ASHRAE Journal - April 2009 - 68

Air Purification to Reduce OA By John Dieckmann, Member ASHRAE; Kurtis McKenney; and James Brodrick, Ph.D., Member ASHRAE uildings require ventilation with outdoor air to maintain a level of indoor air quality that provides a healthy environment for the occupants of the building interior space. In simple terms, carbon dioxide and bioeffluents are generated by building occupants while various gases and particulate matter (PM), some potentially harmful, are emitted by building materials and furnishings, equipment, and cleaning supplies, etc. By exhausting indoor air at an appropriate flow rate and replacing it with clean outdoor air, the concentrations of these occupant- and building material-generated gases can be maintained below objectionable or harmful levels. ASHRAE Standard 62.1-2007 prescribes outdoor ventilation makeup airflow rates and methods to maintain acceptable indoor air quality in commercial and institutional buildings.1 Underscoring the importance of maintaining a healthy indoor environment, Standard 62.1 has been adopted by most state and local building codes. Outdoor ventilation air comes with an energy penalty, including the blower energy required to force outdoor air into the building and exhaust air out of the building, along with the energy needed to cool and dehumidify or heat the outdoor air to indoor comfort conditions. Standard 62.1-2007 provides two alternate paths to provide adequate ventilation and acceptable indoor air quality. • The Ventilation Rate Procedure (VRP) is a prescriptive approach with a table of minimum required outdoor airflow rates per occupant for a variety of commercial, institutional, and multifamily residential occupancies. The airflow rate per square foot of building floor area is based on the design occupancy density and the required flow rate per person, adjusted to reflect the air-distribution system used. • The Indoor Air Quality (IAQ) Procedure provides a framework for an engineered approach to providing acceptable IAQ in a particular building. The building design team is responsible for identifying contaminants of concern, likely rates of generation of these contaminants, and acceptable levels of these contaminants; and defining a system and an outdoor ventilation rate that together will maintain contaminant concentration below the maximum acceptable concentrations. Air purification to remove some or all of the contaminants of concern can be part of the system enabling a reduction in ventilation airflow rates from the levels required by theVRP. 68 ASHRAE Journal B Air purification equipment and technology that can be used is described briefly in the following paragraphs. Particulate matter cleansing can be achieved with mechanical filters, in which PM is removed from an airstream by capturing it on the filter medium; or electronic, electrostatic, or ionizing filters, in which particles are charged and precipitated out of the airstream. The efficiency of these filters can vary significantly and are generally rated for certain particle sizes at specific flow rates or face velocities. They generally have to be cleaned or replaced to avoid increased pressure drop and/or particle offloading. PM filters have generally proven to be practical and are commonly installed in HVAC equipment. Gas-phase adsorbent filters generally use physical adsorption to remove gas phase volatile organic compounds (VOCs) from an airstream. Adsorption occurs when gases bond to surfaces of solid materials via van der Waals forces. Granular activated carbon is the most common adsorption media. Sorbent filters become chemisorbent filters when coated with an agent (e.g., potassium or sodium permanganate) designed to react chemically with VOCs that are less easily physically adsorbed. Sorbent filters generally need to be integrated with fibrous PM filters because the sorbent media alone will typically not remove PM. Adsorbent filter media loses effectiveness and needs to be replaced or regenerated as the adsorption sites are filled with gas phase pollutants. Photocatalytic oxidation (PCO) is an approach to gas-phase air cleaning that uses a monolith reactor coated with a photooxidative catalyst (e.g., titanium dioxide) that is irradiated with ultraviolet light. Simply stated, VOCs interact with the catalyst and the UV light, and the VOCs are broken down into carbon dioxide and water. PCO purification and PCO purification in combination with chemisorbents offer potentially high VOC conversion rates with low pressure drop.2,3 However, they generally need to be combined with PM filters, and by some accounts, seem to be more costly than adsorbent VOC filters on both an installed and annual basis.4 Two other technologies can be used as alternatives to, or in combination with, air purification to reduce the energy impact of outdoor ventilation makeup air, energy recovery ventilation (ERV)5 and demand-controlled ventilation (DCV),6 both of which have been the subject of this column in previous editions of ASHRAE Journal. An ERV exchanges sensible heat and moisture from the incoming outdoor air to the conditioned air being exhausted from the building, reducing the cooling or ashrae.org April 2009
ASHRAE Journal - April 2009

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