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