Re: Potable Water - The Third Way.

I looked up an example
The Katadyn Survivor 35 hand pumped was formerly
called  the PUR Survivor 35 RO.
At 30 strokes/minute for 1.2 gall/hr  - it costs $1500.

Not cheap.
Volume production ought to bring that down a bit?

Brian W

Grease guns are, some of them,  capable of at least. hundreds of psi.
I happen to own a 0-5000 psi gauge. Bought it to check tractor
hydralic systems. I forget just what a grease gun pumped it up to, but
it was a lot.
There is a reverse osmosis watermaker intended for liferaft use, with
a hand pump, and RO takes hundreds of psi. That is what you want, if
you actually need high pressure.

Casady

Hmmmm...here's somebody at least taking a shot at analyzing the
system.    I interpose one or two little comments....

/.../

My, my: "it's just plain wrong":      he said a column of 32 ft, and
you correct him - it's 33 ft.   What a loser he must be!   :-)
But then, you are neglecting to account for the density of SALT water!

Not strictly relevant, but interesting to me at least:
Joseph Priestley kept a water barometer at his house in Birmingham
(before the mob drove him out for his revolutionary sympathies).
Guess how high he had to climb to read the water level?

Uh? Diffusion of water molecules in low pressure air through 66 feet?

Let's say 14 ft, 20 feet even. Now what would the speed be?
Hmmmm. Let's see. Would that speed  be over 500 meters/second?

That's so slow, the time it might take to travel  20 feet,
say 6 meters at 500 m/s might be  12 milliseconds?

Here's a review of the thermo equation.
Just plant the temperature of interest (20 degC say) and the molecular
weght of a water molecule (Hint: its lighter than the average molecule
that makes up air) in the following calculator

  http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html#c4

Hmmmm...a freeboard of eight feet? Some boat! More boat than I've got,
certainly.

The word is "BOIL", not evaporate, not sublime.    If it is not
quickly condensed returning latent heat, the partial pressure rises
quickly sure enough.    Better condense it then! I imagine a central
cold finger of cool salt water in the fresh column might be effective?
(That would however take a hand pump capable of supplying a flow
at 15 psi plus. Like a bicycle pump, or better? )

There you go again - with your really really slow 66 ft diffusion for
condensed water in the fresh column.....

In my experience, the people who talk most about Darwin awards
are completely foggy about how Darwinian selection operates.

"Accidentally spilling all fresh water" , from a "contraption"
Yes, sure.   Can you say, "Straw man?"

Brian W

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.

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.

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.

In reality, though, the columns won't be referenced to the same level,
with the freshwater column being referenced (i.e. the bottom is opened
to) the deck height on the boat. So the freshwater column will be, say
8' higher than the seawater column. The diffusion path is still the
same, but the evacuated seawater column would then be 37', with 29' on
the freshwater side.

No, this does *not* create a vacuum in the sense you seem to mean. It
maintains an equilibrium pressure by lowering the partial pressure of
water vapor generated by the 'boiling' process on the seawater side.

This relates to the critical rate-limiting feature of the system -
maintaining pressure.  When you evaporate, or sublime, water into the
headspace, the pressure in the headspace increases.  Condensation on the
other side lowers the pressure, and an equilibrium pressure will
eventually be established. For any given temperature, the evaporation
rate is going to be limited by the partial pressures at the
headspace/water-surface interface. It's a feedback loop, More
evaporation -> more water vapor molecules liberated to the headspace ->
more pressure in the headspace -> slower evaporation until the pressure
is reduced.  And to reduce the pressure, those molecules have to diffuse
up to 66'.

Yes, VERY slowly. You can increase *throughput* by increasing the column
diameters, but how practical is that on a boat?

Not quite true...you can seal the 'freshwater' column, using only the
column walls for condensation surfaces, until you have sufficient
condensate collected to allow the freshwater column to be opened.

It doesn't *have* to be that way, BUT.... :-)

And what's 'practical' for useability, is impractical for functionality.
There are no 'soft tubing' materials I'm aware of that have anything
approaching decent heat absorbance, conduction, or emissivity
properties, so that will be another very significant rate limiter in the
system.

You actually *can't* pump faster than gravity, unless you want to suck
seawater up the column on the other side.

Doubtful that you'd ever get a solid chunk of salt (and short of having
a bypass circulation loop - cooling the column and further reducing
efficiency - I don't see how a pump could even help the situation), but
of course as the salinity increases, the boiling point increases, and at
some point the process will just stall.  The heat input won't be
sufficient to boil the brine solution.  Then you have to stop, drain,
clean, and start over.  How quickly this happens will depend on column
heights and diameters, but it'll happen at some point. Just another
rate-limiting feature.

All these rate limiters are natures way of saying that there is no
thermodynamic free lunch.  A low energy input system will have a low
output (in terms of whatever work you want the system to do).

Keith Hughes

It works but does it work as well as other methods that are simpler and
easier to implement. Also if you have no fresh water on hand to start
with there is no way to make it work. I can see someone getting a
"Darwin Award" by accidentally spilling all there existing freshwater
supply in a failed attempt to get this contraption going.

