Precoat Filter Cloth 67L: the monofilament support fabric, not a fifth fineness step

Why this cloth does not belong on the micron ladder

Four cloths in this range are placed by their nominal rating: 10, 20, 25 and 25 micron again. It looks obvious to hang the 67L on the end of that list as the coarsest step. That is exactly what has stood in the comparison table for years, and it is the most important technical error in this product group.

The 67L is the one filter press cloth in this range that does not belong on that scale at all, because it works in a different process. In filter press precoat filtration the fabric is not meant to do the fine separation. A layer of filter aid, built up before or during the process, does that. The cloth provides the surface for that layer and should get in the way as little as possible.

A precoat filter press cloth is therefore chosen against different criteria than a cloth for direct cake filtration. Open enough for throughput, smooth enough for an even layer, fine enough for the filter aid to key onto during build-up.

What 46 micron pore size means

For the 67L a pore size of 46 micron is documented. That is a different kind of figure from the nominal 10, 20 and 25 micron of the other cloths: a measured property of the fabric determined by a test method, rather than a classification inside a range.

Two things follow for the selection. First, a 46 micron filter press cloth is not simply a coarser version of the 25 micron grades, because the number carries more weight than a nominal rating. Second, it still does not describe what ends up being retained, because the precoat layer does that job. What the 46 micron say is roughly this: the fabric is open enough not to slow the process much, and closed enough for a common filter aid to form a layer on it.

The most open fabric in the range, and why it is also the lightest

At 275 g/m² the 67L is the lightest cloth in the family and at 380 µm the thinnest. Both follow from the construction: pure monofilament, smooth single strands without the many fine filaments of a multifilament yarn.

The documented air permeability is 111 l(n)·m⁻²·s⁻¹ at 2 mbar. That unit does not look comparable at first, but it 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 the 200 Pa at which the 6L is tested, the two figures can be set side by side. The 67L is around eleven times as permeable as the 6L.

A monofilament filter cloth has one further practical property. Smooth single strands store less solid material between them than a fibrous yarn. That makes cleaning down between cycles easier, and in a process where the layer is rebuilt regularly that goes straight into running time.

Filter aid belongs to the process, not to the delivery

Different filter aids are used to build the precoat layer. Diatomaceous earth, cellulose and perlite based products are common, often in combination.

WINKLER supplies the support fabric. Filter aid is not part of the delivery and we do not select it. In that respect a cloth for diatomaceous earth and a cloth for a cellulose filter aid are the same thing: the same support fabric. Which aid suits you follows from your suspension, your target quality and your own operating rules.

One point for the planning stage. Handling powdered filter aids carries its own occupational safety requirements depending on the product. That assessment is made by the operator on the basis of the relevant safety data sheet.

What happens if the 67L is run without a precoat layer

It happens, and it almost always ends in disappointment. Without a layer the 67L behaves like a very open fabric: high throughput, but noticeably less retention of fine solids than any of the direct grades.

If you want an open cloth for direct cake filtration, the 15L is the one. If you are building a precoat layer, the 67L is right.

Technical data for the 67L precoat filter cloth

Common questions about the precoat filter cloth

The questions here are about the process, the role of the fabric and what is in the box.

What is precoat filtration?

A process in which a filter aid is built into a layer on a support fabric. That layer does the actual fine filtration. It can be laid down before the process or dosed continuously during it.

What is the role of the filter cloth?

It carries the precoat layer, holds it in place and should create as little resistance as possible itself. Fine retention happens in the layer, not in the fabric.

What do the 46 micron pore size mean?

A measured property of the fabric, not the cut-off of the process. It describes the support surface and not the result of the filtration.

Is filter aid included in the delivery?

No. We supply the support fabric. Diatomaceous earth, cellulose based products and perlite come through your existing suppliers. Handling powdered aids follows their own safety requirements.

Can I run the 67L without a precoat layer?

Technically yes, sensibly in most cases no. Without a layer, retention of fine solids is much lower than with the direct grades. For open direct filtration the 15L is the right choice.

Why is the permeability given in a different unit from the other cloths?

Because the datasheet states it that way. Converted, 111 l(n)·m⁻²·s⁻¹ come to roughly 67 l·dm⁻²·min⁻¹. Since the test pressure of 2 mbar is the same as the 200 Pa used for the 6L, those two values are directly comparable.

Are head and end cloths made from the 67L fabric?

Yes, as from all five grades. Special contours are laser cut where needed.