OZONE REMOVAL BY RESIDENTIAL HVAC FILTERS

HVAC filters have a significant influence on indoor air quality. In addition to removing particulate contaminants, filters accumulate a particle layer that can react with ozone. Ozone-particle cake reactions serve as a sink for ozone and a source of secondary carbonyls. The location of filters in air distribution systems can lead to byproducts being distributed throughout a building. Two experiments were performed to determine the ozone removal efficiency on two residential filters that were loaded with particles for one month (#1) and three months (#2). On both filters, the ozone removal efficiency dropped rapidly during the first 30 minutes and then stayed approximately constant for 4 hours. The steady state values of ozone removal efficiency were 10% for Filter #1 and 14% for Filter #2 at a face velocity of 0.81 cm/s. The potential for HVAC filters to affect ozone concentrations in residential buildings was evaluated using a well-mixed reactor model. The tested filters reduced indoor ozone levels slightly when compared to indoor surfaces and an entire HVAC system, although the impact was much smaller than that of the air exchange rate. The influence of continuous particle deposition was not considered here, but would likely increase the impact of ozone interactions on HVAC filters. INDEX TERMS Ozone removal, HVAC filter, Modeling, Experiments, Air exchange rate INTRODUCTION Heating, ventilation, and air conditioning (HVAC) systems play an important role in providing comfort in residential, commercial, and institutional buildings. However, they also play a potentially significant role as conveyers and as sources of indoor air pollution (Pejtersen et al. 1989, Finke and Fitzner 1993, Batterman and Burge 1995). A higher incidence of sick-building syndrome has been observed among office workers in buildings with HVAC systems (Mendell and Smith 1990). The reasons for this observation are not entirely clear, but may include direct (primary) emissions of volatile organic compounds (VOCs) from HVAC components, growth and release of biological agents, and/or emissions of secondary pollutants resulting from heterogeneous chemistry that occurs on surfaces of the HVAC system. Ozone, an oxidant and common indoor and outdoor pollutant, is of particular interest for surface reactions with HVAC components. Byproducts of ozone-surface reactions are likely to include carbonyls that might be irritating to the upper-respiratory system of building occupants. Morrison et al. (1998) observed that exposure to 100 ppb of ozone increased the emission rates of aldehydes from selected HVAC materials such as duct liners, duct sealing caulks and neoprene gaskets. In a laboratory study of ozone interactions with naturally-loaded filters from commercial buildings, Hyttinen et al (2003) observed the consumption of ozone in almost all of the filters. In their field experiments, the reduction in ozone concentrations varied from 8 to 26%; the highest ozone reduction was obtained in an HVAC unit with three stages of filtration. In a separate study, Halás and Bekő (2003) observed poor air quality downstream of filters exposed to ozone or air as compared to filters exposed to nitrogen. In their study, 90% of participants were dissatisfied with the air quality downstream of filters exposed to ozone, compared to 35% dissatisfied before the filters were exposed to ozone. They also showed a relatively high regeneration of the initial ozone removal efficiency after the samples were treated with clean air, nitrogen and heat. They hypothesized that VOCs inside the bulk particle volume slowly diffuse to the external surface following reactions of surface sites with ozone. In this paper we present measured ozone removal efficiency, η, defined as the ratio of the ozone concentration difference across an HVAC filter to the ozone concentration upstream of the filter, for two residential filters. We then employ a well-mixed reactor model to determine the impact of HVAC filter removal * Corresponding author email: jasiegel@mail.utexas. edu Proceedings: Indoor Air 2005 2367 on indoor ozone concentrations. EXPERIMENTAL METHODOLOGY Ozone removal efficiency, η, was used to describe ozone removal across an HVAC filter. Figure 1 depicts the experimental system used to determine η. Figure 1. Schematic of Experimental System A dual section electro-polished stainless steel chamber (28.3 L per section) was separated by the test filter. Small fans were used to mix the air in each section. All air passed through the HVAC filter was pre-filtered through a Polytetrafluoroethylene (PTFE) filter with pore size of 2.0 μm in order to keep particles from depositing on the sample filter. Thereafter, room air through a vacuum pump with fixed flow rate was mixed with ozone generated by an ozone generator (Prozone, Model PZ 6 Air). Mixed air was then conveyed through the chamber system (into the top chamber, through the filter, out of bottom chamber). The air flow rate through the filter was measured with a bubble flow meter (Sensidyne, Model Gilibrator 2) at the outlet of the bottom chamber, as shown in Figure 2. Two fiberglass filters were tested in this study. Both filters came from a residential HVAC system after use for one month (Filter #1) and three months (Filter #2). The temperature and relative humidity of each experiment were approximately 23 °C and 50%, respectively. Ozone concentrations upstream and downstream of the filter were continuously monitored and recorded by two calibrated UV ozone analyzers (2B Technologies, Model 202) with sampling intervals of 10 seconds. Each experiment lasted 4.5 hours. The ozone concentration upstream of Filter #1 increased while conducting the experiment and was approximately 700 ppb by the end of the experiment. The ozone concentration upstream for Filter #2 also increased during the experiment and was approximately 250 ppb by the end of the experiment. RESULTS AND DISCUSSION Some important properties of the tested HVAC filters are described in Table 1. Both filters are spun fiberglass filters that are commonly used in residential heating and cooling systems. When characterized by ASHRAE Standard 52.2 (1999), both filters have a minimum efficiency rating value (MERV) of less than 4. Both filters have a high porosity, as measured by submerging a known volume of bulk filter into a beaker of water and measuring the volume change. Table 1. Description of filters used in testing Filter # Material Thickness Operation Duration MERV Rating Measured Porosity 1 continuous filament spun glass 2.75 cm 4 weeks < 4 0.99 2 continuous filament spun glass 2.75 cm 12 weeks < 4 0.99 Top chamber Bottom chamber Mixing Fan HVAC Filter Covered Sample Port Covered Sample Port O3 Analyzer O3 Generator O3 Analyzer Vacuum Pump