What’s so important about air filtration?

Filters are ubiquitous in air handling systems, so it’s helpful to understand the different options for filters and to learn about how they function

By Kevin Langan, PE, IMEG Corp., Rock Island, Ill. March 17, 2020

Learning objectives

  • Learn fundamentals of how filters remove airborne particles from an airstream.
  • Discuss the different styles, types and ratings of filters used in HVAC systems.
  • See how these fundamentals can be applied in a health care setting.

Almost every air handling system has some level of air cleaning capability. Air filtration refers to removing airborne particles using media filters. From a high level, air filtration is provided in a heating, ventilation and air conditioning system for two main reasons: to protect equipment components from accumulating dirt/debris and to reduce the quantity of air contaminants inside the building environment.

In the most basic sense, air filters work by capturing a portion of the particles in the air that passes through the filter. While the basic concept of air filtration remains the same, the mechanism used to capture particles and the quantity/size of particles captured can vary considerably between different filters.

As described in Chapter 11 of the 2017 ASHRAE Handbook: Fundamentals, air contaminants can either be in the form of particles or gases and vapors. Gases and vapors exist in air as individual molecules, whereas particles are significantly larger than individual molecules. Given their relatively small size, gases and vapors are typically not removed from air through traditional media filtration and are not explored as part of this article.

Airborne particles can consist of many different materials. These particles can either be produced outdoors or inside the building. Outdoor particles can be produced from natural processes (such as wind, volcanic activity or decay of organic materials) or human activities (such as construction, agriculture, industrial plants and transportation). Particles also can be produced inside the building from building occupants, material off-gasing and building activities. Airborne particles can be composed of solids, liquids or a combination of the two and can be broken into the following categories:

  • Dusts, fumes and smokes.
  • Mists, fogs and smogs.
  • Bioaerosols (i.e., viruses, bacteria, mold spores, allergens, dander and endotoxins).

Additionally, the sizes of various airborne particles can vary considerably. See Table 1 for typical sizes of some common airborne particles.

The portion of air particles captured and the size of captured particles determines the removal efficiency of an air filter. The most widely used air filter efficiency scale in the HVAC industry is the minimum efficiency reporting value scale developed by ASHRAE. The MERV scale ranges from MERV 1 to MERV 16, with increasing MERV ratings corresponding to higher capture efficiency of more and smaller particles.

The testing procedures and parameters used to determine the MERV ratings of air filters are governed by ASHRAE Standard 52.2: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. Table 2 provides a high-level summary of the ASHRAE 52.2 MERV performance requirements as well as the types of particles filtered by each rating.

Filtration efficiencies above the highest MERV rating (MERV 16) are typically defined by high-efficiency particulate air and ultralow penetration air filters. HEPA filters are defined by the United States Department of Energy Standard DOE-STD-3020-2015 as filters that remove at least 99.97% of airborne particles with diameters greater than or equal to 0.3 microns. This size range includes all bacteria, mold spores, dander and some smoke and fumes (see Table 1).

ULPA filters remove at least 99.9995% of airborne particles 0.12 microns or larger, with increasing levels of ULPA filtration efficiencies beyond this base level. ULPA filtration levels are defined by the Institute of Environmental Sciences and Technology Recommended Practice IEST-RP-CC001 and European Standard CSN EN 1822-1.

Media filtration mechanisms

There are a variety of mechanisms by which the air particles are captured in a media air filter. In general, media air filters function by forcing air to flow through a fibrous media. As the air flows through this media, some of the particles in the air are captured by the media. As outlined in Chapter 29 of the 2016 ASHRAE Handbook: HVAC Systems and Equipment, the five main mechanisms by which particles are captured by the media are straining, inertial impingement, interception, diffusion and electrostatic effect.

Straining works by forcing an air particle through an opening that’s smaller than the particle, trapping the particle. This mechanism is typically most effective on larger particles.

Inertial impingement works by forcing the air around filter fibers. As the airstream curves around filter fibers, the inertia of the airborne particle can force it into a filter fiber. Depending on the properties of the particle and the fiber surface, the particle may remain stuck to the filter. Inertial impingement is more common with higher velocity permanent media filters.

