Bekaert’s metal fiber filtration media are suitable for both in-depth filtration and surface filtration. Here’s a short explanation of the main differences between these two types of filtration.

In-depth filtration

This is when the contaminants or particles that have to be removed from the flow are captured within the structure of the filter medium. In other words, the particles penetrate the medium and get captured inside.

An in-depth filter has a 3D-structure and mostly consists of multiple layers. For the multiple layer media, the coarser fiber layers are placed at the flow-in side. Coarser particles are stopped by the coarser layer. Only small particles are held in the fine layer. This prevents premature blocking of the medium and increases the dirt holding capacity and on-stream lifetime.

In-depth filtration is mainly used for the filtration of liquids. A typical example is the filtration of polymers.

Since the contaminants penetrate the filter medium, off-line cleaning will be required in order to clean the filter.

Surface filtration

As its name suggests, with surface filtration the particles are stopped at the surface layer of the filter medium. The pore size will determine the size of particles that are stopped.

In many cases, the filter medium will have a multi-layer structure, with the finer fiber layers at the upstream side of the flow. The particles will form a dense cake layer at the surface of the filter. This cake formation can increase the filtration efficiency, as finer particles are retained in the dense cake.

In surface filtration, the cake formation will cause an increasing pressure drop across the filter. In the case of liquids, when the pressure drop becomes too high, a reverse filtrate flow can be initiated to remove the cake. This is called backwashing or back-flushing. In the case of gases, the cake is blown off the candle with a short blow of gas against the hot gas flow. This is called back-pulsing.

Surface filtration can be applied in both liquid and gas filtration, especially for fine filtration.

The equivalent diameters of metal fibers range from 1 to 100 µm. Standard equivalent diameters are 1.5, 2, 4, 6.5, 8, 12, 22, 30 and 40 µm.

A micron, officially called micrometer (µm), is one millionth of a meter. This is one thousandth of a millimeter. A piece of paper is usually about 100 µm thick, a human hair about 70 µm.

Absolute filter rating

The size of the largest pore in a filter is used as a reference to measure the performance of a filter medium. The size of the largest pore in a filter can be determined using the absolute pore size test.

The absolute filter rating (a) is the diameter of the largest hard spherical particle that will pass through the filter element under constant flow.

The definition of the absolute filter rating implies that 100% of the particles larger than a µm will be retained by the filter. Smaller particles are able to pass the filter, but a certain percentage of them will also be retained. The following efficiency graph illustrates this:

Nominal filter rating

The nominal filter rating is an arbitrary value, indicating a particulate size range; the filter manufacturer claims the filter removes some percentage of this size range. The nominal filter rating is defined differently by different manufacturers and is therefore ambiguous. Nominal ratings vary from manufacturer to manufacturer and cannot be used to compare filters. In case of nominal filtration, nominal filter rating refers to the filtration efficiency after cake formation.

Comparison

To compare both the absolute and nominal filter rating, their graphs are compared below. The first graph is one of a filter with an absolute filter rating of 10 µm; it is compared to a graph of a filter with a nominal filter rating of 10 µm at 98%. In this case, the absolute filter rating of the second filter is about 14 µm and the nominal filter rating of the first filter is about 9 µm.

Filtration is basically a process to separate solids from a liquid or gas stream with a porous substrate (medium). 

Separation is the process of converting a mixture or solution of chemical substances into two or more distinct product mixtures. Examples include separating two liquids of an emulsion (water in fuel) or removing liquid mist from a gas.