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Mark Borgerson

In article ,
says...
Mark Borgerson wrote:
In article ,
says...
Mark Borgerson wrote:
In article ,
says...
SNIP
You need to get back to the gas law to see where this error lies. You
have to *create* the vacuum. That requires a HUGE increase in volume
for whatever the initial headspace is. For this to happen you need a
much longer tube to start with.

You seem to have missed the fact that I proposed filling the tubes
completely with water so that the initial head space would be zero.
No, it won't be zero. It can't be. If it is, then you have a solid
liquid stream, and it's just a siphon. You have to have headspace. And
it has to be sufficient to maintain separation of the seawater and
freshwater to prevent contamination when filling the tubes. And it has
to be large enough to prevent percolation carryover when boiling is
initiated.

At that point you release the pressure on the water and it falls
to the point where water weight plus vapor pressure equals 1ATm.
A solid liquid loop will not separate into two separate columns. They
have to be separated by a headspace. You can heat the seawater side and
create a headspace by liberating dissolved gases, then let the columns
drop to create vacuum, but you will have contaminated the freshwater side.

The head space is generated by the evaporation (or boiling) of some of
the water in a column. It's exactly the same principle that you get it
you fill a closed tube full of mercury and then invert it, placing the
end in a reservoir of mercury.

You seem to be forgetting that the whole purpose is to Purify/desalinate
the water. No initial headspace = single process stream = contamination
on the distillate side.

(We call these things barometers.)

Except when we call them Mcleod gauges...

You start with no head space, but when you invert it, VOILA!
head space appears as the mercury sinks to a level where the weight
of the mercury equals the atmospheric pressure. You get a much
better vacuum with mercury, since it has a much lower vapor pressure
at room temperature.

A column of water will behave the same way. The column just has
to be much taller.

True, but you need to keep the context - water purification. The
contamination control features are as crucial to the operational
constraints as are the physical parameters. Thus, you have to *Start*
with headspace. Sure, you could purge the freshwater side until the
contaminants are removed, but by then most, if not all, of your
production will be wasted.

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.

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.

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

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.

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.

Some of the historical references on water barometers mention that,
despite precautions, the water in the barometer eventually got
contaminated with dissolved gases and they lost their accuracy.

Yes, you can only deaerate so far prior to filling. Personally, I've
never seen an absolute pressure water barometer. IME they are primarily
used in an inverted u-tube configuration for DP measurements. BTW,
mercury barometers suffer the same fate, primarily through oxidation of
the mercury, changing the density. Just look at that almost black film
layer on any old barometer.

That is mostly due to contaminants trapped in the glass and impurities
in the mercury. Production barometers didn't use glass that was
heated with a vacuum to remove contaminants.

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.

You may be right, but I doubt it. Unless you control the temperature
versus pressure relationship, which is virtually impossible with any
passive heating process, then I'd expect self quenching would result in
an oscillating system.

What do you mean by "self-quenching"?

Mark Borgerson