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.
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.