I think the RO salesmen have taken lessons form the air compressor people.
Gallons per hour is a lot more useful but gallons per day sounds better.
There are a lot of ways to play with the numbers. The basic physics is hard
to get around but there are a lot of "soft numbers" they can work with.
The simplest way is to overdrive the membranes and change the owner's manual
to require more frequent maintenance. Ideally a membrane should be driven
at 14-15% product but they will work all the way up to 30% or more given the
right conditions. They just won't last as long. Parallel membranes are
more efficient than series membranes because both membranes see the same
salt concentration but they require more flow and therefore more expensive
pumps. Some of the more expensive systems use a multi-stage membrane
arrangement that makes the system more complex but can be 10-20% more
efficient. Also the higher the capacity the more efficient they can be.
The only real way to really get more product for less energy is with a
pressure recovery device. On big commercial systems this is usually an
impulse impeller driven by the discharged brine. They are big heavy,
expensive and don't really do much good until you are in the 50-100 GPM feed
range. The Clark pump as used by Spectra and a few others is the most
efficient and the only recovery device that is practical on a boat..
In most systems the energy in the high pressure brine is lost when it passes
through the pressure regulator. The energy stored as pressure is converted
to velocity and is discharged overboard.. That is about 70% of the total
energy used by the system. The Clark pump is a variation of the old
Worthington shuttle valve duplex boiler feed pump. It has a pair of
cylinders with large pistons and narrow rods. A pair of shuttles control
the flow in and out of the cylinders. At top dead center the rods take up
about 15% of the volume of the cylinder.
Rather than discharging the high pressure brine through a pressure regulator
it is directed to the back side of a piston. The front side of the piston
pushes the new feed water into the membrane. Because of the volume that the
piston rod takes up the amount of feed water pushed into the membrane is
about 15% greater than the brine coming out. . That 15% has to go somewhere
and that is out the product side. After friction losses the Clark pump
can recover about 75% of the brine's energy so the net energy input per
gallon of product is less than half that of a traditional system.
The down side is that a Clark pump of acceptable size for a medium size
yacht can't pump very fast. (Under 1 GPM) They are pretty well limited to
around 6 or 7 gallons of product per hour which means you hear the whine of
the feed pump and click-clack of the Clark pump for hours on end.
--
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...
I still have more questions about RO systems. Manufactures quote their
system production rates in GPD and Amps. I compare them by calculating
product Gallons per Amp-hour, and the rates seem to be all over the board.
For example, Village Marine systems utilize about 2 AH per gallon for
their
Little Wonder Systems. HRO systems are 4+ AH per gallon.
The manufacture's publications make you believe that their systems have
become more efficient through their pump and motor technology. Is there
really that much difference in technology? Some vendors claim to have an
energy recovery system, but I don't understand what this is or how it
works. It certainly doesn't seem to apply to basic systems that I'm
looking at.
Any insights into this?
-- Geoff