Filtration is the mechanical separation of suspended solid particles of a certain size from a viscous phase. The viscous phase flows through a porous material that is permeable.
In surface filtration, primarily particles that are larger than the pore canals are retained on the upper surface of the filter medium. The subsequent particles that deposit on the filter surface form a so-called filter cake.
For depth filtration, additional separation mechanisms apply in addition to the pore geometry. These effects include blocking effect, sedimentation, inertia and diffusion. In cross-flow filtration, the filter medium is placed perpendicular to the flow direction. The particles lodge in the filter surface.
Micron retention defines the diameter of the largest round particle which can pass through the filter medium. The quality of the separation effect is influenced by various factors, for example the particle size and particle shape distribution of the input material, the proportion of solid in the viscous phase, the flow speed, the phase properties, the operating parameters of the filter system and the geometric structure of the filter medium. A clear and comparable specification of a characteristic value is only possible with exact information of the filtration method used.
Pore diameter describes the equivalent diameter of the pore canal within the filtration medium. The resulting distribution function provides information about the separation effect. A proven process is the capillary pressure method.
Geometric pore size
Compared to other filter media, the specific properties of a woven wire mesh may be precisely described and defined by the weave type, wire diameter and mesh count. This distinct advantage can be used to calculate the geometric pore size without having to use exhaustive measuring methods. The resultant value describes the diameter of a round sphere that is just able to pass through the wire mesh. The mathematical equations based on this process were developed by the University of Stuttgart within the scope of AVIF Projects A224 and A251 and experimentally validated via a glass bead test.
When flow takes place across a filter medium, there is a pressure differential between the input and discharge sides, dependent on the filter geometry, ambient operating conditions and phase properties. With solid flow data, the pressure loss coefficient „zeta“ is given as a characteristic value for assessing permeability.
The porosity of a filter medium is the ratio of empty space volume to the total component volume, expressed as a percentage. In contrast with other filter media, for woven wire cloth the entire empty space volume is traversed by the flow.
Measurement Methods for Characterising Filter Media
Bubble Point Methode
This method is used for determining the maximum pore diameter. Preparation involves mounting the sample in a test rig and subjecting it to a wetting liquid with a defined interfacial tension until all the pores of the filter medium are filled. A pressure is then applied on the non-wet side until the first air bubbles force through the mesh against the test liquid. The maximum capillary pressure, the so-called bubble point, is a measure of the filter medium’s equivalent pore diameter.
Determining the equivalent pore diameter is done assuming a circular opening oriented in a horizontal plane. By evaluating the weave type, number of apertures per unit length and wire diameter, the maximum pore diameter can be calculated and thus characterises the largest sphere passing through the filter medium.
Flowrate porometry is an extension of the capillary pressure method and is used for determining the distribution of pores. A sample that is completely filled with a test liquid is subjected to an increasing air volume flow, from the first bubble point and beyond, until all pores flow. To determine this characteristic value, Haver & Boecker use a measurement system with an ultra-fine resolution sensor for the test pressure and air throughput. Pore size distribution: The permeability weighted pore size distribution is calculated from wet-flow (pressure-volumetric flowrate-dependency of the wet sample) and dry-flow (corresponds to a dry sample). Mean Flow Pore Size: the pressure value where the wet-flow is half of the dry-flow. From this the mean pore size can be calculated.
Norm: ASTM F 316-03, ASTM E 1294, and others.
Glass bead test
The glass bead test is based on the classic screening process. Glass beads with a sphericity of almost 1 and known size distribution are placed on the filter medium and screened. Particle passage is evaluated by the largest particle diameter.
The flow-through test is used for determining the technical flow parameters of filter media. Here Haver & Boecker employ an automatic pressure drop test unit that determines the pressure drop behaviour of filter meshes within laminar air throughput. The test results vary and depend on the permeability of the filter medium and can be used for additional calculations (filter performance when used with other Newtonian media, filter layouts and so on).
Residual contamination analysis
This technical cleanliness test is for measuring particle contamination on the surface of filters and filter components. The particles are removed from the test object using a liquid-based extraction process. The extraction fluid is then filtered by a filter membrane that separates the particles, which are then microscopically counted and classified according to size. Haver & Boecker use an automatic residual contamination analysis system that also differentiates between metallic, non-metallic and fibrous material.
Norm: VDA Volume 19 and ISO 16232.
Tensile testing determines the mechanical properties of metallic test specimens eg. elastic limit, yield point, ultimate strength and others, by using an axial loading until rupture (breaking point) is reached. Haver & Boecker conduct tests using the most up-to-date computer-controlled universal testing machines.
Weaving wire: for example incoming good inspection, production testing
Woven wire cloth: for example production testing, end-of-line tests
Norm: DIN EN ISO 6892-1 and others.