Nanofiber Filter Performance

A practical measure of filter efficiency must take into account botii die penetration of particles across the filter, Pt (i.e., the ratio of particles upstream of the filter to that exiting the opposite surface of the filter downstream) and the pressure drop AP across the filter at a given face velocity. Note that Pt= 1—E. A figure of merit (FoM) for the filter can then be defined as 

Numerically, the value of FoM is the gradient of the log-linear plot of penetration vs the pressure drop at constant face velocity. Various phenomena, such as the magnitude of the slip correction factor discussed in the previous sections, tend to increase the FoM as the average fiber diameter is decreased. Ensor et al. (2006) reported the FoM for nanofiber mats to be 

TABLE 8.1 Filtration efficiencies for electrospun nanofiber mats of polysuKone compared to commercial HEPA and ULPA filter paper 

The improvement in performance reported for these filters even in these preliminary experiments is very significant. Despite Ihe unavoidable scatter in data due to variation in packing density and filter thickness, the performance of nanofiber filters appears to well surpass that of the conunercial fiberglass utihty filters. Optimistically, the best data in the table suggest as much as an order of magnitude improvement in FoM for nanofiber mats over HEPA filters. The relative contributions of slip and electrostatic effects to the reported filter performance have not been totally assessed and may depend on the conditions used to eleetrospin eaeh filter mat. It has been estimated that the contribution of electrostatics to the FoM is about 20%. 

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Porosity is one of the important parameter in filter design and filter performance. Past studies have shown that the thickness and porosity of the nanofiber mats can be controlled by changing the deposition rate of nanofibers. Adequate porosity and surface area of the nanofiber mat has turned nanofiber coatings as an important candidate for high performance filters. 

Grafe et al. [62] compared the filtration performances of a composite cellu-lose/nanofiber filter and a standard (wet-laid cellulose) filter and discussed the performance of nanofiber media in a mining vehicle cabin air filter. These two kinds of filters were tested independently on a Caterpillar 992G wheel loader, mounted with two particle sampling devices, one inside the cabin and the other outside. With the standard cellulose filter, the filtration efficiencies of submicron and respirable (>1 p,m) dust were 68 % and 86 %, respectively. However, with the cellulose/nanofiber composite filter, a higher reduction of approximately 92 % has been observed for both the submicron and respirable dusts. Besides, the results showed that submicron salt crystal preferentially collected on nanofibers (compared... 

Various types of fibres could be used in making filter fabrics they include glass fibres, synthehc fibres, ceUulosic fibres (eg, natural wood pulp fibres, viscose fibres, and Lyo-ceU fibres), wool fibres, metal fibres, ceramic fibres, high-performance polymer fibres (eg, inherenfly fire-resistant fibres, chemical resistance fibres, high-strength, and high-modulus fibres), microfibers, and nanofibers. 

A dispersion is instead a mixture in which the less abundant compound is dispersed, but not molecularly dissolved, in the other component. Examples are a dispersion of a solid phase (powder, nanoparticles, nanocrystals, nanotubes, etc) in a solvent, which is called a suspension, or a dispersion of an immiscible liquid phase in a second liquid, which is called an emulsion. Milk and mayonnaise are familiar examples of emulsions. Aerosols are dispersions of tiny liquid droplets or solid particles in a continuous gaseous phase. The science of aerosols is particularly relevant in order to design filter elements able to remove droplets and particles from air, which can be performed with very high efficiency by polymer nanofibers (Section 4.3.1). Finally, another way to indicate homogeneously mixed dispersions, emulsions or aerosols of nano- or microparticles is colloids. Colloidal dispersions of inorganic nanocrystals or organic nanofibers are familiar examples for nanotechnologists. 

Leung WWF, Hung CH, Yuen PT (2010) Effect of face velocity, nanofiber packing density and thickness on filtration performance of filters with nanofibers coated on a substrate. Sep Purif Technol 71(l) 30-37. doi 10.1016/j.seppur.2009.10.017... 

Bjorge D, Daels N, De Vrieze S, Dejans P, Van Camp T, Audenaert W, Hogie J, Westbroek P, De Clerck K, Van Hulle SWH (2009) Performance assessment of electrospun nanofibers for filter applications. Desalination 249 942-948... 

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