Glossary of industrial filtration

The material that actually retains particles: a felt, a woven fabric, a pleated paper, a metal cloth, a bed of carbon. It is what sets the temperature limit, the chemical compatibility and the fineness reached — the frame merely holds it in place. A filter element is chosen by its medium first.

See alsoWeave · Grammage · Surface treatment

A tube of flexible medium, closed at one end, fitted over a metal cage in a dust collector. Dust-laden air enters from the outside, dust builds up on the upstream face, and filtered air leaves through the inside of the tube. Bags are replaced as a complete set, never one at a time: a new element beside clogged ones takes the whole airflow.

See alsoDust collector · Cleaning · Filter medium

A volume of medium sewn into a bag shape, developing a large filter area within a shallow housing depth. In air filtration an element holds several, flexible or rigid. Beware the false friend: in liquid filtration the French word covers a different object, hung by its collar inside a restrainer basket.

See alsoFilter area · Restrainer basket

A cylindrical element closed around a central core, whose medium is pleated, wound or woven according to the fineness sought and the fluid handled. It sits in a filter housing and is replaced whole. Pleating develops area, winding retains within the depth, metal cloth can be cleaned and refitted.

See alsoFilter housing · Filter medium

A technical fabric supplied by the metre or made to size, used on filter presses and belt filters. Its air permeability, its weave and its surface treatment govern how the cake releases and how clear the filtrate runs. A backing cloth protects it and extends its mechanical life.

See alsoWeave · Surface treatment

The structure that holds the medium and seals it against the housing wall. It filters nothing, yet it governs the performance actually obtained: a perfectly efficient element in a leaking frame lets air pass beside the medium. Its material also sets a temperature limit of its own, independent of the medium's.

See alsoFilter medium · Efficiency

An element placed upstream of a finer filter to catch coarse particles and spare it that load. It is almost always cheaper than what it protects: replacing it in time is the simplest way to extend the life of the next stage, and to delay the rise in pressure drop across the whole assembly.

See alsoFilter staging · Pressure drop

Placing several filtration stages in series, from coarsest to finest. Each stage protects the next and only has to retain what the previous one let through. It is staging, not one highly efficient element, that makes an installation last: skipping a stage makes the most expensive element pay for the coarse load.

See alsoPre-filter · Efficiency

The operation that releases the dust layer deposited on the upstream face of an element, bringing the pressure drop back to an acceptable value. It is done by a reverse pulse of compressed air, by continuous reverse air, or by mechanical shaking. The method chosen sets the mechanical strength demanded of the medium, and therefore its selection.

See alsoClogging · Filter bag · Dust collector

The gradual build-up of particles in and on the medium, reducing the passage offered to the fluid and driving the pressure drop up. Clogging is not only a defect: the deposited layer filters in its own right, and the efficiency of a new element is often lower than that of the same element after a few hours in service.

See alsoPressure drop · Cleaning

The pressure difference measured either side of an element. It is the most direct condition indicator: it starts at an initial value, rises with clogging, and reaches the final value beyond which the element is replaced. That final value comes from the builder of the installation and from the medium, never from a general rule.

See alsoClogging · Filtration velocity

The area of medium actually offered to the fluid, far greater than the element's footprint thanks to pleating or pockets. At equal flow, the larger the area, the lower the face velocity — hence a low initial pressure drop and a long service life. It is the quantity sized first on a dust collector.

See alsoFiltration velocity · Pocket

The ratio of the flow handled to the filter area installed. It governs everything else: too high, it compacts the deposited layer, drives the pressure drop up and forces particles into the depth of the medium. The admissible value depends on the dust, the medium and the cleaning method — it does not transfer from one installation to another.

See alsoFilter area · Cleaning

A particle size at which the element retains only a fraction of what reaches it. The word "nominal" therefore commits to partial retention only, and it is not defined the same way by every manufacturer. Two elements quoted at the same nominal rating can behave very differently on the same circuit.

See alsoAbsolute filtration rating · Efficiency

A particle size beyond which retention is near total, established by a standardised test measuring the ratio of particle counts upstream and downstream. It is a far more demanding characteristic than a nominal rating, and the only one to reason from when fluid cleanliness is a process requirement.

See alsoNominal filtration rating · Efficiency

The proportion of what reaches the element that it actually retains. It only means something alongside a particle size and test conditions: an efficiency quoted without saying on what, and measured how, is not a characteristic. Standardised ratings exist precisely to fix those conditions.

See alsoAbsolute filtration rating · Filter staging

The mass of medium per unit area. For the same material and weave, a higher grammage means a thicker, stronger medium, hence better mechanical resistance to cleaning, but also a higher initial pressure drop. It is one of the two or three figures that tell apart two felts of identical appearance.

See alsoFilter medium · Weave

How the yarns of a fabric interlace: plain, twill, satin. It governs the regularity of the openings, the suppleness, the abrasion resistance and how easily the cake releases. Two cloths of the same material and grammage behave differently if their weave differs.

See alsoFilter cloth · Grammage

An operation applied to the medium after manufacture to change its surface behaviour: calendering, singeing, coating, laminated membrane, water- or oil-repellent finish, antistatic treatment. It does not change the material but often changes everything else — cake release, resistance to moisture, dissipation of static charge.

See alsoFilter medium · Filter cloth

An element that gathers finely dispersed droplets in a gas or a liquid into larger ones, which gravity then separates. It does not retain the separated phase: it makes it separable. It is a separation device rather than a particle filter, even though it often fits in the same housing.

See alsoDemister pad · Filter housing

A thick mat of knitted metal or synthetic yarn, set across a gas stream to arrest entrained droplets. Drops catch on the yarns, gather and fall back. Its effectiveness depends on the knit density, the wire diameter and the gas velocity — too fast, and it re-entrains what it has just captured.

See alsoCoalescer

The installation that draws dust out of an airflow and retains it on a set of filter elements, together with its cleaning system and its collection hopper. It is the installation, not the element: you size a dust collector, you replace bags or cartridges.

See alsoFilter bag · Cleaning · Filter area

The pressure vessel that receives one or more cartridges or liquid bags, with its lid, its inlet and outlet nozzles and its pressure tappings. Its material and its seals must withstand the fluid just as the medium does: a compatible housing with an incompatible element makes an incompatible installation.

See alsoCartridge · Restrainer basket

The perforated grid that supports a liquid bag or a filter bag and keeps it from deforming under fluid pressure. Without it, the medium collapses, creases and eventually tears. Its perforation and diameter must match the element: it is a dimension to record before ordering.

See alsoPocket · Filter housing

The fineness of a metal cloth, expressed as the number of wires per unit length. The higher the count, the smaller the openings — but the actual opening also depends on wire diameter and weave: at equal mesh, two cloths may not retain the same thing. The aperture dimension is the figure that matters.

See alsoWeave · Filter medium