Fig 1.
Principle of filtration efficiency enhancement.
Panel A shows how the trajectory of the particle motion changes if we reduce the speed of the air that carries it around the filter fiber (cross-section). At low speeds (face velocities), as a result of Brownian motion, the particle deviates more from its original streamline, and the probability of it being trapped on the fiber surface increases (typically for particles smaller than 1 micron). Reducing the flow rate and thus increasing the filtration efficiency can be achieved by increasing the filter area, as shown in the example in Panel B. Increasing the number of filter layers leads to an exponential decrease in the number of particles passing through the filter and an increase in the filtration efficiency to 99%.
Fig 2.
Characteristics of the knit PES fleece fabric used as a filter material.
Panels A, B and D show the filtration ability of the PES fabric depending on the filter area, which was varied as multiples of the area of 150 cm2 (the size of the mask) and the number of layers of filter material. One layer has a thickness of 2.3 mm. The volumetric flow rate was selected to be 95 L/min. Panel A shows the filtration efficiency for 100 nm particles, Panel B represents the number of particles out of 1,000 that pass through the filter cartridge, and Panel D shows the corresponding pressure drop (Pa) evaluated on the basis of the measured permeability. Panel C shows the experimental curve of the filtration efficiency as a function of the particle diameter for a filter six times the size of a standard mask with eight layers of PES fabric. The blue star represents a measurement of the filtering efficiency performed by an accredited laboratory. Panel E demonstrates a comparison of the filtration efficiencies of two commonly available knit textile materials, namely, PES fleece and cotton, measured for 100 nm particles. Panel F shows the filtration ability of the PES fabric before and after washing at 95°C or boiling cycles.
Fig 3.
Assembly scheme of the filter kit.
Shown are all parts of the filter kit (Panel A) and its gradual assembly (Panel B-E), including a demonstration of how it is to be worn (Panel F). The purpose of the blue porous material on Panel G is to allow air to access the filter cartridge; air is sucked in over its entire surface (flow distributor). On Panel H the cross-section of the filter kit and the direction of air movement are shown.
Fig 4.
Pressure drop of the filter kit and its components.
Shown are the pressure drop of the filter kit and its components at volumetric flow rates of 30 and 95 L/min, which correspond to air inhalation during light and heavy physical exercise, respectively. The cross-section of the filter cartridge is shown. The carrier (backpack) is not shown even though its pressure drop is taken into account.
Table 1.
Summary of the advantages and disadvantages of the designed filter kit.