Using the Aerodynamic Aerosol Classifier (AAC) as a Low-Pass Separator
JULIE PONGETTI, Chris Nickolaus, Jonathan Symonds,
Cambustion Ltd Abstract Number: 599
Working Group: Instrumentation and Methods
AbstractThe Aerodynamic Aerosol Classifier (AAC) was originally designed to transmit particles of a selected aerodynamic diameter in the range 25 nm to >5 μm. This is achieved by passing the polydisperse aerosol through a rotating cylinder (the classifier) where the particles experience opposing centrifugal and drag forces and only those of the selected size follow the correct trajectory into the sample outlet.
While particles larger than the setpoint are lost in the AAC classifier, those smaller than the setpoint remain suspended and can be considered a useful aerosol. It is therefore of interest to investigate the use of the AAC as a low-pass separator by recovering the sheath aerosol which contains all particles smaller than the setpoint. This is possible by disconnecting a readily accessible pipe at the back of the instrument.
Due to the limitations on the flow rates of the sample and sheath, the aerosol will also be diluted. To minimise dilution, it is necessary to use low sheath:sample ratios, hence decreasing the sharpness of the cut-off. Given the extremely high resolution offered by the AAC for monodisperse classification, this reduction is not a concern for a low-pass application.
The transmission efficiency was investigated for different combinations of sample and sheath flows using a second, standard AAC to either scan the output of the low-pass AAC or produce a monodisperse challenge aerosol.
To minimise dilution while maintaining a good resolution, a sheath:sample flow ratio around 3:1 is recommended. Under these settings, the AAC as a low-pass separator showed a sharp cut-off and excellent dilution-corrected transmission (>90%) between 200 nm and 2 μm.
Overall, the AAC can be easily converted into a low-pass separator with freely adjustable setpoint across over one order of magnitude in aerodynamic diameter – offering a flexible alternative to impactors and cyclones.