Pan Filter

Continuously working horizontal vacuum filter, which similar to a disc filter is divided into a number of filter cells with individual filtrate discharges. The solids are discharged horizontally via a discharge screw. Pan filters are employed for easy to filter and rapidly settling suspensions and allow excellent washing of the filter cake in a counter-current washing. Latest design improvements associated with pan filters are the forced feeding system ensuring even filter cakes and FrameTrak for easier and safer attaching of the filter cloth.

Parallel Connection

Parallel arrangement of multiple units of processing equipment when an individual unit is too small to process the total feed, or when quasi-continuous operation has to be realized in the case of discontinuously working units. For the latter, each individual machine is operated in a time-shifted manner for continuous processing of the feed stream.


Equivalent term for solid particles of small dimension in the mm or μm range.

Particle Fraction

Particle collective with a defined property. This can refer to either the particle type or a certain particle size range.

Particle Size

refers to a geometrical particle dimension. Often a definite description is impossible due to an irregular particle shape and one uses a so-called statistical particle diameter or an equivalent diameter.

Particle Size Analysis

Measurement of a particle size distribution. There is a host of different techniques in use that can vary in their dispersivity characteristics as well as in their quantity aspects. An example would be a sieve analysis where the dispersivity characteristic is the sieve hole diameter and the quantity aspect is respectively the volume or the mass of a particle fraction.

Particle Size Distribution

Result of a particle size analysis. The particle size distribution is expressed either as sum or density distribution. The particle sum distribution Q(x) states how many percent of the total particle amount are smaller than the examined particle size x. At the maximum particle size xmax the particle sum is assigned the value Q(xmax) = 1. At the minimum particle size xmin the particle sum has the value Q(xmin) = 0. The particle density q(x) is the particle amount in a differential particle size interval dQ(x) over the differential particle size interval dx. Both distributions are related as follows:


Particle Size Measurement
Particle Size Analysis
Pattern Repeat

Term applied in characterizing a woven filter cloth. The pattern repeat entails the minimum number weft and warp threads cross each other in an uninterrupted sequence when arranged side by side in the weft- and warp direction.

Peeler Centrifuge

Discontinuously working filter centrifuge with a horizontally rotating axis. Its name is derived from the solids discharge knife that touches the filter cake during the drum rotation. This peels the cake off the drum down to a residual product layer, that is left on for the protection of the filter medium. The solids are ultimately discharged via a conveyor chute or a conveyor screw. Additionally, peeler centrifuges can perform intensive cake washing. These centrifuges have diameters of approximately 0.5 m to 2.0 m, can operate at C-values of approx. 400 ÷ 3000, and have a throughput of approx. 0.1 ÷ 50 m.t./hr.

Peeler Knife

Cake removal device in peeler centrifuges. A standard peeler knife reaches across the entire depth of the drum; alternatively, a short knife with axially oscillating movement is used. The peeler knife is either radially pivoted against the cake on the rotating drum or vertically driven against it. To protect the filter medium against damage a thin product layer remains on the drum. The cake discharge with peeler knives at centrifuges is roughly equivalent to the scraper discharge used in drum and disc filters where reverse pneumatic pulsing is not considered.

Peeling Disc

Stationary double disc with curved flow channels in the gap leading to the center of the disc. The rotating liquid surrounding the disc enters into the flow channels and drains through the center of the disc at a high velocity. Such discharge devices are found in disc stack separators and decanter centrifuges as well as centrifugal mixers.

Peeling Pipe

used in centrifuges to transfer clarified liquid from the process chamber. The peeling pipe dips into the rotating liquid and as the kinetic energy of the liquid is sufficiently high an external pump is eliminated. Hence, a peeling pipe or a peeling disc is also dubbed a centrifugal pump.

Peeling Pneumatics

(also known as the Titus-System) is an aspirating pipe used in peeler centrifuges for the solids discharge. Located directly behind the peeler knife this pipe pneumatically takes the peeled solids out of the centrifuge drum. The peeling pneumatics can be especially beneficial for subsequent thermal drying operations.

