In the previous issue, airborne molecular contamination (AMC) was described in terms of its classifications, monitoring, and control considerations and technologies. Part 2 discusses AMC filter types and applications, and Part 3 will focus on specification of an AMC control system and final considerations.

AMC FILTER TYPES
It is well known that a single chemical filter medium may not adequately control multiple contaminants or all classes of AMC. The types and numbers of AMC that one would encounter make it likely that air-cleaning systems need to be equipped with multiple stages of specialized chemical filter media. The preferred system would contain these media in discreet filter beds.

However, it is not always feasible to incorporate an AMC control system with two or more filter beds. Retrofit applications in particular present challenges to the HVAC engineer who is often limited by a lack of physical space for the system, sufficient static pressure in the air-handling system, or budgetary constraints. In these cases, two media may be blended and used as a single stage to reduce the size and/or cost of the system. Properly applied, blended media should not affect the efficiency of the AMC control system; however, the service life will be reduced based on the relative amount of the reduction of each media (Figure 1).

Figure 1: Single media versus media blend performance.

Physical limitations placed on AMC control systems and constant budgetary constraints have spurred the development of many new chemical filtration products. A few manufacturers are applying dry-scrubbing media in various forms and adding them to various filter substrates, but many processes cause the adsorbent materials to become “blinded” or spent through the use of, and/or reaction with, various binders and adhesives in addition to the manufacturing techniques employed. Advances in filter manufacturing technologies have eliminated a number of these deficiencies; however, the appropriateness of one filtration technology or filter type over another for a specific application remains an important question to be answered. Some of the more common AMC filter types in use today will be discussed below.

Adsorbents/Chemisorbents. Traditionally, chemical air filtration has utilized two main processes to remove AMC. One is the reversible physical process of adsorption. The other, which involves adsorption and chemical reactions, is termed chemisorption.

Adsorption can be viewed as a surface phenomenon and the removal capacity of a specific adsorbent is directly related to its total surface area. Therefore, it is important to develop as large an accessible surface area per unit volume as possible. Granular activated carbons (GACs) and activated aluminas are the most common materials that fulfill this requirement.

Because of the relatively weak forces involved, adsorption is (essentially) totally reversible. Thus, the net rate of adsorption depends on the rate at which adsorbate molecules reach the surface of the adsorbent, the percent of those making contact that are adsorbed, and the rate of desorption. However, many other factors can affect the removal of gaseous contaminants by physical adsorption. Among these are the type of adsorbent, adsorbent particle size and bed depth, resistance to airflow ( P), air velocity, concentration and characteristics of the contaminant(s) in the airstream, and the temperature and relative humidity of the airstream.