Feed Cross Section

Opening area of the feed device for the suspension to be separated, in a separation apparatus.

Feed Material

Generalizing term for suspensions fed into process equipment.

Needle Felt
Film Flow Model

The film flow model was conceptualized for the centrifugal demoisturing of a bulk, especially those with a coarser granularity. This model distinguishes between a plug flow mode and a subsequent film flow of the liquid remaining on the particle surfaces. lt was inspired by the image of a plate, that when pulled out of an oil bath shows an oil film varying locally and with time in thickness until an even flow is established.

Filter Aids

Additives to enhance the filterability of a suspensions. Filter aids can be applied as a precoat layer on the filter medium prior to the actual filtration, or as body feed filtration admix to the suspension to be separated. Common filter aid materials are: diatomaceous earth, perlite, wood flour, activated carbon, cellulose fibers and others. They are beneficial for suspensions that form extremely difficult to permeate filter cakes. Major application areas are water treatment and beverage purification.

Filter Area

is the active area in a filtration process, which is covered with a filter cloth. In a drum filter, for example, the entire surface area covered with filter cloth is active as filter area. In a vacuum belt filter with a rubber conveyor belt it includes only that part of the filter cloth that is on the upper side of the filter, whereas the other half is out of use as it is led back on the underside of the belt.

Filter Bag

Filter medium tailored as a bag, as employed in bag filters or disc filters. Depending on the design of a filter apparatus, filter bags are flown through from the inside to the outside or from the outside to the inside.

Filter Cake

A porous layer of solids particles formed on the surface of a filter medium. In order to generate a filter cake, solids bridges have to develop over the media openings thus blocking the following solids. Filter cakes are formed in vacuum filters, pressure filters, press filters, and filter centrifuges. This can occur discontinuously or continuously, and under a constant or changing driving potential. Filter cakes are subjected in general to an extensive mechanical demoisturing after cake formation, and as the case may be are liberated of still remnant mother liquor by washing.

Filter Candle

Cylinder-shaped filter element of a candle filter, mostly consisting of a perforated internal pipe and a filter medium placed on top. As a rule filter candles are permeated from the outside to the inside. Respective to the type of filter medium filter candles can be employed for cake filtration or deep bed filtration. The filtration on a filter candle normally is a discontinuous process.

Filter Cartridge

Changeable, finite filter element, preferably employed for deep bed filtration that is replaced after reaching a critical contamination.

Filter Cell

The filter surface in continuously working rotary filters is divided into single cells, each separately connected to the filtrate pipe system. The filter cells in drum filters are flat, rectangular pans with a plastic cell insert on which the filter cloth is attached, often with a backing cloth between. In disc filters the filter cells are shaped as circular segments and designed to filter on both sides. Usually, they have a perforated metal plate over which the filter cloth is pulled as a tailored bag and attached. The filtrate is drawn oft at the narrow end of the cell, which is called the cell foot.

Filter Centrifuge
Centrifugal Filter
Filter Cloth

Special type of filter medium, consisting of weave. There is a plethora of different materials, different thread or respectively yarn quality as weil as extremely different weave structures. The filter cloth represents the critical interface between separation equipment and suspension. lt must be selected carefully for each individual application to meet the process, mechanical, and chemical demands, respectively. The selection of a filter cloth should never occur solely by theory, but instead has to be supported experimentally.

Filter Cloth Blockage
Filter Cloth Clogging
Filter Cloth Cleaning
Filter Regeneration
Filter Cloth Clogging

A media-fouling mechanism in cake filtration, where over the course of operation more and more solids penetrate the structure of the weave and deposit in it. Such cloggings are some times partially reversible and can be undone by filter regeneration, or if they are irreversible will render the filter cloth unusable, as evidenced by a loss of permeability.

Filter Cloth Resistance

Flow resistance Rm of a filter cloth. lt can be determined directly by Darcy's law, if one permeates the filter cloth with a particle-free liquid at a specified pressure and measures the flow. However, this value is invalid for predicting the filtration performance with a suspension, because the interaction proper between filter cloth and suspension with particles settling in in the cloth structure and especially with a bridge layer forming leads to a relevant filter cloth resistance. Thus, it can only be measured in conjunction with the actual suspension and be estimated for example by the t/V=f(V)-Method.

Filter Dryer

Discontinuously operating pressure Nutsche filter or filter reactor, which offers the possibility of thermal filter cake drying after mechanical demoisturing. Heat transfer is realized by external heating the apparatus walls and/or blowing hot gas through the desaturated filter cake.

