On Sep 22, 10:39 pm, OldNick wrote:
On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
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.

On Thu, 27 Sep 2007 13:54:13 -0000, "jim.isbell"
wrote:

On Sep 22, 10:39 pm, OldNick wrote:
On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
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.

On Thu, 27 Sep 2007 21:25:39 GMT, Glen Walpert
wrote:

On Thu, 27 Sep 2007 13:54:13 -0000, "jim.isbell"
wrote:

On Sep 22, 10:39 pm, OldNick wrote:
On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
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)

Dear Glen Walpert:

"Glen Walpert" wrote in message
...
On Thu, 27 Sep 2007 13:54:13 -0000, "jim.isbell"
wrote:

On Sep 22, 10:39 pm, OldNick wrote:
On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
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.Arti...ticle_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

On Thu, 27 Sep 2007 18:06:47 -0700, "N:dlzc D:aol T:com \(dlzc\)"
wrote:

Dear Glen Walpert:

"Glen Walpert" wrote in message
.. .
On Thu, 27 Sep 2007 13:54:13 -0000, "jim.isbell"
wrote:

On Sep 22, 10:39 pm, OldNick wrote:
On Sat, 22 Sep 2007 10:55:52 -0500, Brian Whatcott
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.Arti...ticle_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.

Dear Glen Walpert:

On Sep 28, 7:09 am, Glen Walpert 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.Arti...ticle_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

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
sal****er 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.

"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
sal****er 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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On Sat, 29 Sep 2007 14:42:17 -0000, "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
sal****er 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

jim wrote:

"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.

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.

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.

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.

The fresh water distills off the top of the
sal****er 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.

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.

as steam to the other side and distills
in the fresh water side....also creating a vacuum.

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'.

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.

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


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.

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.

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.

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



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.

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.


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.

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

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.

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