am, "jim.isbell" wrote:
... The salt water will boil earlier because of the salt content. ... http://en.wikipedia.org/wiki/Boiling-point_elevation -- Tom. |
Potable Water - The Third Way.
On Sat, 29 Sep 2007 19:44:35 -0700, Keith Hughes
wrote: Vic Smith wrote: On Sat, 29 Sep 2007 17:43:02 -0500, Brian Whatcott wrote: On Sat, 29 Sep 2007 20:40:57 GMT, (Richard Casady) wrote: There is a reverse osmosis watermaker intended for liferaft use, with a hand pump, and RO takes hundreds of psi. That is what you want, if you actually need high pressure. Casady I looked up an example The Katadyn Survivor 35 hand pumped was formerly called the PUR Survivor 35 RO. At 30 strokes/minute for 1.2 gall/hr - it costs $1500. Calorie expenditure by the survivor(s) could be a problem here. Oh yeah, right. Now you want to survive also. Geez, what next? :-) The strokes for this RO unit can probably be performed by devising a simple hydraulic pump to move gears, cams, and levers. The pump cylinder itself would probably need an inverted U tube with legs perhaps 32' or 33' long. An initial vacuum might be applied to the top of the U-tube by using a fitting that can be connected to the PUR Survivor 35 RO. Once the water starts flowing through the vane at one end of the U tube, and the vane shaft is turning the gears, cams and levers will be clacking way, running that PUR unit on auto, good as gold. After that it's all gravy until you have to change the membrane. In the meantime you can spend your time fishing until rescued. Sounds like perpetual motion to me, but I'm having a hard time envisioning what you're describing above. Sorry, it was all said jokingly, but appears to be a poor joke. I just went in a circle from the perpetual U-tube distiller to that concept being employed to perpetually pump a purchased RO unit. I never intended to make sense, except maybe to say it's time to go fishing. --Vic |
Potable Water - The Third Way.
On Sat, 22 Sep 2007 17:51:56 -0400, "Wilbur Hubbard"
wrote: When it gets full you haul it up and empty in into your tanks. Reverse osmosis without any energy used to get it. Ain't Wilbur brilliant? You haul it up without using any energy to do it? Absolutely not/ It will take a foot pound for each pound for each foot you haul it. No your basis for perpetual motion will not work. And is the opposite of brilliant. Casady |
Potable Water - The Third Way.
Richard Casady brought forth on stone tablets:
On Sat, 22 Sep 2007 17:51:56 -0400, "Wilbur Hubbard" wrote: When it gets full you haul it up and empty in into your tanks. Reverse osmosis without any energy used to get it. Ain't Wilbur brilliant? You haul it up without using any energy to do it? Absolutely not/ It will take a foot pound for each pound for each foot you haul it. No your basis for perpetual motion will not work. And is the opposite of brilliant. Casady Well, not quite. The harvested fresh water is actually buoyant in the sea water. Hauling up the water is energy free. Hauling up the container and the rope is not, however. With suitable flotation, the container could be made neutral-buoyant, and so hauling it up could be free also, Finally, if the rope were HD polyethylene or something else with about 1.0 density, the rope could be free to hoist too. It would be necessary to attach a weight greater than the weight of water to be harvested to the container in order to get it to sink. This weight would then be disconnected/abandoned before hoisting the recovered water. From an energy standpoint, the investment would be that necessary to cover the friction in the hauling apparatus, and the the invested energy content of the abandoned weight (steel: high, concrete: medium, rock: free). Venting the container to the surface would be impractical. Evacuate it instead. With Wilbur, one must be careful to not discard the wheat with the chaff... bob s/v Eolian Seattle |
Potable Water - The Third Way.
