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

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

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

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

On Sat, 29 Sep 2007 20:40:57 GMT, (Richard
Casady) 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
hydraulic 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


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

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

Dear Richard Casady:

"Richard Casady" wrote in message
.. .
On Sat, 29 Sep 2007 13:57:27 -0500, Brian Whatcott
wrote:

(That would however take a hand pump capable
of supplying a flow at 15 psi plus. Like a bicycle
pump, or better? )

Grease guns are, some of them, capable of at
least. hundreds of psi. I happen to own a 0-5000
psi gauge. Bought it to check tractor hydralic
systems. I forget just what a grease gun pumped
it up to, but it was a lot.

But it was a *very* low flow rate. Brian is talking about "a
few" gallons per minute, to use cooler salt water in a tube on
the "fresh water" side to help carry off waste heat. And it is
going upwards a few tens of feet (then back down), perhaps
starting at atmospheric pressure. It would be hard work,
especially it it had to be kept up for an hour!

There is a reverse osmosis watermaker
intended for liferaft use, with a hand pump, and
RO takes hundreds of psi. That is what you want,
if you actually need high pressure.

I figure you probably can buy a small hand-held "single-shot"
pocket RO unit for just such a purpose...

David A. Smith

On Sat, 29 Sep 2007 12:59:39 -0700, Keith Hughes
wrote:

Gee, I didn't know you were using 'smart' molecules that travel *only*
in the direction you want them too. And they don't bump into each other
in the process. Wow, that's really neat, how did you accomplish that?

I noticed that.

Isn't the speed of sound, whatever it is, what it is because it _is _
the molecular speed? They both vary with temperature but not pressure.

Casady

On Sat, 29 Sep 2007 20:40:57 GMT, (Richard
Casady) wrote:


There is a reverse osmosis watermaker intended for liferaft use, with
a hand pump, and RO takes hundreds of psi. That is what you want, if
you actually need high pressure.

Casady


I looked up an example
The Katadyn Survivor 35 hand pumped was formerly
called the PUR Survivor 35 RO.
At 30 strokes/minute for 1.2 gall/hr - it costs $1500.

Not cheap.
Volume production ought to bring that down a bit?

Brian W

Brian Whatcott wrote:
On Sat, 29 Sep 2007 12:59:39 -0700, Keith Hughes
wrote:


My, my: "it's just plain wrong": he said a column of 32 ft,
Uhmmm, no, he said a "32' column of water". Can you see the difference?

Hmmm..Priestley certainly could. His water barometer had a water
column round 32 or 33 ft high.
How 'bout that! :-)

Quite true, which probably engenders the confusion. However, the
barometer *starts* with an evacuated column. That's how the atmospheric
pressure can push the water 32' up the column - you have 14.7 PSIA to
work with to lift the water. Same for a mercury barometer, or a McCleod
gauge, etc.

Different story than using the water column to *generate* the vacuum.


Gee, I didn't know you were using 'smart' molecules that travel *only*
in the direction you want them too.
....
Keith Hughes



Ho, hum: if half of them go in the wrong direction
until their first collision, it must take them a really, really,
REALLY long time to diffuse through the water vapor/rarified air mix!


Half? More likely 99.99999++% of them will not be traveling parallel to
the axis of the column. Half of those that *are*, are going in the wrong
direction.

How difficult this type of mass transport actually is can be seen by
looking at flow rates for water vapor from lyophilizer chambers to the
condensers (yes, I have done this a lot). Putting a 90° bend in the tube
connecting the chamber (where the ice sits on heated shelves) and the
condenser roughly (very roughly, given the variability of other design
factors) cuts the flow rate in half. That's in a 48" ID tube too! And
pulling vacuum through the condenser to maintain 50-100 microbar
pressure - i.e. maintaining a significant delta-P from chamber to
condenser. And with condenser coils maintained at -65°C.

It may seem counterintuitive, but the molecular motion you referenced
just *causes* the pressure, while providing little impetus for mass
transfer from point A to point B. And once the pressure reaches
equilibrium throughout the system, you have to rely virtually entirely
on diffusion, which is much, much slower.

Keith Hughes