Fabric permeability

Permeability is the ability of a material, in our case a fabric, to allow a flow to cross it through interconnected pores, without altering its internal structure, thus allowing it to perform the function of particle uptake.

What is a fabric  
A weave is essentially a structure of two groups of threads positioned at right angles to each other. The yarns positioned vertically (lengthwise) are called warp yarns, and those positioned crosswise are weft yarns. Both threads are interwoven at right angles to each other to form the weave.

Depending on the evolution in which these threads are interlaced, they will form one structure (pattern) or another, making fabrics with different characteristics and with different filtration characteristics.

The thickness of the yarns, as well as their physical construction (fibre alignment and twist), will affect their performance in terms of filtration characteristics. At the yarn interceptions (4 yarns) a hole, a pore, will be produced. It is through this pore that the air will flow.

The magic of filtration lies here: we must build a pore as large as possible to have a high flow, i.e. a high flow rate, but at the same time the pore must be of a suitable size (small) so that its retention is correct for the process in question.

The permeability of a fabric is defined by the amount of flow (air) that passes through a defined area in a defined space of time, and all this at a given pressure (depression). For example: dm³/dm² x min at 20 mm H2O column.

Factors affecting permeability
The variables that will affect permeability will be the size of the pore and, to a lesser extent, its depth, as a certain level of friction between the air and the outer perimeter of the yarns is expected. It is important to understand that a very open fabric (with a low density of threads) can have the same permeability as a relatively closed fabric (with a high density of threads), since in the former we will have few large pores and in the latter many small ones, but which as a whole could have the same free space.

Particle separation: filtration
The mechanisms of particle separation are basically three: impact (by inertia), interception and diffusion. In the first one, the mechanism is purely physical and in the others, the mechanisms are more complex as they involve forces of attraction, statics and components related to friction or the surface properties of the “substances” in question. At this point we must mention surface filtration or commonly called “cake filtration” (very important in liquid filtration). In this model of filtration, the fabric mainly acts as a “support” so that the filtrate itself accumulates and forms a porous mass that filters by itself.

You can download this article and many others by following this link.