Particulate Air Filtration
Particulate air filters are classified as either mechanical filters or electrostatic filters (electrostatically enhanced filters). Although there are many important performance differences between the two types of filters, both are fibrous media and used extensively in HVAC systems to remove particles, including biological materials, from the air. A fibrous filter is an assembly of fibers that are randomly laid perpendicular to the airflow (Figure 2). The fibers may range in size from less than 1 µm to greater than 50 µm in diameter. Filter packing density may range from 1% to 30%. Fibers are made from cotton, fiberglass, polyester, polypropylene, or numerous other materials.
Fibrous filters of different designs are used for various applications. Flat-panel filters contain all of the media in the same plane. This design keeps the filter face velocity and the media velocity roughly the same. When pleated filters are used, additional filter media areadded to reduce the air velocity through the filter media. This enables the filter to increase collection efficiency for a given pressure drop. Pleated filters can run the range of efficiencies from a minimum efficiency reporting value (MERV) of 6 up to and including high-efficiency particulate air (HEPA) filters. With pocket filters, air flows through small pockets or bags constructed of the filter media. These filters can consist of a single bag or have multiple pockets, and an increased number of pockets increases the filter media surface area. As in pleated filters, the increased surface area of the pocket filter reduces the velocity of the airflow through the filter media, allowing increased collection efficiency for a given pressure drop. Renewable filters are typically low-efficiency media that are held on rollers. As the filter loads, the media are advanced or indexed, providing the HVAC system with a new filter
Four different collection mechanisms govern particulate air filter performance: inertial impaction, interception, diffusion, and electrostatic attraction. The first three of these mechanisms apply mainly to mechanical filters and are influenced by particle size.
- Impaction occurs when a particle traveling in the air stream and passing around a fiber, deviates from the air stream (due to particle inertia) and collides with a fiber.
- Interception occurs when a large particle, because of its size, collides with a fiber in the filter that the air stream is passing through.
- Diffusion occurs when the random (Brownian) motion of a particle causes that particle to contact a fiber.
- Electrostatic attraction, the fourth mechanism, plays a very minor role in mechanical filtration. After fiber contact is made, smaller particles are retained on the fibers by a weak electrostatic force.
As mechanical filters load with particles over time, their collection efficiency and pressure drop typically increase. Eventually, the increased pressure drop significantly inhibits airflow, and the filters must be replaced. For this reason, pressure drop across mechanical filters is often monitored because it indicates when to replace filters.
Conversely, electrostatic filters, which are composed of polarized fibers, may lose their collection efficiency over time or when exposed to certain chemicals, aerosols, or high relative humidities. Pressure drop in an electrostatic filter generally increases at a slower rate than it does in a mechanical filter of similar efficiency. Thus, unlike the mechanical filter, pressure drop for the electrostatic filter is a poor indicator of the need to change filters. When selecting an HVAC filter, you should keep these differences between mechanical and electrostatic filters in mind because they will have an impact on your filter's performance (collection efficiency over time), as well as on maintenance requirements (change-out schedules).