The average human breathes in about 16,000 quarts of air each day. And each quart of air we breathe has about 70,000 visible and invisible particles. The U.S. Environmental Protection Agency (EPA) notes that indoor air is often more polluted (typically two- to five-times more and occasionally 100-plus-times more) than outdoor air. Most of the “respirable” dust and particles people breathe into their lungs is approximately three microns or smaller – a fraction of the size of a grain of sand.
Good indoor air quality (IAQ) depends on a number of factors, including effective filtration, which provides the primary defense for building occupants and HVAC equipment against particular pollutants. Today’s higher standards in filtration, coupled with rigorous attention paid to filter selection and maintenance, helps to produce cleaner, purer air and reduce IAQ problems.
Filter Selection: Efficiency
Figure 1, page 42, illustrates sizes of various particles that may cause IAQ problems. Facility managers/engineers should work to identify the types and sizes of particular pollutants in their buildings to determine the best type of HVAC filter for their needs.
Selecting HVAC filters based on the needs of the facility instead of simply their initial costs will lead to a review of filter efficiency as a determining factor. Filtration efficiency defines how well the filter will remove contaminants. Low-efficiency filters are typically used to keep lint and dust from clogging the heating and cooling coils of an HVAC system. Medium- and high-efficiency filters are typically used to remove bacteria, pollen, soot, and other small particulates.
Initial and sustained efficiency are the primary performance indicators for HVAC filters. Initial efficiency refers to the filter’s efficiency “out-of-the-box.” Sustained efficiency refers to efficiency levels maintained throughout the service life of the filter. Some filters have lower initial efficiency and do not achieve high efficiency until a “dirt cake” has built up on the filter – typically after 30 days. Other filters offer both high initial as well as sustained efficiency, meaning they achieve an ideal performance level early and maintain that performance level.
The Atlanta-based American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has developed two HVAC industry standards that address the efficiency issue: ASHRAE 52.1 and ASHRAE 52.2 (see ASHRAE HVAC Standards, below). In addition to the performance factors measured under ASHRAE 52.1 and 52.2, consider these additional variables when selecting a filter:
Moisture resistance – how high humidity and moisture affect the filter.
Temperature limitations – how the filter performs at application temperature.
Flammability – how the filter performs in flammability tests. Check to see if UL Class I- or Class-II rated filters are needed to conform to local building codes.
Filter Selection: Filter Technology
There are many types of HVAC filters on the market today:
Bulk media and pre-cut pads.
Automatic roll filters.
Disposable panel filters.
Polyester rings, sleeves, and links.
Medium- and high-efficiency bag filters.
Rigid cell filters.
Mini pleated filters.
In most buildings, the best filter choice is a medium-efficiency pleated filter, which has a higher removal efficiency than low-efficiency filters, and more adequately removes the particles that cause IAQ problems, unlike high-efficiency filters, which would clog.
The pleated air filters used in HVAC systems are made with a wide range of materials (media), including fiberglass, polyester, cotton, paper, and synthetic nonwoven materials. Recent advances in nonwoven technologies have allowed for a step-change improvement in both performance and value of synthetic media over the standard cotton/poly blends used for years in HVAC filters.
Unlike traditional cotton/poly media, the synthetic media in more modern filters can be made of thermally bonded, continuous hydrophobic (moisture-repelling) polyolefin fibers that resist shedding and do not absorb moisture. This is important in resisting bacterial growth, and it keeps shed fibers from getting into the HVAC coils or into the breathing air. Moreover, synthetic media can be manufactured without the use of chemical binders, meaning that humidity will not affect the web structure and will not cause the glue to soften and, thus, fibers to shed.
Unlike cotton/poly filter media, which are made with a surface-loading structure, synthetic filter media can be made with a gradient density structure that provides a solid mechanical foundation to maintain high efficiency over the useful life of the filter.
Finally, synthetic filter media have an additional benefit over standard poly/cotton filter media: the ability to apply an electrostatic charge, which yields a higher initial efficiency and enhances the filter’s capture efficiency, especially in the attraction of smaller diameter particles.
