Abstract
Elecotrospun nanofibers,
with fiber diameters of 0.25 microns have been used in industrial, consumer and
defense filtration applications for more than twenty years. Electrospun
nanofibers have fiber diameters that are 5-10 times smaller than the smallest
meltblown fibers available. Recently, Ultra-Web nanofiber filter media has
become available to the non-wovens and filtration industries for a broad range
of filtration applications.
Nanofibers provide dramatic increases in filtration efficiency at relatively small (and in some cases
immeasurable) decreases in permeability. In many laboratory tests and actual
operating environments, nanofiber filter media also demonstrate improved filter life and more contaminant holding capacity. Nanofiber filter media have enabled
new levels of filtration performance in several diverse applications with a
broad range of environments and contaminants. The performance of nanofiber
media in a mining vehicle cabin air filter will be discussed.
Figure 1. Ultra-Web
Nanofiber Filter Media
1. Characteristics Of Small Fibers In Air
Filtration - Theory
Small fibers in the submicron range, in
comparison with larger ones, are well known to provide better filter efficiency
at the same pressure drop in the interception and inertial impaction regimes.
W.C. Hinds shows such effect by assuming non-slip flow at fiber surface.
While smaller fiber size
leads to higher pressure drop, interception and inertial impaction efficiencies
will increase faster, more than compensating for the pressure drop increase.
Thus, in the particle size of interest, i.e., from submicron and up, better
filter efficiency can be achieved at the same pressure drop, or conversely, the
same filter efficiency at lower pressure drop can be achieved with smaller
fiber sizes.
For
nanometer-scale fibers, a second factor has to be taken into account: the
effect of slip flow at the fiber surface. Filtration theory generally relies on
an assumption of continuous flow around the fiber, with a no-slip condition at
the fiber surface. The theory starts to become less valid when the scale of the
fiber becomes small enough that the molecular movements of the air molecules
are significant in relation to the size of the fibers and the flow field. Using
a slip-flow model at the fiber surface can extend the useful range of
continuous flow theory. The Knudsen number is used to describe the importance
of the molecular movements of air molecules at the fiber surface to the overall
flow field. The Knudsen number can be written as
Where
λ is the gas mean free path (the dimension of the non-continuous nature of
the molecules), and rf is the radius of the fiber. When Kn becomes
non-neglible, the continuous flow theory, which does not take into account the
molecular nature of air, starts to become less valid. While there is no exact
Kn above which slip flow will prevail, it generally needs to be considered when
Kn > 0.1, and definitely needs to be considered when Kn is around 0.25. For
air at standard conditions, the mean free path is 0.066 microns; therefore, for
fibers with diameters smaller than 0.5 microns, slip flow must be considered.
In slip flow, the air velocity at the fiber surface is assumed to be non-zero.
