Facility managers are rethinking their facility strategies with an emphasis on mitigating the negative impact of particulate matter on building occupants.
OTHER PARTS OF THIS ARTICLEpt. 1: This pagept. 2: Spotlight on Filtration: Understanding MERV Rating
The need for improved indoor air quality (IAQ) is an increasing concern for maintenance and engineering managers. As a result, many are re-examining their facilities’ ventilation and filtration strategies, with an emphasis on limiting the negative impact of airborne particulates on building users, particulates that include the coronavirus.
These strategies typically include changes to the facility’s HVAC filtration system, the installation of additional filtration systems, increasing the system’s ventilation rates, or all three. Advances in filtration systems and ventilation standards give managers options. Which system is most suitable for a particular facility depends on the requirements of the application.
Static filtration focus
Most building filtration systems consist of static filtration systems – filters whose particle removal mechanism consists of pressing, impacting or sieving. Typical filters are made of spun fiberglass, polyester, pleated paper or cloth. They are cheap and most are disposable. Their effectiveness in removing particles from the air stream is measured by their MERV number. The higher the number, the greater the efficiency of the filter.
The most efficient static filtration filter is the HEPA (High Efficiency Particulate Air) filter. With a MERV rating of 16, HEPA filters can remove 99.9 percent of airstream particulates of 0.3 microns or larger. Not all HVAC systems can use HEPA filters because of the large pressure drop they impose across the filter.
Static filtration systems have been used successfully in HVAC systems for decades, but they have limitations. While they can remove the larger particles from the airstream, most are not effective against pathogens.
A new generation of static filters uses nanofibres. A nanofiber is less than 100 nanometers in diameter, or about one-tenth the diameter of the fibers in conventional static filters. The nanofibers are made of polymers and form a fine mesh that resembles a spider’s web. The spaces between the nanofibers consist of fine pores that are highly efficient at trapping airborne particles, including allergens, mold spores and mites. Most have a MERV score between 11 and 13.
Nanofiber filters cost more than conventional static filters, but offer several benefits. For the same filter efficiency, nanofiber filters have a lower pressure drop than conventional filters, meaning less fan energy is required for the same airflow rate in the system.
The efficiency of a nanofiber filter is relatively constant over its lifetime. In contrast, when a new conventional static filter is installed, the efficiency is lower than a filter that has been in use. As the filter becomes full of trapped particles, the filter efficiency increases. Nanofiber filters work with the same efficiency when they are new as when they have been in use for a while.
Eye on electrostatic
An electrostatic air filter is a two-stage unit that uses static electricity to remove particles from the airflow. The first, or charging stage, consists of ionizer wires connected to a high voltage power supply that gives particles a positive electrostatic charge as they pass through the ionizing field. The second or collector stage consists of oppositely charged metal plates. Particles charged in the first stage are attracted to and adhere to these plates, which are periodically removed and cleaned. Typical MERV ratings are in the range of four to six.
Electrostatic air filters offer several advantages. Since they have an open cell design, they have a low pressure drop, which reduces the energy consumption of the fan. Although the filters are more expensive than static filters, they are washable, which reduces costs in the long run.
One of the biggest drawbacks of an electrostatic air filter is its relatively low MERV rating compared to other filter systems. Another drawback is that using an ionizing field to charge particles can produce ozone, which can be harmful to health, even in small amounts.
Dynamic air filters represent a newer generation of electrostatic filters. A hybrid of electrostatic and static air filters, dynamic air filters avoid the problems of ozone generation by using a low-current, low-voltage power source that supplies power directly to a grid embedded in the filter. The grid polarizes the fibers of the filter, causing them to attract and retain particles in the airflow. Once attached to the filters, the particles attract and bond with other particles. Particles that are too small to be collected become polarized and adhere to other particles and can then be retained by the filter.
Like conventional electrostatic filters, dynamic air filters offer a low pressure drop. Their MERV rating can go up to 13 while producing no ozone. The filters can also remove some odors and volatile organic compounds.
Ultraviolet light systems
More than 150 years ago, researchers discovered that ultraviolet (UV) light was an effective way to disinfect surfaces. Healthcare facilities and water treatment systems have made widespread use of UV-based systems and managers have specified them for building HVAC systems to keep cooling coils free from biological growth.
UV light is effective against viruses because it damages the DNA and RNA of a virus, rendering it inactive. Although the wavelength of UV light ranges from 10 to 400 nanometers (nm), UV-C with a wavelength of 200-280 nm has been shown to be most effective at destroying viruses. With sufficient intensity and long enough exposure, UV-C can destroy up to 99 percent of exposed viruses.
While minimizing biological growth on coils has been the primary application for using UV-C in HVAC systems, the pandemic has focused attention on the use of UV-C light to control the virus in the airflow of the HVAC system. to destroy. UV-C fixtures can be installed in ducts of the HVAC system – usually close to the coil – so that they can expose pathogens in the airstream and any pathogens grown on the coil. Because their effectiveness in destroying pathogens is directly related to exposure intensity and duration, managers must plan installation carefully.
UV-C light is harmful to the eyes and skin and can cause permanent damage, so it is essential that installers provide protection for maintenance personnel. This means installing automatic shut-off switches on all access panels near the lamps, installing UV light blocking filters on all inspection windows, and training any personnel who may be working on the HVAC system near the lamps.
UV-C in-duct systems offer several advantages. Their installation and operating costs are low. They can destroy pathogens in the system. They are also effective in preventing a build-up of biological material on system coils, improving indoor air quality and system performance.
The design and configuration of HVAC systems often do not allow for the changes necessary to improve the IAQ sufficiently without major adjustments. Likewise, some areas in a facility may have more stringent IAQ requirements than the central system can meet. For these applications, managers may consider a number of next-generation portable air filtration systems.
Portable air filtration systems are self-contained units that technicians can move to areas where and when they are needed. System sizes range from desktop units to units that can serve a large room. Technicians can configure them with a variety of filter systems, including high MERV static filters, HEPA filters, UV light systems, and activated carbon filters.
Portable air filters give managers unlimited flexibility in meeting the needs of specific spaces in their facilities. Because they operate independently of a building’s central HVAC system, they can continue to purify the air even when the central HVAC system is down. And they are portable, which increases their potential usefulness.
In recent decades, the design standard for HVAC systems has been to operate systems with a minimum amount of fresh outside air, a tactic that saves heating and cooling energy. But reduced outdoor air levels come at a price. Lower outdoor air levels result in less dilution of contaminants in the air circulated through the system. The pandemic has forced managers to re-examine this position.
ASHRAE Standard 62.1-2019 specifies the minimum ventilation rate, design, installation, commissioning, operation and maintenance of new and existing buildings to provide acceptable indoor air quality. Managers should remember that these are recommended minimum ventilation rates, not target values.
ASHRAE realized things have changed and released “Guidance for Building Operations While COVID-19 Pandemic” in May 2020. The publication offers a number of suggested steps managers can take to improve indoor air quality and mitigate potential transmission of the virus, including:
- Install filters with the highest MERV rating that are compatible with the HVAC system.
- Run HVAC systems longer to improve the filtering process.
- Use portable air purifiers with HEPA filters.
- Use UV-C irradiation in risk areas.
Not all filter systems work effectively in all buildings as HVAC systems may not be able to support them. But if managers understand the type and level of contamination in their building and the level of cleanliness required, they can choose the filter system best suited to their application.