Electrostatic filtration is different than mechanical filtration, which depends on the size of the fiber, size of the particles being filtered, and the physical structure of the media. With mechanical filtration, efficiency tends to build over time as particulates are collected (i.e., the filtration efficiency gets higher as the filter gets dirty).
With electrostatic filtration, filter fibers are charged, thus creating a force that attracts particles. This provides high initial efficiency, and when coupled with a strong mechanical structure, high efficiency can be sustained. The key is to look for a filter that has a strong mechanical structure and is enhanced with an electrostatic charge.
After specifying a filter media and selecting a filter, it’s important to pay close attention to proper filter installation (see Filter Installation, right). The goal is to avoid bypass air by making sure that all the air in the system goes through the filter. Bypass air causes contamination in housings, coils, fans, and ducts.
Some filter manufacturers utilize a two-color filter media construction to help in installation by making it easy to see that, for example, the gold side faces upstream, while the white side faces downstream.
Air filters should be monitored and maintained to provide maximum filtration, while not overtaxing the supply fan capability or leading to “blow-out” situations with no air filtration. Scheduled maintenance or established pressure drops can be determining factors along with specific variations in environmental conditions such as humidity and seasonal changes.
Filter suppliers often recommend installing a differential pressure measurement device across the filter bank to identify the appropriate change-out times. Pressure drop switches may be used to provide an alarm input to a building automation system (BAS) that alerts operators to change the filter. Don’t rely on a visual inspection only, since medium- and high-efficiency filters that appear “dirty” often have not reached their optimum efficiency levels. In addition, normal eyesight can only see particles of about 40 microns in size. Therefore, a filter rated at 10 microns can look dirty, yet still have a useful service life.
As a filter loads with particles, it becomes more efficient at particle removal, but increases the pressure drop through the system, thus reducing air flow. All filters – if loaded to excess – will become deformed, unload dust, and even “blow out” of their filter rack. When filters blow out, bypassing of unfiltered air will lead to clogged coils and dirty air ducts. Flow capabilities of the system fan may also be affected. Filters also should be monitored for bacterial growth. It’s a good idea to remove selected filter elements periodically and send them to a lab for testing when bacteria growth is found.
Proper filter maintenance is crucial to keeping the ductwork clean. If dirt accumulates in the ductwork, and if the relative humidity reaches the dewpoint (so that condensation occurs), then bacteria and mold may grow. This is especially the case in HVAC systems with acoustical duct liners, which are frequently used in air handler fan housings and supply ducts to reduce sound transmission and provide thermal insulation.
For all of these reasons, it is crucial to establish the appropriate filter change-out frequency. However, filters should be changed immediately if they become wet, microbial growth on the filter media is visible, or when filters collapse or become damaged to the extent that air bypasses the media.
Make the job of filter change-out as easy as possible. One tip is to place labels on the housing units with information such as the number and type of filters, date changed, and pressure-drop. Air handlers that are located in difficult-to-access places will be more likely to suffer from poor air filter maintenance and overall poor maintenance. Therefore, quick release and hinged access doors for maintenance are more desirable than bolted access panels when security is not an issue.
When changing the filter, make sure that the replacement filter is of the correct size and compatible with your housing. Review the performance value of the filter to ensure the pressure drop across the filter will not be too great, especially as the filter loads. The greater resistance will reduce the air flow to the unit, creating a negative impact on the unit’s heating/cooling efficiency and energy efficiency.
Time filter change-outs so they occur when the facility is unoccupied. This will help to avoid problems associated with disruptions in the HVAC system, and possible distribution of odors or emissions. If filters must be changed when the facility is occupied, turn off the supply fans to prevent debris from entering the ductwork downstream of the filters. Similarly, the entire filter area should be cleaned and wiped down while fans are off. Use a clean rag instead of compressed air to wipe dust from the inside of the filter housing and around gasket surfaces. When removing a spent filter, take care to avoid dropping contaminants into the ductwork.
At the end of the day, it is important to follow the recommendations of the filter manufacturer/supplier and HVAC system supplier to determine the proper procedures and frequencies for maintaining and changing filters. And don’t forget to fully document all inspections and corrective actions.
Stephanie Earley is market manager, Filtration Products at Kimberly-Clark Corp. (www.kcfiltration.com), Roswell, GA.