Mark Borgerson wrote:

SNIP

With the proper placement of the check valves, I think you could
start with the initial boiling happening in the freshwater side---
after all it is going to boil at a lower temperature.

Well, except that 1) unless you fill the entire apparatus with fresh
water (ignoring for a moment how much fresh water that might take, and
how much system capacity is lost in re-distilling the fresh water), you
haven't eliminated the carryover contamination issue, since you still
have a contiguous water stream, and 2) the freshwater side of the system
is configured for *cooling* and the seawater for heating (passive system
remember), so boiling will always be initiated on the seawater side.

The procedure might look like this:

1 Pump both fresh and salt water to near the top.
2. Shut offf the salt water side pump, but keep the
tube closed at the bottom.
3. Pump a bit more fresh water into the tube---where
it overflows to the sal****er side, displacing
the rest of the air out the check valve.

And where is the barrier layer that keeps salt from moving into the
freshwater?

You now have no air in the tube and a small layer of
fresh water on top of the salt water.

But it won't stay that way. As soon as you begin to heat the seawater,
you'll almost certainly have seawater rising into the fresh (do to the
density change with heating) before you get boiling going on.

4 Release the pressure at the bottom, and the fresh
water at the top will boil and create your head space
with little or no contamination of the freshwater
side.

Yeah, but "little" is not the goal. And you'd have to quantify that
"little" empirically, since there are many factors that contribute to
the process.

5 Apply your heat differential and remove distilled
fresh water as it overflows the reservoir at the
bottom.

This should work until the dissolved gas problem lengthens
the vapor path to the point where you have to start over
at step 1.

Again, the design complexity involved in being able to heat the fresh
side to initiate the boiling there *first*, and then switching to
cooling mode when there is sufficient column separation puts paid to any
thoughts of this being a simple system. And then, you have a very
complex, and Horribly inefficient system.

There are lots of ways that you could make the system work, but why?
The *only* feature this concept has going for it to start with is
simplicity, and basically a passive (save for some human work input) system.

Keith Hughes

In article ,
says...
Mark Borgerson wrote:

SNIP

With the proper placement of the check valves, I think you could
start with the initial boiling happening in the freshwater side---
after all it is going to boil at a lower temperature.

Well, except that 1) unless you fill the entire apparatus with fresh
water (ignoring for a moment how much fresh water that might take, and
how much system capacity is lost in re-distilling the fresh water), you
haven't eliminated the carryover contamination issue, since you still
have a contiguous water stream, and 2) the freshwater side of the system
is configured for *cooling* and the seawater for heating (passive system
remember), so boiling will always be initiated on the seawater side.

The procedure might look like this:

1 Pump both fresh and salt water to near the top.
2. Shut offf the salt water side pump, but keep the
tube closed at the bottom.
3. Pump a bit more fresh water into the tube---where
it overflows to the sal****er side, displacing
the rest of the air out the check valve.

And where is the barrier layer that keeps salt from moving into the
freshwater?

You don't need a barrier if you can do this in a few tens of seconds.

You now have no air in the tube and a small layer of
fresh water on top of the salt water.

But it won't stay that way. As soon as you begin to heat the seawater,
you'll almost certainly have seawater rising into the fresh (do to the
density change with heating) before you get boiling going on.

I think the fresh water on top of the seawater will boil before
the convective mixing will occur. Remember that the seawater
is more dense than the freshwater---which will be as warm as
the seawater on that side of the system.

4 Release the pressure at the bottom, and the fresh
water at the top will boil and create your head space
with little or no contamination of the freshwater
side.

Yeah, but "little" is not the goal. And you'd have to quantify that
"little" empirically, since there are many factors that contribute to
the process.

So quantify away! I can stand to drink water with 0.05 percent
salt (seawater is 3.5% salt).

5 Apply your heat differential and remove distilled
fresh water as it overflows the reservoir at the
bottom.

This should work until the dissolved gas problem lengthens
the vapor path to the point where you have to start over
at step 1.

Again, the design complexity involved in being able to heat the fresh
side to initiate the boiling there *first*, and then switching to
cooling mode when there is sufficient column separation puts paid to any
thoughts of this being a simple system. And then, you have a very
complex, and Horribly inefficient system.

No. I am not talking about heating the fresh water side. I am talking
about moving a half foot of fresh water to the seawater side of the
system and starting the boiling there.

There are lots of ways that you could make the system work, but why?
The *only* feature this concept has going for it to start with is
simplicity, and basically a passive (save for some human work input) system.

I agree. I am simply describing ways to make the system proposed work
as efficiently as possible---which is not very efficient. I think
you would be better off with a few square meters of 1Atm distillation
system sitting on the foredeck. I have seen proposals for systems
where seawater runs across a black plate and the evaporated water
collects and drips off the glass cover. You do need a small pump to
move the seawater to the top of the plate and a lot of sunshine.
But, OTOH, in the areas where I cruise (the Pacific NW), it's
probably easier to put out a clean plastic sheet and collect
rainwater. I just spent a week on a charter yacht in BC, and we
could have collected all the fresh water we needed on one rainy
night!


Mark Borgerson

In article , betwys1
@sbcglobal.net says...
On Mon, 8 Oct 2007 18:22:14 -0700, Mark Borgerson
wrote:
...
If you pump both the fresh and salt water to the top of the U-Tube,
then switch from the pumps to the reservoirs at the bottom,
you won't get a siphon. The boiling of water at the top will
break the siphon action.

Mark Borgerson


I am regretting this already.
But If I repeat this little test, pumping mercury up an inverted
U-tube to 35 inches, when I stop the pump and open the tubes to a
mercury pool, the mercury levels in the two tubes drop to a 29.92
inch column each side. The mercury does not boil.

29.92 inches is 760 mm of mercury, by the way. So boiling is not
essential to breaking a syphon.
Excess height is all that is needed.

That is correct. The difference is that water will boil at
room temperature because the vapor pressure is much higher. Mercury
will not---or at least the evaporation to produce the equilibrium vapor
pressure will not require visible boiling.

Mark Borgerson

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

On Mon, 08 Oct 2007 23:05:14 -0700, Keith Hughes
wrote:

puts paid to any
thoughts of this being a simple system. And then, you have a very
complex, and Horribly inefficient system.

Gee, I thought I was the only one to notice that. No sarcasm intended.

Casady

Dear Richard Casady:

"Richard Casady" wrote in message
...
On Mon, 08 Oct 2007 23:05:14 -0700, Keith Hughes
wrote:

puts paid to any
thoughts of this being a simple system. And then,
you have a very complex, and Horribly inefficient
system.

Gee, I thought I was the only one to notice that.
No sarcasm intended.

Actually, the dicussion has been capable of complexifying even a
paper clip.

David A. Smith