A new methodology for continuous scanning of particle aerodynamic diameter and application to filtration performance assessment of a room air purifier

S. Payne*, J. Symonds, Cambustion Ltd; T. Johnson, Cambridge University, UK

The Aerodynamic Aerosol Classifier (AAC) is a novel instrument that classifies particles based on their aerodynamic diameter. This is accomplished by passing the aerosol between spinning concentric cylinders with a coaxial sheath flow. Particles that have aerodynamic diameters larger than the AAC setpoint are dominated by the centrifugal force and impact the outer cylinder surface, while smaller particles remain entrained in the sheath flow; only particles with a narrow range of aerodynamic diameters pass through the AAC classifier. Particle sizes between 25 nm and 6.7 microns can be selected independent of the aerosol charge state, with a high transmission efficiency limited only by diffusion and impaction losses.

This technique can be used to measure an aerosol’s aerodynamic size distribution by stepping the AAC setpoint while recording the classified particle number concentration (Johnson et al., 2018). However, similar to the Differential Mobility Particle Sizer (DMPS) this stepping procedure requires long measurement times to achieve reasonable spectral resolutions. To overcome this limitation, Johnson et al. (In Preparation) recently developed the scanning AAC inversion. Following a similar methodology as the Scanning Mobility Particle Sizer (SMPS) inversion (Wang & Flagan, 1990), this new approach allows the aerodynamic size distribution to be continuously measured by varying the classifier speed while measuring the classified particle concentration. By continuously varying the classifier speed following an exponential function, the change in the centrifugal force a particle experiences during its classifier residence time is independent of its inlet time. Furthermore, when the scan time is sufficiently longer than the scan speed ramping constant, the scanning AAC transfer function converges to the same shape as the steady state transfer function.

This study uses a scanning AAC to collect accelerated measurements of the ambient air particle size distributions in a room before and after the operation of a commercial room air purifier. A filter sample was taken from the air purifier and measurements of the size-dependent filtration efficiency were made, using the AAC to select monodisperse test particles by aerodynamic diameter. The aim was to assess the air purifier performance by comparing size-dependent penetration measurements with the full real-world ambient spectra...

Session: G17 - Monitoring and Control
Day: 24 October 2019
Time: 14:45 - 16:00 h

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