The materials used in filters and strainers are classified as mechanical, absorbent, or adsorbent. Most strainer material is of the mechanical type, which operates by trapping particles between closely woven metal screens and/or disks, and metal baskets. The mechanical type of material is used mostly where the particles removed from the medium are of a relatively coarse nature.

Absorbent filters are used for most minuteparticle filtration in fluid systems. They are made of a wide range of porous materials, including paper, wood pulp, cotton, yarn, and cellulose.

Figure 9-44.-Filter assembly using a surface-type element.

Paper filters are usually resin-impregnated for strength.

Adsorbent (or active) filters, such as charcoal and fuller's earth, are used mostly in gaseous or vapors systems. This type of filter material should not be used in hydraulic systems since they remove essential additives from the hydraulic fluid.

CONSTRUCTION OF FILTER ELEMENTS

Filter elements are constructed in various ways. The three most common filter element construction types are the surface type (most common), the depth type, and the edge type.

Surface-type filter elements (fig 9-44) are made of closely woven fabric or treated paper with pores to allow fluid to flow through. Very accurate control of the pore size is a feature of the surface-type elements.

A depth-type filter element (fig 9-45) is composed of layers of a fabric or fibers, which provide many tortuous paths for the fluid to flow through. The pores or passages vary in size, and the degree of filtration depends on the flow rate. Increases in flow rate tend to dislodge trapped particles. This filter is limited to low-flow, low pressure-drop conditions.

Figure 9-45.-Depth-type filter element.

Figure 9-46.-Edge-type filter element.

An edge-type filter element fig. 9-46 separates particles rom fluids passing between finely spaced plates. The filter shown features stationary cleaner blades that scrape out the collected contaminants when the handle is twisted to turn the element.