Furnace Filters
Furnace Filter Information and Guide
Furnace Filter Glossary (I - Z)
The following are definitions for common terms related to furnace filters.
Impingement
The process in which particles are removed from an airstream because of their inertia. As air containing a particle flows toward a filter fiber or other collecting surface, the particle does not follow the airstreamlines because of its inertia. Instead it moves in a straight line colliding with the filter fiber or surface to which it may become attached.
Interception
The process in which a particle is removed from an airstream as it follows the streamlines around a filter. The particle comes in contact with a fiber and stays attached to it because the attractive forces between the fiber and the particle are stronger than the forces of disruption of the moving airstream.
Make-Up Air
Unfiltered air that is introduced into a re-circulated air system.
Media
The material in a filter that traps and holds the impurities.
Micron (µm)
(micrometer often abbreviated as um, micron or µ) A unit of measurement of length, equal to one-millionth of a meter (approximately 0.00003937 inch).
Most Penetrating Particle Size (MPPS)
The size of the particles that achieve maximum penetration of the filter medium. Particles that are smaller or larger than the most penetrating size exhibit a lower rate of penetration; the reduced penetration of the smaller particles is due to diffusion mechanisms, while for the large particles it is due to interception and inertial effects. The most penetrating size is a function of the structure of the filter medium, the velocity of the airflow through the filter, and the physical and chemical nature of the particles.
Particle
An object that is solid, liquid or both, usually between 1 nanometer and 1 millimeter in size.
Particle Concentration
The number of particles per unit volume of air.
Particle Size
The apparent maximum linear dimension of a particle in the plane of observation, as observed with an optical microscope, or the equivalent diameter of a particle detected by automatic instrumentation. The equivalent diameter is the diameter of a reference sphere having known properties and producing the same response in the sensing instrument as the particle being measured.
Particulate
An adjective referring to particles, e.g., particulate matter.
Penetration
The exit concentration of a given particulate or gas from an air cleaning device, expressed as percentage of inlet concentration, i.e. 99.99% efficiency = .010% penetration (100% - 99.99% = .010%).
Plenum
An enclosure in which air or other gas is at a pressure greater than that outside the enclosure.
Ultrafine Particles
Particles in the size range from approximately 0.02µm to the upper limit of detectability of the discrete particle counter.
Upflow Furnace
A furnace in which air is drawn in through the sides or bottom and discharged out the top.