Watermakers Pressure vs Output

[dkall]

Another cruiser friend and I were discussing our watermakers. He adjusts his to 800 psi no matter what. I was saying that on mine I adjust it to the rated output of 22 gph no matter what. Brackish water, tropical water, salt water, cool water etc. Obviously temp effect the suggested rated output a bit and I try to adjust for that.

Wondering if there are any watermaker experts here that would know the answer to which is the preferable method of adjustment and why.

His system is rather new and mine is now 7 years old still on the same membrane. Could only estimate how many total gallons output so I won't go there. I flush the membrane after every use. Never pickled it. Triple flush for long term storage. Running now at about 200 ppm plus or minus.

Thanks in advance.

[goboatingnow]

Either approach works, yours maximises power efficiency, his maximises output.

Dave

[Sailormantx]

When I purchased my water maker, I was instructed to set the pressure to 800 psi and the output would vary depending on temperature and salinity. When in fresh or brackish waters, adjust the pressure so the output does not exceed the water makers rates output to avoid damage to the membranes. With my machine, that is 8 gph, and usually about 200 psi in fresh water.
Dave

[Tellie]

Adjusting your system to it's rated output is the best way to run your system.

[colemj]

Technically, the pressure should be adjusted to a specific cross-flow through the membrane. In practice, adjusting to the rated output is pretty much the same thing. Also, in practice in warmish salt water, adjusting to 800psi is pretty much the same thing as both of the above.

The only place where adjusting to a set pressure will be very bad is in brackish or fresh water.

Mark

[SV THIRD DAY]

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 EU2000i generator.
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 water makers:
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.

[colemj]

Rich, for a 6% increase in amperage, one gets a 15% increase in output. Why not run at 900psi instead of 800 - especially since that increase in power consumption is mitigated by the fact that one is running a generator, so that extra 6% power has already been paid for?

Mark

[SV THIRD DAY]

Originally Posted by colemj

Rich, for a 6% increase in amperage, one gets a 15% increase in output. Why not run at 900psi instead of 800 - especially since that increase in power consumption is mitigated by the fact that one is running a generator, so that extra 6% power has already been paid for?

Mark

Good question and there are some water maker companies that use 900PSI as their standard operating pressure to try and eek out every last bit of production they can on an hourly basis but here's our reasons:

No 1: Membrane Life
RO Membrane No 2 in series is already working harder than RO Membrane No 1 because it is seeing about 20% less sea water flow going in combined wtih higher higher TDS. So by driving 15% more fresh water through the membrane you are starting to flirt with scaling and minimum flow problems on membrane No 2. It's a ballancing act of membrane life vs production rates. In a way it was just our "call" to give up the 15% in favor of longer life for the RO Membrane. But at $187 for a replacement membrane...you could make a different call.

No 2: Full Amp Load on the Motor
One of our big goals and design critera was to keep the power requirements as low as possible, to give folks with smaller generators or even a Honda EU2000i the chance to have a high output water maker. So rather than use the more standard 1.5Hp (or even 2.0Hp) motor on a 30GPH water maker and have lots of "power to spare in the motor", we decided to go with a 1.0Hp motor for energy efficiency. So if you bump the operating pressure up to 900PSI, you could be pushing the motor close to it's FLA (Full Load Amp) rating. Now I've ran my unit at 900PSI continually for 7 days (yep..y days of 24/7 running) and 5 years later that motor is still used 3-4 times a week to run my water maker as we live aboard our mooring in Morro Bay. It didn't burn it up or pop the termal trip breaker. But at the end of the day, we made another "call" to not push the motor that extra little bit.

No 3: Warranty and Marketing Reality
There is also some marketing and warranty reality here in why we didn't feel that we "needed" that extra 15% and opted to leave it on the table in exchange for membrane life and perhaps motor life. We are advertizing/selling a 30 gallon per hour water maker that we know damn well will make 33GPH in 68deg F water and closer to 35GPH in the warm waters where most cruiser want to go. So we are under-promising and over-delivering plus at the same time and leaving in a little design wiggle room. To date I have never received a call from a client complaining that their water maker was under producing the spec, but rather I get lots of happy client calls and emails feeling like they snuck out an extra 4-5GPH as a bonus. Which with some of the water makers ours replaced was close to their total hourly output.... Then there is this little thing call a 3 Year Warranty that we offer not just on the water maker, but we are on the only water maker company out there that gives this same 3 year complete replacement warranty on the RO Membrane as well. No prorated membrane warranty that still makes us money when one fails in year one, nope a full replacement. So I want to be damn sure I left enough margin in the design so that I'm not replacing membranes under warranty all the time, which I'm not, so that seems to be working out. Ok...No 3 was a little inside baseball...but hey...I would rather talk water makers than work on this damn MOU/Purchase Contract I have in front of me at the moment to add another well known cruising gear line to the family...opps...that could get me in big trouble if I don't shut up until the deal is done.....

