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.