## 3 Materials and Methods
### Test Rig for Gas-Liquid Coalescing Filters  
The operation of the gas-liquid coalescing media was experimentally verified using the test rig equipped with two housings (two parallel branches):  
1. Original HFP 2000 system from PALAS GmbH, Karlsruhe, Germany with a horizontal vessel for testing circular flat panels.  
\begin{table}
\begin{tabular}{c c c} \hline  & **Layer I** & **Layer II** \\ \hline Average fiber diameter, \(\mu\)m & 0.35 & 11.6 \\ \hline Standard deviation, \(\mu\)m & 0.33 & 1.36 \\ \hline Minimum fiber diameter, \(\mu\)m & 0.08 & 8.23 \\ \hline Maximum fiber diameter, \(\mu\)m & 2.79 & 15.8 \\ \hline \multirow{2}{*}{Material grammage (measured), g/m\({}^{2}\)} & 65 & 85 \\ \cline{2-3}  & & 150 \\ \end{tabular}
\end{table}
Table 1: Specification of the filter media.  
The system is supplied with the pressurized air from the tank (T), which is fed from the screw compressor (COMP). The air passes two cleaning filters and is split into the main line, where the flow is adjusted by the mass flow controller (MFC). The other line of compressed air goes to the aerosol generator (PLG, which stands for the \"particle liquid generator\") through a reducing valve (RC), which allows for adjusting the pressure to a desired level. The aerosol is generated in a Laskin nozzle (a properly designed nozzle immersed in the oil). Then, these two lines merge, and the aerosol flows to a selected filter housing: a horizontal housing (original PALAS part for testing filter media in the form of flat panels) or a vertical housing, in which cartridge elements can be mounted. On the downstream side of the test vessels, the aerosol sample is taken for analysis using a vacuum pump (VAC), which enables the suction of aerosol throughout the chamber of particle counter WELAS 1000 (range 0.2-10.0 \(\upmu\)m). The measurement of the inlet aerosol is carried out using the same sample line but without any filter installed in the housing. The data from the counter are transferred to the PC and analyzed using software provided by PALAS GmbH.  
In experiments, the constant value of superficial velocity equal to 7.5 cm/s was used, which corresponds to the following flow rates of the supplied air:  
* Eighty L/min for flat circular media of the diameter 150 mm (the horizontal filter housing, the filtration material mounted vertically),
* Three hundred sixty L/min for pleated cartridge elements of a height 90 mm, having 44 pleats and a height of pleats 10 mm (the vertical filter housing); the melt-blown drain layer made of polyester (polybutylene terephthalate--PBT) was included on the outlet: grammage \(\sim\)200 g/m\({}^{2}\), the layer thickness \(\sim\)3 mm, and the average fiber diameter around \(\sim\)30 mm.  
The size distribution of the aerosol droplets upstream of the filter is presented in Figure 3. The inlet concentration of oil was approximately 40 mg/m\({}^{3}\).  
Figure 2: The experimental setup for testing gas–liquid coalescing media is as follows: COMP-rotary gas compressor; T—compressed air feed tank; F1, F2—air cleaning filters; MFC—mass flow controller; RV—reducing valve; PLG—aerosol generator; FH—horizontal filter housing (for flat media); FV—vertical filter housing (for cartridge filters); VAC—vacuum pump; WELAS—particle counter.  
The experiment was finished approximately 3 h after the pressure drop was stabilized or when the upper limit of the dP sensor (10 kPa) was exceeded. During experiments, the flow of aerosol was stopped every 30 min for 1 min.