Scholarly record
STUDY OF POROSITY AND PERMEABILITY OF AIR FILTER MATERIAL IN RESPIRATORY PROTECTION FILTERS
Abstract
The objective is the study of filtering tissue permeability and the characterization by means of permeability tests, of the process of coal particles retention in breath protection filters. The porosity of a filter, defined as the ratio between the void volume inside its fibres and the total volume, is a static parameter that provides point source information about the initial conditions of the filtering tissue. When excessive deposition of particles on the fibres happens, there is an increase of resistance to the air flow that can produce discomfort in use and efficiency diminution. For that reason, initial porosity of air filters for breath protection is relatively high, being frequent a range of values between 60\% and 90\%. Filter and respirator efficiency depends primarily on the filtering material used in its elaboration, and on the number and thickness of the different layers they are made of. Generally, the higher the efficiency the bigger the resistance both to breathe and elimination of generated heat and humidity. The permeability of a filter is the flow of air that passes perpendicularly trough the area of the filtrating tissue at atmospheric pressure (EN ISO 9237: 1995). It is, therefore, a dynamic parameter that allows the estimation of maximum time breathing protection equipment can be used, establishing an acceptable limit for permeability or saturation downfall. The permeability of nine probes have been measured against particles P1, P2 and P3, with a fixed test depression of 60 Pa, before and after being used in an atmosphere containing a suspension of coal dust. Brand new filters have different mean values of permeability to the air flow; its value doubles when the efficiency class is changed from P3 (Rm: 0,13) to P2 (Rm: 0,27) and to P1 (Rm: 0,54). In addition, after a time period of use of 120 minutes, the permeability of the filter decreases due to air particles capture, being such a diminution higher in class P2 filters (4,32 \%) respect to class P1 ones (2,68\%) and even lesser in class P3 ones (0,38 \%).
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References5
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