A two-stage 100 l/min circumferential slot virtual impactor system for bioaerosol concentration

Abstract

A two -stage circumferential slot virtual impactor aerosol concentrator system has been developed that is designed for nominal operational conditions of a 2 μm AD cutpoint, an aerosol inflow to the first stage of 100 L/min and a minor flow rate from the second stage of 1 L/min. Each unit was tested separately before being combined in the system. However, because of high inter-stage losses, a sheath air system was inserted between the two stages, wherein a small amount of air was injected into the apex of a cone placed on top of the second stage. The sheath air displaced the stagnation point at the apex of the cone and redirected particles into the sampling zone of the second stage unit. The cutpoint particle size of the system was 2.5 μm AD at the nominal flow rate. The dynamic range (ratio of upper limit to the lower limit of aerodynamic particle diameter associated with transmission efficiencies of 50%) was 5.4, and the largest particle size for which the transmission was at least 50% is 13.6 μm AD. When run at 67 L/min, the cutpoint is 4 μm AD and the dynamic range is 3.75; at 150 L/min the cutpoint is 2.05 μm AD and the dynamic range is not less than 4.74. The pressure drop across the system is 685 Pa (2.75 in. H2O). This yields an ideal power consumption of 0.77 watts.