Filter Fineness

Data about the separation efficiency of a filter medium at filtration. The nominal fineness notes the particle percentage of a certain size that is retained by the medium. However, test materials and experimental conditions influence strongly this value so that nominal filter fineness data from different filter manufacturers are difficult to compare. Information about the test material and experimental conditions has to be included when the stating a nominal filter fineness.

Filter Medium

Porous layer, through which during the filtration the filtrate permeates due to an acting pressure difference, while the solids in the original suspension are retained its surface or in its structure, respectively. Filter media can be of very different types: metallic sieves, textile fabrics, fiber fleeces, felts, paper, membranes, sintered materials, bulk layers, and a host of others are employed. An optimal filter media has maximum separation efficiency, minimal flow resistance, and a long service life. In actual operation a supportable compromise has to be found among these contradicting objectives for each individual separation task.

Filter Medium Resistance
Filter Cloth Resistance
Filter Optimization
Filter Paper

A filter medium, that is preferred in the laboratory for preparative purposes. On the other hand, a technical filter medium is recommended for industrial filter tests, since the filter medium can influence the filtration process significantly.

Filter Performance
Filter Press

One of the most commonly used solid-liquid separation machines for the discontinuous cake filtration and demoisturing of difficult to filter suspensions. A filter press has a frame with a stack of filter plates packed between head and end piece, which are pressed together for sealing. In the case of a chamber filter press rectangular or square plates with a side length of up to 2 m, form filter chambers, lined with a filter medium, into which the feed suspension is pumped. In a frame filter press the filter chambers are created by alternating special frames alternating with the filter plates in the plate package. The cake of membrane filter presses can be squeezed mechanically by a rubber membrane on one side. A filter cycle can take anywhere from of a few minutes to up several hours. Filter presses have areas of up to 1,000 m2 and operate generally with pressures of up to 16 bar. High-pressure filter presses are applied with up to 60 bar. Due to their almost universally applicability for difficult to filtrate suspensions filter presses have an extremely large field of operation.

Filter Press Membrane

Plastic or rubber membranes, encased in a special membrane plate and applied in membrane filter presses. The membrane is pressed with a pressing liquid,
which is pumped into the empty space between membrane and plate wall, against the filter cake and squeezes it mechanically.

Filter Reactor

Discontinuously working pressure filter capable of performing, next to the pure separation task, also such unit operations as reaction, crystallization, extraction, thermal drying et al. Generally, an agitator is installed in filter reactors. Some filter reactor types can be also rotated or tilted for the beneficial execution of specific processing steps like thermal drying of the previously separated particles. Filter reactors offer great advantage if a contamination of the products or an exposure to the environment has to be avoided, because all the different steps take place in a minimized and also weil- isolated space. In addition this process integration reduces independent process steps and thus product losses.

Filter Regeneration

Term used for all types of filtration to describe the restoration of a filter medium's ability to perform after getting clogged with particle deposits or crystallized products. Back flushing, application of ultra sound, blasting with high-pressure water, chemical cleaning with acids, or washing with hot water are some of the measures applied. In extreme cases filter regeneration can mean replacing the filter medium, i.e. when it cannot be cleaned.

Filter Segment

Expression for the filter cells of a disc filter

Filter Tank
Filter Trough
Filter Trough

Tub-shaped container filled with a suspension wherein the filter cells of drum filters or disc filters are periodically immersed in order to form a filter cake. The filter trough generally has either a pendulum-type oscillator, a propeller, or a paddle stirrer to homogenise the suspension and to keep the particles insuspension. In disc filters specially designed troughs are known such as the single trough design and the joint-single trough design, where a mechanical stirring apparatus can be omitted, as the rotational movement of the discs often provides enough mixing of the suspension.


Evaluation criterion for separating of a suspended solid by filtration. Often employed for this is the specific filter cake resistance as determined by a filtration test with the t/V-over-V method.


The liquid, separated by a filtration process, previously having permeated the filter medium.

Filtrate Collector
Filtrate Pipe

Pipe for the drainage of a liquid separated by filtration out of a suspension.

Filtrate Pipe System

for transferring the filtrate out of a filter apparatus. In a more specified usage it refers the filtrate pipes in rotary filters, that come in various manifolded designs for connecting individual filter cells with a single control head of the filter. When sizing the filtrate pipe system the minimization of its pressure loss has priority, as this share of the acting pressure difference is excluded from the actual filtration process. The performance of many filters, however, is limited by an undersized filtrate pipe system dimensioned too narrowly.


