Research Projects

Project at a Glance

Project Status: complete

Title: Investigation of the filtration characteristics of the stacked filter unit

Principal Investigator / Author(s): John, Walter

Contractor: Air and Industrial Hygiene Laboratory, California Department of Health Services

Contract Number: A7-139-30


Research Program Area: Emissions Monitoring & Control

Topic Areas: Stationary Sources


Abstract:

The characteristics of large pore Nuclepore filters have been investigated to evaluate the performance of the Stacked Filter Unit as a particle size -selective sampler. Pore densities, pore diameters, and filter thicknesses were measured for various lots of 5, 8 and 12 m Nuclepore filters. A formula was developed for the pressure drop vs flow rate, this parameter being a good empirical indicator of filtration efficiency.

Nuclepore filtration efficiencies were measured by detecting the penetration of monodisperse particles with an optical counter. For liquid particles the efficiency increases slowly with flow rate, indicating that the principal filtration mechanism is interception. A theoretical calculation in the creeping flow approximation agrees well with the data. Efficiencies for solid particles are much lower than for liquid particles and decrease as the flow rate increases, showing the occurrence of particle bounce. Coating filters with grease greatly reduces the bounce effect. The efficiency and pressure drop change significantly for loadings of only a few particles per pore. Measurements of efficiency before and after loading with ambient particulate matter indicated optimum sampling results for grease-coated 8 m filters at a face velocity of 1.8 cm/s.


 

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