What is a filter?

   A filter is a material or device used to prevent the passage of one or more components of a mixture. Light filters, for example, block off or absorb certain wavelenghts of light. Other filters are porous materials used to separate solid particles from liquids and gases. The gas or liquid passes through the porous material, but the solid particles are trapped in or on one side of the filter. Filters for separating solid particles from liquids and gases are used in research chemistry, in industrial processes, in water purification, and in air conditioners. The process of using such a filter is called filtration.

   A liquid or gas passes through a filter because pressure is greater on one side of the filter than on the other. The pressure difference may be caused by gravity, by pressure applied to the liquid or gas on the upstream side of the filter, or by vacuum applied on the downstream side of the filter.

   Filters that separate particles from gases are usually called air filters or dust collectors. Air filters remove solid particles from air before the air is blown into turbines, dryers, and other industrial machines. Air conditioners use air filters. Most air filters trap solid particles in mats or in masses of porous material. Dust collectors remove dust and other undesirable particles from air or other gases. The air or gas is passed through fabric bags enclosed in a pressure chamber while the particles are trapped inside the bags.

   Filters that separate solids from liquids can be quite complicated. They are used in many industrial and chemical processes. Some of the materials used as filters are paper, carbon, mineral crystals, metallic dost, sand and gravel, organic fibers, and diatomaceous earth. Often mechanical devices are used to separate solids from liquids. The devices usually employ pressure or vacuum.