What Is a String Wound Cartridge Filter?
A string wound cartridge is yarn wound around a core in a graded pattern, loose on the outside and tight toward the core. That structure, and the four components behind it, decide almost everything about how it performs.
How a string wound cartridge is actually built
Continuous yarn, commonly polypropylene, cotton, glass fibre or polyester, is wound around a central core in a diamond or honeycomb pattern on precision-controlled winding equipment. The winding is deliberately graded: looser on the outside, tighter toward the core, so larger particles are captured first at the surface and progressively finer particles are trapped deeper in the structure as the liquid works its way through. That gradient is what gives a string wound cartridge its dirt-holding capacity, and it is why the format is chosen for streams with a heavy or variable solids load rather than for low-contamination polishing duty.
The four components you're actually specifying
The four components of a string wound cartridge
| Component | Common options | What it decides |
|---|---|---|
| Core | Polypropylene, gas-filled polypropylene, stainless steel, tinned steel | Structural strength under pressure and chemical or temperature compatibility |
| Yarn | Cotton, polypropylene, glass fibre, nylon, rayon, polyester | Micron rating, chemical resistance, and temperature range |
| End cap | Matched to housing type | Fit and seal against the housing, no bypass path |
| Gasket | Matched to fluid chemistry | Seal integrity at the knife-edge, the main defence against bypass |
The core carries the structural load under pressure and sets chemical and temperature compatibility: stainless steel and tinned steel suit high-pressure or high-temperature duty, while polypropylene suits general chemical resistance at lower cost. The yarn sets the micron rating and dirt-holding capacity, and its material is chosen for the fluid: polypropylene for general-purpose chemical resistance, glass fibre for high temperature, cotton for potable water and food-grade duty. The end cap and gasket matter as much as the yarn: most cartridge housings use a knife-edge seal, and a gasket that does not seal correctly against that edge lets liquid bypass the filter media entirely, which defeats the filter no matter how good the yarn is.
Micron ratings, and what nominal versus absolute means here
A string wound cartridge is commonly available from about 1 to 100 microns, set by yarn diameter and winding tension. A nominal rating on this media typically retains 50 to 90 percent of contaminants at the stated size, so a nominal 25 micron cartridge quoted at 50 percent efficiency lets roughly half the particles at that size through. An absolute rating is built to retain more than 98.7 percent, which is the number to specify when the downstream process, a membrane, a reactor, a sterile step, cannot tolerate a meaningful bypass fraction.
To put those numbers in scale: a rating around 50 microns is in the range of a human hair's width, 8 microns approaches the size of a red blood cell, and 2 microns starts to reach bacteria-sized particulate. The right rating is the one that matches what your process actually needs removed, not the smallest number on the shelf.
What actually goes wrong, and why
The two failure modes that account for most premature string wound cartridge problems are tunneling and bypass, and both trace back to manufacturing control rather than the format itself. Tunneling happens when yarn that is too smooth or wound under inconsistent tension shifts and rolls under pressure, opening gaps in the weave and letting unfiltered liquid through at a rating the cartridge was never actually holding. Chemical leaching happens when resin binders, lubricants or antistatic agents used in low-grade yarn processing migrate into the filtrate; cartridges free of these agents avoid the problem entirely. Media migration, loose fibres shedding into the filtrate, comes from short-chopped or poorly bonded yarn rather than from the string wound format itself. Correct winding tension, continuous-filament yarn, and a gasket matched to the housing prevent all three.
Where string wound cartridges fit
String wound cartridges are used across water treatment, chemical processing, oil and gas, food and beverage, pharmaceuticals, and electroplating, wherever the stream carries dilute acids or alkalis, organic solvents, petroleum oils, or general sediment at a load that would blind a finer surface filter quickly. They are a pre-filtration stage more often than a final polishing stage, protecting the finer, more expensive element behind them.
Direct answers
Is a string wound filter a depth or a surface filter?
It is a depth filter. The wound yarn is looser on the outside and tighter toward the core, so larger particles are caught first and progressively finer particles are trapped deeper in the structure. That graded structure is what gives a string wound cartridge its high dirt-holding capacity compared to a surface-only filter.
How does a string wound cartridge compare with melt blown and pleated filters?
String wound and melt blown cartridges are both depth filters. String wound generally handles heavier sediment loads and a wider range of core and yarn materials for chemical and temperature compatibility; melt blown offers a more finely graded density in a single fused structure. A pleated cartridge is a surface filter, chosen when the job needs an absolute rating and a longer service life on lower-solids streams rather than raw dirt-holding capacity.
What micron ratings are available in a string wound cartridge?
Commonly from about 1 to 100 microns, depending on winding tension and yarn diameter. A nominal rating typically retains 50 to 90 percent of contaminants at the stated size; an absolute rating is built to retain more than 98.7 percent, which matters when the downstream process cannot tolerate a bypass fraction.