Air Filtration
Air Filtration
“In addition to atmospheric dust, airborne particles can include pollen, mold (fungus) spores, animal dander, insect proteins, pesticides, lead, and infectious bacteria and viruses. Designers can integrate features into the ventilation system that will benefit both the efficiency and longevity of the HVAC system as well as the building occupants. Additionally, these features can reduce the need for expensive cleaning of ductwork and air handlers.”
Filter Efficiency
Air filters should have a dust spot efficiency rating between 35% and 80%, or a Minimum Efficiency Reporting Value (MERV) rating between 8 and 13. The higher the rating, the better the protection for equipment and occupants. A 30% increase in static pressure on a coil is estimated to result in $200 per 10,000 cfm of air movement (at $0.07 per kWh). This does not include the additional cost of cleaning dirty heating or cooling oils, drain pans, or air ducts. Designers should consider using a low-efficiency (~10%) pre-filter upstream of the main filters. Pre-filters are generally easy and inexpensive to replace and will capture a significant amount of particulate mass in the air, thus extending the service life of more expensive main filters.
Pressure Drop
Designing more filter surface area for ventilation systems has two advantages: reducing the number of filter changes each year, thereby lowering labor costs for proper maintenance, and reducing static pressure loss, which saves energy by lowering the power required to operate fans and blowers. Since different filter media generally exhibit a proportional relationship between efficiency and pressure drop rates, the most effective method to reduce pressure drop is designing more filter surface area into the filter system. This can be achieved by specifying filters with larger surface areas, such as pleated or bag filters. Another approach is to increase the number and/or size of filters in the airflow by mounting them in a “V” shape rather than a straight and perpendicular filter rack.
Pressure Monitoring
Consider installing a simple differential pressure gauge on all filter banks. This will prevent school facility staff from having to guess whether a filter needs to be replaced. A gauge with a range of 0 to 1.0 inches wg can prevent premature disposal of filters that still have useful life, saving money and the environment, and can prevent health and maintenance issues caused by overloaded filters bursting. The gauge should be located in a readily accessible position near the air handling unit, visible easily from a standing position.
Air Purification for Gaseous Pollutants
The most effective way to reduce occupants’ exposure to gases and VOCs is to manage and control potential sources of pollution. Filters are available to remove gases and volatile organic pollutants from ventilation air; however, due to cost and maintenance requirements, these systems are generally not used in regularly occupied buildings or schools. In specially designed HVAC systems, permanganate oxidizers and activated carbon can be used for gas removal filters.
Some manufacturers offer “partial bypass” carbon filters and carbon-impregnated filters to reduce volatile organics in ventilation air in office environments. Regular maintenance (replacement or renewal) of gas filters is necessary to ensure the system continues to operate effectively.
Ventilation Controls
Although a typical HVAC system has many controls, the control of outdoor air intake into the building can have a significant impact on Indoor Air Quality (IAQ), but it typically is not part of standard practice. Demand-controlled ventilation is considered as a method for humidity control, but its primary use is not discussed here in another way to improve IAQ, but to reduce outdoor air supply below the recommended minimum for energy savings purposes.