Potable Water - The Third Way.

On Sep 22, 11:12 am, "Roger Long" <stri....RemoveThis@maine.rr.com> wrote:
> There's another neat way you can demonstrate this with minimal equipment.
>
> Take a tight fitting jar and get the water boiling vigerously in it with the
> cap on loose enough to let the steam out. When it is full of dense steam
> and about 1/3 boiling water, remove instantly from heat and tighten cap.
>
> When everything is cooled to room temperature, put an ice cube against the
> jar and the water will start to boil. The ice condenses the water vapor
> further, reducing the pressure to the point where the water will boil at
> room temperature.
>
> I've seen it done and it looks like the ice cube is boiling the water.
>
> My father won a science fair doing this back in the 1930's.
>
> --
> Roger Long

http://www.metacafe.com/watch/414997/boiling_using_ice/

Joe
 >> Stay informed about: Potable Water - The Third Way. 

On Mon, 24 Sep 2007 09:38:16 +0800, OldNick <nsremovable.DeleteThis@iinet.net.au>
wrote stuff
and I replied:

and snickering, snide and childish as well

You are the sort of cliqueish dolt that spoils useful NGs like this.

If you have KF'd me, you simply prove your weak, childish nature.

It's shame. You do actually seem to have a lot of knowledge. IT's a
pity you have to use it to sneer and brag rather thatn help those
"lesser" than you


>On Sun, 23 Sep 2007 18:10:09 GMT, Brian Whatcott
><betwys1.DeleteThis@sbcglobal.net> wrote stuff
>
>and I replied:
>
>You are a rude and arrogant prick
>
>
>>On Sun, 23 Sep 2007 21:57:56 +0800, OldNick <nsremovable.DeleteThis@iinet.net.au>
>>wrote:
>>...
>>>Then how do you keep the freshwater tube so cool? It has to be a _lot_
>>>cooler, not so?
>>
>>
>>I'm probably going to regret responding, but I will anyway, with a
>>question:
>>
>>What is the difference in temperature between steam and water,
>>both at the boiling temperature of water, whatever it may be?
>>
>>Cooler means lower temperature, right?
>>
>>OK you can now answer your own question.
>>I hope.
>>
>>Brian W
>
>Human bevaviour: Bestiality with a brain

Human bevaviour: Bestiality with a brain
 >> Stay informed about: Potable Water - The Third Way. 

On Sep 22, 10:39 pm, OldNick <nsremova....RemoveThis@iinet.net.au> wrote:
> On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
> <betw....RemoveThis@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?


Gravity.
 >> Stay informed about: Potable Water - The Third Way. 

On Thu, 27 Sep 2007 13:54:13 -0000, "jim.isbell"
<jim.isbell.RemoveThis@gmail.com> wrote:

>On Sep 22, 10:39 pm, OldNick <nsremova....RemoveThis@iinet.net.au> wrote:
>> On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
>> <betw....RemoveThis@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?
>
>
>Gravity.
>

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.
 >> Stay informed about: Potable Water - The Third Way. 

Dear Glen Walpert:

"Glen Walpert" <gwalpert DeleteThis @notaxs.com> wrote in message
news:cg6of3hbf72g1rna4bla2bskc8su7jhb70@4ax.com...
> On Thu, 27 Sep 2007 13:54:13 -0000, "jim.isbell"
> <jim.isbell DeleteThis @gmail.com> wrote:
>
>>On Sep 22, 10:39 pm, OldNick <nsremova... DeleteThis @iinet.net.au> wrote:
>>> On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
>>> <betw... DeleteThis @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?
>>
>>
>>Gravity.
>
> 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.

But they don't have to be large, and they don't even have to run
continuously (just frequently). There are also going to be
controls...

You could even run it without a vacuum pump until it shut itself
down, drop and purge the gas bubble, then "forklift" your pipes
back up.

And do it at less than the melting point of plastic (should that
be important).

> 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).

Scaling is real problem too...

> 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.

.... a characteristic article ...
http://www.hcn.org/servlets/hcn.Article?article_id=17136
This was not proposed to be a source of free energy, violate the
second law of thermodynamics, or poke fingers in anyone's eyes.

I think it was something that someone could do fairly cheaply, to
get drinkable water from salt water. In other words "a graduate
or undergraduate college project".

I just wonder if you get any improvement in what is left in the
brine, vs. what also evaporates at the lower temperatures...

