A High-Output, Size-Selective Aerosol Generator

Abstract

An aerosol generator capable of generating a narrow size-distribution aerosol with high mass concentration was designed, fabricated, and tested. Gravity was used to settle out large particles and a virtual impactor with a clean air core was employed to remove small particles. Aerosols generated from a sodium chloride solution (0.05% to 9% by volume) were found to have a mass median aerodynamic diameter (MMAD) in the range from about 1 to 10 μm and a geometric standard deviation (GSD) varying from 1.18 to 1.46. Aerosol generation rate (Ma) varied from about 0.2–24 mg/ min depending upon the final particle size. The overall dimensions of this generator were about 190 cm in height and 15 cm in diameter. Droplet generation was performed with a Delavan simplex nozzle (#30609-5). The selected operating conditions were: atomization pressure ≃ 200 kPa and liquid pressure ≃ — 4 kPa (relative to the ambient pressure). At these operating conditions, the nebulizing air flow rate was 47 1pm and the liquid flow rate was 36 mL/min. The MMAD, GSD, and Ma of the generated liquid aerosol (leaving the virtual impactor) were about 4 μm, 2, and 600 mg/min, respectively. Liquid droplets were segregated by a virtual impactor with a clean air core and evaporated to form solid particles with narrow size distributions. Several sets of acceleration nozzles and collection probes were evaluated. The improved virtual impactor was demonstrated to segregate droplets very efficiently producing an aerosol with a nearly perfect log-normal distribution. Virtual impactor set I was operated at an aerosol flow rate (Qn) = 47 1pm, clean air flow rate (Qc) = 15 1pm, and minor flow rate (Qa) = 8 1pm, while set II was operated at Qn = 47 1pm, Qc = 27 1pm, and Qa = 14 1pm. The liquid aerosol selected by virtual impactor set I had a MMAD of 9.7 μm, GSD of 1.29, and Ma of 120 mg/min, while the liquid aerosol selected by set II had a MMAD of 12.8 μm, GSD of 1.24, and Ma of 71 mg/min.