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Old 01-12-2011, 18:26   #16
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Re: Watermaker Multiple RO Vessels

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Originally Posted by goboatingnow View Post
There is a lower limit to pressure and permeate production falls away quite drastically as you approach it ( see Dows spec sheetsb) Also you need to maintain the 1gpm for filmtec membranes to ensure long life

Note that since its a back pressure system. The system pressure is not a function of the pump. The pump speed only controls flow.

Dave
I understand all of that. So I may have to run closer to the 800psi than expected. But still less than 800, to produce only half the rated output, i'd think?
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Old 01-12-2011, 18:43   #17
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Re: Watermaker Multiple RO Vessels

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Thanks for the detailed answer Tellie. I'm not sure I worded the question very well.

Let's say I have a 60LPH watermaker, with a 10:1 bypass ratio. So the pump delivers 660LPH at 800 psi.

I'm wondering if I changed the membrane to one rated at 120LPH at 800psi, but ran it at a lower pressure, so it was still only producing 60LPH of product, so it would still be running at a 10:1 bypass ratio, because the pump is still delivering 660LPH, but at maybe 650-700psi. (the bypass ratio might possibly be slightly more, since the pump tends to run a little faster at lower pressure.)

But by running at the lower pressure it might use a little less power. (I've observed this - at lower pressure the pump does run faster and draws less amps.)
Seawater at 36,000 PPM typically has an osmotic pressure of about 376 PSI (26.75 Bar). At 800psi from the pump the differential pressure would be 800- 376=424psi.
This is the actual pressure which forces the water to move through the membrane.
If you lower the pressure to 650 psi the diff. pressure would be only 274 psi. If we assume linear relation, you will lower the water production by 35%
T.
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Old 01-12-2011, 19:04   #18
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Re: Watermaker Multiple RO Vessels

G'day, Mates. Just wanted to give heaps of kudos to Tellie. He consistently takes the time to givel his detailed knowledge and experience of RO Watermakers to the benefit of us here on Cruisers Forum. His advice probably even costs him lost revenue. A tip of the hat, mate. Cheers.
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Old 01-12-2011, 19:09   #19
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Originally Posted by 44'cruisingcat

I understand all of that. So I may have to run closer to the 800psi than expected. But still less than 800, to produce only half the rated output, i'd think?
Looking at the spec sheets. I'm not sure there's much to gained in energy consumption. Given the outrageous energy loss in non-energy recovery ROs, I'm not sure it's worth running it on the edge of the envelope.

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Old 01-12-2011, 20:29   #20
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Re: Watermaker Multiple RO Vessels

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Seawater at 36,000 PPM typically has an osmotic pressure of about 376 PSI (26.75 Bar). At 800psi from the pump the differential pressure would be 800- 376=424psi.
This is the actual pressure which forces the water to move through the membrane.
If you lower the pressure to 650 psi the diff. pressure would be only 274 psi. If we assume linear relation, you will lower the water production by 35%
T.
Sounds good to me, since I'd only be after 50% of the membrane's rated production.
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