Gas-Phase Air Filtration and ASHRAE Standard 62: What They Are and How They Work Together by Christopher O. Muller Purafil, Inc. Ever-increasing attention to the economic bottom line presents a difficult situation in the design of commercial buildings. On one hand, designers are pressured to conserve energy and pay attention to the economic environment, while on the other hand the future occupants of such buildings are increasingly concerned about the quality of the indoor environment. ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality, offers a solution to the IAQ problem. However, use of the standards Ventilation Rate Procedure can result in higher energy costs. Air filtration offers the designer a way to improve IAQ by reducing contaminants while reducing energy consumption so that everyone wins. This two-article series focuses first on the procedures established by ASHRAEs Standard 62-1989 and specifically on gas-phase air filtration. The second article (to be published in February) will describe an application of the IAQ Procedure and a case study that illustrates the successful use of gas-phase air filtration to achieve acceptable IAQ and energy savings. ASHRAE Standard 62 - 1989 ASHRAE Standard 62-1989 specifies two procedures to provide acceptable IAQ in buildings. The Ventilation Rate Procedure provides only an indirect solution for the control of indoor contaminants. While it does allow for the use of cleaned, recirculated air, it does not reduce the amount of outdoor air specified in the standard. If cleaned, recirculated air is to be used to reduce the amount of outdoor air required, or for the implementation of energy conservation measures, the IAQ Procedure must be used. The IAQ Procedure provides a direct solution by reducing and controlling the concentrations of air contaminants, through air cleaning, to specified levels. This procedure allows for both quantitative and subjective evaluation of the effectiveness of the air cleaning methods employed. Using this procedure results in the use of less outdoor air than otherwise specified if the resulting air meets the required air quality criteria.

The Ventilation Rate Procedure defines the rate at which ventilation air must be delivered to a space as well as various approaches to conditioning that incoming air. The procedure establishes

the minimum outdoor air quality acceptable for use in ventilation systems outdoor air treatment when necessary ventilation rates for residential, commercial, institutional, vehicular, and industrial

space criteria for variable ventilation when the air volume in the space can be used as a

reservoir to dilute contaminants. Outdoor air must be treated to control contaminants if the outdoor air contaminant levels exceed those listed in the USEPAs National Ambient Air Quality Standards (NAAQS). Properly cleaned air may be used for recirculation. The IAQ Procedure provides a direct solution by reducing and controlling the concentrations of contaminants by means of air cleaning to specified acceptable levels. The amount of outdoor air specified by the Ventilation Rate Procedure in Table 2 of Standard 62-1989 may be reduced by recirculating air from which contaminants have been removed or converted to less objectionable forms. The amount of outdoor air required depends on

contaminant generation in the space contaminant concentrations in the indoor and outdoor air filter location filter efficiency for the contaminants in question ventilation effectiveness supply air recirculation rate fraction recirculated.

The standard lists seven different system configurations (including where filters are placed in the system) and provides equations that allow practitioners to calculate compliance with the IAQ Procedure. Using this procedure allows the amount of outdoor ventilation air to be reduced below standard levels if it can be demonstrated that the resulting air quality meets the required criteria. Gas-Phase Air Filtration Gas-phase air filters are available in a variety of commercial designs usually as packed-bed media filters where the dry, granular gas-phase media is filled in the space between perforated metal or plastic screens. A variety of filter bed types and depths are used. Most, if not all, of these systems are used in tandem with particle removal filters

