Collection efficiency of fiber filters operating on the removal of nano-sized aerosol particles: II. Heterogeneous fibers

The collection efficiency of a fiber filter of cellulose (HEPA) operating in the removal of nano-sized aerosols was studied here. The filter utilized had 0.920 in porosity, 0.4 mm in thickness, and fiber median diameter of 0.45 μm. A TSI electrospray aerosol generator, model 3480, was used to generate monodispersed nanometric particles, with sizes varying from 8.5 to 94.8 nm. The nanoparticles were generated from solutions of NaCl which were atomized as droplets, followed by drying. The filtration device was built in acrylic with a filtration area of 0.4 m2. The filter efficiencies were obtained by counting the particles before and after the filter, using a TSI condensation particle counter, model 3007. The tests were accomplished at gas superficial velocities varying from 0.03 to 0.25 m/s. The results show that the efficiency decrease with increasing particle size, indicating the predominance of the diffusional mechanism in the range studied. Also, the filter efficiency decreased with filtration velocity. The results were compared to theoretical predictions from the literature based on the classical collection mechanisms: diffusional and direct interception. The comparison of the calculated collector efficiency with the experimental results showed that the predictions underestimate the results. A correction was proposed to the existing correlation by defining the filter bed as composed of a number of filters in series, each with the fiber diameter and porosity derived from the fiber size distribution. The proposed modification included no fitting parameter. The results showed a considerable improvement in the prediction correlation.