On Tue, 02 Oct 2007 09:59:46 -0700, RW Salnick
wrote: Richard Casady brought forth on stone tablets: On Sat, 22 Sep 2007 17:51:56 -0400, "Wilbur Hubbard" wrote: When it gets full you haul it up and empty in into your tanks. Reverse osmosis without any energy used to get it. Ain't Wilbur brilliant? You haul it up without using any energy to do it? Absolutely not/ It will take a foot pound for each pound for each foot you haul it. No your basis for perpetual motion will not work. And is the opposite of brilliant. Casady Well, not quite. The harvested fresh water is actually buoyant in the sea water. Hauling up the water is energy free. Hauling up the container and the rope is not, however. With suitable flotation, the container could be made neutral-buoyant, and so hauling it up could be free also, Finally, if the rope were HD polyethylene or something else with about 1.0 density, the rope could be free to hoist too. It would be necessary to attach a weight greater than the weight of water to be harvested to the container in order to get it to sink. This weight would then be disconnected/abandoned before hoisting the recovered water. From an energy standpoint, the investment would be that necessary to cover the friction in the hauling apparatus, and the the invested energy content of the abandoned weight (steel: high, concrete: medium, rock: free). Venting the container to the surface would be impractical. Evacuate it instead. With Wilbur, one must be careful to not discard the wheat with the chaff... bob s/v Eolian Seattle And how much of the time are you sailing in 500 ft deep water, which was the original specification? Bruce in Bangkok (brucepaigeATgmailDOTcom) |
Potable Water - The Third Way.
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Potable Water - The Third Way.
On Wed, 03 Oct 2007 08:13:20 -0700, RW Salnick
wrote: brought forth on stone tablets: On Tue, 02 Oct 2007 09:59:46 -0700, RW Salnick wrote: Richard Casady brought forth on stone tablets: On Sat, 22 Sep 2007 17:51:56 -0400, "Wilbur Hubbard" wrote: When it gets full you haul it up and empty in into your tanks. Reverse osmosis without any energy used to get it. Ain't Wilbur brilliant? You haul it up without using any energy to do it? Absolutely not/ It will take a foot pound for each pound for each foot you haul it. No your basis for perpetual motion will not work. And is the opposite of brilliant. Casady Well, not quite. The harvested fresh water is actually buoyant in the sea water. Hauling up the water is energy free. Hauling up the container and the rope is not, however. With suitable flotation, the container could be made neutral-buoyant, and so hauling it up could be free also, Finally, if the rope were HD polyethylene or something else with about 1.0 density, the rope could be free to hoist too. It would be necessary to attach a weight greater than the weight of water to be harvested to the container in order to get it to sink. This weight would then be disconnected/abandoned before hoisting the recovered water. From an energy standpoint, the investment would be that necessary to cover the friction in the hauling apparatus, and the the invested energy content of the abandoned weight (steel: high, concrete: medium, rock: free). Venting the container to the surface would be impractical. Evacuate it instead. With Wilbur, one must be careful to not discard the wheat with the chaff... bob s/v Eolian Seattle And how much of the time are you sailing in 500 ft deep water, which was the original specification? Bruce in Bangkok (brucepaigeATgmailDOTcom) 500 feet? That's only 83 fathoms. My sailing area is Puget Sound, much of which is 150 fathoms or more. Why? Is Thailand in a skinny water zone? bob s/v Eolian Seattle From Singapore north through either the Gulf of Thailand or up the west coats of Malaysia or most of the western part of Indonesia 150 ft. of water would be deep water. Wilbur's invention isn;t going to work very well over here. Bruce in Bangkok (brucepaigeATgmailDOTcom) |
Potable Water - The Third Way.
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Potable Water - The Third Way.
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Potable Water - The Third Way.