Pendulum Centrifuge

Discontinuously operating filter centrifuge with a vertically or horizontally rotating axis. The drum or basket is rigidly mounted to an oscillating housing, supported by three telescope-type spring legs. Therefore a special foundation for the absorption of vibrations is eliminated. Solids discharge can be performed in conformance to product requirements, e. g. peeler knife, peeling pneumatics, detachable filtering bags, or manual removal. Pendulum centrifuges work in the rpm range of 200 to 3,000, and have drum diameters of 0.3 to 2.0 m. Suspension throughputs of approx. 0.1 ÷ 2 m.t./hr can be realized. They are easy to clean, flexible in operation, and therefore well suited for a quick product change.


are fused, ground, and classified swelling clays of volcanic origin. Their structure is leaf shaped and can be chemically grouped as silicates of sodium, potassium, calcium, or aluminum. They are extracted from open pit mines. Perlites are used as filtering aids to render dense bulk structures more porous.


is the flow resistance of a porous system, termed cake permeability. The specific permeability of a bulk is independent from the layer thickness and determined by the geometry of the pore system. It can be measured in a filter test via the Darcy equation (Darcy’s Law).


The clear liquid produced by microfiltration and ultrafiltration; equivalent terms are filtrate in filter apparatus and centrate in centrifuges.

Permeation Washing

Procedure carried out directly after the filter cake formation to purify the separated particles or to recover valued substances of the mother liquor. Pure liquid, which has to be molecularly mixable with the mother liquor is sprayed on top of the filter cake and permeated through the porous particle bed. After a first stage of displacement washing the rest of impurities has to be removed by diffusion.

pH Value

Negative common logarithm of the hydrogen ion concentration used to quantify the hydrogen or hydroxyl ion concentration in aqueous solutions. pH-values of <7 are described as sour or acidic, pH-values >7 as alkaline or basic. At the pH of 7 a solution is neutral, because the concentrations of hydrogen and hydroxyl ions are equal.

Pilot Plant

Experimental equipment for testing on a semi-technical scale. A pilot plant should be as small as possible in order to minimize the operational efforts, but large enough to provide realistic data for the design of a full scale machine. Especially important is the investigation of operational parameters, such as suspension homogenisation or solids discharge behavior.

Pipe Module

Membrane filter medium assembled in a pipe of several mm or cm diameter for crossflow filtration. They offer in comparison to other module designs only a small membrane area per module volume, but are well suited for applications in microfiltration as they do not make high demands on the pretreatment of feed suspensions to be processed due to their relatively large cross-sectional flow area.

Piston-Spring Model

is used conceptually for describing the consolidation of compressible bulk materials. Accordingly, if a bulk is compacted by pressing the structural pressure and the liquid pressure change inversely in the bulk. The structural pressure rises from zero to the level of the pressing force, whereas the liquid pressure declines from the value of the pressing force at the beginning of the consolidation process down to zero at the equilibrium state.

Plain Weave

Special pattern for a weave representing the tightest and strongest interlacing of warp and weft threads. In earlier times, it was named calico or muslin weave for cotton, cloth weave for wool, and taffeta in silk weaving, respectively. Weaves in plain weave texture show the same appearance on both sides as one weft thread interlaces a warp thread. By interweaving two or more weft threads with two or more warp threads a rib- or basket weave is formed.

Plate Pressure Filter
Leaf Filter

Accordion-style folding of a filter media for filter cartridges. Through pleating a filter cartridge with a large surface area is produced, enabling in return large throughput performances with a low starting pressure loss.

Plug Flow

in a bulk is, in contrast to fingering, characterized by a uniformly progressing liquid front. One differentiates in the modeling of the displacement of liquid in a capillary by centrifugation between plug and film flow.


Accessory for bag filters. Plungers are inserted in a bag to reduce dead space (e.g. glue filtration). They are hollow bodies, which are adjusted in their size to the bag’s interior to reduce the loss of product.

Police Filter

Generally, a discontinuously operating filter located downstream of a separation process for trapping particles that inadvertently can appear in the primary filtrate. Candle and bag filters are typically used as police filters.

Polishing Filter
Police Filter
Polymeric Flocculant

High molecular-weight polymers, such as polyacrylamides, with molecular weights in the magnitude of 107 g/mol. One classifies according to their dissociating groups between cationic, anionic and nonionic polymers. Polymeric flocculants cross-link particles in a suspension that form flocks which enhances their separation in settling as well as in filtrating processes. Polymeric flocculants are offered either as solid granulates or liquid concentrates and have to be conditioned prior to use.


