I think there is a bug in Rosa when it comes to feed pressure. It comes up
with pressures that exactly 1/10th of what I come up with by hand.

Actually the flow rate of the feed water only indirectly effects net product
at a given pressure. What it does do is change the ratio of product to
brine. As the salt content of the brine goes up the required osmotic
pressure goes up. If the system pressure remains the same the product goes
down. Conversely, the more feed water you supply the salt concentration
goes down so the required osmotic pressure goes down. If the system
pressure remains the same product goes up.

You can increase the product up to 25% or so just by increasing system
pressure a bit and cutting back the feed rate but at the price of
considerably shorter membrane life. The membranes are not just super fine
filters. Quantum theory is still black magic to my Newtonian brain so I
don't fully understand it but basically the atoms in the membrane material
have an electrical effect on the salt ions (all salt ions, not just sodium
chloride) that forces them away from the membrane surface. Ions also repel
each other so as the number of salt ions increase they are forced closer to
the membrane and scaling increases.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Geoff Schultz" wrote in message
6...
"Glenn Ashmore" wrote in
news:IkAye.148973$sy6.17500@lakeread04:


"Geoff Schultz" wrote in message
6...
So, my questions are as follows:

1) Are the calculations that I listed above correct?
2) How can I be producing water at the rate that I am?
3)What determines the prodution rate of a membrane assuming that it's
at 800 PSI?
4) Does anyone have a Little Wonder system that actually produces at
the speced rate?
5) They claim to need to test the pressure vessle. What could they
possibly be looking for?

1) Those are the same formulas I am working from. Pretty standard.
You really need .6HP to supply 1.1 GPM @ 850PSI. You can't get more
than about 28 GPH @800PIS out of 1/4HP
2) Sounds like you are overdriving the membranes and they are going
to scale up a lot faster. I ran the numbers you gave through Rosa,
Dow's design program. Even at the designed 10 GPH product @ 15% the
concentrate flow is marginal at best. At half that supply rate
concentrate flow is way low.
3) Salinity of the water, temperature, condition of the membranes,
configuration of the membranes, pressure and rate of feed water. Are
the membranes plumbed in series or parallel? Is the pressure between
800 and 850PSI? Are you pulling from a brackish river outflow or open
seawater?
4) Built my own
5) I have no idea.

Glen,

I've used ROSA also and came up with the basically the same numbers that
I do by hand. I'm running a single 2.5"x40" SW30-2540 membrane and am
pulling in normal seawater at 83F and running it at 825 PSI.

I must admit that one thing thta I've never understood about ROSA is how
to specify the input pressure to the membrane. If I specify a feed
pressure of 825 PSI it shows 100% of the feed water turning into
permeate (product water). I just leave it 0 and it caculates things as
expected at 800 PSI. If I specify 25 PSI for the feed pressure, it
calculates a recovery rate of 44%, which is way high. Any idea on how
to specify an feed pressure of 825 PSI?

Back to one of my main questions. I've never quite understood how RO
membranes work. How is it that the production rate of the membrane
varies based upon the quantity of the water passing over it? The
pressure is the same. Is it the scouring effect of the flowing
raw/waste water?

-- Geoff