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 scrubbers, and other similar systems can be used to remove larger-sized particles, but smaller, nanoparticles will most likely be collected by filtration (see next section, Air Filtration).

2.3.1.2 Air Filtration

Air filtration removes unwanted particulate from an air stream. Particulate air filters are classified as either mechanical or electrostatic filters. Although the two types of filters have important performance differences between them, both are fibrous media or membranes and are used extensively in HVAC and industrial applications. Efficiency is dependent on several factors including fiber diameters, packing density, and material used. A fibrous filter is an assembly of fibers that are randomly laid perpendicular to the airflow. The fibers may range in size from less than 1 μm to greater than 50 μm in diameter. Filter packing density ranges from 1%–30%. Fibers are made from cotton, fiberglass, polyester, polypropylene, or a number of other materials [Davies 1977].

Fibrous filters of different designs are used for various applications. Three types are used for capturing particulate:


 * Flat-panel filters contain all the media in the same plane. This design keeps the filter face velocity and the filter media velocity roughly the same.
 * Pleated filters have additional filter media added to reduce the air velocity through the filter. This allows for an increased collection efficiency for a given pressure drop. Alternatively, pleated filters can be used to reduce the pressure drop for a given airflow velocity because of the larger filter area.
 * Pocket or baghouse filters allow the flow of exhaust air through small pockets or bags consisting of filter media. As with pleated filters, the increased surface area of the pocket filter reduces the velocity of the airflow through the filter media, allowing increased collection efficiency for small particles at a given pressure drop.

Figure 5 presents four different collection mechanisms that govern particulate air filter performance:

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 * Diffusion is the result of the random (Brownian) motion of a particle. The particle may contact a fiber on its path through the filter.
 * Interception occurs when the radius of a particle moving along an air streamline is greater than the distance from the streamline to the surface, thus causing the particle surface to contact the surface of the fiber. The particle adheres to the fiber due to intermolecular forces.
 * Inertial impaction occurs when an air stream bends around a fiber, and a particle traveling in that air stream continues in a straight path due to particle inertia. The particle collides with the fiber and adheres to it due to intermolecular forces.
 * Electrostatic attraction occurs when the particle and the fiber are oppositely charged. As the force of this attraction is governed by the charge-to-mass ratio of the particle, it becomes more effective as particle size decreases.

Current Strategies for Engineering Controls in Nanomaterial Production and Downstream Handling Processes