Experimental assessment of air filter media behaviour with ultrafine particles

This work presents an experimental study on the clogging behaviour, filtration efficiency and airflow resistance of various filter media under controlled aerosol loading and relative humidity (RH). The goal was to assess how RH affects the time to reach a target differential pressure drop in the test rig and to relate this to fractional efficiency, mass deposition and air resistance performance. Four filter media (A, B, C and D) were tested under constant and cyclic RH conditions. At constant RH, clogging time increased with RH: for A and B, raising RH to 60% significantly delayed reaching the final pressure drop. In contrast, C achieved the final pressure drop more quickly, even at 60%. The D medium never reached the target, even at lower RH, due to its low particle capture efficiency. The fractional efficiency increased progressively for A, B, and C media, approaching 100% as clogging increased. The D medium showed irregular trends and nearly constant pressure drop, indicating low ultrafine particle capture. The most penetrating particle size (MPPS) occurred around 110 nm, where efficiency dropped to 70% before rising again due to particle diffusion. Airflow resistance tests before and after clogging showed a consistent correlation between the reference instrument and the test rig used for the fractional efficiency measurement. The particles captured and the mass of the media samples increased with RH, reflecting longer clogging times and larger salt deposits. Samples tested at 60% RH showed the highest mass gain, with A gaining 5% and B 1-2%. Instead, faster clogging or interrupted tests showed lower values. Airflow rate also affected results: higher flow shortened clogging time but reduced the amount of deposited particles. Overnight interruptions led to partial self-cleaning due to residual moisture. We optimised the Scanning Mobility Particle Sizer (SMPS) operation using a 1-minute scan, 0,0508 cm nozzle and 16 channels per decade as the best compromise between resolution and stability. In conclusion, RH strongly influences clogging kinetics: higher humidity delays clogging, especially for low-density media. Denser, charged media exhibit faster clogging and higher efficiency. Accurate RH and flow control are crucial for reliable filter-media assessment. Future work shall include neutralising the test aerosol.