Mark Borgerson wrote:
In article , says... snip 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. How do you get 33' as 1/2 of the diffusion path. A quick thumbnail guesstimation at where equilibrium would likely be reached. I didn't take the time to calculate the exact heights. I think there will be about 33 feet of water in the column on each side Then I think you would be wrong, unless your columns are significantly longer than that, probably more like 50+ feet. ---to provide the weigth that pulls the pressure down. That would leave only about 7 feet of water vapor path on each side of the column. There is no vacuum to hold the water up - the vacuum is what you are trying to *create*. The water columns will drop until there is an equilibrium point reached between the external atmospheric pressure, the height (weight as you state) of the water column, and the pressure in the headspace (the U-tube). The water columns *must* retreat, or the headspace stays at atmospheric pressure. If the tubes are long enough, and the initial column heights are high enough, then when you reach equilibrium, you'll have close to a vacuum and close to 33' water column heights. And a lot more empty headspace than you started with. Use the ideal gas law: PV=nRT For our evacuation purposes, nRT is a constant (#moles is constant, R doesn't change, and assume constant temperature), so if you start with a volume of 1 liter, and a pressure of 14.7 psia, and you want to reduce that pressure to 1.47psia, then you need a 10-fold volume increase. You want to reduce it to 0.147psia? then you need a 100-fold initial-volume increase. I'm not sure that 'diffusion' is the proper term for the motion of the water vapor. After all, the heat engine is providing water vapor on one side and condensing it on the other---so there is a net mass flow and probably a small pressure differential to move the vapor. Well, diffusion is the primary mechanism. What happens when your 'heat engine' creates water vapor? It doesn't just immediately condense on the other side. It creates pressure on the heating side, which does two things. One, it drives both the water columns *downward*, and it raises the boiling point on the seawater side (it does, however, make condensation on the fresh side more efficient as well). You can't look at this as a static system where the pressure stays the same or the column heights stay the same. It's a dynamic system, and will reach an equilibrium point with the columns much lower than the initial starting point, and the headspace pressure much higher. And don't forget, there will also be significant evaporation (due to low partial pressures) on the freshwater side that will be in equilibrium with (and in opposition to) the condensation process. It's not as simple a system as it seems. That's why this system *will* work, but it must work very slowly. Still (pun intended), you need a lot of heat to provide the energy to evaporate the water or it will soon cool to the point where its vapor pressure is reduced and the process slows drastically. My 'guess' would be that the system would end up operating around 4-5psia when equilibrium is reached, which would require a temp of about 60°C (140°F) to maintain boiling. Here in my neck of the woods, our energy from the sun ranges from about 220-360 BTU/ft^2/Hr measured at normal incidence, depending on the time of year. A couple of decades ago I worked at a solar test lab and we tested all kinds of collectors, including swimming pool collectors which are unglazed (i.e. no cover over them to exclude wind). Bare copper tubes, painted black, with no wind, are about 15% efficient at solar absorption (#'s are from my old memory, so...) when the tubings' longitudinal surface is perpendicular to the incident angle. However, with a 3 mph wind (per ASHRAE 95-1981 which we used for indoor system simulations) that efficiency drops to the low single digits. When you factor in off-angle response (i.e. since the tubes won't be on a tracking mount to keep them 'aimed at the sun") the basic efficiency drops from ~15% to probably ~8%, and with the wind, between -3% to 3%. So, using only the tube as a collector is a real challenge. Probably be better using a flat-plate collector as the primary heater, but that's another major addition to the complexity. Of course, too much heat would kill the system with over pressurization. The fact that the water 'boils' near room temperature does not reduce the amount of heat required to change the water from liquid to vapor. No, in fact the lower pressure raises it a bit. Latent Heat of Vaporization for water is inversely proportional to the pressure, albeit the change is less than 10% IIRC. As has been discussed, the simple idea does not address the problems of salt buildup in the seawater side, or the addition of dissolved gasses to the vacuum part of the loop. Non-condensables are a rate limiter for the process, unless you want to spend more energy for vacuum deaeration. With a large enough (or double) sal****er tube you might get a convection cell going with the cold, saltier water sinking and pulling up warmer seawater to the top. Certainly possible, but not easily doable. You could solve the dissolved gas problem by periodically pumping both tubes up enough to displace the accumulated gases. Well, if you added a convection cell as above (another system that requires time to reach an equilibrium condition to work), then the periodic headspace purging would quench both the distillation and the seawater convection systems. In reality, the purging would be likely be very frequent given the size of tubes that would be practical. Now the project is getting complex enough that an RO system starts to look attractive! Yep, sure does. Keith Hughes |
Potable Water - The Third Way.
