Aerosol filtration in nanofiber filter media
Nanofiber filter media are used in air filter applications where a compact filtration system with very high overall and fractional efficiency and low initial pressure drop is required. Moreover, because the aerodynamic drag force on nanofibers is very low compared to the drag on the large dust particles collected on the nanofiber media, this media is ideal for application to self-cleaning filtration. The particles can be easily detached by applying reverse airflow or just shaking the filter.
Fiber diameter is the main variable responsible for filter efficiency and pressure drop. Efficiency increases rapidly with decreasing fiber diameter. The ratio of nanofiber diameter of commercial filter media to fiber diameter of the commonly used cellulose fibers is approximately equal to 1:150. This results in an enormous surface area increase for nanofiber filter media. Surface area for 100 nm nanofibers is approx. 40 square meters/gram of the fibers while only 0.2 square meters/gram for 20-micron cellulose fibers.
Knudsen number, (where= 65.3 nanometers = the mean free path of air molecules under standard conditions and is inversely proportional to the pressure; df= fiber diameter) is used to classify air flow regimes that are listed in the Table.
Pressure drop, that is one of the major filter performance specification, significantly increases with decreasing fiber diameter as a function of 1/df2, until the free molecule regime is reached where pressure drop is a function of 1/df. However, this is a valid dependence only for clean filters. When dust deposits form on nanofibers, this benefit of low-pressure drop diminishes with increasing amount of deposited dust. Moreover, nanofibers capture very fine particles. The pressure drop increases more rapidly for this compacted dust cake. Therefore, it is important to design nanofiber filter media with significantly higher permeability than classical cellulose media to obtain long life filters.
Usually, the permeability of nanofiber filter media is 2-4 times greater than the value for the classical cellulose media, still providing great total and very high initial filter efficiency. Combined mathematical models for granular and fibrous media should be used to optimize nanofiber filter media for specific dust cake filtration.
The picture shows: a) - initial filtration - dust particles are mainly captured by the uniformly distributed nanofibers with an average diameter of 150 nanometers. Picture b) represents dust cake formation on nanofiber filter media, while picture c) shows excessive amount of nanofibers applied to the cellulose substrate. Pressure drop of such media will rapidly increase like in the case of membrane filters.
In air filtration with high aerosol velocity (greater than approximately 5-10 cm/s), the basis weight of 100-300 nanometer fibers should be less than 0.1 grams/ square meter, usually in the range of 0.3 - 0.7 grams per square meter. Cellulose or synthetic media can be used as a substrate.