Brian Whatcott wrote:
On Sat, 22 Sep 2007 08:29:57 -0700, Dan Best
wrote:

...the vast bulk of the energy used by the
distiller is not used in heating the water to the boiling point, but in
performing the phase change from liquid to gas. I believe this is a
constant regardless of the surrounding pressure....

- Dan best

As it happens, you believe wrongly.

But you might be surprized if you took a moment to look up enthalpy of
water, or refer to steam tables, or whatever....

Brian Whatcott Altus OK

Actually, do a search on "triple point" and look at the phase diagram
for water. That gives a good graphical depiction of the
pressure/temperature/phase relationships.

But, as to what the original poster claimed, i.e. that:

Briefly:
Take one forty ft vertical tube filled with saline.
Take one forty ft vertical tube filled with fresh water.
Connect them with a little engineering help - at the top.

And that:

The boiling point of water at the top of a sealed 40 ft column of
water is near ambient.

is simply wrong as stated. If you filled each vertical tube with water,
venting the air at the u-tube connection, then sealed the u-tube *and*
allowed the water to drain down about 30 feet, then you'd pull a
sufficient vacuum. The water, however, would not be anywhere near the
"top of a sealed 40 ft" column.

So, this system will certainly work, but you need to heat the entire
length of the 40' seawater column (to prevent column refluxing). You
also need to cool the condenser side of the system if you want any
efficiency.

Bottom line, you can use a system like this to save energy, but you have
to run it very, very slowly. When the seawater evaporates, the
headspace pressure rises, quenching the process until the condensation
on the other side reduces the headspace pressure again, and that steam
has to travel 60 feet, so it's slowwwww. Diffusion is all you've got to
work with here.

There ain't no free lunch. You want to speed it up, you need more energy
input, either on the heating side, the cooling process, or in the
evacuation process.

Keith Hughes