David A. Smith
 >> Stay informed about: Potable Water - The Third Way. 

On Thu, 27 Sep 2007 21:25:39 GMT, Glen Walpert <gwalpert DeleteThis @notaxs.com>
wrote:

>On Thu, 27 Sep 2007 13:54:13 -0000, "jim.isbell"
><jim.isbell DeleteThis @gmail.com> wrote:
>
>>On Sep 22, 10:39 pm, OldNick <nsremova... DeleteThis @iinet.net.au> wrote:
>>> On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
>>> <betw... DeleteThis @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?
>>
>>
>>Gravity.
>>
>
>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.


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

Bruce in Bangkok
(brucepaigeATgmailDOTcom)
 >> Stay informed about: Potable Water - The Third Way. 

On Thu, 27 Sep 2007 18:06:47 -0700, "N:dlzc D:aol T:com \(dlzc\)"
<dlzc1.TakeThisOut@cox.net> wrote:

>Dear Glen Walpert:
>
>"Glen Walpert" <gwalpert.TakeThisOut@notaxs.com> wrote in message
>news:cg6of3hbf72g1rna4bla2bskc8su7jhb70@4ax.com...
>> On Thu, 27 Sep 2007 13:54:13 -0000, "jim.isbell"
>> <jim.isbell.TakeThisOut@gmail.com> wrote:
>>
>>>On Sep 22, 10:39 pm, OldNick <nsremova....TakeThisOut@iinet.net.au> wrote:
>>>> On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
>>>> <betw....TakeThisOut@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?
>>>
>>>
>>>Gravity.
>>
>> 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.
>
>But they don't have to be large, and they don't even have to run
>continuously (just frequently). There are also going to be
>controls...

The vacuum pumps need to be sized to the load, and it is not a
foregone conclusion that a larger pump running intermittently would be
more efficient than a smaller one running continuosly. Consider also
that the vacuum pump cannot pump out just the noncondensible gasses,
it must pump out the gas mix in the condenser which will be mostly
water vapor - the pumping rate establishes the percentage
noncondensible gasses in the condenser, amd the optimum rate needs to
be established as part of a distillation plant design.

>You could even run it without a vacuum pump until it shut itself
>down, drop and purge the gas bubble, then "forklift" your pipes
>back up.

Does this use less energy per gallon produced?

>And do it at less than the melting point of plastic (should that
>be important).
>
>> 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).
>
>Scaling is real problem too...

True, but one which can be solved by limiting brine concentration and
with chemical treatment and/or periodic cleaning.

>> 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.
>
>... a characteristic article ...
>http://www.hcn.org/servlets/hcn.Article?article_id=17136
>This was not proposed to be a source of free energy, violate the
>second law of thermodynamics, or poke fingers in anyone's eyes.

As usual with this sort of article there are no meaningful numbers
included, perhaps because a complete design analysis has not been
done.

>I think it was something that someone could do fairly cheaply, to
>get drinkable water from salt water. In other words "a graduate
>or undergraduate college project".

Doing an analysis of this approach would be a good student exercise.
Not much point building one without doing the anylysis first - a
complete engineering analysis including the selection or design of all
heat exchangers, mist eliminators, pumps, piping etc., including both
performance and cost calculations. It is always cheaper to optimize a
pencil and paper or computer model than hardware, especially for
something so well understood as heat transfer and fluid flow.

>I just wonder if you get any improvement in what is left in the
>brine, vs. what also evaporates at the lower temperatures...
>
>David A. Smith

I doubt if that would be much of a factor. What contaminants would be
in the feedwater which would evaporate less compared to water as
boiling point is reduced by low pressure?

The biggest issue with distillate quality is carryover; a fine mist of
unevaporated water droplets are inevitably produced by boiling
regardless of temperature, and while most of these can be separated
out, some always make it through to the condenser. This is a big
issue where biological contamination exists in the feedwater,
requiring chlorination of the distillate to make it potable. It might
be possible to eliminate this factor by eliminating the boiling of
bulk liquid, and instead evaporating from a thin film of water flowing
over the heat exchanger surfaces, but I doubt if it would be cost
effective. Perhaps it would be another good student exercise.
 >> Stay informed about: Potable Water - The Third Way. 

