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Boat Forum / Building / October 2007Potable Water - The Third Way.
You've heard all about distilling water, and you've heard all about reverse osmosis, but you haven't heard about low-cost, low energy stills: they are brand new. Briefly: Take one forty ft vertical tube filled with saline. Take one forty ft vertical tube filled with fresh water. Connect them with a little engineering help - at the top. The boiling point of water at sea level pressure is about 100 deg C The boiling point of water at the top of a sealed 40 ft column of water is near ambient. So, it doesn't take much heat to boil the brackish water, and have it pass to the fresh column where it is slightly cooled to hold the near vacuum conditions at the boiling level. [An engineering effort of a U of Utah group I think] Brian Whatcott Altus OK Dear Brian Whatcott: > You've heard all about distilling water, and you've > heard all about reverse osmosis, but you haven't [quoted text clipped - 18 lines] > > [An engineering effort of a U of Utah group I think] There are ship-board distiller units that use an engine to pull a vacuum, and the engine's waste heat to boil that water, to generate drinking water. A little shorter... David A. Smith dlzc <dlzc1@cox.net> wrote in news:1190415672.506271.93890 @k79g2000hse.googlegroups.com: > There are ship-board distiller units that use an engine to pull a > vacuum, and the engine's waste heat to boil that water, to generate > drinking water. A little shorter... > > David A. Smith http://en.wikipedia.org/wiki/Boiling_point "The boiling point of water is 100 °C (212 °F) at standard pressure. On top of Mount Everest the pressure is about 260 mbar (26 kPa) so the boiling point of water is 69 °C. (156.2 °F)." AT 40' ASL, the boiling point must be down to...to....211.95F! Larry  SignatureSearch youtube for "Depleted Uranium" The ultimate dirty bomb...... Dear Larry: > dlzc <dlzc1@cox.net> wrote in news:1190415672.506271.93890 > @k79g2000hse.googlegroups.com: [quoted text clipped - 13 lines] > AT 40' ASL, the boiling point must be down > to...to....211.95F! What Brian left to the reader's imagination, is that the head space of the tubes is at a near perfect vacuum, flooded only with water vapor. You might recall that a perfect vacuum will lift a column of water about 32 feet, on a high pressure day. Or had you not figured that out? David A. Smith > Dear Larry: > [quoted text clipped - 21 lines] > column of water about 32 feet, on a high pressure day. Or had > you not figured that out? Well no, he obviously hadn't figured that out. Nor can anybody figure out what is going to hold a column of water 40 ft high as was stated in the original post. The tubes may be 40 feet but the column of water will be considerably less. How much less will depend on how much energy is heating on the hot side and how much energy is cooling on the cool side. The total amount of energy needed is not going to be any different than any other distilling method. Unless you have the free or cheap sources of cooling and heating at specific temperatures this isn't going to work any better either. -jim > David A. Smith >> What Brian left to the reader's imagination, is that the head >> space of the tubes is at a near perfect vacuum, flooded only with >> water vapor. You might recall that a perfect vacuum will lift a >> column of water about 32 feet, on a high pressure day. Or had >> you not figured that out? >Well no, he obviously hadn't figured that out. Nor can anybody figure >out what is going to hold a column of water 40 ft high as was stated in [quoted text clipped - 7 lines] > >-jim Well, at least this respondent Jim, is operating at shall we say the 7th grade level of science/engineering insight. Like so many other products of the domestic school system, he seems to have a severe case of self-esteem syndrome. Still, he may be retrievable, starting with a science demonstration he may have missed. Place a beaker of water in a bell-jar and pump the air out. When 99% of the air has been pumped out, the water in the beaker is boiling vigorously, until, in the usual way, the beaker boils dry. The beaker feels cool to the touch, naturally. To quote him: "unless I have a cheap source of heating this won't work..." For the $64 prize: NOW do you get it? Brian Whatcott Altus OK There's another neat way you can demonstrate this with minimal equipment. Take a tight fitting jar and get the water boiling vigerously in it with the cap on loose enough to let the steam out. When it is full of dense steam and about 1/3 boiling water, remove instantly from heat and tighten cap. When everything is cooled to room temperature, put an ice cube against the jar and the water will start to boil. The ice condenses the water vapor further, reducing the pressure to the point where the water will boil at room temperature. I've seen it done and it looks like the ice cube is boiling the water. My father won a science fair doing this back in the 1930's. -- Roger Long Sounds neat, haven't seen that one before. Thanks for sharing. > There's another neat way you can demonstrate this with minimal equipment. > [quoted text clipped - 14 lines] > -- > Roger Long "Roger Long" <strider@maine.rr.com> wrote in news:46f53eef$0$5006 $4c368faf@roadrunner.com: > My father won a science fair doing this back in the 1930's. > > -- > Roger Long Was his real name Don Herbert and he worked for GE?... Larry SignatureI don't think I ever missed a Mr Wizard TV show. Of course, if you put the same shows on, today, Mr Wizard would be so far over the heads of the kids with less than Master's Degrees.....they couldn't follow along...