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Flex Fatigue

A failure process for fibres where a repeated cyclic load leads to breaking. This process is influenced by the load and the chemical and physical environment.

Flow Decay

Reduction of the flow rate by plugging or clogging of the filter medium.

Flow Rate

The rate at which a material passes through a system. Usually either mass based or volume based.

Flow Resistance

The resistance to fluid flow presented by e.g. the filter medium.


The rate of flow of mass or volume per unit cross-section normal to the direction of flow.

Frequency Size Distribution

A representation of the distribution of sizes in a sample. The amount of e.g. particles within a given size interval is plotted over size. If the plot includes sizes from minimum to maximum of present sizes, the plot is called Frequency Size Distribution.


Galvanic Corrosion

Galvanic corrosion occurs when two connected different metals are subject to an electrolyte. Due to the electrochemical reaction, the less noble metal will then corrode.

Gauge Pressure

Pressure reading from a gauge. To get the absolute pressure, the ambient pressure must be added to the gauge pressure.


Geometric material designer & material property preDictorA software to virtually generate filter media and predict their properties by using CFD to determine e.g. pressure drop or pore size.

Geometric Poresize

A method of calculating the maximum pore size of a woven mesh by using some basic parameters as input. The formulae are based on the AVIF projects A224 and A251 realized at the University of Stuttgart.

Grade Efficiency

Filter media show different filter effectiveness at different particle sizes. This variation in efficiency is called grade efficiency. Plotting the filtration efficiency over particle sizes gives the ‘grade efficiency curve’.


An alternative word for particle. Usually one that is clearly visible to the naked eye.



The fraction of a filter cake remaining on the filter medium after discharge. Sometimes called residual heel.

HEPAHigh Efficiency Particulate AirHigh Efficiency Particulate Arrestance

HEPA filters are usually throwaway media for air filtration. HEPA filter are classified in DIN EN 779 and EN 1822-1:1998.


Particles consisting of a range of sizes.Same as polydisperse.

Horizontal Belt Filter

A vacuum filter with an endless cloth moving on a support belt. The belt and cloth move continuously around two rollers and across several suction boxes.


Intermediate Blocking Filtration

A blocking mechanism where a particle will block a pore at the surface of a filter medium. Later arriving particles may settle on already deposited particles. Not every particle will block a pore, but there a probability for this to occur.

Irreducible Saturation

The value of saturation of a filter cake at which no further desaturation is possible, other than by evaporative means. Irreducible saturation is best determined experimentally, since calculations are often unreliable.


Knife Discharge

The most common method of cake discharge on rotary drum filters. The knife is places close to the medium and leaves a heel of cake on the cloth to avoid damaging contact. Knife discharge can be supported by blowback if the cake isn’t thick enough to discharge by its own weight.



A flat filter element that contains a filter medium.

Leaf Filter

A filter in the shape of a thin leaf which can be mounted horizontally or vertically.


The weight of particles per unit area deposited on a filter surface.


Having an affinity for a solvent (‘solvent loving’)Is the solvent water the term hydrophilic is used.Opposite of lyophobic.


Lacking an affinity for a solvent (‘solvent hating’)Is the solvent water the term hydrophobic is used.Opposite of lyophilic.


Maximum Pore Size

Diameter of a pore represented as a capillary of circular cross-section with the same capillary pressure as the pore.

Mean Flow Pore Size

Pore size equivalent to the mean flow rating

Mean Flow Rating

Diameter of the pore that divides the volume of flow through all pores into two equal parts.


A term refering to a woven wire medium.

Mesh Count/ Mesh Size

Number of openings per inch or per centimeterSometimes deprecated units like "per rheinisch" or "per frensh" are used.

ModeModal Particle Size

see Average Particle Size


A dispersion in which particles are all of the same size. Usually used when 90% of the particles are within ±10% of the mean.


A single, continuous fibre, usually of a synthetic material.


A fabric yarn made from multiple twisted fibres, usually of a synthetic material.


Newtonian Fluid

A fluid that obeys Newton’s law of viscosity. It shows a linear proportionality between shear stress and shear rate. The constant of proportionality is viscosity.

Nominal (Filter) Rating

Based on filtration tests where 90% (sometimes 95% or 98%) by mass of particles are stopped by the filter. This method is discouraged in favour of the absolute filter rating.

Non-newtonian fluid

A fluid whose viscosity changes with the shear rate. It doesn’t obey Newton’s law of viscosity.

Number Distribution

The relative frequency size distribution by number within a size distribution. Can also be shown as a cumulative size distribution.



Having an affinity for oils (‘oil loving’)Opposite to oleophobic.


