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Mark Borgerson

In article ,
says...

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.

How do you get 33' as 1/2 of the diffusion path. I think there will be
about 33 feet of water in the column on each side---to provide the
weigth that pulls the pressure down. That would leave only about
7 feet of water vapor path on each side of the column.

I'm not sure that 'diffusion' is the proper term for the motion
of the water vapor. After all, the heat engine is providing
water vapor on one side and condensing it on the other---so there
is a net mass flow and probably a small pressure differential to
move the vapor.

Still (pun intended), you need a lot of heat to provide the energy
to evaporate the water or it will soon cool to the point where
its vapor pressure is reduced and the process slows drastically.
The fact that the water 'boils' near room temperature does not
reduce the amount of heat required to change the water from
liquid to vapor.

As has been discussed, the simple idea does not address the problems
of salt buildup in the seawater side, or the addition of dissolved
gasses to the vacuum part of the loop.

With a large enough (or double) sal****er tube you might get a
convection cell going with the cold, saltier water sinking and
pulling up warmer seawater to the top.

You could solve the dissolved gas problem by periodically pumping
both tubes up enough to displace the accumulated gases.

Now the project is getting complex enough that an RO system
starts to look attractive!

Mark Borgerson

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