How to Design an RO / NF System

Step 8 - Select the Staging Ratio (Array Ratio)

Step 1: Consider Feed Source, Feed Quality, Feed / Product Flow, and Required Product Quality
Step 2: Select the Flow Configuration and Number of Passes
Step 3: Select Membrane and Element Type
Step 4: Select Average Membrane Flux (Design Flux)
Step 5: Calculate the Number of Elements Needed
Step 6: Calculate the Number of Pressure Vessels Needed
Step 7: Select the Number of Stages
Step 8: Select the Staging Ratio (Array Ratio)
Step 9: Balance the Permeate Flow Rate
Step 10: Example

Select the Staging Ratio (Array Ratio)

For a system with four vessels in the first stage and two vessels in the second stage the staging ratio is 2:1. A three-stage system with four, three, and two vessels in the first, second, and third stage respectively has a staging ratio of 4:3:2. In brackish water systems, staging ratios between two subsequent stages are usually close to 2:1 for 6-element vessels and less than that for shorter vessels. In two-stage seawater systems with 6-element vessels, the typical staging ratio is 3:2.

Another aspect for selecting a certain arrangement of vessels is the feed flow rate for vessel of the first stage and the concentrate flow rate per vessel of the last stage. Both feed and concentrate flow rate for the system are given (from permeate flow rate and recovery). The number of vessels in the first stage should then be selected to provide a feed flow rate in the range of 35 - 55 gpm (8 - 12 m3h) per 8-inch vessel. Likewise, the number of vessels in the last stage should be selected such that the resultant concentrate flow rate is greater than the minimum of 16 gpm (3.6 m3/h). Flow rate guidelines for different elements are given in the Membrane System Design Guidelines (126KB PDF).

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