Modification of the University of Washington Mark 5 in-stack impactor

Abstract

A 12-stage, medium-flow rate, in-stack, low pressure impactor was designed and built by utilizing the jet plates of the University of Washington Mark 5 in-stack impactor. Impactor design was based on isentropic flow relationships corrected with experimental discharge coefficients measured for stages having similar geometry. Cut diameters were calculated from Stokes equation using the jet core velocity, √Stk 50 = 0.49 and evaluating the slip correction factor and gas viscosity at the upstream stage stagnation gas conditions. Calculated aerodynamic cut diameters of stages 1–12 (stages are numbered starting from the smallest cut diameter) were 0.025, 0.039, 0.067, 0.10, 0.16, 0.27, 0.72, 1.4, 2.4, 4.9, 8.4 and 15.6 μm, when the impactor is operated at the overall outlet to inlet pressure ratio of 0.075 under STP conditions. Operating pressures of the impactor stages were measured and the compressible flow stages 1–6 were calibrated with singly charged DOP-aerosols. Calculated and measured operating pressures agreed to within −9 and +6%. Experimental aerodynamic cut diameters of stages 1–6 were 0.030, 0.043, 0.078, 0.12, 0.30 and 0.36 μm, respectively, being clearly larger than the calculated values. Collection efficiency curves of stages 1–3 and 6 were sharp, having geometric standard deviations in the range 1.13–1.16. Stages 4 and 5 had poorer size classification properties; the geometric standard deviations of their collection efficiency curves were 1.45 and 1.59, respectively. When Stokes numbers were calculated using jet average (adiabatic) velocities and gas stagnation properties upstream the stage, √Stk 50-values of stages 1–4 and 6 were in the range 0.45–0.49. The corresponding value for stage 5 was 0.65. Poor size resolution of stages 4 and 5 and a large √Stk 50-value of stage 5 indicate that the steepness of the collection efficiency curve and cut point Stokes number of the compressible flow multijet impactor stages depend on the distance between the jet and the collection plates and the downstream to upstream stage pressure ratio.