Spiral-wound thin-film composite elements separate dissolved salts from water under pressure. Feed flows along the membrane surface, purified permeate passes through and spirals to the central collection tube, and the rejected salts leave as concentrate. Everything on a data sheet follows from that, including why two elements are only comparable at the same test conditions.
Permeate flow and rejection are test-condition figures, not guarantees
DuPont measures every element on a defined standard solution and prints those conditions on the sheet — for a brackish element typically 2,000 ppm NaCl, 225 psi, 77°F, pH 8, 15% recovery, and for a seawater element 32,000 ppm NaCl, 800 psi, 77°F, 8% recovery. Change the salinity, pressure, temperature or recovery and the delivered flow and rejection change with them. Size a system with projection software using your own water analysis; the headline number is a comparison basis, not a promise about your plant.
Stabilized versus minimum rejection
The stabilized value is what the element is expected to deliver once it has lined out. The minimum is the guaranteed floor for an individual element. Individual permeate flow rates vary around nominal — DuPont states the tolerance on each sheet, commonly no more than 15% below the stated value.
Free chlorine destroys polyamide membrane
Thin-film composite polyamide is oxidised by free chlorine. DuPont's tolerance is typically < 0.1 ppm. Dechlorinate ahead of the membrane and monitor continuously; a single excursion can cost the element bank. Chloramine is not a safe substitute without checking the specific element's guidance.
Element sizes and what the numbers mean
An element code carries its size. A "4040" is 4 in diameter x 40 in long; a "-400" or "-440" is the active area in ft² on an 8 in element. Active area drives flow at a given flux, so a 440 ft² element in the same vessel produces more permeate than a 400 ft² element at equal flux — and fouls faster if the system was designed around the smaller area.
iLEC and interlocking end caps
Some models use interlocking end caps that connect elements directly instead of via a separate interconnector. They are not interchangeable with standard adapters, and they change the stacked length. Check the endcap arrangement against your vessel before ordering.