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Air Filters - Arrestance and Efficiency

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The mission of an air-filter is to remove particulate or gaseous materials from the air-stream and efficiency and arrestance are used to describe filter functionality.

Air Filter Efficiency

Efficiency describes how well an air filter removes microscopic particles - such as dust, dust mites, pollen, mold, bacteria, and smoke.

In the ASHRAE 52.1 Standard test method, efficiency is a measure of ability to remove the staining portion of atmospheric dust from test air - officially termed "Atmospheric Dust Spot Efficiency".

Air filter efficiency can be expressed as

μe = nt / nu = (nu - nd) / nu                                  (1)


μe = air filter efficiency

nt = particles trapped

nu = particles upstream

nd = particles downstream

Air Filter Arrestance

Arrestance is a measure of the ability of an air filtration device to remove synthetic dust from the air.

The arrestance describes how well an air filter removes larger particles - such as dirt, lint, hair, and dust. ASHRAE arrestance is a measure of the ability of a device to remove ASHRAE dust from test air.

The dust holding capacity of a filter is the amount by weight of standard dust the filter will hold without exceeding the resistance 0.18 in W.G. for low-resistance filters or 0.50 in W.G. for medium-resistance filters and 1.0 in W.G. for high-resistance filters.

Filter dust arrestance can be expressed as

μa = 1 - Ca / Cb                                     (2)


μa = dust arrestance

Ca = dust concentration after filter

Cb = dust concentration before filter

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Related Topics

Ventilation Systems

Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

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Particle Sizes

The size of dust particles, pollen, bacteria, virus and many more.


Volume not occupied by solid material.

Ventilation Filters

Classification of air filters used in ventilation systems.

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