Commercial / Institutional Overview
Air filtration systems in spaces such as offices and schools were originally designed primarily to protect HVAC equipment and reduce the need for facility cleaning. In recent years, people have become aware of the issue of indoor air quality. In turn, facility executives have felt increasing ethical, legal and marketing pressures to provide a high-quality indoor air environment for their occupants.
The HVAC system provides heating and cooling, addition or removal of moisture, controlled air movement through fixed or variable air volume, and is the focal point for cleaning air within a building. Indoor air quality is deemed acceptable when the required outdoor air (fresh) supply rates which are specified for different spaces, are provided for the occupied spaces. When there are unusual indoor contaminants, if the occupancy is denser than originally anticipated, or other conditions exist, then higher levels of outdoor air may be required. In no instance is the ventilation rate less than 15 cfm per person.
An air filtration strategy depends on the building’s occupancy, location, specific air quality issues, and the type and operation of the HVAC system. For example, the strategy for an airport terminal, which is subject to high volumes of diesel and jet fuel gases, differs from that of a typical office building.
HVAC systems typically account for about 40% of a commercial building's electricity expenses. HVAC filters play a significant role in the energy used to operate an HVAC system; the lower the filter's resistance to air passing through it, the lower the energy consumption will be. This is pressure drop, the measurement, in inches, of the decrease in air flow through the filter compared to no filter. Ali Sherafat, P.E, explains in Building Operating Management, pressure drop is affected by MERV, dust loading and fan velocity. The higher the MERV, the greater the pressure drop across the surface of the filter. Similarly, as the filter accumulates particles, the pressure drop increases, resulting in decreased air flow through the filter. For example, at the recommended velocity of 500 fpm, the initial pressure drop of a new filter when the fan is running at maximum velocity is approximately 0.35 to 0.75 inches for a MERV 13 filter, 0.85 to 1.0 inch for a MERV 15 filter, and 1.0 to 2.0 inches for HEPA filters.
It might seem that the higher the MERV rating, the higher the operating cost because of the higher initial and replacement costs of the filters, plus the cost of power to operate the higher horsepower fans required to push the air through the filters. But it’s more complicated than that. Consider a comparative analysis of operating costs for medium efficiency versus high efficiency filters in a commercial office building. Most office buildings use variable air volume systems, and, depending on the building’s location, fans operate, on average, at 70 to 90 percent volume. The filters are at their highest pressure drops at maximum volume, thus the difference in operating costs between medium-efficiency and high-efficiency filters will be greater in a building in which the fans average 90 percent volume than in a building in which fans average 70 percent. Regardless of the efficiency of the filter, operating cost is quite specific to the building; there is no rule of thumb. This means the type of filter should be selected based on the specific building location, occupancy, air quality issues and type and operation of the HVAC system. However, it is reasonable to specify a filter of at least MERV 13 to maintain good indoor air quality in common types of spaces. At 85 percent efficiency, the pressure drop is generally less than 0.50 inches, which can be handled by most existing air handling systems. If an owner is considering a higher efficiency system, a life-cycle analysis is strongly recommended to compare benefits against increased operating costs.
Additional variables to consider:
Filtration systems are sized in square feet according to the velocity of the air flow, generally 1 square foot of filter per 500 cfm air flow. Consider an office building that averages 1 cfm per square foot of floor space. Therefore, every 1,000 square feet of office space requires 2 square feet of filter, and a 100,000 square feet office building requires a total of 200 square feet of filter area.
Due to the individual design specifications by engineers and architects, there is no easy rule of thumb to apply for filter requirements or maintenance of filtration media. Considerations by HVAC contractors whose job is to execute maintenance contracts or advise preventative maintenance techniques include:
Figure II-2 in Section Products, Performance & Costs provides a guide to ASHRAE filtration guidelines for most public use building space. These filtration guidelines were established with occupants in mind. Specialty institutions, like museums and libraries, establish guidelines along different parameters.
Very fine dusts and aerosols can adhere to painting and other artworks. Oxides of nitrogen and sulfur dioxide chemically attack canvas fiber and tapestries. Removal of particulate aerosols usually require filters above 90 percent DOP; gaseous contaminants determine appropriate gas phase filtration systems. In libraries, papers of all types can be damaged by oxides of nitrogen and sulfur dioxide. In fact, the recommended maximum allowable concentration of sulfur dioxide in a library environment is less than that for the occupants of the building. Gas purifiers containing alumina impregnated with potassium permanganate are very effective in this application.
Based on the experience of maintenance reviews, quarterly inspections are recommended. Experience shows that filter replacement can vary from every three months to once every two years.