(c; > There's another neat way you can demonstrate this with minimal equipment. > [quoted text clipped - 13 lines] > -- > Roger Long http://www.metacafe.com/watch/414997/boiling_using_ice/ Joe On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott <betwys1@sbcglobal.net> wrote stuff and I replied: But what is the cheap source of getting the vacuum? I figured there had to be a vacuum, although it was not said. But how do you get it? >>Well no, he obviously hadn't figured that out. Nor can anybody figure >>out what is going to hold a column of water 40 ft high as was stated in [quoted text clipped - 27 lines] > >Brian Whatcott Altus OK Human bevaviour: Bestiality with a brain Dear OldNick: > On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott > <betwys1@sbcglobal.net> wrote stuff [quoted text clipped - 3 lines] > I figured there had to be a vacuum, although it was > not said. But how do you get it? Fill the tubes, with a small air bubble in between. Lift the tube at the bubble. No small feat if you are looking to make hundreds of gallons per day, but a one-shot expense... David A. Smith On Sat, 22 Sep 2007 20:43:48 -0700, "N:dlzc D:aol T:com \(dlzc\)" <dlzc1@cox.net> wrote stuff and I replied: Then how do you keep the freshwater tube so cool? It has to be a _lot_ cooler, not so? Is this a continuous or pulsed process? How do you maintain stasis and extract fresh water? If you have a small air bubble, where does the extracted fresh water go? etc >Fill the tubes, with a small air bubble in between. Lift the >tube at the bubble. No small feat if you are looking to make >hundreds of gallons per day, but a one-shot expense... > >David A. Smith Human bevaviour: Bestiality with a brain Dear Old Nick: > On Sat, 22 Sep 2007 20:43:48 -0700, "N:dlzc D:aol T:com > \(dlzc\)" > <dlzc1@cox.net> wrote stuff >>Fill the tubes, with a small air bubble in between. >>Lift the tube at the bubble. No small feat if you >>are looking to make hundreds of gallons per day, >> but a one-shot expense... > and I replied: > > Then how do you keep the freshwater tube so > cool? It has to be a _lot_ cooler, not so? Likely, yes. You could use evaporative cooling (of brine) on that side, and solar heating on the other side. > Is this a continuous or pulsed process? I woudl assume it could be either, depending on the sophisticatioin of your control process. > How do you maintain stasis and extract fresh water? Ever seen a mercury barometer? The bottom end of the tube ends in a "pan" open to atmosphere. The bottom end of both tubes can simply be sunk... one in the ocean, and one in a wet well for a pump station. > If you have a small air bubble, where does the > extracted fresh water go? The air bubble expands when the vaccum is created. > etc etc. Take your shades off, dude. It is coloring everything you see... It isn't the greatest thing since sliced bread. But it is another process, and a viable one. David A. Smith ... >Then how do you keep the freshwater tube so cool? It has to be a _lot_ >cooler, not so? I'm probably going to regret responding, but I will anyway, with a question: What is the difference in temperature between steam and water, both at the boiling temperature of water, whatever it may be? Cooler means lower temperature, right? OK you can now answer your own question. I hope. Brian W Dear Brian Whatcott: > On Sun, 23 Sep 2007 21:57:56 +0800, OldNick > <nsremovable@iinet.net.au> [quoted text clipped - 14 lines] > OK you can now answer your own question. > I hope. You don't happen to like bitters, do you? ;>) David A. Smith On Sun, 23 Sep 2007 11:29:46 -0700, "N:dlzc D:aol T:com \(dlzc\)" <dlzc1@cox.net> wrote: >Dear Brian Whatcott: >You don't happen to like bitters, do you? ;>) > >David A. Smith Angostura I can take or leave: India Pale Ale works for me, but not if I've gone for a Burton. :-) Brian W On Sun, 23 Sep 2007 18:10:09 GMT, Brian Whatcott <betwys1@sbcglobal.net> wrote stuff and I replied: You are a rude and arrogant prick >... >>Then how do you keep the freshwater tube so cool? It has to be a _lot_ [quoted text clipped - 12 lines] > >Brian W Human bevaviour: Bestiality with a brain On Mon, 24 Sep 2007 09:38:16 +0800, OldNick <nsremovable@iinet.net.au> wrote stuff and I replied: and snickering, snide and childish as well You are the sort of cliqueish dolt that spoils useful NGs like this. If you have KF'd me, you simply prove your weak, childish nature. It's shame. You do actually seem to have a lot of knowledge. IT's a pity you have to use it to sneer and brag rather thatn help those "lesser" than you >On Sun, 23 Sep 2007 18:10:09 GMT, Brian Whatcott ><betwys1@sbcglobal.net> wrote stuff [quoted text clipped - 21 lines] > >Human bevaviour: Bestiality with a brain Human bevaviour: Bestiality with a brain > On Mon, 24 Sep 2007 09:38:16 +0800, OldNick <nsremova...