for optimal filtration capabilities and appropriate protection of the gas-phase filter. Gas-phase air filters can be installed in side or front-access housings or other standardized equipment. They can also be installed in some self-contained air cleaner units. These filters are available as refillable or disposable units. The media used in gas-phase filtration, regardless of how installed, use two main processes to remove airborne gaseous contaminants. One is a reversible physical process known as adsorption. The other, which involves adsorption and irreversible chemical reactions, is termed chemisoption. The most common form of gas-phase filtration is adsorption, and by definition, adsorption is the process by which one substance is attracted to and held on the surface of another. Adsorption can occur wherever a material has sufficient attractive force to overcome the kinetic energy of gas molecule. This attraction is evident by the adsorption of cigarette smoke on the interior of an automobile or on a persons clothing. Adsorption is viewed as a surface phenomenon. The removal capacity of an adsorbent is directly related to its total surface area, and in a porous solid adsorbent, the surface extends well into the interior of the solid. Therefore, developing as large an accessible surface area per unit volume as possible is crucial. Granular activated carbons (GACs) are the most common materials that fulfill this requirement. Other commonly used sorbents include activated aluminas. Because of the relatively weak forces involved, adsorption is reversible (Hassler, 1974). Thus, the net rate of adsorption depends on the rate at which gas 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 removal of gaseous contaminants by physical adsorption. Among these are the types of adsorbent, the resistance to airflow, the adsorbent bed depth, the gas velocity, the concentration and characteristics of the contaminants in the space around the adsorbent, the removal efficiency required, and the temperature and relative humidity of the gas stream. Adsorbent materials do not adsorb all contaminant gases equally (Purafil, Inc., 1991; Muller, 1994; and Muller and England, 1995). One way to improve the effectiveness of sorbents for these materials is by the use of various chemical impregnants that react with these less-adsorbable gases. These impregnants react spontaneously and irreversibly with these gases forming stable chemical compounds that are bound to the media or released into the air as CO2, water vapor, or some material more readily adsorbed by

other adsorbents. Therefore, it is not uncommon to have a gas-phase air filtration system that uses a combination of unimpregnated and chemically-impregnated adsorbent medias. In contrast to the reversible process of physical adsorption, chemical adsorption, or chemisorption, is the result of chemical reactions on the surface of the adsorbent. Chemisorption is specific and depends on the chemical nature of both the adsorption

media and the contaminants. It is actually a two-stage process. First, contaminants are physically adsorbed onto the media. Once adsorbed, they react chemically with the media. The chemical impregnant added to the media makes it more or less specific for a contaminant or group of contaminants. Many of the same factors that affect the removal of gases by physical adsorption also affect their removal by chemisorption. One of the more broad-spectrum chemical impregnants in common use is potassium permanganate (KMnO4 ) and is typically used as an impregnant on activated alumina or

other active substrates such as zeolite. Potassium permanganate-impregnated alumina (PIA) is often used in conjunction with GAC to provide a very broad-spectrum gas-phase air filtration system. Just as with other forms of air filtration, there are certain negative as pects of gas-phase air filtration. Particular gases, most importantly, carbon monoxide and carbon dioxide, are not controlled. There is an increased cost in energy to overcome higher pressure drops. And when the media are spent, they must be replaced. Fortunately, the energy cost savings realized by using effective gas-phase filtration systems for air recirculation can far exceed the additional costs. Conclusion ASHRAE 62-1989 achieves a balance between energy consumption and IAQ. Gas-phase filtration can be an effective asset in at least four ways:

by treating contaminated outdoor air so that it can be used more effectively to improve indoor air quality;

by enabling reduction of ventilation down to the 15 cfm minimum levels while using the Ventilation Rate Procedure;

by enabling the use of the IAQ Procedure, which allows filtration to replace required minimums of outdoor air for ventilation; and

by providing a more cost effective means of providing acceptable IAQ compared to increased use of outdoor air.

As illustration, Part 2 of this series will focus on an example of using the IAQ Procedure with gas-phase filtration and a case study of a facility where gas-phase filtration was used to yield documented energy savings and acceptable IAQ.

References Hassler, J.W. Purification with Activated Carbon: Industrial, Commercial, Environmental. New York: Chemical Publishing Co. 363-367. 1974. Muller, C.O. "Gas-Phase Air Filtration: Single Media or Multiple Media Systems. Which Should be Used for IAQ Applications?" Proceedings of IAQ '94: Engineering Indoor Environments. Atlanta: American Society for Heating, Refrigerating, and Air-Conditioning Engineers. 1994. Muller, C.O., and W.G. England. "Achieving Your Indoor Air Quality Goals: Which Filtration System Works Best?" ASHRAE Journal. 24 - 32. February, 1995. Purafil, Inc. Breakthrough Capacity Test Results (typical) @ 99.5% Efficiency, Final Report. 1991.

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