String Wound vs Melt Blown: Choosing the Right Depth Cartridge
String wound and melt blown are both depth cartridges, but they load differently under the same dirty stream. That difference, not brand preference, is what should decide between them.
Two structures, two different paths to the filtrate
A string wound cartridge is yarn of a controlled diameter, wound over a support core in a diamond or "v" pattern that forces the fluid down a long, zigzag path through the media, which is what gives it true depth penetration. A melt blown cartridge is fibre of a controlled size, blown from a nozzle and thermally bonded on a rotating spindle into a dense, uniformly porous structure with no support core needed. The melt blown path to the filtrate is more direct, which sounds like an advantage until you look at where the dirt actually ends up.
Why the same micron rating does not mean the same dirt-holding capacity
String wound compared with melt blown, both in polypropylene media
| Criteria | String wound | Melt blown |
|---|---|---|
| Structure | Yarn wound over a support core in a diamond pattern, a zigzag path | Fibres thermally bonded on a spindle into a dense porous mass, no core needed |
| Path to the filtrate | Long, torturous path, more filtration points along the way | A more direct path through a uniformly porous structure |
| Dirt-holding capacity | Higher: particles penetrate deep into a true depth structure | Lower: particles tend to load the outer layers first, leaving inner layers underused |
| Typical rating basis | Nominal, around 95 percent efficiency | Nominal, around 97 percent efficiency |
| Micron range | About 0.5 to 150 micron | About 1 to 20 micron |
| Temperature | Up to 350C in glass fibre media | Typically up to about 120C |
| Best fit | High solids load, pre-filtration ahead of a costlier fine filter | Lower solids load, end-point or final polishing filtration |
Melt blown media tends to load its outer layers first, leaving the inner layers of the structure underused; string wound's diamond-wound path lets particles penetrate genuinely deep into the media, distributing the load through more of the structure. That is why string wound typically holds more dirt at a comparable rating, and why it is the usual choice ahead of a finer, costlier stage rather than instead of it. Melt blown's advantage shows up elsewhere: a more uniform pore structure at the finer end of its range, and a lower unit cost at that fine rating than an equivalently fine string wound cartridge would carry.
Temperature and chemistry set the outer limits
Standard polypropylene versions of both cartridges behave similarly in moderate chemistry, but the media options diverge at the extremes. Glass fibre string wound cartridges tolerate heat up to roughly 350 degrees Celsius, well beyond melt blown's typical ceiling of around 120 degrees, which makes string wound the default choice for hot process streams regardless of the dirt-holding question. Both are available in polypropylene as the general-purpose option, with string wound adding cotton and glass fibre, and melt blown adding nylon, for chemistry the base polymer cannot handle.
Direct answers
Is melt blown always the finer filter and string wound always the coarser one?
Not by rule. String wound media commonly spans a wider range, from about 0.5 to 150 microns, so a fine string wound cartridge can outperform a coarse melt blown one on the micron number alone. The more reliable distinction is dirt-holding capacity: string wound's true depth structure holds more solids before loading out than melt blown's tendency to load its outer layers first.
Which should I put first if I need to stage two cartridges?
String wound ahead of melt blown, in most cases. Its higher dirt-holding capacity suits it to taking the bulk solids load, protecting the finer, lower-capacity melt blown or pleated stage behind it from loading out prematurely.