Potable Water - The Third Way.

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

dlzc wrote in news:1190415672.506271.93890 @k79g2000hse.googlegroups.com:

"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 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

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

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

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

If the water is cooled SLIGHTLY less than the other side, wouldn't the rate of cryopumping of water vapor be incredibly slow? Isn't the rate of distillation related to the amount of energy put into the system or the difference in temperature between the two columns? Isn't this still distillation? (Excuse the pun.)

Does the absolute pressure of this system really have much of an effect on its efficiency? Doesn't it only affect what portion of the energy is put into cooling rather than heating? Heating water to boil is probably common for distillation because it is more simple and cheaper than cooling but mostly because you can pump enough heat into the water to vaporize a large amount of it and distill quickly.

Mitch

Sounds neat, haven't seen that one before. Thanks for sharing.

Dear Mitch Scherer:

"Slight" is such a hard number, how could you know? Do you think engineering might be involved?

Not the temperature difference, no, because we are talking about a phase change. The heat added to the system comes from ambient, obviously. Rate controlled by the temperature difference between the seawater and ambient.

Excused, and appreciated. ;>)

If you don't have to generate heat, it would be nice... don't you think?

A lot of reasons to do it with heat. Many of them involve sterilization, and other chemical processes. A lot of it has to do with how we discovered fire first... and how to cool much later. I just wonder if they'd still get good separation of water from its solute payload... if alcohol and other organics still would evaporate off at similar rates?

Waste heat from cooling the "pure water" side could be pumped into the seawater side...

David A. Smith

Fraid not. It takes the same ammount of heat to boil water as at 212F. Approximately 1175 BTU/lb. You might save a little not heating the water all the wqy to 212.

Lame suggestion and unworkable on most boats. Ya gotta think outta the box, man.

However, there is another way. I thought it up all by my lonesome. All you need is a reverse osmosis membrane. You put it into a chamber that is vented to atmosphere on the inside and to the ocean on the outside of the membrane. You lower it into the ocean to a depth of only 500 feet and the pressure of the water is enough to push fresh water through the membrane into the chamber. When it gets full you haul it up and empty in into your tanks. Reverse osmosis without any energy used to get it. Ain't Wilbur brilliant?

Wilbur Hubbard

"N:dlzc D:aol T:com \(dlzc\)" wrote in news:pv_Ii.42363$L_ snipped-for-privacy@newsfe16.phx:

There's a limit to the vacuum boiling. After a certain point, water goes from a solid straight to a gas with no liquid state, just like CO2 does at atmospheric pressure.

You can't pull a full vacuum on it and get liquid water.

graph this website, point t.p.)

Larry

"Roger Long" wrote in news:46f53eef$0$5006 $ snipped-for-privacy@roadrunner.com:

Was his real name Don Herbert and he worked for GE?...

Larry

Dear Wilbur Hubbard:

"Wilbur Hubbard" wrote in message news:46f58e32$0$62309$ snipped-for-privacy@auth.newsreader.octanews.com... ...

You are still displacing that much water... not a small feat.

David A. Smith

Dear Larry:

It actually goes into this state at very high pressures too. But you are talking "below" the triple point, which is 0.1degC:

not much of a problem above this temperature.

Not a full vacuum, since water vapor does fill it. And if you boil too fast, you lose your two columns of water. And if you don't refresh the contents of the two columns then you start having scaling issues.

Not a slam dunk, but not a bad idea either.

David A. Smith

When you contradict me, it is always better to check your facts. I do.

Brian Whatcott Altus OK

Dear OldNick:

Fill the tubes, with a small air bubble in between. Lift the tube at the bubble. No small feat if you are looking to make hundreds of gallons per day, but a one-shot expense...

David A. Smith

Dear Old Nick:

Likely, yes. You could use evaporative cooling (of brine) on that side, and solar heating on the other side.

I woudl assume it could be either, depending on the sophisticatioin of your control process.

Ever seen a mercury barometer? The bottom end of the tube ends in a "pan" open to atmosphere. The bottom end of both tubes can simply be sunk... one in the ocean, and one in a wet well for a pump station.

The air bubble expands when the vaccum is created.

etc. Take your shades off, dude. It is coloring everything you see...

It isn't the greatest thing since sliced bread. But it is another process, and a viable one.

David A. Smith

On Sun, 23 Sep 2007 21:57:56 +0800, OldNick wrote: ...

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