Dear Glen Walpert:

On Sep 28, 7:09 am, Glen Walpert <gwalp....RemoveThis@notaxs.com> wrote:
> On Thu, 27 Sep 2007 18:06:47 -0700, "N:dlzc D:aol T:com \(dlzc\)"
....
> >>>Gravity.
>
> >> 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.
>
> >But they don't have to be large, and they don't
> >even have to run continuously (just frequently).
> >There are also going to be controls...
>
> The vacuum pumps need to be sized to the load,
> and it is not a foregone conclusion that a larger
> pump running intermittently would be more efficient
> than a smaller one running continuosly. Consider
> also that the vacuum pump cannot pump out just
> the noncondensible gasses, it must pump out the
> gas mix in the condenser which will be mostly
> water vapor -

So you can get some condensate here, but it will likely have "vacuum
pump oil" in it...

> the pumping rate establishes the percentage
> noncondensible gasses in the condenser, amd
> the optimum rate needs to be established as part
> of a distillation plant design.
>
> > You could even run it without a vacuum pump
> > until it shut itself down, drop and purge the gas
> > bubble, then "forklift" your pipes back up.
>
> Does this use less energy per gallon produced?

Available on a desert island. Simple block and tackle would do.
Since the (de)compression rate is likely low, and the condensation
production rate is necessarily low, if you were not using animal
power, it could be *more* efficient. But you still have to supply or
waste a good deal of heat.

....
> >> 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).
>
> >Scaling is real problem too...
>
> True, but one which can be solved by limiting brine
> concentration and with chemical treatment and/or
> periodic cleaning.

In the case of the marine vacuum distillation unit, they simply have a
constant flow of brine. Probably need to have a "tube within a tube"
to refresh the fluid near the boiling interface.

> >> 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.
>
> >... a characteristic article ...
http://www.hcn.org/servlets/hcn.Article?article_id=17136
> >This was not proposed to be a source of free
> >energy, violate the second law of thermodynamics,
> >or poke fingers in anyone's eyes.
>
> As usual with this sort of article there are no
> meaningful numbers included, perhaps because
> a complete design analysis has not been
> done.

More than likely omitted because:
- the reporter's eyes were glazing over, or
- they are working on a patent (since you can probably even patent
cheese now).

> >I think it was something that someone could do
> >fairly cheaply, to get drinkable water from salt
> >water. In other words "a graduate or
> >undergraduate college project".
>
> Doing an analysis of this approach would be a
> good student exercise. Not much point building
> one without doing the anylysis first - a complete
> engineering analysis including the selection or
> design of all heat exchangers, mist eliminators,
> pumps, piping etc., including both performance
> and cost calculations. It is always cheaper to
> optimize a pencil and paper or computer model
> than hardware, especially for something so well
> understood as heat transfer and fluid flow.
>
> >I just wonder if you get any improvement in
> >what is left in the brine, vs. what also
> >evaporates at the lower temperatures...
>
> I doubt if that would be much of a factor. What
> contaminants would be in the feedwater which
> would evaporate less compared to water as
> boiling point is reduced by low pressure?

Water does get involved in some azeotropes (some alcohols), so
depression of boiling point would not help there. And
thermodynamically, if one of the things you were trying to remove
became a solid at high vacuum (NaOH maybe?) it might help.

> The biggest issue with distillate quality is
> carryover; a fine mist of unevaporated water
> droplets are inevitably produced by boiling
> regardless of temperature, and while most
> of these can be separated out, some always
> make it through to the condenser. This is a
> big issue where biological contamination
> exists in the feedwater,

Such as natural brines...

> requiring chlorination of the distillate to
> make it potable. It might be possible to
> eliminate this factor by eliminating the
> boiling of bulk liquid, and instead
> evaporating from a thin film of water flowing
> over the heat exchanger surfaces, but I
> doubt if it would be cost effective. Perhaps
> it would be another good student exercise.

I wonder if the increased viscosity of droplets at lower temperature
would assist in more efficient removal?

Thanks for the discussion...

David A. Smith
 >> Stay informed about: Potable Water - The Third Way. 

"jim.isbell" wrote:
>
> >
> > Ah well, another great idea skuppered by dat old devil science
> >
> > Bruce in Bangkok
> > (brucepaigeATgmailDOTcom)
>
> 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.

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.


>
> 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.

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

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 >> Stay informed about: Potable Water - The Third Way. 

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


On Sat, 29 Sep 2007 10:04:43 -0700, Keith Hughes
<keithahughes.TakeThisOut@qwest.net> wrote:

>> "jim.isbell" wrote:
/.../

>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.
>


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?


