Understanding filtration › 6. Operating and purchasing

CHAPTER 6.4

Reading a filter cloth specification

“Polypropylene cloth for a 1,200 filter” is not enough to buy a filter cloth. A complete specification avoids cloths that do not fit, distorted comparisons between suppliers and disputes on receipt.

Nicolas Ract · 8 min read · October 2026

What a specification must contain

1. The filter and the drawing

• the type of filter and plates: recessed chamber, membrane, gasketed or non-gasketed plates (see chapters 2.1 and 2.2);
• the plate size and the number of cloths, head and tail plates included;
• a dimensioned drawing of the cloth: outer dimensions, position and diameter of the feed hole and corner holes, cut-outs, tolerances;
• the drawing revision index, so that an old drawing is not reused by mistake.

The sealing method matters: according to a manufacturer's documentation, non-gasketed plates use cloths with eyelets fitted over pins, the seal being provided by the cloth itself (with slight weeping), whereas cloths for gasketed plates carry a bead sewn around the edge that sits in a groove in the plate [1].

2. The fabric

• The material: polypropylene, polyamide, polyester or other, chosen for its chemical and thermal resistance in the process (see chapter 3.1) [4].
• The yarn type and the weave: monofilament, multifilament, spun yarn; plain, twill, satin; or felt (see chapter 3.2).
• The mass per unit area and the thickness, with their tolerances.
• The air permeability, with the test standard and the pressure (see below and chapter 3.3).
• The finish: calendering, heat-setting, surface treatment, and the side concerned (cake side or plate side).

3. The make-up

• the cloth type: single cloth, double cloth joined by a neck, cloth with a sleeve (see chapter 3.4);
• the seams: type, thread used (its material must withstand the process just like the fabric), reinforcements;
• the neck and the feed passage: diameter, length, clamping method;
• the edges: any sealing coating (see chapter 4.3), bead, hem;
• the accessories: eyelets, ties, straps, hook-and-loop strips.

4. The process conditions

These are what allow the supplier to propose a suitable cloth, and to commit to it. State: nature of the product, solids concentration, particle size distribution if known, pH, maximum and usual temperature, washing chemicals and their concentration, filtration and squeezing pressure, cycle time and number of cycles per day, priority objective (filtrate clarity, dry solids content, cycle time, service life). One manufacturer points out that a change of process requires the choice of cloth to be reassessed [1].

5. Special requirements

Food contact, compliance documents, batch traceability, cloth marking, packaging (see appendix B).

6. Acceptance criteria

• dimensional check on a sample of cloths, against the drawing and its tolerances;
• conformity of the fabric reference and the batch, with the batch certificate or data sheet if the supplier issues one;
• quantity, labelling, condition of the packaging;
• for a first purchase: trial fitting of one cloth on a plate before approving the series.

Permeability, pitfall number one

Filter cloths have no “micron rating” in the sense of a cartridge filter: according to one manufacturer, they are characterised by their air permeability [1]. But the value depends on the method. ISO 9237 describes the measurement of the air permeability of fabrics, including technical textiles and nonwovens [2]; the American standard ASTM D737 describes another [3]. Units and test pressures differ depending on the standard and the laboratory.

Two figures can therefore only be compared if they refer to the same standard, the same test pressure and the same unit. Require these three items, and a tolerance.

Common mistakes

Field experience: summary of situations encountered in operations and purchasing.

Questions to ask the supplier

• To which standard and at what pressure is the stated permeability measured?
• What are the tolerances on dimensions, mass per unit area and permeability?
• What material is the sewing thread made of, and does it withstand the same conditions as the fabric?
• Is the proposed fabric held in stock, or woven to order? What lead time, confirmed in writing (see chapter 6.1)?
• Which documents accompany the delivery: data sheet, batch certificate, declaration of conformity if required?
• Does the supplier offer a trial on a few plates before the series (see chapter 6.2)?