Water pressure Reverse Osmosis

Discussion in 'Water Softener Forum, Questions and Answers' started by Bob from accounting, Jun 3, 2020.

  1. Bob from accounting

    Bob from accounting Member

    Joined:
    Mar 5, 2020
    Location:
    California
    Is there a RO posting, if not I hope posting in here is fine.

    1) Can you pump more air pressure into the bladder, over the recommendation. Sacrifice overall volume for increased pressure? Is that an option?

    Or does this compromise the bladder?
     
  2. Bannerman

    Bannerman Well-Known Member

    Joined:
    Mar 19, 2014
    Location:
    Ontario, Canada
    Pressure tanks for RO storage, are equipped with larger bladders since the pressure range is greater than for a well pump pressure tank. When empty, the RO tank precharge is typically preset to 6-7 psi, and the pressure when filled, is normally 60% - 65% of the feed pressure.

    An electric booster pump may be utilized to boost the feed pressure to approx 80 psi when the feed pressure is too low.

    As I seem to recall, your water source is municipal so your home pressure is likely ~60 psi. With 60 psi feed pressure, the pressure at the RO faucet should be about 36 - 40 psi when the storage tank is full. Increasing the tank precharge will increase the pressure when the tank is almost empty, but the maximum pressure will continue to be 36-40 psi as governed by the RO's Automatic Shutoff Valve.

    A further option to increase the pressure at the RO faucet and also increase system efficiency is to install a Permeate Pump after the RO membrane. A permeate pump will use the energy of the wastewater to pump the permeate (RO water) from the membrane into the pressure tank.

    Since the membrane will not need to overcome back pressure from the storage tank, water production will be higher so the storage tank will fill faster and the system will not need to run as long each day for the same water volume. The quality of RO water will also be higher. Because the membrane will be isolated from the pressure in the storage tank, the system pressure may be increased up to 90-95% of feed pressure.

    Here is an animation showing the operation of a permeate pump: https://www.permeate-pump.com/ERP1000ben.htm
     
    Last edited: Jun 3, 2020
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  4. ditttohead

    ditttohead Water systems designer, R&D

    Joined:
    Jan 9, 2012
    Occupation:
    Water systems designer, R&D, Technical Director
    Location:
    Ontario California
    Adding air pressure to the tank does not increase water pressure, it will only reduce water capacity. Remember science class, physics 101. Everything is in balance... air pressure and water pressure on the bladder tank will always be equal. If you put 20 pounds of water pressure into the tank, the air will be compressed to 20 pounds. A larger tank or an extra tank will give you more pressure longer. A delivery pump will give you consistent pressure until the tank is empty. A permeate pump will put your tanks water pressure to near equal with the incoming line pressure rather than 40-60% of line pressure that is common with a standard RO.
     
  5. Bob from accounting

    Bob from accounting Member

    Joined:
    Mar 5, 2020
    Location:
    California
    Dittohead,
    Can you explain this one thing for me again. I am tracking you and Bannerman on everything you're saying but this one difference.
    I was under the impression that the incoming line pressure / aka house water pressure is 60psi, this is independent of the RO tanks output pressure. (the maximum pressure will continue to be governed by the RO's Automatic Shutoff Valve)
    So to be extreme, if your incoming feed pressure was 100psi or 40psi, my question is if the RO tank is at max capacity in both examples, will the RO tank filling that first 12 ounces of water poured at the faucet, will that water pressure be the same in both examples, meaning that the RO tank bladder and volume solely reflect the end result pressure, or is the incoming feed line pressure reflected at the RO faucet when pouring a first glass of water from the RO tank in both examples.
    This is something I was trying to learn more on, if RO Tank pressure is independent of incoming house line pressure for that first glass of water at max capacity. Thanks man, I need to connect the dots in my head here and get my info straight. It will all come together.

    Dittohead "A permeate pump will put your tanks water pressure to near equal with the incoming line pressure rather than 40-60% of line pressure that is common with a standard RO."



     
  6. ditttohead

    ditttohead Water systems designer, R&D

    Joined:
    Jan 9, 2012
    Occupation:
    Water systems designer, R&D, Technical Director
    Location:
    Ontario California
    Not quite following your question but...

    If you incoming water pressure is 60, the ASV will typically maintain your RO tank pressure between 24 and 36 PSI. The ASV will open and close at about 40/60% of incoming line pressure.

    If your incoming pressure were 100 PSI, the RO tank would be maintained about 40-60 PSI.

    The first glass of water will have these pressures, as the tank depletes the air pressure and water pressure will remain the same. Since air compacts, this expansive force is what pushes out the non compressable water from the tank.

    A permeate pump acts as the ASV (sort of) but has a very tight on/off set point, typically 96-96% so the water pressure in the tank will be much higher (the air inside the tank will be compressed more) and the ro will short cycle. This short cycling is the only rel draw back of the permeate pump.

    Larger tanks will maintain higher pressures longer since their is more air to decompress during water draw.

    I hope I explained this well.
     
    old likes this.
  7. old

    old Member

    Joined:
    Feb 27, 2016
    Location:
    NE
    This is an excellent explanation!
     
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