(added on edit)
No 4: Because Dow Said So
No 4 is more of a justification than a real reason, but Dow rates their membranes at 800PSI so it makes a convienient benchmark. Every SW30-2540 that ships out of DOW has been bench tested to 800PSI, so there is some comfort in sticking with that number, or at least not having to justify to skeptics (which is hard enough on everything else already these days) why we would select an operational set-point above Dow's rating. We won't talk about the % recovery rate because that drives the internet engineer experts batshit crazy that start playing with the ROSA software....gulp....duck, run and hide before I get into even more hot water! My problem is I love this stuff and don't know when to shut up and leave Marine RO as Black Magic or Rocket Science...gets me into trouble all the time.

[Oceansailor]

Originally Posted by SV THIRD DAY

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 EU2000i generator.
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 water makers:
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.

Hi Rick,

Thanks for sharing the information. Accordingly to your information you use a 1 HP engine but have a power draw of 13.3 A at 800 PSI??

13.3 A x 110 V = 1463W That is pretty dam close to 2 HP ???

[belizesailor]

Originally Posted by SV THIRD DAY

...
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.

...

Would not plumbing the membranes in parallel, rather than series, help with these issues even for a 2 membrane system?

[SV THIRD DAY]

I’m always happy to talk technical…..so here we go.

Originally Posted by Oceansailor

Hi Rick,
Thanks for sharing the information. Accordingly to your information you use a 1 HP engine but have a power draw of 13.3 A at 800 PSI??
13.3 A x 110 V = 1463W That is pretty dam close to 2 HP ???[IMG]file:///C:\Users\User\AppData\Local\Temp\msohtmlclip1\01\c lip_image001.gif[/IMG]

The calculation you did is why so many people that try to build their own water makers get off track on pump/motor size and then RO Membrane surface area needed for what desired flow rate. They end up with a 1.5Hp or 2.0Hp motor when they could save power by using a 1.0Hp motor. Calculations are nice but this example helps illustrate the calculation weakness when compared to real life testing data. The data was taken using a 1.0Hp Leeson motor and we have hundreds of them out sailing the seas so we know they work and can be ran by a Honda 2000 generator.

BUT I should say the 13.3A motor is not the same motor we use now on the 20 and 30 gallon per hour water makers. Rather than a standard 1.0Hp Leeson motor that uses 13.3A, we changed to a 1.0Hp Leeson “Wattsaver” motor that only uses 9.3A at 800PSI. It costs a bit more, but the amp savings seems worth the cost because the lower amp motor makes it possible to make water AND charge the batteries at the same time with a Honda 2000 generator. It’s what I do aboard and it sure is nice to make 33GPH and top off the batteries!

Originally Posted by belizesailor

Would not plumbing the membranes in parallel, rather than series, help with these issues even for a 2 membrane system?

An important design criteria for RO Membrane long term health (and life) is to keep the brine flow up as high as possible to whisk away the higher salt content sea water. If the brine flow is too low, it will cause too large of a change in the sea water chemistry and you can have excessive calcium precipitation (commonly called “scale”) deposit on the membrane and the membrane will literally plug up.



So take the 1.6GPM high pressure pump flow, if you plumb the membranes in parallel rather than series, you would have 0.8GPM going to each membrane. By plumbing them in series you have 1.6GPM going into membrane one and then about 1.3 GPM going into membrane two. This increased flow rate goes a long way in helping keep the membranes healthy. It’s also why you can’t simply keep adding additional membranes in series, because eventually the sea water going into the down-stream membranes will not get enough flow and will be seeing much saltier water.

[Lancerbye]

Originally Posted by belizesailor

Would not plumbing the membranes in parallel, rather than series, help with these issues even for a 2 membrane system?

They are in parallel usually.

[colemj]

Originally Posted by Lancerbye

They are in parallel usually.

My experience has been the opposite.

Mark

[Oceansailor]

Rick,

Thanks for your quick reply. You are right I was calculating 100 % efficiency. I guess that motor does not exist yet.

No worry about trying to build my own water maker. If I would ever consider upgrading you guys are on top of my list.

[belizesailor]

Originally Posted by Lancerbye

They are in parallel usually.

Really, can you point me to examples by brand that are in parallel?