FILTER EFFICIENCY OF SELECTED HVAC FILTERS

Transcription

1 FILTER EFFICIENCY OF SELECTED HVAC FILTERS ES Moyer *, MA Commodore, J Hayes, and SA Fotta National Institute for Occupational Safety and Health, Division of Respiratory Disease Studies, Laboratory Research Branch, 1095 Willowdale Road, Morgantown, WV, USA ABSTRACT Air quality begins with good engineering air handling system (HVAC) controls. Air handler filters should remove the major particulate matter from incoming ambient air. A study of various NIOSH air handler bag filters was conducted to determine their filter efficiency against submicrometer particles ( micrometers), since submicrometer particles are the most penetrating. Aerosol penetration measurements show an extremely large range of filter efficiency. New filter aerosol penetration ranged from less than one percent to greater than ten percent with penetration depending on flow rate, media surface area, and challenge agent particle size and state (liquid versus solid). Exposed filters (from air handler) showed a significant increase in submicrometer particle filter penetration and a shift in most penetrating aerosol particle size toward the larger particle size range. Both mechanical and electrostatic filters are used in HVAC applications. The electrostatic filters showed significant filter efficiency reduction over the filter s expected service lifetime. INDEX TERMS Air Quality, Filter Efficiency, Submicrometer Aerosol Particles, Electrostatic Filter Media. INTRODUCTION A paper entitled Review of ASHRAE Test Results Versus Real Life Performance (Kern and Jackson, 2000) presented data indicating that Lifetime testing indicates that glass media maintains its efficiency while synthetic media loses efficiency (discharge of enhanced media). Glass media (fiber diameters as fine as one micrometer) is basically mechanical in nature while the synthetic media consists of larger diameter fibers and relies on an electrostatic component to enhance initial filtration efficiency. It has been reported (Gustavsson, 1999) that filter efficiency versus dust load for electrostatic and fiberglass media gave comparable results when tested by the Eurovent 4/9 method. However, work on electrostatic filter media in actual applications (Gustavsson, 1995) (Hauley et al, 1999) has shown a loss of filter efficiency with exposure time. Data on electrostatic filter media used in respirator applications support these findings. Electrostatic respirator filter media exposed to workplace type aerosols resulted in alterations in performance (Blackford, et al, 1986). More recent work (Moyer and Bergman 2000) demonstrated that electrostatic N-95 respirator filter media could be adversely affected by intermittent exposure to sodium chloride aerosol. Further, besides reducing the efficiency of the filter media, a shift of the most penetrating particle size to larger particle diameter has been reported (Martin and Moyer, 2000). Data to quantify the effect that ambient aerosol has on air handler bag filter media needs to be collected. Reliable filter change out schedules for bag filters can only be adequately addressed * Contact author ESM@@cdc.gov 1056

2 by supporting data that will be highly dependent on exposure. Factors which vary significantly are location, flow rates, time of the year, and weather conditions. This study was undertaken to monitor filter efficiency against submicrometer particles and provide industrial hygienists, aerosol researchers, ventilation engineers, and maintenance personnel with vital data upon which to make their decisions. METHODS New and used air handler bag filters were evaluated for aerosol penetration with the dioctyl phthalate (DOP) and/or sodium chloride submicrometer aerosols employed for testing respirator particulate filter efficiency. The aerosol test criteria are specified in the Code of Federal Regulations: Respiratory Protective Devices. Title 42, Part 84. All tests were done employing a TSI model 8110 or 8130 automated filter tester (low flow) for flow rates up to 100 LPM. Higher flow data was collected on a ATI TDA-100P penetrometer (high flow). Both instruments use light-scattering photometer detector systems. Data of filter penetration as a function of particle size was collected on a Model 8160 fractional efficiency filter test system which employs two condensation particle counters (CPC) to monitor upstream and downstream aerosol concentration. The upstream CPC employs a dilution bridge to prevent coincidence problems. The CPCs count the actual number of aerosol particles in the flow stream, instead of using a photometric mass type average. The Model 8160 is capable of testing over a particle size range of to 0.4 micrometers and measure efficiencies to better than percent. New and exposed filters were tested for instantaneous penetration as a function of flow rate. A new piece of filter media was used for each test. Some aerosol loading experiments were performed to determine the effect of aerosol loading on filter penetration. Finally, some samples of filter media were dipped in isopropanol and dried prior to testing in order to determine whether the filter media was electrostatically-enhanced media or basically mechanical. RESULTS AND DISCUSSION Air handler #8 was selected because the filters were due to be changed. In addition, NIOSH was trying to determine which filters gave the best performance and had previously installed three different filters. A six chamber AAF DriPak 2000 Extended Service Filter, 90-95% (AAF) with a total surface area of 2.0 square meters had been installed in 25 of the 32 bank positions. Five 12 chamber Koch Multi Sak 12 FZ 1295-AM, manufacturer code VP (Koch) with a surface area of 4.4 square meters had been initially installed as well. Two Filtration Group wet laid microglass FP Mini-Pleat filters (V-Bank) had been installed sometime during the filter cycle but not initially (surface area 17.9 square meters). All bag filters were changed nine months after installation. Velocity measurements were taken down stream of the 32 filter positions during routine operation of the air handler. The average velocity for the air handler was 2.23 meters per second and the total cross sectional area of the air handler was square meters. Therefore, the average flow for this air handler was 26.6 cubic meters per second. Employing this flow value, the flow through the three types of filters (V-bank, AAF, and Koch) was determined. Knowing the total surface area of each filter type allows one to calculate the face velocity across the surface of each filter. Calculated face velocities were 0.11 meters per second for the V-bank, 0.28 meters per second for the 12 chamber Koch and 0.32 meters per second for the 6 chamber AAF. Since the filter tester has a cross-sectional area of 0.01 square 1057

