Written by: Gene Fitzgerald // Last Updated: Jul 12, 2023
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Figuring out the correct size for your reverse osmosis flow restrictor can be a bit of a hassle.
With our reverse osmosis flow restrictor calculator, you can quickly enter a few numbers to accurately determine what flow restrictor will work for you, ensuring your RO system is functioning as it should!
This reverse osmosis flow restrictor calculator can give you the ideal flow restrictor size based on the gpd rating of your RO membrane. Enter your RO membrane rating in gpd and “3” for the drain-to-pure water ratio, and check the value. Then, enter the RO membrane gpd and “4” for the drain-to-pure water ratio. Your flow restrictor should fall somewhere between the two values.
Why enter “3” and “4” for the drain-to-pure water ratio? It’s the standard range for under sink reverse osmosis system using a storage tank and no pump. Usually, for every one part of water that gets cleaned, about three to four parts go to waste. Naturally, if you know that your RO system has a higher or lower drain-to-pure ratio, you can adjust the value as needed.
By the way, when people talk about how much water a membrane can clean, they usually speak about gallons per day, short gpd. But when they talk about the size of a flow restrictor, they usually use a different measurement: milliliters per minute (mL/min). To make these two measurements match, you can do a little math. If you divide the gallons per day by .38, you’ll get a rough idea of how many milliliters per minute that equals (our calculator does this automatically, of course).
You’ll see numbers on your flow restrictor like 200, 250, 300, 350, 400, or 500. These numbers tell you how much water the flow restrictor can handle in a minute, measured in milliliters (mL).
Now, your RO membrane also has a number. But this number is in gallons per day (gpd), not milliliters per minute.
Usually, the flow restrictor’s number should be 3-4 times bigger than the RO membrane’s number. So, how do we match them up? We need to convert the membrane’s rating into milliliters per minute. Here’s how:
Divide it by 0.38: 100 gpd / 0.38 = 263.16 ml/min
Now multiply that number by your pure-to-drain water ratio, in our example it’s “3”: 263.16 mL/min x 3 = 789.47 mL/min
So, if your RO membrane’s number is around 100 gpd and your pure-to-drain ratio is 3:1, you should use an 800 mL flow restrictor.
Here is a sizing chart that allows you to skip the math:
| Membrane Rating in gpd | Membrane Rating in mL/min | Smallest Flow Restrictor Size | Largest Flow Restrictor Size |
|---|---|---|---|
| 15 | 39.5 | 118.4 | 157.9 |
| 18 | 47.4 | 142.1 | 189.5 |
| 25 | 65.8 | 197.4 | 263.2 |
| 36 | 94.7 | 284.2 | 378.9 |
| 50 | 131.6 | 394.7 | 526.3 |
| 75 | 197.4 | 592.1 | 789.5 |
| 100 | 263.2 | 789.5 | 1052.6 |
| 150 | 394.7 | 1184.2 | 1578.9 |
Oversizing involves opting for a component, such as a flow restrictor or membrane, that surpasses the actual requirements of your system. Although it may seem a good idea to have excess capacity, oversizing can result in inefficiencies, energy wastage, and potential long-term damage. For instance, an oversized RO flow restrictor may fail to generate sufficient backpressure for the proper functioning of the membrane, thereby compromising the quality of filtered water.
Undersizing refers to selecting a component that is smaller or possesses a lower capacity than required. This can result in the component becoming strained, diminishing its lifespan, and potentially leading to system failure. For instance, if a reverse osmosis system flow restrictor is undersized, it may create excessive backpressure that can harm the membrane and reduce the system’s water processing capabilities.
A flow restrictor is a device that controls the water flow rate through an outlet. It’s crucial for systems requiring consistent water volumes like water filters, irrigation systems, and tankless water heaters, helping these appliances operate optimally and preventing damage from excessive flow. Flow restrictors function by adjusting a two-port valve to maintain a steady water flow, irrespective of water pressure changes.
In RO systems, flow restrictors limit wastewater production, maintain membrane pressure, and prevent the system from generating excessive wastewater. By installing a flow restrictor, significant water savings can be achieved.
If you have any questions about our RO flow restrictor calculator please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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When sizing for residential ro membranes you also have to take into account the manufacturer’s test pressure,800ml/m restrictor is a 3:1 ratio for a dow filmtech 50,it would be a 4:1 for a old cta.
ReplyCorrection to previous post a 800ml/m would be a 2.45:1 for old 100gpd ctas,1.8:1 for newer tfms(filmtech).too lean.
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