DEVELOPMENT AND EVALUATION OF A LOW CUTPOINT VIRTUAL IMPACTOR

Designing an impactor with a submicrometer cutpoint is a technical cha llenge, since high jet velocities (resulting in low pressures downstre am of the jet nozzle) or small orifices are required. This paper prese nts the development of a low cutpoint virtual impactor that operates a t a small pressure drop. A parametric study was performed to determine the conditions that affect the performance of the virtual impactor. I n addition, a detailed measurement of the particle losses through the system is presented. By developing a better understanding of the parti cle collection and losses as a function of geometric and flow paramete rs, the design of low cutpoint virtual impactors can be optimized. The results show that the virtual impactor has a cutpoint of 0.12 mu m at a pressure drop of 0.08 atm, with low particle losses, averaging abou t 7% with a maximum of 14% at the 50% cutpoint. The separation charact eristics 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 la rger cutpoint size, a steeper collection efficiency curve and an incre ase in particle losses. The 50% cutpoint increases as the collection-t o-acceleration nozzle diameter ratio (D-r/D-j) increased from a standa rd value of 1.4 to 2. Increasing the jet velocity and therefore the pr essure 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 t he virtual impactor.