@iinet.net.au> > wrote stuff [quoted text clipped - 4 lines] > You are the sort of cliqueish dolt that spoils useful > NGs like this. ... "Go away" ... > If you have KF'd me, you simply prove your weak, > childish nature. ... "Stay and argue with me" ... > It's shame. You do actually seem to have a lot > of knowledge. IT's a pity you have to use it to > sneer and brag rather thatn help those > "lesser" than you ... "I will smear sh.t on you, if you don't argue with me" ... > >Human bevaviour: Bestiality with a brain > > Human bevaviour: Bestiality with a brain How is it that you complain about other's behavior, yet you form not one response but *two*, that show exactly the behavior your signature talks about. Is that signature line a complaint, an acknowledgement, or a promise? Be good to know. Because it seems like we have a choice. David A. Smith > On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott > <betw...@sbcglobal.net> wrote stuff > and I replied: > > But what is the cheap source of getting the vacuum? I figured there > had to be a vacuum, although it was not said. But how do you get it? Gravity. >> On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott >> <betw...@sbcglobal.net> wrote stuff [quoted text clipped - 4 lines] > >Gravity. Wishful thinking. Where are you going to get the feedwater containing no noncondensible gasses in solution? In all real distillation plants a continuosly operating vacuum pump is required to maintain vacuum and prevent the condensers from filling with noncondensible gasses. There is no way you are going to eliminate the vacuum pumps with any kind of inverted tube arrangement. For reasonable efficiency real distillation plants are multi-stage, where the latent heat of condensation from one stage is used to boil feedwater in the next stage, with up to 5 stages being used in larger plants (in the days before reverse osmosis made them uneconomical by comparison). Sucessive stages operate at lower pressures, and corresponding lower temperatures. The 1100 or so BTU required to boil one pound of water can thus boil up to 5 pounds of water instead. You still need enough thermal gradient to get the heat to flow through all those heat exchangers. By using low thermal differentials between the hot and cold ends you either reduce capacity to a pittance or require huge and expensive heat exchangers, in either case not competitive. TANSTAAFL. >>> On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott >>> <betw...@sbcglobal.net> wrote stuff [quoted text clipped - 25 lines] >require huge and expensive heat exchangers, in either case not >competitive. TANSTAAFL. Ah well, another great idea skuppered by dat old devil science :-) Bruce in Bangkok (brucepaigeATgmailDOTcom) > 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 saltwater 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. > > Ah well, another great idea skuppered by dat old devil science :-) > > [quoted text clipped - 12 lines] > 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 [quoted text clipped - 3 lines] > 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 >>> 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 >> saltwater 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 [quoted text clipped - 3 lines] >> 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 [quoted text clipped - 3 lines] >> 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 Hmmmm...here's somebody at least taking a shot at analyzing the system. I interpose one or two little comments.... /.../ >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 >>> saltwater 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 [quoted text clipped - 3 lines] >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/hbase/kinetic/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 [quoted text clipped - 5 lines] >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 > Hmmmm...here's somebody at least taking a shot at analyzing the > system. I interpose one or two little comments.... [quoted text clipped - 9 lines] > > 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? > [quoted text clipped - 10 lines] > > http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/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 [quoted text clipped - 5 lines] > 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? [quoted