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 sal****er 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.

On Sep 29, 12:04 pm, Keith Hughes wrote:
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