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