Portable laboratory pressure filter apparatus of the BOKELA company for investigating vacuum, overpressure, and press type filtrations. lt is based on the principle of the discontinuously working monoplate fitter, has a filter area of 20 cm2 and can be pressurized up to 11 bar. All process steps of the cake fittration as weil as of the cake formation, cake washing, cake demoisturing can be investigated. An electronic measurement data registrating and processing system records the filtration data and issues them in the form of an experimental test protocol. The apparatus corresponds to the VDl-guideline No. 2762: "Mechanical solid-liquid separation by cake filtration. Determination of filter cake resistance."


In the field of solid-liquid separation filtration is defined as the segregation of a suspension into solids and filtrate volumes by means of a porous filter
medium. Both solids and liquid are moved by a driving pressure gradient in co-current flow towards the filter medium. The liquid is able to penetrate the filter medium, while the solids are retained. Filtration is next to the sedimentation one the basic principles applied in solid-liquid separation. According to the mode of execution, one distinguishes further between cake, crossflow, and deep bed filtration modes.

Filtration Pressure

is the active driving pressure gradient during a filtration. The filtration pressure can be generated by a hydrostatic head, centrifugal forces, a vacuum gas overpressure, the head of a slurry pump, or the pressure of an impermeable press membrane. The filtration pressure is either kept constant during filtration, or is increased proportional to the pressure loss increase caused by the cake formation.

Fine Capillary System

In a bulk one typically differentiates between a coarse and a fine capillary system. lt should be pointed out that the term "fine capillary system" can mean different things, i.e. the pore structure of agglomerates, pores in a solids particle, or interstitial liquid remaining in the bulk after a mechanical demoisturing process.

Fine Filter

Class of filters belonging to deep bed filters characterized by solid, porous bodies made of sintered metal, plastic, or ceramic powder. They typically have a mean pore size far below 1 mm, so that they are suitable for deep bed filtration of extremely fine particles down to the µm region. Leaf filters and candle filters with precoat layer are also employed as fine filters. A distinguishing performance objective of fine filters is a filtrate as particle-free as possible.


Particle collective formed during the separation of a feed material with a certain particle size distribution: fines entail the fraction with the smaller size, and the coarse material containing the larger particles. The particle size at the boundary between fines and coarse material is called the cut size.


is a phenomenon in cake filtration often occurring during the washing or the demoisturing. The wash liquid, or the gas, respectively, does not progress in a uniformly even front through the cake, but preferentially penetrates the larger pores, that are always present in normal particle size distributions due to their lower resistance. This can lead to fingering, i.e. over parts of the filter area there is a premature and unwanted break through of the wash liquid (or gas) through the filter medium. Counter measures are limited: an equalization of the cake structure and secondly as high as possible a viscosity of the washing fluid compared to the mother liquor can reduce the fingering effect. Steam pressure filtration has shown to alleviate the fingering when through condensation of the pressurized steam the liquid in the advancing, !arge pores can be replenished.

Fixing Wire

serves as additional attachment of the filter cloth on drum filters, wrapped spiral-like around the cylinder, specially when a compressed air repulsion is utilized.

Flat Bottom Cyclone

Cylindrically shaped hydrocyclone with a flat, non-conical bottom wherein an induced convective flow carries the solids into the center of the flat bottom, from where they can be removed as a thickened sludge. By controlling the sludge height in the cyclone the desired cut size can be freely adjusted within certain limits. Flat bottom cyclones can classify solids up to a cut size of 500µm.


Filter medium made of compressed fiber layers. A fleece compacted by needling is also called needle felt. Fleeces are low cost media, with relatively low strength, however, and lack a regular pore shape or precisely adjustable pore size in contrast to a weave.


Process for the aggregation of primary particles to more or less loosely constructed particle collectives named flocks. Flocculation improves the separation behavior of finely granular suspension through an increase of the sedimentation velocity of the solids or by forming more permeable filter cakes. For the flocculation of a suspension, a parent solution has to be produced at first from a polymeric flocculant, diluted to the operational concentration, and is then added to the suspension to be flocculated. A rapid and thorough admixing is required to bring the polymers uniformly into contact with all particles. A flocculation holding period follows, in which the actual joining of primary particles takes place under low stress.

