Glossary of Filtration Terminology

A housing for a very large collection of filter bag. It's a dust collector, but much larger.


Blocking up or clogging of filter media. Occurs when fine particles of dust become deeply lodged within the structure of the filter media to the extent that black clean of the media cannot remove it.

Can velocity

The upward lift velocity of gas between the base of the filter bags. A high Can velocity will result in heavy abrasion of the filtering media and a high level of dust re-entrainment during cleaning.


The property of certain substances to take up water from the air to form a solution.

The conversion of a solid substance into a liquid as a result of absorption of water vapor from the atmosphere. Since impurities in a solid lower its melting point, the absorbed water causes a decrease in the normal melting point of the solid. If enough water is absorbed to lower the melting point below room temperature, the solid will deliquesce, or turn to liquid. Lithium sulfide and magnesium iodide are examples of deliquescent salts.

Dust collector
A device in which air/gas is filtered. A device in which dust is collected.
Filtration velocity

aka - air to cloth ratio

The speed at which the gas passes through the filter media. Determined by dividing the throughput airflow rate by the nett filtration area.


Absorbing or retaining moisture from the atmosphere.

Nylon is not at all good at absorbing water. It is polymeric structure is hydrophobic (water hating) rather than like cotton which is hydrophilic (water loving). Thats why nylon clothing is so uncomfortable in hot and humid weather. Under those conditions, I will take cotton every time.


The property of certain substances to repel water.


Acid-base reaction between water and ions of a dissolved salt.


A phenomina only found in on-line cleaned filters. Dust moves (migrates) from filter media being cleaned to a filter media this is not being cleaned.

Nett Filtration Area (NFA)

The actual working area through which the air is filtered. Gross Filter Area (GFA) less non working surface area.

Off-line cleaning

Cleaning of the filter media while the filter media is not filtering. This is typically employed in large baghouses, on critical and continuous (24/7) industrial applications. The baghouse is usually compartmentalized into equi-sized cells (modules). These cells (compartments) are typically atleast 1/6th of the total unit size. Each compartment can be individually isolated (using dampers and or poppet valves) such that the compartment can be take off-line as required to allow its filter media to be cleaned. (i.e. have the dust blown and or shaken off the filter media. The frequency of media cleaning is typically determined by the rate of ∆P rise.

Off-line cleaning allows the filter media to its collected dust removed in static air flow conditions. Hence the problems associated with re-entrainment and dust migration are eliminated. The benefits achieved are lower ∆P, reduced energy consumption, extended media life and lower emissions.

On-line cleaning

Cleaning of the filter media while the filter media is filtering. This the most common and most widely used form.

Pressure Drop

aka - DP, Delta P, ∆P, Differential

Measurement of pressure loss caused by the build-up of pollutant on the surface of the filter media. A differential pressure reading taken across filter media. In most cases the loss caused by the filter media and its housing is negligible in comparison to the build-up of dirt.


A type of filter media cleaning methodology that involves using compressed air that is blasted in short pulses, from the clean side of the media. This cleaning air is directed such that is dislodges the collected dust off the dirty side of the media. Advanced pulse-jet compressed air cleaning systems typically arrange the cleaning system in such a manner that the jet of compressed air induces the flow of additional (secondary cleaning air). This improves the cleaning effectiveness, reduces energy consumptions, extends media life and reduces emissions.

Pulse-jet cleaning is used in off-line and on-line baghouses.


Refers to dust that has been collected and cleaned off the filter media, but then re-introduced into the incoming gas stream and re-presented to the filter media. Usually occurs at the base of the filter bags or just below. Typically a consequence of poor dust collector design.