A HIGH-OUTPUT, SIZE-SELECTIVE AEROSOL GENERATOR

An aerosol generator capable of generating a narrow size-distribution aerosol with high mass concentration was designed, fabricated, and tes ted. Gravity was used to settle out targe particles and a virtual impa ctor with a clean air core was employed to remove small particles. Aer osols 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 mu m and a geometric standard deviation (GSD) varying from 1.18 to 1.46. Aerosol generation rate (Ma) varied from a bout 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 noz zle (#30609-5). The selected operating conditions were: atomization pr essure similar or equal to 200 kPa and liquid pressure similar or equa l to -4 kPa (relative to the ambient pressure). At these operating con ditions, the nebulizing air flow rate was 47 Ipm and the liquid flow r ate was 36 mL/min. The MMAD, GSD, and Ma of the generated liquid aeros ol (leaving the virtual impactor) were about 4 mu m, 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 narr ow size distributions. Several sets of acceleration nozzles and collec tion probes were evaluated. The improved virtual impactor was demonstr ated to segregate droplets very efficiently producing an aerosol with a nearly perfect log-normal distribution. Virtual impactor set I was o perated at an aerosol how rate (Qn) = 47 Ipm, clean air flow rate (Qc) = 15 Ipm, and minor flow rate (Qa) = 8 Ipm, while set II was operated at Qn = 47 Ipm, Qc = 27 Ipm, and Qa = 14 Ipm. The liquid aerosol selec ted by virtual impactor set I had a MMAD of 9.7 pm, GSD of 1.29, and M a of 120 mg/min, while the liquid aerosol selected by set II had a MMA D of 12.8 pm, GSD of 1.24, and Ma of 71 mg/min.