Compressed air filters are often a combination of different layers of filter materials. Thus the behaviour of single layers as well as different combinations of filter materials is of interest in the development of new filters.

The test of compressed air filters is described in 2 Standards, ISO 8573-1 to 9 “Compressed air –….. “ describes test methods for the classification of the compressed air quality on site. ISO 12500-1 to 4 “Filters for compressed air – test methods” describes how to characterise the filters regarding their oil mass efficiency under defined conditions in the steady state in part 1) and with regard to their fractional separation efficiency in part 3.

Looking closer into ISO 12500 itself and also the conditions of testing we find the following critical points:

• Standard conditions as e.g testing at a temperature of 20°C (@7 bar overpressure) will give a good comparison for the elements. But these filters are often used at Temperatures up to 100°C what influences the viscosity of separated oil droplets and the pressure drop curve during loading.
• A flat sheet media holder to simulate different layers of media is not given.
• The detection of particle size distribution directly in overpressure is difficult. Most of the optical particle counters cannot work directly in overpressure and a pressure reduction may lead to a change of the particle size distribution.

In this presentation we will outline the construction of a filter test bench with regard to ISO 12500-3 with direct measurement of the particle size distribution in overpressure up to 10 bar and also heatable up to 80°C.

Special care is taken in determining fractional efficiency directly in overpressure and with a pressure reduce in the sampling line. Clearly visible is a significant change of the particle size distribution

A specially constructed filter holder implemented in the bench, designed to perform different forces on the packing of flat sheet media layers to simulate a complete filter element will be discussed. Changes in the filtration behavior due to standard temperature and a temperature of 80°C are also in the focus.