On 2007-10-08 18:17:58 -0400, Mark Borgerson said:

Getting rid of the disssolved gases in the headspace and as bubbles
forming on the sides of the tube is going to be a major headache.

Not a headache, an impossibility (they're not really dissolved at that
point though) :-) That, and the increase in pressure due to water vapor
will make this an oscillating, self-quenching system. It'll require
more and more heat as the partial pressures of the non-condensables
increases, and the column heights will drop as the pressure goes up,
with the diffusion path increasing the whole time.

I agree with that part---except for the oscillation part. I think the
processes are slow enough and the thermal and physical masses are high
enough that the oscillations will be damped out and you will see a slow
change to equilibrium with little or no overshoot.

Though I consider this whole discussion impractical, I haven't seen
anyone mention that the fresh-water side will be drawn down fairly
regularly. And, of course, the sea water side will be replenished from
time to time.

Suck hard enough on the fresh-water side and you get even better
"vacuum" at the top. (Dissolved gasses are likely to be a problem,
though.) Cool the fresh-water side and water vapor will condense there
-- the whole point of the exercise.

Thinking only momentarily on a problem that I have little interest
in... if the fresh-water side is evacuated to the point that the
salt-water side is slightly below the top, every once in a while (or
perhaps often), the fresh-water side will be empty and only the
previously-dissolved gasses evacuated.

The required evacuation pumps and one-way valves sound like the problem
at the moment.

--
Jere Lull
Tanzer 28 #4 out of Tolchester, MD
Xan's new pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI pages: http://homepage.mac.com/jerelull/BVI/

Jere Lull wrote:
On 2007-10-08 18:17:58 -0400, Mark Borgerson said:

snip

Though I consider this whole discussion impractical,

I don't know that I'd call the *discussion* impractical; the device
certainly. Kind of the point of the discussion.

I haven't seen
anyone mention that the fresh-water side will be drawn down fairly
regularly. And, of course, the sea water side will be replenished from
time to time.

Suck hard enough on the fresh-water side and you get even better
"vacuum" at the top. (Dissolved gasses are likely to be a problem,
though.) Cool the fresh-water side and water vapor will condense there
-- the whole point of the exercise.

Thinking only momentarily on a problem that I have little interest in...
if the fresh-water side is evacuated to the point that the salt-water
side is slightly below the top, every once in a while (or perhaps
often), the fresh-water side will be empty and only the
previously-dissolved gasses evacuated.

The required evacuation pumps and one-way valves sound like the problem
at the moment.

The whole exercise was to get a passive system. If you're going to add
a vacuum pump, then you just provide continuous evacuation on the
freshwater side, using a demister that drains into the freshwater pool,
to separate the water vapor from the non-condensables. But if you
accept the need for a pump, why use this rather byzantine approach at all?


Keith Hughes

On Mon, 08 Oct 2007 18:08:51 -0700, Keith Hughes
wrote:

he whole exercise was to get a passive system. If you're going to add
a vacuum pump, then you just provide continuous evacuation on the
freshwater side, using a demister that drains into the freshwater pool,
to separate the water vapor from the non-condensables. But if you
accept the need for a pump, why use this rather byzantine approach at all?

The whole idea here seems ridiculous. This is nothing but a solar
still. Reducing the boiling point is not necessary. All the energy
absorbed, or nearly, will evaporate water. The limiting factor is the
energy input. There is no benefit to making a modest capacity still
thirty feet tall, and skinny. Make it short and fat and save material
and weight. Did anyone mention weight aloft and windage? The hot side
of the skinny job will be well cooled by the surrounding air.

Casady