Filter Press Cloth Types: Choosing a Fabric by Process Goal
The selection does not start at the micron figure
The most frequent enquiry is a single sentence: we need a finer cloth. The second question ought to be: what for? A finer fabric solves exactly one problem, and it often creates another one on the way.
So how to choose a filter press cloth starts with a goal rather than with a comparison of numbers.
| What is not working today | Direction | Grade |
|---|---|---|
| Filtrate stays cloudy, fine solids reach downstream equipment | denser | 3L |
| Filtrate quality falls just short, but the cycle must not get much longer | slightly denser | 6L |
| Process is stable, this is a replacement or a cost baseline | unchanged | 7L |
| Throughput is the bottleneck or the cake sticks | more open | 15L |
| Fine separation is to run through a built-up layer | different process | 67L |
Filter press cloth material: why polypropylene, and what that leaves open
All five grades in this range are polypropylene. PP is dimensionally stable, does not take up water and handles a wide band of aqueous and industrial media. In a chamber filter press that covers most of what actually runs.
It does not settle the material question on its own. Concentration, temperature, pH and any solvents present decide together, and only one grade here carries a documented temperature limit: the 6L at 90 °C. We do not transfer that figure to the other four.
Which means the real material question in this product group is not the polymer. It is the yarn.
Monofilament, multifilament or hybrid
The yarn type is the actual construction difference, and it explains more about how a cloth behaves than any micron figure.
A monofilament filter cloth is woven from smooth single strands. They can be woven into defined structures, and because there are no spaces inside the thread itself, they store less solid material permanently. A monofilament filter press cloth usually cleans down better and holds its permeability longer. At the same thread count it is the more open fabric.
A multifilament filter press cloth uses yarns made of many fine filaments bundled together. Staple fibre yarns of short fibres work in a similar way. Both fill the spaces in the weave more completely and hold back finer material. In exchange they are more inclined to store solids between the filaments.
A mono multifilament filter cloth combines the two, for instance monofilament in the warp and multifilament in the weft. That joins a defined base grid to a more closed surface.
| Grade | Yarn construction | Character |
|---|---|---|
| 3L | Multifilament and staple fibre | densest structure, highest retention, highest resistance |
| 6L | Monofilament warp, multifilament weft | tightest thread grid, fine intermediate step |
| 7L | Multifilament | standard role, no further values documented |
| 15L | Monofilament and multifilament | lightest and most open of the direct cloths |
| 67L | Monofilament | support fabric for precoat layers |
One detail illustrates the whole selection. The 6L has more than twice as many threads per centimetre as the 3L and is still the more open cloth. The 3L reaches its fineness through the fibre, the 6L through the grid. Compare only the micron figure and you never see that difference.
Direct cake filtration or precoat filtration
This decision comes before the fabric choice, because it concerns the process.
In direct cake filtration the fabric holds solids back first. Within a short time a filter cake forms from the process medium and takes over a growing share of the separation. That is why filtrate at the start of a cycle often looks different from filtrate later. The 3L, 6L, 7L and 15L are made for this.
In precoat filtration a filter aid is built into a layer before or during the process. That layer does the fine filtration; the fabric carries it and should slow it as little as possible. The 67L is made for this.
Treating the 67L as a fifth, coarsest fineness step is the most common selection error in this product group. Without a precoat layer it behaves like a very open fabric.
The six pieces of information that make a selection reliable
When you write to us, these help more than any micron specification.
Medium and solid. What is being filtered, which solid, at what concentration.
Particle distribution. Even, high fine fraction, flocculating or sludgy.
Process data. Temperature, pH, pressing pressure, any solvents or additives.
Target. Filtrate quality, throughput, cake residual moisture or service life.
Current problem. What is not working today, and since when.
Plant. Plate size, position in the pack, inlet geometry and the cloth in use now.
A comparison under real plant conditions is almost always more informative than a laboratory figure. If you are torn between two grades, a set of cloths for a trial is the most practical route.
Four selection mistakes worth avoiding
Finer is better. A denser cloth raises resistance. If throughput or residual moisture is critical, it only moves the problem.
Two cloths with the same micron figure are interchangeable. The 7L and the 15L both carry 25 micron nominal and behave differently in service.
The air permeability figure predicts throughput. It describes dry fabric. In operation the filter cake sets the resistance.
A new cloth fixes a process problem. If the cake sticks every cycle or the cloth blinds quickly, the cause is often upstream of the fabric.
