Accelerated air filter ageing with a synthetic nano-aerosol

To predict the energy impact of air filters for general ventilation it is necessary to estimate correctly the airflow resistance variation of the filtering elements throughout their service life. The particle size distribution of the loading aerosol is essential for determining the kinetics of the air filter loading and clogging process. Current test standards simulate the ageing of air filters by dispersing synthetic dusts in the test duct to load the filter. This experimental procedure determines a “test dust capacity”, commonly referred to as “dust holding capacity”. However, standardized test dusts and typical urban atmospheric aerosols exhibit particle size distributions completely different. In fact, test dusts have particle size distributions shifted towards much larger particle sizes, while the atmospheric aerosols contain a large number of nanoparticles. Therefore, the main purpose of the data provided by the accelerated ageing procedure provided by current test methods is to compare the performance of an air filter against other competitors. To acknowledge this important limitation, test standards include a disclaimer to highlight the difference between the data obtained in the laboratory test procedure and the actual operation performance. Hence, the energy implications of air filtration equipment cannot be reliably predicted by current standards. We can overcome such limitation by using synthetic aerosols with particle size distributions comparable to typical urban atmospheric aerosols to load air filters in the laboratory. By simulating operating conditions similar to reality, we can predict more accurately the airflow resistance trend during the filter service life and consequently optimize the time interval for changing air filters too. Moreover, the data obtained in realistic conditions can help establish a meaningful energy labeling system. However, until now no standardized method to produce typical atmospheric aerosols is available. This is mainly due to the lack of the ability to generate and control large number of nanoparticles suspended in the test air, in a repeatable and reproducible way. We describe a method for generating large amounts of nanoparticles to obtain a loading aerosol with a particle size distribution similar to a typical urban atmospheric one, but with much higher concentration. The aim is to reproduce in a few hours what actually happens to a filter in one year of actual service. We compare different loading curves obtained by clogging air filters with currently standardized synthetic dusts and a synthetic nano-aerosol made up mainly by salt nanoparticles generated by combustion. The results are discussed and the fundamental differences between the laboratory results are highlighted.