In recent years, there has been an increasing demand for HVAC filters with adsorptive effect resulting in the standardization of a test method for these filters and filter media in ISO 10121, Parts 1 and 2. In 2022, Part 3 of ISO 10121 also introduced a classification system for adsorptive HVAC filters. This classification of adsorptive filters is based on the results of breakthrough tests with 9 ppm toluene, SO₂, NO₂ or 3 ppm ozone on the test filters. The filters are classified according to how long the filter separates the supplied test gas with an efficiency of at least 50 %. Following the evaluation of parts 1 and 2 of ISO 10121 in previous research projects, the project reported here focuses on the third part of the standard.

An overview is provided how the results of the standard breakthrough tests are used to determine the classification result. The individual classification parameters, performance class or duty level, integrated removal efficiency, initial removal efficiency and retention capacity are presented and the practical significance of the classification parameters is explained for various filter designs (compact filters, pocket filters, filter cartridges). In addition, the challenges involved in the practical implementation of the classification tests are shown with several examples.

One of the main challenges turned out to be the practical implementation of the tests with ozone. The provision of 3 ppm ozone with air flow rates of typically up to 3400 m³/h requires high-performance ozone generators. Another difficulty is the long test duration for ozone tests which is caused by the following reason: To obtain the performance class, the filtration efficiency for the specified test gas is considered as a function of the amount of test gas added (dose) per cross section of the filter. In this way, also filters with different nominal volume flow rates and installation cross-sections can be compared with each other. The performance class is determined by the dose at which the filter efficiency falls below 50 % during the breakthrough test. ISO 10121-3 specifies dose limits that classify the filter as Light Duty (LD), Medium Duty (MD) or Heavy Duty (HD). The molar dose limits are the same for all four test substances. Since the standard specifies a three times lower test gas concentration for ozone as for the other test gases the duration of the filter test with ozone is correspondingly longer. The test times for filters with a high ozone separation efficiency and/or low volume flow rates can by far exceed one working day, which is difficult to realize due to the special safety requirements for handling ozone.

In summary, the evaluation of the pure realization of the classification procedure and the basic practicability of the tests according to ISO 10121-3 has led to a positive result. The benefits of the classification are unquestioned; for the first time it is possible to compare the performance of adsorptive filters with different geometries and nominal volume flow rates. However, the test procedure on filters is quite complex and expensive because of the required test facility dimensions, large air volumes which have to be conditioned and large quantities of test substances that have to be provided. For this reason, it was also investigated to what extent and under what conditions the tests on media samples from filters lead to comparable classifications as the tests on filter elements. The results on the significance of determining the flow rate through the media sample during the test and the reference area for calculating the area-specific dose are discussed.