Air Filter

Air pollution is an increasing concern all over the world due to its adverse effects on human health. It claims thousands of lives every year in countries like USA, Europe, Australia, Japan, China and the Netherlands. Air pollution due to particulate matter (PM) and gaseous pollutants can cause asthma, nausea, skin irritation, high blood pressure, cancer, birth defects along with respiratory and cardiovascular diseases. The severity of health hazard depends on exposure level and nature of air pollutants.
Fibrous filters are often used when particles of 1 µm or smaller are to be removed from a gas flow with high efficiency. A fibrous filter usually consists of loosely packed fibers in which the inter-fiber distance is large compared to the size of the particles. A typical structure of a fibrous filter is shown in Figure 1a. During filtration, aerosol particles are captured by the fibers and deposited on the surface of the fibers, while the open space between fibers provide paths for air flow (Figure 1 b). Aerosol particles are removed from the gas stream by the fibers through direct interception, Brownian diffusion, inertia impact, gravity settling, or electrostatic deposition (Figure 1 c). The mechanisms that play a role in the filtration of aerosol particles depend on the size of the fiber, gas velocity, and particle size. Figure 1 d shows typical filter efficiencies for these mechanisms and the total efficiency. The efficacy of an air filter depends on the type of air pollutant and can be tuned by the pollutant capturing mechanism. Smaller fiber diameter can increase the specific surface area of filter media that improves filtration performance. Hence, nanofibers have received increased attention in air filtration applications.
A number of techniques are available to fabricate nanofibers. These include conjugate spinning (sea-island technique), chemical vapor deposition, phase separation (sol–gel process), drawing, self-assembly, melt-blowing and electrospinning. Among these, electrospinning is a versatile and widely accepted process for producing air filter media.
Electrospun nanofibers are suitable for air filtration due to their small pore size and high specific surface area. The average pore size of nanofiber membranes can be 4–100 times smaller than microfiber membranes which can capture dust particles on its surface and ultimately improves filtration efficiency. The specific surface area can be 1000 times higher than microfibers due to micropores (less than 2 nm) and mesopores (2–50 nm) generation in the fiber structure during electrospinning. Hence, a small layer of electrospun fibers can significantly improve filtration efficiency [1, 2].

Figure 2 shows the efficiency and pressure drop of a common air filter and nanofiber coated air filter. As shown in the figure, FNM’s air filter (FreshAir®) which is coated by a layer of nanofiber has much higher efficiency than common (non-coated) air filter, while the pressure drop of nanofiber coated air filter is not increased significantly.

A comparison between the performance of air filter nanofiber coated media (by FNM Co.) and cellulose / synthetic media (blank substrate) has been done and the data are presented in Table 1.

References
1. Kadam, Vinod V., Lijing Wang, and Rajiv Padhye. "Electrospun nanofibre materials to filter air pollutants–A review." Journal of Industrial Textiles 47, no. 8 (2018): 2253-2280.
2. Li, Peng, Chunya Wang, Yingying Zhang, and Fei Wei. "Air filtration in the free molecular flow regime: a review of high‐efficiency particulate air filters based on carbon nanotubes." Small 10, no. 22 (2014): 4543-4561.