>>> The fresh water distills off the top of the
>>> saltwater column then migrates

>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.


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


>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.


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


>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.

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? )


> 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'.

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


>> I can see someone getting a
>> "Darwin Award" by accidentally spilling all their existing freshwater
>> supply in a failed attempt to get this contraption going.

>It doesn't *have* to be that way, BUT....

>Keith Hughes


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
 >> Stay informed about: Potable Water - The Third Way. 

Brian Whatcott wrote:
> Hmmmm...here's somebody at least taking a shot at analyzing the
> system. I interpose one or two little comments....
>
>
> On Sat, 29 Sep 2007 10:04:43 -0700, Keith Hughes
> <keithahughes.DeleteThis@qwest.net> wrote:
>
>>> "jim.isbell" wrote:
> /.../
>
>> 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.
>>
>
>
> My, my: "it's just plain wrong": he said a column of 32 ft,

Uhmmm, no, he said a "32' column of water". Can you see the difference?

> and
> you correct him - it's 33 ft.

You clearly need to re-read the paragraph whose point you're mangling.
The 33' you quote is not a correction to the OP, but a point for further
discussion (which you misunderstand later on).

> What a loser he must be!
> But then, you are neglecting to account for the density of SALT water!

And "about 0.8psia" makes what claim of precision?
>
<snip>
>
>> Yes, and this "migration" is simple diffusion.

<snip>

>
> 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

Gee, I didn't know you were using 'smart' molecules that travel *only*
in the direction you want them too. And they don't bump into each other
in the process. Wow, that's really neat, how did you accomplish that?
Barring that, of what value, or relevance is the above?

Here's a little thought experiment for you: Given the random
distribution of molecular velocities and directions (look at the
Boltzman and Maxwell discussion on the site you referenced, for
example), you can pick a point say 1 foot above the water surface, in
the center of the tube (NOTE: 1 foot is an Example). From this point,
every molecule has *almost* the same chance of going up, down, north,
south, east, west, or any direction in between right? If you say "no",
look at your own reference again. Given this, tell me again how the
molecular speed is *proportional* to the diffusion rate? (Hint: It's not)

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

Mayhap a definition of "example" could be of help to you?

>
>
>> 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.
>
> The word is "BOIL",

*If* the temp is high enough, yes. This system, in the currently
discussed configuration, is most likely to oscillate between boiling and
evaporation. I mentioned sublimation since posters continually reference
"ice to steam" in this context, and the concept is the same.

> not evaporate, not sublime. If it is not
> quickly condensed returning latent heat, the partial pressure rises
> quickly sure enough.

So if you condense it quickly enough, then there is no pressure rise?
Right. Look at your own reference above then, and tell me where the
energy went between boiling and condensing. That was a GAS law you were
citing no?

And "quickly condensed returning latent heat" has a name; refluxing.
Refluxing = no distilled water. Get the drift?

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

There you go again with your absurd assumption that all the kinetic
energy of the gas translates into motion in one direction only...

>
>>> I can see someone getting a
>>> "Darwin Award" by accidentally spilling all their existing freshwater
>>> supply in a failed attempt to get this contraption going.
>
>> It doesn't *have* to be that way, BUT....
>
>> Keith Hughes
>
>
> In my experience, the people who talk most about Darwin awards
> are completely foggy about how Darwinian selection operates.

Yeah, that's likely. I don't think we ever talked about Darwin when I
was getting my biology degree...

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

You also seem to need a refresher on how email/newsgroup postings are
structured. A casual glance would show that the quote you're mocking
was not mine.

Keith Hughes
 >> Stay informed about: Potable Water - The Third Way. 

>
> Ah well, another great idea skuppered by dat old devil science
>
> Bruce in Bangkok
> (brucepaigeATgmailDOTcom)


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.
 >> Stay informed about: Potable Water - The Third Way. 

On Sat, 29 Sep 2007 14:42:17 -0000, "jim.isbell"
<jim.isbell DeleteThis @gmail.com> wrote:

>
>>
>> Ah well, another great idea skuppered by dat old devil science
>>
>> Bruce in Bangkok
>> (brucepaigeATgmailDOTcom)
>
>
>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.

What you describe is just a still, and a 32 ft inverted U will change
nothing. Solar stills are not new. A tall boiler connected to a very
tall cond
 >> Stay informed about: Potable Water - The Third Way.