Lacking an affinity for oils (‘oil hating’)Opposite to oleophilic.

Open Area

The pore area of a filter medium. Usually given as a percentage of the total area.


Open Field Operation And ManipulationA general purpose CFD simulation software


Particle Counter

A device to count the particles in a fluid.

Particle Size

The value of the physical dimension of a particle. Unless the particle is of spherical shape, it isn’t possible to state a single value. For those particles the method of measurement must be stated.

Particle Size Analysis

The process of measuring the size of a particle or the size distribution of many particles. Methods include microscopy (e.g. optical), electrical sensing (e.g. Coulter Counter), light diffraction, light scattering and photon correlation spectroscopy (PCS).

Particle Size Distribution

A table of numbers or a graph giving the frequency of particles of a chosen size range.

Pick Count

Used for woven filter media, giving the number of weft wires per unit length.

Pitting Corrosion

When the protective passive film of a metal is damaged and can’t be regenerated faster than corrosion occurs, a pit is formed with time. Once formed, the corrosion continuous at an ever growing rate. In extreme cases this will lead to perforation of the metal.

Pitting Resistance Equivalent Number (PREN)

A number representing the resistance of a metal against pitting corrosion. The higher the number, the more resistant the metal is.The PREN can be calculated from the contents of the alloy.PREN = %Cr + 3.3 * %Mo + 16 * %N


A fold formed by doubling a sheet of material back on itself. Pleating leads to an increased filter surface within a given volume. A pleat is a single fold.

Pleat SupportPleat Spacer

Used to prevent a pleat from collapsing.


see Heterodisperse


A hole or cavity in a body

Pore Flow

A convectional transport mechanism through a porous material.

Pore Size

The size of a pore in a porous material. The size and/or size distribution can be measure by a porometer.

Pore Size â?? Absolute Rating

The pore size at which particles equal or greater are retained with 100% efficiency.

Pore Size Distribution

Similarly to particle size distribution, this describes the distribution of pores of specified sizes. This affects filtration performance as the ratio of pore size to particle size influences plugging and bridging. Can be measure by a porometer.

Pore Size Nominal Rating

The pore size at which particles are retained with an efficiency less than 100% (typically 90 to 98%).

Pore Velocity

The velocity of a fluid flow through the pores of a filter cake.


A measuring device to determine the pore size distribution.


A device to measure the pore volume and hence the porosity of a filter medium. Often (incorrectly) used as term to describe a porometer.


The ratio of the void volume to the total volume of a material.


The physical unit calculated by force per unit area.SI Unit to be used is Pascal (Pa).

Pressure Drop

The loss of pressure in a fluid which flows through an obstacle, e.g. filter medium.

Pressure FilterPressure Filtration

A class of filters that uses pressure above the filter media as driving force for the fluid to pass the media.

Primary Particle Size

The size of an individual particle, e.g. prior to agglomeration.



The process of particles, which have been deposited on a filter medium, reenter the flow, e.g. by sudden change in flow velocity or vibrations.

Resonant Frequency

The natural frequency of a system where the largest amplitudes occur. Resonance can be used on a filter medium to reduce cake formation and fouling.


The study of flow and deformation properties of matter, such as elasticity, plasticity and viscosity, under the influence of an applied stress.See Newtonian Fluid, Non-Newtonian Fluid



A device containing apertures of uniform shape and size arranged in a rectangular manner.In the paper industry used as synonym for transport band.

Sieve Diameter

The minimum width of the square apertures of a sieve.

Size Fraction

The fraction of a system of particles limited by two given sizes.


The process of measuring the size or size distribution.

Standard Blocking Filtration

A blocking mechanism in liquid filtration where it is assumed that particles stick to the walls of the pores, leading to decreased flow and increased pressure drop. This will either lead to total blockage of the filter medium or a breakthrough which will contaminate the filtrate. Standard blocking filtration is favoured by low concentration of particles in the feed.

Stress Corrosion Cracking (SCC)

A form of corrosion that appears in chloride or halide environments when the metal is subjected to tensile stresses. High nickel contents promote this kind of corrosion.


Tex System

Metric system of units describing the linear densitiy of a fibre.1 tex = weight in gramms of 1000 m of fibre1 decitex (dtex) = weight in gramms of 10000 m of fibre


UHEPAUltra High Efficiency Particulate AirULPAUltra low penetration air

see HEPA


Describes the approaching fluid or flow before the filter.


Wet Sieving

Sieving with the aid of a liquid. Measuring the particle distribution usually leads to bigger particles passing the filter compared to a dry sieving of the same filter medium.

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