text clipped - 13 lines] > 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 [quoted text clipped - 6 lines] > 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 >> 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 >>> 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. [quoted text clipped - 4 lines] > 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 >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 >(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 >>(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 [quoted text clipped - 5 lines] > >Casady How interesting! Sure enough, a grease gun can usually put up 1000's of psi. and a low volume RO with such a hand pump seems like a reasonably economic proposition. Wonder how much they cost. Must take a look! Brian W Dear Richard Casady: >>(That would however take a hand pump capable >>of supplying a flow at 15 psi plus. Like a bicycle [quoted text clipped - 5 lines] > 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 >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. Not cheap. Volume production ought to bring that down a bit? Brian W >>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 [quoted text clipped - 6 lines] >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 >>> 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 [quoted text clipped - 8 lines] >> > 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. [quoted text clipped - 7 lines] > 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 >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 Dear Richard Casady: >>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 > Dear Richard Casady: > [quoted text clipped - 10 lines] > > 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 N:dlzc D:aol T:com (dlzc) brought forth on stone tablets: > Dear Richard Casady: > [quoted text clipped - 11 lines] > > David A. Smith ... and much more importantly, making the process viable with far lower quality heat. bob s/v Eolian Seattle >>>> 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 [quoted text clipped - 25 lines] >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 >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 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 saltwater 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. > >>> Ah well, another great idea skuppered by dat old devil science :-) > [quoted text clipped - 118 lines] > > Keith Hughes In rec.boats.cruising jim.isbell <jim.isbell@gmail.com> 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. Dear jim.isbell: > I give up. My Masters degree in Physics is of > no value here. My Bachelors degree in Math [quoted text clipped - 3 lines] > 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 Wow, Pearls! Let this swine take a look and see if he can root them out... > 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> > [quoted text clipped - 4 lines] > 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 [quoted text clipped - 4 lines] > 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 >... The salt water will boil > earlier because of the salt content. ... http://en.wikipedia.org/wiki/Boiling-point_elevation -- Tom. > 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 [quoted text clipped - 24 lines] > Put a pot on the stove and then before it comes to a boil add salt. > Voila, it begins to boil. BZZZZT!!! FALSE!!! Salt water DOES NOT boil at a lower temperature than fresh water. It boils at a higher temperature. When you add salt to heated water, it appears to boil because the water is superheated near the bottom of the pan and the salt crystals provide "nuclei" that start the boiling process. You can easily verify this by starting with two pans of water, one salty and one fresh and heating them to their respective boiling points and measuring the temperature. More explicit instructions, aimed at middle school students are at: http://aquarius.nasa.gov/pdfs/prop_fresh_sea.pdf <snip> Mark Borgerson <<SNIP>> 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-Co/Colligative-Properties.html >Boiling Point Elevation >The boiling point of a solution is higher than that of the pure [quoted text clipped - 3 lines] >From: >http://www.chemistryexplained.com/Ce-Co/Colligative-Properties.html Actually, no. Ethylene glycol in its pure liquid state boils near 200 degC http://www.dow.com/ethyleneglycol/about/properties.htm It is usually cut to 50% dilution for use as an antifreeze. Brian Whatcott Altus OK >> Boiling Point Elevation >> The boiling point of a solution is higher than that of the pure [quoted text clipped - 11 lines] > > Brian Whatcott Altus OK Since he used the term "solvent", the assumption, at least on my end, is that he's talking about a solvent/solute system, not a solution of miscible liquids. Keith Hughes > >Boiling Point Elevation > >The boiling point of a solution is higher than that of the pure [quoted text clipped - 9 lines] > > It is usually cut to 50% dilution for use as an antifreeze. True---when mixing liquids, the boiling point is somewhere between the boiling points of the two. Radiators and cooling systems are pressurized so that the system can have an elevated boiling point. Mark Borgerson > >>> Ah well, another great idea skuppered by dat old devil science :-) > >>> [quoted text clipped - 27 lines] > 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. I think there will be about 33 feet of water in the column on each side---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. 