In article ,
says... Mark Borgerson wrote: In article , says... snip 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. How do you get 33' as 1/2 of the diffusion path. A quick thumbnail guesstimation at where equilibrium would likely be reached. I didn't take the time to calculate the exact heights. I think there will be about 33 feet of water in the column on each side Then I think you would be wrong, unless your columns are significantly longer than that, probably more like 50+ feet. ---to provide the weigth that pulls the pressure down. That would leave only about 7 feet of water vapor path on each side of the column. There is no vacuum to hold the water up - the vacuum is what you are trying to *create*. The water columns will drop until there is an equilibrium point reached between the external atmospheric pressure, the height (weight as you state) of the water column, and the pressure in the headspace (the U-tube). The water columns *must* retreat, or the headspace stays at atmospheric pressure. If the tubes are long enough, and the initial column heights are high enough, then when you reach equilibrium, you'll have close to a vacuum and close to 33' water column heights. And a lot more empty headspace than you started with. I see the problem. I am assuming that you completely fill a 40 foot tube with water using a pump capable of providing about 16-20 PSIG. That fills the tube completely with water--at which point you close the tube (with a one-way valve). When you release the pressure at the bottom end, the water falls to the point where the weight of the water column is one atm (about 14.7PSIA) minus the vapor pressure of water at 20deg C. The vapor pressure of water at 20C is about 17.5mmHg, or about 2.3% of the 760mmHg standard atmosphere. Since a mercury has a density 13.6, the column of water will be 13.6 * (760- 17.6)mm high. That's 10.1m high, or about 33.12 feet high. In a 40-foot tube, that would leave about 7 feet of water vapor at the top of the tube and 33 feet of water below the vapor. Use the ideal gas law: PV=nRT For our evacuation purposes, nRT is a constant (#moles is constant, R doesn't change, and assume constant temperature), so if you start with a volume of 1 liter, and a pressure of 14.7 psia, and you want to reduce that pressure to 1.47psia, then you need a 10-fold volume increase. You want to reduce it to 0.147psia? then you need a 100-fold initial-volume increase. What is the 1 liter to which you refer? This is not a closed system---the tube is open to a reservoir at atmospheric pressure at the bottom. I'm assuming that you start with a head space (or initial volume) of zero. You then simply have to evaporate enough water to fill the top of the tube with water vapor to the point where vapor pressure + water weight = 1ATM. I'm not sure that 'diffusion' is the proper term for the motion of the water vapor. After all, the heat engine is providing water vapor on one side and condensing it on the other---so there is a net mass flow and probably a small pressure differential to move the vapor. Well, diffusion is the primary mechanism. What happens when your 'heat engine' creates water vapor? It doesn't just immediately condense on the other side. It creates pressure on the heating side, which does two things. One, it drives both the water columns *downward*, and it raises the boiling point on the seawater side (it does, however, make condensation on the fresh side more efficient as well). You can't look at this as a static system where the pressure stays the same or the column heights stay the same. It's a dynamic system, and will reach an equilibrium point with the columns much lower than the initial starting point, and the headspace pressure much higher. Well, not much higher----only about 