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Old 18-05-2019, 13:48   #1
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Upgrading just the pump on a watermaker?

I have a 150GPD S&K DC watermaker aboard. It is basically a 1/2 HP Leeson motor connected to a Wanner F20S (.56GPM) pump and going through a 21" membrane. 6.5 gallons per hour takes a lot of hours to refill tanks. I'd like to see if I can just upgrade the pump and possibly get more gallons per hour out of it? Even if I could get 8-12 gallons per hour out of it, that would be worth it to me. Rich's excellent post on common DIY mistakes to avoid had a table in it which seems to indicate I could get a .8 gallon per minute pump on the motor I have now.

0.5gpm = 1/3Hp
0.8gpm = 1/2Hp
1.6gpm = 1.0Hp
2.3gpm = 1.5Hp
3.5gpm = 2.0Hp
4.2gpm = 2.5Hp

Any recommendations for a pump that will give me .8 GPM on a 1/2 HP motor?
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Old 18-05-2019, 13:57   #2
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Re: Upgrading just the pump on a watermaker?

Sorry... it doesn't work that way.

If you are running the membranes at their rated pressure, they are the limiting factor NOT the pump. You can't just push more water through.

Also, you shouldn't be "refilling" the tanks. You should be running the watermaker every day. It is far better for the membranes to run for one hour every day than for 8 hours once a week!
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Old 18-05-2019, 14:04   #3
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Re: Upgrading just the pump on a watermaker?

Upgrade the pump and the membrane then? What is the theoretical max of a 21" membrane?

I understand it is better to run them more often than less often, however it is a certain amount of trouble to get it going and to flush it at the end. My hope was to run it about every 3 days for a few hours.

I wonder how does Sunpure claim to get 8-12 GPH out of a 21" membrane? http://sunpurewatermakers.com/estima...12-gpm-system/
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Old 18-05-2019, 14:34   #4
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Re: Upgrading just the pump on a watermaker?

If you use a Filmtec SW30-2521 membrane, the industry standard unit, it is rated to produce 300 gallons per day (12.5 gallons per hour) at 800 PSI. That requires a feed flow of 6 gallons per minute (360 gallons per hour). Roughly 30 gallons of feed is needed for 1 gallon of product water.

If that pressure and feed flow are correct, but you do not get that product rate, then you probably need to clean or replace the membrane.

The membranes are pretty sensitive, and really do not work well or last long if operated very far outside their specifications, either lower OR higher.

If you want to increase your capacity, you can double up on the 21 inch membranes, or go to a single 40 inch unit. Either will roughly double output if you can feed them 12 gpm at 800 psi.

If you flush with fresh water after use, then your three day operating cycle is going to be OK. Don't routinely leave them full of salt water for three days in tropical temperatures. Stuff grows on the membranes pretty quickly, and that is very bad for them.
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Old 18-05-2019, 14:59   #5
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Re: Upgrading just the pump on a watermaker?

You are right that my membrane needs to be replaced, but even new the system is only rated for 6.5 GPH of product. I am getting up to 800 PSI.

Maybe I'm confused, according to your calculations my .56 GPM pump is only 33.6 gallons per hour, which should only get me about 1 gallon per hour of product, so why am I getting 6.5?

I'm Ok with upgrading the membrane, like I said even 8 or 10 GPH would be a substantial improvement over what I have now. Can you tell me what pump I would need (that would run on my 1/2HP motor)?
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Old 18-05-2019, 15:11   #6
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Re: Upgrading just the pump on a watermaker?

Get bigger or multiple membranes. But at some point both pump and membranes are going to matter.
Let's see... for comparison:
My Cat had an AC 1.5 HP motor and 3 ~two foot membranes, I think it was 35 gal per hour.
I converted that pump onto an engine and had one long membrane which made about 20 gal per hour.
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Old 18-05-2019, 17:59   #7
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Re: Upgrading just the pump on a watermaker?

Isn't there also a rule about needing enough flow to "scour" the membrane to keep it from building up scale? It seems like I've read something about this somewhere, that production might go up by adding more or longer membranes, but life expectancy might go down.
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Old 18-05-2019, 18:40   #8
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Re: Upgrading just the pump on a watermaker?

Most of what has been said is correct, except for the 30 gls of water to make 1 gl.
It’s more like 3 to 1.
My 1.6 gpm pump will produce .6gpm of product water.
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Old 18-05-2019, 20:00   #9
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Re: Upgrading just the pump on a watermaker?

