Development and evaluation of a low cutpoint virtual impactor

Designing an impactor with a submicrometer cutpoint is a technical challenge, since high jet velocities (resulting in low pressures downstream of the jet nozzle) or small orifices are required. This paper presents the development of a low cutpoint virtual impactor that operates at a small pressure drop. A parametric study was performed to determine the conditions that affect the performance of the virtual impactor. In addition, a detailed measurement of the particle losses through the system is presented. By developing a better understanding of the particle collection and losses as a function of geometric and flow parameters, the design of low cut-point virtual impactors can be optimized. The results show that the virtual impactor has a cutpoint of 0.12 μm at a pressure drop of 0.08 atm, with low particle losses, averaging about 7% with a maximum of 14% at the 50% cutpoint. The separation characteristics of the virtual impactor are similar to those predicted by the theory. Reducing the minor flow ratio from 0.2 to 0.1 results in a larger cutpoint size, a steeper collection efficiency curve and an increase in particle losses. The 50% cutpoint increases as the collection-to-acceleration nozzle diameter ratio (D_r/D_j) increased from a standard value of 1.4 to 2. Increasing the jet velocity and therefore the pressure drop across the impactor's nozzle results in a decrease in the 50% cutpoint, as well as in a decrease in the particle losses within the virtual impactor.