Potable Water - The Third Way.

In article <470b1a19$0$505$815e3792@news.qwest.net>,
keithahughes.TakeThisOut@qwest.net says...
> Mark Borgerson wrote:
>
> <SNIP>
>
> > With the proper placement of the check valves, I think you could
> > start with the initial boiling happening in the freshwater side---
> > after all it is going to boil at a lower temperature.
>
> Well, except that 1) unless you fill the entire apparatus with fresh
> water (ignoring for a moment how much fresh water that might take, and
> how much system capacity is lost in re-distilling the fresh water), you
> haven't eliminated the carryover contamination issue, since you still
> have a contiguous water stream, and 2) the freshwater side of the system
> is configured for *cooling* and the seawater for heating (passive system
> remember), so boiling will always be initiated on the seawater side.
> >
> > The procedure might look like this:
> >
> > 1 Pump both fresh and salt water to near the top.
> > 2. Shut offf the salt water side pump, but keep the
> > tube closed at the bottom.
> > 3. Pump a bit more fresh water into the tube---where
> > it overflows to the saltwater side, displacing
> > the rest of the air out the check valve.
>
> And where is the barrier layer that keeps salt from moving into the
> freshwater?

You don't need a barrier if you can do this in a few tens of seconds.
> >
> > You now have no air in the tube and a small layer of
> > fresh water on top of the salt water.
>
> But it won't stay that way. As soon as you begin to heat the seawater,
> you'll almost certainly have seawater rising into the fresh (do to the
> density change with heating) before you get boiling going on.

I think the fresh water on top of the seawater will boil before
the convective mixing will occur. Remember that the seawater
is more dense than the freshwater---which will be as warm as
the seawater on that side of the system.
> >
> > 4 Release the pressure at the bottom, and the fresh
> > water at the top will boil and create your head space
> > with little or no contamination of the freshwater
> > side.
>
> Yeah, but "little" is not the goal. And you'd have to quantify that
> "little" empirically, since there are many factors that contribute to
> the process.

So quantify away! I can stand to drink water with 0.05 percent
salt (seawater is 3.5% salt).
> >
> > 5 Apply your heat differential and remove distilled
> > fresh water as it overflows the reservoir at the
> > bottom.
> >
> > This should work until the dissolved gas problem lengthens
> > the vapor path to the point where you have to start over
> > at step 1.
>
> Again, the design complexity involved in being able to heat the fresh
> side to initiate the boiling there *first*, and then switching to
> cooling mode when there is sufficient column separation puts paid to any
> thoughts of this being a simple system. And then, you have a very
> complex, and Horribly inefficient system.

No. I am not talking about heating the fresh water side. I am talking
about moving a half foot of fresh water to the seawater side of the
system and starting the boiling there.
>
> There are lots of ways that you could make the system work, but why?
> The *only* feature this concept has going for it to start with is
> simplicity, and basically a passive (save for some human work input) system.

I agree. I am simply describing ways to make the system proposed work
as efficiently as possible---which is not very efficient. I think
you would be better off with a few square meters of 1Atm distillation
system sitting on the foredeck. I have seen proposals for systems
where seawater runs across a black plate and the evaporated water
collects and drips off the glass cover. You do need a small pump to
move the seawater to the top of the plate and a lot of sunshine.
But, OTOH, in the areas where I cruise (the Pacific NW), it's
probably easier to put out a clean plastic sheet and collect
rainwater. I just spent a week on a charter yacht in BC, and we
could have collected all the fresh water we needed on one rainy
night!


Mark Borgerson

On Mon, 08 Oct 2007 17:36:59 -0700, Keith Hughes
<keithahughes.DeleteThis@qwest.net> wrote:


>... If you *pump* the water up
>the columns, evacuating the headspace as you go, until the columns meet,
>you can siphon pretty much any height you want.
...
>Keith Hughes


No, you cannot syphon to any height.
Yes, you can pump to any height, within reason.
There is a difference.
If you pump to 40 feet head, quite filling an inverted u-tube, for
instance, when the pump is halted, the water falls until a particular
maximum height is no longer exceeded, if the foot of the u-tube is
opened to water whose surface is at atmospheric pressure..

Brian W