Air filters installed in ventilation systems face various types of aerosols during their service life, both in residential and in commercial buildings. Their particle size is the most important characteristic and ranges from a few nanometers to a few micrometers. Different physicochemical properties, such as phase state, hygroscopicity, and morphology are also important to determine the impact of particulate matter on the behavior of air filters during their service life. Therefore, the performance of air filters installed in a Heating, Ventilation and Air-Conditioning (HVAC) system is strongly dependent on the properties of the particles captured during their service life and not only on the characteristics of the materials and technologies used to manufacture the air cleaning equipment.
Current laboratory test methods for evaluating HVAC filter performance include the determination of their fractional particle removal efficiency on a limited size range, typically between 300 nm and 10000 nm. Such information is useful and meaningful for clean filters. However, air filters performance (i.e. airflow resistance and removal efficiency) changes during their service life because of particle loading. For this reason, air filters are artificially clogged in laboratory with the intent to compare one product to another and to predict their behavior while they age in HVAC systems. Current standardized air filter loading procedures use synthetic dusts with particle size distributions very different from typical urban atmospheric aerosols. Consequently, the results obtained in this way differ from the air filter performance measured in real HVAC installations and the designers cannot use them to predict quantitatively the in-situ air filter performance. Standards writers are aware of the problem and this limitation is stated explicitly in EN ISO 16890 and ANSI/ASHRAE 52.2 standards. However, there is a need to perform the test in a short time. Moreover, the filtration industry consolidated this approach during the past decades and a lot of data is available with those dusts.
To improve the prediction of the size-resolved efficiency and the loading kinetics of HVAC filters we need improved test methodologies. ASHRAE is promoting the development of a new method to age air filters as part of ASHRAE Guideline Project 35 “Method for Determining the Energy Consumption Caused by Air- Cleaning and Filtration Devices”. Research teams in USA and Italy are attempting to improve the loading procedure to age HVAC filters using aerosols with more realistic particle size distributions. If successful, the new ageing procedure could provide a reliable prediction tool for evaluating the airflow resistance during the filter service life. In this way, we could optimize the performance of air cleaning equipment in realistic conditions.
In this paper, we summarize current laboratory ageing procedures. We compare the airflow resistance trend in an HVAC system monitored for more than one year, with the results from an ISO 16890 laboratory test on the same air filter. We discuss the difference between the mass increase values causing the same airflow resistance increase.
To reduce the difference between actual data and laboratory simulation results, we present an emerging technique with a preliminary evaluation of a new thermal flame generator for challenging HVAC filters with sub-micron potassium chloride aerosol at high mass concentrations. The thermal aerosol generator is able to reproduce the sub-micron urban atmospheric aerosol mass size distribution and it is a promising technique to solve some of the problems stated above.