6.6.8 Filtration Systems
To offer effective protection, filtration systems should be specific to the particular contaminant’s physical state and size. Chemical vapor/gas filtration (a.k.a. air cleaning) is currently a very expensive task (high initial and recurring costs) with a limited number of design professionals experienced in its implementation. Specific expertise should be sought if chemical filtration is desired. Possible application of the air cleaning approach to collective protection zones (with emergency activation) can assist in significantly reducing the cost though the protection is limited to the reduced size of the zone.
Most “traditional” HVAC filtration systems focus on aerosol type contaminants. The CBR threats in this category include radioactive “dirty bombs”, bio-aerosols, and some chemical threats. Riot-control agents and low-volatility nerve agents, for example, are generally distributed in aerosol form; however, a vapor component of these chemical agents could pass through a filtration system. HEPA filtration is currently considered adequate by most professionals to achieve sufficient protection from CBR particulates and aerosols. However, HEPA filtration systems generally have a higher acquisition cost than traditional HVAC filters and they cause larger pressure drops within the HVAC system, resulting in increased energy requirements to maintain the same design airflow rate. Due to recent improvements in filter media development, significant improvements in aerosol filtration can be achieved at relatively minimal increases in initial and operating costs. Also important is that incremental increases in filtration efficiency will generally provide incremental increases in protection from the aerosol contaminant.
In 1999, the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) released Standard 52.2-1999. This standard provides a system for rating filters that quantifies filtration efficiency in different particle size ranges to provide a composite efficiency value named the Minimum Efficiency Reporting Value (MERV). MERV ratings range between 1 and 20 with a higher MERV indicating a more efficient filter. Using the MERV rating table, a desired filter efficiency may be selected according to the size of the contaminant under consideration. For example, a filter with a MERV of 13 or more will provide a 90% or greater reduction of most CBR aerosols (generally considered to be at least 1-3 um in size or larger) within the filtered airstream with much lower acquisition and maintenance costs than HEPA filtration.
Efficiency of filtration systems is not the only concern. Air can become filtered only if it actually passes through the filter. Thus, filter-rack design, gasketing, and good quality filter sources should all play a role in minimizing bypass around the filter. The use of return-air filtration systems and the strategic location of supply and return systems should also be carefully employed to maximize effective ventilation and filtration rates.