17.5 mmHG higher. But that IS a lot higher than zero! ;-) And don't forget, there will also be significant evaporation (due to low partial pressures) on the freshwater side that will be in equilibrium with (and in opposition to) the condensation process. It's not as simple a system as it seems. That's why this system *will* work, but it must work very slowly. Still (pun intended), you need a lot of heat to provide the energy to evaporate the water or it will soon cool to the point where its vapor pressure is reduced and the process slows drastically. My 'guess' would be that the system would end up operating around 4-5psia when equilibrium is reached, which would require a temp of about 60°C (140°F) to maintain boiling. AHA!, you're assuming a much higher operating temperature than me. I was assuming something on the order of 20 to 25C. You're going to have to add to your energy budget the heat necessary to raise the water temperature from 20C to 60C, then. If the equilibrium pressure is really 1/3ATM, then there will be about 20 feet of water in the 40-foot tube and 20 feet of vapor. If you're going to work at those temperatures and pressures, you probably need only a 22-foot tube. Here in my neck of the woods, our energy from the sun ranges from about 220-360 BTU/ft^2/Hr measured at normal incidence, depending on the time of year. A couple of decades ago I worked at a solar test lab and we tested all kinds of collectors, including swimming pool collectors which are unglazed (i.e. no cover over them to exclude wind). Bare copper tubes, painted black, with no wind, are about 15% efficient at solar absorption (#'s are from my old memory, so...) when the tubings' longitudinal surface is perpendicular to the incident angle. However, with a 3 mph wind (per ASHRAE 95-1981 which we used for indoor system simulations) that efficiency drops to the low single digits. When you factor in off-angle response (i.e. since the tubes won't be on a tracking mount to keep them 'aimed at the sun") the basic efficiency drops from ~15% to probably ~8%, and with the wind, between -3% to 3%. So, using only the tube as a collector is a real challenge. Probably be better using a flat-plate collector as the primary heater, but that's another major addition to the complexity. Of course, too much heat would kill the system with over pressurization. The fact that the water 'boils' near room temperature does not reduce the amount of heat required to change the water from liquid to vapor. No, in fact the lower pressure raises it a bit. Latent Heat of Vaporization for water is inversely proportional to the pressure, albeit the change is less than 10% IIRC. As has been discussed, the simple idea does not address the problems of salt buildup in the seawater side, or the addition of dissolved gasses to the vacuum part of the loop. Non-condensables are a rate limiter for the process, unless you want to spend more energy for vacuum deaeration. With a large enough (or double) sal****er tube you might get a convection cell going with the cold, saltier water sinking and pulling up warmer seawater to the top. Certainly possible, but not easily doable. You could solve the dissolved gas problem by periodically pumping both tubes up enough to displace the accumulated gases. Well, if you added a convection cell as above (another system that requires time to reach an equilibrium condition to work), then the periodic headspace purging would quench both the distillation and the seawater convection systems. In reality, the purging would be likely be very frequent given the size of tubes that would be practical. Now the project is getting complex enough that an RO system starts to look attractive! Yep, sure does. Keith Hughes Mark Borgerson |
Potable Water - The Third Way.
