The structure of the filter material can be divided into two types: surface type and depth type.

(1) surface type

The surface type filter material is a fine filter mesh woven with a wire, and is also used in other forms such as wire winding. The filtration accuracy is determined by the size of the woven holes or the size of the wire gap. Any particles larger than the size or clearance of the aperture can not pass through just the ideal state, which is not practical in practice. The woven filter or wire is wound into a filter element having the same size of pores, and its filtering action only occurs on the surface of the filter screen, so it is called surface type filtration. As the dirt accumulates on the surface of the screen, its pores are constantly blocked, so that the pressure drop is gradually increased; the filtration efficiency is also lowered because the particles trapped in the mesh may be squeezed out of the mesh at any time. If it is desired to control the pressure drop within a certain range, the filter element should have a sufficiently large filtration area. This filter element can be reused after cleaning.

(2) Deep filter media

The deep filter media is made of fiber papermaking and has a certain thickness of filter media. The properties of this filter are quite different from those of surface type filtration. Its shape of the eyelet, the size is irregular, uneven, and the inner part is complicated by bending and twisting. Its filtering effect occurs not only on the surface of the filter medium but also in the depth of the filter medium, so it is called depth type filtration. Since the filter medium has a certain thickness and the passage is in all directions, its dirt holding capacity is much larger than that of the surface type filter, and the pressure drop is also much lower than that of the surface type filter. In addition, because of its wide range of pore size distribution, it can also filter out a large number of fine particles.

For the depth filter, the fiber diameter constituting the filter medium has a direct influence on the filtration efficiency and other properties. A filter layer made of a relatively small diameter fiber can form more through holes in a unit area, and can also form more channels in the interior. Therefore, its filtration efficiency and dirt holding capacity are higher than those of the coarser fiber. For the same reason, its pressure drop is also smaller than the latter. Therefore, the filter element made of fine fiber has a long working life.