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Originally Posted by a64pilot View Post
Most of what has been said is correct, except for the 30 gls of water to make 1 gl.
It’s more like 3 to 1.
My 1.6 gpm pump will produce .6gpm of product water.
I guess the real question is what flow rate do you need to keep up to keep the membrane healthy?
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Old 18-05-2019, 20:19   #10
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Re: Upgrading just the pump on a watermaker?

Pumping more water at same 800psi results in more product output. The only difference between a 30gph and 40gph watermaker is the motor going from 1hp to 1.5hp. I have not seen any 12gpm hp pumps on sailboats yet. I think that would be in liters?

RO membranes are crossflow filters the need the flow to scour clean. This is why the 1st membrane in a 2 in series system should last longer (it has more crossflow) but alas for me it was the other way around.
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Old 19-05-2019, 08:27   #11
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Re: Upgrading just the pump on a watermaker?

Lots of numbers being thrown around and now I'm confused. I've been running water makers on boats since 1995 and had always heard/read that to maximize the life of the membrane you wanted at least 10 times as much water flowing across the membrane as you get in product. Another way of saying this is that product water output should be no more than 10% of the total input water. I size my systems so product water is about 6% of the total flow (input) and I've been running the same membranes for years. Am I off base on this <10% product water rule???
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Old 19-05-2019, 08:38   #12
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Re: Upgrading just the pump on a watermaker?

Quote:
Originally Posted by s/v Jedi View Post
Pumping more water at same 800psi results in more product output. The only difference between a 30gph and 40gph watermaker is the motor going from 1hp to 1.5hp. I have not seen any 12gpm hp pumps on sailboats yet. I think that would be in liters?



RO membranes are crossflow filters the need the flow to scour clean. This is why the 1st membrane in a 2 in series system should last longer (it has more crossflow) but alas for me it was the other way around.


Then there is the quandary of why not drive up the pressure to increase output?
The membranes are good up to 1000 PSI from data I can find.
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Old 19-05-2019, 09:44   #13
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Re: Upgrading just the pump on a watermaker?

Quote:
Originally Posted by sailingunity View Post
Upgrade the pump and the membrane then? What is the theoretical max of a 21" membrane?

I understand it is better to run them more often than less often, however it is a certain amount of trouble to get it going and to flush it at the end. My hope was to run it about every 3 days for a few hours.

I wonder how does Sunpure claim to get 8-12 GPH out of a 21" membrane? 8 TO 12 GPH DC SYSTEM/DESALINATOR – SUN PURE WATER MAKERS
If you run your watermaker every day or two you don’t need to flush it. I’ve been using mine for two years and I only flush it if I know I won’t be able to run it for a few days.
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Old 19-05-2019, 09:53   #14
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Re: Upgrading just the pump on a watermaker?

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Originally Posted by JSLExplores View Post
If you run your watermaker every day or two you don’t need to flush it. I’ve been using mine for two years and I only flush it if I know I won’t be able to run it for a few days.
That is good to know!

Still it would be nice to know we could run it for an hour instead of 2. It's not the quietest thing in the world!
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Old 19-05-2019, 12:41   #15
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Re: Upgrading just the pump on a watermaker?

Let me clarify a few things about the cross flow membranes. The key figure of merit is the recovery ratio, or how much product you can make from the feed water. The membrane optimal recovery is 8-10%, which means you feed 10 gallons for 1 gallon of product water. Then the quality, scaling, etc. is optimal. When you reduce the flow rate, the quality decreases, the scaling increases, etc. but it all happens very slowly. Most ordinary small scale watermakers operate at around 30% recovery, basically trading off some scaling for energy efficiency and compactness. Dow says so in their product literature, specifically, they say that if you operate your membranes above 20% recovery, the lifespan may be limited to less than 3 years. I have seen some people go to 45% recovery and all is good, although the quality of the water begins to decrease. This is also a variable because one can make 500 ppm water for washing and cleaning dishes and 150 ppm water for drinking as long as you can change the rpms of the pump which should be possible with a dc motor. Even if you have to replace the membrane more often, it is only $200 every two years vs. $1,500 for the new pump today.

To cut the long story short, my suggestion would be to increase the voltage of your 1/2 hp motor gradually and observe the quality and quantity of the product water. Then you can graph the recovery ratio vs. the water quality for your system. This is all you need to do, basically you need a step up transformer. Worst case, you burn your motor (very unlikely), it will cost $150 to replace.

If you still want to buy a new pump, I would be happy to buy the current one from you
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