Aerosol Collection Efficiency of a Graded Metal-Fiber Filter at High Airflow Velocity (10 m s^–1)

Abstract

A non-woven graded metal-fiber filter was characterized for its collection efficiency as a function of particle size at an airflow velocity of 10 m s −1 . Efficiencies were determined by generating a monodisperse aerosol of polystyrene latex (PSL) spheres and oleic acid liquid particles of measured concentration with and without the filter present. For PSL spheres ≤0.7 μm, the collection efficiencies follow theoretical single-fiber collision efficiency (Ptak and Jaroszczyk 1990). Above 0.7 μm diameter (Stokes number 0.8), PSL collection efficiency exhibits a general decreasing trend with increasing particle size suggesting particle bounce. Although this trend is consistent with measured single-fiber collection (collision and adhesion) efficiency for Stokes numbers in the range of 0.2 to 22 (Rembor et al. 1999; Ptak and Jaroszczyk 1990), the magnitude of the filter efficiency is less. For 1 μm to 4 μm PSL, the average collection efficiency is 0.43 (expanded uncertainty (U) of 0.08, n = 6). In the case of particles having very different surface adhesion, e.g., oleic acid (sticky) aerosol, the collection efficiency for ≥1 μm diameter particles is nearly 1.00. This is in accordance with the theoretical collision efficiency as expected for particles with high adhesion. PSL tests performed on a filter coated with a synthetic-oil (aerosol) deposit (32.9 μg mm −2 ± 0.4 μg mm −2 ) improved the collection efficiency for PSL ≥ 0.7 μm to nearly 1.00. Particles that are deposited by interception-inertial impaction at 10 m s −1 airflow velocity depend on the particle and fiber adhesion properties.