Mark Borgerson wrote:
In article , says... Mark Borgerson wrote: In article , says... snip snip How do you get 33' as 1/2 of the diffusion path. A quick thumbnail guesstimation at where equilibrium would likely be reached. I didn't take the time to calculate the exact heights. I think there will be about 33 feet of water in the column on each side Then I think you would be wrong, unless your columns are significantly longer than that, probably more like 50+ feet. ---to provide the weigth that pulls the pressure down. That would leave only about 7 feet of water vapor path on each side of the column. There is no vacuum to hold the water up - the vacuum is what you are trying to *create*. The water columns will drop until there is an equilibrium point reached between the external atmospheric pressure, the height (weight as you state) of the water column, and the pressure in the headspace (the U-tube). The water columns *must* retreat, or the headspace stays at atmospheric pressure. If the tubes are long enough, and the initial column heights are high enough, then when you reach equilibrium, you'll have close to a vacuum and close to 33' water column heights. And a lot more empty headspace than you started with. I see the problem. I am assuming that you completely fill a 40 foot tube with water using a pump capable of providing about 16-20 PSIG. That fills the tube completely with water--at which point you close the tube (with a one-way valve). Uhmm, a manual valve is a manual valve. A "one-way" valve is a checkvalve, and you wouldn't need to close it. When you release the pressure at the bottom end, the water falls to the point where the weight of the water column is one atm (about 14.7PSIA) minus the vapor pressure of water at 20deg C. The vapor pressure of water at 20C is about 17.5mmHg, or about 2.3% of the 760mmHg standard atmosphere. Since a mercury has a density 13.6, the column of water will be 13.6 * (760- 17.6)mm high. That's 10.1m high, or about 33.12 feet high. In a 40-foot tube, that would leave about 7 feet of water vapor at the top of the tube and 33 feet of water below the vapor. Ahh, no. See below... Use the ideal gas law: PV=nRT For our evacuation purposes, nRT is a constant (#moles is constant, R doesn't change, and assume constant temperature), so if you start with a volume of 1 liter, and a pressure of 14.7 psia, and you want to reduce that pressure to 1.47psia, then you need a 10-fold volume increase. You want to reduce it to 0.147psia? then you need a 100-fold initial-volume increase. What is the 1 liter to which you refer? It is an example, for illustration purposes. It's the headspace (i.e. the amount of volume *not* filled with water, prior to closing the valve and letting the water columns 'fall'). The point is, whatever your starting headspace volume is, to get anywhere near a vacuum, the *VOLUME* of the headspace must increase 100 fold. That is the relevance of the ideal gas law. If you start with a 100ml headspace, then to get a decent vacuum, the water columns have to drop to a point where the headspace is 10L. *AT THAT POINT* you have sufficient vacuum to support a water column of around 30'. But the columns have dropped significantly to achieve that vacuum, and thus the columns must be much higher, as must the initial water column height. The *only* way the headspace volume increases is if the water columns drop significantly, and the only way significant vacuum is created is if the sealed volume increases tremendously. This is not a closed system---the tube is open to a reservoir at atmospheric pressure at the bottom. I'm assuming that you start with a head space (or initial volume) of zero. You then simply have to evaporate enough water to fill the top of the tube with water vapor to the point where vapor pressure + water weight = 1ATM. I'm not sure that 'diffusion' is the proper term for the motion of the water vapor. After all, the heat engine is providing water vapor on one side and condensing it on the other---so there is a net mass flow and probably a small pressure differential to move the vapor. Well, diffusion is the primary mechanism. What happens when your 'heat engine' creates water vapor? It doesn't just immediately condense on the other side. It creates pressure on the heating side, which does two things. One, it drives both the water columns *downward*, and it raises the boiling point on the seawater side (it does, however, make condensation on the fresh side more efficient as well). You can't look at this as a static system where the pressure stays the same or the column heights stay the same. It's a dynamic system, and will reach an equilibrium point with the columns much lower than the initial starting point, and the headspace pressure much higher. Well, not much higher----only about 17.5 mmHG higher. But that IS a lot higher than zero! ;-) No, a lot higher. You're confusing vapor pressure with the "steam" pressure during distillation. Huge difference. Vapor pressure is the countervailing force (fighting condensation as it were) on the FRESH water side of the system (and the seawater side). Vapor pressure in both columns will be about the same, so you have to boil the seawater column to get any significant vapor transfer. This results in *much* higher pressure, which lowers the columns and increases the vapor path and.... And don't forget, there will also be significant evaporation (due to low partial pressures) on the freshwater side that will be in equilibrium with (and in opposition to) the condensation process. It's not as simple a system as it seems. That's why this system *will* work, but it must work very slowly. Still (pun intended), you need a lot of heat to provide the energy to evaporate the water or it will soon cool to the point where its vapor pressure is reduced and the process slows drastically. My 'guess' would be that the system would end up operating around 4-5psia when equilibrium is reached, which would require a temp of about 60°C (140°F) to maintain boiling. AHA!, you're assuming a much higher operating temperature than me. Yes, because you're mistaking the amount of "vacuum" you'll have available when the system reaches equilibrium. I was assuming something on the order of 20 to 25C. Then you are assuming an almost perfect vacuum, which can't happen since the boiling Must significantly raise the headspace pressure. You're going to have to add to your energy budget the heat necessary to raise the water temperature from 20C to 60C, then. If the equilibrium pressure is really 1/3ATM, then there will be about 20 feet of water in the 40-foot tube and 20 feet of vapor. If you're going to work at those temperatures and pressures, you probably need only a 22-foot tube. You need to get back to the gas law to see where this error lies. You have to *create* the vacuum. That requires a HUGE increase in volume for whatever the initial headspace is. For this to happen you need a much longer tube to start with. Keith Hughes |
Potable Water - The Third Way.
