Wash Bar

Device especially applied at drum filters for the washing of a filter cake. Pipes with spray nozzles running across the drum distribute the wash liquid as uniform as possible film over the cake surface.

Wash Degree

Residual portion of a substance dissolved in the mother liquor that is to be washed out of a filter cake. The wash degree is normalized by assigned values of 1 before the washing and 0 after the total removal of the undesired substances, respectively.

Wash Liquid

Used for the removal of undesired substances, which are dissolved in the mother liquor of a bulk. The bulk can either be permeated by the wash liquid (see displacement washing) or the demoistured bulk is re-suspended in the wash liquid separately and filtered again. The latter case causes a dilution effect (see dilution washing).

Wash Pipe

Horizontal pipe with a single nozzle at the end to spray a wash liquid on a filter cake in pusher centrifuges etc.

Wash Ratio

Characteristic measure in displacement washing for the consumption of wash liquid. The wash ratio relates the volume of wash liquid to the pore volume in a filter cake. Ideally, the ratio would be one as the pore volume would be replaced just once by wash liquid. In reality, however, wash ratios can be considerably higher.

Cake Washing
Washing Efficiency

Term that means the portion of dissolved substance removed by an amount of wash liquid that corresponds to a wash ratio of 1. It is also used to describe the efficiency of dissolved substance removal at the end of the cake washing process, equally to the wash degree.

Water Content
Residual Moisture
Water Value

Reference value for the permeation rating of filter media. It is determined with particle free water under defined conditions and by relating the passed volume to the surface. The water value serves mainly for the comparative ranking of filter media; it is of little value regarding the filtration performance with an actual suspension.


Textile material, fabricated in a weaving mill, from two thread systems crossing each other in a rectangular pattern in a weave. Transversely to the in longitudinal direction running warp (warp threads) the weft is woven in and out repeatedly at the weave’s edges or ledges.

Wedge Wire Filter

Discontinuously working, candle-shaped filter for purifying large liquid volumes with low particle contamination. The filtration process on the slit- shaped filter openings is stopped when a pre-set value for the developing pressure loss is reached. Then the wedge filter is regenerated mechanically by means of brushes or scrapers, mostly while filtering. The gap width of the filter sieve has to be adjusted for the particle size to be retained.

Wedge Wire Sieve

Metallic, rigid and non-woven filter medium with slot- shaped openings. Wedge wire sieves are used as filter media for instance in screen centrifuges or worm extruders. These sieves are considerably more robust and resistant against abrasion than a weave.

Wedge Zone

A second demoisturing section in a double belt press following the pre-demoisturing straining zone where the bulk is compacted by additional pressing and shearing on its way through the roller press system.

Weir Disc

Device used in sedimentation centrifuges (see decanter) for liquid level control in the machine. The weir disc can be designed either for a fixed level or through height-adjustable radial slots for a variable filling.


Spreading of a liquid on a solid surface. A liquid that spreads spontaneously across the entire body is called a completely wetting or spreading liquid. The contact angle at the three-phase boundary of solid, liquid and gas is in this case 0o. Glass, water and air form a completely wetting system. If the contact angle is smaller than 90o a liquid shows wetting behavior. Contact angles larger than 90o define the non-wetting class; the system glass-mercury-air would be an example. Wetting is of importance in solid-liquid separation due to its direct influence on the capillary pressure and therefore it plays a role in demoisturing of filter cakes.

Wetting Angle
Wing-Shear Strength

Test method for measuring the aggregate strength of moist bulk materials designated for landfilling. A wing probe is inserted into a moist bulk material sample that has been prepared according to specifications and the torque needed to rotate the probe is measured. A minimum value of approx. 2N/m2 suffices for landfilling. The wing-shear strength depends on the type of material and its residual moisture.

Worm Extruder

Continuously working press filter made up of a screw in a perforated cylinder with contracting cross sectional area. The screw conveys the feed material axially and compresses it continuously. The separated liquid permeates the filter medium (e.g. gap sieve) and flows to the outside. Worm extruders are preferably employed with substances that are coarse, compressible and often fibrous in addition. Known applications extend as far as to the separation of liquid manure.

Worm Screen Centrifuge

Continuously working filter centrifuge with a conically widening, cantilevered sieve basket, in which the solids transport is facilitated with a conveyer screw rotating at a differential number of revolutions. Worm screen centrifuges have drum diameters of approx. 0.2 ÷ 1.0 m and operate with C-values of approx. 200÷3000. Their suspension throughput is approx.1.0 ÷ 80 m.t./hr. Particles to be separated should be larger 100 μm. Cake washing is limited due to the drum internals.

Wound Module
Spiral Module
Wrapped Module
Spiral Module
Wyckoff & Botset Equation

The permeability of filter cakes changes for both the liquid and the gaseousphase flows with a change in the saturation degree during a gas differential pressure demoisturing of the system. As for the permeability of the gaseous phase pg(S) as a function of the saturation degree S, it is according to Wyckoff & Botset the product of the single phase permeability pc and a relative gas permeability prel,g(S) ranging from 0 to 100%:

The permeability for the liquid phase pL(S), in turn, also as a function of the saturation degree S, is formed as the product of the single phased permeability pc and a relative liquid permeability prel,L(S) which also can attain values between 0 and 100%:

The function prel,L(S) can be described by the Brutsaert equation.

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