Sizing Flow Restrictors


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Every reverse osmosis unit has a device called a flow restrictor that determines the amount of water that flows to drain. In general, the flow restrictor must be sized to fit the membrane production size. The flow to drain is usually about four times the membrane production rating, although the drain flow ratio often decreases a bit as membranes get larger.

Part of the difficulty in understanding this ratio is that membrane production is usually given in gallons per day (gpd), while flow restrictors are usually sized in milliliters per minute (mlm). To convert, multiply mlm by .38 and the result, roughly, will be gallons per day.

For example, the standard flow restrictor used for an 18 gallon per day membrane is a 200 mlm restrictor.

200 times .38 is 76. Thus, an 18 gpd membrane needs about a 70 gallon per day flow restrictor. 4 times 18 = 72. That's about the normal ratio.

The list below represents a rough effort to match membranes and flow restrictors. If your water is very clean and has low TDS (total dissolved solids), especially low hardness, you might run your unit a little leaner (with a smaller sized flow restrictor) than indicated. But remember — saving water can be false economy, because if you run the unit too lean both product water quality and membrane life will be adversely affected.

The chart below covers our inline capillary flow restrictors.

Membrane Size Use Flow Restrictor
18 gpd 200
25 gpd 250
36 gpd 250 or 360
50 gpd 420 or 525
75 gpd 525 or 800
100 gpd 800

These are inline capillary systems that install into the drain tube itself. Just clip the 1/4" drain tube and push the two ends of the tube into the flow restrictor.

Insert Style Flow Restrictors

Many RO units have small insert style flow restrictors that are inserted into the 1/8" X 1/4" elbow that connects the drain tube to the membrane vessel. If you install a new capillary inline flow restrictor, you should remove the original insert restrictor.

If you're replacing your RO membrane, you can use any of the membrane sizes above in a standard membrane housing. Just make sure you change the flow restrictor as well so that it will match the membrane.

If you're replacing an existing insert style restrictor, choose one that matches your membrane production, AND your fitting style. If the tube pushes into the drain fitting on your membrane housing, use a JOHN GUEST style flow restrictor; if the tube attaches with a plastic nut, choose a JACO style flow restrictor.

For size, please refer to this sizing chart:

Membrane Size Use Flow Restrictor
10 or 12 gpd 50 Gal.
18 gpd 70 Gal.
25 gpd 100 Gal.
36 gpd 100 or 125 Gal.
50 gpd 150 or 200 Gal.
75 gpd Use A Capillary Style Restrictor
100 gpd Use A Capillary Style Restrictor

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