Use channel flow area in the calculation
For a multichannel module, add the internal flow area of all channels receiving the measured flow. Identical circular channels have total area = channel count × π × diameter²/4. Use manufacturer-supplied geometry and hydraulic dimensions when channels are non-circular or their distribution requires another approach.
In an ideal example, 100 channels of 4 mm diameter provide approximately 0.001257 m². A loop flow of 6 m³/h equals 0.001667 m³/s and gives a mean velocity of 1.33 m/s. This assumes uniform distribution and open channels and does not recommend that velocity. Membrane surface area is not channel flow area.
Separate loop circulation from new feed
High pump circulation is neither the same permeate production nor the same fresh-water consumption. Define external inputs and outputs, flow per module and the series or parallel arrangement. When permeate is extracted along the module, axial flow can change between inlet and concentrate outlet.
Record inlet-to-outlet pressure loss separately from mean TMP. A velocity calculated using the original channel count does not establish actual distribution if channels become blocked. Check flow-meter location, especially where bypasses, return branches and control valves alter which flow actually passes through the membrane channels.
Measure energy and filtration together
Compare measured electrical power, specific flux, quality and cleaning frequency with the same water and concentration. Basic liquid power is flow multiplied by pressure loss using consistent SI units. Electrical input additionally depends on the actual efficiencies and operating conditions of the pump and motor.
Doubling velocity does not imply twice the useful flux. Flux may approach a plateau while losses continue increasing. Conversely, very low velocity may increase downtime and cleaning. Trial changes within the approved range over repeated cycles rather than relying only on measurements immediately after a membrane clean.
Select a sustainable operating point
Include the highest campaign concentration and relevant temperatures because viscosity changes circulation, losses and permeate production. Request curves or pilot results linking these variables to a defined product quality. Where reverse pulsing is permitted, measure its consumption and operating effect separately from continuous circulation.
The quotation should state pump and module limits, instrumentation and net-output acceptance. Compare cost per useful cubic metre, retentate management and cleaning capability. Provide geometry, pressures, circulation, analysis and the intended concentration target so that a suitable sustainable-flux trial can be defined for your stream.
Common questions
Should circulation be divided by membrane surface area?
No. Axial velocity uses hydraulic channel cross-section. Membrane surface area is used for specific permeate flux.
Does higher velocity always produce more water?
No. It may mainly increase pressure loss and energy. Measure net performance over the intended operating campaign.