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Old 26-07-2009, 19:11   #1
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Watermaker Membrane - Change to Larger = Increased Output?

I have a 500 gal per day watermaker that I removed from my boat that worked but was a 220 volt unit and am replaceing it with a used 12 volt 150 gallon per day unit. I was wondering if I installed the 500 gallon per day membrane on the 150 gallon per day unit, would it make more water?
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Old 26-07-2009, 20:24   #2
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Nope. It's the pressure that makes the fresh water not the membrane. The high capacity membrane needs the huge pump to make it work. For the most part you match the membrane to the high pressure pump. The rest of the gear is not unimportant but can use 12 volts with less of a problem. You are now making a new water maker from scratch.
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Old 26-07-2009, 22:26   #3
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I'm not the absolute authority on this subject but it's my impression that you would make more water. I have a component 600 gpd unit and the membrane is captive in the membrane chamber and the entire chamber is pressurized. It seems that the larger the membrane the more water you make. The pump puts out more pressure than you need anyway and on my unit I hand regulate it, the excess water goes through the chamber and over the side. I have seen some units composed of several smaller membrane chambers connected in series and I was once told that the capacity could be increased by adding additional chambers but have no first hand knowledge of that. I hope someone else will jump in here who really knows. I hate to be over ruling some one here because I really don't know
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Old 27-07-2009, 01:44   #4
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It gets down to flow rate of High pressure pump as you can only recover a percentage of FW say 10% so it's hard to get a 12volt HP pump to deliver enough water
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Old 27-07-2009, 02:34   #5
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Don't have one myself, so no real world practical experience, but was interested in trying to make one so have done some reading. You need a minimum pressure, and a minimum flow rate of waste brine water. Below a certain pressure, you won't get water through the membrane, below a certain (manufacturer recommended) brine flow rate you clog the membrane. If you have too small a pump, and attempt to open the brine valve enough to get the required flow for the membrane you will exceed the pumping rate of the pump and the pressure will fall. If you close the valve enough to maintain pressure, you will have too low a flow and clog the membrane. (I think in reality most systems use a back pressure valve so you set the pressure and it varies the flow to maintain that pressure.) This site says that about 15% of the water used makes fresh water. So pump flow capacity (in GPM) has to be at least (water made GPM)/ 0.15.

http://www.westward-ii.com/PDF's/How...watermaker.pdf

http://www.wcponline.com/pdf/0907Wirth.pdf

At some point if you change the membrane size too much, other parts will become too small, like the brine regulater might be too small to allow enough flow for the capacity of the system.

John
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Old 27-07-2009, 06:52   #6
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Quote:
Originally Posted by quartersplash View Post
I have a 500 gal per day watermaker that I removed from my boat that worked but was a 220 volt unit and am replaceing it with a used 12 volt 150 gallon per day unit. I was wondering if I installed the 500 gallon per day membrane on the 150 gallon per day unit, would it make more water?

This depends on what brand and model 150GPD watermaker you have. If it's a Spectra 150 then yes it might very well be possible with some minor modification to the unit to use the larger membrane. It won't make 500GPD but it would double the production rate of the 150.
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Old 27-07-2009, 08:06   #7
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I added a third membrane to my 110v. Village Marine watermaker and increased output from 25 to 38 g/hr.
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Old 27-07-2009, 08:53   #8
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I added a third membrane to my 110v. Village Marine watermaker and increased output from 25 to 38 g/hr.
How did you go about it? Did you know the pump capacity was high enough for adding another membrane?

Or was willing to possibly replace membranes more often due to low brine flow causing premature clogging of the membrane?

Or measured the brine flow rate?

Or just stuck it in, it worked, the end?

Also how long have you operated it this way?

John
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Old 27-07-2009, 09:58   #9
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pressure is pressure, regardless of container size. if your pump will do the pressure it will do the pressure. The problem is Volume. How much capacity your pump can flow at pressure is important. My catamaran had a 120 volt unit. I took the high volume pump and parts and mounted it on the engine with an electric clutch. Eventually I eliminated three short membranes and got one long one. The amount of water didnt SEEM to change, but I never kept very close track as it made about 25 gallons an hour.
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Old 27-07-2009, 10:59   #10
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Watermaker permeate vs. retentate quantities are dependent on 'transmembrane pressure' VERSUS total membrane surface area. Simple speak: a larger surface area membrane will/can pass the same volume of water at LOWER transmembrane pressure as a smaller surface area membrane at higher pressure. Yes, you can use the larger surface area membrane, and be able to run it at lower pressure to result in the SAME permeate/retentate flow rate ... but you may have to increase the throttle valve 'backpressure' The advantage of running the larger membrane will be ... increased time in service before non-bacteria fouling; disadvantage will be tendency to become bio-fouled faster due to lower internal flow velocity (lower 'sweep velocity). Cheechako has it correct ... and you adjust that pressure by the throttle valve.
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Old 27-07-2009, 11:03   #11
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No problem w/ pressure - dialed in about 800 with adjustment on the pump manifold valve.
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Old 27-07-2009, 12:01   #12
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As to the OP, it is going to depend on what type of 150GPD watermaker he is trying to drive the larger surfaced membranes with. Not all units use a back pressure type valve. Not enough information is given by the OP.
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Old 27-07-2009, 12:18   #13
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A semi-related query

With no intent to hyjack this thread (if a moderator feels it is, then let's start another thread), what are the pros and cons of a 12v DC watermaker system vs a 230AC (or in US a 120AC ) watermaker system?

I've heard a few thoughts, though have no idea whether that was marketing/sales "fiction" or fact.
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Old 27-07-2009, 14:11   #14
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BlueSovereign they all work the same and that's the power to drive the pump only. A power take off from the engine means that you have to have the pump near the engine so you need some engine room space and a little extra HP if you run it under way, under power. The 12/24/etc means to have to have it closer to batteries or run heavy battery cables if at any distance, and AC allows you to put it much further from the power source but you need an ample AC power source.

My unit is AC and I originally intended to power it through my 2500W inverter off my batteries but it drew 19 amps AC to start and about 17 amps AC to run and with the inverter loss the 200+ amps DC was too great a draw on my batteries so I have to run it on my genset.

quartersplash, You have some good answers above here and they all seem to know as much if not more than I do. My unit is an Aqua Marine made in Deer Harbor WA. It's a small Company and the guy has always seemed willing to discuss watermakers, so I'm pretty sure he would talk to you about this subject.

Good luck

Joe S
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Old 27-07-2009, 15:06   #15
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Originally Posted by BlueSovereign View Post
With no intent to hyjack this thread (if a moderator feels it is, then let's start another thread), what are the pros and cons of a 12v DC watermaker system vs a 230AC (or in US a 120AC ) watermaker system?

I've heard a few thoughts, though have no idea whether that was marketing/sales "fiction" or fact.
It boils down to how you power your boat and how energy conscious you want or need to be. Are you a big generator guy or a solar panel, wind generator guy or somewhere in between?
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