That makes no sense. You are going to have a hard time pumping water out
of the fresh water side any faster than gravity can deliver it. The
salty side OTOH,  if you rely only on gravity to feed it, will become a
solid block of salt once you have evaporated enough water from it.

-jim

A 32' column of water is a continuous vacuum pump.  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.  The fresh water distills off the top of the
saltwater column then migrates as steam to the other side and distills
in the fresh water side....also creating a vacuum.  You draw off the
fresh water on one side and pump salt water into the other side.  The
salt water side is painted black to absorb sun heat and the fresh
water side is painted white to reflect the suns heat.  You only need a
few degrees difference for distillation and the vacuum creates the
boiling at low temperatures...even ice will change state to steam in a
vacuum.  The idea works.

In a practical sense, I would use soft tubing for the sides and a
solid "U" shaped piece of copper tubing for the top center with a ring
soldered to it so it could be hoisted up the mast of a sailboat.  It
would take a 30 to 40 foot mast to do the job.  The bottom end of the
salt water tube could go to a through hull for a continuous supply of
salt water and the bottom end of the fresh water tube could go to a
small pump to remove the water without breaking the vacuum.

Ah well, another great idea skuppered by dat old devil science :-)

Bruce in Bangkok
(brucepaigeATgmailDOTcom)

Wishful thinking.  Where are you going to get the feedwater containing
no noncondensible gasses in solution?  In all real distillation plants
a continuosly operating vacuum pump is required to maintain vacuum and
prevent the condensers from filling with noncondensible gasses.  There
is no way you are going to eliminate the vacuum pumps with any kind of
inverted tube arrangement.

For reasonable efficiency real distillation plants are multi-stage,
where the latent heat of condensation from one stage is used to boil
feedwater in the next stage, with up to 5 stages being used in larger
plants (in the days before reverse osmosis made them uneconomical by
comparison).  Sucessive stages operate at lower pressures, and
corresponding lower temperatures.  The 1100 or so BTU required to boil
one pound of water can thus boil up to 5 pounds of water instead.

You still need enough thermal gradient to get the heat to flow through
all those heat exchangers.  By using low thermal differentials between
the hot and cold ends you either reduce capacity to a pittance or
require huge and expensive heat exchangers, in either case not
competitive.  TANSTAAFL.

Gravity.

On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
<betwys1@sbcglobal.net> wrote stuff
and I replied:

But what is the cheap source of getting the vacuum? I figured there
had to be a vacuum, although it was not said. But how do you get it?

Human bevaviour: Bestiality with a brain

Well, at least this respondent Jim, is operating at shall we say the
7th grade level of science/engineering insight.    Like so many other
products of the domestic school system, he seems to have a severe
case of self-esteem syndrome.  

 Still, he may be retrievable, starting with a science demonstration
he may have missed.   Place a beaker of water in a bell-jar and pump
the air out.

When 99% of the air has been pumped out, the water in the beaker is
boiling vigorously, until, in the usual way, the beaker boils dry.
The beaker feels cool to the touch, naturally.

To quote him: "unless I have a cheap source of heating this won't
work..."

    For the $64 prize:   NOW do you get it?

Brian Whatcott    Altus OK

Well no, he obviously hadn't figured that out. Nor can anybody figure
out what is going to hold a column of water 40 ft high as was stated in
the original post. The tubes may be 40 feet but the column of water will
be considerably less. How much less will depend on how much energy is
heating on the hot side and how much energy is cooling on the cool side.
The total amount of energy needed is not going to be any different than
any other distilling method.
    Unless you have the free or cheap sources of cooling and heating at
specific temperatures this isn't going to work any better either.

-jim

Dear Larry:

What Brian left to the reader's imagination, is that the head
space of the tubes is at a near perfect vacuum, flooded only with
water vapor.  You might recall that a perfect vacuum will lift a
column of water about 32 feet, on a high pressure day.  Or had
you not figured that out?

David A. Smith

dlzc <dlzc1@cox.net> wrote in news:1190415672.506271.93890
@k79g2000hse.googlegroups.com:

http://en.wikipedia.org/wiki/Boiling_point

"The boiling point of water is 100 °C (212 °F) at standard pressure. On
top of Mount Everest the pressure is about 260 mbar (26 kPa) so the
boiling point of water is 69 °C. (156.2 °F)."

AT 40' ASL, the boiling point must be down to...to....211.95F!

Larry

Dear Brian Whatcott:

There are ship-board distiller units that use an engine to pull a
vacuum, and the engine's waste heat to boil that water, to generate
drinking water.  A little shorter...

David A. Smith

You've heard all about distilling water, and you've heard all about
reverse osmosis,  but you haven't heard about low-cost, low energy
stills: they are brand new.

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.

The boiling point of water at sea level pressure is about 100 deg C

The boiling point of water at the top of a sealed 40 ft column of
water is near ambient.
So, it doesn't take much heat to boil the brackish water, and have it
pass to the fresh column where it is slightly cooled to hold the near
vacuum conditions at the boiling level.

[An engineering effort of a U of Utah group I think]

Brian Whatcott   Altus OK