The five ways particles are filtered from the air

Removing particles are often a key role to meeting certain indoor air quality and critical environment goals. This is particularly the case when dealing with clean rooms or hospitals. There are a number of different types of pollutant particles that can affect air quality; particulate matter and gaseous pollutants.

Particulate matter includes dust, smoke, pollen, animal dander, tobacco smoke, particles generated from combustion appliances such as cooking stoves, and particles associated with tiny organisms such as dust mites, molds, bacteria, and viruses.

Gaseous pollutants come from combustion processes. Sources include gas cooking stoves, vehicle exhaust, and tobacco smoke. They also come from building materials, furnishings, and the use of products such as adhesives, paints, varnishes, cleaning products, and pesticides.

While there are a number of ways to filter particles from the air, mechanical filters are the most common. They use fibers to catch particles as air is forced through an HVAC system. These filters catch particles in five possible ways:

  1. Straining: larger particles are physically trapped between fibers in a filter
  2. Inertial Impaction: particles impact and are “stuck” in the fiber
  3. Interception: particles following a line of flow in the air stream come within one radius of a fiber and adhere to it
  4. Diffusion: an enhancing mechanism that is a result of the collision by the smallest particles, especially those below 0.1 µm in diameter, which are thereby impeded and delayed in their path through the filter; this behavior is similar to Brownian motion and raises the probability that a particle will be stopped by either of the two mechanisms above; it becomes dominant at lower air flow velocities
  5. Electrostatic attraction: positively charge filters attract negatively charged particles like a magnet

Diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm.

Particles in between (0.3 μm) are too small to be captured by interception and impaction, but too large to be captured by diffusion. Particles of this size  are by far the most difficult to remove, or the prestigious title of Most Penetrating Particle Size (MPPS).

Are you trying to determine particle size and concentration in your facility to determine the right type of filter?

Raeco can provide a number of instruments to accomplish this.

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