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. 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. 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. 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. With a large enough (or double) saltwater tube you might get a convection cell going with the cold, saltier water sinking and pulling up warmer seawater to the top. You could solve the dissolved gas problem by periodically pumping both tubes up enough to displace the accumulated gases. Now the project is getting complex enough that an RO system starts to look attractive! Mark Borgerson <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 [quoted text clipped - 5 lines] > > 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) saltwater 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 > <snip> > [quoted text clipped - 30 lines] > 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 > [quoted text clipped - 4 lines] > 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 [quoted text clipped - 12 lines] > 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 [quoted text clipped - 10 lines] > 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 [quoted text clipped - 51 lines] > > Keith Hughes Mark Borgerson >> <snip> <snip> >>> How do you get 33' as 1/2 of the diffusion path. >> A quick thumbnail guesstimation at where equilibrium would likely be [quoted text clipped - 22 lines] > 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 [quoted text clipped - 8 lines] > 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 >> [quoted text clipped - 6 lines] > > 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. [quoted text clipped - 21 lines] > 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 [quoted text clipped - 10 lines] > > 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 [quoted text clipped - 4 lines] > 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 <<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. At that point you release the pressure on the water and it falls to the point where water weight plus vapor pressure equals 1ATm. 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. As soon as you release the pressure and start warming the seawater side, bubbles are going to form all along the tube as the temperature rises and the pressure is less than 1ATM except at the bottom of the tube. Mark Borgerson > <<SNIP>> >> You need to get back to the gas law to see where this error lies. You [quoted text clipped - 4 lines] > 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 [quoted text clipped - 14 lines] > 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 > > <<SNIP>> > >> You need to get back to the gas law to see where this error lies. You [quoted text clipped - 19 lines] > create a headspace by liberating dissolved gases, then let the columns > drop to create vacuum, but you will have contaminated the freshwater side. The head space is generated by the evaporation (or boiling) of some of the water in a column. It's exactly the same principle that you get it you fill a closed tube full of mercury and then invert it, placing the end in a reservoir of mercury. (We call these things barometers.) You start with no head space, but when you invert it, VOILA! head space appears as the mercury sinks to a level where the weight of the mercury equals the atmospheric pressure. You get a much better vacuum with mercury, since it has a much lower vapor pressure at room temperature. A column of water will behave the same way. The column just has to be much taller. Some of the historical references on water barometers mention that, despite precautions, the water in the barometer eventually got contaminated with dissolved gases and they lost their accuracy. > > At that point, you essentially have two water barometers, > > interconnected at the top. One is salty and warm, and [quoted text clipped - 21 lines] > increases, and the column heights will drop as the pressure goes up, > with the diffusion path increasing the whole time.