Types of Air Filters

Filters are usually classified as:
  • Fibrous Filters
  • Porous Membrane Filters
  • Capillary Porous Membrane Filters
  • Fabric Filters

The most important types are fibrous and porous membrane filters.

Fibrous Filters

Fibrous filters consist of a mat of fine fibers arranged in such a way that most are perpendicular to the direction of air flow. It is mostly air inside. Thus the porosity is high -- from 70% to more than 99%. The size of the fibers ranges from submicrometers to 100 mm. The most common types of fibers in fibrous filters are cellulose (wood) fibers, glass fibers, and plastic fibers. The air velocity inside fibrous filter is often in the order of 10 cm/s.

Porous Filters

Porous membrane filters have a structure as seen in the figure on the right. The porosity is less, ranging from 50% to 90%. Due to the complex pore structure, the gas inside follows an irregular path. These filters have high efficiency and greater pressure drop than other types of filters. Porous filters include cellulose esters, sintered metals, polyvinyl chloride, Teflon, and other plastics.

Capillary Porous Membrane Filters

This type of filter has a matrix of cylindrical pores of uniform diameter and approximately perpendicular to the filter surface. These filters are particularly used for collecting particles in a scanning electron microscope because of their smooth surface.

Fabric Filters

These types of filters are often confused with fibrous filters. They are especially useful in industrial cleaning of high dust concentrations. They usually consist of large parallel fabric bags, woven or felted. Their efficiency depends upon the dust layer that is built up on the fabrics. The porous dust layer supported by the fabric is what makes the filter efficient. A simplified example is a home vacuum cleaner.