What we tell our clients is to just stick with 800PSI with the exception being in fresh or brackish waters, then let your production rate limit your operating pressure. Could we come up with a smart phone
ap that takes sea water ppm, temp, membrane surface area, and the moon's tidal phase into account to adjust the 800PSI up or down as needed? Well I guess, but....operational Simplicity to me wins out.
As a science geek, I like to look at data, so here is some real life testing data from the testing lab (IE my boat while anchored in San Diego
, Bay August 2009).
Sea Water temp was 20 degs C and the data is from a single
40" Dow Filmtec SW30-2540 Membrane being fed with a 1.6GPM high pressure pump powered by a 1.0Hp motor
. So you can see the effect of PSI on fresh water production rate and power usage quite nicely.
700PSI: 12.4A and 16.1GPH
750PSI: 12.5A and 18.7GPH
800PSI: 13.3A and 20.2GPH
850PSI: 13.5A and 22.2GPH
900PSI: 13.8A and 24.1GPH
"But Rich, what happens if you add a second 40" RO Membrane in series to the first?"
Well I'm glad you asked because that just happens to be our most popular selling water maker the SM30. Same pump and motor
and same sea water temps.
700PSI: 12.7A and 28.2GPH
750PSI: 13.0A and 32.2GPH
800PSI: 13.3A and 33.6GPH
850PSI: 13.6A and 34.3GPH
900PSI: 14.1A and 38.6GPH
The two membrane data was for you Mark...
You are looking at the averages of 12 data readings for each test point taken over 4 days of testing on 4 different RO membranes. It's fun to play scientist.
The motor was our old motor, which wasn't a Wattsaver, so now with the new motor your 800PSI Amp draw is 9.3A...so that's a pretty big power savings, especially for folks powering the water maker with the Honda
While in the warmer waters of Mexico
, the 33.6GPH production pumps up to about 35-36GPH and the 20.2GPH up to 23-24GPH.
At the risk of over-commenting and giving too much information, the obvious questions comes up.
"Why not add a 3rd RO Membrane and make ever more water?"
The answer boils down to the fact that RO Membranes must have enough flow through them to carry away the higher salt
content on the sea water side of the membrane without it precipitating out on the RO Membrane and forming "scale" or clogging the membrane due to concentration and water chemistry changes of the brine. In english
, there isn't enough flow to support the 3rd RO Membrane from the 1.6GPM pump, it will clog up and die an early death.
Four things govern the fresh water production rate for marine
1. Membrane Surface Area
2. System Operating Pressure (PSI)
3. High Pressure Sea Water flow rate (GPM)
3. Concentration of Salt
in sea water (PPM)
4. Sea Water Temperature.
You can move the first three around in your water maker design but there are operation limits and constraints that we try to work within to balance the things such as: Power Usage, System Cost, and certainly membrane life.
Ok, just one more tidbit to save me from doing "real work' that I should be doing today. There are LOTS of internet
and cruising myths as to why water makers increase their fresh water production in warmer waters and decrease it in cooler waters. If you ask the folks at Dow Chemical, who make the membranes, they will tell you that the No 1 reason has to do with molecular kenitics. The water molecules are vibrating and moving faster in warm waters than they are in colder water. So their "odds" of passing through the molecular pores of the RO Membrane increase. It's also why the TDS tends to go up in the warmer waters as well. There....I promise I'm done.