Development of a High Volume Slit Nozzle Virtual Impactor to Concentrate Coarse Particles

This paper presents the development and evaluation of a high volume slit nozzle virtual impactor that can be used to concentrate coarse particles (2.5-10 θ m) for inhalation studies. A variety of physical design and flow parameters were evaluated for their effects on the virtual impactor performance. To increase the impactor sampling flow, both multinozzle and longer slit configurations were developed and tested. The use of a longer slit nozzle made it possible to increase the inlet flow more effectively than using multiple nozzles. The effect of the slit nozzle length (from 0.762 cm up to 6.35 cm) on the impactor performance was examined. It was observed that increasing the slit length did not significantly diminish the performance of the impactor to concentrate coarse particles. Furthermore, a wider slit (W₁ = 0.363 cm) that operates at a slightly higher pressure drop was used to increase the sampling flow rate. Although the wider slit presented greater losses for fine particles, the concentration factor for coarse particles was just as good as with the original narrower slit (W₁ = 0.305 cm). Finally, it was demonstrated that a single-stage slit impactor (L = 6.35 cm, W₁ = 0.363 cm, Q₀ = 870 L/min) can be used to increase the coarse particle concentration by a factor of 15 and to supply a flow of 43.5 L/min for animal or human exposure studies. A higher concentration factor, up to 50, was achieved by using a smaller minorto-total flow ratio.