Boiling Point Elevation
The boiling point of a solution is higher than that of the pure solvent. Accordingly, the use of a solution, rather than a pure liquid, in antifreeze serves to keep the mixture from boiling in a hot automobile engine. As with freezing point depression, the effect depends on the number of solute particles present in a given amount of solvent, but not the identity of those particles. If 10 grams (0.35 ounces) of sodium chloride are dissolved in 100 grams (3.5 ounces) of water, the boiling point of the solution is 101.7°C (215.1°F; which is 1.7°C (3.1°F) higher than the boiling point of pure water). The formula used to calculate the change in boiling point ( Tb) relative to the pure solvent is similar to that used for freezing point depression: Tb = i Kb m, where Kb is the boiling point elevation constant for the solvent (0.52°C·kg/mol for water), and m and i have the same meanings as in the freezing point depression formula. Note that Tb represents an increase in the boiling point, whereas Tf represents a decrease in the freezing point. As with the freezing point depression formula, this one is most accurate at low solute concentrations. From: http://www.chemistryexplained.com/Ce...roperties.html |
Potable Water - The Third Way.
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Potable Water - The Third Way.
Mark Borgerson wrote:
In article , says... SNIP You need to get back to the gas law to see where this error lies. You have to *create* the vacuum. That requires a HUGE increase in volume for whatever the initial headspace is. For this to happen you need a much longer tube to start with. You seem to have missed the fact that I proposed filling the tubes completely with water so that the initial head space would be zero. No, it won't be zero. It can't be. If it is, then you have a solid liquid stream, and it's just a siphon. You have to have headspace. And it has to be sufficient to maintain separation of the seawater and freshwater to prevent contamination when filling the tubes. And it has to be large enough to prevent percolation carryover when boiling is initiated. At that point you release the pressure on the water and it falls to the point where water weight plus vapor pressure equals 1ATm. A solid liquid loop will not separate into two separate columns. They have to be separated by a headspace. You can heat the seawater side and create a headspace by liberating dissolved gases, then let the columns drop to create vacuum, but you will have contaminated the freshwater side. At that point, you essentially have two water barometers, interconnected at the top. One is salty and warm, and one is fresh and cold. Neither need be too much longer than 33 feet. The actual height of the water will be less than 32 feet by a factor dependent on the temperature of the water in the warm side. The real practical problem lies in the addition of the dissolved gases in the seawater to the water vapor in the headspace. What we have here is a rather inefficient degassing column. I spent a lot of time degassing seawater while working on my MS in chemical oceanography. I was trying to measure the dissolved hydrogen in seawater, and the oxygen, nitrogen, methane, and other gases kept getting in the way! Getting rid of the disssolved gases in the headspace and as bubbles forming on the sides of the tube is going to be a major headache. Not a headache, an impossibility (they're not really dissolved at that point though) :-) That, and the increase in pressure due to water vapor will make this an oscillating, self-quenching system. It'll require more and more heat as the partial pressures of the non-condensables increases, and the column heights will drop as the pressure goes up, with the diffusion path increasing the whole time. Keith Hughes |
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