Primarily devices for removing particles from aerosols. Filters utilize a variety of microscopic forms and a variety of mechanisms to accomplish this. Most common are fibrous filters, in which the fibers are of cellulose (paper filters), but almost any fibrous material, including glass fiber, wool, asbestos , and finely spun polymers, has been used. Microscopically, these fibers collect fine particles because fine particles vibrate around their average position due to collision with air molecules (Brownian motion). These vibrations are likely to cause them to collide with the fibers as they pass through the filter. Larger particles are removed because, as the air stream carrying them passes through the filter, some of the particles are intercepted as they pass close to the fibers and touch them. Other particles are in air streams that would cause them to miss the fibers, but when the air stream bends to go around the fibers the momentum, of the particles is too much to let them remain with the stream, so that they are "centrifuged out" onto the fibers (impaction). By electrophoresis, still other particles may be attracted to the fibers by electric charges of opposite sign on the particles and on the fibers. Finally, particles may simply be larger than the space between fibers, and be sifted out of the air in a process called sieving.
Filters are also formed by a process in which polymers such as cellulose esters are made into a film out of a solution in an organic solvent containing water. As the solvent evaporates, a point is reached at which the water separates out as microscopic droplets, in which the polymer is not soluble. The final result is a film of polymer full of microscopic holes where the water droplets once were. Such filters can have pore sizes from a small fraction of a micrometer to a few micrometers. (One micrometer equals 0.00004 in) These are called membrane filters.
Another form of membrane filter is formed from the polymer called polycarbonate. A thin film of this material is fastened to a surface of uranium metal and placed in a nuclear reactor for a time. In the reactor, the uranium undergoes nuclear fission , and gives off particles called fission fragments, atoms of the elements formed when the uranium atoms split. Every place that an atom from the fissioning uranium passes through the film is disturbed on a molecular scale. After removal from the reactor, if the polymer sheet is soaked in alkali, the disturbed material is dissolved. The amount of material dissolved is controlled by the temperature of the solution and the amount of time the film is treated. Since the fission fragments are very energetic, they travel in straight lines, and so the holes left after the alkali treatment are very straight and round. Again, pore sizes can be from a small fraction of a micrometer to a few micrometers. These filters are known by their trade name, Nuclepore.
In both types of membrane filters, the small pore size increases the role of sieving in particle removal. Because of their very simple structure, Nuclepore filters have been much studied to understand filtration mechanisms, since they are far easier to represent mathematically than a random arrangement of fibers.
It was mentioned above that small particles are collected because of their Brownian motion, while larger particles are removed by interception, impaction, and sieving. Under many conditions, a particle of intermediate size may pass through, too large for Brownian diffusion, and too small for impaction, interception, or sieving. Hence many filters may show a penetration maximum for particles of a few tenths of a micrometer. For this reason, standard methods of filter testing specify that the aerosol test for determining the efficiency of filters should contain particles in that size range. This phenomenon has also been used to select relatively uniform particles of that size out of mixtures of many sizes.
In circumstances where filter strength is of paramount importance, such as in industrial filters where a large air flow must pass through a relatively small filter area, filters of woven cloth are used, made of materials ranging from cotton to glass fiber and asbestos, these last for use when very hot gases must be filtered. The woven fabric itself is not a particularly good filter, but it retains enough particles to form a particle cake on the surface, and that soon becomes the filter. When the cake becomes thick enough to slow airflow to an unacceptable degree, the air flow is interrupted briefly, and the filters are shaken to dislodge the filter cake, which falls into bins at the bottom of the filters. Then filtration is resumed, allowing the cloth filters to be used for months before being replaced. A familiar domestic example is the bag of a home vacuum cleaner. Cement plants and some electric power plants use dozens of cloth bags up to several feet in diameter and more than ten feet (three meters) in length to remove particles from their waste gases.
Otherwise poor filters can be made efficient by making them thick. A glass tube can be partially plugged with a wad of cotton or glass fiber, then nearly filled with crystals of sugar or naphthalene and used as a filter; this is advantageous since sugar can be dissolved in water, or naphthalene will sublime away if gently heated, leaving behind the collected particles.
[James P. Lodge Jr. ]