Air Filters - Arrestance and Efficiency
Air filters efficiency and arrestance.
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)
where
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)
where
μa = dust arrestance
Ca = dust concentration after filter
Cb = dust concentration before filter
