What is an RO Flow Restrictor and When to Change?
What is Flow Restrictor?
A flow restrictor, as the name suggests, restricts the flow of RO reject water.
The flow restrictor maintains high pressure inside the RO membrane (a precondition for RO purification) by creating back pressure on the membrane and also controls the recovery ratio of the purifier.
If the flow restrictor is not used then the high-pressure water coming in from the booster pump will flow unrestricted from the reject outlet of the RO membrane.
This will result in low pressure inside the RO membrane and higher wastage of water. RO membrane won’t be able to work if the pressure inside the RO membrane is not high.
See the below water flow diagram of an RO water purifier to understand the working of a flow restrictor.
Flow restrictors are generally available in different flow ratings and must be properly sized according to the capacity of the RO membrane.
How to Find a Matching Flow Restrictor?
The ratings that you see on flow restrictors, like FR 200, FR 300, FR 450, indicate the flow rate, in mL per minute (Millilitres Per Minute).
RO membrane capacity is given in GPD (Gallons Per Day).
1 Gallon is approximately equal to 3.78 litres.
Generally, the flow restrictor value should be 4 times the production rating or the capacity of the membrane.
In order to find a matching flow restrictor, first, we need to convert the flow restrictor’s capacity value into GPD.
This is quite simple, and all you need to do is simply multiply the flow restrictor capacity in mL per minute by 0.38 to convert it into GPD.
Example of Flow Restrictor for 800 mL is:
800 x 0.38 = 304 GPD
As we know the flow restrictor rating should be 4 times the RO membrane.
Dividing the above rating by 4 i.e. 304/4, we get 76 GPD. So for an RO membrane of 75-80 GPD, you should use an 800 mL flow restrictor.
In India, most technicians are using Flow Restrictor 450 with 75 GPD or 80 GPD RO membrane. But, we recommend a flow restrictor of at least 550 ml.
The recommended flow restrictor sizes are listed in the below table.
| Membrane Size (in GPD) | Flow Restrictor (mL per minute) |
| 18 | 200 |
| 25 | 250 |
| 36 | 250 or 360 |
| 50 | 420 or 525 |
| 75 | 550 to 800 |
| 80 | 800 |
| 100 | 1000 |
If you are looking to replace the flow restrictor of your water purifier then you can check on Amazon.in.
What if I use a Flow Restrictor of a Higher or Lower Capacity?
The RO membrane GPD and flow restrictor capacity must be matched to keep a proper balance between water wastage and purification quality. If they are not matched, this can cause excessive wastage of water or may reduce the life of the RO membrane.
Under Sizing
If you are using a lower capacity flow restrictor for a higher capacity membrane then it will result in early choking of the RO membrane, reduced membrane life, and an increase in the TDS value of the purified water.
Over-Sizing
And if you use a higher capacity flow restrictor then it will result in higher wastage of water and lower pressure inside the RO membrane. Low pressure inside an RO membrane can have an adverse effect on the quality of purification.
When to Change the Flow Restrictor?
We recommend that the flow restrictor should be replaced every time an RO membrane is replaced. Because over time, like RO membrane and filters, flow restrictors can also get choked because of scale build-up.
And if the new RO membrane is of a different capacity then also the flow restrictor must be changed according to the capacity of the RO membrane.
“As we know the flow restrictor rating should be 4 times the RO membrane.”
We know, but on what is this 4 based? Why 4?
I am confused because an engineer told me 75% recovery for industrial, and 50% recovery for residential are typical. But this 4 factor makes that 4X the concentrate!
Eg., I have a 50 GPD membrane, and for 6 years, I have had a 400 mL/min concentrate flow restrictor. I noticed this time, buying filters and things, that a restrictor of 550 mL/min was recommended.
So I started asking questions, and this engineer told me I should want 50% recovery to avoid membrane scaling and biological fouling.
Okay, so 50 GPD of concentrate X 2.63 = 132 mL/min. Four times that is about 550 mL/min.
So what are we saying here – that to get a concentrate flow of 132 mL/min, I need a restrictor rated at 4 times that? Or, are we saying my engineer is just wrong?
What’s the 4??
Please follow the table given above for the correct sizing of the flow restrictor.
Hello, I want to ask something, does flow 300 produce less waste water or does flow 600?
The sizing of the flow restrictor depends on the capacity of the RO Membrane. But generally speaking, flow restrictor 600 will produce more wastewater as compared to flow restrictor 300. Please read the concept of over-sizing and under-sizing given above.
Make sure you flush your ro filters out before installing them , if it is a granulated carbon filter, the fines can clog the aso valve and the restrictor, flushing may be difficult because the filter may be encapsulated. The manufacture uses air to blow out fines but sometimes doesn’t do a good job.
Comment for Raghavan, flow restrictor fr1000 is a 3.6:1.4 ratio for a 100gpd membrane, a nominal size, you can use 1052 also, it is a 4:1 ratio restrictor.800 is OK also, it is a 4.1:0.9 ratio. It’s probably better to use the 800 ml/m restrictors richer restriction and membrane will last longer.
Restrictor size is an abstract number unless you know or assign a membrane test pressure to it. It would be like a house without an address. Membrane test pressures are 50,60, and 65. The restrictor size is calibrated to the test pressure of the membrane. For instance, if you have a 50 gpd membrane with a test pressure of 65, you would divide your water pressure by the test pressure times the restriction value of the gpd rating of the membrane, for Example (my water pressure) 60 divided by 65 (test pressure) times 500 (3.6 ratio for a 50 god membrane) equals 600 ml/m, this is your calibration figure for a 50 god membrane with 65 psi test pressure. you can make your own restrictor, with a needle valve by knowing this information.
Sir, read your full chart. You said 100GPD RO matched flow restrictor is FR800.
But as per technical calculation = 100 GPD RO is matched with FR1052. In market FR1000 is available. May I know why you did not recommended FR1000 for 100 GPD to?
Thank you for writing to us. You are right. Earlier FR1000 flow restrictor was not easily available in the market. We have now updated the table.
What if the pump pressure increased from 100 to 150 psi
Sir
Can we use 36v adapter or SMPS for a 24 v 1.4A ro pump. The technican has fitted such adapter.
If flow resistor get removed then will the ro tank fill or not? Plz answer
What the flow arrow indicates .
What will be the arrow head position
The flow arrows indicate the direction of water flow.
Very useful and good info, thanks.
My RO just stopped sending purified water out.
I found that the wastewater just kept coming out and realized it could be restrictor not building pressure so I closed the waste line fully choking it and water came out of the membrane. I am not sure if Restrictors can also fail to leak continuously though as per the details here it might fail by chocking itself with scales in water. Can there be other problems like a membrane or pump pressure issues here?