Filter Press Cloth Specifications: What Micron Rating, Pore Size and Air Permeability Actually Say
Three numbers, three different statements
Filter cloth datasheets carry figures that look at first as though they describe the same thing. They do not.
| Figure | What it is | What it is not |
|---|---|---|
| Nominal rating in micron | a supplier’s classification within a range | a measured or tested cut-off |
| Pore size in micron | a measured property of the fabric from a test method | a statement about the result of the filtration |
| Air permeability | a test value on dry, flat fabric at a defined pressure | the liquid throughput of a working press |
The distinction is not academic. It decides whether two numbers can be compared at all.
The nominal micron rating
When a cloth is listed at 10, 20 or 25 micron, that is as a rule a nominal figure. It places the product inside a range and supports the statement that this cloth is denser than that one.
What it does not support is the statement that particles above that size are retained and particles below it are not. A woven fabric has no single uniform opening but a distribution, and with multifilament and staple fibre yarns that distribution is especially irregular.
Then there is the cake. Within a short time a layer builds on the fabric out of the process medium itself, and that layer separates more finely than the fabric does. Across much of a cycle the cake is doing the filtering, not the cloth. What the fabric mainly decides is how quickly that layer forms and how it comes away again.
That is also why a filter press cloth micron size on its own is a weak basis for a purchase. A search for a micron filter cloth in the abstract turns up figures from very different measurement conventions, and a 50 micron filter cloth from one supplier is not comparable with a 46 micron pore size from another.
The measured pore size
A stated pore size is a different case. It comes out of a test method and therefore carries more weight than a nominal figure.
In the WINKLER range the 67L is the only grade with such a figure: 46 micron pore size. That is precisely why it cannot be lined up with the nominal 10, 20 and 25 micron of the other cloths. They are two different types of statement.
Reading the 46 micron as the coarsest step draws the wrong conclusion twice over: the value is measured differently, and the product belongs to a different process.
Air permeability and its test pressure
Air permeability states how much air passes through a given area of dry fabric in a given time at a given pressure. All three quantities are part of the figure.
The test pressure is where comparisons fail most often. Two values in the same unit are only comparable if they were measured at the same pressure. In this range it looks like this.
| Grade | Figure | Test pressure |
|---|---|---|
| 3L | 3 ± 1 l·dm⁻²·min⁻¹ | not documented |
| 6L | 6 l·dm⁻²·min⁻¹ | 200 Pa |
| 7L | not documented | not documented |
| 15L | 15 ± 6 l·dm⁻²·min⁻¹ | not documented |
| 67L | 111 l(n)·m⁻²·s⁻¹ | 2 mbar |
The 67L carries a different unit but converts: 111 litres per square metre and second come to roughly 67 litres per square decimetre and minute. And because 2 mbar is the same pressure as 200 Pa, that value sits directly alongside the 6L figure. The 67L is around eleven times as permeable as the 6L.
For the 3L and the 15L the test condition is missing from the papers we hold. Their figures of 3 and 15 are plausible against each other but should not be set against the other two without that reservation.
Why filter press cloth permeability does not give you a throughput
Three reasons, in order of weight.
The filter cake. Once a load-bearing layer has formed it sets the resistance. With strongly compressible cakes the fabric’s share of the total resistance is small within a short time.
Air is not liquid. Viscosity, wetting and surface tension act differently in a suspension than in dry air. A fabric that passes air well can behave differently with a particular liquid.
The plant. Pressing pressure, chamber geometry, pump curve and how the cycle is run all help decide the volume flow you actually get.
None of that makes the figure useless. An air permeability filter cloth comparison is the best available yardstick between fabric grades, as long as it is treated for what it is: a material property.
Taking the tolerances seriously
Three grades carry a tolerance on the datasheet, and it is not small. For the 15L, 15 ± 6 means the value may lie anywhere from 9 to 21; between the bottom and top ends there is more than a factor of two. For the 3L, 3 ± 1 runs from 2 to 4, again a factor of two.
For sizing that means two things. Do not calculate with the mean as though it were exact. And do not choose between two grades on a difference that is smaller than the tolerance.
What counts in the end
A number on a datasheet describes a piece of fabric under test conditions. Your process consists of a particular suspension, a particular cake, a particular press and a particular way of running it.
Which is why every honest selection ends on the same sentence: a comparison on the real medium says more than any catalogue value.
