A Breath of Fresh Air: Cleaning the Air with Gas-Phase Filtration
By Paula P. Gillette, PE, CDT, HFDP, LEED AP BD+C
MasterSpec® Engineering Specification Writer
With the advent and implementation of sustainable design practices, the quality of indoor air has improved as compared with what was once commonplace. Yet, airborne contaminants remain a concern within as well as around the built environment. Gas-phase filtration further improves indoor air quality, in addition to cleaning contaminated exhaust air.
There are three major focus areas for application of gas-phase filtration:
- Commercial/Institutional Building Applications: Gas-phase filtration increases air quality and eliminates odors in places such as office buildings, hospitals, airports, loading docks, and morgues (autopsy) by removing general VOCs and ozone.
- Containment Applications: Gas-phase filtration contains or reduces the spread of hazardous gaseous contaminants in spaces such as cancer and chemical agent research laboratories, chemotherapy drug compounding laboratories, radiopharmaceutical facilities, and nuclear power plants.
- Commercial/Industrial Process or Storage Applications: Gas-phase filtration protects equipment and personnel from corrosive gas by-products in data centers, fertilizer plants, petrochemical production facilities, pulp and paper mills, manufacturing facilities (especially semi-conductor and steel production), and critical telecommunications buildings. Gas-phase filtration prolongs product storage life in food storage and archival storage applications.
Airborne contaminants can be classified as solid (fixed volume and shape), liquid (fixed volume but takes the shape of the container/enclosure), or gas (no fixed volume or shape). Gases are characterized by the greatest freedom of molecules and the lack of any inherent fixed shape and volume.
An aerosol is an airborne particle that is a suspension of fine solid or liquid particles, generally varying in size from 0.1 to 10 micrometers. Airborne dust is a common aerosol. An aerosol smaller than 0.1 micrometers behaves like a gas. Vapors are liquids in their gaseous form (such as water vapor) created by evaporating liquids. Vapors can be brought back to their original liquid form by increasing pressure while keeping temperature constant. Vapors are not a separate state of matter, as are liquids, solids, and gases. A bad smelling vapor is frequently referred to as fumes (e.g., gasoline fumes). Particulate filtration is accomplished with particulate filters of minimum efficiency reporting value (MERV), High Efficiency Particulate Air (HEPA), or Ultra Low Penetration Air (ULPA) rating.
Gas-Phase Filtration Media
Activated carbon, alumina, and zeolite, and impregnated activated carbon, alumina, and zeolite are examples of media used for gas-phase filtration. The gaseous "filtration" process is actually "adsorption" (for activated carbon, alumina, and zeolite) or "chemisorption" (for impregnated activated carbon, alumina, and zeolite). Adsorption is attraction of gaseous contaminants that "stick to" the surfaces of the activated carbon (or alumina or zeolite) pores without altering the composition of either. In chemisorption, new chemical bonds are formed between gaseous contaminants and the impregnated activated carbon (or alumina or zeolite).
Activated carbon (or alumina or zeolite) is carbon (or aluminum oxide or aluminosilicate) that has been processed to increase its pore surface area to provide additional surface area for contaminant gas adsorption. Common applications are for removal of VOCs and ozone.
Impregnated activated carbon (or alumina or zeolite) is activated carbon (or aluminum oxide or aluminosilicate) that has been impregnated with various chemicals selected to bond with specific contaminated gases that do not adsorb well to carbon (or alumina or zeolite). Examples include caustic sodium hydroxide impregnated carbon used for acid gas removal, phosphoric acid impregnated activated carbon used for amine and ammonia removal, and potassium permanganate impregnated activated alumina or zeolite used for acid gas and formaldehyde removal.
Activated and impregnated materials also can be mixed to target either a wide or narrow range of contaminants. For instance, a 50/50 blend of activated carbon and potassium permanganate impregnated activated alumina or zeolite is used to remove VOCs, ozone, acid gas, and formaldehyde. Whetlerite carbon (a carbon impregnated with special salts and metals such as zinc, copper, and silver) is used to remove chlorine, hydrogen cyanide, phosgene (military-type chemical agent gases).
Gas-Phase Filter Types
Filtration media come in several forms, such as flat or pleated panel filters, v-cell filters, rigid cell box filters, and deep-v filters.
Gas-phase panel filters are flat panel or pleated, multilayer filters with an inlet layer of non-woven polyester fibers and a filtration layer, bonded or supported by a grid, of adsorbent media of variable depth. This is typical construction for a simple activated carbon filter for light-duty ozone and odor filtration. These are available as either a combination MERV/gas-phase filter or in series with a MERV-rated filter. Not all MERV ratings are available in the combination type.
A step up from a gas-phase panel filter is a gas-phase v-cell filter. The media in these filters are shaped in a v-configuration, with panels of honeycombed paper and nylon mesh or mini-pleats on each side of the adsorbent media. They are deeper than the gas-phase panel filters and require a support structure that can be made of high-density polystyrene (HIPS), acrylonitrile butadiene styrene (ABS), or galvanized steel. They are available in combinations of activated and impregnated media and are used to adsorb VOCs, ozone, acid gas, ammonia, amine, and formaldehyde. These filters can be installed in series with MERV-rated filters.
A gas-phase rigid cell box filter uses a non-woven material with plastic spacers that keep deep pleats in position. It can be combination MERV rated or in series with a MERV-rated filter. Not all MERV ratings are available in the combination type. This type of filter can use either activated carbon or a blend of activated carbon and potassium permanganate to adsorb VOCs, ozone, acid gas, and formaldehyde.
For heavy-duty dangerous and hazardous contaminant removal, a deep-v gas-phase adsorber can be specified. This type of filter includes a frame consisting of trays or cassettes that contain granular or pelleted adsorbent media placed between screens. This type more readily permits media to be customized for an application that may include removal of less common contaminate gases.
For heavy-duty dangerous and hazardous containment-required gaseous contaminants, High Efficiency Gas Adsorber (HEGA) filters are appropriate. HEGAs are gas-phase filters that are tested and rated to strict standards. HEGAs contrast with standard gas-phase filters in the way MERV-rated filters contrast with HEPA filters. For dangerous and hazardous containment-required contaminants, side-access bag-in/bag-out containment housings are recommended to protect maintenance personnel during filter change-out operation.
For disposal of filters, those made of HIPS or ABS may be able to be incinerated depending on local jurisdiction restrictions and adsorbed contaminants. Verify with local authorities having jurisdiction that the absorbed contaminants meet the requirements to be incinerated or determine alternative means of disposal.
Prefilters may be used to keep particulate from collecting on carbon surfaces. For some types of carbon media, manufacturers recommend final filters downstream to eliminate carryover carbon dusting, but this is no longer as much of an issue with most types of adsorber media as it has been in the past. However, for particularly hazardous and dangerous contaminants, HEPA filters may be recommended downstream of the gas-phase filter (for some types of HEGA filter applications) to retain any contaminated dust that may be released from the HEGAs.
Access is an important consideration. Standard gas-phase filters are sometimes mounted within an air handling unit filter access section. Some modules are designed for side-access filter housing. Some are designed for front or rear access. With so many options for gas-phase filtration, it has never been easier to get a breath of fresh air.