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Buckeye Hydro
12/08/2012, 04:21 AM
The "optimum" pressure for RODI systems seems to be an issue that keeps coming up on RC. There's a few things to consider here, including the best pressure for the membrane, and the pressure ratings for other system components.

Filmtec 75 gpd membranes are spec'ed with a max pressure of 300 psi. In general, the higher the pressure the better the membrane will perform in terms of how fast it produces clean water, and how clean the water is (the rejection curve goes pretty flat however at less than 100 psi). Now, in practical terms, no one has pressure approaching 300 psi in their plumbing. How about if you have a booster pump? Chances are it will max out at 120 psi or less.

I can hear the question already - "I thought the optimum pressure for Filmtec membranes was 50 psi - that's what the manufacturer says." Well, not really. What Filmtec says is that if you provide a 75 gpd membrane feedwater at 50 psi, 77 degrees F, and 250 ppm; and configure the system to run at 15% recovery, it will produce 75 gallons per day, plus or minus 20%, at the spec'ed rejection rate. Is 50 psi the "optimum" pressure? No. It just happens to be one of the standard test conditions used by the manufacturer in their testing. Most membrane manufacturers other than Filmtec use a test pressure of 60 psi.

Remember that your flow restrictor is designed to provide about a 4:1 ratio of concentrate to permeate assuming you have "standard conditions" (50 or 60 psi, 77 degree water, ...). So if you operate your system at much higher pressure, you may have to adjust/change your flow restrictor so that you stay near the recommended 4:1 ratio.

Also be aware that other components in your RO or RODI system can't handle anything close to the 300 psi your membrane can. The weakest link in most systems in terms of ability to withstand pressure is the clear housings. This is why we always recommend installation of booster pumps after the prefilters.

Russ

Haxer
05/09/2013, 12:12 PM
Having a Filmtec 75 membrane myself and on well water at 30-40 psi I am looking at booster pumps. What would be the "ideal" psi ... Would the 120 ps1 max of most pumps dramatically shorten their lifespan if ran that high?

Buckeye Hydro
05/09/2013, 02:25 PM
No - the pressure won't bother the membrane a bit. Just make sure you don't expose other components of your system to pressure above what they can handle.

http://i224.photobucket.com/albums/dd293/BuckeyeFS/Instructions/AquatecComponentsDiagram-1.jpg (http://s224.photobucket.com/user/BuckeyeFS/media/Instructions/AquatecComponentsDiagram-1.jpg.html)