\ I agree with that part---except for the oscillation part. I think the processes are slow enough and the thermal and physical masses are high enough that the oscillations will be damped out and you will see a slow change to equilibrium with little or no overshoot. Mark Borgerson >>> 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. [quoted text clipped - 10 lines] > enough that the oscillations will be damped out and you will see a slow > change to equilibrium with little or no overshoot. Though I consider this whole discussion impractical, I haven't seen anyone mention that the fresh-water side will be drawn down fairly regularly. And, of course, the sea water side will be replenished from time to time. Suck hard enough on the fresh-water side and you get even better "vacuum" at the top. (Dissolved gasses are likely to be a problem, though.) Cool the fresh-water side and water vapor will condense there -- the whole point of the exercise. Thinking only momentarily on a problem that I have little interest in... if the fresh-water side is evacuated to the point that the salt-water side is slightly below the top, every once in a while (or perhaps often), the fresh-water side will be empty and only the previously-dissolved gasses evacuated. The required evacuation pumps and one-way valves sound like the problem at the moment. SignatureJere Lull Tanzer 28 #4 out of Tolchester, MD Xan's new pages: http://web.mac.com/jerelull/iWeb/Xan/ Our BVI pages: http://homepage.mac.com/jerelull/BVI/ <snip> > Though I consider this whole discussion impractical, I don't know that I'd call the *discussion* impractical; the device certainly. Kind of the point of the discussion. > I haven't seen > anyone mention that the fresh-water side will be drawn down fairly [quoted text clipped - 14 lines] > The required evacuation pumps and one-way valves sound like the problem > at the moment. The whole exercise was to get a passive system. If you're going to add a vacuum pump, then you just provide continuous evacuation on the freshwater side, using a demister that drains into the freshwater pool, to separate the water vapor from the non-condensables. But if you accept the need for a pump, why use this rather byzantine approach at all? Keith Hughes >he whole exercise was to get a passive system. If you're going to add >a vacuum pump, then you just provide continuous evacuation on the >freshwater side, using a demister that drains into the freshwater pool, >to separate the water vapor from the non-condensables. But if you >accept the need for a pump, why use this rather byzantine approach at all? The whole idea here seems ridiculous. This is nothing but a solar still. Reducing the boiling point is not necessary. All the energy absorbed, or nearly, will evaporate water. The limiting factor is the energy input. There is no benefit to making a modest capacity still thirty feet tall, and skinny. Make it short and fat and save material and weight. Did anyone mention weight aloft and windage? The hot side of the skinny job will be well cooled by the surrounding air. Casady >>> <<SNIP>> >>>> You need to get back to the gas law to see where this error lies. You [quoted text clipped - 22 lines] > you fill a closed tube full of mercury and then invert it, placing the > end in a reservoir of mercury. You seem to be forgetting that the whole purpose is to Purify/desalinate the water. No initial headspace = single process stream = contamination on the distillate side. (We call these things barometers.) Except when we call them Mcleod gauges... > You start with no head space, but when you invert it, VOILA! > head space appears as the mercury sinks to a level where the weight [quoted text clipped - 4 lines] > A column of water will behave the same way. The column just has > to be much taller. True, but you need to keep the context - water purification. The contamination control features are as crucial to the operational constraints as are the physical parameters. Thus, you have to *Start* with headspace. Sure, you could purge the freshwater side until the contaminants are removed, but by then most, if not all, of your production will be wasted. > Some of the historical references on water barometers mention that, > despite precautions, the water in the barometer eventually got > contaminated with dissolved gases and they lost their accuracy. Yes, you can only deaerate so far prior to filling. Personally, I've never seen an absolute pressure water barometer. IME they are primarily used in an inverted u-tube configuration for DP measurements. BTW, mercury barometers suffer the same fate, primarily through oxidation of the mercury, changing the density. Just look at that almost black film layer on any old barometer. > I agree with that part---except for the oscillation part. I think > the processes are slow enough and the thermal and physical masses > are high enough that the oscillations will be damped out and you > will see a slow change to equilibrium with little or no overshoot. You may be right, but I doubt it. Unless you control the temperature versus pressure relationship, which is virtually impossible with any passive heating process, then I'd expect self quenching would result in an oscillating system. Keith Hughes |
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