Components of Good Indoor Air Quality
Good IAQ (indoor air quality) is an important component of a healthy indoor environment. Poor air quality can lead to lost worker productivity and tenant health complaints such as allergic reactions, nausea, chronic fatigue, or respiratory problems.
One of the key elements to good IAQ is controlling airborne contaminants (the other elements are maintaining acceptable temperature and relative humidity, and adequate distribution of ventilation air).
Airborne Contaminants
The contaminants that produce indoor air pollution include carbon dioxide, smoke, dust, odors, and biological contaminants such as mold and bacteria. Some of the most dangerous pollutants result from the gas released by materials used in newly-finished construction and space furnishings. Indoor air may contain tobacco smoke, gaseous emissions from fabrics, furniture, and adhesives, as well as offensive odors from equipment, solvents, and people.
IAQ problems can develop because of improper maintenance, modifications to controls, and improper design of HVAC systems. Improper maintenance can cause missing, dirty, and/or collapsed filters. Habitats for microbial growth are often created by standing water in condensate pans and clogged condensate lines, by moisture blowby from fan coils into ducts, and by standing water at outside air intakes.
To prevent or reduce IAQ problems, you should operate and maintain HVAC systems properly. By correcting temperature and RH levels, replacing filters as needed, and cleaning mechanical rooms, ducts, and equipment, you can reduce IAQ problems. The condensate pan should be treated yearly with tablets that reduce bacteria growth and prevent the growth of algae. You can also eliminate sources of indoor pollutants by identifying the composition of construction materials, furnishings, and other supplies brought into the building. This process includes assuring that outside air is not brought in from areas near standing water, water spray, or exhaust outlets. You should also provide adequate and effective ventilation. One technique for controlling odors and contamination is to dilute them with outside air. It is also essential to educate the building staff and occupants about IAQ issues, sources and effects of pollutants, and control measures.
Contaminant Removal
Three basic types of contaminants need to be removed from the air.
- Solid and liquid particles: Various particles and fibers, often the products of combustion
- Gases and vapors: nicotine, formaldehyde, and other VOCs (volatile organic compounds)
- Microbiological contaminants (sometimes referred to as bio-aerosols): mold, fungus, bacteria, animal feces, insect body parts, plant parts, or any part of a living or once-living organism
These contaminants are most often removed with filters or, in the case of bacteria, other types of cleaning devices. The four principal types of filters are:
- viscous impingement filters
- fibrous filters
- membrane filters
- electrostatic precipitators
Viscous Impingement Filters
Viscous impingement filters effectively clean air where there are heavy dust concentrations. These filters use adhesive-coated baffles to divide and deflect the latent airstream dust particles mechanically. After being separated from the air, the particles are trapped in the viscous film. These filters have a large capacity to hold dust. Changing the thickness and density of the impinging media can affect cleaning efficiency and resistance to airflow.
Fibrous Filters
Fibrous air filters are made of nonwoven webs of glass polymeric, metal, or natural fibers. Fiber diameters range from about 0.2 to 1,000 micrometers. Media fibers are usually bonded together to give dimensional stability and to resist compression. Practical media are made with as much open space as possible to allow air passage at low-pressure drops. Fibrous air filters come in four basic types: panel filters, pleated-media filters, bag filters, and roll filters.
- Panel filters: Panel filters have an open-filter media with low airflow resistance. The media are held in thin panes, and filters are placed so they are perpendicular to the main airflow direction. This ensures the velocity through the filter media is equal to the duct velocity.
- Pleated-media filters: Pleated-media filters use a dense, high-resistance media with low airflow velocities. The filter media are held more or less rigidly in parallel planes by spacers. These spacers can be corrugated paper, aluminum inserts, rivets, or threads attached to either the surface of the filter media or corrugations of the media itself.
- Bag filters: Bag filters have a media that is formed into tubes or pockets. These filters are closed at one end and sealed at the opposite end into a header plate or frame. The purposes of this construction are the same as those of pleating: to expand the media area, to reduce the airflow velocity, and to enhance pressure drop. These filters are usually collapsible and are sometimes held up by wire frame supports.
- Roll filters: Roll filters use compressible panel-type class or polymer fiber, which is supplied in the same manner as rolled camera film. A clean filter is usually placed into the air-filtering zone by an automatic transport mechanism activated by a clock or sensor of resistance to dust load.
Membrane Filters
Membrane air filters are made from very thin polymer sheets with extremely small airflow passages. They trap particles on thin film, or sponge-like membranes. They are available in four forms: collimated pore membranes, granular symmetric membrane filters, granular asymmetric membrane filters, and fiber membrane filters.
- Collimated pore membranes: Collimated pore membranes have micrometer diameter holes running straight through the membrane. These membranes have a high solid material content and high airflow resistance but can also offer absolute removal of particles larger than their tube diameters.
- Granular symmetric membrane filters: Granular symmetric membrane filters are formed like sand beds or sponges on a microscopic scale.
- Granular asymmetric membrane filters: Granular asymmetric membrane filters have a relatively open, sponge-like structure that acts like a support for a very thin face layer containing ultrafine pores. This thin layer is the actual filter.
- Fiber membrane filters: Fiber membrane filters are smaller in structure than the fibrous media previously described. Membranes are usually pleated and are formed into panels or cylindrical cartridge filters for gas filtration. The fragility of the fiber membranes, their cost, and the relatively high pressure drop for a given flow limit the application of this filter to the production of ultra-pure gases for air sampling and air pollution control equipment. Future developments may make it practical for ventilation applications.
Electrostatic Precipitators
Electrostatic precipitators electrically charge particles and attract the charged particles to plates. They operate in two stages. During the first stage, unclean air is passed through an ionizing zone where dust particles are electrically charged. The ionizer consists of fine tungsten wires that are suspended parallel to and midway between grounded struts. Each wire is maintained at a high voltage (5 to 14 kV, depending on the spacing). During the second stage, the air, now containing positively charged particles, flows through a plate package whose plates are alternately grounded and charged so that fields perpendicular to the airflow exist between the plates. These fields drive the charged dust particles to the plates. Here, the particles are retained with the help of a low-flammability viscous liquid.
Electrostatic precipitators can filter about 90 percent of the fine dust, smoke, and fumes from the air within overall pressure drop of no more than 3 millimeters. This pressure drop will not increase appreciably as grease builds up on the plates. After a period of usage, the plates must be washed, cleaned, and recoated using adhesive.
Electrostatic filters operate at voltages and amperages that vary depending on the manufacturer. These filters use alternating current or direct current at levels that can be lethal; therefore, adequate safety measures must be taken. High voltage cables must be sufficiently insulated and run through a metal conduit. All plenum doors accessing electrostatic filters must be provided with closures that turn off power supply capacitors and delay entry for approximately fifteen seconds. This allows the power supply capacitors to bleed before anyone can enter the plenum.
Conclusion
The removal of contaminants from the air is a vital component of a well-functioning IAQ system. By employing the proper filters, and with the proper maintenance, crucial solid and liquid contaminant particles will be kept at bay—and away from tenants. The result will be a healthier, more productive workspace, a win-win for everyone involved.
This article is excerpted from BOMI International’s Air Handling, Water Treatment, and Plumbing Systems. The guide can be purchased by calling 1—800—235—2664, or by visiting www.bomi.org.