Liquid pool in the cylindrical part of a Decanter


In solid-liquid separation a pore describes the void space surrounded by solids in filter media or in bulk materials. The geometry of such pores is often quite complex and exact description can not be given. Hence, the pore size, i.e. the pore diameter, is often represented by substitute values such as the circle equivalent to the pore cross sectional area. A measuring method for the determination of pore sizes in filter media is the bubble point test.

Pore Diameter
Pore Flow

relates to the flow of a fluid in the pores of a filter medium or a bulk. The empty pipe velocity of the pore system is calculated as the product of the mean pore flow velocity and the mean porosity. Known equations for describing the pore flow in bulk materials are respectively the Carman & Kozeny-equation and the Gupte-equation.

Pore Size
Pore Size Distribution

The pores in a filter medium or in bulks are always more or less a size distributed that can be approximated in the same form as a particle size distribution, as a pore sum distribution, or pore density distribution as a function of the pore size.


Measurement techniques for the pore size or the pore size distribution in filter media or bulks. In mercury porosimetry the evacuated structure of a porous system is successively filled. With the bubble point procedure a porous body filled with a wetting liquid is stepwise demoistured by application of a steadily increasing gas pressure differential. Both procedures utilize the connection between capillary pressure and pore size given by the Laplace-equation.


The porosity ε of a bulk is defined as the ratio of void volume Vv and total volume Vtot of a bulk, which in turn is the sum of void- and solids volume Vs:

The porosity is directly connected to the void ratio e:

Potential Vortex

Flow form (also known as Helmholtz vortex) which develops in hydrocyclones within the so-called primary vortex. The rotational velocity increases radially inwards up to a maximum. For such a flow the momentum equation based on the tangential velocity v and the radius r yields the following relation for different radii:

For frictionless flow the exponent m equals 1. For the frictional flow of suspensions in hydrocyclones a value of m = 0.5 is often used.


Apparatus employed for screening of oversize particles to protect the subsequent separation equipment. Pre-filters are installed upstream of disc stack separators and hydrocyclones to prevent potential blocking of their discharge nozzles.


The pre-treatment of a suspension, where as much as possible of the particle-free liquid is removed from a diluted suspension by relatively simple means, in order to reduce the burden on the often considerably more complex equipment for the following mechanical demoisturing, or to allow their application in the first place. For instance, pusher centrifuges require a certain minimum feed concentration for proper operation, i.e. to prevent an excessive increase of the solids loss through the sieve as well as to be able to form and demoisture the cake in the relatively short residence time. Gravitational thickeners, decanters, thickening filters, and crossflow filters are used as thickeners. The thickening process is often combined with a flocculation to ease the separation of the mostly extremely small particles.


Solidification of dissolved compounds by adding a suitable substance (i.e. precipitation agents). The insoluble solid precipitates can then be removed by a solid-liquid separation.

Precoat Filtration
Precoat Drum Filter

Variant of drum filters specially designed for precoat filtration

Precoat Filtration

Another term for clarifying filtration where a coarse porous layer of a filter aid is filtered first on a cake-forming vacuum or overpressure filter. Through this auxiliary layer the actual suspension is then filtered. Aside from surface filtration there is also a deep bed filtration component. Generally precoat filtration is applied when a clear liquid product with as few particles as possible is needed. Important fields of application are in the beverage and food industries, as well as in biotechnology. Discontinuous candle filters, leaf filters, as well as continuous drum filters are employed. The disc stack separator and the crossflow filter equipped with membranes are competing separation options.

Precoat Layer

A filter layer, formed by cake filtration from a filter aid, through which the process suspension is filtered during a precoat filtration. Precoat materials are diatomaceous earth, activated carbon, perlite, wood flour et al; they usually offer a large specific surface for stopping large amounts of pollutants. Aside from the purely mechanical retention, adsorption can be utilized to remove compounds that are dissolved or form colloids in a liquid

Press Belt

employed on drum filters or belt filters to cover partially the demoisturing section of the filter cake for mechanical compression. The press belt, usually designed as a rubber belt, is guided over a roller system to the filter cake, where it is pressed on by press rollers. As long as the press belt seals on the edge of the filter cake, the existing gas pressure differential can also be utilized for pressing, thereby acting not only linearly but also uniformly over the entire surface. Such a pre-press system is of advantage when the filter cake has a tendency for shrinkage crack formation as it can be stabilized by a certain pre-compression.

