Industrial air filters are devices designed to remove solid particulates and molecular contaminants for the purpose of improving air quality in a system or environment.  Air filters typically consist of a sturdy frame, filled with some type of filter media, which is sealed to prevent leaks between the frame and media. Some filters may also have a faceguard — a screen attached to the filter to protect the media during handling — and/or a gasket to prevent leaks between the filter frame and its housing.

The image below illustrates these components on a typical air filter.

Image credit: Cambridge Filter

This image also illustrates typical air filter operation: air flows into the upstream side, contaminants are removed, and clean air exits through the downstream side.

While most air filters share the common components shown on the square filter above, the filters themselves may be manufactured in a variety of forms to suit the unique qualities of the filtration system of application. Typical form factors include bag filters, cartridge filters, and filter mats.

Filtration Methods

Air filters can be classified into three groups based on their filtration method: mechanical, electrostatic, and electronic.

Mechanical Filtration

Mechanical filters simply capture contaminants on the filter media and can work in three different ways, depending on the size of the particles being filtered:

• Straining occurs when contaminant particles are larger than the space between the media fibers, causing them to collect on the filter media. Straining is effective for filtering larger particles such as lint or hair.
• Interception occurs when particles, following the air stream, come in contact with media fibers and remain trapped there due to Van der Waal force, which describes the molecular attraction present.
• Diffusion is effective when filtering very small particles and is often employed by HEPA and ULPA filters. In diffusion, the particles are so small that they move erratically within the air stream, causing them to become stuck to media fibers.

Three methods of mechanical filtration (left to right): straining, interception, and diffusion.

Image credit: Cambridge Filter

Electrostatic Filtration

Electrostatic filters use electrostatically charged media fibers which attract and trap airborne particles. These filters typically use polypropylene or polyurethane filter medias. Electrostatic filters are capable of 94-96% efficiency in eliminating contaminants under proper operating conditions.

A cross-sectional view of a typical electrostatic filter. Image credit: S.I. Rosati

Electronic Filtration

Electronic filters consist of a series of electrodes inside the filter grid. When current is applied to the electrodes, particles drawn into the filter will cause a voltaic arc and take on a positive charge. The particles are then attracted to and trapped within a negatively-charged group of electrodes or a disposable/cleanable collector pad. Electronic filters are very effective at filtering small particles but are also noisy, relatively expensive, and difficult to service compared with mechanical or electrostatic types.

An example of an electronic air filter. Image credit: DwellSmart

Particle Size

As indicated above, the size of the contaminants to be filtered has a great impact upon the choice of a filter and media type, especially when dealing with mechanical filters. The chart below shows the range of sizes for common airborne contaminants. More detailed information about particle sizes can be found here.

Image credit: Vokes Filtration

Specifications

Media Type

An air filter may feature one of several different types of filtration media, depending on its intended application and the type and size of particle to be filtered. Common media types are described below.

• Acrylic media is made of synthetic polymers containing at least 85% acrylonitrile material.
• Activated carbon/charcoal filters are sometimes referred to as chemical filters. Activated carbon is a highly absorbent form of granular carbon used to filter small particles, odors, and other contaminants.
• Aluminum screen media is used to filter large, visible particles. These filters are often used in conjunction with fans where maximum airflow is required.
• Fiberglass media leverages the gradient density of fiberglass to trap progressively small particles.
• Paper media is comprised of compressed fibers and is capable of filtering small particles.
• Polyurethane foam is ideal for many filtration applications. These filters are safe for general handling, are non-allergenic and non-toxic, and are frequently used as fan filters.
• Poly/cotton media is common in many filtration uses and is comprised of polyester and cotton fibers bonded with ethylene vinyl chloride or poly vinyl acetate.

Filter media can also be specified by the number of material layers it contains. For example, a 1-ply filter contains one layer of filter media, while a 5-ply filter contains five layers.

MERV

Some filters may be provided with a maximum efficiency reporting value (MERV), which rates the performance of air purifiers and filters. MERV ratings are most frequently given to large air purification systems designed to provide whole-house or -building filtration, whereas smaller filters are usually supplied with a clean air delivery rating (CADR). A larger MERV rating is given to a filter which can filter smaller contaminants with higher efficiency. For example, a MERV 20 (the best possible rating) filter is capable of eliminating at least 99.999% of particles smaller than 0.3 μm; for reference, this would include all smoke particles, carbon dust, sea salt, and unattached viruses. A MERV 20 filter would be appropriate for critical uses such as pharmaceutical manufacturing, cleanrooms, and electronics manufacturing. By contrast, a MERV 4 (the lowest possible rating) filter is only rated to remove some particles larger than 10 μm; for reference, these particles include moss, pollen, mites, large dust particles, and carpet fibers. A MERV 4 filter is suited for basic residential uses such as air conditioner filtering.

For more about MERV ratings, please visit Air Purifier Guide's MERV page.

HEPA Filters

High efficiency particulate air (HEPA) filters are specialized filters which conform to the US Department of Energy (DOE) standards. In general, HEPA filters must be capable of removing 99.97% of particles greater than 0.3 μm. These filters are closely related to ultra-low particulate air (ULPA) filters.

All HEPA filters are disposable, rigid frame filters that typically feature media comprised of interwoven fiberglass fibers. Because the media functions as a depth filter (in that the fibers are randomly arranged), particles are retained throughout the filter medium rather than just on the surface.

More information about HEPA and ULPA filters can be found at IHS Engineering360's HEPA/ULPA Filters Selection Guide.

Applications

Air filters can be used in any application requiring the use of clean air. Devices and locations that use air filters include cars, gas turbines, air conditioners, industrial and commercial buildings, hospitals, vacuum cleaners, and food processing equipment.

The table below lists some common applications and approximate filter requirements.

Standards

Air filters may be manufactured and tested using various standards, including:

• BS EN 1822 - European standard for HEPA/ULPA filters
• BS EN ISO 16890-1 - Air filters for general ventilation Part 1: Technical specifications, requirements and classification system based upon particulate matter efficiency (ePM)
• ISO 14644 - Cleanrooms and associated environments

References

Engineering Toolbox - Air filter efficiency and arrestance