Filtration and ventilation strategies in the COVID-19 era
Facility managers are re-examining their facility strategies with a focus on limiting the negative impact of airborne particles on building occupants.
OTHER PARTS OF THIS ARTICLEpt. 1: This pagept. 2: Spotlight on Filtration: Understanding the MERV Rating
The need to improve indoor air quality (IAQ) is a growing concern for those responsible for maintenance and engineering. As a result, many are re-examining their facilities’ ventilation and filtration strategies with a focus on limiting the negative impact of airborne particles on building occupants, particles that include the coronavirus.
These strategies typically include modifications to the facility’s HVAC filtration system, installing additional filtration systems, increasing system ventilation rates, or all three. Advances in filtration systems and ventilation standards provide managers with options. The most suitable system for a particular installation depends on the requirements of the application.
Static filtration hearth
Most building filtration systems consist of static filtration systems – filters whose particulate removal mechanism consists of filtering, impacting, or sieving. Typical filters are made from spun fiberglass, polyester, pleated paper, or fabric. They are inexpensive 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 High Efficiency Particulate Filter (HEPA). With a MERV rating of 16, HEPA filters are capable of removing 99.9% of particles in the air stream of 0.3 microns or larger. Not all HVAC systems can use HEPA filters due to the high 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 larger particles from the airflow, most are ineffective against pathogens.
A new generation of static filters uses nanofibers. A nanofiber is less than 100 nanometers in diameter, or about a tenth of the diameter of fibers found in conventional static filters. Constructed from polymers, nanofibers form a fine mesh similar to a spider’s web. The spaces between nanofibers are made up of fine pores which are very effective at capturing airborne particles including allergens, mold spores and dust mites. Most have a MERV rating between 11 and 13.
Nanofiber filters cost more than conventional static filters, but they offer several advantages. For the same filtering efficiency, nanofiber filters have a lower pressure drop than conventional filters, which means lower energy requirements for the fan for the same airflow through the system.
The efficiency of a nanofiber filter is relatively constant throughout its lifetime. On the other hand, when a new conventional static filter is installed, its efficiency is lower than that which was in service. As the filter becomes charged with captured particles, its filtering 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 electrostatics
An electrostatic air filter is a two-stage unit that uses static electricity to remove particles from the air flow. The first stage, or charge stage, consists of ionization wires connected to a high voltage power supply that imparts a positive electrostatic charge to the particles as they pass through the ionizing field. The second stage, or collector, is made up of metal plates of opposite charges. The charged particles in the first stage are attracted and attach 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. Because they are of open cell design, they have low pressure drop, which reduces fan power consumption. Although filters are more expensive than static filters, they are washable, reducing costs in the long run.
One of the biggest drawbacks of an electrostatic air filter is its relatively low MERV rating compared to other filtration systems. Another drawback is that its use of an ionizing field to charge particles can produce ozone, which can be harmful to health, even in small amounts.
Dynamic air filters represent a new generation of electrostatic filters. A hybrid of electrostatic and static air filters, dynamic air filters avoid problems with ozone generation by using a low-current, low-voltage power source that applies power directly to a grid built into the filter. The grid polarizes the fibers of the filter, causing them to attract and hold particles in the air flow. Once attached to filters, particles attract and bind to other particles. Particles too small to collect polarize and adhere to other particles and can then be retained by the filter.
Like conventional electrostatic filters, dynamic air filters offer low pressure drop. Their MERV rating can reach 13 while not producing ozone. Filters can also remove some odors and volatile organic compounds.
Ultraviolet light systems
Over 150 years ago, researchers discovered that ultraviolet (UV) light was an effective way to disinfect surfaces. Healthcare facilities and water purification systems have widely used 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. While the wavelengths of UV light range from 10 to 400 nanometers (nm), UV-C with a wavelength of 200 to 280 nm has been shown to be the most effective at destroying viruses. With sufficient intensity and long enough exposure, UV-C can destroy up to 99% of exposed viruses.
While minimizing biological growth on coils has been the primary application for the use of UV-C in HVAC systems, the pandemic has drawn attention to the use of UV-C light to destroy the virus. in the airflow of the HVAC system. UV-C devices can be installed in the ducts of the HVAC system – typically, near the coil – so they can expose pathogens in the airflow and those that may have grown on the coil. Since their effectiveness in destroying pathogens is directly related to the intensity and duration of exposure, managers must plan the installation carefully.
UV-C light is harmful to eyes and skin and can cause permanent damage, so it is essential that installers protect maintenance personnel. This means providing automatic shut-off switches on all access panels near the lamps, installing UV filters on all inspection windows, and training personnel who may be working on the HVAC system near the lamps.
UV-C systems in ducts 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 the system coils, thereby improving IAQ and system performance.
The design and configuration of HVAC systems often does not allow for the changes necessary to sufficiently improve IAQ without major modifications. Likewise, some areas of a facility may have more stringent IAQ requirements than the central system can meet. For these applications, managers can consider a number of next generation portable air filtration systems.
Portable air filtration systems are stand-alone units that technicians can move to areas where and when they’re needed. System sizes range from office units to those that can serve a large room. Technicians can configure them with a variety of filtering systems, including high MERV static filters, HEPA filters, UV light systems, and activated carbon filters.
Portable air filters give managers unlimited flexibility to meet the needs of specific areas of their facilities. Since they operate independently of a building’s central HVAC system, they can continue to purify the air even when the central HVAC system is not operating. And they’re portable, which increases their potential usefulness.
For the past several 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 reducing outdoor air levels comes at a price. Lower outdoor air levels result in less dilution of contaminants in the air circulated by the system. The pandemic has forced managers to reconsider this position.
ASHRAE Standard 62.1-2019 specifies the minimum ventilation rate, design, installation, commissioning, operation and maintenance of the air purification system for new and existing buildings to provide a IAQ acceptable. Managers should remember that these are recommended minimum ventilation rates, not target rates.
Realizing that things have changed, ASHRAE released “Guidelines for Construction Operations During the COVID-19 Pandemic” in May 2020. The publication offers a number of actions managers can take to improve IAQ and limit possible transmission of the virus, including:
- Install the highest MERV filters compatible with the HVAC system.
- Run HVAC systems longer hours to improve the filtering process.
- Use portable air purifiers with HEPA filters.
- Use UV-C irradiation in high risk areas.
Not all filtration systems work effectively in all buildings, as HVAC systems might not be able to support them. But if managers understand the type and level of contamination present in their building and the level of cleanliness required, they can select the most appropriate filtration system for their application.