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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

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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.

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