Interception works similarly to inertial impingement, but with some important differences. With interception, the airstream curves around filter fibers. The airborne particles get close enough to the fibers that they make contact and adhere through molecular interactions. The contact between the particle and filter fiber occurs more on the side than the front of the fiber, which differs from inertial impingement. Also, because interception requires close interaction between particles and filter media, it’s more common with low-velocity extended media filters.

The diffusion capture mechanism is essentially an extension of the interception capture mechanism. Diffusion works through the normal erratic movement of very small particles. This erratic movement can bring particles close enough to filter fibers to be captured via interception. The buildup of particles on filter media actually increases the effectiveness of diffusion capture. Diffusion is more effective at lower velocities and with smaller particle that move via diffusion.

Electrostatic effect works by giving the filter media an electrostatic charge, which can attract and capture particles with the opposite charge. The filter’s electrostatic charge can be passive or active. A passive charge is added during the manufacturing of the filter or is generated in the filter media as air passes through it. On the other hand, an active charge is added to the filter with a power source.

Filter styles

While the particle capture mechanisms of all media filters fall within the five categories laid out, filters can be constructed in many different ways. Depending on the application, several variables surrounding the design of a filter can be adjusted to achieve a balance between filtration efficiency, air resistance, particle holding capacity, leakage potential and cost.

As a filter holds onto more particles, its filtration efficiency actually increases due to smaller and smaller airways, but so does its air resistance. A filter with higher particle holding capacity allows it to trap more particles while minimizing the increase in air resistance. This increases the life of the filter, decreasing operating costs in the form of filter replacements.

Also, a filter usually has a maximum air resistance for which it’s designed. A pressure drop across the filter that is higher than this design value can result in damage to the filter. Some filter design variables that can be adjusted include: media material, media assembly style, filter installation style, edge sealing style and filter size.

Fiberglass is the most common filter media material used in commercial HVAC systems. There are several properties of fiberglass media that can be adjusted based on the requirements of a filter. Some examples of these properties include average diameter and length of the glass fibers and the packing density of the glass fibers. In general, denser packing of the glass fibers can offer better filtration efficiency but will have a higher air resistance. Coarser fibers are used for lower filtration efficiencies while finer fibers are used for higher filtration efficiencies.

Fiberglass fibers used in HEPA and ULPA filters are especially fine and are often referred to as microglass. Some filters even contain different types and densities of fiberglass throughout the depth of the filter. These variations can allow for increased particle holding capacity.

Note that there are also other filter media materials besides fiberglass. Permanent media filters, also referred to as washable filters, often have media consisting of metal and/or plastic filaments arranged in a weave. These filters typically have relatively low efficiencies, with MERV ratings between 1 and 4, but they are designed so they can be cleaned of captured particles and reinstalled.

HEPA and ULPA filters can be constructed of plastic fibers, such as polypropylene and expanded polytetrafluoroethylene (i.e., Teflon). Expanded PTFE offers some advantages over microglass filter media, such as more uniform fiber size distribution, smaller fiber size, increased media strength and more chemical resistance. These properties offer lower air resistance and less potential for filter damage at higher airflows and pressure drops. Additionally, some filter medias are embedded with antimicrobial materials to help maintain the integrity of the filter throughout its life.

In addition to the media material itself, there are many variations of media arrangement within a filter.

The simplest arrangement of filter media is a flat panel across the face of the filter, with or without a supporting material included to hold the media in place. The flat panel media arrangement is used with lower efficiency filters with minimal filter depth. This style of filter is not commonly seen in health care central air handling systems.

Perhaps the most common style for arranging media in a filter is to form the media into a series of pleats. This effectively increases the surface area of the filter, which helps to reduce air resistance and increase particle holding capacity. A supporting structure is sometimes required to maintain the arrangement of the pleats. The style and material of the pleat support structure depends on the spacing of the pleats, the maximum resistance rating of the filter and the depth of the filter.

Thin filters (e.g., depths less than 4 inches) with efficiency ratings less than MERV 8 might have self-supported pleats or a cardboard pleat support structure. Deep filters (e.g., 6 or 12 inches deep) with higher efficiencies might have pleat supports/separators constructed of aluminum or plastic. Some deep filters are constructed of multiple v-banks of thin pleated media filters. The v-banks are supported by a rigid plastic or metal structure. This arrangement can achieve a higher amount of media area than a typical deep pleat filter, increasing dust holding capacity and decreasing air resistance.