Flocculation Agents

Chemicals, which upon addition to a suspension lead to the aggregation of individual particles to larger particle collectives, also called flocks. Flocculation agents mostly are long-chained polymers which carry electric charges which are either neutral (nonionic), anionic, or cationic with respect to the solids to be flocculated. Polymerie flocculants are supplied in form of powders or concentrated solutions. Generally, they can't be applied in the food sector. Flocculation agents are a significant operating cost factor in wastewater treatment. They are applied in particular with thickeners, decanter centrifuges, sieve belt presses, and filter presses. Many applications are also found in area of continuous vacuum filtration.

Flocculation Aids
Flocculation Agents
Flocculation Holding Period

is an aggregate collective of primary particles. The adhesion of particles in a flock is effected by either influencing the electric surrounding of the particles (agglomeration), or by adding polymeric flocculating agents (flocculation). A flock settles faster than the single particles would, ties in the finest, suspended matter, and produces during cake filtration a cake structure with a higher permeability. Sediments, produced out of flocks or filter cakes are considerably more compressible than structures, formed out of comparable single particles

Flock Density

Packing density of primary particles in a flock. The more compacted a flock is, the higher is its strength against mechanical stress. For this reason the flock density of a flocculated suspension is increased prior to feeding into a centrifuge by a shearing pre-treatment, e.g. in a cylindrical stirrer.

Flock Factor

used in the Richardson & Zaki-equation to determine the volume fraction of flocks in a flocculated suspension. The volumetric flock concentration CvF is derived as the product of solids volume concentration Cv and flock factor k:


Flotation is generally based on the fact, that suspended particles have a smaller spec. weight than the surrounding liquid phase and thus are rising up against the direction of earth gravity or a centrifugal field. lf particles show a larger spec. weight than the sourrounding liquid phase, they nevertheless can rise up against gravity to the top of a so called "flotation cell" as a froth by adhering gas bubbles. lndustrially above all flotation in aqueous suspensions is used for sorting particles of different material but also for paper deinking or waste water treatment. Non wetting particles are floated by adhering gas bubbles and wetting particles are setting down. Wettability of particles can be adjusted by surfactants.

Flow Moisture Point

Test method for estimating the stability of a wet bulk material under an alternating load. A semi-spherically shaped test sample of moist material is exposed to defined vibrations on an oscillating table. At some moisture content the bulk starts to deform and begins to flow. This test method serves the purpose to assure stable storage conditions during transport of moist bulk materials by truck, train, or ship.


General term for a liquid or a gas.

Forced Discharge

through a special discharge device for demoistured solids to guaranty the safe removal of the product under any circumstance, e.g. a discharge screw.

Forced Feeding System

Specially designed slurry feeding device for equal slurry distribution over the whole filter area of BoVac filters developed by the BOKELA company. Four equal slurry sub-flows to 4 annuli of equal filter area lead to an even distribution of the slurry on the pan table. Thus, an absolutely even filter cake is formed with constant cake thickness across the pan radius what is a precondition for effective cake washing with minimum consumption of wash liquid.

Forward Edge
Leading Edge

Dirt layer, formed by biological activity, on membranes. Fouling can lead to a clogging of the membrane pores and therewith to a decrease of the permeate flow. Fouling can be abated by different chemical, physical and mechanical regeneration measures.


Clearly differentiated subset in a particle collective defined by their particle size or type of particle.

Fractional Grade Efficiency

Term for describing the separation characteristic T(x) of a separation apparatus also called Tromp curve. lt represents the particle amount MG (x) with the defined particle size x as percentage of the total amount MA(x) of a particle collective, which is discharged in the coarse material of the separating apparatus.

Frame Filter Press

Oldest design of a filter press characterized by the feature, that the chamber for holding the filter cake is formed by a frame, which constitutes the plate package, together with alternating filter plates between frames. A disadvantage of the frame filter press is the solids discharge, which has to be manually performed by breaking the cake out of the frame. An automation of this step was initially introduced with the chamber filter press.


Patented technology for simplified and improved attaching of filter cloth to drum filters and pan filters developed by BOKELA and ClearEdge. Frame­Trak consists of a special designed steel profile, an extruded elastomer polymer profile with cloth inserts and an elastomer polymer profile, which acts as a locking strip. lt replaces the traditional caulking method or bolt down clamping, makes re-clothing easier and faster and eliminates Operational Health & Safety issues associated with the existing methods of re-clothing.


Force of resistance, counteracting the movement of one body along the surface of another one. According to Coulomb's law, the frictional force F depends on the coefficient of friction µ and the normal force N, with which the surfaces in contact press against each other. However, it is not dependent on the size of these surfaces:


As long as there is no movement, i.e. at adhesive friction, µ is larger than after the onset of sliding, i.e. during sliding friction.

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