Flow and Pressure Balancing in a Fluid Interface Impactor

Abstract

Fluid interface impactors, such as virtual and opposing jet impactors, require particular attention to flow and pressure balance in the incoming and outgoing airstreams. The relationship of flow rate to pressure drop has been studied in an opposing jet aerosol classifier of rectangular design. In tests of the sensitivity of classifier operation, changes in pressure differences across the inlets and the outlets were associated with changes in the flow balances of 0.5 and -1.0%/mm w.g., respectively. These flow sensitivities may be used in conjunction with cut size sensitivities to conclude that the appropriate measurements to use for classifier control are the static pressure differences between the two inlets and between the two outlets, rather than the mass flow rates of the four jets. For aerosol size classification tests, a nebulizer-virtual impactor polydisperse aerosol generator was designed to reduce the concentration of small particles and make the generator compatible with dynamic optical particle counting. The aerodynamic cut diameter is Stk ^1/2 ₅₀ = 0.49 ± 0.08 for the standard pressure balance condition. Classification sharpness, using 84 and 16% efficiency sizes, is 1.43 ± 0.03 at the standard condition and can be reduced to 1.11 ± 0.02 at other conditions.