## 3 Results and Discussion  
Filter bags from two installations operating in the same furniture factory in different technological lines were used for the test. Thus, data were obtained on the characteristics of the formation of air flow resistance through bags made of the same non-woven fabric installed in filters differently loaded with air carrying various types of dust waste. Before testing the total pressure drop \(\Delta\mathrm{p_{C}}\) for individual bags, the pressure drop across the bags \(\Delta\mathrm{p_{0}}\) during the clean air flow was determined. This test showed an increase in resistance with increasing bag life in an industrial filter. The pressure drop values for the filter bags from both filter baghouses are shown in Figures 5 and 6. The formation of the pressure drop during the flow of clean air through the filter bags is very similar to the results obtained in previous studies on the use of two-layer filter media for the separation of wood dust [26]. The value of the K\({}_{0}\) coefficient equal to 1276 [Pa\(\cdot\)s\(\cdot\)m\({}^{-1}\)] was read from plot of the clean bag, which had not been used before, as the value of the proportionality coefficient of the regression equation. On the basis of the results obtained in accordance with the Formula (1), the value of the pressure drop across the clean bag \(\Delta\mathrm{p_{0}}\) was calculated for the assumed filtration velocity of 0.0405 m\(\cdot\)s\({}^{-1}\). This pressure drop is 51.7 Pa. This value was further used to determine the dust cake resistance coefficient. The operation time of the bags in an industrial installation increases the resistance of clean air flow through the tested filter fabric. This is certainly due to the presence of dust inside the bag structure that was previously used in an industrial dust collector. Higher resistances were obtained for the bags working in filter 2 in the dust extraction installation of the CNC drilling machine line. The pulse-jet baghouse in this installation works in lighter conditions (filtration velocity and less waste than in the narrow surfaces processing line) and the bags are cleaned less frequently. This shows that the frequency of cleaning pulses has a decisive influence on the air flow resistance. In the second stage of the test, the total pressure drop was measured during the experiments with the use of filter bags previously working in industrial filters for 67, 133 and 272 days and a clean bag, not previously used. The same test wood dust was used in all the experimental air purification processes. Figures 7 and 8 show the maximum pressure drop \(\Delta\mathrm{p_{C}}\) in individual filtration cycles for the tested filter bags from baghouses 1 and 2 respectively. Both figures show the pressure drop curves for bags that were used in industrial installations. The course of the increase in total pressure drop during experimental filtration processes is characteristic for this type of filtration process. Figure 4: Particle-size distributions of the dust by sieving method. Figure 5: Pressure drop across clean filter media from narrow surfaces treatment line. Figure 6: Pressure drop across clean filter media from the line of drilling centers. Figure 7: Flow resistance \(\Delta\)pc through the filter fabric from narrow surfaces treatment line. In the initial phase, the pressure drop increases quite quickly, but with the time of the experiments, the increase in the pressure drop gets smaller. This proves the stabilization of the dust cake formed on the surface of the filter medium and thus the stabilization of the flow conditions in the entire system. For bags from both lines it was observed that the longer the fabric was used in the industrial installation, the higher the air flow resistance during laboratory tests. Comparing both lines, it can be concluded that slightly higher resistances were observed for the bags of the CNC drilling machine line. However, in the case of the longest operation life of 272 days, this difference was significant.