Calculation of Leakage and Particle Loss in Filter Cassettes

Experimental evidence of aerosol bypass leakage around the filter in plastic filter cassettes prompted an investigation using computational fluid dynamics to explain particle penetration through the leak. Axi-symmetric models of a cassette with several leak dimensions were constructed. The models predicted that submicrometer particles penetrated the leak, but that larger particles impacted on the filter surface. Experimental data from another study clearly indicated that larger solid particles were being lost from the surface of the filter during sampling. When particle bounce was invoked as an explanation for this loss of sampled solid particles, the theoretical loss from the filter in cassettes with large leaks exhibited characteristics similar to the experimental data. For small leaks, the mass loss behavior appeared to be more complex.