Air-filtration technology has evolved tremendously since the advent of air filtration during the 1930s. The air-filter-selection process has evolved as well, not only protecting HVAC equipment, but protecting building occupants, reducing energy consumption, and minimizing environmental impacts.

With a variety of filtration technologies and styles to choose from, as well as industry-standard criteria for judging filtration performance, engineers now can select air filters to maintain HVAC-system cleanliness, improve indoor-air quality (IAQ), and reduce energy consumption simultaneously.

A HISTORY

Air filters were developed for use in the forced-air heating systems that were popular in commercial buildings during the 1930s. The filters primarily were designed to reduce the risk of building fires by protecting HVAC-system heating elements from dust. At the time, air filters consisted of loose mats of fibrous glass, animal and synthetic fibers, metal mesh, and various fabrics of cellulose and other porous materials.

As air-conditioning systems became more prevalent in commercial buildings during the 1940s and 1950s, filters were used more for HVAC-system hygiene, protecting cooling coils from dust and preventing fouling. These filters were made with the same materials used in the industry's earliest filters.

During the 1980s and 1990s, public concerns about IAQ began to emerge. The filter industry started to look at a filter's ability not only to protect HVAC equipment, but to clean air streams. During this period, the industry developed standard methods for testing filtration efficiency. With the advent of ASHRAE Standard 52.2, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size, minimum-efficiency reporting values (MERVs) became the accepted method of comparing filtration efficiencies over a range of particle sizes.

Today, air filtration is even more evolved. Fueled by various green-building initiatives and rising energy costs, commercial and institutional facilities are casting a more critical eye toward the roles air filters play in energy conservation, sustainable building design, and occupant health and productivity. Also, the terrorist attacks of Sept. 11, 2001, and the anthrax incidents that followed have created a focus on the use of air filters as protection against chemical or biological attacks.

HOW FILTERS WORK

There are four primary methods of particle capture for particulate air filters:

Straining

Straining occurs when a particle is larger than the opening between fibers and cannot pass through. It is an ineffective method of filtration because the majority of particles are smaller than the spaces between fibers.

Inertial impaction

Also called impingement, inertial impaction describes the capture mechanism of large, high-density particles. As air flows through a filter, it must make adjustments and change direction to flow around the filter's fibers. Large particles resist changes in an air stream's direction, and their inertia causes them to collide with and remain attached to a filter-media fiber.

Interception

Interception occurs when a particle follows an air stream, but comes in contact with a fiber. If the forces of attraction between a fiber and a particle are greater than the air stream's velocity, the particle will adhere to the fiber. Interception is enhanced when the sizes of a fiber and particle are comparable.

Diffusion

Diffusion takes place when very small particles are influenced by molecular collisions (called Brownian movement), causing a particle to strike a filter-media fiber at random and remain attached by van der Waals force.

AIR-FILTRATION EFFICIENCY

The first step in determining the best type of HVAC filter for a particular facility or application is to consider the conditioned space's activities and the types and sizes of particles those activities generate.

Removal of all airborne contaminants is not practical in most cases, so once problematic pollutants are identified, a filter's efficiency can be selected. Filtration efficiency defines how well a filter cleans indoor air by removing airborne particles.

Particle-capture efficiency is the primary performance indicator for HVAC filters. Once particle-capture efficiency is specified, other filter characteristics should be reviewed and compared to improve cost in use, such as a filter's resistance to airflow, which is proportional to the energy consumed by the filter. Energy expenditures can account for about 81 percent of an air-filtration system's annual operating costs, while its purchase price and maintenance can account for about 18.5 percent. The filter's recommended life should be a factor because of replacement-product and labor costs. The use of filter-media fibers that do not absorb moisture or shed also should be considered.

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