3 meters, a test flow of 65 LPM was used for the V-bank, 170 LPM for the Koch and 190 LPM for the AAF to match face velocities in the air handler. Because actual flow rates may vary, each filter media was tested over an extremely large flow rate range to completely characterize the media. Data for control and exposed filter samples are presented in Table 1 (AAF), Table II (Koch) and Table III (V-Bank). Table I. AAF Filter Media Efficiency Results (Average) Flow Rate AAF Control 9 Month Location 2-1B 9 Month Location 2-2H LPM Low Flow High Flow Low Flow High Flow Low Flow High Flow

4 Table II. Koch Filter Media Efficiency Results (Average) Flow Rate Koch Control % 9 Month Location 2-2C 9 Month Location 2-2B Efficiency LPM Low Flow High Flow Low Flow High Flow Low Flow High Flow

5 Table III. V-Bank Filter Media Efficiency Results (Average) Flow Rate V-Bank Control V-Bank Location 1 V-Bank Location 2 LPM Low Flow High Flow Low Flow High Flow Low Flow High Flow The data confirm that two of the media (Koch and AAF) are electrostatic type media that lose filter efficiency with exposure to ambient aerosol particles. This was also confirmed by DOP loading studies that show continuing filter efficiency reduction with DOP loading. Treatment with isopropanol significantly reduced the filter media efficiency. Further, there was a shift of the most penetrating particle size to a larger particle size with this reduction in filter efficiency at low flow conditions. But, the microglass media, which is basically mechanical, did not suffer from the same reduction in filter efficiency with aerosol loading or isopropanol treatment. CONCLUSIONS AND IMPLICATIONS The nine-month use period for the two electrostatic filters appears to be too long and the change out schedule for this type filter should be reduced. In fact, the electrostatic filters showed a reduction in filter efficiency of approximately 50 percent with exposure. The only acceptable way to determine filter change out schedule is by the collection of objective data (an optical particle counter with size selective capabilities is acceptable). Routine measurement in the air handler of upstream and downstream particle counts allows one to collect objective data and set an acceptable change out schedule. It should be noted that only filter efficiency was discussed but the filters need to be properly sealed in the air handler to prevent major leak sites around the filter media. 1060

6 REFERENCES Blackford DB, Bostock GJ, Brown RC, et al Alteration in the Performance of Electrostatic Filters Caused by Exposure to Aerosols. In Proceedings of the Fourth World Filtration Congress, pp Ostende, Belgium Gustavsson J Test Methods and Classification of Air Filters, Camfil, Sweden. Gustavsson J Air Filters for Ventilation Systems Laboratory and In Situ Testing, International Nonwovens Journal. Vol. 8 (2). Hanley JT, et al The Effect of Loading Dust Type on the Filtration Efficiency of Electrostatically-Charged Filters. Indoor Air 99. Vol. 4 p 73. Edinburgh. Kern CF and Jackson FL Review of ASHRAE Test Results Versus Real Life performance. In Book of Papers International Nonwovens Technical Conference. pp , Dallas, Texas. Martin SB and Moyer ES Electrostatic Respirator Filter Media: Filter Efficiency and Most Penetrating Particle Size Effects. Appl. Occup. Environ. Hyg. Vol 15(8), pp Moyer ES and Bergman MS Electrostatic N-95 Respirator Filter Media Efficiency Degradation Resulting from Intermittent Sodium Chloride Aerosol Exposure. Appl. Occup. Environ. Hyg. Vol 15 (8), pp