Air Filters - Arrestance and Efficiency

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

μ_e = n_t / n_u

     = (n_u - n_d) / n_u                                  (1)

where

μ_e = air filter efficiency

n_t = particles trapped

n_u = particles upstream

n_d = 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

μ_a = 1 - C_a / C_b                                     (2)

where

μ_a = dust arrestance

C_a = dust concentration after filter

C_b = dust concentration before filter

• Particle Sizes - Sizes of airborne particle as dust, pollen bacteria, virus and many more