How to Design an RO / NF System

Step 5 - Calculate the Number of Elements Needed

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

Calculate the Number of Elements Needed

Total number of elements needed = (design permeate flow rate) / (design flux) / (active membrane surface area of selected element)

Tip: For 8-inch elements, model number indicates active membrane surface area. (e.g. FILMTEC™ BW30-400 element has 400 ft2 of active membrane surface area.

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