Thin pleated filters are typically referred to as “pleated filters” whereas deep pleated filters are referred to as “box filters” or “cartridge filters.” These types of filters are most commonly seen in health care central air handling systems.

Another filter media arrangement style is the bag filter. Bag filters consist of multiple media pockets supported by a frame on the front face of the filter. As air flows through the filter, it inflates the media pockets. This style of filter can accommodate relatively deep filters and high filter efficiencies, but not HEPA/ULPA ratings. These filters are susceptible to leakage and damage from impacts or moisture. They are seen in health care HVAC systems, but are not as common as pleated and cartridge filters.

Filters can be installed into air handling systems in two main ways: side-load and front-load. As implied by the name, side-load filters are loaded from the side of the air handling system. The filters are inserted in one side of the unit, with additional filters pushed into the side of previously loaded filters until they cover the entire width of the air tunnel.

This installation style in convenient from a maintenance standpoint, but it does have drawbacks. The seals between the filters are only achieved by the sides of the filter frames being pushed against each other. Ultimately, this does not achieve an airtight seal and will result in unfiltered air bypassing the filters. This is typically acceptable for low efficiency filters (i.e., MERV 8 or less) but is less acceptable for higher efficiency filters (i.e., MERV 13 and higher).

Front-load filters are installed into the air handling system from the front. Each filter is mounted into a grid-style frame that covers the face of the air tunnel. This requires more work to install the filter and requires a higher cost filter. However, this installation style offers much better sealing around the edges of the filter. Each filter is able to be sealed to the filter frame, minimizing the bypass of unfiltered air. Because of this feature, front-load filters are much preferred for high-efficiency filters.

There are also various options for the sealing mechanism around the edge of a filter. For low-efficiency side-load filters, there may not be any sealing mechanism, other than the flat frame of the filter. Front-load filters can be provided with a gasket material on the filter header, where the filter contacts the air handling system filter support frame. This gasket can be especially effective because the airflow against the front face of the filter pushes the gasket tighter against the support frame.

For HEPA/ULPA filters, a gel-seal with knife-edge design can be provided to seal around the filter. This sealing system includes a channel filled with gel. A knife-edge or thin metal protrusion is inserted into the gel when the filter is installed. This seal results in virtually no air leakage around the edges of the filter. A gel-seal with knife-edge system is basically required for any HEPA/ULPA filter that will be field tested for performance to HEPA/ULPA standards.

Experience has shown that a standard filter gasket will allow too much particle leakage for the filter to pass HEPA/ULPA performance requirements. This is especially applicable for pharmacy clean rooms within health care facilities. Per pharmaceutical regulations, the HEPA filters serving these spaces have to be tested biannually. Typically, it’s difficult to get a high-performing gel-seal with knife-edge system in a central air handling unit, so the HEPA filters perform best when installed in ceiling mounted laminar flow diffusers or fan filter units.

Health care filtration requirements

The main code/standard governing required filtration levels in health care facilities is ASHRAE Standard 170: Ventilation of Health Care Facilities. ASHRAE 170 is adopted as code in numerous jurisdictions and compliance with the standard is required for Centers for Medicare and Medicaid Services reimbursements.

ASHRAE 170 lists required minimum filter efficiencies for systems serving various types of spaces within a health care facility. The minimum filter efficiencies are listed for two filter banks: the filter bank upstream of the main heating and cooling coils (normally referred to as the “pre-filter”) and the filter bank downstream of all wet cooling coils (normally referred to as the “final filter”). In general, a minimum filter efficiency of MERV 7 is required for pre-filters and MERV 14 is required for final filters. Some spaces do not require final filters, while other spaces require higher filtration levels. Refer to Table 3 for a summary of the filter efficiency requirements included in the 2017 version of ASHRAE 170.

In addition to ASHRAE 170, there are additional standards that place filtration requirements on spaces within a health care facility. The most notable examples are USP General Chapter <797> Pharmaceutical Compounding – Sterile Preparations and USP General Chapter <800> Hazardous Drugs – Handling in Healthcare Settings, which govern the compounding of drugs in pharmacies. These regulations require HEPA filtration for most compounding clean rooms.

Author Bio: Kevin Langan is a senior mechanical engineer at IMEG Corp. He serves as project manager and lead mechanical engineer on a variety of health care projects.