RO / DI Flow Restrictor Question

 

I just ordered, received and installed new pre-filters, new membrane and new DI cartridge on my RO unit.  I ordered a 50 gpd, but received a 75 gpd membrane.  The flow restrictor I currently have is 800 ml.  Will this cause any problems?   I know that if the flow restrictor is too small or too large you can prematurely foul the membrane.

 

THANKS PEOPLE!

 

The Flow Restrictor has two functions:
First, to create pressure throughout the membrane element.
Second and more importantly, to maintain a desired ratio of reject flow (to drain) and permeate (product) flow.

Residential membranes require a reject flow rate of at least 3-6 times the product flow rate.  Normally this ratio is maintained at approximately 3 to 4:1.  Therefore, a membrane rated at 10 gallons per 24 hour day should be sized with a flow restrictor of at least 50 gallons per day.

Under-sizing the flow restrictor in order to run the system at higher recoveries results in premature scaling of the membrane, and shorter membrane life, and/or an increase in higher-TDS product water.  Additionally, scaling of the flow restrictor will alter the flow rate to drain, and decrease the ratio, causing the above problems.  To avoid this, flow restrictors should be changed regularly.
There are two types of flow restrictors:
Encapsulated Capillary Type.  Comes in Male, and Quick Connect Ends
catalogridge Type.  Fits into an elbow fitting then screws into the brine side of the pressure vessel. 
 
The best flow restrictor to use is the capillary type, as it is less likely to scale as quickly as the catalogridge type.  The flow restrictor should be changed every time the membrane is changed.

 Flow Restrictor Sizing
 
Flow restrictors are rated by a flow rate expressed as milliliters/minute (ml/min).  The following table gives a guideline for selecting a flow restrictor.

 

Membrane Product Output (GPD)	Restrictor Flow (ml/min)
	5:1 Ratio	3:1 Ratio
7	100	60
10	150	90
16	200	120
18	250	150
24	300	180
36	420	252
50	550	330
75	850	510
100	950	570

A standard flow restrictor that closely meets the calculated brine flow should be selected.  The poorer the feed water quality, the higher the reject to product ratio should be.  Lowering the ratio below the recommended level will result in faster fouling of the membrane and shorter membrane life.  In addition to this, when designing a system, check to ensure that the desired reject to product ratio can be met at the operating pressure the system will be running at.

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