Performance evaluation of commonly used impaction substrates under various loading conditions

This paper presents the results from the systematic performance evaluation tests of commonly used impaction substrates including uncoated metal plate, teflon membrane filter, greased metal plate, oiled sintered porous metal, and polyurethane foam (PUF), at various loading conditions. A single stage impactor and artificially generated aerosol composed of polydisperse glass beads were used to test the substrates. Significant particle bounce was observed for both the uncoated metal plate and Teflon filter substrate for particles larger than the impactor cutpoints (d50) at very low loading conditions. Particle bounce decreased with increasing particle loading until it reaches a plateau at a loaded mass of approximately 3 mg. A shift of collection efficiency curve was also observed, including a decrease of d50 for both the uncoated metal and the teflon filter. Despite the fact that the collection efficiency for particles larger than d50 remained high (95%) and constant up to about 30 mg of collected mass on the greased and the oiled sintered metal, an indication for minimum particle bounce and re-entrainment, a decrease of the d50 was also observed. However, while the changes in collection efficiency curve with increased loading and the consequent decreases of d50 were significant for the greased and oiled sintered metal substrate, for the PUF substrate, neither substantial particle-bounce nor significant changes in collection efficiency were observed. For PUF, the collection efficiency remained high (98%) for particles larger than the d50 for upto 30 mg of collected mass. The changes of the d50 on PUF are considerably less than those for the flat and rigid substrates. These results comprise a comprehensive confirmation that the nature of the impaction substrate has a significant effect on the particle bounce and particle separation performance of conventional inertial impactors.