Press Filter Automat

Special design of a membrane filter press, with a vertical plate package and horizontal filter chambers. The filter cloth is continuous that zig-zags through all chambers. After the opening of the plate package the cloth is transported by one plate length forward, and the cakes are safely discharged to the sides of the chambers. Filter cakes can be washed, pressed and demoistured with gas pressure. Press filter automated machines are built with up to 150 m2 filter area and work with gas pressures of up to approx. 6 bar. The horizontal position of the filter plates promotes the consistent product distribution in the filter chamber as a well as the efficient washing of the cake.

Press Filtration

A process, where the filter cake, after formation, is either compressed by the suspension, or undergoes additional pressing (filter press), or by a mechanically applied pressing power (membrane filter press), for subsequent demoisturing. Depending on the filter design, pressing powers of a few bars up to high pressures of 150 bars are applied. Press filtration is obviously applicable only if the filter cakes display a distinct compressibility. This is especially the case with extremely fine grained (x<10 μm), non-rigid (organic substances), or strongly flocculated particles. This compression is also called consolidation.

Press Roller

Device to apply pressing power on demoistured filter cakes. In double belt presses, the cake which is held between two filter belts moves through a narrow slot formed by rollers facing each other, alternatively a pressure is created by winding the belts around a roller. On drum filters and belt filters, press rollers are installed as an additional facility for the post-demoisturing of the cake during the vacuum filtration. Often they are combined with a press belt which they press onto the cake.


Physical parameter, defined as the quotient of a force and the area, on which this force acts against the area’s normal line. The pressure is expressed in the following units: 1Pa =1 N/m² = 10-5 bar = 7.5 10-3 mmHG (Torr) = 1.45 10-4psi = 9.869 10-6 atm.

Pressure Difference

Notion in solid-liquid separation for the acting pressure difference across a filter medium, representing the driving potential for the separation of a suspension that is supplied by an outside pressure source.

Pressure Drop
Pressure Loss
Pressure Filter

Filters where the driving potential is a pneumatic or hydraulic pressure above atmospheric pressure. Pressure filters are employed as cake filters or deep bed filters for both discontinuous and continuous operation. The term is physically not clearly specified, as for example crossflow filters are usually not characterized as pressure filters.

Pressure Leaf Filter
Leaf Filter
Pressure Loss

The loss of pressure caused by fluid friction in a permeated system, such as a pipe or a filter cake.

Primary Particles

When particles in a suspension are present as solid agglomerates, the originally individual particles that make up the agglomerate are called primary particles.

Primary Vortex

Flow in a hydrocyclone developing immediately behind the suspension inlet. At the opposite end of the hydrocyclone, at the throttled apex nozzle, the flow direction is reversed, and the liquid leaves the cyclone in a secondary vortex through the vortex finder. Coarse particles in primary vortices are separated onto the outside at the cyclone wall, while the fine particles follow the liquid flow on the inside. The primary vortex is a potential vortex, meaning the flow velocity increases towards the inside. The maximum flow velocity is attained at the radius of the vortex finder, where also the cut size is determined.

Product Moisture

Amount of liquid which still remains in the separated solids after the solid-liquid separation. It is reported, respectively, as residual moisture, as dry substance content, as load, or as saturation degree.

Product Parameters

are intrinsic product properties that influence the separation process. Product parameters are for instance the dynamic viscosity ηL of the liquid or its surface tension γL, the particle size x and the particle size distribution Q(x), and the suspension concentration cV.

Shear Thinning
Pulp Density
Suspension Density
Pusher Centrifuge

Continuously operating, cantilevered filter centrifuge with solids discharge by means of an axially reciprocating pusher plate. They are offered in single- or multi-stage design with either cylindrical or conic-cylindrical drums. Pusher centrifuges are also suited for an intensive washing of the product due to the cantilevered support and the free accessibility of the processing space. Pusher centrifuges as a rule separate particles with diameters larger than 80 μm. They operate with C-values of 200 ÷ 2500, have drum diameters of 0.15 ÷ 1.5 m, and can handle suspension streams of 0.5÷100m.t/hr. To assure good filtrate clarity, these machines require feed concentrations of approx. 10 ÷ 40 %, which is why they are often combined with a pre-thickener.

Pusher Plate

Axially reciprocating, circular transfer device in pusher centrifuges. During each forward stroke the previously formed filter cake is pushed a step toward the open front end of the centrifuge drum where the filter cake ultimately breaks.

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