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Thread: RO UNITS: Temperature and Pressure Correction to Determine GPD

  1. #1
    Registered Member RandalB's Avatar
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    Default RO UNITS: Temperature and Pressure Correction to Determine GPD

    Hi all,
    Here's another informative thread brought to you by The Simpydiscus.com Team.

    Right now, throughout most of the country, RO users are scratching their heads and wondering why their RO units aren't producing enough water and are producing tons of waste water.

    The answer is simple, Lower Feed water temperature.

    Temperature and Pressure of feed water are two critical factors in Reverse osmosis efficiency. If they are both low, your RO unit will not perform well or efficiently.

    OK, so how do you determine how much water your RO unit will produce?

    All RO membrane elements are rated at 77F. Below this, they lose GPD. There is a simple way to determine how much water will be produced. Consult the following chart and select the closest temperature. Divide the GPD rating of the Membrane by the TCF (Temperature Correction Factor) and you'll have the GPD your RO unit is going to produce at temperature.
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    Last edited by RandalB; 11-10-2004 at 12:51 PM.

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    Registered Member RandalB's Avatar
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    Default Re:RO UNITS: Temperature and Pressure Correction to Determine GPD

    Ok,
    You'll notice that the closer you get to 77F, the more efficient the Membrane becomes. This is due to the density of the water at temp. What about the other way? If you make the water warmer, will the membrane get even better? The Answer is yes, up to 113F where most household membranes are destroyed. Here's the chart going the other way:
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    Last edited by RandalB; 11-10-2004 at 12:54 PM.

  3. #3
    Registered Member RandalB's Avatar
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    Default Re:RO UNITS: Temperature and Pressure Correction to Determine GPD

    That's easy enough..

    How 'bout something a little harder? Get your pencil and calculator out and we'll learn how to determine Net Pressure effects on the membrane.

    Ok, for the purpose of this excercise; we have a Dow Filmtec membrane rated at 100GPD. Our feed water is 43F, our TDS is 178 and our feed pressure is 40PSI. (These are my current water conditions BTW..)

    First, Determine the Temperature corrected rating of the membrane (TCF adjusted flow). Consult the chart, and you'll see that the closest we come to 43 is 42.8 with a TCF (Temp Correction Factor) of 2.38. So the TCF rating of the membrane is: 42.02 GPD (100/2.38)

    Next, Determine the approximate Osmotic Pressure of our feed water with the following formula: Op (osmotic Pressure) =TDS/100 so, 178/100=1.78

    Next Calculate the Net Pressure Rating (Pr) of the membrane. This is the Rated Pressure minus the Osmotic Pressure (Op) . Filmtecs are rated at 50PSI so;
    50-1.78= 48.22

    Next, Calculate the Operating condition Net pressure (PoP) Since we are operating unboosted in this instance,
    Applied Pressure (40) - Osmotic Pressure (1.78) = 38.22

    Ok, Confused yet? Not to worry, One more step to go.

    Now, Expected water flow at operating conditions is determined by the following formula: (TCF Adjusted flow)x(PoP)/(Pr)

    So (TCF Adjusted flow: 42.02) x (PoP 38.33) / (Pr 48.22) = 33.40 GPD

    As you can see, cold, lower pressure water causes dramatic drop off of Ro performance. It also lowers efficiency which results in more waste water. Increasing the pressure or temp will help overcome the effects of cold water and low pressure.

    One more?

    OK, Let's use the handy-dandy Aquatec 8800 booster pump and increase the water pressure to 100PSI

    So:
    Pr= 50-1.78 or 48.22
    PoP= 100-1.78 (100 is the applied (boosted) pressure) or 98.22
    Rated Flow is 42.02 from chart.

    Using the above equation: (TCF Adjusted flow)42.02x (PoP) 98.22 / (Pr) 48.22 = 85.59GPD

    Got it? It gives me a headache doing this stuff after 0100, but what else to do while filling tanks?

    HTH,
    RandalB

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    Registered Member ronrca's Avatar
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    Default Re:RO UNITS: Temperature and Pressure Correction to Determine GPD


    Thanks Randal!
    So when connecting the RO, why not use hot/cold and set the temp at 77F? Or is it better just to use cold?

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    Registered Member RandalB's Avatar
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    Default Re:RO UNITS: Temperature and Pressure Correction to Determine GPD

    Ron,
    It's because the vagarities of plumbing in some houses and the fact that Hot water tanks have a finite supply of hot water. If you are mixing and someone flushes the toilet, the RO unit gets hot water for several seconds which damages the membrane. Sure, you can get a mixing valve, and there are several suggestions for preheating the water with coils and etc, all of which do help. Most people's big contention is the use of all the hot water to drive the RO unit and an unhappy fish hating spouse wanting to take a shower. Of course, if you have a tankless Heater like I do ( ;D) it's not an issue. The danger of damaging the membrane is my top concern.

    If asked, I recommend installing to the cold water line only.

    HTH,
    RandalB

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    Default Re:RO UNITS: Temperature and Pressure Correction to Determine GPD

    Randal,

    Thanks for taking the time to explain the details of RO unit operation. This certainly will help me in determining how big a RO unit I will need and how to get the most performance from it. Adam

  7. #7
    Registered Member ronrca's Avatar
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    Default Re:RO UNITS: Temperature and Pressure Correction to Determine GPD

    Thanks Randal! Very good point! Its pretty much the same problem when filling the storage tanks and someone else wants to use water. In my case, I fill the storage barrels early in the morning (around 5am actually) so by the time anyone is up, the barrels are full. I suppose timing would be of concern so why not fill the RO during the night? ;D Works unless someone goes on the bathroom in the night I suppose! LOL!

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