Filtration is the other main function of a filter-drier and is accomplished by different methods. Some driers use only one method; others may use a combination of methods.

There are two primary means of mechanical filtration: surface and depth.

The simplest form of surface filtration is the screen. The screen is usually a woven wire mesh that catches particles that are larger than the holes in the screen. Until the screen has captured enough particles to provide a layer across the entire surface, particles that are smaller than the holes will pass through the screen. In addition, a particle longer than a hole can pass through if its cross-section is smaller than the hole.

As layers of contaminant cover the screen, it will become a depth filter as the layer of contaminant will act as a filter to remove smaller particles that would ordinarily pass through the screen. This layering of contaminant will continue until the pressure drop across the screen reaches the point at which the refrigerant flashes into vapor.

Depth filtration takes different forms. The most common depth filters are:

• Bonded desiccant cores
• Rigid fiber glass filters bonded with phenolic resin; and
• Fiberglass pad filters.

Depth filters force the fluid and contaminant to take an indirect route through the filter. Contaminants are trapped in the maze of openings that are spread throughout the filter. Depending on the type of filter, the openings will vary significantly.